ANAEROBIC AND AEROBIC OXIDATION OF GLUCOSE. ALTERNATIVE WAYS OF MONOSACCHARIDE METABOLISM. STUDING OF BIOSYNTHESIS AND CATABOLISM OF GLYCOGEN. REGULATION OF GLYCOGEN METABOLISM. BIOSYNTHESIS OF GLUCOSE – GLUCONEOGENESIS. METABOLISM OF LIPIDS: DIGESTION, ABSORBTION, RESYNTHESIS IN THE INTESTINAL WALL. METABOLISM OF LIPIDS: OXIDATION AND BIOSYNTHESIS OF FATTY ACIDS, TRIACYLGLYCEROLS AND PHOSPHOLIPIDS. BIOSYNTHESIS AND BIOTRANSFORMATION OF CHOLESTEROL. METABOLISM OF KETONE BODIES. REGULATION AND DISORDERS OF LIPID METABOLISM. DIGESTION OF PROTEINS. GENERAL PATHWAYS OF AMINO ACIDS TRANSFORMATION. DETOXIFICATION OF AMMONIA AND BIOSYNTHESIS OF UREA.
Carbohydrates have the general molecular formula CH2O, and thus were once thought to represent "hydrated carbon". However, the arrangement of atoms in carbohydrates has little to do with water molecules.
Starch and cellulose are two common carbohydrates. Both are macromolecules with molecular weights in the hundreds of thousands. Both are polymers (hence "polysaccharides"); that is, each is built from repeating units, monomers, much as a chain is built from its links.
The monomers of both starch and cellulose are the same: units of the sugar glucose.
Three common sugars share the same molecular formula: C6H12O6. Because of their six carbon atoms, each is a hexose.
· glucose, "blood sugar", the immediate source of energy for cellular respiration
· galactose, a sugar in milk (and yogurt), and
· fructose, a sugar found in honey.
Although all three share the same molecular formula (C6H12O6), the arrangement of atoms differs in each case. Substances such as these three, which have identical molecular formulas but different structural formulas, are known as structural isomers.
Glucose, galactose, and fructose are "single" sugars or monosaccharides. Two monosaccharides can be linked together to form a "double" sugar or disaccharide.
Three common disaccharides:
· sucrose — common table sugar = glucose + fructose
· lactose — major sugar in milk = glucose + galactose
· maltose — product of starch digestion = glucose + glucose
Although the process of linking the two monomers is rather complex, the end result in each case is the loss of a hydrogen atom (H) from one of the monosaccharides and a hydroxyl group (OH) from the other. The resulting linkage between the sugars is called a glycosidic bond. The molecular formula of each of these disaccharides is
C12H22O11 = 2 C6H12O6 − H2O
All sugars are very soluble in water because of their many hydroxyl groups. Although not as concentrated a fuel as fats, sugars are the most important source of energy for many cells.
Carbohydrates provide the bulk of the calories (4 kcal/gram) in most diets, and starches provide the bulk of that. Starches are polysaccharides.
Starches are polymers of glucose. Two types are found:
· amylose consists of linear, unbranched chains of several hundred glucose residues (units). The glucose residues are linked by a glycosidic bond between their #1 and #4 carbon atoms.
· amylopectin differs from amylose in being highly branched. At approximately every thirtieth residue along the chain, a short side chain is attached by a glycosidic bond to the #6 carbon atom (the carbon above the ring). The total number of glucose residues in a molecule of amylopectin is several thousand.
Starches are insoluble in water and thus can serve as storage depots of glucose. Plants convert excess glucose into starch for storage. The image shows starch grains (lightly stained with iodine) in the cells of the white potato. Rice, wheat, and corn are also major sources of starch in the human diet.
Before starches can enter (or leave) cells, they must be digested. The hydrolysis of starch is done by amylases. With the aid of an amylase (such as pancreatic amylase), water molecules enter at the 1 -> 4 linkages, breaking the chain and eventually producing a mixture of glucose and maltose. A different amylase is needed to break the 1 -> 6 bonds of amylopectin.
Animals store excess glucose by polymerizing it to form glycogen. The structure of glycogen is similar to that of amylopectin, although the branches in glycogen tend to be shorter and more frequent.
Glycogen is broken back down into glucose when energy is needed (a process called glycogenolysis).
· phosphate groups — not water — break the 1 -> 4 linkages
· the phosphate group must then be removed so that glucose can leave the cell.
The liver and skeletal muscle are major depots of glycogen.
There is some evidence that intense exercise and a high-carbohydrate diet ("carbo-loading") can increase the reserves of glycogen in the muscles and thus may help marathoners work their muscles somewhat longer and harder than otherwise. But for most of us, carbo loading leads to increased deposits of fat.
Cellulose is probably the single most abundant organic molecule in the biosphere. It is the major structural material of which plants are made. Wood is largely cellulose while cotton and paper are almost pure cellulose.
Like starch, cellulose is a polysaccharide with glucose as its monomer. However, cellulose differs profoundly from starch in its properties.
· Because of the orientation of the glycosidic bonds linking the glucose residues, the rings of glucose are arranged in a flip-flop manner. This produces a long, straight, rigid molecule.
· There are no side chains in cellulose as there are in starch. The absence of side chains allows these linear molecules to lie close together.
· Because of the many -OH groups, as well as the oxygen atom in the ring, there are many opportunities for hydrogen bonds to form between adjacent chains.
The result is a series of stiff, elongated fibrils — the perfect material for building the cell walls of plants.
This electron micrograph (courtesy of R. D. Preston) shows the cellulose fibrils in the cell wall of a green alga. These long, rigid fibrils are a clear reflection of the nature of the cellulose molecules of which they are composed.
Digestion of Dietary Carbohydrates
Dietary carbohydrate from which humans gain energy enter the body in complex forms, such as disaccharides and the polymers starch (amylose and amylopectin) and glycogen. The polymer cellulose is also consumed but not digested. The first step in the metabolism of digestible carbohydrate is the conversion of the higher polymers to simpler, soluble forms that can be transported across the intestinal wall and delivered to the tissues. The breakdown of polymeric sugars begins in the mouth. Saliva has a slightly acidic pH of 6.8 and contains lingual amylase that begins the digestion of carbohydrates. The action of lingual amylase is limited to the area of the mouth and the esophagus; it is virtually inactivated by the much stronger acid pH of the stomach. Once the food has arrived in the stomach, acid hydrolysis contributes to its degradation; specific gastric proteases and lipases aid this process for proteins and fats, respectively. The mixture of gastric secretions, saliva, and food, known collectively as chyme, moves to the small intestine.
The main polymeric-carbohydrate digesting enzyme of the small intestine is -amylase. This enzyme is secreted by the pancreas and has the same activity as salivary amylase, producing disaccharides and trisaccharides. The latter are converted to monosaccharides by intestinal saccharidases, including maltases that hydrolyze di- and trisaccharides, and the more specific disaccharidases, sucrase, lactase, and trehalase. The net result is the almost complete conversion of digestible carbohydrate to its constituent monosaccharides. The resultant glucose and other simple carbohydrates are transported across the intestinal wall to the hepatic portal vein and then to liver parenchymal cells and other tissues. There they are converted to fatty acids, amino acids, and glycogen, or else oxidized by the various catabolic pathways of cells.
Oxidation of glucose is known as glycolysis.Glucose is oxidized to
either lactate or pyruvate. Under aerobic conditions, the dominant product in
most tissues is pyruvate and the
pathway is known as aerobic glycolysis.
When oxygen is depleted, as for instance during prolonged vigorous exercise, the
dominant glycolytic product in many tissues is lactate and the process is known as anaerobic glycolysis.
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Aerobic glycolysis of glucose to pyruvate, requires two equivalents of ATP to activate the process, with the subsequent production of four equivalents of ATP and two equivalents of NADH. Thus, conversion of one mole of glucose to two moles of pyruvate is accompanied by the net production of two moles each of ATP and NADH.
Glucose + 2 ADP + 2 NAD+ + 2 Pi -----> 2 Pyruvate + 2 ATP + 2 NADH + 2 H+
The NADH generated during glycolysis
is used to fuel mitochondrial ATP synthesis via oxidative phosphorylation, producing either
two or three equivalents of ATP depending upon whether the glycerol phosphate shuttle or the malate-aspartate shuttle is used to
transport the electrons from cytoplasmic NADH into the mitochondria. The net
yield from the oxidation of 1 mole of glucose to 2 moles of pyruvate is,
therefore, either 6 or 8 moles of ATP. Complete oxidation of the 2 moles of
pyruvate, through the TCA cycle, yeilds an additional 30 moles of
ATP; the total yield, therefore being either 36 or 38 moles of ATP from the
complete oxidation of 1 mole of glucose to CO2 and H2O.
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The pathway of glycolysis can be seen as consisting of 2 separate phases. The first is the chemical priming phase requiring energy in the form of ATP, and the second is considered the energy-yielding phase. In the first phase, 2 equivalents of ATP are used to convert glucose to fructose 1,6-bisphosphate (F1,6BP). In the second phase F1,6BP is degraded to pyruvate, with the production of 4 equivalents of ATP and 2 equivalents of NADH.
Pathway of glycolysis from glucose to pyruvate. Substrates and products are in blue, enzymes are in green. The two high energy intermediates whose oxidations are coupled to ATP synthesis are shown in red (1,3-bisphosphoglycerate and phosphoenolpyruvate).
The Hexokinase Reaction:
The ATP-dependent phosphorylation of glucose to form glucose 6-phosphate (G6P)is the first reaction of glycolysis, and is catalyzed by tissue-specific isoenzymes known as hexokinases. The phosphorylation accomplishes two goals: First, the hexokinase reaction converts nonionic glucose into an anion that is trapped in the cell, since cells lack transport systems for phosphorylated sugars. Second, the otherwise biologically inert glucose becomes activated into a labile form capable of being further metabolized.
Four mammalian isozymes of hexokinase are known (Types I - IV), with the Type IV isozyme often referred to as glucokinase. Glucokinase is the form of the enzyme found in hepatocytes. The high Km of glucokinase for glucose means that this enzyme is saturated only at very high concentrations of substrate.
Comparison of the activities of hexokinase and glucokinase. The Km for hexokinase is significantly lower (0.1mM) than that of glucokinase (10mM). This difference ensures that non-hepatic tissues (which contain hexokinase) rapidly and efficiently trap blood glucose within their cells by converting it to glucose-6-phosphate. One major function of the liver is to deliver glucose to the blood and this in ensured by having a glucose phosphorylating enzyme (glucokinase) whose Km for glucose is sufficiently higher that the normal circulating concentration of glucose (5mM).
This feature of hepatic glucokinase allows the liver to buffer blood glucose. After meals, when postprandial blood glucose levels are high, liver glucokinase is significantly active, which causes the liver preferentially to trap and to store circulating glucose. When blood glucose falls to very low levels, tissues such as liver and kidney, which contain glucokinases but are not highly dependent on glucose, do not continue to use the meager glucose supplies that remain available. At the same time, tissues such as the brain, which are critically dependent on glucose, continue to scavenge blood glucose using their low Km hexokinases, and as a consequence their viability is protected. Under various conditions of glucose deficiency, such as long periods between meals, the liver is stimulated to supply the blood with glucose through the pathway of gluconeogenesis. The levels of glucose produced during gluconeogenesis are insufficient to activate glucokinase, allowing the glucose to pass out of hepatocytes and into the blood.
The regulation of hexokinase and glucokinase activities is also different. Hexokinases I, II, and III are allosterically inhibited by product (G6P) accumulation, whereas glucokinases are not. The latter further insures liver accumulation of glucose stores during times of glucose excess, while favoring peripheral glucose utilization when glucose is required to supply energy to peripheral tissues.
The second reaction of glycolysis is an isomerization, in which G6P is converted to fructose 6-phosphate (F6P). The enzyme catalyzing this reaction is phosphohexose isomerase (also known as phosphoglucose isomerase). The reaction is freely reversible at normal cellular concentrations of the two hexose phosphates and thus catalyzes this interconversion during glycolytic carbon flow and during gluconeogenesis.
6-Phosphofructo-1-Kinase (Phosphofructokinase-1, PFK-1):
The next reaction of glycolysis involves the utilization of a second ATP to convert F6P to fructose 1,6-bisphosphate (F1,6BP). This reaction is catalyzed by 6-phosphofructo-1-kinase, better known as phosphofructokinase-1 or PFK-1. This reaction is not readily reversible because of its large positive free energy (G0' = +5.4 kcal/mol) in the reverse direction. Nevertheless, fructose units readily flow in the reverse (gluconeogenic) direction because of the ubiquitous presence of the hydrolytic enzyme, fructose-1,6-bisphosphatase (F-1,6-BPase).
The presence of these two enzymes in the same cell compartment provides an example of a metabolic futile cycle, which if unregulated would rapidly deplete cell energy stores. However, the activity of these two enzymes is so highly regulated that PFK-1 is considered to be the rate-limiting enzyme of glycolysis and F-1,6-BPase is considered to be the rate-limiting enzyme in gluconeogenesis.
Aldolase catalyses the hydrolysis of F1,6BP into two 3-carbon products: dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P). The aldolase reaction proceeds readily in the reverse direction, being utilized for both glycolysis and gluconeogenesis.
Triose Phosphate Isomerase: \
The two products of the aldolase reaction equilibrate readily in a reaction catalyzed by triose phosphate isomerase. Succeeding reactions of glycolysis utilize G3P as a substrate; thus, the aldolase reaction is pulled in the glycolytic direction by mass action principals.
The second phase of glucose catabolism features the energy-yielding glycolytic reactions that produce ATP and NADH. In the first of these reactions, glyceraldehyde-3-P dehydrogenase (G3PDH) catalyzes the NAD+-dependent oxidation of G3P to 1,3-bisphosphoglycerate (1,3BPG) and NADH. The G3PDH reaction is reversible, and the same enzyme catalyzes the reverse reaction during gluconeogenesis.
The high-energy phosphate of 1,3-BPG is used to form ATP and 3-phosphoglycerate (3PG) by the enzyme phosphoglycerate kinase. Note that this is the only reaction of glycolysis or gluconeogenesis that involves ATP and yet is reversible under normal cell conditions. Associated with the phosphoglycerate kinase pathway is an important reaction of erythrocytes, the formation of 2,3-bisphosphoglycerate, 2,3BPG (see Figure below) by the enzyme bisphosphoglycerate mutase. 2,3BPG is an important regulator of hemoglobin's affinity for oxygen. Note that 2,3-bisphosphoglycerate phosphatase degrades 2,3BPG to 3-phosphoglycerate, a normal intermediate of glycolysis. The 2,3BPG shunt thus operates with the expenditure of 1 equivalent of ATP per triose passed through the shunt. The process is not reversible under physiological conditions.
The pathway for 2,3-bisphosphoglycerate (2,3-BPG) synthesis within erythrocytes. Synthesis of 2,3-BPG represents a major reaction pathway for the consumption of glucose in erythrocytes. The synthesis of 2,3-BPG in erythrocytes is critical for controlling hemoglobin affinity for oxygen. Note that when glucose is oxidized by this pathway the erythrocyte loses the ability to gain 2 moles of ATP from glycolytic oxidation of 1,3-BPG to 3-phosphoglycerate via the phosphoglycerate kinase reaction.
Phosphoglycerate Mutase and Enolase:
The remaining reactions of glycolysis are aimed at converting the relatively low energy phosphoacyl-ester of 3PG to a high-energy form and harvesting the phosphate as ATP. The 3PG is first converted to 2PG by phosphoglycerate mutase and the 2PG conversion to phosphoenoylpyruvate (PEP) is catalyzed by enolase
The final reaction of aerobic glycolysis is catalyzed by the highly regulated enzyme pyruvate kinase (PK). In this strongly exergonic reaction, the high-energy phosphate of PEP is conserved as ATP. The loss of phosphate by PEP leads to the production of pyruvate in an unstable enol form, which spontaneously tautomerizes to the more stable, keto form of pyruvate. This reaction contributes a large proportion of the free energy of hydrolysis of PEP.
Under aerobic conditions, pyruvate in most cells is further metabolized via the TCA cycle. Under anaerobic conditions and in erythrocytes under aerobic conditions, pyruvate is converted to lactate by the enzyme lactate dehydrogenase (LDH), and the lactate is transported out of the cell into the circulation. The conversion of pyruvate to lactate, under anaerobic conditions, provides the cell with a mechanism for the oxidation of NADH (produced during the G3PDH reaction) to NAD+; which occurs during the LDH catalyzed reaction. This reduction is required since NAD+ is a necessary substrate for G3PDH, without which glycolysis will cease. Normally, during aerobic glycolysis the electrons of cytoplasmic NADH are transferred to mitochondrial carriers of the oxidative phosphorylation pathway generating a continuous pool of cytoplasmic NAD+.
Aerobic glycolysis generates substantially more ATP per mole of glucose oxidized than does anaerobic glycolysis. The utility of anaerobic glycolysis, to a muscle cell when it needs large amounts of energy, stems from the fact that the rate of ATP production from glycolysis is approximately 100X faster than from oxidative phosphorylation. During exertion muscle cells do not need to energize anabolic reaction pathways. The requirement is to generate the maximum amount of ATP, for muscle contraction, in the shortest time frame. This is why muscle cells derive almost all of the ATP consumed during exertion from anaerobic glycolysis.
The reactions catalyzed by hexokinase, PFK-1 and PK all proceed with a relatively large free energy decrease. These nonequilibrium reactions of glycolysis would be ideal candidates for regulation of the flux through glycolysis. Indeed, in vitro studies have shown all three enzymes to be allosterically controlled.
Regulation of hexokinase, however, is not the major control point in glycolysis. This is due to the fact that large amounts of G6P are derived from the breakdown of glycogen (the predominant mechanism of carbohydrate entry into glycolysis in skeletal muscle) and, therefore, the hexokinase reaction is not necessary. Regulation of PK is important for reversing glycolysis when ATP is high in order to activate gluconeogenesis. As such this enzyme catalyzed reaction is not a major control point in glycolysis. The rate limiting step in glycolysis is the reaction catalyzed by PFK-1.
PFK-1 is a tetrameric enzyme that exist in two conformational states termed R and T that are in equilibrium. ATP is both a substrate and an allosteric inhibitor of PFK-1. Each subunit has two ATP binding sites, a substrate site and an inhibitor site. The substrate site binds ATP equally well when the tetramer is in either conformation. The inhibitor site binds ATP essentially only when the enzyme is in the T state. F6P is the other substrate for PFK-1 and it also binds preferentially to the R state enzyme. At high concentrations of ATP, the inhibitor site becomes occupied and shifting the equilibrium of PFK-1 comformation to that of the T state decreasing PFK-1's ability to bind F6P. The inhibition of PFK-1 by ATP is overcome by AMP which binds to the R state of the enzyme and, therefore, stabilizes the conformation of the enzyme capable of binding F6P. The most important allosteric regulator of both glycolysis and gluconeogenesis is fructose 2,6-bisphosphate, F2,6BP, which is not an intermediate in glycolysis or in gluconeogenesis.
Regulation of glycolysis and gluconeogenesis by fructose 2,6-bisphosphate (F2,6BP). The major sites for regulation of glycolysis and gluconeogenesis are the phosphofructokinase-1 (PFK-1) and fructose-1,6-bisphosphatase (F-1,6-BPase) catalyzed reactions. PFK-2 is the kinase activity and F-2,6-BPase is the phosphatase activity of the bi-functional regulatory enzyme, phosphofructokinase-2/fructose-2,6-bisphosphatase. PKA is cAMP-dependent protein kinase which phosphorylates PFK-2/F-2,6-BPase turning on the phosphatase activity. (+ve) and (-ve) refer to positive and negative activities, respectively.
The synthesis of F2,6BP is catalyzed by the bifunctional enzyme phosphofructokinase-2/fructose-2,6-bisphosphatase (PFK-2/F-2,6-BPase). In the nonphosphorylated form the enzyme is known as PFK-2 and serves to catalyze the synthesis of F2,6BP by phosphorylating fructose 6-phosphate. The result is that the activity of PFK-1 is greatly stimulated and the activity of F-1,6-BPase is greatly inhibited.
Under conditions where PFK-2 is active, fructose flow through the PFK-1/F-1,6-BPase reactions takes place in the glycolytic direction, with a net production of F1,6BP. When the bifunctional enzyme is phosphorylated it no longer exhibits kinase activity, but a new active site hydrolyzes F2,6BP to F6P and inorganic phosphate. The metabolic result of the phosphorylation of the bifunctional enzyme is that allosteric stimulation of PFK-1 ceases, allosteric inhibition of F-1,6-BPase is eliminated, and net flow of fructose through these two enzymes is gluconeogenic, producing F6P and eventually glucose.
The interconversion of the bifunctional enzyme is catalyzed by cAMP-dependent protein kinase (PKA), which in turn is regulated by circulating peptide hormones. When blood glucose levels drop, pancreatic insulin production falls, glucagon secretion is stimulated, and circulating glucagon is highly increased. Hormones such as glucagon bind to plasma membrane receptors on liver cells, activating membrane-localized adenylate cyclase leading to an increase in the conversion of ATP to cAMP (see diagram below). cAMP binds to the regulatory subunits of PKA, leading to release and activation of the catalytic subunits. PKA phosphorylates numerous enzymes, including the bifunctional PFK-2/F-2,6-BPase. Under these conditions the liver stops consuming glucose and becomes metabolically gluconeogenic, producing glucose to reestablish normoglycemia.
Representative pathway for the activation of cAMP-dependent protein kinase (PKA). In this example glucagon binds to its' cell-surface receptor, thereby activating the receptor. Activation of the receptor is coupled to the activation of a receptor-coupled G-protein (GTP-binding and hydrolyzing protein) composed of 3 subunits. Upon activation the alpha subunit dissociates and binds to and activates adenylate cyclase. Adenylate cylcase then converts ATP to cyclic-AMP (cAMP). The cAMP thus produced then binds to the regulatory subunits of PKA leading to dissociation of the associated catalytic subunits. The catalytic subunits are inactive until dissociated from the regulatory subunits. Once released the catalytic subunits of PKA phosphorylate numerous substrate using ATP as the phosphate donor.
Regulation of glycolysis also occurs at the step catalyzed by pyruvate kinase, (PK). The liver enzyme has been most studied in vitro. This enzyme is inhibited by ATP and acetyl-CoA and is activated by F1,6BP. The inhibition of PK by ATP is similar to the effect of ATP on PFK-1. The binding of ATP to the inhibitor site reduces its affinity for PEP. The liver enzyme is also controlled at the level of synthesis. Increased carbohydrate ingestion induces the synthesis of PK resulting in elevated cellular levels of the enzyme.
A number of PK isozymes have been described. The liver isozyme (L-type), characteristic of a gluconeogenic tissue, is regulated via phosphorylation by PKA, whereas the M-type isozyme found in brain, muscle, and other glucose requiring tissue is unaffected by PKA. As a consequence of these differences, blood glucose levels and associated hormones can regulate the balance of liver gluconeogenesis and glycolysis while muscle metabolism remains unaffected.
In erythrocytes, the fetal PK isozyme has much greater activity than the adult isozyme; as a result, fetal erythrocytes have comparatively low concentrations of glycolytic intermediates. Because of the low steady-state concentration of fetal 1,3BPG, the 2,3BPG shunt (see diagram above) is greatly reduced in fetal cells and little 2,3BPG is formed. Since 2,3BPG is a negative effector of hemoglobin affinity for oxygen, fetal erythrocytes have a higher oxygen affinity than maternal erythrocytes. Therefore, transfer of oxygen from maternal hemoglobin to fetal hemoglobin is favored, assuring the fetal oxygen supply. In the newborn, an erythrocyte isozyme of the M-type with comparatively low PK activity displaces the fetal type, resulting in an accumulation of glycolytic intermediates. The increased 1,3BPG levels activate the 2,3BPG shunt, producing 2,3BPG needed to regulate oxygen binding to hemoglobin.
Genetic diseases of adult erythrocyte PK are known in which the kinase is virtually inactive. The erythrocytes of affected individuals have a greatly reduced capacity to make ATP and thus do not have sufficient ATP to perform activities such as ion pumping and maintaining osmotic balance. These erythrocytes have a short half-life, lyse readily, and are responsible for some cases of hereditary hemolytic anemia.
The liver PK isozyme is regulated by phosphorylation, allosteric effectors, and modulation of gene expression. The major allosteric effectors are F1,6BP, which stimulates PK activity by decreasing its Km(app) for PEP, and for the negative effector, ATP. Expression of the liver PK gene is strongly influenced by the quantity of carbohydrate in the diet, with high-carbohydrate diets inducing up to 10-fold increases in PK concentration as compared to low carbohydrate diets. Liver PK is phosphorylated and inhibited by PKA, and thus it is under hormonal control similar to that described earlier for PFK-2.
Muscle PK (M-type) is not regulated by the same mechanisms as the liver enzyme. Extracellular conditions that lead to the phosphorylation and inhibition of liver PK, such as low blood glucose and high levels of circulating glucagon, do not inhibit the muscle enzyme. The result of this differential regulation is that hormones such as glucagon and epinephrine favor liver gluconeogenesis by inhibiting liver glycolysis, while at the same time, muscle glycolysis can proceed in accord with needs directed by intracellular conditions.
Pyruvate is the branch point molecule of glycolysis. The ultimate fate of pyruvate depends on the oxidation state of the cell. In the reaction catalyzed by G3PDH a molecule of NAD+ is reduced to NADH. In order to maintain the re-dox state of the cell, this NADH must be re-oxidized to NAD+. During aerobic glycolysis this occurs in the mitochondrial electron transport chain generating ATP. Thus, during aerobic glycolysis ATP is generated from oxidation of glucose directly at the PGK and PK reactions as well as indirectly by re-oxidation of NADH in the oxidative phosphorylation pathway. Additional NADH molecules are generated during the complete aerobic oxidation of pyruvate in the TCA cycle. Pyruvate enters the TCA cycle in the form of acetyl-CoA which is the product of the pyruvate dehydrogenase reaction. The fate of pyruvate during anaerobic glycolysis is reduction to lactate.
During anaerobic glycolysis, that period of time when glycolysis is proceeding at a high rate (or in anaerobic organisms), the oxidation of NADH occurs through the reduction of an organic substrate. Erythrocytes and skeletal muscle (under conditions of exertion) derive all of their ATP needs through anaerobic glycolysis. The large quantity of NADH produced is oxidized by reducing pyruvate to lactate. This reaction is carried out by lactate dehydrogenase, (LDH). The lactate produced during anaerobic glycolysis diffuses from the tissues and is transproted to highly aerobic tissues such as cardiac muscle and liver. The lactate is then oxidized to pyruvate in these cells by LDH and the pyruvate is further oxidized in the TCA cycle. If the energy level in these cells is high the carbons of pyruvate will be diverted back to glucose via the gluconeogenesis pathway.
Mammalian cells contain two distinct types of LDH
subunits, termed M and H. Combinations of these different subunits generates
LDH isozymes with different characteristics. The H type subunit predominates in
aerobic tissues such as heart muscle (as the H4 tetramer) while the M subunit
predominates in anaerobic tissues such as skeletal muscle as the M4 tetramer).
H4 LDH has a low Km for pyruvate and also is inhibited by high
levels of pyruvate. The M4 LDH enzyme has a high Km for pyruvate and
is not inhibited by pyruvate. This suggsts that the H-type LDH is utilized for
oxidizing lactate to pyruvate and the M-type the reverse.
Animal cells (primarily hepatocytes) contain the cytosolic enzyme alcohol dehydrogenase (ADH) which oxidizes ethanol to acetaldehyde. Acetaldehyde then enters the mitochondria where it is oxidized to acetate by acetaldehyde dehydrogenase (AcDH).
Acetaldehyde forms adducts with proteins, nucleic acids and other compounds, the results of which are the toxic side effects (the hangover) that are associated with alcohol consumption. The ADH and AcDH catalyzed reactions also leads to the reduction of NAD+ to NADH. The metabolic effects of ethanol intoxication stem from the actions of ADH and AcDH and the resultant cellular imbalance in the NADH/NAD+. The NADH produced in the cytosol by ADH must be reduced back to NAD+ via either the malate-aspartate shuttle or the glycerol-phosphate shuttle. Thus, the ability of an individual to metabolize ethanol is dependent upon the capacity of hepatocytes to carry out eother of these 2 shuttles, which in turn is affected by the rate of the TCA cycle in the mitochondria whose rate of function is being impacted by the NADH produced by the AcDH reaction. The reduction in NAD+ impairs the flux of glucose through glycolysis at the glyceraldehyde-3-phosphate dehydrogenase reaction, thereby limiting energy production. Additionally, there is an increased rate of hepatic lactate production due to the effect of increased NADH on direction of the hepatic lactate dehydrogenase (LDH) reaction. This reverseral of the LDH reaction in hepatocytes diverts pyruvate from gluconeogenesis leading to a reduction in the capacity of the liver to deliver glucose to the blood.
In addition to the negative effects of the altered NADH/NAD+ ratio on hepatic gluconeogenesis, fatty acid oxidation is also reduced as this process requires NAD+ as a cofactor. In fact the opposite is true, fatty acid synthesis is increased and there is an increase in triacylglyceride production by the liver. In the mitocondria, the production of acetate from acetaldehyde leads to increased levels of acetyl-CoA. Since the increased generation of NADH also reduces the activity of the TCA cycle, the acetyl-CoA is diverted to fatty acid synthesis. The reduction in cytosolic NAD+ leads to reduced activity of glycerol-3-phosphate dehydrogenase (in the glcerol 3-phosphate to DHAP direction) resulting in increased levels of glycerol 3-phosphate which is the backbone for the synthesis of the triacylglycerides. Both of these two events lead to fatty acid deposition in the liver leading to fatty liver syndrome.
If for no other reason, it is because of the demands of the brain for oxidizable glucose that the human body exquisitely regulates the level of glucose circulating in the blood. This level is maintained in the range of 5mM.
Nearly all carbohydrates ingested in the diet are converted to glucose following transport to the liver. Catabolism of dietary or cellular proteins generates carbon atoms that can be utilized for glucose synthesis via gluconeogenesis. Additionally, other tissues besides the liver that incompletely oxidize glucose (predominantly skeletal muscle and erythrocytes) provide lactate that can be converted to glucose via gluconeogenesis.
Maintenance of blood glucose homeostasis is of paramount importance to the survival of the human organism. The predominant tissue responding to signals that indicate reduced or elevated blood glucose levels is the liver. Indeed, one of the most important functions of the liver is to produce glucose for the circulation. Both elevated and reduced levels of blood glucose trigger hormonal responses to initiate pathways designed to restore glucose homeostasis. Low blood glucose triggers release of glucagon from pancreatic -cells. High blood glucose triggers release of insulin from pancreatic -cells. Additional signals, ACTH and growth hormone, released from the pituitary act to increase blood glucose by inhibiting uptake by extrahepatic tissues. Glucocorticoids also act to increase blood glucose levels by inhibiting glucose uptake. Cortisol, the major glucocorticoid released from the adrenal cortex, is secreted in response to the increase in circulating ACTH. The adrenal medullary hormone, epinephrine, stimulates production of glucose by activating glycogenolysis in response to stressful stimuli.
Glucagon binding to its' receptors on the surface of liver cells triggers an increase in cAMP production leading to an increased rate of glycogenolysis by activating glycogen phosphorylase via the PKA-mediated cascade. This is the same response hepatocytes have to epinephrine release. The resultant increased levels of G6P in hepatocytes is hydrolyzed to free glucose, by glucose-6-phosphatase, which then diffuses to the blood. The glucose enters extrahepatic cells where it is re-phosphorylated by hexokinase. Since muscle and brain cells lack glucose-6-phosphatase, the glucose-6-phosphate product of hexokinase is retained and oxidized by these tissues.
In opposition to the cellular responses to glucagon (and epinephrine on hepatocytes), insulin stimulates extrahepatic uptake of glucose from the blood and inhibits glycogenolysis in extrahepatic cells and conversely stimulates glycogen synthesis. As the glucose enters hepatocytes it binds to and inhibits glycogen phosphorylase activity. The binding of free glucose stimulates the de-phosphorylation of phosphorylase thereby, inactivating it. Why is it that the glucose that enters hepatocytes is not immediately phosphorylated and oxidized? Liver cells contain an isoform of hexokinase called glucokinase. Glucokinase has a much lower affinity for glucose than does hexokinase. Therefore, it is not fully active at the physiological ranges of blood glucose. Additionally, glucokinase is not inhibited by its product G6P, whereas, hexokinase is inhibited by G6P.
One major response of non-hepatic
tissues to insulin is the recruitment, to the cell surface, of glucose
transporter complexes. Glucose transporters comprise a family of five members, GLUT-1 to GLUT-5. GLUT-1 is
ubiquitously distributed in various tissues. GLUT-2 is found primarily in
intestine, kidney and liver. GLUT-3 is also found in the intestine and GLUT-
Hepatocytes, unlike most other cells,
are freely permeable to glucose and are, therefore, essentially unaffected by
the action of insulin at the level of increased glucose uptake. When blood
glucose levels are low the liver does not compete with other tissues for
glucose since the extrahepatic uptake of glucose is stimulated in response to
insulin. Conversely, when blood glucose levels are high extrahepatic needs are
satisfied and the liver takes up glucose for conversion into glycogen for future needs. Under conditions
of high blood glucose, liver glucose levels will be high and the activity of
glucokinase will be elevated. The G6P produced by glucokinase is rapidly
converted to G1P by phosphoglucomutase, where it can then be incorporated into
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Digestion & Absorption of Proteins & Carbohydrates
Digestion and Absorption of Proteins
1. Humans must ingest proteins, carbohydrates and lipids to maintain tissue and organ functions.
2. Most of these nutrients consist of large polymers that must be broken down before they can be made available to the intestinal cells for transport.
3. Dietary proteins are cleaved by hydrolases with specificity for the peptide bond (peptidases).
4. Endopeptidases (aka Proteases): attack internal protein bonds liberating large peptide fragments.
Exopeptidases: cleave off one amino acid at a time from the....
NH3+, aminopeptidases or COO- terminus, carboxypeptidase.
5. Endo- and Exopeptidases work in concert
The Big Picture:
Protein Digestion and Absorption
Gastric (Stomach) Digestion:
1. Gastric HCl is responsible for the low pH <2 of gastric juice.
2. Gastric acid kills microorganisms and denatures dietary proteins preparing them for hydrolysis by proteases.
3. Gastric juices contain the acid stable proteases of the pepsin family, which produce large peptide fragments and some free amino acids.
4. Protein digestion at this stage is partial, as the amino acids enter the duodenum, they trigger the release of cholectystokinin-pancreozymin (CCK-PZ) into the bloodstream.
This release initiates the secretion of protease zymogens from the pancreas and releases of enteropeptidase in the gut.
1. The pancreatic juice is rich in the proenzymes of endopeptidase and carboxypeptidases.
2. Enteropeptidase activates pancreatic trypsinogen to trypsin.
3. Trypsin autocatalytically activates more trypsinogen and other proenzymes, liberating chymotrypsin, elastase and the carboxypeptidases A and B.
Secretion and Activation of Pancreatic Proteases:
Digestion at the Brush Border (surface of intestinal epithelial cells):
1. Since pancreatic juice does not contain appreciable aminopeptidase activity, final digestion of di- and small peptides depends on brush border enzymes.
2. The surface of intestinal epithelial cells is rich in endopeptidases and aminopeptidases.
3. The end products of cell surface digestion are free amino acids and di- and tripeptides.
1. Following digestion, amino acids and small peptides are co-absorbed w/ sodium via group specific amino acid or peptide transport systems.
2. These processes are carrier mediated, discriminating between natural, L amino acids and D-amino acids, require energy (from the Na+ gradient, Na-K ATPase) and physiologic temperatures.
At least five brush border transport systems exist:
1. neutral amino acids (uncharged aliphatic and aromatic)
2. basic amino acids (Lys, Arg, Cys, Cys-Cys)
3. acidic amino acids (Asp, Glu)
4. imino acids (Pro), Hydroxyproline)
5. di- and tripeptides
1. Hartnup Disease:
Genetic defect in the neutral amino acid transporter.
Symptoms: dermatitis due to tryptophan malabsorption ("niacin" flush)
Consequences: not serious di- and tripeptide absorption supply minimal amounts of dietarily essential neutral amino acids.
Precursor to kidney stones
Symptoms: painful kidney stone formation due to malabsoprtion of cystine (two disulfide linked cysteines)
Destruction and flattening of the intestinal villi resulting in generalized malabsorption.
Causes: bacterial infection or gluten (contained in certain grains such as wheat and barley) sensitivity.
Digestion and Absorption of Carbohydrates
1. Carbohydrates provide a major component of the daily caloric requirement, ~40%.
2.Distinguish between mono-, di- and polysaccharides.
Monosaccharides- do not need hydrolysis prior to absorption.
Disaccharides- require brush border enzymes.
Polysaccharides- require brush border enzymes, as well as, pancreatic amylase and salivary amylase for digestion.
Hydrolyzed by -amylase into Maltotriose, -Limit Dextrin, Maltose, Glucose
-1,4-glucosidic linkages (non-branching, amylose) and branched chains -1,6 linkages (branch points, amylopectin)
Present in saliva and pancreatic juice.
Specific for internal -1,4-glucosidic bonds.
Brush Border Carbohydrate Digestion:
Final hydrolysis of di- and oligosaccharides to monosaccharides is carried out by -glucosidases on the surface of the small intestine.
Monosaccharides are absorbed by carrier mediated transport.
At least two types are known:
1. Na+monosaccharide transporter
2. Na+ independent, diffusion type monosaccharide transport system
1. Di-, oligo- and polysaccharides that are not hydrolyzed by -amylase and/or brush border enzymes cannot be absorbed.
2. These carbohydrates reach the lower tract of the intestine which contains bacteria.
3. The bacteria utilize many of the remaining carbohydrate, metabolizing them and producing by- products such as: hydrogen gas, methane and carbon dioxide.
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Carbohydrates are one of three macronutrients that provide the body with energy (protein and fats being the other two). The chemical compounds in carbohydrates are found in both simple and complex forms, and in order for the body to use carbohydrates for energy, food must undergo digestion, absorption, and glycolysis. It is recommended that 55 to 60 percent of caloric intake come from carbohydrates.
Carbohydrates are a main source of energy for the body and are made of carbon, hydrogen, and oxygen. Chlorophyll in plants absorbs light energy from the sun. This energy is used in the process of photosynthesis, which allows green plants to take in carbon dioxide and release oxygen and allows for the production of carbohydrates. This process converts the sun's light energy into a form of chemical energy useful to humans. Plants transform carbon dioxide (CO2) from the air, water (H2O) from the ground, and energy from the sun into oxygen (O2) and carbohydrates (C6H12O6) (6 CO2 + 6 H2O + energy = C6H12O6 + 6 O2). Most carbohydrates have a ratio of 1:2:1 of carbon, hydrogen, and oxygen, respectively.
Humans and other animals obtain carbohydrates by eating foods that contain them. In order to use the energy contained in the carbohydrates, humans must metabolize, or break down, the structure of the molecule in a process that is opposite that of photosynthesis. It starts with the carbohydrate and oxygen and produces carbon dioxide, water, and energy. The body utilizes the energy and water and rids itself of the carbon dioxide.
Simple carbohydrates, or simple sugars, are composed of monosaccharide or disaccharide units. Common monosaccharides (carbohydrates composed of single sugar units) include glucose, fructose, and galactose. Glucose is the most common type of sugar and the primary form of sugar that is stored in the body for energy. It sometimes is referred to as blood sugar or dextrose and is of particular importance to individuals who have diabetes or hypoglycemia. Fructose, the primary sugar found in fruits, also is found in honey and high-fructose corn syrup (in soft drinks) and is a major source of sugar in the diet of Americans. Galactose is less likely than glucose or fructose to be found in nature. Instead, it often combines with glucose to form the disaccharide lactose, often referred to as milk sugar. Both fructose and galactose are metabolized to glucose for use by the body.
Oligosaccharides are carbohydrates made of two to ten monosaccharides. Those composed of two sugars are specifically referred to as disaccharides, or double sugars. They contain two monosaccharides bound by either an alpha bond or a beta bond. Alpha bonds are digestible by the human body, whereas beta bonds are more difficult for the body to break down.
There are three particularly important disaccharides: sucrose, maltose, and lactose. Sucrose is formed when glucose and fructose are held together by an alpha bond. It is found in sugar cane or sugar beets and is refined to make granulated table sugar. Varying the degree of purification alters the
Monosaccharide or disaccharide
Beet sugar (cane sugar)
Disaccharide (fructose and glucose)
Similar to white and powdered sugar, but varied degree of purification
Disaccharide (fructose and glucose)
Similar to white and powdered sugar, but varied degree of purification
High-fructose corn syrup
Very sweet and inexpensive
Fructose and glucose
Disaccharide (glucose and glucose)
Formed by the hydrolysis of starch, but sweeter than starch
Disaccharide (fructose and glucose)
Disaccharide (glucose and galactose)
Made in mammary glands of most lactating animals
Disaccharide (fructose and glucose)
Similar to white and brown sugar, but varied degree of purification
Disaccharide (fructose and glucose)
Similar to brown and powdered sugar, but varied degree of purification
SOURCE: Mahan and Escott-Stump, 2000; Northwestern University; Sizer and Whitney, 1997; and Wardlaw and Kessel, 2002.
final product, but white, brown, and powdered sugars all are forms of sucrose. Maltose, or malt sugar, is composed of two glucose units linked by an alpha bond. It is produced from the chemical decomposition of starch, which occurs during the germination of seeds and the production of alcohol. Lactose is a combination of glucose and galactose. Because it contains a beta bond, it is hard for some individuals to digest in large quantities. Effective digestion requires sufficient amounts of the enzyme lactase.
Complex carbohydrates, or polysaccharides, are composed of simple sugar units in long chains called polymers. Three polysaccharides are of particular importance in human nutrition: starch, glycogen, and dietary fiber.
Starch and glycogen are digestible forms of complex carbohydrates made of strands of glucose units linked by alpha bonds. Starch, often contained in seeds, is the form in which plants store energy, and there are two types: amylose and amylopectin. Starch represents the main type of digestible complex carbohydrate. Humans use an enzyme to break down the bonds linking glucose units, thereby releasing the sugar to be absorbed into the bloodstream. At that point, the body can distribute glucose to areas that need energy, or it can store the glucose in the form of glycogen.
Glycogen is the polysaccharide used to store energy in animals, including humans. Like starch, glycogen is made up of chains of glucose linked by alpha bonds; but glycogen chains are more highly branched than starch. It is this highly branched structure that allows the bonds to be more quickly broken down by enzymes in the body. The primary storage sites for glycogen in the human body are the liver and the muscles.
Another type of complex carbohydrate is dietary fiber. In general, dietary fiber is considered to be polysaccharides that have not been digested at the point of entry into the large intestine. Fiber contains sugars linked by bonds that cannot be broken down by human enzymes, and are therefore
Pastas and whole-grain breads contain complex carbohydrates, which are long strands of glucose molecules. Nutritionists recommend that 55–60 percent of calories come from carbohydrates, and especially complex carbohydrates.
[Photograph by James Noble. Corbis. Reproduced by permission.]
labeled as indigestible. Because of this, most fibers do not provide energy for the body. Fiber is derived from plant sources and contains polysaccharides such as cellulose, hemicellulose, pectin, gums, mucilages, and lignins.
The indigestible fibers cellulose, hemicellulose, and lignin make up the structural part of plants and are classified as insoluble fiber because they usually do not dissolve in water. Cellulose is a nonstarch carbohydrate polymer made of a straight chain of glucose molecules linked by beta bonds and can be found in whole-wheat flour, bran, and vegetables. Hemicellulose is a nonstarch carbohydrate polymer made of glucose, galactose, xylose, and other monosaccharides; it can be found in bran and whole grains. Lignin, a noncarbohydrate polymer containing alcohols and acids, is a woody fiber found in wheat bran and the seeds of fruits and vegetables.
In contrast, pectins, mucilages, and gums are classified as soluble fibers because they dissolve or swell in water. They are not broken down by human enzymes, but instead can be metabolized (or fermented) by bacteria present in the large intestine. Pectin is a fiber made of galacturonic acid and other monosaccharides. Because it absorbs water and forms a gel, it is often used in jams and jellies. Sources of pectin include citrus fruits, apples, strawberries, and carrots. Mucilages and gums are similar in structure. Mucilages are dietary fibers that contain galactose, manose, and other monosaccharides; and gums are dietary fibers that contain galactose, glucuronic acid, and other monosaccharides. Sources of gums include oats, legumes, guar, and barley.
Digestion and Absorption
Carbohydrates must be digested and absorbed in order to transform them into energy that can be used by the body. Food preparation often aids in the digestion process. When starches are heated, they swell and become easier for the body to break down. In the mouth, the enzyme amylase, which is contained in saliva, mixes with food products and breaks some starches into smaller units. However, once the carbohydrates reach the acidic environment of the stomach, the amylase is inactivated. After the carbohydrates have passed through the stomach and into the small intestine, key digestive enzymes are secreted from the pancreas and the small intestine where most digestion and absorption occurs. Pancreatic amylase breaks starch into disaccharides and small polysaccharides, and enzymes from the cells of the small-intestinal wall break any remaining disaccharides into their monosaccharide components. Dietary fiber is not digested by the small intestine; instead, it passes to the colon unchanged.
Sugars such as galactose, glucose, and fructose that are found naturally in foods or are produced by the breakdown of polysaccharides enter into absorptive intestinal cells. After absorption, they are transported to the liver where galactose and fructose are converted to glucose and released into the bloodstream. The glucose may be sent directly to organs that need energy, it may be transformed into glycogen (in a process called glycogenesis) for storage in the liver or muscles, or it may be converted to and stored as fat.
The molecular bonds in food products do not yield high amounts of energy when broken down. Therefore, the energy contained in food is released within cells and stored in the form of adenosine triphosphate (ATP), a high-energy compound created by cellular energy-production systems. Carbohydrates are metabolized and used to produce ATP molecules through a process called glycolysis.
Glycolysis breaks down glucose or glycogen into pyruvic acid through enzymatic reactions within the cytoplasm of the cells. The process results in the formation of three molecules of ATP (two, if the starting product was glucose). Without the presence of oxygen, pyruvic acid is changed to lactic acid, and the energy-production process ends. However, in the presence of oxygen, larger amounts of ATP can be produced. In that situation, pyruvic acid is transformed into a chemical compound called acetyle coenzyme A, a compound that begins a complex series of reactions in the Krebs Cycle and the electron transport system. The end result is a net gain of up to thirty-nine molecules of ATP from one molecule of glycogen (thirty-eight molecules of ATP if glucose was used). Thus, through certain systems, glucose can be used very efficiently in the production of energy for the body.
times, carbohydrates have been incorrectly labeled as "fattening."
Evidence actually supports the consumption of more, rather than less, starchy
foods. Carbohydrates have four calories per gram, while dietary fats
contribute nine per gram, so diets high in complex carbohydrates are likely to
provide fewer calories than diets high in fat. Recommendations are for 55 to 60
percent of total calories to come from carbohydrates (approximately 275 to
Low-carbohydrate diets, such as the Atkins and South Beach diets, are based on the proposition that it's not fat that makes you fat. Allowing dieters to eat steak, butter, eggs, bacon, and other high-fat foods, these diets instead outlaw starches and refined carbohydrates on the theory that they are metabolized so quickly that they lead to hunger and overeating. This theory, which was first popularized in the nineteenth century, came under scathing criticism from the medical establishment during the early 1970s when Dr. Robert Atkins published the phenomenally popular low-carb diet bearing his name. According to the American Medical Association (AMA), the Atkins diet was a "bizarre regimen" that advocated "an unlimited intake of saturated fats and cholesterol-rich foods" and therefore presented a considerable risk of heart disease. Most doctors recommended instead a diet low in fat and high in carbohydrates, with plenty of grains, fruits, and vegetables and limited red meat or dairy products. This became the received wisdom during the 1980s, at the same time that the U.S. waistline began to expand precipitously. As dieters found that weight loss was difficult to maintain on a low-fat diet, low-carb diets regained popularity—with as many as 30 million people trying a low-carb diet in 2003. Several small-scale studies began to suggest that a low-carb diet may indeed be effective and may not have the deleterious effects its detractors have claimed; other research found that any benefits of a low-carb diet are short-lived, and that the negative effects will take decades to become evident. The National Institutes of Health has pledged $2.5 million for a five-year study of the Atkins diet with 360 subjects. While the results of this and other large-scale studies are awaited, many researchers stress that the key issue in maintaining a healthy weight is the number of calories consumed, not the type of calories. The National Academy of Sciences recommends that adults obtain 45 to 65 percent of their calories from carbohydrates, 20 to 35 percent from fat, and 10 to 35 percent from protein.
is important to consume a minimum amount of carbohydrates to prevent ketosis,
a condition resulting from the breakdown of fat for energy in the absence of
carbohydrates. In this situation, products of fat breakdown, called ketone
bodies, build up in the blood and alter normal pH balance. This can be
particularly harmful to a fetus. To avoid ketosis, daily carbohydrate intake
should include a minimum of 50 to
The exchange system is composed of lists that describe carbohydrate, fat, and protein content, as well as caloric content, for designated portions of specific foods. This system takes into account the presence of more than one type of nutrient in any given food. Exchange lists are especially useful for individuals who require careful diet planning, such as those who monitor intake of calories or certain nutrients. It is particularly useful for diabetics, for whom carbohydrate intake must be carefully controlled, and was originally developed for planning diabetic diets.
Diabetes, Carbohydrate-Modified Diets, and Carbohydrate Counting
Diabetes is a condition that alters the way the body handles carbohydrates. In terms of diet modifications, diabetics can control blood sugar levels by appropriately managing the carbohydrates, proteins, and fats in their meals. The amount of carbohydrates, not necessarily the source, is the primary issue. Blood glucose levels after a meal can be related to the process of food preparation, the amount of food eaten, fat intake, sugar absorption, and the combination of foods in the meal or snack.
One method of monitoring carbohydrate levels—carbohydrate counting—assigns a certain number of carbohydrate grams or exchanges to specific foods. Calculations are used to determine insulin need, resulting in better control of blood glucose levels with a larger variety of foods. Overall, diabetic diets can include moderate amounts of sugar, as long as they are carefully monitored.
Nomenclature of carbohydrates - the naming of carbohydrates - much more information than most of us need
1. Glucose - a monosaccharide - what you measure in your blood.
The chemical formula for glucose is C6H12O6.
2. Sucrose - a disaccharide - table sugar. Is made of one molecule of glucose and one fructose.
3. Starch - amylose and amylopectin - a polysaccharide - potatoes.
Amylose is a straight chain of glucose molecules linked together.
Amylopectin is a branched chain of glucose molecules.
4. Glycolysis is the breakdown of glucose to release its energy in the form of ATP.
metabolism - a slide
presentation from the
more chemistry than most people will want.
Glycogen is the storage form of glucose in animals and humans which is analogous to the starch in plants. Glycogen is synthesized and stored mainly in the liver and the muscles. Structurally, glycogen is very similar to amylopectin with alpha acetal linkages, however, it has even more branching and more glucose units are present than in amylopectin. Various samples of glycogen have been measured at 1,700-600,000 units of glucose.
The structure of glycogen consists of long polymer chains of glucose units connected by an alpha acetal linkage. The graphic on the left shows a very small portion of a glycogen chain. All of the monomer units are alpha-D-glucose, and all the alpha acetal links connect C # 1 of one glucose to C # 4 of the next glucose.
The branches are formed by linking C # 1 to a C # 6 through an acetal linkages. In glycogen, the branches occur at intervals of 8-10 glucose units, while in amylopectin the branches are separated by 12-20 glucose units.
Acetal Functional Group:
Carbon # 1 is called the anomeric carbon and is the center of an acetal functional group. A carbon that has two ether oxygens attached is an acetal.
The Alpha position is defined as the ether
oxygen being on the opposite side of the ring as the C #
Starch vs. Glycogen:
Plants make starch and cellulose through the photosynthesis processes. Animals and human in turn eat plant materials and products. Digestion is a process of hydrolysis where the starch is broken ultimately into the various monosaccharides. A major product is of course glucose which can be used immediately for metabolism to make energy. The glucose that is not used immediately is converted in the liver and muscles into glycogen for storage by the process of glycogenesis. Any glucose in excess of the needs for energy and storage as glycogen is converted to fat.
Start with G-6-P, again note that this molecule is at a metabolic crossroads. First convert to G-1-P using Phosphoglucomutase:
This reaction is very much like PGA Mutase, requiring the bis phosphorylated intermediate to form and to regenerate the phosphorylated enzyme intermediate. Again a separate "support" enzyme, Phosphoglucokinase, is required to form the intermediate, this time using ATP as the energy source:
Note that this reaction is easily reversible, though it favors G-6-P.
UDP-glucose pyrophosphorylase, which catalyzes the next reaction, has a near zero DG° ':
It is driven to completion by the hydrolysis of the PPi to 2 Pi by Pyrophosphatase with a DG° ' of about -32 kJ (approx. one ATP's worth of energy).
Finally glycogen is synthesized with Glycogen Synthase:
UDPGlucose + (glucose)n Æ UDP + (glucose)n+1
This reaction is favored by a DG° ' of about 12 kcal, thus the overall synthesis of glycogen from G-1-P is favored by a standard free energy of about 40 kJ. Note that the glucose is added to the non-reducing end of a glycogen strand, and that there is a net investment of 2 ATP equivalents per glucose (ATP to ADP and UTP to UDP, regenerated with ATP to ADP). Note also that glycogen synthase requires a 'primer.' That is it needs to have a glycogen chain to add on to. What happens then in new cells to make now glycogen granules? Can use a special primer protein (glycogenin). Thus glycogen granules have a protein core.
These reactions will give linear glycogen strands, additional reactions are required to produce branching. Branching enzyme [amylo-a-(1,4) to a-(1,6)-transglycosylase] transfers a block of residues from the end of one chain to another chain making a 1,6-linkage (cannot be closer than 4 residues to a previous branch). (For efficient release of glucose residues it has been determined that the optimum branching pattern is a new branch every 13 residues, with two branchs per strand.)
Glycogen is broken down using Phosphorylase to phosphorylize off glucose residues:
(glucose)n + Pi Æ (glucose)n-1 + G-1-P
Note that no ATP is required to recover Glucose phosphate from glycogen. This is a major advantage in anaerobic tissues, get one more ATP/glucose (3 instead of 2!). [Phosphorylase was originally thought to be the synthetic as well as breakdown enzyme since the reaction is readily reversible in vitro. However it was found that folks lacking this enzyme - McArdle's disease - can still make glycogen, though they can't break it down.]
Glycogen synthesis and degradation occurs in the liver cells. It is here that the hormone insulin (the primary hormone responsible for converting glucose to glycogen) acts to lower blood glucose concentration. Insulin stimulates glycogen synthesis; thereby, inhibiting glycogen degradation as shown in the figure. 3
Crystal structure of glycogen synthase: homologous enzymes catalyze glycogen synthesis and degradation
Alejandro Buschiazzo, Juan E Ugalde, Marcelo E Guerin, William Shepard, Rodolfo A Ugalde and Pedro M Alzari
Molecular surface representation of the GS core, showing the equivalent position of the arginine clusters in the mammalian/yeast (GT3) allosteric site (in red) with respect to the active center. Assuming an extended main-chain conformation, approximate distances are shown for two relevant phosphorylation sites, one in the N-terminal (2a) and the other in the C-terminal (3a) extensions of GT3 enzymes.
2. Liver - excess glycose production - gluconeogenesis and glycogenolysis
In order to provide glucose for vital functions such as the metabolism of RBC's and the CNS during periods of fasting (greater than about 8 hrs after food absorption in humans), the body needs a way to synthesis glucose from precursors such as pyruvate and amino acids. This process is referred to as gluconeogenesis. It occurs in the liver and in kidney. Most of Glycolysis can be used in this process since most glycolytic enzymes are reversible. However three irreversible enzymes must be bypassed in gluconeogenesis vs. glycolysis: Hexokinase, Phosphofructokinase, and Pyruvate kinase. Phosphofructokinase, and/or hexokinase must also be bypassed in converting other hexoses to glucose.
Let's begin with pyruvate. How is pyruvate converted to PEP without using the pyruvate kinase reaction? Formally, pyruvate is first converted to oxaloacetate, which is in turn converted to PEP. In the first reaction of this process Pyruvate carboxylase adds carbon dioxide to pyruvate with the expenditure of one ATP equivalent of energy. Biotin, a carboxyl-group transfer cofactor in animals, is required by this enzyme:
The reaction takes place in two parts on two different sub-sites on the enzyme. In the first part biotin attacks bicarbonate with a simultaneous attack/hydrolysis by bicarbonate on ATP, resulting in the release of ADP and inorganic phosphate (note the coupling by the enzyme of independent processes in this reaction):
Note that the 14 Angstrom arm of biocytin allows biotin to move between the two sites, in this case carrying the activated carboxyl group. In the second site a pyruvate carbanion then attacks the activated carboxyl group, regenerating the biotin cofactor and releasing oxaloacetate:
Investigation of mechanisms of metabolism hormonal regulation and significance in medical practice.
Investigation of mechanisms of metabolism hormonal regulation and significance in medical practice.
Insulin. Chemical structure: protein. Insulin is formed in b-cells of Langerhans islets (specialized endocrine regions of the pancreas).
Proinsulin is the biosynthetic precursor of insulin.
Effect of insulin on carbohydrate metabolism:
- increases the permeability of cell membranes for glucose;
- activates the first enzyme of glycolysis - glucokinase and prevent the inactivation of hexokinase;
- activates some enzymes of Krebs cycle (citrate synthase);
- activates the pentose phosphate cycle;
- activates glycogen synthetase;
- activates pyruvate dehydrogenase and a-ketoglutarate dehydrogenase;
- inhibits the gluconeogenesis;
- inhibits the decomposition of glycogen.
Effect of insulin on protein metabolism:
- increases the permeability of cell membranes for amino acids;
- activates synthesis of proteins and nucleic acids;
- inhibits the gluconeogenesis.
Effect of insulin on lipid metabolism:
- enhances the synthesis of lipids;
- promotes the lipid storage activating the carbohydrate decomposition;
- inhibits the gluconeogenesis.
Effect of insulin on mineral metabolism:
- activates Na+, K+-ATP-ase (transition of K into the cells and Na from the cells).
Target tissue for insulin - liver, muscles and lipid tissue.
The release of insulin from pancreas depends on the glucose concentration in the blood. Some other hormones, sympathetic and parasympathetic nervous system also can influence on the rate of insulin secretion.
The deficiency of insulin causes diabetes mellitus.
Insulin is destroyed in the organism by the enzyme insulinase that is produced by liver.
Other names: insulin
Taxa expressing: Homo sapiens; homologs: in metazoan taxa from invertebrates to
Antagonists: glucagon, steroids, most stress hormomes
Insulin (from Latin insula, "island", as it is produced in the Islets of Langerhans in the pancreas) is a polypeptide hormone that regulates carbohydrate metabolism. Apart from being the primary agent in carbohydrate homeostasis, it has effects on fat metabolism and it changes the liver's activity in storing or releasing glucose and in processing blood lipids, and in other tissues such as fat and muscle. The amount of insulin in circulation has extremely widespread effects throughout the body.
Insulin is used medically to treat some forms of diabetes mellitus. Patients with type 1 diabetes mellitus depend on external insulin (most commonly injected subcutaneously) for their survival because of an absolute deficiency of the hormone. Patients with type 2 diabetes mellitus have insulin resistance, relatively low insulin production, or both; some type 2 diabetics eventually require insulin when other medications become insufficient in controlling blood glucose levels.
Insulin's structure varies slightly between species of animal. Insulin from animal sources differs somewhat in regulatory function strength (ie, in carbohydrate metabolism) in humans because of those variations. Porcine (pig) insulin is especially close to the human version.
Discovery and characterization
In 1869 Paul Langerhans, a medical student in Berlin, was studying the structure of the pancreas (the jelly-like gland behind the stomach) under a microscope when he identified some previously un-noticed tissue clumps scattered throughout the bulk of the pancreas. The function of the "little heaps of cells," later known as the Islets of Langerhans, was unknown, but Edouard Laguesse later suggested that they might produce secretions that play a regulatory role in digestion.
In 1889, the Polish-German physician Oscar Minkowski in collaboration with Joseph von Mehring removed the pancreas from a healthy dog to test its assumed role in digestion. Several days after the dog's pancreas was removed, Minkowski's animal keeper noticed a swarm of flies feeding on the dog's urine. On testing the urine they found that there was sugar in the dog's urine, establishing for the first time a relationship between the pancreas and diabetes. In 1901, another major step was
taken by Eugene Opie, when he clearly established the link between the Islets of Langerhans and diabetes: Diabetes mellitus ... is caused by destruction of the islets of Langerhans and occurs only when these bodies are in part or wholly destroyed. Before his work, the link between the pancreas and diabetes was clear, but not the specific role of the islets.
The structure of insulin.
The left side is a space-filling model of the insulin monomer, believed to be biologically active. Carbon is green, hydrogen white, oxygen red, and nitrogen blue. On the right side is a cartoon of the insulin hexamer, believed to be the stored form. A monomer unit is highlighted with the A chain in blue and the B chain in cyan.
Yellow denotes disulfide bonds, and magenta spheres are zinc ions.Over the next two decades, several attempts were made to isolate whatever it was the islets produced as a potential treatment. In 1906 George Ludwig Zuelzer was partially successful treating dogs with pancreatic extract but was unable to continue his work. Between 1911 and 1912, E.L. Scott at the University of Chicago used aqueous pancreatic extracts and noted a slight diminution of glycosuria but was unable to convince his director of his work's value; it was shut down. Israel Kleiner demonstrated similar effects at Rockefeller University in 1919, but his work was interrupted by World War I and he did not return to it. Nicolae Paulescu, a professor of physiology at the University of Medicine and Pharmacy in Bucharest, published similar work in 1921 that had been carried out in France. Use of his techniques was patented in Romania, though no clinical use resulted. It has been argued ever since that he is the rightful discoverer.
In October 1920, Frederick Banting was reading one of Minkowski's papers and concluded that it is the very digestive secretions that Minkowski had originally studied that were breaking down the islet secretion(s), thereby making it impossible to extract successfully. He jotted a note to himself Ligate pancreatic ducts of the dog. Keep dogs alive till acini degenerate leaving islets. Try to isolate internal secretion of these and relieve glycosurea.
The idea was that the pancreas's internal secretion, which supposedly regulates sugar in the bloodstream, might hold the key to the treatment of diabetes.
He travelled to Toronto to meet with J.J.R. Macleod, who was not entirely impressed with his idea – so many before him had tried and failed. Nevertheless, he supplied Banting with a lab at the University, an assistant (medical student Charles Best), and 10 dogs, then left on vacation during the summer of 1921. Their method was tying a ligature (string) around the pancreatic duct, and, when examined several weeks later, the pancreatic digestive cells had died and been absorbed by the immune system, leaving thousands of islets. They then isolated an extract from these islets, producing what they called isletin (what we now know as insulin), and tested this extract on the dogs. Banting and Best were then able to keep a pancreatectomized dog alive all summer because the extract lowered the level of sugar in the blood.
Computer-generated image of insulin hexamers highlighting the threefold symmetry, the zinc ions holding it together, and the histidine residues involved in zinc binding.Macleod saw the value of the research on his return but demanded a re-run to prove the method actually worked. Several weeks later it was clear the second run was also a success, and he helped publish their results privately in Toronto that November. However, they needed six weeks to extract the isletin, which forced considerable delays. Banting suggested that they try to use fetal calf pancreas, which
had not yet developed digestive glands; he was relieved to find that this method worked well. With the supply problem solved, the next major effort was to purify the extract. In December 1921, Macleod invited the biochemist James Collip to help with this task, and, within a month, the team felt ready for a clinical test.
On January 11, 1922, Leonard Thompson, a 14-year-old diabetic who lay dying at the Toronto General Hospital, was given the first injection of insulin. However, the extract was so impure that Thompson suffered a severe allergic reaction, and further injections were canceled. Over the next 12 days, Collip worked day and night to improve the ox-pancreas extract, and a second dose injected on the 23rd. This was completely successful, not only in not having obvious side-effects, but in completely eliminating the glycosuria sign of diabetes. However, Banting and Best never worked well with Collip, regarding him as something of an interloper, and Collip left the project soon after.
The exact sequence of amino acids comprising the insulin molecule, the so-called primary structure, was determined by British molecular biologist Frederick Sanger. It was the first protein to have its sequence be determined. He was awarded the 1958 Nobel Prize in Chemistry for this work.
In 1969, after decades of work, Dorothy Crowfoot Hodgkin determined the spatial conformation of the molecule, the so-called tertiary structure, by means of X-ray diffraction studies. She had been awarded a Nobel Prize in Chemistry in 1964 for the development of crystallography.
Rosalyn Sussman Yalow received the 1977 Nobel Prize in Medicine for the development of the radioimmunoassay for insulin.
Insulin undergoes extensive posttranslational modification along the production pathway. Production and secretion are largely independent; prepared insulin is stored awaiting secretion. Both C-peptide and mature insulin are biologically active. Cell components and proteins in this image are not to scale.
Within vertebrates, the similarity of insulins is very close. Bovine insulin differs from human in only three amino acid residues, and porcine insulin in one. Even insulin from some species of fish is similar enough to human to be effective in humans. The C-peptide of proinsulin (discussed later), however, is very divergent from species to species.
In mammals, insulin is synthesized in the pancreas within the beta cells (β-cells) of the islets of Langerhans.
One to three million islets of Langerhans (pancreatic islets) form the endocrine part of the pancreas, which is primarily an exocrine gland. The endocrine portion only accounts for 2% of the total mass of the pancreas. Within the islets of Langerhans, beta cells constitute 60–80% of all the cells.
In beta cells, insulin is synthesized from the proinsulin precursor molecule by the action of proteolytic enzymes, known as prohormone convertases (PC1 and PC2), as well as the exoprotease carboxypeptidase E. These modifications of proinsulin remove the center portion of the molecule, or C-peptide, from the C- and N- terminal ends of the proinsulin. The remaining polypeptides (51 amino acids in total), the B- and A- chains, are bound together by disulfide bonds. Confusingly, the primary sequence of proinsulin goes in the order "B-C-A", since B and A chains were identified on the basis of mass, and the C peptide was discovered after the others.
Effect of insulin on glucose uptake and metabolism. Insulin binds to its receptor which in turn starts many protein activation cascades. These include: translocation of Glut-4 transporter to the plasma membrane and influx of glucose, glycogen synthesis, glycolysis and fatty acid synthesis.
Effect of insulin on glucose uptake and metabolism. Insulin binds to its receptor which in turn starts many protein activation cascades. These include: translocation of Glut-4 transporter to the plasma membrane and influx of glucose, glycogen synthesis , glycolysis and fatty acid synthesis.
Control of cellular intake of certain substances, most prominently glucose in muscle and adipose tissue (about ⅔ of body cells).
Increase of DNA replication and protein synthesis via control of amino acid uptake.
Modification of the activity of numerous enzymes (allosteric effect).
The actions of insulin on cells include:
Increased glycogen synthesis – insulin forces storage of glucose in liver (and muscle) cells in the form of glycogen; lowered levels of insulin cause liver cells to convert glycogen to glucose and excrete it into the blood.
This is the clinical action of insulin which is directly useful in reducing high blood glucose levels as in diabetes.
Increased fatty acid synthesis – insulin forces fat cells to take in blood lipids which are converted to triglycerides; lack of insulin causes the reverse.
Increased esterification of fatty acids – forces adipose tissue to make fats (ie, triglycerides) from fatty acid esters; lack of insulin causes the reverse.
Decreased proteinolysis – forces reduction of protein degradation; lack of insulin increases protein degradation.
Decreased lipolysis – forces reduction in conversion of fat cell lipid stores into blood fatty acids; lack of insulin causes the reverse.
Decreased gluconeogenesis – decreases production of glucose from various substrates in liver; lack of insulin causes glucose production from assorted substrates in the liver and elsewhere.
Increased amino acid uptake – forces cells to absorb circulating amino acids; lack of insulin inhibits absorption.
Increased potassium uptake – forces cells to absorb serum potassium; lack of insulin inhibits absorption.
Arterial muscle tone – forces arterial wall muscle to relax, increasing blood flow, especially in micro arteries; lack of insulin reduces flow by allowing these muscles to contract.
Regulatory action on blood glucose
human blood glucose levels normally remain within a narrow range. In most humans this varies from about 70 mg/dl to perhaps 110 mg/dl (3.9 to 6.1 mmol/litre) except shortly after eating when the blood glucose level rises temporarily. This homeostatic effect is the result of many factors, of which hormone regulation is the most important.
It is usually a surprise to realize
how little glucose is actually maintained in the blood, and body fluids. The
control mechanism works on very small quantities. In a healthy adult male of
growth hormone men). A more familiar comparison may
There are two types of mutually antagonistic metabolic hormones affecting blood glucose levels: catabolic hormones (such as glucagon, growth hormone, and catecholamines), which increase blood glucose and one anabolic hormone (insulin), which decreases blood glucose
Mechanisms which restore satisfactory blood glucose levels after hypoglycemia must be quick, and effective, because of the immediate serious consequences of insufficient glucose (in the extreme, coma, less immediately dangerously, confusion or unsteadiness, amongst many other effects). This is because, at least in the short term, it is far more dangerous to have too little glucose in the blood than too much. In healthy individuals these mechanisms are indeed generally efficient, and symptomatic hypoglycemia is generally only found in diabetics using insulin or other pharmacologic treatment. Such hypoglycemic episodes vary greatly between persons and from time to time, both in severity and swiftness of onset. In severe cases prompt medical assistance is essential, as damage (to brain and other tissues) and even death will result from sufficiently low blood glucose levels.
Diabetes causes an excessive amount of glucose to remain in the blood stream which may cause damage to the blood vessels. Within the eye the damaged vessels may leak blood and fluid into the surrounding tissues and cause vision problems.
Mechanism of glucose dependent insulin releaseBeta cells in the islets of Langerhans are sensitive to variations in blood glucose levels through the following mechanism (see figure to the right):
Mechanism of glucose dependent insulin release
Glucose enters the beta cells through the glucose transporter GLUT2
Glucose goes into the glycolysis and the respiratory cycle where multiple high-energy ATP molecules are produced by oxidation
Dependent on blood glucose levels and hence ATP levels, the ATP controlled potassium channels (K+) close and the cell membranes depolarize
On depolarisation, voltage controlled calcium channels (Ca2+) open and calcium flows into the cells
An increased calcium level causes activation of phospholipase C, which cleaves the membrane phospholipid phosphatidyl inositol 4,5-bisphosphate into inositol 1,4,5-triphosphate and diacylglycerol.
Inositol 1,4,5-triphosphate (IP3) binds to receptor proteins in the membrane of endoplasmic reticulum (ER). This allows the release of Ca2+ from the ER via IP3 gated channels, and further raises the cell concentration of calcium.
Significantly increased amounts of calcium in the cells causes release of previously synthesised insulin, which has been stored in secretory vesicles
This is the main mechanism for release of insulin and regulation of insulin synthesis. In addition some insulin synthesis and release takes place generally at food intake, not just glucose or carbohydrate intake, and the beta cells are also somewhat influenced by the autonomic nervous system. The signalling mechanisms controlling this are not fully understood.
Other substances known which stimulate insulin release are acetylcholine, released from vagus nerve endings (parasympathetic nervous system), cholecystokinin, released by enteroendocrine cells of intestinal mucosa and glucose-dependent insulinotropic peptide (GIP). The first of these act similarly as glucose through phospholipase C, while the last acts through the mechanism of adenylate cyclase.
The sympathetic nervous system (via α2-adrenergic agonists such as norepinephrine) inhibits the release of insulin.
When the glucose level comes down to the usual physiologic value, insulin release from the beta cells slows or stops. If blood glucose levels drop lower than this, especially to dangerously low levels, release of hyperglycemic hormones (most prominently glucagon from Islet of Langerhans' alpha cells) forces release of glucose into the blood from cellular stores, primarily liver cell stores of glycogen. By increasing blood glucose, the hyperglycemic hormones correct life-threatening hypoglycemia. Release of insulin is strongly inhibited by the stress hormone norepinephrine (noradrenaline), which leads to increased blood glucose levels during stress.
There are special transporter proteins in cell membranes
through which glucose from the blood can enter a cell. These transporters are, indirectly, under insulin control in certain body cell types (eg, muscle cells). Low levels of circulating insulin, or its absence, will prevent glucose from entering those cells (eg, in untreated Type 1 diabetes). However, more commonly there is a decrease in the sensitivity of cells to insulin (eg, the reduced insulin sensitivity characteristic of Type 2 diabetes), resulting in decreased glucose absorption. In either case, there is 'cell starvation', weight loss, sometimes extreme. In a few cases, there is a defect in the release of insulin from the pancreas. Either way, the effect is, characteristically, the same: elevated blood glucose levels.
Activation of insulin receptors leads to internal cellular mechanisms which directly affect glucose uptake by regulating the number and operation of protein molecules in the cell membrane which transport glucose into the cell. The genes which specify the proteins which make up the insulin receptor in cell membranes have been identified and the structure of the interior, cell membrane section, and now, finally after more than a decade, the extra-membrane structure of receptor (Australian researchers announced the work 2Q 2006).
Two types of tissues are most strongly influenced by insulin, as far as the stimulation of glucose uptake is concerned: muscle cells (myocytes) and fat cells (adipocytes). The former are important because of their central role in movement,
Together, they account for about two-thirds of all cells in a typical human body.
Although other cells can use other fuels for a while growth hormone
(most prominently fatty acids), neuron
breathing, circulation, etc, and the latter because they accumulate excess food energy against future ..
depend on glucose as a source of energy in the non-starving human. They do not require insulin to absorb glucose, unlike muscle and adipose tissue, and they have very small internal stores of glycogen. Glycogen stored in liver cells (unlike glycogen stored in muscle cells) can be converted to glucose, and released into the blood, when glucose from digestion is low or absent, and the glycerol backbone in triglycerides can also be used to produce blood glucose.
Exhaustion of these sources can, either temporarily or on a sustained basis, if reducing blood glucose to a sufficiently low level, first and most dramatically manifest itself in impaired functioning of the central nervous system – dizziness, speech problems, even loss of consciousness, are not unknown. This is known as hypoglycemia or, in cases producing unconsciousness, "hypoglycemic coma" (formerly termed "insulin shock" from the most common causative agent). Endogenous causes of insulin excess (such as an insulinoma)
are very rare, and the overwhelming majority of hypoglycemia cases are caused by human action (e.g., iatrogenic, caused by medicine) and are usually accidental. There have been a few reported cases of murder, attempted murder, or suicide using insulin overdoses, but most insulin shocks appear to be due to mismanagement of insulin (didn't eat as much as anticipated, or exercised more than expected), or a mistake (e.g., 20 units of insulin instead of 2).
Possible causes of hypoglycemia include:
Oral hypoglycemic agents (e.g., any of the sulfonylureas, or similar drugs, which increase insulin release from beta cells in response to a particular blood glucose level).
External insulin (usually injected subcutaneously).
Ingestion of low-carbohydrate sugar substitutes (animal studies show these can trigger insulin release (albeit in much smaller quantities than sugar) according to a report in Discover magazine, August 2005, p18).
Diabetes mellitus – general term referring to all states characterized by hyperglycemia.
For the disease characterized by excretion of large amounts of very dilute urine, see diabetes insipidus. For diabetes mellitus in pets, see diabetes in cats and dogs.
Diabetes mellitus (IPA pronunciation: is a metabolic disorder characterized by hyperglycemia (high blood sugar) and other signs, as distinct from a single illness or condition.
The World Health Organization recognizes three main forms of diabetes: type 1, type 2, and gestational diabetes (occurring during pregnancy),[ which have similar signs, symptoms, and consequences, but different causes and population distributions. Ultimately, all forms are due to the beta cells of the pancreas being unable to produce sufficient insulin to prevent hyperglycemia Type 1 is usually due to autoimmune destruction of the pancreatic beta cells which produce insulin. Type 2 is characterized by tissue-wide insulin resistance and varies widely; it sometimes progresses to loss of beta cell function. Gestational diabetes is similar to type 2 diabetes, in that it involves insulin resistance; the hormones of pregnancy cause insulin resistance in those women genetically predisposed to developing this condition.
Types 1 and 2 are incurable chronic conditions, but have been treatable since insulin became medically available in 1921, and are nowadays usually managed with a combination of dietary treatment, tablets (in type 2) and, frequently, insulin supplementation. Gestational diabetes typically resolves with delivery.
Diabetes can cause many complications. Acute complications (hypoglycemia, ketoacidosis or nonketotic hyperosmolar coma) may occur if the disease is not adequately controlled. Serious long-term complications include cardiovascular disease (doubled risk), chronic renal failure (diabetic nephropathy is the main cause of dialysis in developed world adults), retinal damage (which can lead to blindness and is the most significant cause of adult blindness in the non-elderly in the developed world), nerve damage (of several kinds), and microvascular damage, which may cause erectile dysfunction (impotence) and poor healing. Poor healing of wounds, particularly of the feet, can lead to gangrene which can require amputation — the leading cause of non-traumatic amputation in adults in the developed world. Adequate treatment of diabetes, as well as increased emphasis on blood pressure control and lifestyle factors (such as smoking and keeping a healthy body weight), may improve the risk profile of most aforementioned complications.
The term diabetes (Greek: διαβήτης) was coined by Aretaeus of Cappadocia. It is derived from the Greek word διαβαίνειν, diabaínein that literally means "passing through," or "siphon", a reference to one of diabetes' major symptoms—excessive urine production. In 1675 Thomas Willis added the word mellitus to the disease, a word from Latin meaning "honey", a reference to the sweet taste of the urine. This sweet taste had been noticed in urine by the ancient Greeks, Chinese, Egyptians, and Indians. In 1776 Matthew Dobson confirmed that the sweet taste was because of an excess of a kind of sugar in the urine and blood of people with diabetes.
The ancient Indians tested for diabetes by observing whether ants were attracted to a person's urine, and called the ailment "sweet urine disease" (Madhumeha). The Korean, Chinese, and Japanese words for diabetes are based on the same ideographs which mean "sugar urine disease".
Diabetes, without qualification, usually refers to diabetes mellitus, but there are several rarer conditions also named diabetes. The most common of these is diabetes insipidus (insipidus meaning "without taste" in Latin) in which the urine is not sweet; it can be caused by either kidney (nephrogenic DI) or pituitary gland (central DI) damage.
The term "type 1 diabetes" has universally replaced several former terms, including childhood-onset diabetes, juvenile diabetes, and insulin-dependent diabetes. "Type 2 diabetes" has also replaced several older terms, including adult-onset diabetes, obesity-related diabetes, and non-insulin-dependent diabetes. Beyond these numbers, there is no agreed standard. Various sources have defined "type 3 diabetes" as, among others:
Insulin-resistant type 1 diabetes (or "double diabetes")
Type 2 diabetes which has progressed to require injected insulin.
Latent autoimmune diabetes of adults (or LADA or "type 1.5" diabetes)
The distinction between what is now known as type 1 diabetes and type 2 diabetes was first clearly made by Sir Harold Percival (Harry) Himsworth, and published in January 1936.
Other landmark discoveries include: identification of the first of the sulfonylureas in 1942 the determination of the amino acid order of insulin (by Sir Frederick Sanger, for which he received a Nobel Prize) the radioimmunoassay for insulin, as discovered by
identification of the first thiazolidinedione as an effective insulin sensitizer during the 1990s .
Type 1 diabetes mellitus
Type 1 diabetes mellitus—formerly known as insulin-dependent diabetes (IDDM), childhood diabetes or also known as juvenile diabetes, is characterized by loss of the insulin-producing beta cells of the islets of Langerhans of the pancreas leading to a deficiency of insulin. It should be noted that there is no known preventative measure that can be taken against type 1 diabetes. Most people affected by type 1 diabetes are otherwise healthy and of a healthy weight when onset occurs. Diet and exercise cannot reverse or prevent type 1 diabetes. Sensitivity and responsiveness to insulin are usually normal, especially in the early stages. This type comprises up to 10% of total cases in North America and Europe, though this varies by geographical location. This type of diabetes can affect children or adults but was traditionally termed "juvenile diabetes" because it represents a majority of cases of diabetes affecting children.
The main cause of beta cell loss leading to type 1 diabetes is a T-cell mediated autoimmune attack. The principal treatment of type 1 diabetes, even from the earliest stages, is replacement of insulin. Without insulin, ketosis and diabetic ketoacidosis can develop and coma or death will result.
Currently, type 1 diabetes can be treated only with insulin, with careful monitoring of blood glucose levels using blood testing monitors. Emphasis is also placed on lifestyle adjustments (diet and exercise). Apart from the common subcutaneous injections, it is also possible to deliver insulin by a pump, which allows continuous infusion of insulin 24 hours a day at preset levels and the ability to program doses (a bolus) of insulin as needed at meal times. An inhaled form of insulin, Exubera, was approved by the FDA in January 2006.
Type 1 treatment must be continued indefinitely. Treatment does not impair normal activities, if sufficient awareness, appropriate care, and discipline in testing and medication is taken. The average glucose level for the type 1 patient should be as close to normal (80–120 mg/dl, 4–6 mmol/l) as possible. Some physicians suggest up to 140–150 mg/dl (7-7.5 mmol/l) for those having trouble with lower values, such as frequent hypoglycemic events. Values above 200 mg/dl (10 mmol/l) are often accompanied by discomfort and frequent urination leading to dehydration. Values above 300 mg/dl (15 mmol/l) usually require immediate treatment and may lead to ketoacidosis. Low levels of blood glucose, called hypoglycemia, may lead to seizures or episodes of unconsciousness.
Type 2 diabetes mellitus
Main article: Diabetes mellitus type 2
Type 2 diabetes mellitus—previously known as adult-onset diabetes, maturity-onset diabetes, or non-insulin-dependent diabetes mellitus (NIDDM)—is due to a combination of defective insulin secretion and insulin resistance or reduced insulin sensitivity (defective responsiveness of tissues to insulin), which almost certainly involves the insulin receptor in cell membranes. In the early stage the predominant abnormality is reduced insulin sensitivity, characterized by elevated levels of insulin in the blood. At this stage hyperglycemia can be reversed by a variety of measures and medications that improve insulin sensitivity or reduce glucose production by the liver, but as the disease progresses the impairment of insulin secretion worsens, and therapeutic replacement of insulin often becomes necessary. There are numerous theories as to the exact cause and mechanism for this resistance, but central obesity (fat concentrated around the waist in relation to abdominal organs, and not subcutaneous fat, it seems) is known to predispose individuals for insulin resistance, possibly due to its secretion of adipokines (a group of hormones) that impair glucose tolerance. Abdominal fat is especially active hormonally. Obesity is found in approximately 55% of patients diagnosed with type 2 diabetes. Other factors include aging (about 20% of elderly patients are diabetic in North America) and family history (Type 2 is much more common in those with close relatives who have had it), although in the last decade it has increasingly begun to affect children and adolescents, likely in connection with the greatly increased childhood obesity seen in recent decades in some places.
Type 2 diabetes may go unnoticed for years in a patient before diagnosis, as visible symptoms are typically mild or non-existent, without ketoacidotic episodes, and can be sporadic as well. However, severe long-term complications can result from unnoticed type 2 diabetes, including renal failure, vascular disease (including coronary artery disease), vision damage, etc.
Fetal/neonatal risks associated with GDM include congenital anomalies such as cardiac, central nervous system, and skeletal muscle malformations. Increased fetal insulin may inhibit fetal surfactant production and cause respiratory distress syndrome. Hyperbilirubinemia may result from red blood cell destruction. In severe cases, perinatal death may occur, most commonly as a result of poor placental profusion due to vascular impairment. Induction may be indicated with decreased placental function. Cesarean section may be performed if there is marked fetal distress or an increased risk of injury associated with macrosomia, such as shoulder dystocia.
Both type 1 and type 2 diabetes are at least partly inherited. Type 1 diabetes appears to be triggered by some (mainly viral) infections, or in a less common group, by stress or environmental exposure (such as exposure to certain chemicals or drugs). There is a genetic element in individual susceptibility to some of these triggers which has been traced to particular HLA genotypes (i.e., the genetic "self" identifiers relied upon by the immune system). However, even in those who have inherited the susceptibility, type 1 diabetes mellitus seems to require an environmental trigger. A small proportion of people with type 1 diabetes carry a mutated gene that causes maturity onset diabetes of the young (MODY).
When the glucose concentration in the blood is high (ie, above the "renal threshold"), reabsorption of glucose in the proximal renal tubuli is incomplete, and part of the glucose remains in the urine (glycosuria). This increases the osmotic pressure of the urine and thus inhibits the resorption of water by the kidney, resulting in an increased urine producton (polyuria) and an increased fluid loss. Lost blood volume will be replaced osmotically from water held in body cells, causing dehydration and increased thirst.
Prolonged high blood glucose causes glucose absorption and so shape changes in the shape of the lens in the eye, leading to vision changes. Blurred vision is a common complaint leading to a diabetes diagnosis; Type 1 should always be suspected in cases of rapid vision change. Type 2 is generally more gradual, but should still be suspected.
A rarer, but equally severe, possibility is hyperosmolar nonketotic state, which is more common in type 2 diabetes, and is mainly the result of dehydration due to loss of body water. Often, the patient has been drinking extreme amounts of sugar-containing drinks, leading to a vicious circle in regard to the water loss.
The diagnosis of type 1 diabetes, and many cases of type 2, is usually prompted by recent-onset symptoms of excessive urination (polyuria) and excessive thirst (polydipsia), often accompanied by weight loss. These symptoms typically worsen over days to weeks; about 25% of people with new type 1 diabetes have developed some degree of diabetic ketoacidosis by the time the diabetes is recognized. The diagnosis of other types of diabetes is usually made in other ways. The most common are ordinary health screening, detection of hyperglycemia when a doctor is investigating a complication of longstanding, though unrecognized, diabetes, and new signs and symptoms due to the diabetes, such as vision changes or unexplainable fatigue.
Diabetes screening is recommended for
many people at various stages of life, and for those with any of several risk
factors. The screening test varies according to circumstances and local policy,
and may be a random blood glucose test, a fasting blood glucose test, a blood
glucose test two hours after
The complications of diabetes are far less common and less severe in people who have well-controlled blood sugar levels. In fact, the better the control, the lower the risk of complications.
Hence patient education, understanding, and participation is vital. Healthcare professionals treating diabetes also often attempt to address health issues that may accelerate the deleterious effects of diabetes. These include smoking (stopping), elevated cholesterol levels (control or reduction with diet, exercise or medication), obesity (even modest weight loss can be beneficial), high blood pressure (exercise or medication if needed), and lack of regular exercise.
To monitor the amount of glucose within the blood a person with diabetes should test their blood regularly. The procedure is quite simple and can often be done at home.
Main articles: Diabetic ketoacidosis , Nonketotic hyperosmolar coma , Hypoglycemia and Diabetic coma
Diabetic ketoacidosis (DKA) is an acute, dangerous complication and is always a medical emergency. On presentation at hospital, the patient in DKA is typically dehydrated and breathing both fast and deeply. Abdominal pain is common and may be severe. The level of consciousness is typically normal until late in the process, when lethargy (dulled or reduced level of alertness or consciousness) may progress to coma. Ketoacidosis can become severe enough to cause hypotension, shock, and death. Prompt proper treatment usually results in full recovery, though death can result from inadequate treatment, delayed treatment or from a variety of complications. It is much more common in type 1 diabetes than type 2, but can still occur in patients with type 2 diabetes.
Nonketotic hyperosmolar coma
While not generally progressing to coma, this hyperosmolar nonketotic state (HNS) is another acute problem associated with diabetes mellitus. It has many symptoms in common with DKA, but an entirely different cause, and requires different treatment. In anyone with very high blood glucose levels (usually considered to be above 300 mg/dl (16 mmol/l)), water will be osmotically drawn out of cells into the blood. The kidneys will also be "dumping" glucose into the urine, resulting in concomitant loss of water, and causing an increase in blood osmolality. If fluid is not replaced (by mouth or intravenously), the osmotic effect of high glucose levels combined with the loss of water will eventually result in very high serum osmolality (ie, dehydration). The body's cells will become progressively dehydrated as water is taken from them and excreted. Electrolyte imbalances are also common, and dangerous. This combination of changes, especially if prolonged, will result in symptoms of lethargy (dulled or reduced level of alertness or consciousness) and may progress to coma. As with DKA urgent medical treatment is necessary, especially volume replacement. This is the 'diabetic coma' which more commonly occurs in type 2 diabetics.
Etiology, pathogenesis and genetics of diabetes mellitus.
Proinsulin consists of three domains: an amino-terminal B chain, a carboxy-terminal A chain and a connecting peptide in the middle known as the C peptide.
Within the endoplasmic reticulum, proinsulin is exposed to several specific endopeptidases which excise the C peptide, thereby generating the mature form of insulin. Insulin and free C peptide are packaged in the Golgi into secretory granules which accumulate in the cytoplasm.
Control of Insulin Secretion
Insulin is secreted in primarily in response to elevated blood concentrations of glucose. This makes sense because insulin is "in charge" of facilitating glucose entry into cells. Some neural stimuli (e.g. sight and taste of food) and increased blood concentrations of other fuel molecules, including amino acids and fatty acids, also promote insulin secretion.
Our understanding of the mechanisms behind insulin secretion remain somewhat fragmentary. Nonetheless, certain features of this process have been clearly and repeatedly demonstrated, yielding the following model:
Glucose is transported into the B cell by facilitated diffusion through a glucose transporter; elevated concentrations of glucose in extracellular fluid lead to elevated concentrations of glucose within the B cell.
A person with diabetes constantly manages their blood's sugar (glucose) levels. After a blood sample is taken and tested, it is determined whether the glucose levels are low or high. If glucose levels are too low carbohydrates are ingested.If glucose in the blood is too high, the appropriate amount of insulin is administered into the body such as through an insulin pump.
Elevated concentrations of glucose within the B cell ultimately leads to membrane depolarization and an influx of extracellular calcium. The resulting increase in intracellular calcium is thought to be one of the primary triggers for exocytosis of insulin-containing secretory granules. The mechanisms by which elevated glucose levels within the B cell cause depolarization is not clearly established, but seems to result from metabolism of glucose and other fuel molecules within the cell, perhaps sensed as an alteration of ATP:ADP ratio and transduced into alterations in membrane conductance.
Stimulation of insulin release is readily observed in whole animals or people. The normal fasting blood glucose concentration in humans and most mammals is 80 to 90 mg per 100 ml, associated with very low levels of insulin secretion.
Immediately after the increasing the level of glycemia begins, plasma insulin levels increase dramatically. This initial increase is due to secretion of preformed insulin, which is soon significantly depleted. The secondary rise in insulin reflects the considerable amount of newly synthesized insulin that is released immediately. Clearly, elevated glucose not only simulates insulin secretion, but also transcription of the insulin gene and translation of its mRNA.
Physiologic effects opf insulin
Stand on a streetcorner and ask people if they know what insulin is, and many will reply, "Doesn't it have something to do with blood sugar?" Indeed, that is correct, but such a response is a bit like saying "Mozart? Wasn't he some kind of a musician?"
Insulin is a key player in the control of intermediary metabolism. It has profound effects on both carbohydrate and lipid metabolism, and significant influences on protein and mineral metabolism. Consequently, derangements in insulin signalling have widespread and devastating effects on many organs and tissues.
The Insulin Receptor
Like the receptors for other protein hormones, the receptor for insulin is embedded in the plasma membrane. The insulin receptor is composed of two alpha subunits and two beta subunits linked by disulfide bonds. The alpha chains are entirely extracellular and house insulin binding domains, while the linked beta chains penetrate through the plasma membrane.
The insulin receptor is a tyrosine kinase. In other words, it functions as an enzyme that transfers phosphate groups from ATP to tyrosine residues on intracellular target proteins. Binding of insulin to the alpha subunits causes the beta subunits to phosphorylate themselves (autophosphorylation), thus activating the catalytic activity of the receptor. The activated receptor then phosphorylates a number of intracellular proteins, which in turn alters their activity, thereby generating a biological response.
Several intracellular proteins have been identified as phosphorylation substrates for the insulin receptor, the best-studied of which is insulin receptor substrate 1 or IRS-1. When IRS-1 is activated by phosphorylation, a lot of things happen. Among other things, IRS-1 serves as a type of docking center for recruitment and activation of other enzymes that ultimately mediate insulin's effects.
The action of insuin
Insulin is an anabolic hormone (promotes the synthesis of carbohydrates, proteins, lipids and nucleic acids).
The most important target organs for insulin action are:
The brain and blood cells are unresponsive to insulin.
Insulin and Carbohydrate Metabolism
Glucose is liberated from dietary carbohydrate such as starch or sucrose by hydrolysis within the small intestine, and is then absorbed into the blood. Elevated concentrations of glucose in blood stimulate release of insulin, and insulin acts on cells thoughout the body to stimulate uptake, utilization and storage of glucose. The effects of insulin on glucose metabolism vary depending on the target tissue.
The effects of insulin on carbohydrate metabolism include:
1. Insulin facilitates entry of glucose into muscle, adipose and several other tissues.
The only mechanism by which cells can take up glucose is by facilitated diffusion through a family of hexose transporters. In many tissues - muscle being a prime example - the major transporter used for uptake of glucose (called GLUT4) is made available in the plasma membrane through the action of insulin.
In the absense of insulin, GLUT4 glucose transporters are present in cytoplasmic vesicles, where they are useless for transporting glucose. Binding of insulin to receptors on such cells leads rapidly to fusion of those vesicles with the plasma membrane and insertion of the glucose transporters, thereby giving the cell an ability to efficiently take up glucose. When blood levels of insulin decrease and insulin receptors are no longer occupied, the glucose transporters are recycled back into the cytoplasm.
It should be noted here that there are some tissues that do not require insulin for efficient uptake of glucose: important examples are brain and the liver. This is because these cells don't use GLUT4 for importing glucose, but rather, another transporter that is not insulin-dependent.
2. Insulin stimulates the liver to store glucose in the form of glycogen .
A large fraction of glucose absorbed from the small intestine is immediately taken up by hepatocytes, which convert it into the storage polymer glycogen.
Insulin has several effects in liver which stimulate glycogen synthesis. First, it activates the enzyme hexokinase, which phosphorylates glucose, trapping it within the cell. Coincidently, insulin acts to inhibit the activity of glucose-6-phosphatase. Insulin also activates several of the enzymes that are directly involved in glycogen synthesis, including phosphofructokinase and glycogen synthase. The net effect is clear: when the supply of glucose is abundant, insulin "tells" the liver to bank as much of it as possible for use later.
3. Insulin inhibits glucose formation – from glycogen (glycogenolysis) and – from amino-acid precursors (glyconeogenesis).
As aresult - well-known effect of insulin is to decrease the concentration of glucose in blood, which should make sense considering the mechanisms described above. Another important consideration is that, as blood glucose concentrations fall, insulin secretion ceases. In the absense of insulin, a bulk of the cells in the body become unable to take up glucose, and begin a switch to using alternative fuels like fatty acids for energy. Neurons, however, require a constant supply of glucose, which in the short term, is provided from glycogen reserves.
In the absense of insulin, glycogen synthesis in the liver ceases and enzymes responsible for breakdown of glycogen become active. Glycogen breakdown is stimulated not only by the absense of insulin but by the presence of glucagon, which is secreted when blood glucose levels fall below the normal range.
Insulin and Protein Metabolism:
1. Insulin transfers of amino acids across plasma membranes.
2. Insulin stimulates of protein synthesis.
3. Insulin inhibites of proteolysis.
The metabolic pathways for utilization of fats and carbohydrates are deeply and intricately intertwined. Considering insulin's profound effects on carbohydrate metabolism, it stands to reason that insulin also has important effects on lipid metabolism. Important effects of insulin on lipid metabolism include the following:
1. Insulin promotes synthesis of fatty acids in the liver. As discussed above, insulin is stimulatory to synthesis of glycogen in the liver. However, as glycogen accumulates to high levels (roughly 5% of liver mass), further synthesis is strongly suppressed.
When the liver is saturated with glycogen, any additional glucose taken up by hepatocytes is shunted into pathways leading to synthesis of fatty acids, which are exported from the liver as lipoproteins. The lipoproteins are ripped apart in the circulation, providing free fatty acids for use in other tissues, including adipocytes, which use them to synthesize triglyceride.
2. Insulin inhibits breakdown of fat in adipose tissue (lipolisis) by inhibiting theintracellular lipase that hydrolyzes triglycerides to release fatty acids.
Insulin facilitates entry of glucose into adipocytes, and within those cells, glucose can be used to synthesize glycerol. This glycerol, along with the fatty acids delivered from the liver, are used to synthesize triglyceride within the adipocyte. By these mechanisms, insulin is involved in further accumulation of triglyceride in fat cells.
From a whole body perspective, insulin has a fat-sparing effect. Not only does it drive most cells to preferentially oxidize carbohydrates instead of fatty acids for energy, insulin indirectly stimulates accumulation of fat is adipose tissue.
1. Insulin stimulates the intracellular flew of potassium, phosphate and magnesium in the heart.
2. Insulin inhibits inotropic and chronoropic action (unrelated to hypoglycemia).
The action of insulin can be decreased by:
- glucagons: stimulates glycogenolysis and glyconeogenesis;
- somatostatin: inhibits secretion of insulin and regulates glucose absorption from alimentary tract into blood;
- glucocorticoids: decrease of glucose utilization by tissues, stimulate glycogenolysis and glyconeogenesis, increase lipogenesis (in patients with insulinorsistancy);
- katecholamines (adrenaline): inhibits β-cells secretion, stimulates glycogenolysis and ACTH secretion;
- somatotropin: stimulates α-cells (which secret glucagon), increases activity of enzymes which destroy the insulin, stimulates glyconeogenesis, increases of glucose exit from the liver veins into blood, decreases of glucose utilization by tissues;
- ACTH: stimulates glucocorticoides secretion and β-cells secretion;
- thyroid hormones: increase glucose absorption into blood, stimulate glycogenolysis, inhibit fat formation from the carbohydrates.
Absolute insulin insufficiency means that pancreas produce insulin in very low quantities or doesn’t produce it at all (due to destruction of beta-cells by inflammative, autoimmune process or surgery).
Relative insulin insufficiency means that pancreas produces or can produce insulin but it doesn’t “work”. (The pathologic process can be on the next levels:
- beta cells: they can be not sensitive for the high level of glycemia;
- insulin: abnormal insulin, insulin antibodies, contrainsulin hormones, absence of enzyme, which activates proinsulin (into insulin));
- receptors (decreased receptor number or diminished binding of insulin).
Type 1, or insulin-dependent diabetes mellitus is characterized by pancreatic islet beta cell destruction and absolute insulinopenia.
Type I Diabetes
In response to high levels of glucose in the blood, the insulin-producing cells in the pancreas secrete the hormone insulin. Type I diabetes occurs when these cells are destroyed by the body's own imune system.
This individuals are ketosis prone under basal conditions. The onset of the disease is generally in youth, but it can occur at any age. Patients have dependence on daily insulin administration for survival.
Current formulation of the pathogenesis of type 1 DM includes the following:
1. A genetic predisposition, conferred by diabetogenic genes on the short arm of chromosome C, either as part of it or in close proximity to the major histocompatibility complex (MMHC) region (more than 95 % of type 1 diabetes individuals are HLA DR3, DR4 or DR3/DR4; on the other hand, HLA DR2 confers protection against the development of type 1 DM);
2. Putative environmental triggers (possibly viral infections (Coxsackie B, rubella, mumps) or chemical toxins (nitrosourea compounds)) that in genetically susceptible individuals might play a role in initiating the disease process.
3. An immune mechanism gone awry, either initiation of immune destruction or loss of tolerance, leading to slow, progressive loss of pancreatic islet beta cells and eventual clinical onset of type 1 diabetes.
Stages of type 1 DM development (by Flier, 1986)
I. A genetic predisposition or changes of immunity.
II. Putative environmental triggers.
III. Active autoimmune insulities with β-cells destruction.
IV. Progression of autoimmune insulities with destruction of >50 % of β-cells.
V. Development of manifest DM.
VI. Total β-cells destruction.
I. A genetic predisposition or changes of immunity.
II. Putative environmental triggers.
III. Active autoimmune insulities with β-cells destruction.
IV. Progression of autoimmune insulities with destruction of >50 % of β-cells.
V. Development of manifest DM.
VI. Total β-cells destruction.
Type 2 or non-insulin-dependent diabetes mellitus is the most common form of diabetes, accounting for 95 – 90 % of the diabetic population. (Video) Most investigators agree that genetic factors underlie NIDDM, but it is probably not caused by defects at a single gene locus. Obesity, diet, physical activity, intrauterine environment, and stress are among the most commonly implicated environmental factors which play a role in the development of the disease. In patients with type 2 DM mostly we can find relative insulin insufficiency (when pancreatic gland secrets insulin but it can have changed structure or weight, or circulating enzymes and antibodies destroy normal insulin, or there are changes of insulin receptors).
Pathogenetic and clinical difference of type 1 and type 2 DM.
I. Type 1 of DM (destruction of β-cells which mostly leads to absolute insulin insufficiency):
II. Type 2 of DM (resistance to insulin and relative insulin insufficiency or defect of insulin secretion with or without resistance to insulin).
III. Other specific types:
- genetic defects of β-cells function;
- genetic defects of insulin action;
- pancreatic diseases (chronic pancreatitis; trauma, pancreatectomy; tumor of pancreatic gland; fibrocalculosis; hemochromatosis);
- endocrine disease;
- drug exposures;
- infections and others.
Diabetes can affect every part of the body, including the skin. The skin is a common target of DM As many as one third of people with diabetes will have a skin disorder caused or affected by diabetes at some time in their lives. In fact, such problems are sometimes the first sign that a person has diabetes. Luckily, most skin conditions can be prevented or easily treated if caught early.
Some of these problems are skin conditions anyone can have, but people with diabetes get more easily. These include bacterial infections, fungal infections, and itching. Other skin problems happen mostly or only to people with diabetes. These include diabetic dermopathy, necrobiosis lipoidica diabeticorum, diabetic blisters, and eruptive xanthomatosis.
Several kinds of bacterial infections occur in people with diabetes. One common one are styes. These are infections of the glands of the eyelid. Another kind of infection are boils, or infections of the hair follicles. Carbuncles are deep infections of the skin and the tissue underneath. Infections can also occur around the nails.
Inflamed tissues are usually hot, swollen, red, and painful. Several different organisms can cause infections. The most common ones are the Staphylococcus bacteria, also called staph.
Once, bacterial infections were life threatening, especially for people with diabetes. Today, death is rare, thanks to antibiotics and better methods of blood sugar control.
But even today, people with diabetes have more bacterial infections than other people do.
The culprit in fungal infections of people with diabetes is often Candida albicans. This yeast-like fungus can create itchy rashes of moist, red areas surrounded by tiny blisters and scales. These infections often occur in warm, moist folds of the skin. Problem areas are under the breasts, around the nails, between fingers and toes, in the corners of the mouth, under the foreskin (in uncircumcised men), and in the armpits and groin.
Common fungal infections include jock itch, athlete's foot, ringworm (a ring-shaped itchy patch), and vaginal infection that causes itching.
Localized itching is often caused by diabetes. It can be caused by a yeast infection, dry skin, or poor circulation. When poor circulation is the cause of itching, the itchiest areas may be the lower parts of the legs.
Diabetes can cause changes in the small blood vessels. These changes can cause skin problems called diabetic dermopathy.
Dermopathy often looks like light brown, scaly patches. These patches may be oval or circular. Some people mistake them for age spots. This disorder most often occurs on the front of both legs. But the legs may not be affected to the same degree. The patches do not hurt, open up, or itch.
Necrobiosis Lipoidica Diabeticorum
Another disease that may be caused by changes in the blood vessels is necrobiosis lipoidica diabeticorum (NLD). NLD is similar to diabetic dermopathy. The difference is that the spots are fewer, but larger and deeper.Iit consists of skin necrosis with lipid infiltration and is also characteristically found in the pretibial area. The lesions resemble red plaques with distinct border.s
NLD often starts as a dull red raised area. After a while, it looks like a shiny scar with a violet border. The blood vessels under the skin may become easier to see. Sometimes NLD is itchy and painful. Sometimes the spots crack open.
NLD is a rare condition. Adult women are the most likely to get it. As long as the sores do not break open, you do not need to have it treated. But if you get open sores, see your doctor for treatment.
Thickening of the arteries - atherosclerosis - can affect the skin on the legs. People with diabetes tend to get atherosclerosis at younger ages than other people do.
As atherosclerosis narrows the blood vessels, the skin changes. It becomes hairless, thin, cool, and shiny. The toes become cold. Toenails thicken and discolor. And exercise causes pain in the calf muscles because the muscles are not getting enough oxygen.
Because blood carries the infection-fighting white cells, affected legs heal slowly when the skin in injured. Even minor scrapes can result in open sores that heal slowly.
People with neuropathy are more likely to suffer foot injuries. These occur because the person does not feel pain, heat, cold, or pressure as well. The person can have an injured foot and not know about it. The wound goes uncared for, and so infections develop easily. Atherosclerosis can make things worse. The reduced blood flow can cause the infection to become severe.
Allergic skin reactions can occur in response to medicines, such as insulin or diabetes pills. You should see your doctor if you think you are having a reaction to a medicine. Be on the lookout for rashes, depressions, or bumps at the sites where you inject insulin.
Diabetic Blisters (Bullosis Diabeticorum)
Rarely, people with diabetes erupt in blisters. Diabetic blisters can occur on the backs of fingers, hands, toes, feet, and sometimes, on legs or forearms.
These sores look like burn blisters. They sometimes are large. But they are painless and have no redness around them. They heal by themselves, usually without scars, in about three weeks. They often occur in people who have diabetic neuropathy. The only treatment is to bring blood sugar levels under control.
Eruptive xanthomas are usually associated with very high serum triglycerides or chylimicrones. They may occur in familial chylomicronaemia syndrome, lipoprotein lipase deficiency, severe familial hypertriglyceridemia, excess alcohol intake, severe uncontrolled diabetes. Treatment is to correct the underlying condition. Lowering triglycerides will result in the clearance of the lesions.
Pict. This shown classic xanthelasma around the eye. It may be associated with genetic hyperlipidaemias, although it may occur with diabetes, biliary cirrhosis or without any associated conditions.
Eruptive xanthomatosis is another condition caused by diabetes that's out of control. It consists of firm, yellow, pea-like enlargements in the skin. Each bump has a red halo and may itch. This condition occurs most often on the backs of hands, feet, arms, legs, elbows, knees and buttocks.
The disorder usually occurs in young men with type 1 diabetes. The person often has high levels of cholesterol and fat (particularly hyperchylomicronemia) in the blood. Like diabetic blisters, these bumps disappear when diabetes control is restored.
Sometimes, people with diabetes develop tight, thick, waxy skin on the backs of their hands. Sometimes skin on the toes and forehead also becomes thick. The finger joints become stiff and can no longer move the way they should. Rarely, knees, ankles, or elbows also get stiff.
This condition happens to about one third of people who have type 1 diabetes. The only treatment is to bring blood sugar levels under control.
Disseminated Granuloma Annulare
In disseminated granuloma annulare, the person has sharply defined ring-shaped or arc-shaped raised areas on the skin. These rashes occur most often on parts of the body far from the trunk (for example, the fingers or ears). But sometimes the raised areas occur on the trunk. They can be red, red-brown, or skin-colored.
Subcutaneous adipose tissue
The abdomen type of obesity is common in patients with type 2 DM. Sometimes generalized subcutaneous adipose tissue atrophy can be observed in diabetics.
Bones and joints
Osteoporosis, osteoarthropaphy, diabetic chairopathy (decreasing of the movements of joints) can be find in patients with DM also.
Diabetic Blood Circulation in Foot
People with diabetes are at risk for blood vessel injury, which may be severe enough to cause tissue damage in the legs and feet.
The heart, arteries, arterioles, and capillaries can be affected. Cardiovascular changes tend to occur earlier in patients with DM when compared with individuals of the same age. Several factors play a role in the high incidence of coronary artery disease seen in patients with DM. These include age of the patient, duration and severity of the diabetes, and presence of other risk factors such as hypertension, smoking and hyperlipoproteinemia. It has been suggested that in some patients with DM, involvement of the small vessels of the heart can lead to cardiomyopathy, independent of narrowing of the major coronary arteries. Myocardial infarction is responsible for at least half of deaths in diabetic patients, and mortality rate for the diabetics is higher than that for nondiabetics of the same age who develop this complication.
Hypertension is common in patients with DM, particularly in the presence of renal disease (as a result of atherosclerosis, destruction of juxtaglomerular cells, sympathetic-nervous-system dysfunction and volume expansion).
Atherosclerosis of femoral, popliteal and calf larger arteries may lead to intermittent claudication, cold extremities, numbness, tingling and gangrene.
Mucomycosis of the nasopharinx, sinusitis, bronchitis, pneumonia, tuberculosis are more common in patients with diabetes than in nondiabetics.
Kidneys and urinary tract
Renal disease include diabetic nephropathy, necrosing renal papillitis, acute tubular necrosis, lupus erythematosus, acute poststreptococcal and membranoproliferative glomerulonephritis, focal glomerulosclerosis, idiopathic membranous nephropathy, nonspecific immune complex glomerulonephritides, infections can occur in any part of the urinary tract. Last are caused when bacteria, usually from the digestive system, reach the urinary tract. If bacteria are growing in the urethra, the infection is called urethritis. The bacteria may travel up the urinary tract and cause a bladder infection, called cystitis. An untreated infection may go farther into the body and cause pyelonephritis, a kidney infection. Some people have chronic or recurrent urinary tract infections.
Complications of the eyes include: ceratities, retinatis, chorioretinatis, cataracts. The last one occurs commonly in the patients with long-standing DM and may be related to uncontrolled hyperglycemia (glucose metabolism by the lens does not require the presence of insulin. The epithelial cells of the lens contain the enzyme aldose reductase, which converts glucose into sorbitol. This sugar may be subsequently converted into fructose by sorbitol dehydrogenase. Sorbitol is retained inside the cells because of its difficulty in transversing plasma membranes. The rise in intracellular osmolality leads to increased water uptake and swelling of the lens).
The diagnosis of DM
The diagnosis of DM may be straightforward or very difficult.
(The presence of the marked hyperglycemia, glucosuria, polyuria, polydipsia, polyphagia, lethargy, a tendency to acquire infections, and physical findings consistent with the disease should offer no difficulty in arriving at the correct diagnosis. On the other hand, mild glucose intolerance in the absence of symptoms or physical findings does not necessarily indicate that DM is present.)
The diagnosis of DM include:
I. Clinical manifestations of DM.
II. Laboratory findings.
1) fasting serum glucose (if the value is over 6,7 mmol/l (120 mg/dl) on two or more separate days, the patient probably has DM);
2) the glucose tolerance test (GTT):
If the diagnosis is still in doubt, then perform a GTT.
The long-term degenerative changes in the blood, vessels, the heart, the kidneys, the nervous system, and the eyes as responsible for the most of the morbidity and mortality of DM. There is a causal relationship and the level of the metabolic control.
It is usually asymptomatic until end stage renal disease develops, but it can course the nephrotic syndrome prior to the development of uremia. Nephropathy develops in 30 to 50 % of type 1 DM patients and in small percentage of type 2 DM patients. Arteriolar hyalinosis, a deposition of hyaline material in the lumen of the afferent and efferent glomerular arterioles, is an almost pathognomic histologic lesion of DM.
In the first few years of type 1 DM there is hyperfiltration which declines fairly steadily to return to a normal value at approximately 10 years (blue line). After sbout 10 years there is sustained proteinurea and by approximately 14 years it has reached nephritic stage (red line). Renal function continues to decline, with the end stage being reached at approximately 16 years
Atherosclerosis of large vessels (macroangiopathy) leads to intermittent claudication, cold extremities and other symptoms which can be also find while arteriols and capillaries are affected (microangiopathy).
Ischemic heart disease.
1. Cardiovascular changes tend to occur earlier in patients with DM when compared with individuals of the same age.
2. Frequency of myocardial infarction (MI) and mortality is higher in diabetics than that in nondiabetis og the same age.
3. The prognosis is even worse if ketoacidosis, or other complications of DM are present.
4. Diabetic patients have more complications of MI (arrhythmias, cardiogenic shock and others) than nondiabetic ones.
5. Often can observe atypical forms (without pain).
6. Male : female = 1 : 1 (nondiabetics = 10 : 1).
It is an old clinical observation that the symptoms of neuropathic dysfunction improve with better control of DM, lending support to the idea that hyperglycemia plays an important role. Accumulation of sorbitol and fructose in the diabetic nerves leads to damage of the Schwann cells and segmental demyelination.
Classification of diabetic neuropathy.
I. Encephalopathy (central neyropathy) is characterized by decreased memory, headache, unadequate actions and others.
II. Peripheral polyneuropathy (radiculoneuropathy). There are three types of radiculoneuropathy:
- distal polyradiculoneuropathy (It is characterized by symmetrical sensory loss, pain at night and during the rest, hyporeflexia, decreased responce touch, burning of heels and soles. The skin becomes atrophic, dry and cold, hair loss may be prominent. The decreased response to touch and pain predisposes to burns and ulcers of the legs and toes.);
- truncal polyradiculoneuropathy (It is an asymmetric, and characterized by pain (which is worse at night), paresthesia and hyperesthesia; muscular weakness involves the muscles of the anterior thigh; reflexes are decreased; weight loss is common.);
- truncal monoradiculoneuropathy (It is usually involves thorasic nerves and the findings are limited to the sensory abnormalities in a radicular distribution.).
III. Visceral dysfunction:
Neuropathic arthropathy (Charcot’s joints)
When you want to lift your arm or take a step, your brain sends nerve signals to the appropriate muscles. Internal organs like the heart and bladder are also controlled by nerve signals, but you do not have the same kind of conscious control over them as you do over your arms and legs. The nerves that control your internal organs are called autonomic nerves, and they signal your body to digest food and circulate blood without your having to think about it. Your body's response to sexual stimuli is also involuntary, governed by autonomic nerve signals that increase blood flow to the genitals and cause smooth muscle tissue to relax. Damage to these autonomic nerves is what can hinder normal function.
1) gastrointestinal tract:
- esophageal neuropathy (It is characterized by segmental distribution with low or absent resting pressure in the low or absent resting pressure in the lower esophageal sphincter and by absence of peristalsis in the body of the esophagus.);
- diabetic gastroparesis (It leads to the irregular food absorption and is characterized by nausea, vomiting, early satiety, bloating and abdomen pain.);
- involvement of the bowel (It is characterized by diarrhea (mostly at night time, postural diarrhea), constipation, malabsorption and fecal incontinence;
2) cardiovascular system:
- orthostatic hypotension (It is characterized by dizziness, vertigo, faintness, and syncope upon assumption of the upright posture and is caused by failure of peripheral arteriolar constriction.);
- tachicardia (but it does not occur in response to hypotension because of sympathetic involvement).
3) urinary tract:
- Bladder dysfunction can have a profound effect on quality of life. Diabetes can damage the nerves that control bladder function. Men and women with diabetes commonly have bladder symptoms that may include a feeling of urinary urgency, frequency, getting up at night to urinate often, or leakage of urine (incontinence). These symptoms have been called overactive bladder. Less common but more severe bladder symptoms include difficulty urinating and complete failure to empty (retention). These symptoms are called a neurogenic bladder. Some evidence indicates that this problem occurs in both men and women with diabetes at earlier ages than in those without diabetes.
is characterized by painless swelling of the feet without edema or signs of infection. The foot becomes shorter and wider, eversion, external rotation, and flattening of the longitudinal arch. This arthropathy is associated with sensory involvelvement, particularly impairment of afferent pain proprioceptive impulses.
Appearance of diabetic foot is caused by a combination of vascular insufficiency, neuropathy, and infection.
Sensibility partly decreased or normal decreased or absent
Hypoglycemia, or abnormally low blood glucose, is a complication of several diabetes treatments. It may develop if the glucose intake does not cover the treatment. The patient may become agitated, sweaty, and have many symptoms of sympathetic activation of the autonomic nervous system resulting in feelings similar to dread and immobilized panic. Consciousness can be altered, or even lost, in extreme cases, leading to coma and/or seizures, or even brain damage and death. In patients with diabetes, this can be caused by several factors, such as too much or incorrectly timed insulin, too much exercise or incorrectly timed exercise (exercise decreases insulin requirements) or not enough food (actually an insufficient amount of glucose producing carbohydrates in food). In most cases, hypoglycemia is treated with sugary drinks or food. In severe cases, an injection of glucagon (a hormone with the opposite effects of insulin) or an intravenous infusion of glucose is used for treatment, but usually only if the person is unconscious. In hospital, intravenous dextrose is often used.
Coronary artery disease, leading to angina or myocardial infarction ("heart attack")
Stroke (mainly the ischemic type)
Peripheral vascular disease, which contributes to intermittent claudication (exertion-related foot pain) as well as diabetic foot.
Diabetic myonecrosis ('muscle wasting')
Diabetic foot, often due to a combination of neuropathy and arterial disease, may cause skin ulcer and infection and, in serious cases, necrosis and gangrene. It is the most common cause of adult amputation, usually of toes and or feet, in the developed world.
Lipids are water-insoluble organic biomolecules that can be extracted from cells and tissues by nonpolar solvents, e.g., chloroform, ether, or benzene.
Lipids are an amphiphilic class of hydrocarbon-containing organic compounds. Lipids are categorized by the fact that they have complicated solvation properties, giving rise to lipid polymorphism. Lipid molecules have these properties because they consist largely of long hydrocarbon tails which are lipophilic in nature as well as polar headgroups (e.g. phosphate-based functionality, and/or inositol based functionality). In living organisms, lipids are used for energy storage, serve as the structural components of cell membranes, and constitute important signalling molecules. Although the term lipid is often used as a synonym for fat, the latter is in fact a subgroup of lipids called triglycerides.
There are several different families or classes of lipids but all derive their distinctive properties from the hydrocarbon nature of a major portion of their structure.
Biological functions of lipids
Biological molecules that are insoluble in aqueous solutions and soluble in organic solvents are classified as lipids. The lipids of physiological importance for humans have four major functions:
Lipids have several important biological functions, serving
(1) as structural components of membranes,
(2) as storage and transport forms of metabolic fuel,
(3) as a protective coating on the surface of many organisms, and
(5) as cell-surface components concerned in cell recognition, species specificity, and tissue immunity. Some substances classified among the lipids have intense biological activity; they include some of the vitamins and hormones.
Although lipids are a distinct class of biomolecules, we shall see that they often occur combined, either covalently or through weak bonds, with members of other classes of biomolecules to yield hybrid molecules such as glycolipids, which contain both carbohydrate and lipid groups, "and lipoproteins, which contain both lipids and proteins. In such biomolecules the distinctive chemical and physical properties of their components are blended to fill specialized biological functions.
Lipids have been classified in several different ways. The most satisfactory classification is based on their backbone structures:
1. Simple lipids:
2. Complex lipids:
Lipids usually contain fatty acids as components. Such lipids are called saponifiable lipids since they yield soaps (salts of fatty acids) on alkaline hydrolysis. The other great group of lipids which do not contain fatty acids and hence are nonsapomfiable.
Let us first consider the structure and properties of fatty acids, characteristic components of all the complex lipids.
Fatty acids and glycerides
Fatty acids fill two major roles in the body:
· 1. as the components of more complex membrane lipids.
· 2. as the major components of stored fat in the form of triacylglycerols.
Fatty acids are long-chain hydrocarbon molecules containing a carboxylic acid moiety at one end. The numbering of carbons in fatty acids begins with the carbon of the carboxylate group. At physiological pH, the carboxyl group is readily ionized, rendering a negative charge onto fatty acids in bodily fluids.
Fatty acids that contain no carbon-carbon double bonds are termed saturated fatty acids; those that contain double bonds are unsaturated fatty acids. The numeric designations used for fatty acids come from the number of carbon atoms, followed by the number of sites of unsaturation (eg, palmitic acid is a 16-carbon fatty acid with no unsaturation and is designated by 16:0). The site of unsaturation in a fatty acid is indicated by the symbol and the number of the first carbon of the double bond (e.g. palmitoleic acid is a 16-carbon fatty acid with one site of unsaturation between carbons 9 and 10, and is designated by 16:19).
Saturated fatty acids of less than eight carbon atoms are liquid at physiological temperature, whereas those containing more than ten are solid. The presence of double bonds in fatty acids significantly lowers the melting point relative to a saturated fatty acid.
The majority of body fatty acids
are acquired in the diet. However, the lipid biosynthetic capacity of the body
(fatty acid synthase and other fatty acid modifying enzymes) can supply the
body with all the various fatty acid structures needed. Two key exceptions to
this are the highly unsaturated fatty acids know as linoleic acid and linolenic
acid, containing unsaturation sites beyond carbons 9 and 10. These two fatty
acids cannot be synthesized from precursors in the body, and are thus
considered the essential fatty acids;
essential in the sense that they must be provided in the diet. Since plants are
capable of synthesizing linoleic and linolenic acid humans can aquire these
fats by consuming a variety of plants or else by eating the meat of animals
that have consumed these plant fats.
Chemically, fatty acids can be described as long-chain monocarboxylic acids and have a general structure of CH3(CH2)nCOOH. The length of the chain usually ranges from 12 to 24, always with an even number of carbons. When the carbon chain contains no double bonds, it is a saturated chain. If it contains one or more such bonds, it is unsaturated. The presence of double bonds generally reduces the melting point of fatty acids. Furthermore, unsaturated fatty acids can occur either in cis or trans geometric isomers. In naturally occurring fatty acids, the double bonds are in the cis-configuration.
Glycerides are lipids possessing a glycerol (propan-1, 2, 3-triol) core structure with one or more fatty acyl groups, which are fatty acid-derived chains attached to the glycerol backbone by ester linkages. Glycerides with three acyl groups (triglycerides or neutral fats) are the main storage form of fat in animals and plants.
An important type of glyceride-based molecule found in biological membranes, such as the cell's plasma membrane and the intracellular membranes of organelles, are the phosphoglycerides or glycerophospholipids. These are phospholipids that contain a glycerol core linked to two fatty acid-derived "tails" by ester or, more rarely, ether linkages and to one "head" group by a phosphate ester linkage. The head groups of the phospholipids found in biological membranes are phosphatidylcholine (also known as PC, and lecithin), phosphatidylethanolamine (PE), phosphatidylserine and phosphatidylinositol (PI). These phospholipids are subject to a variety of functions in the cell: for instance, the lipophilic and polar ends can be released from specific phospholipids through enzyme-catalysed hydrolysis to generate secondary messengers involved in signal transduction. In the case of phosphatidylinositol, the head group can be enzymatically modified by the addition of one, two or three phosphate groups, this constituting another mechanism of cell signalling. While phospholipids are the major component of biological membranes, other non-glyceride lipid components like sphingolipids and sterols (such as cholesterol in animal cell membranes) are also found in biological membranes.
A biological membrane is a form of lipid bilayer, as is a liposome. Formation of lipid bilayers is an energetically-favoured process when the glycerophospholipids described above are in an aqueous environment. In an aqueous system, the polar heads of lipids orientate towards the polar, aqueous environment, while the hydrophobic tails minimise their contact with water. The lipophilic tails of lipids (U) tend to cluster together, forming a lipid bilayer (1) or a micelle (2). Other aggregations are also observed and form part of the polymorphism of amphiphile (lipid) behaviour. The polar heads (P) face the aqueous environment, curving away from the water. Phase behaviour is a complicated area within biophysics and is the subject of current academic research.
Micelles and bilayers form in the polar medium by a process known as the lipophilic effect. When dissolving a lipophilic or amphiphilic substance in a polar environment, the polar molecules (i.e. water in an aqueous solution) become more ordered around the dissolved lipophilic substance, since the polar molecules cannot form hydrogen bonds to the lipophilic areas of the amiphphile. So, in an aqueous environment the water molecules form an ordered "clathrate" cage around the dissolved lipophilic molecule.
The self-organisation depends on the concentration of the lipid present in solution. Below the critical micelle concentration, the lipids form a single layer on the liquid surface and are (sparingly) dispersed in the solution. At the first critical micelle concentration (CMC-I), the lipids organise in spherical micelles, at given points above this concentration, other phases are observed (see lipid polymorphism).
Although fatty acids occur in very large amounts as building-block components of the saponifiable lipids, only traces occur in free (unesterified) form in cells and tissues. Well over 100 different kinds of fatty acids have been isolated from various lipids of animals, plants, and microorganisms. All possess a long hydrocarbon chain and a terminal carboxyl group. The hydrocarbon chain may be saturated, as in palmitic acid, or it may have one or more double bonds, as in oleic acid; a few fatty acids contain triple bonds. Fatty acids differ from each other primarily in chain length and in the number and position of their unsaturated bonds. They are often symbolized by a shorthand notation that designates the length of the carbon chain and the number, position, and configuration of the double bonds. Thus palmitic acid (16 carbons, saturated) is symbolized 16:0 and oleic acid [18 carbons and one double bond (cis) at carbons 9 and 10] is symbolized 18:1. It is understood that the double bonds are cis (see below) unless indicated otherwise.
Some generalizations can be made on the different fatty acids of higher plants and animals. The most abundant have an even number of carbon atoms with chains between 14 and 22 carbon atoms long, but those with 16 or 18 carbons predominate. The most common among the saturated fatty acids are palmitic acid (Cis) and stearic acid (Cis) and among the unsaturated fatty acids oleic acid (Cis). Unsaturated fatty acids predominate over the saturated ones, particularly in higher
plants and in animals living at low temperatures. Unsaturated fatty
acids have lower
melting points than saturated fatty acids of the same chain length. In most
monounsaturated (monoenoic) fatty acids of higher organisms there is a double
bond between carbon atoms 9 and
There are two kinds of fats,
saturated and unsaturated. Unsaturated fats have at least one
double bond in one of the fatty acids. A double bond happens when two electrons
are shared or exchanged in a bond. They are much stronger than single bonds. Saturated
fats have no double bonds.
Fats have a lot of energy stored up in their molecular bonds. That's why the human body stores fat as an energy source. When it needs extra fuel, your body breaks down the fat and uses the energy. Where one molecule of sugar only gives a small amount of energy, a fat molecule gives off many times more.
Saturated fatty acid
Unsaturated monoenic fatty acid
Unsaturated polienic fatty acid
All Lipids are hydrophobic: that’s the one property they have in common. This group of molecules includes fats and oils, waxes, phospholipids, steroids (like cholesterol), and some other related compounds.
Structure of Fatty Acids
Fats and oils are made from two kinds of molecules: glycerol (a type of alcohol with a hydroxyl group on each of its three carbons) and three fatty acids joined by dehydration synthesis. Since there are three fatty acids attached, these are known as triglycerides. “Bread” and pastries from a “bread factory” often contain mono- and diglycerides as “dough conditioners.” Can you figure out what these molecules would look like? The main distinction between fats and oils is whether they’re solid or liquid at room temperature, and this, as we’ll soon see, is based on differences in the structures of the fatty acids they contain.
Essential fatty acids
When weanling or immature rats are placed on a fat-free diet, they grow poorly, develop a scaly skin, lose hair, and ultimately die with many pathological signs. When linoleic acid is present in the diet, these conditions do not develop. Linolenic acid and arachidonic acid also prevent these symptoms. Saturated and monounsaturated fatty acids are inactive. It has been concluded that mammals can synthesize saturated and monounsaturated fatty acids from other precursors but are unable to make linoleic and linolenic acids. Fatty acids required in the diet of mammals are called essential fatty acids. The most abundant essential fatty acid in mammals is linoleic acid, which makes up from 10 to 20 percent of the total fatty acids of their triacylglycerols and phosphoglycerides. Linoleic and linolenic acids cannot be synthesized by mammals but must be obtained from plant sources, in which they are very abundant. Linoleic acid is a necessary precursor in mammals for the biosynthesis of arachidonic acid, which is not found in plants.
The terms saturated, mono-unsaturated, and poly-unsaturated refer to the number of hydrogens attached to the hydrocarbon tails of the fatty acids as compared to the number of double bonds between carbon atoms in the tail. Fats, which are mostly from animal sources, have all single bonds between the carbons in their fatty acid tails, thus all the carbons are also bonded to the maximum number of hydrogens possible. Since the fatty acids in these triglycerides contain the maximum possible amouunt of hydrogens, these would be called saturated fats. The hydrocarbon chains in these fatty acids are, thus, fairly straight and can pack closely together, making these fats solid at room temperature. Oils, mostly from plant sources, have some double bonds between some of the carbons in the hydrocarbon tail, causing bends or “kinks” in the shape of the molecules. Because some of the carbons share double bonds, they’re not bonded to as many hydrogens as they could if they weren’t double bonded to each other. Therefore these oils are called unsaturated fats. Because of the kinks in the hydrocarbon tails, unsaturated fats can’t pack as closely together, making them liquid at room temperature. Many people have heard that the unsaturated fats are “healthier” than the saturated ones. Hydrogenated vegetable oil (as in shortening and commercial peanut butters where a solid consistency is sought) started out as “good” unsaturated oil. However, this commercial product has had all the double bonds artificially broken and hydrogens artificially added (in a chemistry lab-type setting) to turn it into saturated fat that bears no resemblance to the original oil from which it came (so it will be solid at room temperature).
Although the specific functions of essential fatty acids in mammals were a mystery for many years, one function has been discovered. Essential fatty acids are necessary precursors in the biosynthesis of a group of fatty acid derivatives called prostaglandins, hormonelike compounds which in trace amounts have profound effects on a number of important physiological activities.
Physical and chemical properties of fatty acids
Saturated and unsaturated fatty acids have quite different conformations. In saturated fatty acids, the hydrocarbon tails are flexible and can exist in a very large number of conformations because each single bond in the backbone has complete freedom of rotation. Unsaturated fatty acids, on the other hand, show one or more rigid kinks contributed by the nonrotating double bond(s).
Unsaturated fatty acids undergo addition reactions at their double bonds. Quantitative titration with halogens, e.g., iodine or bromine, can yield information on the relative number of double bonds in a given sample of fatty acids or lipid.
Fat is also known as a triglyceride. It is made up of a molecule known as glycerol that is connected to one, two, or three fatty acids. Glycerol is the basis of all fats and is made up of a three-carbon chain. It connects the fatty acids together. A fatty acid is a long chain of carbon atoms connected to each other.
Fatty acid esters of the alcohol glycerol are called acylglycerols or glycerides; they are sometimes referred to as "neutral fats," a term that has become archaic. When all three hydroxyl groups of glycerol are esterified with fatty acids, the structure is called a triacylglycerol:
Although the name "triglyceride" has been traditionally used to designate these compounds, an international nomenclature commission has recommended that this chemically inaccurate term no longer be used. Triacylglycerols are the most abundant family of lipids and the major components of depot or storage lipids in plant and animal cells. Triacylglycerols that are solid at room temperature are often referred to as "fats" and those which are liquid as "oils." Diacylgiycerols (also called diglycerides) and monoacylgiycerols (or monoglycerides) are also found in nature, but in much smaller amounts.
Triacylglycerols occur in many different types, according to the identity and position of the three fatty acid components esterified to glycerol. Those with a single kind of fatty acid in all three positions, called simple triacylglycerols, are named after the fatty acids they contain. Examples are tristearoylglycerol, tripalmitoylglycerol, and trioleoylglycerol; the trivial and more commonly used names are tristearin, tripalmitin, and trioiein, respectively. Mixed triacylglycerols contain two or more different fatty acids. The naming of mixed triacylglycerols can be illustrated by the example of 1-palmitoyldi-stearoylglycerol (trivial name, 1-palmitodistearin). Most natural fats are extremely complex mixtures of simple and mixed triacylglycerols.
Properties of triacylglycerols
The melting point of triacylglycerols is determined by their fatty acid components. In general, the melting point increases with the number and length of the saturated fatty acid components. For example, tripalmitin and tristearin are solids at body temperature, whereas triolein and trilinolein are liquids. All triacylglycerols are insoluble in water and do not tend by themselves to form highly dispersed micelles. However, diacylglycerols and monoacylglycerols have appreciable polarity because of their free hydroxyl groups and thus can form micelles. Diacyl- and monoacylglycerols find wide use in the food industry in the production of more homogeneous and more easily processed foods; they are completely digestible and utilized biologically. Acylglycerols are soluble in ether, chloroform, benzene, and hot ethanol. Their specific gravity is lower than that of water. Acylglycerols undergo hydrolysis when boiled with acids or bases or by the action of lipases, e.g., those present in pancreatic juice. Hydrolysis with alkali, called saponification, yields a mixture of soaps and glycerol.
occur in animals in something called hormones. The basis of a steroid
molecule is a four-ring structure, one with five carbons and three with six
carbons in the rings. You may have heard of steroids in the news. Many body
builders and athletes use anabolic steroids to build muscle mass. The steroids
make their body want to add more muscle than they normally would be able to. The body builders wind up stronger and bulkier (but
Never take drugs to enhance your body. Those body builders are actually hurting their bodies. They can't see it because it is slowly destroying their internal organs and not the muscles. When they get older, they can have kidney and liver problems. Some even die.
The important class of lipids called steroids are actually metabolic derivatives of terpenes, but they are customarily treated as a separate group. Steroids may be recognized by their tetracyclic skeleton, consisting of three fused six-membered and one five-membered ring, as shown in the diagram to the right. The four rings are designated A, B, C & D as noted, and the peculiar numbering of the ring carbon atoms (shown in red) is the result of an earlier misassignment of the structure. The substituents designated by R are often alkyl groups, but may also have functionality. The R group at the A:B ring fusion is most commonly methyl or hydrogen, that at the C:D fusion is usually methyl. The substituent at C-17 varies considerably, and is usually larger than methyl if it is not a functional group. The most common locations of functional groups are C-3, C-4, C-7, C-11, C-12 & C-17. Ring A is sometimes aromatic. Since a number of tetracyclic triterpenes also have this tetracyclic structure, it cannot be considered a unique identifier.
Steroids are widely distributed in
animals, where they are associated with a number of physiological processes.
Examples of some important steroids are shown in the following diagram.
Different kinds of steroids will be displayed by clicking the "Toggle
Structures" button under the diagram. Norethindrone is a synthetic
steroid, all the other examples occur naturally. A common strategy in pharmaceutical chemistry is to take a natural
compound, having certain desired biological properties together with undesired
side effects, and to modify its structure to enhance the desired
characteristics and diminish the undesired. This is sometimes accomplished by
trial and error.
The generic steroid structure drawn above has seven chiral stereocenters (carbons 5, 8, 9, 10, 13, 14 & 17), which means that it may have as many as 128 stereoisomers. With the exception of C-5, natural steroids generally have a single common configuration. This is shown in the last of the toggled displays, along with the preferred conformations of the rings.
Chemical studies of the steroids were very important to our present understanding of the configurations and conformations of six-membered rings. Substituent groups at different sites on the tetracyclic skeleton will have axial or equatorial orientations that are fixed because of the rigid structure of the trans-fused rings. This fixed orientation influences chemical reactivity, largely due to the greater steric hindrance of axial groups versus their equatorial isomers. Thus an equatorial hydroxyl group is esterified more rapidly than its axial isomer.
Steroids are complex ethers of cyclic spirits sterols and fatty acids. Sterols are
derivatives of the saturated tetracylic hydrocarbon cyclopentanoperhydrophenanthrene:
The general structure of cholesterol consists of two six-membered rings side-by-side and sharing one side in common, a third six-membered ring off the top corner of the right ring, and a five-membered ring attached to the right side of that.
The central core of this molecule, consisting of four fused rings, is shared by all steroids, including estrogen (estradiol), progesterone, corticosteroids such as cortisol (cortisone), aldosterone, testosterone, and Vitamin D. In the various types of steroids, various other groups/molecules are attached around the edges. Know how to draw the four rings that make up the central structure.
Cholesterol is not a “bad guy!” Our bodies make about
Many people have hear the claims that egg yolk contains too much cholesterol, thus should not be eaten. An interesting study was done at Purdue University a number of years ago to test this. Men in one group each ate an egg a day, while men in another group were not allowed to eat eggs. Each of these groups was further subdivided such that half the men got “lots” of exercise while the other half were “couch potatoes.” The results of this experiment showed no significant difference in blood cholesterol levels between egg-eaters and non-egg-eaters while there was a very significant difference between the men who got exercise and those who didn’t.
A great many
different steroids, each with a distinctive function or activity, have been
isolated from natural sources. Steroids differ in the number and position of
double bonds, in the type, location, and number of substituent functional
groups, in the configuration of the bonds between the substituent groups and
the nucleus, and in the configuration of the rings in relation to each other. Cholesterol
is the most abundant steroid in animal tissues. Cholesterol and lanosterol
are members of a large subgroup of steroids called the sterols. They are steroid alcohols containing a hydroxyl group
at carbon 3 of ring A and a branched aliphatic chain of eight or more carbon
atoms at carbon 17. They occur either as free alcohols or as long-chain fatty
acid esters of the hydroxyl group at carbon 3; all are solids at room
temperature. Cholesterol melts at
Cholesterol is the precursor of many other steroids in animal tissues, including the bile acids, detergentlike compounds that aid in emulsification and absorption of lipids in the intestine; the androgens, or male sex hormones; the estrogens, or female sex hormones; the progestational hormone progesterone; and the adrenocortical hormones. Among the most important steroids are a group of compounds having vitamin D activity.
Waxes are water-insoluble, solid esters of higher fatty acids with long-chain monohydroxylic fatty alcohols or with sterols. They are soft and pliable when warm but hard when cold. Waxes are found as protective coatings on skin, fur, and feathers, on leaves and fruits of higher plants, and on the exoskeleton of many insects. The major components of beeswax are palmitic acid esters of long-chain fatty alcohols with 26 to 34 carbon atoms. Lanolin, or wool fat, is a mixture of fatty acid esters of the sterols lanosterol and agnosterol.
Waxes are used to
coat and protect things in nature. Bees make wax. Your ears make wax. Plant
leaves even have wax on the outside of their leaves. It can be used for
structures such as the bees' honeycombs. Waxes can also be used for protection.
Plants use wax to stop evaporation
of water from their leaves.
Prostaglandins Thromboxanes & Leukotrienes
The members of this group of structurally related natural hormones have an extraordinary range of biological effects. They can lower gastric secretions, stimulate uterine contractions, lower blood pressure, influence blood clotting and induce asthma-like allergic responses. Because their genesis in body tissues is tied to the metabolism of the essential fatty acid arachadonic acid (5,8,11,14-eicosatetraenoic acid) they are classified as eicosanoids. Many properties of the common drug asprin result from its effect on the cascade of reactions associated with these hormones.
The metabolic pathways by which arachidonic acid is converted to the various eicosanoids are complex and will not be discussed here. A rough outline of some of the transformations that take place is provided below. It is helpful to view arachadonic acid in the coiled conformation shown in the shaded box.
The basic structure of phospolipids is very similar to that of the triacylglycerides except that C-3 (sn3)of the glycerol backbone is esterified to phosphoric acid. The building block of the phospholipids is phosphatidic acid which results when the X substitution in the basic structure shown in the Figure below is a hydrogen atom. Substitutions include ethanolamine (phosphatidylethanolamine), choline (phosphatidylcholine, also called lecithins), serine (phosphatidylserine), glycerol (phosphatidylglycerol), myo-inositol (phosphatidylinositol, these compounds can have a variety in the numbers of inositol alcohols that are phosphorylated generating polyphosphatidylinositols), and phosphatidylglycerol.
Phosphoglycerides are characteristic major components of cell membranes; only very small amounts of phosphoglycerides occur elsewhere in cells.
Phospholipids are made from glycerol, two fatty acids, and (in place of the third fatty acid) a phosphate group with some other molecule attached to its other end. The hydrocarbon tails of the fatty acids are still hydrophobic, but the phosphate group end of the molecule is hydrophilic because of the oxygens with all of their pairs of unshared electrons. This means that phospholipids are soluble in both water and oil.
An emulsifying agent is a substance which is soluble in both oil and water, thus enabling the two to mix. A “famous” phospholipid is lecithin which is found in egg yolk and soybeans. Egg yolk is mostly water but has a lot of lipids, especially cholesterol, which are needed by the developing chick. Lecithin is used to emulsify the lipids and hold them in the water as an emulsion. Lecithin is the basis of the classic emulsion known as mayonnaise.
Our cell membranes are made
mostly of phospholipids arranged in a double
layer with the tails from both layers “inside” (facing toward each other) and the heads facing “out” (toward the watery environment) on both surfaces.
In phosphoglycerides one of the primary hydroxyl groups of glycerol is esterified to phosphoric acid; the other hydroxyl groups are esterified to fatty acids. The parent compound of the series is thus the phosphoric ester of glycerol.
Because phosphoglycerides possess a polar head in addition to their nonpolar hydrocarbon tails, they are called amphipathic or polar lipids. The different types of phosphoglycerides differ in the size, shape, and electric charge of their polar head groups.
The parent compound of the phosphoglycerides is phosphatidic acid, which contains no polar alcohol head group. It occurs in only very small amounts in cells, but it is an important intermediate in the biosynthesis of the phosphoglycerides.
The most abundant phosphoglycerides in higher plants and animals are phosphatidylethanoamme and phosphatidylchohne, which contain as head groups the amino alcohols ethanoiamine and choline, respectively. (The new names recommended for these phosphoglycerides are ethanolamine phosphoglyceride and choline phosphoglyceride, but they have not yet gained wide use. The old trivial names are cephalin and lecithin, respectively.) These two phosphoglycerides are major components of most animal cell membranes.
In phosphqtidylserine, the hydroxyl group of the amino acid L-serine is esterified to the phosphoric acid.
Closely related to phosphatidylglycerol is the more complex lipid cardiolipin, also called diphosphatidylglycerol, which consists of a molecule of phosphatidylglycerol in which the 3'-hydroxyl group of the second glycerol moiety is esterified to the phosphate group of a molecule of phosphatidic acid. The backbone of cardiolipin thus consists of three molecules of glycerol joined by two phosphodiester bridges; the two hydroxyl groups of both external glycerol molecules are esterified with fatty acids. Cardiolipin is present in large amounts in the inner membrane of mitochondria; it was first isolated from heart muscle, in which mitochondria are abundant.
Lipid Soluble Vitamins
The essential dietary
substances called vitamins are commonly
classified as "water soluble" or "fat soluble". Water
soluble vitamins, such as vitamin C, are rapidly eliminated from the body and
their dietary levels need to be relatively high. The recommended daily
allotment (RDA) of vitamin C is 100 mg, and amounts as large as 2 to
Vitamin A 800 μg ( upper
limit ca. 3000 μg)
Vitamin D 5 to 10 μg ( upper limit ca. 2000 μg)
Vitamin E 15 mg ( upper limit ca.
Vitamin K 110 μg ( upper limit not specified)
From this data it is clear that vitamins A and D, while essential to good health in proper amounts, can be very toxic. Vitamin D, for example, is used as a rat poison, and in equal weight is more than 100 times as poisonous as sodium cyanide. From the structures shown here, it should be clear that these compounds have more than a solubility connection with lipids. Vitamins A is a terpene, and vitamins E and K have long terpene chains attached to an aromatic moiety. The structure of vitamin D can be described as a steroid in which ring B is cut open and the remaining three rings remain unchanged. The precursors of vitamins A and D have been identified as the tetraterpene beta-carotene and the steroid ergosterol, respectively.
Phosphoglycerides have variations in the size, shape, polarity, and electric charge and it plays a significant role in the structure of various types of cell membranes.
Phosphoglycerides can be hydrolyzed by specific phospholipases, which have become important tools in the determination of phosphoglyceride structure. Phospholipase A1 specifically removes the fatty acid from the 1 position and phospholipase A2 from the 2 position. Removal of one fatty acid molecule from a phosphoglyceride yields a lysophosphoglyceride, e.g., lysophosphatidyl-ethanolamine. Lysophosphoglycerides are intermediates in phosphoglyceride metabolism but are found in cells or tissues in only very small amounts; in high concentrations they are toxic and injurious to membranes. Phospholipase B can bring about successive removal of the two fatty acids of phosphoglycerides. Phospholipase C hydrolyzes the bond between phosphoric acid and glycerol, while phospholipase D removes the polar head group to leave a phosphatidic acid.
Sphingolipids are composed of a backbone of sphingosine which is derived itself from glycerol. Sphingosine is N-acetylated by a variety of fatty acids generating a family of molecules referred to as ceramides. Sphingolipids predominate in the myelin sheath of nerve fibers. Sphingomyelin is an abundant sphingolipid generated by transfer of the phosphocholine moiety of phosphatidylcholine to a ceramide, thus sphingomyelin is a unique form of a phospholipid.
The other major class of sphingolipids (besides the sphingomyelins) are the glycosphingolipids generated by substitution of carbohydrates to the sn1 carbon of the glycerol backbone of a ceramide. There are 4 major classes of glycosphingolipids:
n Cerebrosides: contain a single moiety, principally galactose.
n Sulfatides: sulfuric acid esters of galactocerebrosides.
n Globosides: contain 2 or more sugars.
n Gangliosides: similar to globosides except also contain sialic acid.
Glycosyldiqcylglycerols contain a sugar in glycosidic linkage with the unesterified 3-hydroxyl group of diacylglycerols. A common example is galactosyldiacylglycerol, found in higher plants and also in neural tissue of vertebrates.
This class of glycolipids contains one or more neutral sugar residues as their polar head groups and thus has no electric charge; they are called neutral glycosphingolipids. The simplest of these are the cerebrosides, which contain as their polar head group a monosaccharide bound in beta-glycosidic linkage to the hydroxyl group of ceramide. The cerebrosides of the brain and nervous system contain D-galactose and are therefore called galactocerebrosides. Cerebrosides are also present in much smaller amounts in nonneural tissues of animals, where, because they usually contain D-glucose instead of D-galactose, they are called glucocerebrosides.
Sulfate esters of galactocerebrosides (at the 3 position of the D-galactose) are also present in brain tissue; they are called sulfotides.
The neutral glycosphingolipids are important cell-surface components in animal tissues. Their nonpolar tails presumably penetrate into the lipid bilayer structure of cell membranes, whereas the polar heads protrude outward from the surface. Some of the neutral glycosphingolipids are found on the surface of red blood cells and give them blood-group specificity.
Acidic glycosphingolipids (gangliosides)
Gangliosides contain in their oligosaccharide head groups one or more residues of a sialic acid, which gives the polar head of the gangliosides a net negative charge at pH 7.0. The sialic acid usually found in human gangliosides is N-acetylneuraminic acid. Gangliosides are most abundant in the gray matter of the brain, where they constitute 6 percent of the total lipids, but small amounts are also found in nonneural tissues.
Function of glycosphingolipids
Although glycosphingolipids are only minor constituents of membranes, they appear to be extremely important in a number of specialized functions. Because gangliosides are especially abundant in nerve endings, it has been suggested that they function in the transmission of nerve impulses across synapses. They are also believed to be present at receptor sites for acetylcholine and other neurotransmitter substances. Some of the cell-surface glycosphingolipids are concerned not only in blood-group specificity but also in organ and tissue specificity. These complex lipids are also involved in tissue immunity and in cell-cell recognition sites fundamental to the development and structure of tissues. Cancer cells, for example, have characteristic glycosphingolipids different from those in normal cells.
The lipids discussed up to this point contain fatty acids as building blocks, which can be released on alkaline hydrolysis. The simple lipids contain no fatty acids. They occur in smaller amounts in cells and tissues than the complex lipids, but they include many substances having profound biological activity—vitamins, hormones, and other highly specialized fat-soluble biomolecules.
Prostaglandins are a family of fatty acid derivatives which have a variety of potent biological activities of a hormonal or regulatory nature. Prostaglandins function as regulators of metabolism in a number of tissues and in a number of ways.
All the natural prostaglandins are biologically derived by cyclization of 20-carbon unsaturated fatty acids, such as arachidonic acid, which is formed from the essential fatty acid linoleic acid. The prostaglandins differ from each other with respect to their biological activity, although all show at least some activity in lowering blood pressure and inducing smooth muscle to contract. Some, like PGE2, antagonize the action of certain hormones. PGE2 and PGE2a may find clinical use in inducing labor and bringing about therapeutic abortion.
Digestion of fats
By far the most common of the diet are the neutral fats, also known as triglycerides, each molecule of which is composed of a glycerol nucleus and three fatty acids, as illustrated. Neutral fat is found in food of both animal and and plant origin. In the usual diet are also small quantities of phospholipids, cholesterol, and cholesterol esters.
Digestion of fats in the intestine. A small amount of short chain triglycerides is digested in the stomach by gastric lipase.
Emulsification of fat by bile acids. The first in fat digestion is to break the fat globules into s sizes so that the water-soluble digestive enzymes act on the globule surfaces. This process is called emulsification of the
fat, and it is achieved under
the presence of bile acids. Bile contain a large quantity of bile salts, mainly in the form of ionized sodium salts.
The carboxyl and other parts of the bile salt molecule are highly soluble in water, whereas most of the sterol portion of the bile is highly soluble in fat. Therefore, the fat-soluble portion of the bile salt dissolves in the surface layer of the fat globule and polar portion of the bile salt is soluble in the surrounding fluids. This effect decreases the interfacial tension of the fat. When the interfacial tension of a globule is low, globule is broken up into many minute particles. The total surface area of the particles in the intestinal contents is inversely proportional to the diameters of the particles. The lipases are water-soluble compounds and can act on the fat globules only on their surfaces. Consequently, it can be readily understood how important detergent function of bile salts is for the digestion of fats.
Digestion of fats by pancreatic lipase. The most important enzyme for the digestion of fats is pancreatic lipase in the pancreatic juice. However, the cells of the small intestine also contain a minute quantity of lipase known as enteric lipase. Both liiese act alike to cause hydrolysis of fat.
Products of fat digestion. Most of the triglycerides of the diet are split into free fatty acids and monoglycerides.
Role of bile salts in accelerating fat digestion — formation of micelles. The hydrolysis of triglycerides highly reversible process; therefore, accumulation of monoglycerides and free fatty acids very quickly blocks further digestion. The bile salts play an important role in removing the monoglycerides and free fatty acids from the vicinity of the digesting fat globules almost as rapidly as these end-products of digestion are formed. This occurs in the following way: bile salts have the propensity to form micelles, which are small spherical globules composed of 20 to 40 molecules of bile salt. These develop because each bile salt molecule is composed of a sterol nucleus, most of which is highly fat-soluble, and a polar group that is highly water-soluble. The sterol nuclei of the 20 to 40 bile salt molecules of the micelle aggregate together to form a small fat globule in the middle of the micelle. This aggregation causes the polar groups to project outward to cover the surface of the micelle During triglyceride digestion, as rapidly as the monoglycerides and free fatty acids are formed they become dissolved in the fatty portion of the micelles, which immediately reduces these end-products of digestion in the vicinity of the digesting fat globules. The bile salt micelles also act as a transport medium to carry the monoglycerides and the free fatty acids, both of which would otherwise be relatively insoluble, to the brush borders of the epithelial cells. There the monoglycerides and free fatty acids are absorbed. On delivery of these substances to the brush border, the bile salts are again released
back into the chyme to be used again and again for this "ferrying" process.
Digestion of Cholesterol Esters and Phospholipids. Most of the cholesterol in the diet is in the form of cholesterol esters, which are combinations of free cholesterol and one molecule of fatty acid. And phospholipids also contain fatty acid chains within their molecules. Both the cholesterol esters and the phospholipids are hydrolyzed by lipases in the pancreatic secretion that free the fatty acids — the enzyme cholesterol ester hydrolase to hydrolyze the cholesterol ester and phospholipase A to hydrolyze the phospholipid.
The bile salt micelles play identically the same role in "ferrying" free cholesterol as they play in "ferrying" monoglycerides and free fatty acids. Indeed, this role of the bile salt micelles is absolutely essential to the absorption of cholesterol because essentially no cholesterol is absorbed without the presence of bile salts. On the other hand, as much as 60 per cent of the triglycerides can be digested and absorbed even in the absence of bile salts.
Absorption of fats
Monoglycerides and fatty acids - both of digestive end-products - become dissolved in the lipid portion of the micelles. Because of the molecular dimension of these micelles, only 2.5 nanometers, and also because of their highly charged, they are soluble in the chyme. Micelles contact with the surfaces of the brush border even penetrating into the recesses , agitating microvilli.
The micelles then diffuse back through the chyme and absorb still more monoglycerides and fatty acids, and similarly transport these also to the epithelial cells. Thus, the bile acids perform a "ferrying" function, which is highly important for fat absorption. In the presence of an abundance of bile acids, approximately 97 per cent of the fat is absorbed; in the absence of bile acids, only 50 to 60 per cent is normally absorbed.
The mechanism for absorption of the monoglycerides and fatty acids through the brush border is based entirely on the fact that both these substances are highly lipid-soluble. Therefore, they become dissolved in the membrane and simply diffuse to the interior of the cell. The undigested triglycerides and the diglycerides are both also highly soluble in the lipid membrane of the epithelial cell. However, only small quantities of these are normally absorbed because the bile acid micelles will not dissolve either triglycerides or diglycerides and therefore will not ferry them to the epithelial membrane.
After entering the epithelial cell, the fatty acids and monoglycerides are taken up by the smooth endoplasmic reticulum, and here they are mainly recombined to form new triglycerides. However, a few of the monoglycerides are further digested into glycerol and fatty acids by an epithelial cell lipase. Then, the free fatty acids are reconstituted by the smooth endoplasmic reticulum into triglycerides. Most of the glycerol that is utilized for this purpose is synthesized de novo from alpha-glycerophosphate, this synthesis requiring both energy from ATP and a complex of enzymes to catalyze the reactions. Once formed, the triglycerides aggregate within the endoplasmic reticulum into globules along with absorbed cholesterol, absorbed phospholipids, and small amounts of newly synthesized cholesterol and phospholipids. The phospholipids arrange themselves in these globules with the fatty portion of the phospholipid toward the center and the polar portions located on the surface. This provides an electrically charged surface that makes these globules miscible with the fluids of the cell. In addition, small amounts of lipoprotein, also synthesized by the endoplasmic reticulum, coat part of the surface of each globule. In this form the globule diffuses to the side of the epithelial cell and is excreted by the process of cellular exocytosis into the space between the cells; from there it passes into the lymph in the central lacteal of the villus. These globules are then called chylomicrons.
Transport of the Chylomicrons in the Lymph. From the sides of the epithelial cells the chylomicrons wend their way into the central lac-teals of the villi and from here are propelled, along with the lymph, by the lymphatic pump upward through the thoracic duct to be emptied into the great veins of the neck. Between 80 and 90 per cent of all fat absorbed from the gut is absorbed in this manner and is transported to the blood by way of the thoracic lymph in the form of chylomicrons.
Direct Absorption of fatty acids into the portal blood. Small quantities of short chain fatty acids, such as those from butterfat, are absorbed directly into the portal blood rather than being converted into triglycerides and absorbed into the lymphatics. The cause of this difference between short and long chain fatty acid absorption is that the shorter chain fatty acids are more water-soluble and are not reconverted into triglycerides by the endoplasmic reticulum. This allows direct diffusion of these fatty acids from the epithelial cells into the capillary blood of the dlood.
Catabolism of triacylglycerols
Dietary acylglycerols undergo hydrolysis in the small intestine by the action of lipases, e.g., those present in pancreatic juice. Lipase digests the triacylglycerols to 2-monoglycerols, glycerol and free fatty acids. These components are absorbed and metabolized in the enterocytes, blood and liver. In the enterocytes and liver the specific for organism acylglycerols are synthesized. Then these are accumulated in adipose tissue and in much smaller quantity in other organs.
Fermentative hydrolysis of in adipocytes and other cells is implemented in several stages. Diacylglycerols, monoacylglycerols, glycerol and free fatty acids are formed in this process:
Fatty acids play an extremely important part as an energy-rich fuel in higher animals and plants since large amounts can be stored in cells in the form of triacylglycerols. Triacylglycerols are especially well adapted for this role because they have a high energy content (about 9 kcal/g) and can be accumulated in nearly anhydrous form as intracellular fat droplets. In contrast, glycogen and starch can yield only about 4 kcal/g; moreover, since they are highly hydrated, they cannot be stored in such concentrated form. Fatty acids provide up to 40 percent of the total fuel requirement in man on a normal diet.
Mammalian tissues normally contain only vanishingly small amounts of free fatty acids, which are in fact somewhat toxic. By the action of hormonally controlled lipases free fatty acids are formed from triacylglycerols in fat or adipose tissue. The free fatty acids are then released from the tissue, become tightly bound to serum albumin, and in this form are carried via the blood to other tissues for oxidation. Fatty acids delivered in this manner are first enzymatically "activated" in the cytoplasm and then enter the mitochondria for oxidation.
Long-chain fatty acids are oxidized to CO2 and H2O in nearly all tissues of vertebrates except the brain. Some tissues, such as heart muscle, obtain most of their energy from the oxidation of fatty acids. The mobilization, distribution, and oxidation of fatty acids are integrated with the utilization of carbohydrate fuels; both are under complex endocrine regulation.
The pathway of fatty acid oxidation.
Knoop postulated that fatty acids are oxidized by b-oxidation, i.e., oxidation at the b carbon to yield a b-keto acid, which was assumed to undergo cleavage to form acetic acid and a fatty acid shorter by two carbon atoms.
Outline of the fatty acid oxidation cycle.
Before oxidation, long-chain fatty acids from the cytosol must undergo a rather complex enzymatic activation, followed by transport across the mitochondrial membranes into the major compartment. There the fatty acyl group is transferred to intramitochondrial coenzyme A, yielding a fatty acyl-CoA thioester. The subsequent oxidation of the fatty acyl-CoA takes place entirely in the mitochondrial matrix. The fatty acyl-CoA is dehydrogenated by removal of a pair of hydrogen atoms from the a and b carbon atoms (atoms 2 and 3) to yield the a,b-unsaturated acyl-CoA. This is then enzymatically hydrated to form a b-hydroxyacyl-CoA, which in turn is dehydrogenated in the next step to yield the b-ketoacyl-CoA. It then undergoes enzymatic cleavage by reaction with a second molecule of CoA. One product is acetyl-CoA, derived from carbon atoms 1 and 2 of the original fatty acid chain. The other product, a long-chain saturated fatty acyl-CoA having two fewer carbon atoms than the original fatty acid, now becomes the substrate for another round of reactions, beginning at the first dehydrogenation step and ending with the removal of a second two-carbon fragment as acetyl-CoA. At each passage through this spiral the fatty acid chain loses a two-carbon fragment as acetyl-CoA. The 16-carbon palmitic acid thus undergoes a total of seven such cycles, to yield altogether 8 molecules of acetyl-CoA and 14 pairs of hydrogen atoms. The palmitate must be primed or activated only once, since at the end of each round the shortened fatty acid appears as its CoA thioester.
The hydrogen atoms removed during the dehydrogenation of the fatty acid enter the respiratory chain; as electrons pass to molecular oxygen via the cytochrome system, oxidative phosphorylation of ADP to ATP occurs. The acetyl-CoA formed as product of the fatty acid oxidation system enters the tricarboxylic acid cycle.
Activation and entry of fatty acids into mitochondria.
There are three stages in the entry of fatty acids into mitochondria from the extramitochondrial cytoplasm: (1) the enzymatic ATP-driven esterification of the free fatty acid with extramitochondrial CoA to yield fatty acyl-CoA, a step often referred to as the activation of the fatty acid, (2) the transfer of the acyl group from the fatty acyl-CoA to the carrier molecule carnitine, followed by the transport of the acyl carnitine across the inner membrane, and (3) the transfer of the acyl group from fatty acyl carnitine to intramitochondrial CoA.
Activation of fatty acids.
At least three different enzymes catalyze formation of acyl-CoA thioesters, each being specific for a given range of fatty acid chain length. These enzymes are called acyl-CoA synthetases. Acetyl-CoA synthetase activates acetic, propionic, and acrylic acids, medium-chain acyl-CoA synthetase activates fatty acids with 4 to 12 carbon atoms, and long-chain acyI-CoA synthetase activates fatty acids with 12 to 22 or more carbon atoms. The last two enzymes activate both saturated and unsaturated fatty acids. Otherwise the properties and mechanisms of all three synthetases, which have been isolated in highly purified form, are nearly identical. The overall reaction catalyzed by the ATP-linked acyl-CoA synthetases is:
RCOOH + ATP + CoA–SH Û RCO—S—CoA + AMP + PP
Fatty acids acyl-CoA
In this reaction a thioester linkage is formed between the fatty acid carboxyl group and the thiol group of CoA; the ATP undergoes pyrophosphate cleavage to yield AMP and inorganic pyrophosphate.
The acyl-CoA synthetases are found in the outer mitochondrial membrane and in the endoplasmic reticulum.
Transfer to carnitine.
Long-chain saturated fatty acids have only a limited ability to cross the inner membrane as CoA thioesters, but their entry is greatly stimulated by carnitine.
The stimulation of fatty acid oxidation by carnitine is due to the action of an enzyme carnitine acyltransferase, which catalyzes transfer of the fatty acyl group from its thioester linkage with CoA to an oxygen-ester linkage with the hydroxyl group of carnitine. The acyl carnitine ester so formed then passes through the inner membrane into the matrix, presumably via a specific transport system.
Transfer to intramitochondrial CoA.
In the last stage of the entry process the acyl group is transferred from carnitine to intramitochondrial CoA by the action of a second type of carnitine acyltransferase located on the inner surface of the inner membrane:
Acyl carnitine + CoA Û acyl-CoA + carnitine
This complex entry mechanism, often called the fatty acid shuttle, has the effect of keeping the extramitochondrial and intramitochondrial pools of CoA and of fatty acids separated. The intramitochondrial fatty acyl-CoA now becomes the substrate of the fatty acid oxidation system, which is situated in the inner matrix compartment.
The first dehydrogenation step in fatty acid oxidation.
Following the formation of intramitochondrial acyl-CoA, all subsequent reactions of the fatty acid oxidation cycle take place in the inner compartment. In the first step the fatty acyl-CoA thioester undergoes enzymatic dehydrogenation by acyl-CoA dehydrogenase at the a and b carbon atoms (carbons 2 and 3) to form enoyl-CoA as product. The double bond formed in this reaction has the trans geometrical configuration. Recall, however, that the double bonds of the unsaturated fatty acids of natural fats nearly always have the cis configuration.
There are four different acyl-CoA dehydrogenases, each specific for a given range of fatty acid chain lengths. All contain tightly bound flavin adenine dinucleotide (FAD) as prosthetic groups. The FAD becomes reduced at the expense of the substrate, a process that probably occurs through distinct one-electron steps.
The FADH2 of the reduced acyl-CoA dehydrogenase cannot react directly with oxygen but donates its electrons to the respiratory chain via a second flavoprotein, electron-transferring flavoprotein, which in turn passes the electrons to some carrier of the respiratory chain.
The double bond of the enoyl-CoA ester is then hydrated to form 3-hydroxyacyl-CoA by the enzyme enoyl-CoA hydratase.
The addition of water across the trans double bond is stereo-specific and results in the formation of the L-stereoisomer of the 3-hydroxyacyl-CoA.
The second dehydrogenation step.
In the next step of the fatty acid oxidation cycle, the 3-hydroxyacyl-CoA is dehydrogenated to form 3-ketoacyl-CoA) by 3-hydroxyacyl-CoA dehydrogenase. NAD+ is the specific electron acceptor. The reaction is:
This enzyme is relatively nonspecific with respect to the length of the fatty acid chain but is absolutely specific for the l stereoisomer. The NADH formed in the reaction donates its electron equivalents to the NADH dehydrogenase of the mitochondrial respiratory chain.
The cleavage step.
In the last step of the fatty acid oxidation cycle, which is catalyzed by acetyl-CoA acetyltransferase, more commonly known as thiolase, the 3-ketoacyl-CoA undergoes cleavage by interaction with a molecule of free CoA to yield the carboxyl-terminal two-carbon fragment of the fatty acid as acetyl-CoA. The remaining fatty acid, now shorter by two carbon atoms, appears as its coenzyme A thioester.
This cleavage reaction, also called a thiolysis or a thiolytic cleavage, is analogous to hydrolysis. Since the reaction is highly exergonic, cleavage is favored. There appear to be two (perhaps three) forms of the enzyme, each specific for different fatty acid chain lengths.
The balance sheet.
We have described one turn of the fatty acid oxidation cycle, in which one molecule of acetyl-CoA and two pairs of hydrogen atoms have been removed from the starting long-chain fatty acyl-CoA. The overall equation for one turn of the cycle, starting from palmitoyl-CoA, is
Palmitoyl-CoA + CoA + FAD+ + NAD+ + H2O ®
myristoyl-CoA + acetyl-CoA + FADH2 + NADH2
We can now write the equation for the seven turns of the cycle required to convert one molecule of palmitoyl-CoA into eight molecules of acetyl-CoA:
Palmitoyl-CoA + 7CoA + 7FAD+ + 7NAD+ + 7H2O ®
8 acetyl-CoA + 7FADH2 + 7NADH2 + 7H+
Each molecule of FADH2 donates a pair of electron equivalents to the respiratory chain at the level of coenzyme Q; thus two molecules of ATP are generated during the ensuing electron transport to oxygen. Similarly, oxidation of each molecule of NADH2 by the respiratory chain results in formation of three molecules of ATP. Hence, a total of five molecules of ATP is formed by oxidative phosphorylation per molecule of acetyl-CoA cleaved.
The seven turns of the cycle required to convert one molecule of palmitoyl-CoA rsults in the formation of 5 x 7 = 35 ATP.
The eight molecules of acetyl-CoA formed in the fatty acid cycle may now enter the tricarboxylic acid cycle. The degradation of 1 molecule of acetyl-CoA in tricarboxylic acid cycle results in the formation of 12 molecules of ATP. 8 molecules of acetyl-CoA give 96 molecules of ATP.
Thus, the total output of energy in full cleavage of 1 molecule of palmitoyl-CoA is: 35 + 96 = 131 molecules of ATP.
Since one molecule of ATP is in effect utilized to form palmitoyl-CoA from palmitate, the net yield of ATP per molecule of palmitate is 130.
Oxidation of unsaturated fatty acids.
Unsaturated fatty acids, such as oleic acid, are oxidized by the same general pathway as saturated fatty acids, but two special problems arise. The double bonds of naturally occurring unsaturated fatty acids are in the cis configuration, whereas the unsaturated acyl-CoA intermediates in the oxidation of saturated fatty acids are trans, as we have seen. Moreover, the double bonds of most unsaturated fatty acids occur at such positions in the carbon chain that successive removal of two-carbon fragments from the carboxyl end yields a D3-unsaturated fatty acyl-CoA rather than the D2 fatty acyl-CoA serving as the normal intermediate in the fatty acid cycle.
These problems have been resolved with the discovery of an auxiliary enzyme, enoyl-CoA isomerase, which catalyzes a reversible shift of the double bond from the D3-cis to the D2-trans configuration. The resulting D2-trans-unsaturated fatty acyl-CoA is the normal substrate for the next enzyme of the fatty acid oxidation sequence, enoyl-CoA hydratase, which hydrates it to form L-3-hydroxyacyl-CoA. The complete oxidation of oleyl-CoA to nine acetyl-CoA units by the fatty acid oxidation cycle thus requires an extra enzymatic step catalyzed by the enoyl-CoA isomerase, in addition to those steps required in the oxidation of saturated fatty acids.
Polyunsaturated fatty acids, such as linoleic acid, require a second auxiliary enzyme to complete their oxidation, since they contain two or more cis double bonds. When three successive acetyl-CoA units are removed from linoleyl-CoA, a D3-cis double bond remains, as in the case of oleyl-CoA. This is then transformed by the enoyl-CoA isomerase described above to the D2-trans isomer. This undergoes the usual reactions, with loss of two acetyl-CoA's, leaving an eight-carbon D2-unsaturated acid. Note, however that the double bond of the latter is in the cis configuration. Although the D2-cis double bond can be hydrated by enoyl-CoA hydratase, the product is the D stereoisomer of a 3-hydroxyacyl-CoA, not the L stereoisomer normally formed during oxidation of saturated fatty acids. Utilization of the d stereoisomer requires a second auxiliary enzyme, 3-hydroxyacyl-CoA epimerase, which catalyzes epi-merization at carbon atom 3 to yield the l isomer. The product of this reversible reaction is then oxidized by the L-specific 3-hydroxyacyl-CoA dehydrogenase and cleaved by thiolase to complete the oxidation cycle. The remaining six-carbon saturated fatty acyl-CoA derived from linoleic acid can now be oxidized to three molecules of acetyl-CoA. These two auxiliary enzymes of the fatty acid oxidation cycle make possible the complete oxidation of all the common unsaturated fatty acids found in naturally occurring lipids. The number of ATP molecules yielded during the complete oxidation of an unsaturated fatty acid is somewhat lower than for the corresponding saturated fatty acid since unsaturated fatty acids have fewer hydrogen atoms and thus fewer electrons to be transferred via the respiratory chain to oxygen.
Oxidation of odd-carbon fatty acids and the fate of propionyl-CoA
Odd-carbon fatty acids, which are rare but do occur in some marine organisms, can also be oxidized in the fatty acid oxidation cycle. Successive acetyl-CoA residues are removed until the terminal three-carbon residue pro-pionyl-CoA is reached. This compound is also formed in the oxidative degradation of the amino acids valine and isoleucine. Propionyl-CoA undergoes enzymatic carboxylation in an ATP-dependent process to form Ds-methylmaionyl-CoA, a reaction catalyzed by propionyl-CoA corboxylase. This enzyme contains biotin as its prosthetic group. In the next step Ds-methylmalonyl-CoA undergoes enzymatic epimerization to LR-methylmalonyl-CoA, by action of methyimaionyl-CoA racemase. In the next reaction step, catalyzed by methylmalonyl-CoA mutase, LR-methylmalonyl-CoA is isomerized to succinyl-CoA, which may then undergo deacylation by reversal of the succinyl-CoA synthetase reaction to yield free succinate, an intermediate of the tricarboxylic acid cycle.
Methylmalonyl-CoA mutase requires as cofactor coenzyme B12. Study of this intramolecular reaction with isotope tracers has revealed that it takes place by the migration of the entire —CO—S—CoA group from carbon atom 2 of methylmalonyl-CoA to the methyl carbon atom in exchange for a hydrogen atom.
Patients suffering from pernicious anemia, who are deficient in vitamin B12 because of their lack of intrinsic factor, excrete large amounts of methylmalonic acid and its precursor propionic acid in the urine, showing that in such patients the coenzyme B12-dependent methylmalonyl-CoA mutase reaction is defective.
Glycerol formed in cleavage of tryacylglycerols enter catabolism or use for biosynthesis of glycerides again. Before including of glycerol in metabolism it is activated by ATP to glycerol-3-phosphate by action of glycerol phosphokinase:
Glycerol-3-phosphate is oxidized by glycerophosphate dehydrogenase and glyceroaldehyde-3-phosphate is produced:
Glyceroaldehyde-3-phosphate is the central metabolite of glycolysis.
The biosynthesis of lipids is a prominent metabolic process in most organisms. Because of the limited capacity of higher animals to store polysaccharides, glucose ingested in excess of immediate energy needs and storage capacity is converted by glycolysis into pyruvate and then acetyl-CoA, from which fatty acids are synthesized. These in turn are converted into triacylglycerols, which have a much higher energy content than polysaccharides and may be stored in very large amounts in adipose or fat tissues. Triacylglycerols are also stored in the seeds and fruits of many plants.
The formation of the various phospholipids and sphingolipids of cell membranes is also an important biosynthetic process. These complex lipids undergo continuous metabolic turnover in most cells.
Biosynthesis of saturated fatty acids
The biosynthesis of saturated fatty acids from their ultimate precursor acetyl-CoA occurs in all organisms but is particularly prominent in the liver, adipose tissues, and mammary glands of higher animals. It is brought about by a process that differs significantly from the opposed process of fatty acid oxidation. In the first place total biosynthesis of fatty acids occurs in the cytosol, whereas fatty acid oxidation occurs in the mitochondria. Second, the presence of citrate is necessary for maximal rates of synthesis of fatty acids, whereas it is not required in fatty acid oxidation. Perhaps the most unexpected difference is that CO2 is essential for fatty acid synthesis in cell extracts, although isotopic CO2 is not itself incorporated into the newly synthesized fatty acids. These and many other observations have revealed that fatty acid synthesis from acetyl-CoA takes place with an entirely different set of enzymes from those employed in fatty acid oxidation.
In the overall reaction of fatty acid synthesis, which is catalyzed by a complex multienzyme system in the cytosol, the fatty-acid synthetase complex, acetyl-CoA derived from carbohydrate or amino acid sources is the ultimate precursor of all the carbon atoms of the fatty acid chain. However, of the eight acetyl units required for biosynthesis of palmitic acid, only one is provided by acetyl-CoA; the other seven arrive in the form of malonyl-CoA, formed from acetyl-CoA and HCO3- in a carboxylation reaction. One acetyl residue and seven malonyl residues undergo successive condensation steps, with release of seven molecules of CO2, to form palmitic acid; the reducing power is furnished by NADPH:
Acetyl-CoA + 7 malonyl-CoA + 14NADPH + 14H+ ®
CH3(CH2)14COOH + 7CO2 + 8CoA + 14NADP+ + 6H2O
The single molecule of acetyl-CoA required in the process serves as a primer, or starter; the two carbon atoms of its acetyl group become the two terminal carbon atoms (15 and 16) of the palmitic acid formed. Chain growth during fatty acid synthesis thus starts at the carboxyl group of acetyl-CoA and proceeds by successive addition of acetyl residues at the carboxyl end of the growing chain. Each successive acetyl residue is derived from two of the three carbon atoms of a malonic acid residue entering the system in the form of malonyl-CoA; the third carbon atom of malonic acid, i.e., that of the unesterified carboxyl group, is lost as CO2. The final product is a molecule of palmitic acid.
A distinctive feature of the mechanism of fatty acid biosynthesis is that the acyl intermediates in the process of chain lengthening are thio esters, not of CoA, as in fatty acid oxidation, but of a low-molecular-weight conjugated protein called acyl carrier protein (ACP). This protein can form a complex or complexes with the six other enzyme proteins required for the complete synthesis of palmitic acid. In most eukariotic cells all seven proteins of the fatty acid synthetase complex are associated in a multienzymes cluster.
In most organisms the end product of the fatty-acid synthetase system is palmitic acid, the precursor of all other higher saturated fatty acids and of all unsaturated fatty acids.
The carbon source for fatty acid synthesis
The ultimate source of all the carbon atoms of fatty acids is acetyl-CoA, formed in the mitochondria by the oxidative decarboxylation of pyruvate, the oxidative degradation of some of the amino acids, or by the b-oxidation of long-chain fatty acids.
Acetyl-CoA itself cannot pass out of the mitochondria into the cytosol; however, its acetyl group is transferred through the membrane in other chemical forms. Citrate, formed in mitochondria from acetyl-CoA and oxaloacetate, may pass through the mitochondrial membrane to the cytoplasm via the tricarboxylate transport system. In the cytosol acetyl-CoA is regenerated from citrate by ATP-citrate lyase, also called citrate cleavage enzyme, which catalyzes the reaction:
In a second pathway the acetyl group of acetyl-CoA is enzymatically transferred to carnitine, which acts as a carrier of fatty acids into mitochondria preparatory to their oxidation. Acetylcarnitine passes from the mitochondrial matrix through the mitochondrial membrane into the cytosol; acetyl-CoA is then regenerated by transfer of the acetyl group from acetylcarnitine to cytosol CoA.
Before the acetyl groups of acetyl-CoA can be utilized by the fatty-acid synthetase complex, an important preparatory reaction must take place to convert acetyl-CoA into malonyl-CoA, the immediate precursor of 14 of the 16 carbon atoms of palmitic acid. Malonyl-CoA is formed from acetyl-CoA and bicarbonate in the cytosol by the action of acetyl-CoA carboxylase, a complex enzyme that catalyzes the reaction:
The carbon atom of the CO2 becomes the distal or free carboxyl carbon of malonyl-CoA. However, the above equation give only the overall reaction, the sum of at least three intermediate reactions.
Acetyl-CoA carboxylase contains biotin as its prosthetic group. The carboxyl group of biotin is bound in amide linkage to the e-amino group of a specific lysine residue of a subunit of the enzyme. The covalently bound biotin serves as an intermediate carrier of a molecule of CO2.
The reaction catalyzed by acetyl-CoA carboxylase, an allosteric enzyme, is the primary regulatory, or rate-limiting, step in the biosynthesis of fatty acids. Acetyl-CoA carboxylase is virtually inactive in the absence of its positive modulators citrate or isocitrate. The striking allosteric stimulation of this enzyme by citrate accounts for the fact that citrate is required for fatty acid synthesis in cell extracts without being used as a precursor.
Acetyl-CoA carboxylase occurs in both an inactive monomeric form and an active polymeric form. As it occurs in the avian liver, the inactive enzyme monomer has a molecular weight of 410,000 and contains one binding site for CO2 (that is, one biotin prosthetic group), one binding site for acetyl-CoA, and one for citrate. Citrate shifts the equilibrium between the inactive monomer and the active polymer, to favor the latter.
Polymeric acetyl-CoA carboxylase of animal tissues consists of long filaments of enzyme monomers; each monomer unit contains a molecule of bound citrate. The length of the polymeric form varies, but on the average each filament contains about 20 monomer units, has a particle weight of some 8 megadaltons, and is about 400 nm long. Such filaments have been studied in the electron microscope and have actually been observed in the cytoplasm of adipose cells.
The acetyl-CoA carboxylase reaction is complex. In fact, the monomeric unit of the enzyme contains four different subunits. The sequence of reactions in the formation of malonyl-CoA has been deduced from study of the four subunits of the monomer. One of these subunits, biotin carboxylase (BC), catalyzes the first step of the overall reaction, namely, the carboxylation of the biotin residue covalently bound to the second subunit, which is called biotin carboxyl-carrier protein (BCCP). The second step in the overall reaction is catalyzed by the third type of subunit, called carboxyl transferase (CT). In these reactions the biotin residue of the carboxyl carrier protein serves as a swinging arm to transfer the bicarbonate ion from the biotin carboxylase subunit to the acetyl-CoA bound to the active site of the carboxyltransferase subunit. The change from the inactive monomeric form of acetyl-CoA carboxylase to the polymeric, active form of the enzyme occurs when citrate is bound to the fourth subunit of each monomeric unit.
Acyl carrier protein (ACP)
Acyl carrier protein, universally symbolized as ACP, was first isolated in pure form from E. coli and has since been studied from many other sources. The E. coli ACP is a relatively small (mol wt 10,000), heat-stable protein containing 77 amino acid residues, whose sequence has been established, and a covalently attached prosthetic group.
The single sulfhydryl group of ACP, to which the acyl intermediates are esterified, is contributed by its prosthetic group, a molecule of 4'-phosphopantetheine, which is covalently linked to the hydroxyl group of serine residue 36 of the protein. The 4'-phosphopantetheme moiety is identical with that of coenzyme A, from which it is derived. The function of ACP in fatty acid synthesis is analogous to that of CoA in fatty acid oxidation: it serves as an anchor to which the acyl intermediates are esterified.
The priming reaction
To prime the fatty-acid synthetase system, acetyl-CoA first reacts with the sulfhydryl group of ACP by the action of one of the six enzymes of the synthetase system, ACP-acyltransferase, which catalyzes the reaction:
The malonyl transfer step
In the next reaction, catalyzed by ACP malonyltransferase, malonyl-S-CoA formed in the acetyl-CoA carboxylase reaction reacts with the —SH group of the 4'-phosphopantetheine arm of ACP, with loss of free CoA, to form malonyl-S-ACP:
Malonyl—S—CoA + ACP—SH Û malonyl—S—ACP + CoA—SH
As a result of this step and of the preceding priming reaction, a malonyl group is now esterified to ACP and an acetyl group is esterified to an —SH group on the ACP molecule.
The condensation reaction
In the next reaction of the sequence, catalyzed by b-ketoacyl-ACP synthase, the acetyl group esterified to the cysteine residue is transferred to carbon atom 2 of the malonyl group on ACP, with release of the free carboxyl group of the malonyl residue as CO2:
Study of the reaction equilibrium has revealed the probable basis for the biological selection of malonyl-CoA as the precursor of two-carbon residues for fatty acid synthesis. If acetoacetyl-CoA were to be formed from two molecules of acetyl-CoA by the action of acetyl-CoA acetyltransferase,
Acetyl—S—CoA + acetyl—S—CoA Û acetoacetyl—S—CoA + CoA—SH
the reaction would be endergonic, with its equilibrium lying to the left.
The first reduction reaction
The acetoacetyl-S-ACP now undergoes reduction by NADPH to form b-hydroxybutyryl-S-ACP. This reaction is catalyzed by b-ketoacyl-ACP reductase:
The dehydration step
b-Hydroxybutyryl-S-ACP is next dehydrated to the corresponding unsaturated acyl-S-ACP, namely, crotonyl-S-ACP, by b-hydroxyacyl—ACP-dehydratase:
The second reduction step
Crotonyl-S-ACP is now reduced to butyryl-S-ACP by enoil-ACP reductase (NADPH); the electron donor is NADPH in animal tissues:
This reaction also differs from the corresponding reaction of fatty acid oxidation in mitochondria in that a pyridine nucleotide rather than a flavoprotein is involved. Since the NADPH-NADP+ couple has a more negative standard potential than the fatty acid oxidizing flavoprotein, NADPH favors reductive formation of the saturated fatty acid.
The formation of butyryl-ACP completes the first of seven cycles en route to palmitoyl-S-ACP. To start the next cycle the butyryl group is transferred from —SH group of phosphopantetheine to the —SH group of cysteine, thus allowing —SH group of ACP phosphopantetheine to accept a malonyl group from another molecule of malonyl-CoA.
Then the cycle repeats, the next step being the condensation of malonyl-S-ACP with butyryl-S-ACP to yield b-ketohexanoyl-S-ACP and CO2.
After seven complete cycles, palmitoyl-ACP is the end product. The palmitoyl group may be removed to yield free palmitic acid by the action of a thioesterase, or it may be transferred from ACP to CoA, or it may be incorporated directly into phosphatidic acid in the pathway to phospholipids and triacylglycerols.
It is remarkable that in most organisms the fatty-acid synthetase system stops with the production of palmitic acid and does not yield stearic acid, which has only two more carbon atoms than palmitic acid and thus does not differ greatly in physical properties.
Saturated fatty acids having an odd number of carbon atoms, which are found in many marine organisms, are also made by the fatty-acid synthetase complex. In this case the synthesis is primed by a starter molecule of propionyl-S-ACP (instead of acetyl-S-ACP), to which are added successive two-carbon units via condensations with malonyl-S-ACP.
We can now write the overall equation for palmitic acid biosynthesis starting from acetyl-S-CoA:
8 Acetyl—S—CoA + 14NADPH + 14H+ + 7ATP + H2O ®
palmitic acid + 8CoA + 14NADP+ + 7ADP + 7P.
The 14 molecules of NADPH required for the reductive steps in the synthesis of palmitic acid arise largely from the NADP-dependent oxidation of glucose 6-phosphate via the phosphogluconate pathway. Liver, mammary gland, and adipose tissue of vertebrates, which have a rather high rate of fatty acid biosynthesis, also have a very active 6-phosphogluconate pathway.
The enzymatic steps leading to the biosynthesis of palmitic acid differ from those involved in oxidation of palmitic acid in the following respects:
1. Their intracellular location.
2. The type of acyl-group carrier.
3. The form in which the two-carbon units are added or removed.
4. The pyridine nucleotide specificity of the b-ketoacyl-b-hydroxyacyl reaction.
5. The stereoisomeric configuration of the b-hydroxyacyl intermediate.
6. The electron donor-acceptor system for the crotonyl-butyryl step.
These differences illustrate how two opposing metabolic processes may proceed independently of each other in the cell.
Elongation of saturated fatty acids in mitochondria and microsomes
Palmitic acid, the normal end product of the fatty-acid synthetase system, is the precursor of the other long-chain saturated and unsaturated fatty acids in most organisms. Elongation of palmitic acid to longer-chain saturated fatty acids, of which stearic acid is most abundant, occurs by the action of two different types of enzyme systems, one in the mitochondria and the other in the endoplasmic reticulum.
In mitochondria palmitic and other saturated fatty acids are lengthened by successive additions to the carboxyl-terminal end of acetyl units in the form of acetyl-CoA; malonyl-ACP cannot replace acetyl-CoA. The mitochondrial elongation pathway occurs by reactions similar to those in fatty acid oxidation. Condensation of palmityl-CoA with acetyl-CoA yields b-ketostearyl-CoA, which is reduced by NADPH to b-hydroxystearyl-CoA. The latter is dehydrated to the unsaturated stearyl-CoA, which is then reduced to yield stearyl-CoA at the expense of NADPH. This system will also elongate unsaturated fatty acids.
Microsome preparations can elongate both saturated and unsaturated fatty acyl-CoA esters, but in this case malonyl-CoA rather than acetyl-CoA serves as source of the acetyl groups. The reaction sequence is identical to that in the fatty-acid synthetase system except that the microsomal system employs CoA and not ACP as acyl carrier.
Formation of monoenoic acids
Palmitic and stearic acids serve as precursors of the two common monoenoic (monounsaturated) fatty acids of animal tissues, namely, poimitoleic and oleic acids, both of which possess a cis double bond in the D9 position. Although most organisms can form palmitoleic and oleic acids, the pathway and enzymes employed differ between aerobic and anaerobic organisms. In vertebrates (and most other aerobic organisms) the D9 double bond is introduced by a specific monooxygenase system; it is located in the endoplasmic reticulum of liver and adipose tissue. One molecule of molecular oxygen (O2) is used as the acceptor for two pairs of electrons, one pair derived from the palmitoyl-CoA or stearyl-CoA substrate and the other from NADPH, which is a required coreductant in the reaction. The transfer of electrons in this complex reaction involves a microsomal electron-transport chain which carries electrons from NADPH (or NADH) to microsomal cytochrome b5 via cytochrome b5 reductase, a flavoprotein. A terminal cyanide-sensitive factor (CSF), a protein, is required to activate the acyl-CoA and the oxygen.
The overall reaction for palmitoyl-CoA is:
Palmitoyl—CoA + NADPH + H+ + O2 ® palmitoleyl—CoA + NADP+ + 2H2O.
Formation of polyenoic acids
Bacteria do not contain polyenoic acids; however, these acids are abundant both in higher plants and in animals. Mammals contain four distinct families of polyenoic acids, which differ in the number of carbon atoms between the terminal methyl group and the nearest double bond. These families are named from their precursor fatty acids, namely, palmitoleic, oleic, linoleic, and linolenic acids. All polyenoic acids found in mammals are formed from these four precursors by further elongation and/or desaturation reactions. Two of these precursor fatty acids, linoleic and linolenic acids, cannot be synthesized by mammals and must be obtained from plant sources; they are therefore called essential fatty acids.
The elongation of chains of polyenoic acids occurs at the carboxyl end by the mitochondrial or microsomal systems described above. The desaturation steps occur by the action of the cytochrome b5-oxygenase system with NADPH as coreductant of oxygen, like the steps in the formation of palmitoleic and oleic acids, also described above.
Arachidonic acid is the most abundant polyenoic acid. When young rats are placed on diets deficient in essential fatty acids, they grow slowly and develop a scaly dermatitis and thickening of the skin. This condition can be relieved by administration not only of linoleic or linolenic acid but also of arachidonic acid. The essential fatty acids and some of their derivatives serve as precursors of the prostaglandins.
In plants linoleic and linolenic acids are synthesized from oleic acid via aerobic desaturation reactions catalyzed by specific oxygenase systems requiring NADPH as coreductant.
The double bonds of naturally occurring fatty acids do not in general undergo hydrogenation to yield more completely saturated fatty acids; only a few organisms appear to carry out this process. Unsaturated fatty acids, however, are completely oxidized by the fatty acid oxidation system.
In most organisms the conversion of saturated to unsaturated fatty acids is promoted by low environmental temperatures. This is an adaptation to maintain the melting point of the total cell lipids below the ambient temperature; unsatu-rated fatty acids have lower melting points than saturated. In some organisms the enzymes involved in fatty acid desaturation increase in concentration in response to low temperatures; in others the unsaturated fatty acids are inserted into lipids at increased rates.
Biosynthesis of triacylglycerols
The triacylglycerols, which function as depot, or storage, lipids, are actively synthesized in the cells of vertebrates, particularly liver and fat cells, as well as those of higher plants. Bacteria in general contain relatively small amounts of triacyglycerols.
In higher animals and plants two major precursors are required for the synthesis of triacylglycerols: L-glycerol 3-phosphate and fatty acyl-CoA.. L-Glycerol 3-phosphate is derived from two different sources. Its normal precursor is dihydroxyacetone phosphate, the product of the aldolase reaction of glycolysis. Dihydroxyacetone phosphate is reduced to L-glycerol 3-phosphate by the NAD-linked glycerol- 3-phosphate dehydrogenase of the cytosol:
Dihydroxyacetone phosphate + NADH + H+ ® L-glycerol 3-phosphate + NAD+
It may also be formed from free glycerol arising from degradation of triacylglycerols, through the action of glycerol kinase:
ATP + glycerol ® L-glycerol 3-phosphate + ADP
The first stage in triacyglycerol formation is the acylation of the free hydroxyl groups of glycerol phosphate by two molecules of fatty acyl-CoA to yield first a lysophosphotidic acid and then a phosphatidic acid:
Glycerol phosphate Lysophosphotidic acid
Lysophosphotidic acid Phosphatidic acid
Free glycerol is not acylated. These reactions occur preferentially with 16- and 18-carbon saturated and unsaturated acyl-CoA.
Phosphatidic acids occur only in trace amounts in cells, but they are important intermediates in the biosynthesis of triacylglycerols and phosphoglycerides.
In the pathway to triacylglycerols, phosphatidic acid undergoes hydrolysis by phosphatidate phosphatase to form a diacylglycerol:
Phosphatidic acid Diacylglycerol
The diacylglycerol then reacts with a third molecule of a fatty acyl-CoA to yield a triacylglycerol by the action of diacylglycerol acyltransferase:
In the intestinal mucosa of higher animals, which actively synthesizes triacylglycerols during absorption of fatty acids from the intestine, another type of acylation reaction comes into play. Monoacylglycerols formed during intestinal digestion may be acylated directly by acylglycerol palmitoyltrans-ferase and thus phosphatidic acid is not an intermediate:
Monoacylglycerol + palmitoyl-CoA ® diacylglycerol + CoA
In storage fats of animal and plant tissues the triacylglycerols are usually mixed, i.e., contain two or more different fatty acids.
PROTEIN CATABOLISM – hydrolysis breaks peptide bonds yielding amino acids
AMINO ACID CATABOLISM - (requires B6)
PROTEIN CATABOLISM – attaches an amino group of an amino acid to a keto acid converting a keto acid
into an amino acid. The original amino acid becomes a keto acid.
1. New amino acid can be used for synthesis
2. Keto acid can be broken down in the TCA cycle
DEAMINATION – uses deaminase, water & NAD
1. breaks down an amino acid into a keto acid
and an ammonia.
2. liver cells convert ammonia to urea via the
PROTEIN ANABOLISM – dehydration synthesis
A. Amination – attaches amino group to a keto
B. Ten essential amino acids
C. Deficiency diseases
D. Genetic metabolic disorder - PKU
The digestive process breaks down food by chemical and mechanical action into substances that can pass into the bloodstream and be processed by body cells.
Certain nutrients, such as salts and minerals, can be absorbed directly into the circulation. Fat, complex carbohydrates, and proteins are broken down into smaller molecules before being absorbed.
Fat is split into glycerol and fatty acids; carbohydrates are split into monosaccharide sugars; and proteins are split into linked amino acids called peptides, and then into individual amino acids.
Food is chewed with the teeth and mixed with saliva. The enzyme amylase, present in saliva, begins the breakdown of starch into sugar. Each lump of soft food, called a bolus, is swallowed and propelled by contractions down the oesophagus into the stomach.
Pepsin is an enzyme produced when pepsinogen, a substance secreted by the stomach lining, is modified by hydrochloric acid (also produced by the stomach lining).
Pepsin breaks proteins down into smaller units, called polypeptides and peptides. Lipase is a stomach enzyme that breaks down fat into glycerol and fatty acids. The acid produced by the stomach also kills bacteria.
Lipase, a pancreatic enzyme, breaks down fat into glycerol and fatty acids. Amylase, another enzyme produced by the pancreas, breaks down starch into maltose, a disaccharide sugar. Trypsin and chymotrypsin are powerful pancreatic enzymes that split proteins into polypeptides and peptides.
Maltase, sucrase, and lactase are enzymes produced by the lining of the small intestine. They convert disaccharide sugars into monosaccharide sugars. Peptidase, another enzyme produced in the intestine, splits large peptides into smaller peptides and then into amino acids.
Undigested food enters the large intestine, where water and salt are absorbed by the intestinal lining. The residue, together with waste pigments, dead cells , and bacteria, is pressed into faeces and stored for excretion.
Humans are totally dependent on other organisms for converting atmospheric nitrogen into forms available to the body. Nitrogen fixation is carried out by bacterial nitrogensases forming reduced nitrogen, NH4+ which can then be used by all organisms to form amino acids.
Overview of the flow of nitrogen in the biosphere. Nitrogen, nitrites and nitrates are acted upon by bacteria (nitrogen fixation) and plants and we assimilate these compounds as protein in our diets. Ammonia incorporation in animals occurs through the actions of glutamate dehydrogenase and glutamine synthase. Glutamate plays the central role in mammalian nitrogen flow, serving as both a nitrogen donor and nitrogen acceptor.
Reduced nitrogen enters the human body as dietary free amino acids, protein, and the ammonia produced by intestinal tract bacteria. A pair of principal enzymes, glutamate dehydrogenase and glutamine synthatase, are found in all organisms and effect the conversion of ammonia into the amino acids glutamate and glutamine, respectively. Amino and amide groups from these 2 substances are freely transferred to other carbon skeletons by transamination and transamidation reactions.
Representative aminotransferase catalyzed reaction.
Aminotransferases exist for all amino acids except threonine and lysine. The most common compounds involved as a donor/acceptor pair in transamination reactions are glutamate and -ketoglutarate (-KG), which participate in reactions with many different aminotransferases. Serum aminotransferases such as serum glutamate-oxaloacetate-aminotransferase (SGOT) (also called aspartate aminotransferase, AST) and serum glutamate-pyruvate aminotransferase (SGPT) (also called alanine transaminase, ALT) have been used as clinical markers of tissue damage, with increasing serum levels indicating an increased extent of damage. Alanine transaminase has an important function in the delivery of skeletal muscle carbon and nitrogen (in the form of alanine) to the liver. In skeletal muscle, pyruvate is transaminated to alanine, thus affording an additional route of nitrogen transport from muscle to liver. In the liver alanine transaminase tranfers the ammonia to -KG and regenerates pyruvate. The pyruvate can then be diverted into gluconeogenesis. This process is refered to as the glucose-alanine cycle.
The reaction catalyzed by glutamate dehydrogenase is:
Glutamate dehydrogenase can utilize either NAD orNADP as cofactor. In the forward reaction as shown above glutamate dehydrogenase is important in converting free ammonia and -ketoglutarate (-KG) to glutamate, forming one of the 20 amino acids required for protein synthesis. However, it should be recognized that the reverse reaction is a key anapleurotic process linking amino acid metabolism with TCA cycle activity.In the reverse reaction, glutamate dehydrogenase provides an oxidizable carbon source used for the production of energy as well as a reduced electron carrier, NADH. As expected for a branch point enzyme with an important link to energy metabolism, glutamate dehydrogenase is regulated by the cell energy charge. ATP and GTP are positive allosteric effectors of the formation of glutamate, whereas ADP and GDP are positive allosteric effectors of the reverse reaction. Thus, when the level of ATP is high, conversion of glutamate to -KG and other TCA cycle intermediates is limited; when the cellular energy charge is low, glutamate is converted to ammonia and oxidizable TCA cycle intermediates. Glutamate is also a principal amino donor to other amino acids in subsequent transamination reactions. The multiple roles of glutamate in nitrogen balance make it a gateway between free ammonia and the amino groups of most amino acids.
The reaction catalyzed by glutamine synthase is:
glutamate + NH4+ + ATP -------> glutamine + ADP + Pi + H+
The glutamine synthatase reaction is also important in several respects. First it produces glutamine, one of the 20 major amino acids. Second, in animals, glutamine is the major amino acid found in the circulatory system. Its role there is to carry ammonia to and from various tissues but principally from peripheral tissues to the kidney, where the amide nitrogen is hydrolyzed by the enzyme glutaminase (reaction below); this process regenerates glutamate and free ammonium ion, which is excreted in the urine.
glutamine + H2O -------> glutamate + NH3
Note that, in this function, ammonia arising in peripheral tissue is carried in a nonionizable form which has none of the neurotoxic or alkalosis-generating properties of free ammonia.
contains both glutamine synthetase and glutaminase
but the enzymes are localized in different cellular segments. This ensures that
the liver is neither a net producer nor consumer of glutamine. The differences
in cellular location of these two enzymes allows the liver to scavange ammonia
that has not been incorporated into urea. The enzymes of the urea cycle are
located in the same cells as those that contain glutaminase. The result
of the differential distribution of these two hepatic enzymes makes it possible
to control ammonia incorporation into either urea or glutamine, the latter
leads to excretion of ammonia by the kidney.
When acidosis occurs the body will divert more glutamine from the liver to the kidney. This allows for the conservation of bicarbonate ion since the incorporation of ammonia into urea requires bicarbonate (see below). When glutamine enters the kidney, glutaminase releases one mole of ammonia generating glutamate and then glutamate dehydrogenase releases another mole of ammonia generating a-ketoglutarate. The ammonia will ionizes to ammonium ion ( NH4+) which is excreted. The net effect is a reduction in the pH (see also Kidneys and Acid-Base Balance).
While glutamine, glutamate, and the remaining nonessential amino acids can be
made by animals, the majority of the amino acids found in human tissues
necessarily come from dietary sources (about 400g of protein per day). Protein
digestion begins in the stomach, where a proenzyme called pepsinogen
is secreted, autocatalytically converted to Pepsin A, and used
for the first step of proteolysis. However, most proteolysis takes place in the
duodenum as a consequence of enzyme activities secreted by the pancreas. All of
the serine proteases and the zinc peptidases of pancreatic secretions are
produced in the form of their respective proenzymes. These proteases are both
endopeptidase and exopeptidase, and their combined action in the intestine
leads to the production of amino acids, dipeptides, and tripeptides, all of
which are taken up by enterocytes of the mucosal wall.
A circuitous regulatory pathway leading to the secretion of proenzymes into the intestine is triggered by the appearance of food in the intestinal lumen. Special mucosal endocrine cells secret the peptide hormones cholecystokinin (CCK) and secretin into the circulatory system. Together, CCK and secretin cause contraction of the gall bladder and the exocrine secretion of a bicarbonate-rich, alkaline fluid, containing protease proenzymes from the pancreas into the intestine. A second, paracrine role of CCK is to stimulate adjacent intestinal cells to secrete enteropeptidase, a protease that cleaves trypsinogen to produce trypsin. Trypsin also activates trypsinogen as well as all the other proenzymes in the pancreatic secretion, producing the active proteases and peptidases that hydrolyze dietary polypeptides.
Subsequent to luminal hydrolysis, small peptides and amino acids are transferred through enterocytes to the portal circulation by diffusion, facilitated diffusion, or active transport. A number of Na+-dependent amino acid transport systems with overlapping amino acid specificity have been described. In these transport systems, Na+ and amino acids at high luminal concentrations are co-transported down their concentration gradient to the interior of the cell. The ATP-dependent Na+/K+ pump exchanges the accumulated Na+ for extracellular K+, reducing intracellular Na+ levels and maintaining the high extracellular Na+ concentration (high in the intestinal lumen, low in enterocytes) required to drive this transport process.
Transport mechanisms of this nature are ubiquitous in the body. Small peptides are accumulated by a proton (H+) driven transport process and hydrolyzed by intracellular peptidases. Amino acids in the circulatory system and in extracellular fluids are transported into cells of the body by at least 7 different ATP-requiring active transport systems with overlapping amino acid specificities.
Hartnup disorder is an autosomal recessive impairment of neutral amino acid transport affecting the kidney tubules and small intestine. It is believed that the defect lies in a specific system responsible for neutral amino acid transport across the brush-border membrane of renal and intestinal epithelium. The exact defect has not yet been characterized. The characteristic diagnostic feature of Hartnup disorder is a dramatic neutral hyperaminoaciduria. Additionally, individuals excrete indolic compounds that originate from the bacterial degradation of unabsorbed tryptophan. The reduced intestinal absorption and increased renal loss of tryptophan lead to a reduced availability of tryptophan for niacin and nicotinamide nucleotide biosynthesis. As a consequence affected individuals frequently exhibit pellegra-like rashes. .
Many other nitrogenous compounds are found in the intestine. Most are bacterial products of protein degradation. Some have powerful pharmacological (vasopressor) effects.
Products of Intestinal Bacterial Activity
Indole and skatole
All amino acids
Prokaryotes such as E. coli can make the carbon skeletons of all 20 amino acids and transaminate those carbon skeletons with nitrogen from glutamine or glutamate to complete the amino acid structures. Humans cannot synthesize the branched carbon chains found in branched chain amino acids or the ring systems found in phenylalanine and the aromatic amino acids; nor can we incorporate sulfur into covalently bonded structures. Therefore, the 10 so-called essential amino acids must be supplied from the diet. Nevertheless, it should be recognized that,depending on the composition of the diet and physiological state of an individual,one or another of the non-essential amino acids may also become a required dietary component. For example, arginine is not usually considered to be essential, because enough for adult needs is made by the urea cycle.
However, the urea cycle generally does not provide sufficient arginine for the needs of a growing child.
To take a different type of example, cysteine and
tyrosine are considered non-essential but are formed from the essential amino
acids methionine and phenylalanine, respectively. If sufficient cysteine and
tyrosine are present in the diet, the requirements for methionine and
phenylalanine are markedly reduced; conversely, if methionine and phenylalanine
are present in only limited quantities, cysteine and tyrosine can become
essential dietary components. Finally, it should be recognized that if the -keto acids corresponding to the carbon skeleton of
the essential amino acids are supplied in the diet, aminotransferases in the
body will convert the keto acids to their respective amino acids, largely
supplying the basic needs.
Unlike fats and carbohydrates, nitrogen has no designated storage depots in the body. Since the half-life of many proteins is short (on the order of hours), insufficient dietary quantities of even one amino acid can quickly limit the synthesis and lower the body levels of many essential proteins. The result of limited synthesis and normal rates of protein degradation is that the balance of nitrogen intake and nitrogen excretion is rapidly and significantly altered. Normal, healthy adults are generally in nitrogen balance, with intake and excretion being very well matched. Young growing children, adults recovering from major illness, and pregnant women are often in positive nitrogen balance. Their intake of nitrogen exceeds their loss as net protein synthesis proceeds. When more nitrogen is excreted than is incorporated into the body, an individual is in negative nitrogen balance. Insufficient quantities of even one essential amino acid is adequate to turn an otherwise normal individual into one with a negative nitrogen balance. The biological value of dietary proteins is related to the extent to which they provide all the necessary amino acids. Proteins of animal origin generally have a high biological value; plant proteins have a wide range of values from almost none to quite high. In general, plant proteins are deficient in lysine, methionine, and tryptophan and are much less concentrated and less digestible than animal proteins. The absence of lysine in low-grade cereal proteins, used as a dietary mainstay in many underdeveloped countries, leads to an inability to synthesize protein (because of missing essential amino acids) and ultimately to a syndrome known as kwashiorkor, common among children in these countries.
The amino acids arginine, methionine and phenylalanine are considered essential for reasons not directly related to lack of synthesis. Arginine is synthesized by mammalian cells but at a rate that is insufficient to meet the growth needs of the body and the majority that is synthesized is cleaved to form urea. Methionine is required in large amounts to produce cysteine if the latter amino acid is not adequately supplied in the diet. Similarly, phenyalanine is needed in large amounts to form tyrosine if the latter is not adequately supplied in the diet.
Removal of Nitrogen from Amino Acids
Nitrogen elimination begins intracellularly with protein degradation. There are two main routes for converting intracellular proteins to free amino acids: a lysosomal pathway, by which extracellular and some intracellular proteins are degraded, and cytosolic pathways that are important in degrading proteins of intracellular origin. In one cytosolic pathway a protein known as ubiquitin is activated by conversion to an AMP derivative, and cytosolic proteins that are damaged or otherwise destined for degradation are enzymically tagged with the activated ubiquitin. Ubiquitin-tagged proteins are then attacked by cytosolic ATP-dependent proteases that hydrolyze the targeted protein, releasing the ubiquitin for further rounds of protein targeting.
The dominant reactions involved in removing amino acid nitrogen from the body are known as transaminations. This class of reactions funnels nitrogen from all free amino acids into a small number of compounds; then, either they are oxidatively deaminated, producing ammonia, or their amine groups are converted to urea by the urea cycle. Transaminations involve moving an -amino group from a donor a-amino acid to the keto carbon of an acceptor -keto acid. These reversible reactions are catalyzed by a group of intracellular enzymes known as aminotransferases (transaminases), which employ covalently bound pyridoxal phosphate as a cofactor (see reaction mechanism).
Aminotransferases exist for all amino acids except threonine and lysine. The most common compounds involved as a donor/acceptor pair in transamination reactions are glutamic acid and -ketoglutaric acid, which participate in reactions with many different aminotransferases. Serum aminotransferases such as serum glutamate - oxaloacetate - aminotransferase (SGOT) have been used as clinical markers of tissue damage, with increasing serum levels indicating an increased extent of damage.
A small but clinically important amount of creatinine is excreted in the urine daily, and the creatinine clearance rate is often used as an indicator of kidney function. The first reaction in creatinine formation is the transfer of the amido (or amidine) group of arginine to glycine, forming guanidinoacetate. Subsequently, a methyl group is transferred from the ubiquitous 1-carbon-donor S-adenosylmethionine to guanidinoacetate to produce creatine (from which phosphocreatine is formed), some of which spontaneously cyclizes to creatinine, and is eliminated in the urine. The quantity of urine creatinine is generally constant for an individual and approximately proportional to muscle mass. In individuals with damaged muscle cells, creatine leaks out of the damaged tissue and is rapidly cyclized, greatly increasing the quantity of circulating and urinary creatinine.
Because of the participation of -ketoglutarate in numerous transaminations, glutamate is a prominent intermediate in nitrogen elimination as well as in anabolic pathways. Glutamate formed in the course of nitrogen elimination is either oxidatively deaminated by liver glutamate dehydrogenase, forming ammonia, or converted to glutamine by glutamine synthase and transported to kidney tubule cells. There the glutamine is sequentially deamidated by glutaminase and deaminated by kidney glutamate dehydrogenase.
The ammonia produced in the latter two reactions is
excreted as NH4+ in the urine, where it helps maintain
urine pH in the normal range of pH 4 to pH 8. The extensive production of
ammonia by peripheral or liver glutamate dehydrogenase is not
feasible because of the highly toxic effects of circulating ammonia. Normal serum
ammonium concentrations are in the range of 20-40 mmol, and an increase in
circulating ammonia to about 400 mmol causes alkalosis and neurotoxicity.
A final, therapeutically useful amino acid-related reaction is the amidation of aspartic acid to produce asparagine. The enzyme asparagine synthase catalyzes the ATP, requiring the transamidation reaction shown below:
Most cells perform this reaction
well enough to produce all the asparagine they need. However, some leukemia
cells require exogenous asparagine, which they obtain from the plasma.
Chemotherapy using the enzyme asparaginase takes advantage of
this property of leukemic cells by hydrolyzing serum asparagine to ammonia and
aspartic acid, thus depriving the neoplastic cells of the asparagine that is
essential for their characteristic rapid growth.
In the peroxisomes of mammalian tissues, especially liver, there are 2 stereospecific amino acid oxidases involved in elimination of amino acid nitrogen. D-amino acid oxidase is an FAD-linked enzyme, and while there are few D-amino acids that enter the human body the activity of this enzyme in liver is quite high. L-amino acid oxidase is FMN-linked and has broad specificity for the L amino acids.A number of substances, including oxygen, can act as electron acceptors from the flavoproteins. If oxygen is the acceptor the product is hydrogen peroxide, which is then rapidly degraded by the catalases found in liver and other tissues. Missing or defective biogenesis of peroxisomes or L-amino acid oxidase causes generalized hyper-aminoacidemia and hyper-aminoaciduria, generally leading to neurotoxicity and early death.
Amino Acid Biosynthesis
Glutamate and Aspartate
Glutamate and aspartate are synthesized from their widely distributed -keto acid precursors by simple 1-step transamination reactions. The former catalyzed by glutamate dehydrogenase and the latter by aspartate aminotransferase, AST.Aspartate is also derived from asparagine through the action of asparaginase. The importance of glutamate as a common intracellular amino donor for transamination reactions and of aspartate as a precursor of ornithine for the urea cycle is described in the Nitrogen Metabolism page.
Aside from its role in protein synthesis, alanine is second only to glutamine in prominence as a circulating amino acid. In this capacity it serves a unique role in the transfer of nitrogen from peripheral tissue to the liver. Alanine is transferred to the circulation by many tissues, but mainly by muscle, in which alanine is formed from pyruvate at a rate proportional to intracellular pyruvate levels. Liver accumulates plasma alanine, reverses the transamination that occurs in muscle, and proportionately increases urea production. The pyruvate is either oxidized or converted to glucose via gluconeogenesis. When alanine transfer from muscle to liver is coupled with glucose transport from liver back to muscle, the process is known as the glucose-alanine cycle. The key feature of the cycle is that in 1 molecule, alanine, peripheral tissue exports pyruvate and ammonia (which are potentially rate-limiting for metabolism) to the liver, where the carbon skeleton is recycled and most nitrogen eliminated.There are 2 main pathways to production of muscle alanine: directly from protein degradation, and via the transamination of pyruvate by glutamate-pyruvate aminotransferase (also called alanine transaminase, ALT).
glutamate + pyruvate <-------> -KG + alanine
The sulfur for cysteine synthesis comes from the essential amino acid methionine. A condensation of ATP and methionine catalyzed by methionine adenosyltransferase yields S-adenosylmethionine (SAM or AdoMet).
Biosynthesis of S-adenosylmethionine, SAM
SAM serves as a precurosor for numerous methyl transfer reactions (e.g. the conversion of norepinephrine to epinenephrine, see Specialized Products of Amino Acids). The result of methyl transfer is the conversion of SAM to S-adenosylhomocysteine. S-adenosylhomocysteine is then cleaved by adenosylhomocyteinase to yield homocysteine and adenosine. Homocysteine can be converted back to methionine by methionine synthase, a reaction that occurs under methionine-sparing conditions and requires N5-methyl-tetrahydrofolate as methyl donor. This reaction was discussed in the context of vitamin B12-requiring enzymes in the Vitamins page. Transmethylation reactions employing SAM are extremely important, but in this case the role of S-adenosylmethionine in transmethylation is secondary to the production of homocysteine (essentially a by-product of transmethylase activity). In the production of SAM all phosphates of an ATP are lost: one as Pi and two as PPi. It is adenosine which is transferred to methionine and not AMP. In cysteine synthesis, homocysteine condenses with serine to produce cystathionine, which is subsequently cleaved by cystathionase to produce cysteine and -ketobutyrate. The sum of the latter two reactions is known as trans-sulfuration. Cysteine is used for protein synthesis and other body needs, while the ketobutyrate is decarboxylated and converted to propionyl-CoA. While cysteine readily oxidizes in air to form the disulfide cystine, cells contain little if any free cystine because the ubiquitous reducing agent, glutathione effectively reverses the formation of cystine by a non-enzymatic reduction reaction.
Utilization of methionine in the synthesis of cysteine
The 2 key enzymes of this pathway, cystathionine synthase and cystathionase (cystathionine lyase), both use pyridoxal phosphate as a cofactor, and both are under regulatory control. Cystathionase is under negative allosteric control by cysteine, as well, cysteine inhibits the expression of the cystathionine synthase gene. Genetic defects are known for both the synthase and the lyase. Missing or impaired cystathionine synthase leads to homocystinuria and is often associated with mental retardation, although the complete syndrome is multifaceted and many individuals with this disease are mentally normal. Some instances of genetic homocystinuria respond favorably to pyridoxine therapy, suggesting that in these cases the defect in cystathionine synthase is a decreased affinity for the cofactor. Missing or impaired cystathionase leads to excretion of cystathionine in the urine but does not have any other untoward effects. Rare cases are known in which cystathionase is defective and operates at a low level. This genetic disease leads to methioninuria with no other consequences.
Tyrosine is produced in cells by hydroxylating the essential amino acid phenylalanine. This relationship is much like that between cysteine and methionine. Half of the phenylalanine required goes into the production of tyrosine; if the diet is rich in tyrosine itself, the requirements for phenylalanine are reduced by about 50%. Phenylalanine hydroxylase is a mixed-function oxygenase: one atom of oxygen is incorporated into water and the other into the hydroxyl of tyrosine. The reductant is the tetrahydrofolate-related cofactor tetrahydrobiopterin, which is maintained in the reduced state by the NADH-dependent enzyme dihydropteridine reductase.
Biosynthesis of tyrosine from phenylalanine
Missing or deficient phenylalanine hydroxylase leads to the genetic disease known as phenlyketonuria (PKU), which if untreated leads to severe mental retardation. The mental retardation is caused by the accumulation of phenylalanine, which becomes a major donor of amino groups in aminotransferase activity and depletes neural tissue of -ketoglutarate. This absence of -ketoglutarate in the brain shuts down the TCA cycle and the associated production of aerobic energy, which is essential to normal brain development.
The product of phenylalanine transamination, phenylpyruvic acid, is reduced to phenylacetate and phenyllactate, and all 3 compounds appear in the urine. The presence of phenylacetate in the urine imparts a "mousy" odor. If the problem is diagnosed early, the addition of tyrosine and restriction of phenylalanine from the diet can minimize the extent of mental retardation.
In other pathways, tetrahydrobiopterin is a cofactor. The effects of missing or defective dihydropteridine reductase cause even more severe neurological difficulties than those usually associated with PKU caused by deficient hydroxylase activity.
Glutamate is the precursor of both proline and ornithine, with glutamate semialdehyde being a branch point intermediate leading to one or the other of these 2 products. While ornithine is not one of the 20 amino acids used in protein synthesis, it plays a significant role as the acceptor of carbamoyl phosphate in the urea cycle. Ornithine serves an additional important role as the precursor for the synthesis of the polyamines. The production of ornithine from glutamate is important when dietary arginine, the other principal source of ornithine, is limited. The fate of glutamate semialdehyde depends on prevailing cellular conditions. Ornithine production occurs from the semialdehyde via a simple glutamate-dependent transamination, producing ornithine.
Ornithine synthesis from glutamate
When arginine concentrations become elevated, the ornithine contributed from the urea cycle plus that from glutamate semialdehyde inhibit the aminotransferase reaction, with accumulation of the semialdehyde as a result. The semialdehyde cyclizes spontaneously to 1pyrroline-5-carboxylate which is then reduced to proline by an NADPH-dependent reductase.
The main pathway to serine starts with the glycolytic intermediate 3-phosphoglycerate.
An NADH-linked dehydrogenase converts 3-phosphoglycerate into a keto acid, 3-phosphopyruvate, suitable for subsequent transamination. Aminotransferase activity with glutamate as a donor produces 3-phosphoserine, which is converted to serine by phosphoserine phosphatase.
The main pathway to glycine is a 1-step reaction catalyzed by serine hydroxymethyltransferase.
This reaction involves the transfer of the hydroxymethyl group from serine to the cofactor tetrahydrofolate (THF), producing glycine and N5,N10-methylene-THF. Glycine produced from serine or from the diet can also be oxidized by glycine cleavage complex, GCC, to yield a second equivalent of N5,N10-methylene-tetrahydrofolate as well as ammonia and CO2.
Glutamate is synthesized by the reductive amination of -ketoglutarate catalyzed by glutamate dehydrogenase; it is thus a nitrogen-fixing reaction. In addition, glutamate arises by aminotransferase reactions, with the amino nitrogen being donated by a number of different amino acids. Thus, glutamate is a general collector of amino nitrogen. Aspartate is formed in a transamintion reaction catalyzed by aspartate transaminase, AST. This reaction uses the aspartate -keto acid analog, oxaloacetate, and glutamate as the amino donor. Aspartate can also be formed by deamination of asparagine catalyzed by asparaginase. Asparagine synthetase and glutamine synthetase, catalyze the production of asparagine and glutamine from their respective amino acids. Glutamine is produced from glutamate by the direct incorporation of ammonia; and this can be considered another nitrogen fixing reaction. Asparagine, however, is formed by an amidotransferase reaction. Aminotransferase reactions are readily reversible. The direction of any individual transamination depends principally on the concentration ratio of reactants and products. By contrast, transamidation reactions, which are dependent on ATP, are considered irreversible. As a consequence, the degradation of asparagine and glutamine take place by a hydrolytic pathway rather than by a reversal of the pathway by which they were formed. As indicated above, asparagine can be degraded to aspartate.
Amino Acid Catabolism
Glutaminase is an important kidney tubule enzyme involved in converting glutamine (from liver and from other tissue) to glutamate and NH3+, with the NH3+ being excreted in the urine. Glutaminase activity is present in many other tissues as well, although its activity is not nearly as prominent as in the kidney. The glutamate produced from glutamine is converted to -ketoglutarate, making glutamine a glucogenic amino acid. Asparaginase is also widely distributed within the body, where it converts asparagine into ammonia and aspartate. Aspartate transaminates to oxaloacetate, which follows the gluconeogenic pathway to glucose. Glutamate and aspartate are important in collecting and eliminating amino nitrogen via glutamine synthetase and the urea cycle, respectively. The catabolic path of the carbon skeletons involves simple 1-step aminotransferase reactions that directly produce net quantities of a TCA cycle intermediate. The glutamate dehydrogenase reaction operating in the direction of -ketoglutarate production provides a second avenue leading from glutamate to gluconeogenesis.
Alanine is also important in intertissue nitrogen transport as part of the glucose-alanine cycle. Alanine's catabolic pathway involves a simple aminotransferase reaction that directly produces pyruvate. Generally pyruvate produced by this pathway will result in the formation of oxaloacetate, although when the energy charge of a cell is low the pyruvate will be oxidized to CO2 and H2O via the PDH complex and the TCA cycle. This makes alanine a glucogenic amino acid.