Anaerobic and aerobic oxidation of glucose. Studing of biosynthesis and catabolism of glycogen. gluconeogenesis.
Metabolism of lipids. Metabolism of ketone bodies and cholesterol.
Digestion of proteins. General pathways of amino acids transformation. Detoxification of ammonia and biosynthesis of urea.
Foods high in carbohydrate include fruits, sweets, soft drinks, breads, pastas, beans, potatoes, bran, rice, and cereals. Carbohydrates are a common source of energy in living organisms; however, no carbohydrate is an essential nutrient in humans.
Carbohydrates are not necessary building blocks of other molecules, and the body can obtain all its energy from protein and fats.[ The brain and neurons generally cannot burn fat for energy, but use glucose or ketones. Humans can synthesize some glucose (in a set of processes known asgluconeogenesis) from specific amino acids, from the glycerol backbone in triglycerides and in some cases from fatty acids. Carbohydrate and protein contain 4 calories per gram, while fats contain 9 calories per gram. In the case of protein, this is somewhat misleading as only some amino acids are usable for fuel.
Organisms typically cannot metabolize all types of carbohydrate to yield energy. Glucose is a nearly universal and accessible source of calories. Many organisms also have the ability to metabolize other monosaccharides and Disaccharides, though glucose is preferred. In Escherichia coli, for example, the lac operon will express enzymes for the digestion of lactose when it is present, but if both lactose and glucose are present the lac operon is repressed, resulting in the glucose being used first. Polysaccharides are also common sources of energy. Many organisms can easily break down starches into glucose, however, most organisms cannot metabolize cellulose or other polysaccharides like chitin and arabinoxylans. These carbohydrates types can be metabolized by some bacteria and protists. Ruminants and termites, for example, use microorganisms to processcellulose. Even though these complex carbohydrates are not very digestible, they represent an important dietary element for humans, called dietary fiber. Fiber enhances digestion, among other benefits.
Based on the effects on risk of heart disease and obesity, the Institute of Medicine recommends that American and Canadian adults get between 45–65% of dietary energy from carbohydrates. The Food and Agriculture Organization and World Health Organization jointly recommend that national dietary guidelines set a goal of 55–75% of total energy from carbohydrates, but only 10% directly from sugars (their term for simple carbohydrates).
Functions of Carbohydrate
All animals derive the major portion of their food calories from the different types of Carbohydrates in their diets. Most of the energy for the metabolic activities of the cell in all organisms is derived from the oxidation of Carbohydrate. Important functions of Carbohydrate are that of storing food, acting as a framework in body, performs are listed below.
Carbohydrate functions as Bio Fuel
Carbohydrate functions as an energy source of the body and acts as Bio fuel.Step wise details for the process of production of energy are discussed below.
· Polysaccharides such as starch and glycogen are first hydrolyzed by enzymes to Glucose.
· Glucose is the transported from one cell to another by blood in case of animals and cell sap in case of plants.
· Glucose is then oxidized to produce carbon dioxide and water.
· Energy is released in this process which is used for functioning of the cells.
Carbohydrate functions as Primary Source of Energy
The process of production of energy by carbohydrates is described in above steps. Now it is important to note, that fats and proteins can also be burned to provide energy but carbohydrate functions as primary source of energy. Fats are only burned if there is non availability of carbohydrates. When fat is burned in absence of carbohydrates, toxic compounds like called ketone bodies are produced. Accumulation of these ketone bodies over long period causes a condition called Ketosis. In this condition blood becomes unable to carry oxygen properly and this can be fatal. Thus, one of important function of carbohydrate is help burn fat properly.
Carbohydrate functions as storage food
Different forms of Carbohydrate are stored in living organism as storage food.
· Polysaccharide starch acts as storage food for plants.
· Glycogen stored in liver and muscles acts as storage food for animals.
· Inulin acts as storage food of dahlias, onion and garlic.
Thus carbohydrate performs the function of storing food.
Carbohydrate functions as framework in body
Different Carbohydrates especially Polysaccharides act as framework in living organism.
· Cellulose forms cell wall of plant cell along with hemicelluloses and Pectin
· Chitin forms cell wall of fungal cell and exoskeleton of arthropods
· Peptidoglycan forms cell wall of bacteria and cyanobacteria.
Thus carbohydrates function as contributing material to the cellular structure.
Carbohydrate functions as Anticoagulant
Heparin is a polysaccharide (carbohydrate) which acts as anticoagulant and prevents intravascular clotting.
Carbohydrate functions as Antigen
Many antigens are glycoprotein (which contains oligosaccharide) in nature and give immunological properties to the blood.
Carbohydrate functions as Hormone
Many Hormones like FSH (Follicular Stimulating Hormone which takes part in ovulation in females) and LH (Leutinizing Hormone) are glycoprotein and help in reproductive processes.
Carbohydrates provide raw material for industry
Carbohydrates are an important component of many industries like textile, paper, lacquers and breweries.
Agar is polysaccharide used in culture media, laxative and food.
Cellulose acts as roughage of food. It stimulates peristalsis movement and secretion of digestive enzymes.
Hyaluronic acid found in between joints acts as synovial fluid and provides frictionless movement.
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.
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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The Energy Derived from Glucose Oxidation
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.
The Individual Reactions of Glycolysis
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.
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)
basic amino acids (
3. acidic amino acids (Asp, Glu)
4. imino acids (Pro), Hydroxyproline)
5. di- and tripeptides
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.
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.
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
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.
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
diets, such as the Atkins and
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.
Carbohydrate metabolism - a slide presentation from the
more chemistry than most people will want.
Pentose Phosphate Pathway
The Pentose Phosphate Pathway (also called Phosphogluconate Pathway, or Hexose Monophosphate Shunt) is depicted with structures of intermediates in Fig. 23-25 p. 863 of Biochemistry, by Voet & Voet, 3rd Edition. The linear portion of the pathway carries out oxidation and decarboxylation of glucose-6-phosphate, producing the 5-C sugar ribulose-5-phosphate.
Glucose-6-phosphate Dehydrogenase catalyzes oxidation of the aldehyde (hemiacetal), at C1 of glucose-6-phosphate, to a carboxylic acid in ester linkage (lactone). NADP+ serves as electron acceptor.
6-Phosphogluconolactonase catalyzes hydrolysis of the ester linkage (lactone) resulting in ring opening. The product is 6-phosphogluconate. Although ring opening occurs in the absence of a catalyst, 6-Phosphogluconolactonase speeds up the reaction, decreasing the lifetime of the highly reactive, and thus potentially toxic, 6-phosphogluconolactone.
Phosphogluconate Dehydrogenase catalyzes oxidative decarboxylation of 6-phosphogluconate, to yield the 5-C ketose ribulose-5-phosphate. The hydroxyl at C3 (C2 of the product) is oxidized to a ketone. This promotes loss of the carboxyl at C1 as CO2. NADP+ again serves as oxidant (electron acceptor).
Reduction of NADP+ (as with NAD+) involves transfer of 2e- plus 1H+ to the nicotinamide moiety.
NAD+ and NADP+ differ only in the presence of an extra phosphate on the adenosine ribose of NADP+. This difference has little to do with redox activity, but is recognized by substrate-binding sites of enzymes. It is a mechanism for separation of catabolic and synthetic pathways.
NADPH, a product of the Pentose Phosphate Pathway, functions as a reductant in various synthetic (anabolic) pathways, including fatty acid synthesis.
NAD+ serves as electron acceptor in catabolic pathways in which metabolites are oxidized. The resultant NADH is reoxidized by the respiratory chain, producing ATP.
Glucose-6-phosphate Dehydrogenase is the committed step of the Pentose Phosphate Pathway. This enzyme is regulated by availability of the substrate NADP+. As NADPH is utilized in reductive synthetic pathways, the increasing concentration of NADP+ stimulates the Pentose Phosphate Pathway, to replenish NADPH.
The remainder of the Pentose Phosphate Pathway accomplishes conversion of the 5-C ribulose-5-phosphate to the 5-C product ribose-5-phosphate, or to the 3-C glyceraldehyde-3-phosphate and the 6-C fructose-6-phosphate (reactions 4 to 8 p. 863).
Additional enzymes include Ribulose-5-phosphate Epimerase, Ribulose-5-phosphate Isomerase, Transketolase, and Transaldolase.
· Epimerase interconverts the stereoisomers ribulose-5-phosphate and xylulose-5-phosphate.
· Isomerase converts the ketose ribulose-5-phosphate to the aldose ribose-5-phosphate.
· Both reactions involve deprotonation to form an endiolate intermediate, followed by specific reprotonation to yield the product. Both reactions are reversible.
Transketolase and Transaldolase catalyze transfer of 2-C and 3-C molecular fragments respectively, in each case from a ketose donor to an aldose acceptor. D. E. Nicholson has suggested that the names of these enzymes should be changed, since Transketolase actually transfers an aldol moiety (glycoaldehyde) and Transaldolase actually transfers a ketol moiety (dihydroxyacetone). However the traditional enzyme names are used here.
Transketolase transfers a 2-C fragment from xylulose-5-phosphate to either ribose-5-phosphate or erythrose-4-phosphate.
Thiamine pyrophosphate binds at the active sites of enzymes in a "V" conformation. The amino group of the aminopyrimidine moiety is close to the dissociable proton, and serves as the proton acceptor. This proton transfer is promoted by a glutamate residue adjacent to the pyrimidine ring.
The thiazolium carbanion (ylid) that results from proton dissociation reacts with the carbonyl C of xylulose-5-P to form an addition compound.
The positively charged N in the thiazole ring acts as an electron sink, promoting C-C bond cleavage. The 3-C aldose glyceraldehyde-3-phosphate is released. A 2-C fragment remains on TPP.
Completion of the reaction is by reversal of these steps. The 2-C fragment condenses with one of the aldoses erythrose-4-phosphate (4-C) or ribose-5-phosphate (5-C) to form a 6-C or 7-C ketose-phosphate product.
Transfer of the 2-C fragment to the 5-C aldose ribose-5-phosphate yields sedoheptulose-7-phosphate (see above). Transfer instead to the 4-C aldose erythrose-4-phosphate yields fructose-6-phosphate. See also diagram p. 865.
Transaldolase catalyzes transfer of a 3-C dihydroxyacetone moiety, from sedoheptulose-7-phosphate to glyceraldehyde-3-phosphate.
The e-amino group of an active site lysine residue reacts with the carbonyl C of sedoheptulose-7-phosphate to form a protonated Schiff base intermediate.
Aldol cleavage results in release of erythrose-4-phosphate. The Schiff base stabilizes the carbanion on C3.
Completion of the reaction occurs by reversal, as the carbanion attacks instead the aldehyde carbon of the 3-carbon aldose glyceraldehyde-3-phosphate to yield the 6-carbon fructose-6-phosphate. See also diagram p. 866.
Explore at right the structure of E. coli Transaldolase, crystallized with a modified active site intermediate. The structure of human Transaldolase has also been determined, and it exhibits a similar a,b barrel structure.
The flow of
C5 + C5 � C3 + C7 (Transketolase)
C3 + C7 � C6 + C4 (Transaldolase)
C5 + C4 � C6 + C3 (Transketolase)
3 C5 � 2C6 + C3 (Overall)
Glucose-6-phosphate may be
regenerated from either the 3-C product glyceraldehyde-3-phosphate or
the 6-C product fructose-6-phosphate, via enzymes of Gluconeogenesis.
In the diagram at right, IS = Isomerase, EP = Epimerase, TK = Transketolase, TA = Transaldolase.
Depending on relative needs of a cell for ribose-5-phosphate, NADPH, and ATP, the Pentose Phosphate Pathway can operate in various modes, to maximize different products. There are three major scenarios:
1. Ribulose-5-phosphate may be converted to ribose-5-phosphate, a substrate for synthesis of nucleotides and nucleic acids. The pathway also produces some NADPH.
2. Glyceraldehyde-3-phosphate and fructose-6-phosphate, formed from the 5-carbon sugar phosphates, may be converted to glucose-6-phosphate for reentry into the linear portion of the Pentose Phosphate Pathway, maximizing formation of NADPH.
3. Glyceraldehyde-3-phosphate and fructose-6-phosphate, formed from the 5-carbon sugar phosphates, may enter Glycolysis, for synthesis of ATP. The pathway also produces some NADPH.
Ribose-1-phosphate generated during catabolism of nucleosides also enters the Glycolytic pathway in this way, after first being converted to ribose-5-phosphate. Thus the Pentose Phosphate Pathway serves as an entry into Glycolysis for both 5-carbon and 6-carbon sugars.
Glutathione is a tripeptide that includes a glutamate residue linked by an isopeptide bond involving the side-chain carbonyl group. Its functional group is a cysteine thiol.
One role of glutathione is degradation of hydroperoxides that arise spontaneously in the oxygen-rich environment within red blood cells. Hydroperoxides can react with double bonds in fatty acid moieties of membrane lipids, making membranes leaky.
Glutathione Peroxidase catalyzes degradation of organic hydroperoxides by reduction, as two glutathione molecules are oxidized to a disulfide. In the reaction summary below, glutathione is represented as GSH.
2 GSH + ROOH � GSSG + ROH + H2O
Glutathione Peroxidase contains the trace element selenium as a functional group. The enzyme's primary structure includes an analog of cysteine, selenocysteine, with Se replacing S.
Regeneration of reduced glutathione requires NADPH, produced within erythrocytes in the Pentose Phosphate Pathway. Glutathione Reductase catalyzes:
GSSG + NADPH + H+ � 2 GSH + NADP+
Genetic deficiency of Glucose-6-phosphate Dehydrogenase can lead to a hemolytic anemia, because of an inadequate supply of NADPH within red blood cells. The effect of a partial deficiency of Glucose-6-phosphate Dehydrogenase activity is exacerbated by substances that lead to increased production of peroxides (e.g., the antimalarial primaquine).
Fig. The pathway of glycolysis.
Fig. Summary of glycolysis.
Investigation of catabolism and biosynthesis of glycogen.
Regulation of glycogen metabolism.
Biosynthesis of glucose – gluconeogenesis.
Hormonal adjusting and pathologies of carbohydrate metabolism.
Metabolic disorders. Diabetes Mellitus.
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
Alejandro Buschiazzo, Juan E Ugalde, Marcelo E Guerin, William Shepard, Rodolfo A Ugalde and Pedro M Alzari
Figure 6. 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.
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
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
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
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
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
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 help --
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
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.
Insulin and Lipid Metabolism
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:
Appearance of diabetic foot is caused by a combination of vascular insufficiency, neuropathy, and infection.
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.
Lipids are a large group of substances which dissolve well in organic solvents (acetone, chloroform, benzene) and they are insoluble or slightly soluble in water. They contain acids with long chains (= fatty acids).
Repeate the composition and function of lipids, structure and classification of fatty acids (FA).
Biological roles of lipids:
1. Lipids are an important source of energy. They are the energy reserve of the body. Adipocytes (fat cells) are specialized for fat storage. Lipids serve as metabolic fuel. Their components are oxidized in the mitochondria to water and CO2. In the process, ATP is produced in large amounts.
2. Amphipathic lipids (phospholipids, glycolipids) are building blocks of cellular membranes.
3. Lipids are excellent insulators.
Classification and metabolism of lipids:
I. Simple lipids:
Triacylglycerols (TAG, fats) are esters of glycerol and three fatty acids.
Waxes are esters of fatty alcohol and fatty acids and they are present in plants.
Degradation of TAG in adipose tissue (see Fig. 1)
The degradation of fats in adipose tissues (= lipolysis) is catalyzed by enzyme hormone sensitive lipase Õ fatty acids released by adipose tissue are transported in the blood in unesterified form = free fatty acids (FFA) – in complexes with albumin. Hormone-sensitive lipase is activated by hormones glucagon and catecholamines. This enzyme is inhibited by insulin.
Fig. 1: Hormone-induced fatty acid mobilization in adipose tissue
Figure is found on http://web.indstate.edu/thcme/mwking/fatty-acid-oxidation.html
The tissues take up fatty acids from the blood to rebuild fats or to obtain energy from their oxidation. Metabolism of fatty acids is especially intensive in the liver (hepatocytes).
In the cell, fatty acids are activated by conversion to their CoA derivatives ñ acyl-CoAs are formed in cell cytoplasm (reaction takes place on outer mitochondria membrane and is catalyzed by enzyme acyl-CoA-synthetase). Transfer of acyl-CoAs from cytoplasm to the mitochondrial matrix is performed by a carnitine transporter Õ carnitine carries acyl residues through the inner mitochondrial membrane Õ matrix.
Fig. 2: Function of carnitine transporter – transport of fatty acids from cytoplasm into matrix of mitochondria.
Figure is found on http://web.indstate.edu/thcme/mwking/fatty-acid-oxidation.html
β - oxidation of fatty acids is the most important process for degradation of fatty acids. This pathway
occurs in mitochondrial matrix. It is an oxidative reaction cycle in which C2 units are succesively released in the form of acetyl-CoA. Cleavage of the acetyl groups starts between C2 (α) and C3 (β) = β – oxidation
Reactions of β – oxidation (see Fig. 3):
1. Oxidation (= dehydrogenation) of an activated fatty acid (= acyl-CoA) is catalyzed by enzyme acyl-CoA dehydrogenase. In the process, two protons with their electrons are transferred to FAD Õ FADH2, which passes them to the respiratory chain.
2. Hydration (= addition of water molecule) is catalyzed by enoyl-CoA hydratase.
3. Oxidation. Acceptor for reducing equivalents is NAD+ Õ NADH + H+, which passes them to the respiratory chain.
4. Thiolysis (thioclastic cleavage) - activated β-ketoacid is cleaved by acyl-CoA acetyl transferase (thiolase) in the presence of CoA. The products are acetyl-CoA and an activated fatty acid with one less C2 unit then the original acid.
For the complete degradation of long chain fatty acids, the cycle must go through multiple rounds. (for example: for stearyl-CoA (C18) eight cycles are required).
The acetyl-CoA formed is condensed with an oxaloacetate to form citrate, which than enters to the citric acid cycle.
Regulation of β-oxidation:
Regulation is performed on the carnitine acyltransferase I level. This enzyme is inhibited by malonyl-CoA (= an intermediate of fatty acid synthesis).
Degradation of palmitic acid (
FADH2 gives 2 ATP
NADH + H+ gives 3 ATP
5 ATP in one oxidation cycle
for complete degradation of palmitic acid the cycle must go 7 times ñ 7 x 5 = 35 ATP
The product of β - oxidation is acetyl-CoA, in case palmitic acid is yielded 8 acetyl-CoAs (12 ATP per 1 acetyl-CoA) ñ 8 x 12 = 96 ATP
Total: 35 ATP + 96 ATP = 131 ATP - 2 ATP for activation of palmitic acid = 129 ATP
Fig. 3: β-oxidation of fatty acids in mitochondrion.
Figure is found on http://www.peroxisome.org/Scientist/Biochemistry/boxidationtext.html
β - oxidation is the main pathway for degradation of fatty acids. But there are also other special pathways for degradation of a) unsaturated fatty acids and b) degradation of odd-numbered fatty acids.
a) degradation of unsaturated fatty acids
Unsaturated FA usually contain cis double bond at positions 9 or 12 (i. e. linoleic acid). Cis double bonds have to convert into trans double bonds and degradation of these FA occurs via β - oxidation.
b) degradation of odd numbered fatty acids
These fatty acids are degradated by β - oxidation as the "normal" even-numbered FA. The products are n acetyl-CoAs and propionyl-CoA. Propionyl-CoA is carboxylated to form methylmalonyl-CoA, which is converted to succinyl-CoA (= intermediate of CAC).
α- and ω- oxidation are only minor importance. The α- oxidation of FA serves for degradation of methyl-branched fatty acids. ω - oxidation serves for oxidation of the end of the fatty acid.
Fatty acid synthesis
The biosynthesis of FA is catalyzed by enzyme fatty acid synthase. It is a multifunctional enzyme that is located in the cytoplasm of the cell.
Fatty acid synthase is composed of 2 identical peptide chains. Each of peptide chains catalyzes 7 different partial reactions that are required for palmitate synthesis. Each half of fatty acid synthase contains a cysteine residue (Cys-SH) and a 4´phosphopantetheine group (Pan-SH), which is very similar to CoA, is bound to a domain of the enzyme that is called as the acyl-carrier protein (ACP).
Biosynthesis of palmitate (see Fig. 4) :
Acetyl-CoA is transferred to the functional cysteine residue of the enzyme.
1. Acetyl-CoA is carboxylated by HCO3- to yield malonyl-CoA by enzyme acetyl-CoA carboxylase. Enzyme acetyl-CoA carboxylase is a regulatory enzyme of fatty acid synthesis. It is activated by citrate and inhibited by end-product = palmitoyl-CoA.
2. Malonyl unit is transferred to the 4´phosphopantetheine group of ACP. Another acetyl group is transferred to cysteine residue.
3. This acetyl group is transferred to the malonyl unit, during this process the carboxyl group is cleaved off as CO2.
4. Reduction of 3-oxogroup by NADPH + H+ Õ NADP+
6. Reduction by NADPH + H+ Õ NADP+ to yield of fatty acid with 4 carbons (= butyryl)
Butyryl unit is relocated from ACP to the functional cysteine Õ ACP can bind another malonyl residue Õ after 7 cycles the product = palmitate is released. One turn of the "cycle" adds a -CH2-CH2- unit to the growing acyl chain.
Most of the reductant, NADPH + H+, is supplied by the pentose phosphate pathway.
Acetyl-CoA (= principal substrate of fatty acid synthesis) is produced in the pyruvate dehydrogenase reaction (located in mitochondria). It is transported from mitochondria into cytoplasm via the citrate shuttle (acetyl-CoA + oxaloacetate → citrate).
Fatty acid synthesis is activated by hormone insulin and inhibited by hormones glucagon and catecholamines.
Fig. 4: Biosynthesis of FA (1 cycle)
Figure is found on http://188.8.131.52/bio/Courses/biochem2/FattyAcid/FASynthesis.html
Metabolism of triacylglycerols (TAG)
TAG are esters of glycerol and three FA.
Monoacylglycerol – one FA is esterified with glycerol. Esterification with futher FA gives diacylglycerols, and ultimately triacylglycerols.
TAG are synthetized in the liver cells and fat cells. Adipocytes containing large droplets of TAG. TAG are degradated (hydrolysis) to glycerol and FA in adipocytes. These hydrolysis is catalyzed by hormone-sensitive lipase. FA are released into the blood Õ FA + albumin complexes Õ to the liver, heart muscle, kidneys, skeletal muscle. Glycerol is transferred to the liver by blood.
Biosynthesis of TAG:
a) de novo synthesis of TAG occurs in cytoplasm and endoplasmatic reticulum of liver and fat cells. This way is less usual (see Fig. 5).
b) reacylation from monoacylglycerols occurs in endoplasmatic reticulum of enterocytes. Monoacylglycerols enter to the enterocytes from lumen of intestine by diffusion through cytoplasmic membrane.
Fig. 5: De novo synthesis of TAG from dihydroxyacetone phosphate with formation of phosphatidic acid
Figure is found on http://web.indstate.edu/thcme/mwking/lipid-synthesis.html
II. Complex lipids
Complex lipids contain alcohol (glycerol, sphingosine), fatty acids and polar residue (phosphate residue, amino alcohol, sugar).
a) Phospholipids are the main constituents of cellular membranes.
Composition of phospholipids: glycerol + 2 fatty acids + phosphate group. They contain a phosphate residue. Phosphate residue is esterified with a hydroxyl group at C-3 of glycerol. This residue gives a negative charge to phospholipids.
Phosphatidic acid (phosphatidate) is the simpliest phospholipid, it is phosphate ester of diacylglycerol. It is an important intermediate in the biosynthesis of fats and phospholipids. All other phospholipids are derived from phosphatidic acid by esterification of the phosphate group with the –OH group of an amino alcohol (choline, ethanolamine, serine) or inositol.
Phosphatidylcholine (= lecithin) is the most abundant phospholipid in membranes.
Phosphatidylethanolamine (cephalin) has an ethanol-amine residue.
Phosphatidylinositol contains inositol (= sugar-like alcohol).
Biosynthesis of phospholipids:
Biosynthesis of complex lipids begins with glycerol-3-P. Glycerol-3-P is esterified with a long chain fatty acid at C-1. This intermediate is esterified with another long chain fatty acid to form phosphatidate. Phosphatidates are key molecules in the biosynthesis of fats, phospholipids and glycolipids.
For the synthesis of fats the phosphate group of phosphatides is first removed by hydrolysis Õ diacylglycerol is then converted to a triacylglycerol (TAG) by transfer of a futher fatty acid from acyl-CoA.
For the synthesis of phosphatidylcholine: phosphate group of phosphatides is first removed by hydrolysis Õ diacylglycerol Õ CDP-choline is transferred to the diacylglycerol ñ phosphatidylcholine.
Phosphatidylethanolamine is synthetized from a diacylglycerol and CDP-ethanolamine.
Degradation of phospholipids:
Degradation of phospholipids is catalyzed by enzymes phospholipases. Phospholipases are divided according to the type of the bond which is cleaved.
Phospholipase A1 catalyzes the cleavage of fatty acid from phospholipid in position 1.
Phospholipase A2 catalyzes the cleavage of fatty acid from phospholipid in position 2.
Phospholipase C catalyzes the cleavage of phosphate group from phospholipid.
b) Sphingophospholipids are found in large amounts in the brain and nervous tissue.
In these compounds, sphingosine (an amino alcohol with a long side chain) replaces glycerol and one of the acyl residues. Amide bond formation between sphingosine and a fatty acid yields ceramide
(= precursor of the sphingolipids).
Composition of sphingophospholipids: sphingosine + fatty acid + phosphate residue + amino alcohol or sugar alcohol
Sphingomyelin (= the most important sphingolipid) contain choline (= amino alcohol) which is connected to the phosphate group of ceramide part.
c) Glycolipids are present in all tissues on the outer surface of the plasma membrane.
These lipids are composed of sphingosine + fatty acid + sugar or oligosaccharide residue. The phosphate group is absent.
Galactosylceramides and glucosylceramides are examples of glycolipids. The sugar can be esterified with sulfuric acid ñ sulfatides.
Gangliosides are the most complex glycolipids. They form a large family of membrane lipids with receptor function.
III. Isoprenoids and steroids
All lipids are derived from acetyl-CoA („activated acetic acid“). The major pathway leads from acetyl-CoA to fatty acids. Their CoA-derivatives are the basic building blocks for fats, phospholipids, glycolipids and other derivatives. The second pathway leads from acetyl-CoA to isopentenyl diphosphate, the basic building block for the isoprenoids and steroids.
Isoprenoids are derived from isoprene (= 2-methyl-1,3-butadiene). From activated isoprene, the main pathway leads to geraniol and farnesol. Farnesol is converted to squalene Õ cholesterol and the steroids.
Some of the isoprenoids have essential roles in metabolism, but cannot be synthetized by animals. This group includes vitamins A, D, E and K. Vitamin D is now usually classified as a steroid hormone.
Steroids are divided into 3 classes: sterols, bile acids, steroid hormones.
A. Sterols are steroid alcohols. The most important sterol in animals is cholesterol. Cholesterol is present in all animal tissues. It is a major constituent of cellular membranes. Cholesterol is esterified with fatty acid to form esters (in lipoproteins). Cholesterol is a normal constituent of the bile.
B. Bile acids are synthetized from cholesterol in the liver. Their structures are derived from cholesterol. Bile acids increase the solubility of cholesterol and promote the digestion of lipids in the intestine.
(i. e. cholic and chenodeoxycholic acid).
C. Steroid hormones are a group of lipophilic signal molecules, which regulate metabolism, growth and reproduction. The steroid hormones of vertebrate animals are progesterone, estradiol, testosterone, aldosterone, cortisol and calcitriol.
Cholesterol biosynthesis (see Fig. 6):
Cholesterol biosynthesis is located in the smooth ER and this pathway can be divided into 4 phases:
1. Formation of mevalonate: 3 acetyl-CoA are converted to 3-hydroxy-3-methyl-glutaryl-CoA
(3-HMG-CoA). The conversion of 3-HMG-CoA to mevalonate is catalyzed by enzyme 3-HMG-CoA reductase (= key regulatory enzyme in cholesterol biosynthesis). Synthesis of the enzyme is inhibited by final products of the pathway (cholesterol). Action of this enzyme is regulated by hormones – insulin and thyroxine stimulate the enzyme and glucagon inhibits it.
2. Formation of isopentenyl diphosphate: The conversion of mevalonate to isopentenyl diphosphate
Formation of squalene:
2 Isopentenyl diphosphate are converted
to geranyl diphosphate (
Formation of cholesterol: Cholesterol (
Reducing agent for these reactions is NADPH + H+.
Fig. 6: Cholesterol biosynthesis
Figure is found on http://web.indstate.edu/thcme/mwking/cholesterol.html
Lipid metabolism and formation of ketone bodies
The liver is the most important site for the formation of FA, fats, ketone bodies and cholesterol. The metabolism of lipids in the liver is closely linked to carbohydrate and amino acid metabolism.
In the well-fed (= absorptive) state, the liver converts Glc via acetyl-CoA into FA. FA are converted into fats and phospholipids.
In the postabsorptive state, especially during prolonged fasting, starvation, or in the case of diabetes mellitus, there is a shift of lipid metabolism. Since Glc and lipids are no longer being supplied in the diet, the organism has to fall back on its own reserves. Under these conditions, the adipose tissue releases fatty acids that are taken up by the liver from the blood. FA are degraded to acetyl-CoA, and finally converted to ketone bodies.
Ketone bodies are acetoacetate, 3-hydroxybutyrate and acetone.
Biosynthesis of ketone bodies (see Fig. 7):
1. When the concentration of acetyl-CoA in the liver mitochondria is high, two molecules condense to form acetoacetyl-CoA.
2. The addition of a futher acetyl group gives 3-hydroxy-3-methyl-glutaryl-CoA, which removes acetyl-CoA to yield acetoacetate.
Acetoacetate can be converted to 3-hydroxybutyrate by reduction or breaks down to acetone by nonenzymatic decarboxylation.
Compounds acetoacetate, 3-hydroxybutyrate and acetone are called ketone bodies. The ketone bodies are released by the liver into the blood, in which they are soluble. Levels of ketone bodies in the blood are elevated during periods of starvation. Acetoacetate and 3-hydroxybutyrate then serve as key metabolites in energy production. Acetone, which has no metabolic significance, is exhaled via lungs. After 1-2 weeks of starvation, the nerve tissue also begins to utilize the ketone bodies as energy sources.
The excess acids (acetoacetate, 3-hydroxybutyrate) in the blood decreases pH ñ ketoacidosis.
Fig. 7: Ketogenesis = formation of ketone bodies in the liver
Figure is found on http://en.wikipedia.org/wiki/Ketogenesis
Lipoproteins are spherical complexes of lipids and proteins. They consist of a core of apolar lipids (= TAG and acyl esters of cholesterol) and a shell made up of phospholipids and apoproteins (Apo A, B, C, E). The shell is polar on its outside, and thus keeps the lipids dissolved in the plasma (see Fig. 8).
Fig. 8: Structure of LDL.
Figure is found on http://www.rpi.edu/dept/bcbp/molbiochem/MBweb
Lipoprotein complexes are divided into 5 different groups according to their increasing density
(see Fig. 9):
● chylomicrons and chylomicrons remnants transport dietary lipids from intestine to tissues. In peripheral vessels – particularly in muscle and adipose tissue enzyme lipoprotein lipase on the surface of the vascular endothelia hydrolyzes most of TAG → chylomicrons remnants → liver.
● VLDL (= very low density lipoproteins) are formed in the liver and can be converted to IDL or LDL.
VLDL transport TAG, cholesterol and phospholipids to other tissues. They are gradually converted into IDL and LDL under the influence of lipoprotein lipase.
● IDL (= intermediate density lipoproteins)
● LDL (= low density lipoproteins) are produced from VLDL and supply the cholesterol to tissues.
● HDL (= high density lipoproteins) transport excess of cholesterol from peripheral tissues back to the liver. Cholesterol is acylated (+ acyl = esterification) by enzyme lecithin cholesterol acyltransferase (LCAT). Cholesterol esters can be transported in the core of the lipoproteins.
Fig. 9: Overview of lipoprotein functions
Figure is found on http://courses.cm.utexas.edu/archive/Fall1997/CH339K/Browning/lec34/lec34.htm
Composition of the blood lipoproteins
The major components of lipoproteins are triacylglycerols, cholesterol, cholesterol esters, phospholipids, and proteins. Purified proteins (apoproteins) are designated A, B, C, and E.
Component Chylomicrons VLDL IDL LDL HDL
Triacylglycerol 85% 55% 26% 10% 8%
Protein 2% 9% 11% 20% 45%
Type B,C,E B,C,E B,E B A,C,E
Cholesterol 1% 7% 8% 10% 5%
Cholesterol ester 2% 10% 30% 35% 15%
Phospholipid 8% 20% 23% 20% 25%
Able to control
What you eat.Certain foods have types of fat that raise your cholesterol level.
· Saturated fat raises your low-density lipoprotein (LDL) cholesterol level more than anything else in your diet
· Trans fatty acids (transfats) are made when vegetable oil is hydrogenated to harden it. Transfatty acids raise cholesterol levels
· Cholesterol is found in foods that come from animal sources, for example, egg yolks, meat, and cheese
Weight. Being overweight tends to increase your LDL level, lower your high-density lipoprotein (HDL) level, and increase your total cholesterol level.
Activity level. Lack of regular exercise can lead to weight gain, which could raise your LDL cholesterol level. Regular exercise can help you lose weight and lower your LDL level. It can also help you raise your HDL level.
Unable to control.
Heredity.High blood cholesterol can run in families. An inherited genetic condition (familial hypercholesterolemia) results in very high LDL cholesterol levels. It begins at birth, and may result in a heart attack at an early age.
Age. Starting at puberty, men have lower levels of HDL than women. As women and men get older, their LDL cholesterol levels rise. Younger women have lower LDL cholesterol levels than men, but after age 55, women have higher levels than men.
A thorough examination of the state of the live blood allows the doctor to consider various nutritional factors that often prove to be the underlying causes of chronic ill-health. Furthermore, the patient's physical response to a recommended course of treatment can be monitored visually. Improvements in the condition of the blood can be seen, sometimes within a few days and usually within weeks. This gives patients tremendous encouragement to continue with their regimen of supplementation and diet.
The importance of obesity, a sedentary lifestyle, very high fat diet, and intake of large concentrations of refined carbohydrates should not be underestimated as causes of severe hypertriglyceridemia. Instituting a program of progressive aerobic and toning exercise, weight loss, and dietary management can significantly lower triglyceride levels and, in some cases, normalize them.
During pregnancy, severe hypertriglyceridemia is an unusual complication and may cause pancreatitis.
· Many case reports have been published describing interventions to manage this condition.
· Most commonly, a very low-fat diet was sufficient to control triglycerides and prevent pancreatitis.
· Intermittent and, in persistent cases, continuous total parenteral nutrition has been used—usually in the third trimester.
To treat hyperlipidemia, a diet low in total fat, saturated fat, and cholesterol is recommended, along with reducing or avoiding alcohol intake. The American Heart Association (AHA) endorses the following dietary recommendations for people with high blood cholesterol:
· Total fat: 25% of total calories
· Saturated fat: less than 7% total calories
· Polyunsaturated fat: up to 10% total calories
· Monounsaturated fat: up to 20% total calories
· Carbohydrates: 50-60% total calories
· Protein: ~15% total calories
· Cholesterol: less than 200 mg/dL
· Soluble fiber such as psyllium: 10- 25g
Categories of appropriate foods include:
· Lean meat/fish: less than 5 oz/day
· Eggs: less than 2 yolks per week (whites unlimited)
· Low fat dairy products (<1% fat): 2-3 servings/day
· Grains, especially whole grains: 6-8 tsp/day
· Vegetables: less than 6 servings per day
· Fruits: 2-5 servings per day
These recommendations translate into the following practical dietary guidelines:
· Select only the leanest meats, poultry, fish and shellfish. Choose chicken and turkey without skin or remove skin before eating. Some fish, like cod, have less saturated fat than either chicken or meat.
· Limit goose and duck. They are high in saturated fat, even with the skin removed.
· Some chicken and turkey hot dogs are lower in saturated fat and total fat than pork and beef hot dogs. There are also lean beef hot dogs and vegetarian (tofu) franks that are low in fat and saturated fat.
· Dry peas, beans and tofu can be used as meat substitutes that are low in saturated fat and cholesterol. Dry peas and beans also have a lot of fiber, which can help to lower blood cholesterol.
· Egg yolks are high in dietary cholesterol. A yolk contains about 213 mg. They should be limited to no more than 2 per week, including the egg yolks in baked goods and processed foods. Egg whites have no cholesterol, and can be substituted for whole eggs in recipes.
· Like high fat meats, regular dairy foods that contain fat, such as whole milk, cheese, and ice cream, are also high in saturated fat and cholesterol. However, dairy products are an important source of nutrients and the diet should include 2 to 3 servings per day of low-fat or nonfat dairy products.
· When shopping for hard cheeses, select them fat-free, reduced fat, or part skim.
· Select frozen desserts that are lower in saturated fat, such as ice milk, low-fat frozen yogurt, low-fat frozen dairy desserts, sorbets, and popsicles.
· Saturated fats should be replaced with unsaturated fats. Select liquid vegetable oils that are high in unsaturated fats, such as canola, corn, olive, peanut, saf-flower, sesame, soybean, and sunflower oils.
· Limit butter, lard, and solid shortenings. They are high in saturated fat and cholesterol.
· Select light or nonfat mayonnaise and salad dressings.
· Fruits and vegetables are very low in saturated fat and total fat, and have no cholesterol. Fruits and vegetables should be eaten as snacks, desserts, salads, side dishes, and main dishes.
· Breads, cereals, rice, pasta, grains, dry beans, and peas are high in starch and fiber and low in saturated fat and calories. They also have no dietary cholesterol, except for some bakery breads and sweet bread products made with high fat, high cholesterol milk, butter and eggs.
· Select whole grain breads and rolls whenever possible. They have more fiber than white breads.
· Most dry cereals are low in fat. Limit high-fat granola, muesli, and cereal products made with coconut oil and nuts, which increases the saturated fat content.
· Limit sweet baked goods that are made with saturated fat from butter, eggs, and whole milk such as croissants, pastries, muffins, biscuits, butter rolls, and doughnuts.
· Snacks such as cheese crackers, and some chips are often high in saturated fat and cholesterol. Select rather low-fat ones such as bagels, bread sticks, cereals without added sugar, frozen grapes or banana slices, dried fruit, non-oil baked tortilla chips, popcorn or pretzels.
Reductions in circulating cholesterol levels can have profound positive impacts on cardiovascular disease, particularly on atherosclerosis, as well as other metabolic disruptions of the vasculature. Control of dietary intake is one of the easiest and least cost intensive means to achieve reductions in cholesterol. Recent studies in laboratory rats has demonstrated an additional benefit of reductions in dietary cholesterol intake. In these animals it was observed that reductions in dietary cholesterol not only resulted in decreased serum VLDLs and LDLs, and increased HDLs but DNA synthesis was also shown to be increased in the thymus and spleen. Upon histological examination of the spleen, thymus and lymph nodes it was found that there was an increased number of immature cells and enhanced mitotic activity indicative of enhanced proliferation. These results suggest that a marked reduction in serum LDLs, induced by reduced cholesterol intake, stimulates enhanced DNA synthesis and cell proliferation.
Drug treatment to lower plasma lipoproteins and/or cholesterol is primarily aimed at reducing the risk of athersclerosis and subsequent coronary artery disease that exists in patients with elevated circulating lipids. Drug therapy usually is considered as an option only if non-pharmacologic interventions (altered diet and exercise) have failed to lower plasma lipids.
Nicotinic acid: Nicotinic acid reduces the plasma levels of both VLDLs and LDLs by inhibiting hepatic VLDL secretion, as well as suppressing the flux of FFA release from adipose tissue by inhibiting lipolysis. In addition, nicotinic administration strongly increases the circulating levels of HDLs. Patient compliance with nicotinic acid administration is sometimes compromised because of the unpleasant side-effect of flushing (strong cutaneous vasodilation). Recent evidence has shown that nicotinic acid binds to and activates the G-protein coupled receptor identified as GPR109A (also called HM74A or PUMA-G). The identity of a receptor to which nicotinic acid binds allows for the development of new drug therapies that activate the same receptor but that may lack the negative side-effect of flushing associated with nicotinic acid. Because of its ability to cause large reductions in circulating levels of cholesterol, nicotinic acid is used to treat Type II, III, IV and V hyperlipoproteinemias.
Pathologies of lipids metabolism:
Obesity is a condition in which the natural energy reserve, stored in the fatty tissue of humans and other mammals, is increased to a point where it is a risk factor for certain health conditions or increased mortality. Obesity develops from the interaction of individual biology and the environment. Excessive body weight has been shown to predispose to various diseases, particularly cardiovascular diseases, diabetes mellitus type 2, sleep apnea, and osteoarthritis. Obesity is both an individual clinical condition and is increasingly viewed as a serious public health problem.
People that are overweight can cause a lot of strain on the heart. The extra weight forces the heart to work harder, making it less effective at pumping out blood to through the arteries. In addition, too much weight can lead to an increase in blood pressure and blood cholesterol, and create a higher risk of diabetes. A carefully regulated diet with a limited amount of fat and alcohol intake along with a regular exercise regimen can help with weight loss.
Fatty degeneration of liver
It is a recognized fact that all-seed diets, which are not only high in fat, but deficient in many essential nutrients, predispose psittacine birds to fatty liver degeneration. It would seem that the prevalence of this condition in all companion birds, including cockatiels, would be decreasing since the advent of pelleted diets, but this does not appear to be happening. Is this because we cockatiel breeders do not recognize this condition when it appears and kills our birds? Is it because we don’t know enough about what causes the disease? Is it because we don’t generally recognize the benefits of pellets? Is it because, in our pursuit of "substance" in show birds, we don’t differentiate between a bird with a large frame and one that is fat?
The main symptom that brought the problem to my attention was a substantial decline in weight, although I did not notice a change in eating habits. As all my birds are kept in large flights when not breeding, I did not notice this until I removed the birds from the flight in order to set them up for breeding. (Foods present in the flight cages are: pellets at all times, seed three times a week, and cooked grain and vegetable "soft food" 3 times a week.) At that time, their weight loss led me to take them to the doctor, where blood tests revealed liver problems. Although I did medicate both birds, and tried very hard to get them to eat a healthier diet in order to possibly reverse the course of this disease, they both died within two months of diagnosis. Necropsy did confirm the cause of death as fatty degeneration and failure of the liver.
According to the lipid hypothesis, abnormally high cholesterol levels (hypercholesterolemia), or, more correctly, higher concentrations of LDL and lower concentrations of functional HDL are strongly associated with cardiovascular disease because these promote atheroma development in arteries (atherosclerosis). This disease process leads to myocardial infarction (heart attack), stroke and peripheral vascular disease. Since higher blood LDL, especially higher LDL particle concentrations and smaller LDL particle size, contribute to this process more than the cholesterol content of the LDL particles, LDL particles are often termed "bad cholesterol" because they have been linked to atheroma formation. On the other hand, high concentrations of functional HDL, which can remove cholesterol from cells and atheroma, offer protection and are sometimes referred to colloquially as "good cholesterol". These balances are mostly genetically determined but can be changed by body build, medications, food choices and other factors.
Conditions with elevated concentrations of oxidized LDL particles, especially "small dense LDL" (sdLDL) particles, are associated with atheroma formation in the walls of arteries, a condition known as atherosclerosis, which is the principal cause of coronary heart disease and other forms of cardiovascular disease.
In contrast, HDL particles (especially large HDL) have been identified as a mechanism by which cholesterol and inflammatory mediators can be removed from atheroma. Increased concentrations of HDL correlate with lower rates of atheroma progressions and even regression.
Elevated levels of the lipoprotein fractions, LDL, IDL and VLDL are regarded as atherogenic (prone to cause atherosclerosis). Levels of these fractions, rather than the total cholesterol level, correlate with the extent and progress of atherosclerosis. Conversely, the total cholesterol can be within normal limits, yet be made up primarily of small LDL and small HDL particles, under which conditions atheroma growth rates would still be high. In contrast, however, if LDL particle number is low (mostly large particles) and a large percentage of the HDL particles are large, then atheroma growth rates are usually low, even negative, for any given total cholesterol concentration. Recently, a post-hoc analysis of the IDEAL and the EPIC prospective studies found an association between high levels of HDL cholesterol (adjusted for apolipoprotein A-I and apolipoprotein B) and increased risk of cardiovascular disease, casting doubt on the cardioprotective role of "good cholesterol".
Abnormally low levels of cholesterol are termed hypocholesterolemia. Research into the causes of this state is relatively limited, but some studies suggest a link with depression, cancer and cerebral hemorrhage. Generally, the low cholesterol levels seem to be a consequence of an underlying illness, rather than a cause.
Some cholesterol derivatives, (among other simple cholesteric lipids) are known to generate the liquid crystalline cholesteric phase. The cholesteric phase is in fact a chiral nematic phase, and changes colour when its temperature changes. Therefore, cholesterol derivatives are commonly used in liquid crystal thermometers and temperature-sensitive paints.
When carbohydrates enter the body, they are broken down into glucose (sugar), which is absorbed into the blood. Upon absorption, the pancreas secretes the hormone insulin, which allows the glucose to be absorbed into the body’s tissues and cells. Diabetes results when the body is unable to produce sufficient amounts of insulin or does not respond to the insulin produced.
As a result, there is a glucose buildup in the body. This buildup can cause an increase in high blood pressure, high levels of LDL cholesterol, and obesity, which all contribute to cardiovascular disease. A controlled diet, regular exercise, and blood glucose testing, as well as oral medication and insulin injections can help patients with diabetes.
Digestion of proteins. General pathways of amino acids transformation.
Proteins are essential nutrients for the human body. They are one of the building blocks of body tissue, and can also serve as a fuel source. As fuel, proteins contain 4 kcal per gram, just like carbohydrates and unlike lipids, which contain 9 kcal per gram.
Proteins are polymer chains made of amino acids linked together by peptide bonds. In nutrition, proteins are broken down in the stomach during digestion by enzymes known as proteases into smaller polypeptides to provide amino acids for the body, including the essential amino acids that cannot be biosynthesized by the body itself.
Amino acids can be divided into three categories: essential amino acids, non-essential amino acids and conditional amino acids. Essential amino acids cannot be made by the body, and must be supplied by food. Non-essential amino acids are made by the body from essential amino acids or in the normal breakdown of proteins. Conditional amino acids are usually not essential, except in times of illness, stress or for someone challenged with a lifelong medical condition.
Essential amino acids are leucine, isoleucine, valine, lysine, threonine, tryptophan, methionine, phenylalanine and histidine. Non-essential amino acids include alanine, asparagine, aspartic acid and glutamic acid. Conditional amino acids include arginine, cysteine, glutamine, glycine, proline, serine, and tyrosine.
Amino acids are found in animal sources such as meats, milk, fish and eggs, as well as in plant sources such as whole grains, pulses, legumes, soy, fruits, nuts and seeds. Vegetarians and vegans can get enough essential amino acids by eating a variety of plant proteins.
Protein functions in body
Protein is a nutrient needed by the human body for growth and maintenance. Aside from water, proteins are the most abundant kind of molecules in the body. Protein can be found in all cells of the body and is the major structural component of all cells in the body, especially muscle. This also includes body organs, hair and skin. Proteins also are utilized in membranes, such as glycoproteins. When broken down into amino acids, they are used as precursors to nucleic acid, co-enzymes, hormones, immune response, cellular repair and molecules essential for life. Finally, protein is needed to form blood cells.
Proteins are very important molecules in our cells. They are involved in virtually all cell functions. Each protein within the body has a specific function. Some proteins are involved in structural support, while others are involved in bodily movement, or in defense against germs. Proteins vary in structure as well as function. They are constructed from a set of 20 amino acids and have distinct three-dimensional shapes. Below is a list of several types of proteins and their functions.
Antibodies - are specialized proteins involved in defending the body from antigens (foreign invaders). They can travel through the blood stream and are utilized by the immune system to identify and defend against bacteria, viruses, and other foreign intruders. One way antibodies counteract antigens is by immobilizing them so that they can be destroyed by white blood cells.
Contractile Proteins - are responsible for movement. Examples include actin and myosin. These proteins are involved in muscle contraction and movement.
Enzymes - are proteins that facilitate biochemical reactions. They are often referred to as catalysts because they speed up chemical reactions. Examples include the enzymes lactase and pepsin. Lactase breaks down the sugar lactose found in milk. Pepsin is a digestive enzyme that works in the stomach to break down proteins in food.
Hormonal Proteins - are messenger proteins which help to coordinate certain bodily activities. Examples include insulin, oxytocin, and somatotropin. Insulin regulates glucose metabolism by controlling the blood-sugar concentration. Oxytocin stimulates contractions in females during childbirth. Somatotropin is a growth hormone that stimulates protein production in muscle cells.
Structural Proteins - are fibrous and stringy and provide support. Examples include keratin, collagen, and elastin. Keratins strengthen protective coverings such as hair, quills, feathers, horns, and beaks. Collagens and elastin provide support for connective tissues such as tendons and ligaments.
Storage Proteins - store amino acids. Examples include ovalbumin and casein. Ovalbumin is found in egg whites and casein is a milk-based protein.
Transport Proteins - are carrier proteins which move molecules from one place to another around the body. Examples include hemoglobin and cytochromes. Hemoglobin transports oxygen through the blood. Cytochromes operate in the electron transport chain as electron carrier proteins.
Protein function in exercise
Proteins are one of the key nutrients for success in terms of sports. Amino acids, the building blocks of proteins, are used for building tissue, including muscle, as well as repairing damaged tissues. Proteins usually only provide a small source of fuel for the exercising muscles, being used as fuel typically only when carbohydrates and lipid resources are low.
Animal sources of protein.
A wide range of foods are a source of protein. The best
combination of protein sources depends on the region of the world, access,
cost, amino acid types and nutrition balance, as well as acquired tastes. Some foods
are high in certain amino acids, but their digestibility and the anti-nutritional factors present in these
foods make them of limited value in human nutrition. Therefore, one must
consider digestibility and secondary nutrition profile such as calories,
cholesterol, vitamins and essential mineral density of the protein source. On a
worldwide basis, plant protein foods contribute over 60 percent of the per capita
supply of protein, on average. In
Meat, eggs and fish are sources of complete protein. Milk and milk-derived foods are also good sources of protein.
Whole grains and cereals are another source of proteins. However, these tend to be limiting in the amino acid lysine or threonine, which are available in other vegetarian sources and meats. Examples of food staples and cereal sources of protein, each with a concentration greater than 7 percent, are (in no particular order) buckwheat, oats, rye, millet, maize (corn), rice, wheat, spaghetti, bulgar, sorghum, amaranth, and quinoa.
Vegetarian sources of proteins include legumes, nuts, seeds and fruits. Legumes, some of which are called pulses in certain parts of the world, have higher concentrations of amino acids and are more complete sources of protein than whole grains and cereals. Examples of vegetarian foods with protein concentrations greater than 7 percent include soybeans, lentils, kidney beans, white beans, mung beans, chickpeas, cowpeas, lima beans, pigeon peas, lupines, wing beans, almonds, Brazil nuts, cashews, pecans, walnuts, cotton seeds, pumpkin seeds, sesame seeds, and sunflower seeds.
Plant sources of protein.
Food staples that are poor sources of protein include
roots and tubers such as yams, cassava
and sweet potato.
Plantains, another major staple, are also a
poor source of essential amino acids. Fruits, while rich in other essential
nutrients, are another poor source of amino acids per
A good source of protein is often a combination of various foods, because different foods are rich in different amino acids. A good source of dietary protein meets two requirements:
· The requirement for the nutritionally indispensable amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) under all conditions and for conditionally indispensable amino acids (cystine, tyrosine, taurine, glycine, arginine, glutamine, proline) under specific physiological and pathological conditions
· The requirement for nonspecific nitrogen for the synthesis of the nutritionally dispensable amino acids (aspartic acid, asparagine, glutamic acid, alanine, serine) and other physiologically important nitrogen-containing compounds such as nucleic acids, creatine, and porphyrins.
Healthy people eating a balanced diet rarely need protein supplements. Except for a few amino acids, most are readily available in human diet. The limiting amino acids are lysine, threonine, tryptophan and sulfur-containing amino acids.
The table below presents the most important food groups as protein sources, from a worldwide perspective. It also lists their respective performance as source of the commonly limiting amino acids, in milligrams of limiting amino acid per gram of total protein in the food source. The green highlighted cells represent the protein source with highest density of respective amino acid, while the yellow highlighted cells represent the protein source with lowest density of respective amino acid. The table reiterates the need for a balanced mix of foods to ensure adequate amino acid source.
Protein milkshakes, made from protein powder (center) and milk (left), are a common bodybuilding supplement.
Protein powders – such as casein, whey, egg, rice and soy – are processed and manufactured sources of protein. These protein powders may provide an additional source of protein for bodybuilders. The type of protein is important in terms of its influence on protein metabolic response and possibly on the muscle's exercise performance. The different physical and/or chemical properties within the various types of protein may affect the rate of protein digestion. As a result, the amino acid availability and the accumulation of tissue protein is altered because of the various protein metabolic responses.
Different proteins have different levels of biological availability (BA) to the human body. Many methods have been introduced to measure protein utilization and retention rates in humans. They include biological value, net protein utilization, and PDCAAS (Protein Digestibility Corrected Amino Acids Score) which was developed by the FDA as an improvement over the Protein efficiency ratio (PER) method. These methods examine which proteins are most efficiently used by the body. The PDCAAS rating is a fairly recent evaluation method; it was adopted by the US Food and Drug Administration (FDA) and the Food and Agricultural Organization of the United Nations/World Health Organization (FAO/WHO) in 1993 as "the preferred 'best'" method to determine protein quality. These organizations have suggested that other methods for evaluating the quality of protein are inferior.
An education campaign launched by the United States Department of Agriculture about 100 years ago, on cottage cheese as a lower-cost protein substitute for meat.
Considerable debate has taken place regarding issues surrounding protein intake requirements. The amount of protein required in a person's diet is determined in large part by overall energy intake, the body's need for nitrogen and essential amino acids, body weight and composition, rate of growth in the individual, physical activity level, individual's energy and carbohydrate intake, as well as the presence of illness or injury. Physical activity and exertion as well as enhanced muscular mass increase the need for protein. Requirements are also greater during childhood for growth and development, during pregnancy or when breast-feeding in order to nourish a baby, or when the body needs to recover from malnutrition or trauma or after an operation.
If enough energy is not taken in through diet, as in the process of starvation, the body will use protein from the muscle mass to meet its energy needs, leading to muscle wasting over time. If the individual does not consume adequate protein in nutrition, then muscle will also waste as more vital cellular processes (e.g. respiration enzymes, blood cells) recycle muscle protein for their own requirements.
According to US & Canadian Dietary Reference Intake guidelines, women
aged 19–70 need to consume
Several studies have concluded that active people and athletes may require elevated protein intake (compared to 0.8 g/kg) due to increase in muscle mass and sweat losses, as well as need for body repair and energy source. Suggested amounts vary between 1.6 g/kg and 1.8 g/kg, while a proposed maximum daily protein intake would be approximately 25% of energy requirements i.e. approximately 2 to 2.5 g/kg. However, many questions still remain to be resolved.
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.
Protein deficiency and malnutrition can lead to variety of ailments including mental retardation and kwashiorkor. Symptoms of kwashiorkor include apathy, diarrhea, inactivity, failure to grow, flaky skin, fatty liver, and edema of the belly and legs. This edema is explained by the action of lipoxygenase on arachidonic acid to form leukotrienes and the normal functioning of proteins in fluid balance and lipoprotein transport.
Although protein energy malnutrition is more common in low-income countries, children from higher-income countries are also affected, including children from large urban areas in low socioeconomic neighborhoods. This may also occur in children with chronic diseases, and children who are institutionalized or hospitalized for a different diagnosis. Risk factors include a primary diagnosis of mental retardation, cystic fibrosis, malignancy, cardiovascular disease, end stage renal disease, oncologic disease, genetic disease, neurological disease, multiple diagnoses, or prolonged hospitalization. In these conditions, the challenging nutritional management may get overlooked and underestimated, resulting in an impairment of the chances for recovery and the worsening of the situation.
Deficiency of protein leads to following:
1. Shortage of protein leads to retardation of growth and in extreme cases failure of growth. This is manifested as marasmus and kwashiorkor among infants and children.
2. Protein deficiency affects the intestinal mucosa and the gland that secret digestive enzymes. This results in the failure to digest and absorb the food, consequently leading to diarrhea and loss of fluid and electrolyte.
3. The normal structure and function of liver is disturbed leading fat accumulation and fatty livers. Liver fails to synthesis plasma albumin thus leading to Oedema.
4. Muscle wasting and anemia due to the shortage of hemoglobin are common feature due to the deficiency of protein.
6. The amino acids presents in the protein help in tissue synthesis during growth period e.g. infancy childhood and adolescence. The body goes into negative N2 -balance due to the shortage of protein in the diet. This results in muscle wastage
7. Proteins from important constituents of hormones. How-ever the deficiency of proteins leads to no marked and characteristic changes in the functioning of endocrine glands.
8. Proteins furnish 10-12per cent of calories required daily. However the major part of proteins is essentially for body-building purposes only.
The nitrogen balance index (NBI) is used to evaluate the amount of protein used by the body in comparison with the amount of protein supplied from daily food intake. The body is in the state of nitrogen (or protein) equilibrium when the intake and usage of protein is equal. The body has a positive nitrogen balance when the intake of protein is greater than that expended by the body. In this case, the body can build and develop new tissue. Since the body does not store protein, the overconsumption of protein can result in the excess amount to be converted into fat and stored as adipose tissue.
Blood urea nitrogen can be used in estimating nitrogen balance.
A positive value is often found during periods of growth, tissue repair or pregnancy.
This means that the intake of nitrogen into the body is greater than the loss of nitrogen from the body, so there is an increase in the total body pool of protein.
The body has a negative nitrogen balance when the intake of protein is less than that expended by the body. In this case, protein intake is less than required, and the body cannot maintain or build new tissues.
A negative nitrogen balance represents a state of protein
deficiency, in which the body is breaking down tissues faster than they are
being replaced. The ingestion of insufficient amounts of protein, or food with
poor protein quality, can result in serious medical conditions in which an
individual's overall health is compromised. The immune system is
severely affected; the amount of blood plasma decreases, leading to medical
conditions such as anemia or edema; and the body becomes vulnerable to infectious
diseases and other serious conditions. Protein malnutrition in infants is
called kwashiorkor, and it poses a major health problem in developing
countries, such as Africa, Central and South America, and certain parts of
A negative value can be associated with burns, fevers, wasting diseases and other serious injuries and during periods of fasting. This means that the amount of nitrogen excreted from the body is greater than the amount of nitrogen ingested.
It can be used in the evaluation of malnutrition.
Essential and nonessential amino acids.
THE TWENTY AMINO ACIDS
Linear structure formula (atom composition and bonding)
SOURCE: Institute for Chemistry
consist of a sufficient and balanced supply of both essential and nonessential amino acids in order to ensure high levels of protein production.
Essential vs. Nonessential Amino Acids
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.
The quality of protein depends on the level at which it provides the nutritional amounts of essential amino acids needed for overall body health, maintenance, and growth. Animal proteins, such as eggs, cheese, milk, meat, and fish, are considered high-quality, or complete, proteins because they provide sufficient amounts of the essential amino acids. Plant proteins, such as grain, corn, nuts, vegetables and fruits, are lower-quality, or incomplete, proteins because many plant proteins lack one or more of the essential amino acids, or because they lack a proper balance of amino acids. Incomplete proteins can, however, be combined to provide all the essential amino acids, though combinations of incomplete proteins must be consumed at the same time, or within a short period of time (within four hours), to obtain the maximum nutritive value from the amino acids. Such combination diets generally yield a high-quality protein meal, providing sufficient amounts and proper balance of the essential amino acids needed by the body to function.
Protein function in the organism.
All enzymes are proteins.
Storing amino acids as nutrients and as building blocks for the growing organism.
Transport function (proteins transport fatty acids, bilirubin, ions, hormones, some drugs etc.).
Proteins are essential elements in contractile and motile systems (actin, myosin).
Protective or defensive function (fibrinogen, antibodies).
Some hormones are proteins (insulin, somatotropin).
Structural function (collagen, elastin).
Nitrogenous balance. Protein standards of nutrition.
The ratio between the amounts of nitrogen entered the organism and nitrogen removed from the organism is called nitrogenous balance. It may be positive, negative and neutral (zero).
Positive nitrogenous balance – the amount of nitrogen entered the organism is more than amount of nitrogen removed from the organism. It occurs in young growing organism, during recovering after severe diseases, at the using of anabolic medicines.
Negative nitrogenous balance – the amount of nitrogen removed from the organism is more than amount of nitrogen entered the organism. It occurs in senile age, destroying of malignant tumor, vast combustions, poisoning by some toxins.
Zero nitrogenous balance – the amount of nitrogen removed from the organism is equal to the amount of nitrogen entered the organism. It occurs in healthy adult people.
The amount of proteins ingested each day should be in the range from 80
Essential and unessential amino acids.
Essential amino acids are those amino acids which are not synthesized in organism and must come with food. Unessential amino acids can be synthesized in organism from another compounds. There are 10 essential amino acids: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
Chemical composition of digestive juices.
Gastric juice contains water, enzymes,
hydrochloric acid, mineral salts and other compounds. About
Intestinal juice contains water, enzymes, bile, mineral salts, some tissue hormones and other compounds.
Proteolytic enzymes and their activation.
Three enzymes are in gastric juice: pepsin, gastricsin and rennin. All these enzymes cleave proteins or peptides.
The main enzyme is pepsin. It is an endopeptidase that cleaves peptide bonds in the interior of protein substrate. Especially actively pepsin splits the peptide bonds between aromatic amino acids as well as the bonds between residues of alanine, serin and cysteine. The optimal pH for pepsin activity – 1,5-2. Thus, hydrochloric acid is necessary for pepsin functioning.
Special cells in the gastric mucosa produce pepsinogen. Pepsinogen is converted to pepsin in the gastric cavity. For this process the ready pepsin and hydrochloric acid are necessary (autocatalysis). The peptide chain is split off the molecule of pepsinogen and rest part gains the enzyme activity. This part is called pepsin.
Optimal pH for gastricsin is 2,0-3,0. The ratio between gastricsin and pepsin in gastric juice is 1:5,5. This ratio can be changed in some pathological states.
Rennin also possesses a proteolytic activity and causes a rapid coagulation of ingested casein. But this enzyme plays important role only in children because the optimal pH for it is 5-6. Such pH is typical for gastric juice of children.
The role of hydrochloric acid in digestion.
Denaturate proteins (denaturated proteins easier undergo digestion by pepsin than native proteins).
Stimulates the activity of pepsin.
Hydrochloric acid has bactericidial properties.
Stimulates the peristalsis.
Regulate the enzyme function of pancreas.
Protein digestion in the small intestine.
Most of the digestion of proteins is carried out in the small intestine by pancreatic enzymes trypsin, chymotrypsin, carboxypeptidases and aminopeptidases.
PH of intestinal juice is 7-8.
Trypsinogen is produced by pancreas. This is not active form of enzyme. Another enzyme enterokinase is secreted from the mucosal cells of the small intestine. Enterokinase split off polipeptade chain from trypsinogen and as result the active trypsin is formed. The optimal pH for trypsin – 7,8. Trypsin acts as endopeptidase and splits about 1/3 of all peptide bonds of protein molecule.
Chymotrypsin is also produced by pancreas in inactive form (chymotrypsinogen). Under the effect of trypsin it undergoes the hydrolysis and active form chymotrypsin is produced. Chymotrypsin like trypsin also is endopeptidase. It makes deeper hydrolysis of protein and splits about 50 % of all peptide bonds basically those bonds that are formed between aromatic amino acids.
Peptides of different size and structure are finally decompoused in small intestine by peptidases. Peptidases splitting the amino acid from the end of free COOH group are called carboxipeptidases and those, which split amino acid residue from the end of free NH2 group are called aminopeptidases.
The splitting of elastin in an intestine is catalyzed by elastase and collagen is decomposed by collagenase.
Digestion of protein takes place not only in the intestinal cavity but also on the surface of mucosa cells. The final hydrolysis takes place by the action of hydrolases of the intestinal mucosa as the peptide comes in contact with the epithelial border or within the mucosal cell.
The kinds of gastric juice acidity. There are four main parameters of gastric juice acidity – (1) free hydrochloric acid; (2) bound hydrochloric acid; (3) total hydrochloric acid;(4) total acidity.
Free hydrochloric acid – HCl that is contained in gastric juice in dissociated state.
Bound hydrochloric acid – HCl bound with proteins in gastric juice.
Total hydrochloric acid - free hydrochloric acid and bound hydrochloric acid together.
Total acidity of gastric juice - total hydrochloric acid and other acidic compounds (phosphate, lactic acid etc.).
THE KINDS OF GASTRIC JUICE ACIDITY.
Gastric acid is a digestive fluid, formed in the stomach. It has a pH of 1.5 to 3.5 and is composed of hydrochloric acid (HCl) (around 0.5%, or 5000 parts per million) as high as 0.1 N, and large quantities of potassium chloride (KCl) and sodium chloride (NaCl). The acid plays a key role in digestion of proteins, by activating digestive enzymes, and making ingested proteins unravel so that digestive enzymes break down the long chains of amino acids.
Gastric acid is produced by cells lining the stomach, which are coupled to systems to increase acid production when needed. Other cells in the stomach produce bicarbonate, a base, to buffer the fluid, ensuring that it does not become too acidic. These cells also produce mucus, which forms a viscous physical barrier to prevent gastric acid from damaging the stomach. Cells in the beginning of the small intestine, or duodenum, further produce large amounts of bicarbonate to completely neutralize any gastric acid that passes further down into the digestive tract.
Gastric acid is produced by parietal cells (also called oxyntic cells) in the stomach. Its secretion is a complex and relatively energetically expensive process. Parietal cells contain an extensive secretory network (called canaliculi) from which the gastric acid is secreted into the lumen of the stomach. These cells are part of epithelial fundic glands in the gastric mucosa. The pH of gastric acid is 1.35 to 3.5  in the human stomach lumen, the acidity being maintained by the proton pump H+/K+ ATPase. The parietal cell releases bicarbonate into the blood stream in the process, which causes a temporary rise of pH in the blood, known as alkaline tide.
The resulting highly acidic environment in the stomach lumen causes proteins from food to lose their characteristic folded structure (or denature). This exposes the protein's peptide bonds. The chief cells of the stomach secrete enzymes for protein breakdown (inactive pepsinogen and rennin). Hydrochloric acid activates pepsinogen into the enzyme pepsin, which then helps digestion by breaking the bonds linking amino acids, a process known as proteolysis. In addition, many microorganisms have their growth inhibited by such an acidic environment, which is helpful to prevent infection.
Your stomach is where the food you eat is broken down into smaller pieces. This action is called digestion.
Stomach (gastric) juice is the term used to describe the chemicals that break down food in the stomach. These include hydrochloric acid and an enzyme called pepsin. Gastric juice is sometimes referred to as stomach acid, although not all of the substances in gastric juice are acidic.
Measurement methods of the gastric juice acidity.
Gastric juice acidity can be measured either using pH-meter or titrating the gastric juice by sodium hydroxide in the presence of special indicators.
The acidity is expressed in titration units (according to quantity of milliliters of 0,1 n NaOH solution used for the titration of 1000 ml of the gastric juice) or in mM/l (1 unit of titration corresponds to 1 mM/l of hydrochloric acid).
The contents of free hydrochloric acid in gastric juice is measured in quantity of ml of 0,1 n NaOH used for the neutralization of 1000 ml of gastric juice in the presence of indicator p-dimethylaminoazobenzol. The normal free hydrochloric acid level is 20-40 mM/l.
The total acidity is measured in quantity of ml of 0,1 n NaOH used for the neutralization of 1000 ml of the gastric juice in phenolphthalein presence. The normal total acidity - 40-60 mM/l.
Hypoaciditas, hyperaciditas, anaciditas, hypochlorhydria, hyperchlorhydria, achlorhydria, achilia.
Hypoaciditas – the decrease of total acidity of gastric juice. Such state occurs in the chronic atrophic gastritis.
Hyperaciditas – the increase of total acidity of gastric juice. It can be observed in ulcer disease, hyperacidic gastritis.
Anaciditas – absence of acidity of gastric juice. Such state occurs in chronic atrophic gastritis and cancer of stomach.
Achilia - absence of acidity and pepsin in gastric juice. Cancer of stomach can causes this state.
Hypochlorhydria – the decrease of hydrochloric acid content in gastric juice. Very often hypochlorhydria occurs in chronic atrophic gastritis.
Hyperchlorhydria - the increase of hydrochloric acid content in gastric juice. It can be observed in ulcer disease, hyperacidic gastritis.
Pathological components of gastric juice and their determination.
Pathological components of gastric juice can be blood, lactic acid and bile.
Blood in gastric juice ocurs in stomach cancer, ulcer of stomach mucosa, bleeding from veins of esophagus, swallowing of blood.
Lactic acid in gastric juice occurs in sharp depression of gastric juice acidity. It can be observed in chronic atrophic gastritis and cancer of stomach.
Bile can be present in gastric juice in duodeno-gastric reflux.
Mechanism of amino acid absorbtion.
Most proteins are absorbed in the form of amino acids. However, small quantities of dipeptides and even tripeptides are also absorbed, and extreme minute quantities of whole proteins can be absorbed by the process of pinocytosis.
Amino acid transport either ceases or is greatly reduced wherever active sodium transport is blocked. Therefore, it is assumed that the energy required for transport of amino acids is actually provided by the sodium transport system. A theory that attempts to explain this is the following: it is known that the carrier protein for transport of amino acids is present in the brush border of the epithelial cell. However, this carrier will not transport the amino acids in the absence of sodium transport. Therefore, it is believed that the carrier protein has receptor sites for both amino acid and a sodium ion, and that it will not transport either of these to the interior of the epithelial cell until both receptor sites are simultaneously filled. The energy to cause movement of the carrier from the exterior of the membrane to the interior is derived from the differences in sodium concentration between the outside am inside. That is, as sodium diffuses to the inside of the cell it "drags" the amino acid along with it, thus providing the energy for transport of the amino acid. For obvious reasons, this explanation is called the sodium cotransport theory for amino acid transport; it is also called secondary active transport of amino acid.
Absorption of amino acids through the intestine mucosa can occur far more rapidly than protein can be digested in the lumen of the intestine. As a result the normal rate of absorption is determined not by the rate at which they can be absorbed but by the rate at which they can be released from the proteins during digestion. Thus, no free amino acids can be found in the intestine during digestion – that is, they are absorbed as rapidly as they are formed.
Since most protein digestion occurs in the upper small intestine most protein absorption occurs in the duodenun and jejunum.
The ways of entry and using of amino acids in tissue.
The sources of amino acids: 1) absorption in the intestine;
2) formation during the protein decomposition;
3) synthesis from the carbohydrates and lipids.
Using of amino acids: 1) for protein synthesis;
2) for synthesis of nitrogen containing compounds (creatine, purines, choline, pyrimidine);
3) as the source of energy (oxidation – deamination, transamination, decarboxilation);
4) for the gluconeogenesis;
5) for the formation of biologically active compounds.
Deamination of amino acids.
Deamination - eliminating of amino group from amino acid with ammonia formation. There are four types of deamination: oxidative, reduction, hydrolitic and intramolecular.
Oxidative deamination. Some amino acids, for example glutamate, may undergo oxidative deamination catalyzed by the pyridine-linked dehydrogenase, which is present in both the cytosol and mitochondria of the liver.
Glutamate a-iminoglutaric acid a-ketoglutarate
It is thought that the first step in the glutamate dehydrogenase reaction is the formation of a-iminoglutarate by dehydrogenation, followed by hydrolytic decomposition of the imino acid to the keto acid.
The NADH formed is ultimately oxidized by the electron-transport chain. L-Glutamate dehydrogenase plays a central role in amino acid deamination because in most organisms glutamate is the only amino acid that has such an active dehydrogenase.
Many organisms contain flavin-linked amino acid oxidases, which also catalyze oxidative deamination of amino acids, but they play a relatively minor role. One, L-amino acid oxidase, is specific for deamination of L-amino acids and promotes the reaction:
L-Amino acid + H2O + E—FMN ® a-keto acid + NH3 + E—FMNH2
L-Amino acid oxidase contains tightly bound FMN as the prosthetic group and is present in the endoplasmic reticulum of the liver and kidneys.
The other flavoenzyme functioning in oxidative deamination is D-amino acid oxidase, present in the liver and kidneys, which catalyzes the oxidation of D-amino acids:
D-Amino acid + H2O + E—FAD ® a-keto acid + NH3 + E—FADH2
D-Amino acid oxidase contains FAD as prosthetic group.
R-CH(NH2)-COOH + NADH2 ® R-CH2-COOH + NH3 + NAD+
Amino acid fatty acid
R-CH(NH2)-COOH + H2O ® R-CH(OH)-COOH + NH3
Amino acid oxiacid
R-CH2-CH(NH2)-COOH ® R-CH-CH-COOH + NH3
Amino acid unsaturated fatty acid
The role of vitamins in deamination of amino acids.
Vitamin B5 (NAD, dehydrogenase) and B2 (FAD and FMN, amino acid oxidase) take part in the deamination processes.
The urea cycle
Urea formation, which takes place in the liver of ureotelic organisms, is brought about by the urea cycle, a cyclic pathway first postulated by H.A.Krebs.
The first amino group entering the urea cycle arises as free ammonia following the oxidative deamination of glutamate in liver mitochondria.
The free ammonia so formed is then utilized, together with carbon dioxide, to form carbamoyl phosphate, a very unstable compound, in a complex reaction catalyzed by carbamoyl phosphate synthetase (ammonia), present in the mitochondrial matrix:
Two molecules of ATP are required to form each molecule of carbamoyl phosphate in this reaction, which is essentially irreversible. The formation of carbamoyl phosphate by this pathway in the mitochondria is specialized for urea synthesis.
The carbamoyl phosphate generated in the mitochondria now donates its carbamoyl group to ornithine, which is formed in the cytosol but enters the mitochondrion via a specific inner-membrane transport system. The product is citruiline:
This reaction is catalyzed by ornithine carbamoyltransferase of the mitochondrial matrix. The citrulline formed now leaves the mitochondrial matrix and passes to the cytosol, where the remaining reactions of the urea cycle take place.
The second amino group required for urea synthesis now arrives in the form of aspartate, which in turn acquired it from glutamate by the action of aspartate transaminase in the cytosol. The amino group of aspartate condenses reversibly with the carbamoyl carbon atom of citrulline in the presence of ATP to form argininosuccinate; this reaction is catalyzed by argininosuccinate synthetase:
In the next reaction argininosuccinate undergoes a elimination reaction by the action of argininosuccinate lyase to form free arginine and fumarate:
The arginine formed in this reaction becomes the immediate precursor of urea, whereas the fumarate returns to the pool of tricarboxylic acid cycle intermediates.
Up to this point the reaction sequence is that employed by all organisms capable of the biosynthesis of arginine. However, only ureotelic animals possess large amounts of arginase, which cleaves urea from arginine and regenerates ornithine, a reaction taking place in the cytosol:
25-30 g/day of urea is excreted in normal conditions.
The increase of urea in urine occurs in high fever, malignant anemia, poisoning by phosphorus, intensive decomposition of protein in organism. The decrease of urea in urine occurs in liver diseases, kidney unsufficiency, acidosis.
Urea Cycle Defects (UCDs)
A complete lack of any one of the enzymes of the urea cycle will result in death shortly after birth. However, deficiencies in each of the enzymes of the urea cycle, including N-acetylglutamate synthase, have been identified. These disorders are referred to as urea cycle disorders or UCDs. A common thread to most UCDs is hyperammonemia leading to ammonia intoxication with the consequences described below. Deficiencies in arginase do not lead to symptomatic hyperammonemia as severe or as commonly as in the other UCDs.
Clinical symptoms are most severe when the UCD is at the level of carbamoyl phosphate synthetase I. Symptoms of UCDs usually arise at birth and encompass, ataxia, convulsions, lethargy, poor feeding and eventually coma and death if not recognized and treated properly. In fact, the mortality rate is 100% for UCDs that are left undiagnosed. Several UCDs manifest with late-onset such as in adulthood. In these cases the symptoms are hyperactivity, hepatomegaly and an avoidance of high protein foods.
In general, the treatment of UCDs has as common elements the reduction of protein in the diet, removal of excess ammonia and replacement of intermediates missing from the urea cycle. Administration of levulose reduces ammonia through its action of acidifying the colon. Bacteria metabolize levulose to acidic byproducts which then promotes excretion of ammonia in the feces as ammonium ions, NH4+. Antibiotics can be administered to kill intestinal ammonia producing bacteria. Sodium benzoate and sodium phenylbutyrate can be administered to covalently bind glycine (forming hippurate) and glutamine (forming phenylacetylglutamine), respectively. These latter compounds, which contain the ammonia nitrogen, are excreted in the feces. Dietary supplementation with arginine or citrulline can increase the rate of urea production in certain UCDs.
Table of Urea Cycle Defects
Type I Hyperammonemia
Carbamoylphosphate synthetase I
with 24h - 72h after birth infant becomes lethargic, needs stimulation to feed, vomiting, increasing lethargy, hypothermia and hyperventilation; without measurement of serum ammonia levels and appropriate intervention infant will die: treament with arginine which activates N-acetylglutamate synthetase
N-acetylglutamate synthetase Deficiency
severe hyperammonemia, mild hyperammonemia associated with deep coma, acidosis, recurrent diarrhea, ataxia, hypoglycemia, hyperornithinemia: treatment includes administration of carbamoyl glutamate to activate CPS I
Type 2 Hyperammonemia
most commonly occurring UCD, only X-linked UCD, ammonia and amino acids elevated in serum, increased serum orotic acid due to mitochondrial carbamoylphosphate entering cytosol and being incorporated into pyrimidine nucleotides which leads to excess production and consequently excess catabolic products: treat with high carbohydrate, low protein diet, ammonia detoxification with sodium phenylacetate or sodium benzoate
episodic hyperammonemia, vomiting, lethargy, ataxia, siezures, eventual coma: treat with arginine administration to enhance citrulline excretion, also with sodium benzoate for ammonia detoxification
Argininosuccinate lyase (argininosuccinase)
episodic symptoms similar to classic citrullinemia, elevated plasma and cerebral spinal fluid argininosuccinate: treat with arginine and sodium benzoate
rare UCD, progressive spastic quadriplegia and mental retardation, ammonia and arginine high in cerebral spinal fluid and serum, arginine, lysine and ornithine high in urine: treatment includes diet of essential amino acids excluding arginine, low protein diet
Earlier it was noted that ammonia was neurotoxic. Marked brain damage is seen in cases of failure to make urea via the urea cycle or to eliminate urea through the kidneys. The result of either of these events is a buildup of circulating levels of ammonium ion. Aside from its effect on blood pH, ammonia readily traverses the brain blood barrier and in the brain is converted to glutamate via glutamate dehydrogenase, depleting the brain of α-ketoglutarate. As the α-ketoglutarate is depleted, oxaloacetate falls correspondingly, and ultimately TCA cycle activity comes to a halt. In the absence of aerobic oxidative phosphorylation and TCA cycle activity, irreparable cell damage and neural cell death ensue. In addition, the increased glutamate leads to glutamine formation. This depletes glutamate stores which are needed in neural tissue since glutamate is both a neurotransmitter and a precursor for the synthesis of α -aminobutyrate, GABA, another neurotransmitter. Therefore, reductions in brain glutamate affect energy production as well as neurotransmission.
All tissues have some capability for synthesis of the non-essential amino acids, amino acid remodeling, and conversion of non-amino acid carbon skeletons into amino acids and other derivatives that contain nitrogen. However, the liver is the major site of nitrogen metabolism in the body. In times of dietary surplus, the potentially toxic nitrogen of amino acids is eliminated via transaminations, deamination, and urea formation; the carbon skeletons are generally conserved as carbohydrate, via gluconeogenesis, or as fatty acid via fatty acid synthesis pathways. In this respect amino acids fall into three categories: glucogenic, ketogenic, or glucogenic and ketogenic. Glucogenic amino acids are those that give rise to a net production of pyruvate or TCA cycle intermediates, such as α-ketoglutarate or oxaloacetate, all of which are precursors to glucose via gluconeogenesis. All amino acids except lysine and leucine are at least partly glucogenic. Lysine and leucine are the only amino acids that are solely ketogenic, giving rise only to acetylCoA or acetoacetylCoA, neither of which can bring about net glucose production.
A small group of amino acids comprised of isoleucine, phenylalanine, threonine, tryptophan, and tyrosine give rise to both glucose and fatty acid precursors and are thus characterized as being glucogenic and ketogenic. Finally, it should be recognized that amino acids have a third possible fate. During times of starvation the reduced carbon skeleton is used for energy production, with the result that it is oxidized to CO2 and H2O.
Diagnostic role of determination of AlAT and AsAT
Aspartate transaminase (AST), also called aspartate aminotransferase (AspAT/ASAT/AAT) or serum glutamic oxaloacetic transaminase (SGOT), is a pyridoxal phosphate (PLP)-dependent transaminase enzyme. AST catalyzes the reversible transfer of an α-amino group between aspartate and glutamate and, as such, is an important enzyme in amino acid metabolism. AST is found in the liver, heart, skeletal muscle, kidneys, brain, and red blood cells, and it is commonly measured clinically as a marker for liver health.
Aspartate (Asp) + α-ketoglutarate ↔ oxaloacetate + glutamate (Glu)
Reaction catalyzed by aspartate aminotransferase
As a prototypical transaminase, AST relies on PLP as a cofactor to transfer the amino group from aspartate or glutamate to the corresponding ketoacid. In the process, the cofactor shuttles between PLP and the pyridoxamine phosphate (PMP) form. The amino group transfer catalyzed by this enzyme is crucial in both amino acid degradation and biosynthesis. In amino acid degradation, following the conversion of α-ketoglutarate to glutamate, glutamate subsequently undergoes oxidative deamination to form ammonium ions, which are excreted as urea. In the reverse reaction, aspartate may be synthesized from oxaloacetate, which is a key intermediate in the citric acid cycle.
Two isoenzymes are present in a wide variety of eukaryotes. In humans:
These isoenzymes are thought to have evolved from a common ancestral AST via gene duplication, and they share a sequence homology of approximately 45%.
AST has also been found in a number of microorganisms, including E. coli, H. mediterranei, and T. thermophilus. In E. coli, the enzyme is encoded by the aspCgene and has also been shown to exhibit the activity of an aromatic-amino-acid transaminase.
AST is similar to alanine transaminase (ALT) in that both enzymes are associated with liver parenchymal cells. The difference is that ALT is found predominantly in the liver, with clinically negligible quantities found in the kidneys, heart, and skeletal muscle, while AST is found in the liver, heart (cardiac muscle), skeletal muscle, kidneys, brain, and red blood cells. As a result, ALT is a more specific indicator of liver inflammation than AST, as AST may be elevated also in diseases affecting other organs, such as myocardial infarction, acute pancreatitis, acute hemolytic anemia, severe burns, acute renal disease, musculoskeletal diseases, and trauma.
AST was defined as a biochemical marker for the diagnosis of acute myocardial infarction in 1954. However, the use of AST for such a diagnosis is now redundant and has been superseded by the cardiac troponins.
AST (SGOT) is commonly measured clinically as a part of diagnostic liver function tests, to determine liver health.
6 - 34 IU/L
8 - 40 IU/L
Alanine transaminase or ALT is a transaminase It is also called serum glutamic pyruvic transaminase (SGPT) or alanine aminotransferase (ALAT).
ALT (and all transaminases) require the coenzyme pyridoxal phosphate, which is converted into pyridoxamine in the first phase of the reaction, when an amino acid is converted into a keto acid.
It is commonly measured clinically as a part of a diagnostic evaluation of hepatocellular injury, to determine liver health. When used in diagnostics, it is almost always measured in international units/liter (U/L). While sources vary on specific normal range values for patients, 10-40 U/L is the standard normal range for experimental studies. Alanine transaminase shows a marked diurnal variation.
Significantly elevated levels of ALT(SGPT) often suggest the existence of other medical problems such as viral hepatitis, diabetes, congestive heart failure, liver damage, bile duct problems, infectious mononucleosis, or myopathy. For this reason, ALT is commonly used as a way of screening for liver problems. Elevated ALT may also be caused by dietary choline deficiency. However, elevated levels of ALT do not automatically mean that medical problems exist. Fluctuation of ALT levels is normal over the course of the day, and ALT levels can also increase in response to strenuous physical exercise.
When elevated ALT levels are found in the blood, the possible underlying causes can be further narrowed down by measuring other enzymes. For example, elevated ALT levels due to liver-cell damage can be distinguished from biliary duct problems by measuring alkaline phosphatase. Also, myopathy-related ALT levels can be ruled out by measuring creatine kinase enzymes. Many drugs may elevate ALT levels, including Zileuton, omega-3-acid ethyl esters (Lovaza), anti-inflammatory drugs, antibiotics, cholesterol medications, some antipscyhotics such as Risperidone, and anti-convulsants.
For years, the American Red Cross used ALT testing as part of
the battery of tests to ensure the safety of its blood supply by deferring
donors with elevated ALT levels. The intent was to identify donors potentially
infected with Hepatitis C because there was no specific test for that disease
at the time. Prior to July 1992, widespread blood donation testing in the
There are 20 standard amino acids in proteins, with a variety of carbon skeletons. Correspondingly, there are 20 different catabolic pathways for amino acid degradation. In humans, these pathways taken together normally account for only 10 to 15% of the body's energy production. Therefore, the individual amino acid degradative pathways are not nearly as active as glycolysis and fatty acid oxidation. In addition, the activity of the catabolic pathways can vary greatly from one amino acid to the next, depending upon the balance between requirements for biosynthetic processes and the amounts of a given amino acid available. For this reason, we shall not examine them all in detail. The 20 catabolic pathways converge to form only five products, all of which enter the citric acid cycle. From here the carbons can be diverted to gluconeogenesis or ketogenesis, or they can be completely oxidized to CO2 and H2O.
All or part of the carbon skeletons of ten of the amino acids are ultimately broken down to yield acetyl-CoA. Five amino acids are converted into α-ketoglutarate, four into succinyl-CoA, two into fumarate, and two into oxaloacetate. The individual pathways for the 20 amino acids will be summarized by means of flow diagrams, each leading to a specific point of entry into the citric acid cycle. In these diagrams the amino acid carbon atoms that enter the citric acid cycle are shown in color. Note that some amino acids appear more than once, reflecting the fact that different parts of their carbon skeletons have different fates. Some of the enzymatic reactions in these pathways that are particularly noteworthy for their mechanisms or their medical significance will be singled out for special discussion.
Several Enzyme Cofactors Play Important Roles in Amino Acid Catabolism
A variety of interesting chemical rearrangements are found among the amino acid catabolic pathways. Before examining the pathways themselves, it is useful to note classes of reactions that recur and to introduce the enzymatic cofactors required. We have already considered one important class, the transamination reactions requiring pyridoxal phosphate. Another common type of reaction in amino acid catabolism is a one-carbon transfer. One-carbon transfers usually involve one of three cofactors: biotin, tetrahydrofolate, or S-adenosylmethionine.
These cofactors are used to transfer one-carbon groups in different oxidation states. The most oxidized state of carbon, CO2, is transferred by biotin. The remaining two cofactors are especially important in amino acid and nucleotide metabolism.
Tetrahydrofolate is generally involved in transfers of one-carbon groups in the intermediate oxidation states, and S-adenosylmethionine in transfers of methyl groups, the most reduced state of carbon.
Tetrahydrofolate (H4 folate) consists of a substituted pteridine, p-aminobenzoate, and glutamate linked together. This cofactor is synthesized in bacteria and its precursor, folate, is a vitamin for mammals. The one-carbon group, in any of three oxidation states, is bonded to N-5 or N-10 or to both. The most reduced form of the cofactor carries a methyl group, a more oxidized form carries a methylene group, and the most oxidized forms carry a methenyl, formyl, or formimino group. The different forms of tetrahydrofolate are interconvertible and serve as donors of one-carbon units in a variety of biosynthetic reactions. The major source of one-carbon units for tetrahydrofolate is the carbon removed in the conversion of serine to glycine, producing N5,N10-methylenetetrahydrofolate.
Although tetrahydrofolate can carry a methyl group at N-5, the methyl group's transfer potential is insufficient for most biosynthetic reactions. S-Adenosylmethionine is more commonly used for methyl group transfers. It is synthesized from ATP and methionine by the action of methionine adenosyl transferase. This reaction is unusual in that the nucleophilic sulfur atom of methionine attacks at the 5' carbon of the ribose moiety of ATP, releasing triphosphate, rather than attacking at one of the phosphorus atoms. The triphosphate is cleaved to Pi and PPi on the enzyme, and the PPi is later cleaved by inorganic pyrophosphatase, so that three bonds, two of which are high-energy bonds, are broken in this reaction. The only other reaction known in which triphosphate is displaced from ATP occurs in the synthesis of coenzyme B12.
S-Adenosylmethionine is a potent alkylating agent by virtue of its destabilizing sulfonium ion. The methyl group is subject to attack by nucleophiles and is about 1,000 times more reactive than the methyl group of N5-methyltetrahydrofolate.
Transfer of a methyl group from S-adenosylmethionine to an acceptor yields S-adenosylhomocysteine, which is subsequently broken down to homocysteine and adenosine. Methionine is regenerated by the transfer of a methyl group to homocysteine in a reaction catalyzed by methionine synthase. One form of this enzyme is common in bacteria and uses N5-methyltetrahydrofolate as a methyl donor. Another form that occurs in bacteria and mammals uses methylcobalamin derived from coenzyme B12. This reaction and the rearrangement of L-methylmalonyl-CoA to succinyl-CoA are the only coenzyme Bl2-dependent reactions known in mammals. Methionine is reconverted to S-adenosylmethionine to complete an activated methyl cycle.
Tetrahydrobiopterin is another cofactor introduced in these pathways, but it is not involved in one-carbon transfers. Tetrahydrobiopterin is structurally related to the flavin coenzymes, and it participates in biological oxidation reactions. It belongs to a widespread class of biological compounds called pterins, and we will consider its mode of action when we discuss phenylalanine degradation.
The carbon skeletons of ten amino acids yield acetyl-CoA, which enters the citric acid cycle directly. Five of the ten are degraded to acetyl-CoA via pyruvate. The other five are converted into acetyl-CoA and/or acetoacetyl-CoA, which is then cleaved to form acetyl-CoA.
The five amino acids entering via pyruvate are alanine, glycine, serine, cysteine, and tryptophan. In some organisms threonine is also degraded to form acetyl-CoA, in humans it is degraded to succinyl-CoA, as described later. Alanine yields pyruvate directly on transamination with a-ketoglutarate, and the side chain of tryptophan is cleaved to yield alanine and thus pyruvate. Cysteine is converted to pyruvate in two steps, one to remove the sulfur atom, the other a transamination. Serine is converted to pyruvate by serine dehydratase. Both the β-hydroxyl and the α-amino groups of serine are removed in this single PLP-dependent reaction. Glycine has two pathways. It can be converted into serine by enzymatic addition of a hydroxymethyl group. This reaction, catalyzed by serine hydroxymethyl transferase, requires the coenzymes tetrahydrofolate and pyridoxal phosphate. The second pathway for glycine, which predominates in animals, involves its oxidative cleavage into CO2, NH4+ , and a methylene group (-CH2-). This readily reversible reaction, catalyzed by glycine synthase, also requires tetrahydrofolate, which accepts the methylene group. In this oxidative cleavage pathway the two carbon atoms of glycine do not enter the citric acid cycle. One is lost as CO2, and the other becomes the methylene group of N5,N10- methylene- tetrahydrofolate, which is used as a one-carbon group donor in certain biosynthetic pathways.
Portions of the carbon skeleton of six amino acids-tryptophan, lysine, phenylalanine, tyrosine, leucine, and isoleucine-yield acetyl-CoA and/or acetoacetyl-CoA; the latter is then converted into acetyl-CoA. Some of the final steps in the degradative pathways for leucine, lysine, and tryptophan resemble steps in the oxidation of fatty acids. The breakdown of two of these six amino acids deserves special mention.
The catabolism of arginine begins within the context of the urea cycle. It is hydrolyzed to urea and ornithine by arginase.
The Arginine Metabolic Pathways: Nitric Oxide Synthase and Arginase
Two important metabolic pathways use the amino acid arginine as the precursor: the enzyme nitric oxide synthase, which converts arginine to nitric oxide, and citrulline and the enzyme arginase, which converts arginine to ornithine and urea. The latter is part of a pathway for detoxifying ammonia. Ornithine is also part of a proliferative pathway that is involved in cell division and tissue regeneration. Arginase II is the form of arginase that is thought to be involved in the synthesis of polyamines, which control cell proliferation and collagen production. It is most highly expressed in the prostate and kidney.1
There has been considerable recent publication of papers on nitric oxide synthase because of the importance of nitric oxide in functions such as (importantly) vasodilation (endothelial function). Scientists have found that an inadequate supply of arginine or too little of the cofactor tetrahydrobiopterin (which one paper reports may be mimicked by folic acid2) results in an “uncoupling” of nitric oxide synthase from the production of nitric oxide, producing superoxide anion instead. Not only is there a reduction in the production of nitric oxide when nitric oxide synthase is uncoupled, but oxidative stress is greatly increased.
Now, another major mechanism of decreased production of nitric oxide has been reported: an increase in the arginase pathway for the use of arginine. Recent studies have reported increases in arginase in conditions including reperfusion injury, asthma, psoriasis, arthritis, and human breast cancer. (Since arginase II is highly expressed in the prostate, it would be interesting to see whether there is increased expression in prostate cancer.) The increased arginase decreases arginine availability to be converted to nitric oxide, as well as increasing ornithine that can be converted into polyamines, procellular proliferation factors. In psoriasis, for example, there is hyperproliferation of keratinocytes. In the arthritis paper, it was reported that arginase II could be induced ex vivo (outside the body) by inflammatory factors such as PGE2 and LPS (lipopolysaccharide, from bacteria). Ornithine, produced by arginase, is necessary for the production of collagen, which occurs in rheumatoid arthritis.
Ornithine, in excess of urea cycle needs, is transaminated to form glutamate semialdehyde.
Nitric Oxide Synthases (NOS)
Is an important cellular-signaling molecule, a potent vasodilatator due to the smooth muscle relaxation. It also inhibits platelet adherence and aggregation, reduces adherence of leukocytes to the endothelium. Furthermore, NO has been shown to inhibit DNA synthesis and mitogenesis, and the proliferation of vascular smooth muscle cells. These antiproliferative effects are likely to be mediated by cyclic GMP.
Nitric Oxide Synthases from the biochemical point of view, are a family of complex enzymes catalyzing the oxidation of L-arginine to form NO and L-citrulline. The three human NOS isoforms identified to date are: eNOS (endothelial NOS), nNOS (neuronal NOS), and iNOS (inducible NOS). Their genes are found on human chromosomes 7, 12, and 17, respectively, and so they were named for the tissue in which they were first cloned and characterized. vasculoprotective effect of individual NOS isoforms in human organism is not sufficiently clarified yet. Endothelial NOS (eNOS) and neuronal NOS (nNOS) are constitutively expressed, mainly in endothelial cells and nitrergic nerves, respectively, synthesizing a small amount of NO under basal conditions and on stimulation by various agonists. By contrast, inducible NOS (iNOS) is expressed when stimulated by inflammatory stimuli, synthesizing a large amount of NO in a transient manner. The knowledge of nitric oxide synthases (NOSs) is of extreme scientific importance, not only for understanding new pathophysiological mechanisms but also as a target for therapeutic intervention.
The role of NO in regulating vascular tone and mediating platelet function is attributable to the ongoing activity of eNOS. It is pharmacologically identical with previously isolated EDRF (endothelium-derived releasing factor), exprimed by the intact endothelium. Inactivation of the eNOS pathway limits the contribution of NO to vessel homeostasis and results in increased vascular tone and platelet adhesion and aggregation. The activity of eNOS is regulated by the intracellular free calcium concentration and calcium- calmodulin complexes. Endothelial NOS is a constitutively expressed protein predominantly associated with the particulate subcellular fraction, suggesting that the native enzyme is a membrane-bound protein. A detailed analysis of the membrane association of eNOS showed that this enzyme is localized to the Golgi apparatus as well as to specific structures in the plasmalemmal membrane called caveolae. The association of eNOS with a region of the plasma membrane in which several key signal-transducing complexes are concentrated (such as G-proteins) is likely to have profound repercussions on enzyme activity as well as on its accessibility to intracellular mechanisms of the pathway release, including mechanisms independent of intracellular calcium release.
Neuronal Constitutive Nitric Oxide Synthase (nNOS) is present in central and peripheral neuronal cells and certain epithelial cells. Its activity is also regulated by Ca2+ and calmodulin. Its functions include long-term regulation of synaptic transmission in the central nervous system, central regulation of blood pressure, smooth muscle relaxation, and vasodilation via peripheral nitrergic nerves. It has also been implicated in neuronal death in cerebrovascular stroke. NO plays also an important role in the pathophysiology of some neurodegenerative diseases. The presence of NO and NOS should be proved indirectly through the histochemic positivity of nicotinamide dinucleotide phosphate diaphorase (NADPHd). It was proposed that nerve stimulation directly activated the release of NO from nitrergic nerves and, in fact, NO appears to be the dominant neurotransmitter responsible for the nerve-mediated, endothelium-independent vasodilation.
Glycine is classified as a glucogenic amino acid, since it can be converted to serine by serine hydroxymethyltransferase, and serine can be converted back to the glycolytic intermediate, 3-phosphoglycerate or to pyruvate by serine/threonine dehydratase.
Nevertheless, the main glycine catabolic pathway leads to the production of CO2, ammonia, and one equivalent of N5,N10-methyleneTHF by the mitochondrial glycine cleavage complex.
Hyperglycinemia refers to a condition where glycine is elevated in the blood.
- Propionic acidemia, also known as "ketotic glycinemia"
- Glycine encephalopathy, also known as "non-ketotic hyperglycinemia".
Glycine encephalopathy (also known as non-ketotic hyperglycinemia or NKH) is a rare autosomal recessive disorder of glycine metabolism. After phenylketonuria, glycine encephalopathy is the second most common disorder of amino acid metabolism. The disease is caused by defects in the glycine cleavage system, an enzyme responsible for glycine catabolism. There are several forms of the disease, with varying severity of symptoms and time of onset. The symptoms are exclusively neurological in nature, and clinically this disorder is characterized by abnormally high levels of the amino acid glycine in bodily fluids and tissues, especially the cerebral spinal fluid.
Glycine encephalopathy is sometimes referred to as "nonketotic hyperglycinemia" (NKH), as a reference to the biochemical findings seen in patients with the disorder, and to distinguish it from the disorders that cause "ketotic hyperglycinemia" (seen in propionic acidemia and several other inherited metabolic disorders). To avoid confusion, the term "glycine encephalopathy" is often used, as this term more accurately describes the clinical symptoms of the disorder. Glycine is metabolized in the body to end products of carbon dioxide and ammonia. The glycine cleavage system, which is responsible for glycine metabolism in the mitochondrion is made up of four protein subunits.
Propionic acidemia, also known as propionic aciduria, propionyl-CoA carboxylase deficiency and ketotic glycinemia, is an autosomal recessive metabolic disorder, classified as a branched-chain organic acidemia.
The disorder presents in the early neonatal period with progressive encephalopathy. Death can occur quickly, due to secondary hyperammonemia, infection, cardiomyopathy, or basal ganglial stroke.
Propionic Acidemia is a rare disorder that is inherited from both
parents. Being autosomal recessive, neither parent shows symptoms, but both
carry a defective gene responsible for this disease. It takes two faulty genes
to cause PA, so there is a
Glutamine/Glutamate and Asparagine/Aspartate 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.
The conversion of serine to glycine and then glycine oxidation to CO2 and NH3, with the production of two equivalents of N5,N10-methyleneTHF, was described above. Serine can be catabolized back to the glycolytic intermediate, 3-phosphoglycerate, by a pathway that is essentially a reversal of serine biosynthesis. However, the enzymes are different. Serine can also be converted to pyruvate through a deamination reaction catalyzed by serine/threonine dehydratase.
Glutamine is converted to glutamate by glutaminase or several other enzymes by the removal of the amide nitrogen. Proline is first converted to a Schiff base and then converted by hydrolysis to glutamate-5-semialdehyde. All of these changes occur on the same carbon. Arginine and histidine contain 5adjacent carbons and a sixth carbon attached through a nitrogen attom. The catabolism of these amino acids is thus slightly more complicated than glutamine or proline. Arginine is converted to ornithine and urea. Ornithine is furthere transaminated to produce glutamate-5-semialdehyde. Glutamate-5-semialdehyde is converted to glutamate. The enzymes involved in the steps of the histidine pathway are listed in the box in the lower right corner of the diagram. Tetrahydrofolate is the cofactor in the final step converting histidine to glutamate. Transamination or deamination of glutamate produces a-ketoglutarate which feeds into the citric acid cycle.
Glutamate semialdehyde can serve as the precursor for proline biosynthesis as described above or it can be converted to glutamate. Proline catabolism is a reversal of its synthesis process. The glutamate semialdehyde generated from ornithine and proline catabolism is oxidized to glutamate by an ATP-independent glutamate semialdehyde dehydrogenase. The glutamate can then be converted to ketoglutarate in a transamination reaction. Thus arginine, ornithine and proline, are glucogenic.
Hyperprolinemia is an excess of a particular protein building block (amino acid), called proline, in the blood. This condition generally occurs when proline is not broken down properly by the body. There are two inherited forms of hyperprolinemia, called type I and type II.
People with hyperprolinemia type I often do not show any symptoms, although they have proline levels in their blood between 3 and 10 times the normal level. Some individuals with hyperprolinemia type I exhibit seizures, intellectual disability, or other neurological or psychiatric problems.
Hyperprolinemia type II results in proline levels in the blood between 10 and 15 times higher than normal, and high levels of a related compound called pyrroline-5-carboxylate. This form of the disorder has signs and symptoms that vary in severity, and is more likely than type I to involve seizures or intellectual disability.
Hyperprolinemia can also occur with other conditions, such as malnutrition or liver disease. In particular, individuals with conditions that cause elevated levels of lactic acid in the blood (lactic acidemia) may have hyperprolinemia as well, because lactic acid inhibits the breakdown of proline.
Mutations in the ALDH4A1 and PRODH genes cause hyperprolinemia.
Inherited hyperprolinemia is caused by deficiencies in the enzymes that break down (degrade) proline. Hyperprolinemia type I is caused by a mutation in the PRODH gene, which provides instructions for producing the enzyme proline oxidase. This enzyme begins the process of degrading proline by starting the reaction that converts it to pyrroline-5-carboxylate.
Hyperprolinemia type II is caused by a mutation in the ALDH4A1 gene, which provides instructions for producing the enzyme pyrroline-5-carboxylate dehydrogenase. This enzyme helps to break down the pyrroline-5-carboxylate produced in the previous reaction, converting it to the amino acid glutamate. The conversion between proline and glutamate, and the reverse reaction controlled by different enzymes, are important in maintaining a supply of the amino acids needed for protein production, and for energy transfer within the cell.
A deficiency of either proline oxidase or pyrroline-5-carboxylate dehydrogenase results in a buildup of proline in the body. A deficiency of the latter enzyme leads to higher levels of proline and a buildup of the intermediate breakdown product pyrroline-5-carboxylate, causing the signs and symptoms of hyperprolinemia type II.
Histidine catabolism begins with release of the α-amino group catalyzed by histidase, introducing a double bond into the molecule. As a result, the deaminated product, urocanate, is not the usual α-keto acid associated with loss of α-amino nitrogens. The end product of histidine catabolism is glutamate, making histidine one of the glucogenic amino acids. Another key feature of histidine catabolism is that it serves as a source of ring nitrogen to combine with tetrahydrofolate (THF), producing the carbon THF intermediate known as N5-formiminoTHF. The latter reaction is one of two routes to N5-formiminoTHF. The principal genetic deficiency associated with histidine metabolism is absence or deficiency of the first enzyme of the pathway, histidase.
The resultant histidinemia is relatively benign. The disease, which is
of relatively high incidence (
Histamine, biologically active substance found in a great variety of living organisms. It is distributed widely, albeit unevenly, throughout the animal kingdom and is present in many plants and bacteria and in insect venom. Histamine is chemically classified as an amine, an organic molecule based on the structure of ammonia (NH3). It is formed by the decarboxylation (the removal of a carboxyl group) of the amino acid histidine.
Histamine is synthesized in all tissues, but is particularly abundant in skin, lung and gastrointestinal tract. Mast cells, which are present in many tissues, are a prominent source of histamine, but histamine is also secreted by a number of other immune cells. Mast cells have surface receptors that bind immunoglobulin E, and when antigen crosslinks IgE on the mast cell surface, they respond by secreting histamine, along with a variety of other bioactive mediators.
Valine, Leucine and Isoleucine Catabolism
This group of essential amino acids are identified as the branched-chain amino acids, BCAAs. Because this arrangement of carbon atoms cannot be made by humans, these amino acids are an essential element in the diet. The catabolism of all three compounds initiates in muscle and yields NADH and FADH2 which can be utilized for ATP generation. The catabolism of all three of these amino acids uses the same enzymes in the first two steps. The first step in each case is a transamination using a single BCAA aminotransferas