Biosynthesis and biotransformation of cholesterol. Metabolism of ketonе bodies. Regulation and disorders of lipid metabolism.
Synthesis of cholesterol.
There are three stage in cholesterol synthesis. (1) acetic acid is converted to mevalonic acid, (2) mevalonic acid is converted into squalene, and (3) squalene is converted into cholesterol.
Mevalonic acid is formed by condensation of three molecules of acetyl-CoA. The key intermediate in this process is b-hydroxy-b-methylglutaryl-CoA (HMG-CoA), which is formed as follows:
Acetyl-CoA Acetyl-CoA Acetoacetyl-CoA
b-hydroxy-b-methylglutaryl-CoA
The enzyme is called b-hydroxy-b-methylglutaryl-CoA synthase.
The b-hydroxy-b-methylglutaryl-CoA undergoes an irreversible two-step reduction of one of its carboxyl groups to an alcohol group, with concomitant loss of CoA, by the action of hydroxymethylglutaryl-CoA reductase, to yield mevalonate:
Mevalonate is phosphorylated by ATP, first to the 5-monophosphate ester and then to the 5-pyrophosphomevalonic acid:
5-pyrophosphomevalonic acid
A third phosphorylation, at carbon atom 3, yields a very unstable intermediate which loses phosphoric acid and decarboxylates to form 3-isopentenyl pyrophosphate, which isomerizes to 3,3-dimethylallyl pyrophosphate.
3,3-dimethylallyl pyrophosphate
In the next several reactions 3,3-dimethylallyl pyrophosphate is converted to squalene. In the last stage of cholesterol biosynthesis, squalene undergoes attack by molecular oxygen, undergoes cyclization and lanosterol is formed. Lanosterol is transformed to cholesterol.
Transport forms of cholesterol in blood, content of cholesterol in blood, biological role of cholesterol.
LDL are formed in liver and transport cholesterol from liver to peripheral tissue. LDL is taken up by various tissues and provides cholesterol, which the tissue utilize.
HDL picks up cholesterol from cell membranes or from other lipoproteins. Cholesterol is converted to cholesterol esters by the lecithin:cholesterol acyltransferase (LCAT) reaction. The cholesterol esters may be transferred to other lipoproteins or carried by HDL to the liver, where they are hydrolyzed to free cholesterol, which is used for synthesis of VLDL or converted to bile salts.
The content of cholesterol in blood plasma – 3-8 mmol/l.
Biological role of cholesterol:
- building blocks of membranes;
- synthesis of steroid hormones;
- synthesis of bile acids;
- synthesis of vitamin D;
- cholesterol is often deposited on the inner walls of blood vessels, together with other lipids, a condition known as atherosclerosis, which often leads to occlusion of blood vessels in the heart and the brain, resulting in heart attacks and strokes, respectively.
Transport forms of lipids
Certain lipids associate with specific proteins to form lipoprotein systems in which the specific physical properties of these two classes of biomolecules are blended. In these systems the lipids and proteins are not covalently joined but are held together largely by hydrophobic interactions between the nonpolar portions of the lipid and the protein components.
Transport
lipoproteins of blood plasma. The plasma lipoproteins are complexes in which
the lipids and proteins occur in a relatively fixed ratio. They carry
water-insoluble lipids between various organs via the blood, in a form with a
relatively small and constant particle diameter and weight. Human plasma
lipoproteins occur in four major classes that differ in density as well as
particle size. They are physically distinguished by their relative rates of
flotation in high gravitational fields in the ultracentrifuge.
The blood lipoproteins serve to transport water-insoluble triacylglycerols and cholesterol from one tissue to another. The major carriers of triacylglyeerols are chylomicrons and very low density lipoproteins (VLDL).
The triacylglycerols of the chylomicrons and VLDL are digested in capillaries by lipoprotein lipase. The fatty acids that are produced are utilized for energy or converted to triacylglycerols and stored. The glycerol is used for triacylglycerol synthesis or converted to DHAP and oxidized for energy, either directly or after conversion to glucose in the liver. The remnants of the chylomicrons are taken up by liver cells by the process of endocytosis and are degraded by lysosomal enzymes, and the products are reused
by the cell.
VLDL is converted to intermediate density lipoproteins (IDL), which is degraded by the liver or converted in blood capillaries to low density lipoproteins LDL by further digestion of triacylglycerols.
LDL is taken up by various tissues and provides cholesterol, which the tissue utilize
High density lipoproteins (HDL) which is synthesized by the liver, transfers apoproteins to ehylomicrons and VLDL.
HDL picks up cholesterol from cell membranes or from other lipoproteins. Cholesterol is converted to cholesterol esters by the lecithin:cholesterol acyltransferase (LCAT) reaction. The cholesterol esters may be transferred to other lipoproteins or carried by HDL to the liver, where they are hydrolyzed to free cholesterol, which is used for synthesis of VLDL or converted to bile salts.
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%
Chylomicrons are the least dense of the blood lipoproteins because they have the most triacylglycerol and the least protein.
VLDL is more dense than chylomicrons but still has a high content of triacylglycerol.
IDL, which is derived from VLDL, is more dense than chylomicrons but still has a high content of triacylglycerol.
LDL has less triacylglycerol and more protein and, therefore, is more dense than the IDL from which it is derived. LDL has the highest content of cholesterol and its esters.
HDL is the most dense lipoprotein. It has the lowest triacylglycerol and the highest protein content.
Metabolism of Chylomicrons
Chylomicrons are synthesized in intestinal epithelial cells. Their triacylglycerols are derived from dietary lipid, and their major apoprotein is apo B-48.Chylomicrons travel through the lymph into the blood. In peripheral tissues, particularly adipose and muscle, the triacylglyerols are digested by lipoprotein lipase.The chylomicron remnants interact with receptors on liver cells and are taken+ up by endocytosis. The contents are degraded by lysosomal enzymes, and the products (amino acids, fatty acids, glycerol, and cholesterol) are released into the cytosol and reutilized.
Metabolism of VLDL
VLDL is synthesized in the liver, particularly after a high-carbohydrate meal. It is formed from triacylglycerols that are package with cholesterol, apoproteins (particularly apo B-100), and phospholipids and it is released into the blood.
In peripheral tissues, particularly adipose and muscle, VLDL triacylglycerols are digested by lipoprotein lipase, and VLDL is converted to IDL.
IDL returns to the liver, is taken up by endocytosis, and is degraded by lysosomal enzymes.
IDL may also be further degraded by lipoprotein lipase, forming LDL.
LDL reacts with receptors on various cells, is taken up by endocytosis and is digested by lysosomal enzymes.
Cholesterol, released from cholesterol esters by a lysosomal esterase, can be used for the synthesis of cell memmbranes or bile salts in the liver or steroid hormones in endocrine tissue.
Metabolism of HDL.
HDL is synthesized by the liver and released into the blood as disk-shaped particles. The major protein of HDL is apo A.
HDL cholesterol, obtained from cell membranes or from other lipoproteins, is converted to cholesterol esters. As cholesterol esters accumulate in the core of the lipoprotein, HDL particles become spheroids.
HDL particles are taken up by the liver by endocytosis and hydrolyzed by lysosomal enzymes. Cholesterol, released from cholesterol esters may be packaged by the liver in VLDL and released into the blood or converted to bile salts and secreted into the bile.
Nutrition and health
Lipids play diverse and
important roles in nutrition and health. Many lipids are absolutely essential for
life,
however, there is also considerable
awareness that abnormal levels of certain lipids, particularly cholesterol (in hypercholesterolemia)
and, more recently, trans fatty acids, are risk factors for heart disease and other diseases. We
need fats in our bodies and in our diet. Animals in general use fat for energy
storage because fat stores 9 KCal/g of energy. Plants, which don’t move around, can
afford to store food for energy in a less compact but more easily accessible
form, so they use starch (a carbohydrate, NOT A LIPID) for energy storage.
Carbohydrates and proteins store only 4 KCal/g of energy, so fat stores over
twice as much energy/gram as other sources of energy.
We need fats in our bodies and in our diet. Animals in general use fat for energy storage because fat stores 9 KCal/g of energy. Plants, which don’t move around, can afford to store food for energy in a less compact but more easily accessible form, so they use starch (a carbohydrate, NOT A LIPID) for energy storage. Carbohydrates and proteins store only 4 KCal/g of energy, so fat stores over twice as much energy/gram as fat. By the way, this is also related to the idea behind some of the high-carbohydrate weight loss diets.
The human body burns carbohydrates and fats for fuel in a given proportion to each other. The theory behind these diets is that if they supply carbohydrates but not fats, then it is hoped that the fat needed to balance with the sugar will be taken from the dieter’s body stores. Fat is also is used in our bodies to a) cushion vital organs like the kidneys and b) serve as insulation, especially just beneath the skin.
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Ketone bodies: structure.
In many vertebrates the liver has the enzymatic capacity to divert some of the acetyl-CoA derived from fatty acid or pyruvate oxidation, presumably during periods of excess formation, into free acetoacetate and b-hydroxybutyrate, which are transported via the blood to the peripheral tissues, where they may be oxidized via the tricarboxylic acid cyrcle.
These compounds, together with acetone, are collectively called the ketone bodies.
acetoacetate b-hydroxybutyrate acetone
Ketogenesis: mechanism, localization, biological role.
Free acetoacetate, which is the primary source of the other ketone bodies, is formed from acetoacetyl-CoA. Some of the acetoacetyl-CoA arises from the last four carbon atoms of a long-chain fatty acid after oxidative removal of successive acetyl-CoA residues in the mitochondrial matrix. However, most of the acetoacetyl-CoA formed in the liver arises from the head-to-tail condensation of two molecules of acetyl-CoA derived from fatty acid oxidation by the action of acetyl-CoA acetyltransferase:
acetyl-CoA acetyl-CoA acetoacetyl-CoA
The acetoacetyl-CoA formed in these reactions then undergoes loss of CoA, a process called deacylation, to yield free acetoacetate in a special pathway taking place in the mitochondrial matrix. It involves the enzymatic formation and cleavage of b-hydroxy-b-methylglutaryl-CoA, an intermediate which also serves as a precursor of sterols.
acetoacetyl-CoA acetyl-CoA b-hydroxy-b-methylglutaryl-CoA
b-hydroxy-b-methylglutaryl-CoA acetoacetate acetyl-CoA
The free acetoacetate so produced is enzymatically reduced to D-b-hydroxybutyrate by the NAD-linked b-hydroxybutyrate dehydrogenase, which is located in the inner mitochondrial membrane.
acetoacetate b-hydroxybutyrate
The mixture of free acetoacetate and b-hydroxybutyrate resulting from these reactions may diffuse out of the liver cells into the bloodstream, to be transported to the peripheral tissues.
The mechanism of acetoacetate utilizing in tissues (ketolysis).
In the peripheral tissues the b-hydroxybutyrate is oxidized to acetoacetate, which is then activated by transfer of CoA from succinyl-CoA. The succinyl-CoA required arises from the oxidation of a-ketoglutarate.
Another way of acetoacetate activation in peripheral tissues is the direct interaction of acetoacetate with ATP and CoA-SH:
The acetoacetyl-CoA formed in the peripheral tissues by these reactions then undergoes thiolytic cleavage to two molecules of acetyl-CoA, which then may enter the tricarboxylic acid cycle.
The mechanism of the increase of ketone bodies content in blood at diabetus mellitus and starvation.
Normally the concentration of ketone bodies in the blood is rather low (10-20 mg/l), but in fasting or in the disease diabetes mellitus, it may reach very high levels. This condition, known as ketosis, arises when the rate of formation of the ketone bodies by the liver exceeds the capacity of the peripheral tissues to utilize them, with a resulting accumulation in the blood and excretion via the kidneys (in normal the content of ketone bodies in urine is up to 50 mg/day).
The utilization of acetyl-CoA in tricarboxylic acid cycle depends on the availability of oxaloacetate in cell. The formation of oxaloacetate depends on quantity of pyruvate, which is formed from glucose. In fasting or diabetus mellitus the entering of glucose into cells is inhibited, oxaloacetate enters the gluconeogenesis process and is not available for the interaction with acetyl CoA in citrate synthase reaction. In this metabolic state acetyl-CoA is used for the ketone bodies formation. The accumulation of ketone bodies is also promotted by b-oxidation of fatty acids due to the stimulation of lipolysis in adipose tissue in glucose starvation conditions.
The effect of nervous system on lipid metabolism.
Sympathetic nervous system activates the splitting of triacylglycerol (lipolysis) and oxidation of fatty acids.
Parasympathetic nervous system promotes the synthesis of lipids and cholesterol in organism.
Endocrine regulation of lipid metabolism.
The effect of somatotropic hormone on lipid metabolism:
- stimulates lipolysis;
- stimulates the oxidation of fatty acids.
Prolactin.
- stimulates synthesis of lipids in mammary glands.
Lipotropic hormone.
- stimulates the mobilization of lipids from depot.
Thyroxine and triiodthyronine.
- activate the lipid oxidation and mobilization.
Insulin.
- enhances the synthesis of lipids;
- promotes the lipid storage activating the carbohydrate decomposition;
- inhibits the gluconeogenesis.
Glucagon.
- activates the lipolisis;
Lipocain.
- activates the formation of phospholipids in liver and stimulates the action of lipotropic alimentary factors;
- activates the oxidation of fatty acids in liver.
Epinephrine.
- activates the tissue lipase, mobilization of lipids and oxidation of fatty acids.
Glucocorticoids.
- promote the absorption of lipids in intestine;
- activate lipolysis;
- activate the conversion of fatty acids in carbohydrates.
Sex hormones.
- enhance the oxidation of lipids;
- inhibit the synthesis of cholesterol.
Interrelationship of carbohydrate and lipid metabolism.
Interrelationship of carbohydrate metabolism with lipid metabolism has two directions: 1-st – convertion of carbohydrates to lipids, 2-nd - convertion of lipids to carbohydrates.
Transformation of carbohydrates to lipids.
1. Glycerol phosphate can be produced from dihydroxiacetone monophosphate. The last is the common intermediate product of lipid and carbohydrate metabolism. Glycerol phosphate is the active form of glycerol and can react with active forms of fatty acids resulting in synthesis of triacylglycerols and complex lipids.
2. Biosynthesis of fatty acids takes place from acetyl-CoA which is formed in oxidative decarboxilation of pyruvate. Pyruvate is the central intermediate product of carbohydrate metabolism.
3. Carbohydrates are also source of hydrogen atoms, which are necessary for fatty acids synthesis. For this purpose the hydrogen atoms of reduced coenzymes NADPH2 are used. NADPH2 are usually produced in pentose phosphate cycle.
Transformation of lipids to carbohydrates.
The formation of carbohydrates from other compounds is called gluconeogenesis.
1. In b-oxidation of fatty acids the acetyl-CoA is formed. Acetyl-CoA can’t be converted directly to pyruvate. But it enter the tricarboxilic acid cycle and some intermediates of this cycle can be used for gluconeogenesis.
2. Small amount of carbohydrates can be also synthesised from glycerol by means of its oxidation to dihydroxiacetone monophosphate and glycerolaldehyde phosphate, which are the intermediates metabolites of glycolysis.
