BIOSYNTHESIS OF CHOLESTEROL

Cholesterol is the major sterol in the animal tissues.
Cholesterol is present in tissues and in plasma either as free cholesterol or as a storage form, combined with a long-chain fatty acid as cholesteryl ester.
In plasma, both forms are transported in lipoproteins
removed from tissues by plasma high-density lipoprotein (HDL) and transported to the liver, where it is eliminated from the body either unchanged or after conversion to bile acids in the process known as reverse cholesterol transport

1. BIOSYNTHESIS OF CHOLESTEROL PREPARED BY; MISS RABIA KHAN BABER COURSE TITLE : BIOCHEMISTRY

2. LEARNING OBJECTIVES OF PPT

3. INTRODUCTION TO CHOLESTROL METABOLISM Cholesterol is the major sterol in the animal tissues. Cholesterol is present in tissues and in plasma either as free cholesterol or as a storage form, combined with a long-chain fatty acid as cholesteryl ester. In plasma, both forms are transported in lipoproteins removed from tissues by plasma high-density lipoprotein (HDL) and transported to the liver, where it is eliminated from the body either unchanged or after conversion to bile acids in the process known as reverse cholesterol transport

4. STRUCTURE OF CHOLESTROL 4 non aromatic rings named as A,B,C,D 27 Carbon compound 1 double bond between C 5&6 1 side chain 1 hydroxyl group at C-3

5. SOURCES OF CHOLESTROL Cholesterol is derived from Diet De novo synthesis From the hydrolysis of cholesteryl esters The liver and intestine account for approximately 10% each of total synthesis in humans. Virtually all tissues containing nucleated cells are capable of cholesterol synthesis, which occurs in the endoplasmic reticulum and the cytosol .

6. FUNCTIONS OF CHOLESTROL Cholesterol is the most abundant sterol in humans and performs a number of essential functions. It is a major constituent of the plasma membrane and of plasma lipoproteins. It is a precursor of bile salts, It is a precursor of steroid hormones that include adrenocortical hormones, sex hormones, placental hormones etc Also a precursor of vitamin D It is required for the nerve transmission

7. STEROID HORMONE PRODUCTION All steroid hormones are derived form cholesterol In the cortex of adrenal glands two classes of hormones are synthesized – mineralocorticoids and glucocorticoids In the male and female gonads – sex hormones are produced Sex hormones include – progesterone, androgens and estrogens

8. BIOSYNTHESIS OF CHOLESTROL Slightly less than half of the cholesterol in the body derives from biosynthesis  de novo . Biosynthesis in the liver accounts for approximately 10%, and in the intestines approximately 15%, of the amount produced each day. The cholesterol biosynthesis pathway involves enzymes that are in the cytoplasm, microsomes (ER), and peroxisomes . Synthesis of cholesterol, like that of most biological lipids, begins from the two-carbon acetate group of acetyl- CoA . The initial steps in the pathway of cholesterol biosynthesis are collectively called the mevalonate pathway which itself culminates with the synthesis of the isoprenoid molecule, isopentenyl pyrophosphate (IPP).

9. STEP # 1 HMG-COA SYNTHESIS Initially, two molecules of acetyl- CoA condense to form Acetoacetyl-CoA catalyzed by cytosolic thiolase . Acetoacetyl-CoA condenses with a further molecule of acetyl- CoA catalyzed by HMG- CoA synthase to form HMG- CoA

10. HMG- CoA is then converted to mevalonate by HMG- CoA reductase . HMGR is bound in the endoplasmic reticulum. HMGR requires NADPH as a cofactor and two moles of NADPH are consumed during the conversion of HMG- CoA to mevalonate The final step the reduction of HMG-CoA to mevalonate, catalyzed by HMG-CoA reductase. STEP#2 MEVALONATE SYNTHESIS

11. STEP#3 IPP SYNTHESIS Conversion of mevalonate into activated isoprene units Isoprene containing molecules are important intermediates in cholesterol biosynthesis

12. Mevalonate is phosphorylated by 2 sequential P i transfers from ATP, yielding the pyrophosphate derivative. ATP-dependent decarboxylation, with dehydration, yields isopentenyl pyrophosphate. H 2 C C CH 3 H O 2 C  O O 2 C H C H O H O  O O H 2 C CH 2 CH 2 O P O P O  O  CH 3 C H 2 C C CH 3 H O 2 C  O O 2 O  O O C H C H O P O P O  O  CO 2 5-pyrophosphomevalonate ATP ADP + P i mevalonate 2 ATP (2 steps) 2 ADP isopentenyl pyrophosphate

13. Isopentenyl Pyrophosphate Isomerase inter-converts isopentenyl pyrophosphate & dimethylallyl pyrophosphate . O  O O CH 3 C H 2 C CH 2 CH 2 O P O P O  O  O  O O CH 3 C H 3 C CH CH 2 O P O P O  O  isopentenyl p y r o p h o s p h ate dimethylallyl p y rop h o s p h a t e

14. STEP#4 SYNTHESIS OF SQUALENE Polymerization of six 5-carbon isoprene units to form the 30-carbon linear structure of squalene

15. O  O O O  O O H 2 C CH 3 C CH 2 CH 2 O P O P CH 3 H 3 C C CH CH 2 O P O P O  O  O  O CH 3 H 3 C C CH CH 2 CH 2 CH 3 C CH CH 2 O P O P O  O  PP i O  O O H 2 C CH 3 C CH 2 CH 2 O P O P CH 3 H 3 C C CH CH 2 CH 2 PP i O  CH 3 CH 3 C CH CH 2 CH 2 C CH CH 2 O P O P O  O  dimethylallyl pyrophosphate O  O  isopentenyl pyrophosphate O O  O  isopentenyl pyrophosphate O O geranyl pyrophosphate farnesyl pyrophosphate Each condensation involves a carbocation formed as PP i is eliminated.

16. O  O O CH CH 2 O P O P O  O  CH 3 CH 3 CH 2 C CH CH 2 CH 2 C CH 3 2 H 3 C C CH CH 2 O O 2 H 2 O H O H + N A D PH NADP + + 2 PP i N A D P + NADPH 2 farnesyl pyrophosphate squalene 2,3-oxidosqualene lanosterol Squalene Synthase: Head-to-head condensation of 2 farnesyl pyrophosphate, with reduction by NADPH, yields squalene.

17. Cyclization of squalene forms the four rings of the steroid nucleus. Subsequent modifications leads to the final product, cholesterol. STEP#5 SYNTHESIS OF CHOLESTROL

18. Conversion of lanosterol to cholesterol involves 19 reactions, catalyzed by enzymes in ER membranes. Additional modifications yield the various steroid hormones or vitamin D. H O H O lanosterol cholesterol 19 steps

19. REGULATION OF CHOLESTEROL SYNTHESIS Normal healthy adults synthesize cholesterol at a rate of approximately 1g/day and consume approximately 0.3g/day. A relatively constant level of cholesterol in the blood (150–200 mg/ dL ) is maintained primarily by controlling the level of  de novo  synthesis. The level of cholesterol synthesis is regulated in part by the dietary intake of cholesterol. Cholesterol from both diet and synthesis is utilized in the formation of membranes and in the synthesis of the steroid hormones and bile acids. The greatest proportion of cholesterol is used in bile acid synthesis.

20. VARIATION OF SERUM CHOLESTEROL LEVELS High cholesterol concentration is found in: Diabetes mellitus Nephrotic syndrome Obstructive jaundice Familial hypercholesterolemia Biliary cirrhosis Hypothyroidism

21. HYPOCHOLESTEROLEMIA Low serum cholesterol concentration is observed in- Hyperthyroidism Malnutrition Malabsorption Anemia Physiologically lower levels are found in children Persons on cholesterol lowering drugs

22. The rate limiting step in the pathway to cholesterol is catalyzed by HMG- CoA reductase Its activity is modulated over a 100-fold range REGULATION OF HMGR ACTIVITY AND LEVELS.

23. CHOLESTEROL MADE IN THE LIVER IS EXPORTED Much of cholesterol synthesis takes place in the liver Cholesterol is exported in two forms Bile salts – amphipathic cholesterol derivatives that aid lipid digestion Cholesteryl esters – transported and secreted in lipoprotein particles to other tissues that use cholesterol or are stored in the liver

24. THE UTILIZATION OF CHOLESTEROL Cholesterol is transported in the plasma predominantly as cholesteryl esters associated with lipoproteins. Dietary cholesterol is transported from the small intestine to the liver within chylomicrons . Cholesterol synthesized by the liver, as well as any dietary cholesterol in the liver that exceeds hepatic needs, is transported in the serum within LDL. The liver synthesizes VLDL and these are converted to LDL through the action of endothelial cell-associated lipoprotein lipase. Cholesterol found in plasma membranes can be extracted by HDL

25. DEGRADATION OF CHOLESTEROL The ring structure of cholesterol cannot be metabolized to CO 2 and H 2 in humans . The intact sterol ring is eliminated from the body by: Conversion to bile acids, which are excreted in feces Secretion of cholesterol into the bile, which transports it to the intestine for elimination