Dr.S.Chakravarty M.D

1. BIOSYNTHESIS OF CHOLESTEROL Dr.S.Chakravarty M.D

2. Can you explain how workouts can decrease serum cholesterol ?

3. Specific learning objectives At the end of this session a student will be able to :- • Describe the importance of cholesterol and its synthesis briefly. • List the products derived from cholesterol as a substrate • Describe the mechanism of action of HMG-coAreductase inhibitors(STATINS )and explanation of its side effects • Describe the regulation of HMG-coA reductase in detail

4. INTRODUCTION • Cholesterol  exclusively ANIMAL sterol !! • No plant food has cholesterol !! • GO GREEN !! • VERY important biological molecule. • Role in Membrane Structure & fluidity of cell membrane. • Precursors for synthesis – • a) Steroid Hormones • b) Bile Acids • c) Vitamin D

5. BIOMEDICAL IMPORTANCE OF CHOLESTEROL • Cholesterol level >200 mg/dl leads to hypercholesterolemia which is a risk factor for Atherosclerosis. • Cholesterol is only excreted through BILE and a very less amount of bile acids are actually excreted. • Hereditary defects in cholesterol metabolism leads to Fredrickson’s hyperlipidaemias(Type II)

6. Structure of Cholesterol(27 C) 3 5 6 Entire molecule made up of 2 Carbon units – Acetyl CoA

7. SITE: Liver(10%), Intestine(10%), Adrenal Cortex, Skin, Testis & Aorta (virtually all tissues containing nucleated cells are capable of synthesizing cholesterol.) SUBCELLULAR SITE : Endoplasmic Reticulum KEY ENZYME :HMGCoA Reductase

8. PRECURSOR : Acetyl CoA • Acetyl CoA derived from :- • Oxidation reaction • Fed state :- Glucose Puruvate Acetyl CoA(MAIN ) • Fasting state :- FA oxidation (MINOR) • Imp :- Ethanol metabolism also produces acetyl CoA INSULIN

9. SUBSTANCES REQUIRED • Acetyl CoA from Glycolysis • ATP  Lots of them • NADPH ( which pathway produces it ?) Sources of NADPH HMP Shunt Malic enzyme Cytoplasmic Isocitrate dehydrogenase

10. Kaplan step 1 USMLE notes KAPLAN USMLE STEP 1 LECTURE NOTES Minor(only when cell divides its needed)

11. BIOSYNTHESIS OF CHOLESTEROL STEP 1 Acetyl CoA (2C) Acetoacetyl CoA (4C) Acetyl CoA 3-Hydroxy-3-methylglutaryl-CoA (HMGCoA) (6C) Mevalonate(6C) Acetyl CoA Thiolase CoA.SH HMG CoA Synthase CoA.SH 2NADPH + 2H+ HMG CoA reductase 2NADP+ + CoA.SH STEP 2

12. STEP 3 Mevalonate Isopentenyl pyrophosphate (5C) Geranyl pyrophosphate (10C) Farnesyl pyrophosphate (15C) 2 mole ATP Co2 Mg+2 Isopentenyl pyrophosphate Prenylated proteins Isopentenyl pyrophosphate STEP 3 • Used for synthesis of • Prenylated proteins • Dolicol • 3) Coenzyme Q

13. Not required to know the steps !

15. Kaplan step 1 USMLE notes KAPLAN USMLE STEP 1 LECTURE NOTES STATIN Side effects :- Rhabdomyolysis Hepatitis Minor(only when cell divides its needed) USMLE concept! Minor Minor Reabsorption by Enterohepatic circulation  Inhibits 7αhydroxylase= Decreased excretion of cholesterol from body Minor

16. REGULATION OF CHOLESTEROL SYNTHESIS • HMG-CoA reductase is the rate limiting & primary regulated step • Partly regulated by dietary intake of cholesterol • –INHIBITS ITS OWN BIOSYNTHEIS AT GENETIC LEVEL • (DECREASE TRANSCRIPTION AND HENCE TRANSLATION-LONG TERM REGULATION) • Rapid regulation of HMG-CoA reductase occurs thro, inhibition by phosphorylation (Short term regulation by covalent modification).

17. REGULATION OF HMG CoA REDUCTASE BY COVALENT MODIFICATION Harper 28th

18. Cholesterol utilization in cell • Incorporated in cell membrane • Esterification and storage • Synthesis of steroid hormones • Export from the cell in HDL

19. Cholesterol balance at the cellular level STORAGE FORM Harper, 28th edition

20. How Cholesterol makes steroids ??

21. EXCRETION OF CHOLESTEROL a) 1 gm eliminated from body per day b) 50% Excreted through bile by conversion to bile acids c) 50% in feces as Coprostanol Cholesterol Coprostanol d) Bile salts not reabsorbed or their derivatives are excreted in feces Intestinal bacteria

22. CLINICAL SIGNIFICANCE OF CHOLESTEROL Normal Range : 150 – 200 mg/dl Hypercholesterolemia : Diabetes Mellitus Nephrotic syndrome Hypothyroidism Obstructive jaundice Hyperlipidaemia ( Familial Hypercholesterolemia) Type II Atherosclerosis Additional factors for Coronary Artery Disease include – Lifestyle : Cigarette Smoking, Coffee drinking Emotional Stress, Male Gender, Obesity Lack of exercise, High Blood Pressure etc

23. MCQ1 Why are dietary fibers mainly helpful in decreasing cholesterol? • A. They promote the GI motility and decrease cholesterol absorption. • B. They are not digested and they don’t produce Acetyl CoA upon metabolism • C. They cause a fullness of stomach and decrease appetite • D. They bind to bile salts and decreases their reabsorption • E. They decrease cholesterol absorption from gut

24. MCQ 1 • The anticholesterolemic action of simvastatin is based on its effectiveness as a competitive inhibitor of rate-limiting enzyme in cholesterol biosynthesis. The reaction product normally produced by this enzyme is • A. Squalene • B.Methylmalonate • C.Lanosterol • D.Mevalonate • E.acetoacetate

25. MCQ2Which of the following substances is decreased if HMG CoA reductase inhibitors are used for treatment of hypercholesterolemia? • A. Acetyl CoA • B. Malonyl CoA • C. CoQ • D. SuccinylCoA • E. ATP

26. How can exercise and active life style decrease serum cholesterol levels? The starting point Acetyl CoA is directed to TCA cycle for ATP production and less of it is available for cholesterol biosynthesis. Also during exercise the metabolic state of the body is different …

27. Formation of Bile acids

28. Bile acids • Primary bile acids : Cholic acid Chenodeoxycholic acid • Secondary bile acids: Deoxycholic acid Lithocholic acid

29. BILE ACID METABOLISM • Synthesizedin liverfrom cholesterol • Rate limiting step catalyzed by 7 α-hydroxylase • Requires O2, NADPH, cytochrome P450 • Primary bile acids are Cholic acid & Chenodeoxycholic acid • Primary bile acids modified by intestinal bacteria ( undergo deconjugation & 7 α-dehydroxylation) • Secondary bile acids are Deoxycholic acid & Lithocholic acid

30. Vit. C 12α-hydroxylase Taurine Glycine CoA-SH CoA-SH Taurocholic acid Glycocholic acid Tauro- & Glyco- Chenodeoxycholic acid Deconjugation 7 α-dehydroxylation Deconjugation 7 α-dehydroxylation Deoxycholic acid Lithocholic acid SYNTHESIS OF BILE ACIDS

31. REGULATION OF BILE ACID SYNTHESIS • Rate limiting step – 7 α-hydroxylase reaction • Dietary cholesterol induces gene for 7 α-hydroxylase • Suppressed by bile acids via activation of protein kinase C • Return of bile acids to liver via enterohepatic circulation is an important control • If interrupted leads to activation of 7 α-hydroxylase • 7 α-hydroxylase controlled by covalent phosphorylation & dephosphorylation

32. albumin

33. SIGNIFICANCE OF BILE ACID SYNTHESIS • Bile acids perform four physiologically functions : • Their synthesis and subsequent excretion in the feces represent the only significant mechanism for the elimination of excess cholesterol. • 2.Bile acids and phospholipids solubilize cholesterol in the bile, thereby preventing precipitation of cholesterol in gallbladder. • 3. Facilitate digestion of dietary triacylglycerols by acting as emulsifying agents that render fats accessible to pancreatic lipases. • 4. Facilitate intestinal absorption of fat-soluble vitamins.

34. LABORATORY INVESTIGATIONS • Blood Glucose Level • Serum Lipid Profile – • a) Measured Parameters - • 1) Serum Total Cholesterol Level (150-200 mg/dl) • 2) Serum Triglycerides Level (<150mg/dl) • 3) Serum HDL Cholesterol Level (35-75 mg/dl) • b) Calculated Parameter – • 1) Serum LDL Cholesterol Level (<130 mg/dl) • Recent markers for atherosclerosis- • i) Sr. Homocysteine ( 5-15mmol/L) • ii) Sr Lipoprotein (a) (<30mg/dl)