1. Lipogenesis. Metabolism of cholesterol.1

2. Functions of Cholesterol
a precursor of steroid hormones (progesterone, testosterone, estradiol, cortisol, etc.)
a precursor of bile acids
a precursor of vitamin D
important component of many mammalian membranes (modulates the fluidity) 2

3. Sources of Cholesterol
from the diet
can be synthesized de novo (about 800 mg of cholesterol per day) in the liver (major site) in the intestine
Liver-derived and dietary cholesterol are both delivered to body cells by lipoproteins 3

4. Synthesis of Cholesterol
Three stages of cholesterol biosynthesis
1. Synthesis of isopentenyl pyrophosphate, that is the key building block of cholesterol, from acetyl CoA
2. Condensation of six molecules of isopentenyl pyrophosphate to form squalene
3. Squalene cyclizes and the tetracyclic product is converted into cholesterol
Acetyl CoA (C2) Isopentenyl pyrophosphate (C5)
Squalene (C30) Cholesterol (C27) 4

5. A. Stage 1: Acetyl CoA to Isopentenyl Pyrophosphate
All carbons of cholesterol come from cytosolic acetyl CoA (transported from mitochondria via citrate transport system) 5

6. Sequential condensation of three molecules of acetyl CoA
Two molecules of acetyl CoA condense to form acetoacetyl CoA.
Enzyme – thiolase. 6

7. Enzyme: HMG-CoA synthase
Acetoacetyl CoA reacts with acetyl CoA and water to give 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) and CoA.
Enzyme: HMG-CoA synthase 7

8. In cytoplasm 3-Hydroxy-3-methylglutaryl CoA is reduced to mevalonate.
Enzyme: HMG-CoA reductase
In mitochondria 3-Hydroxy-3-methylglutaryl CoA is cleaved to acetyl CoA and acetoacetate.
Enzyme: HMG-CoA lyase. 8

9. HMG-CoA reductase
HMG-CoA reductase is an integral membrane protein in the endoplasmic reticulum
Primary site for regulating cholesterol synthesis
Cholesterol-lowering statin drugs (e.g. Lovastatin) inhibit HMG-CoA reductase
Lovastatin resembles mevalonate 9

10. Mevalonate is converted into 3-isopentenyl pyrophosphate in three consecutive reactions requiring ATP and decarboxylation.
Isopentenyl pyrophosphate is a key building block for cholesterol and many other important biomolecules. 10

11. Isopentenyl Pyrophosphate to Squalene
Stage 2:
Isopentenyl Pyrophosphate to Squalene
Isopentenyl pyrophosphate is isomerized to dimethylallyl pyrophosphate. 11

12. C5 units isopentenyl pyrophosphate react with C5 units dimethylallyl pyrophosphate to yield C10 compound geranyl pyrophosphate 12

13. C10 compound geranyl pyrophosphate reacts with C5 units isopentenyl pyrophosphate and C15 compound is formed - farnesyl pyrophosphate. 13

14. Reductive tail-to-tail condensation of two molecules of farnesyl pyrophosphate results in the formation of C30 compound squalene 14

15. Squalene to Cholesterol
C. Stage 3:
Squalene to Cholesterol
Squalene activated by conversion into squalene epoxide.
Squalene epoxide is cyclized to lanosterol. 15

16. Lanosterol is converted into cholesterol in a multistep process.16

17. THE REGULATION OF CHOLESTEROL BIOSYNTHESIS
Regulatory enzyme - 3-hydroxy-3-methylglutaryl CoA reductase.
Tetrameric enzyme.
NADPH - coenzyme 17

18. HMG CoA reductase is controlled in multiple ways:
The rate of synthesis of reductase mRNA is controlled by the sterol regulatory element binding protein (SREBP).
When cholesterol levels fall this protein migrates to the nucleus and enhance transcription.
The rate of translation of reductase mRNA is inhibited by cholesterol
The degradation of the reductase is controlled.
The increase of cholesterol concentration makes the enzyme more susceptible to proteolysis.
4. Phosphorylation decreases the activity of the reductase.
Enzyme is switched off by an AMP-activated protein kinase. Thus, cholesterol synthesis ceases when the ATP level is low. 18

19. Products of Cholesterol Metabolism19