Cholesterol & Bile Acids

1. What are the major physiological functions of cholesterol?

• Cell membrane component: regulates membrane fluidity.

• Precursor for vitamin D

• Precursor for steroid hormones

• Precursor for bile acids/bile salts

• Liver is central to cholesterol homeostasis

2. What structural features define a sterol?

• Steroid nucleus = 4 fused rings (A–D)

• OH group at C3

• Aliphatic side chain ≥8 carbons at C17

• Cholesterol has a Δ5 double bond.

3. What is a cholesterol ester and why is it important?

• Cholesterol with its C3-OH esterified to a fatty acid.

• More hydrophobic than free cholesterol.

• Used for storage and transport.

4. What is the difference between free cholesterol and cholesterol ester?

• Free cholesterol: C3-OH is unesterified → important in membranes.

• Cholesterol ester: C3-OH has fatty acid attached → storage/transport form

5. How does cholesterol affect membrane fluidity?

• Cholesterol acts as a fluidity buffer, preventing membranes from becoming excessively fluid or rigid.

• Especially abundant in plasma membranes and myelin.

6. What important molecules are synthesized from cholesterol?

• Vitamin D

• Steroid hormones: cortisol, aldosterone, progesterone, testosterone, estradiol

• Bile acids/bile salts

7. How is cholesterol obtained and eliminated by the body?

• Dietary cholesterol is absorbed in the intestine.

• Most tissues can synthesize cholesterol; liver is the major source.

• Cholesterol is eliminated primarily after conversion to bile acids and fecal excretion.

8. What transporter absorbs dietary cholesterol, and what drug targets it?

• NPC1L1 transports cholesterol into enterocytes.

• Ezetimibe inhibits NPC1L1 → ↓ intestinal cholesterol absorption.

9. How do plant sterols affect cholesterol levels?

• Plant sterols are poorly absorbed.

• They compete with cholesterol for intestinal absorption and promote cholesterol return to the intestinal lumen.

• Result: ↓ dietary cholesterol absorption.

10. What are three general strategies for lowering body cholesterol?

• ↓ intestinal cholesterol absorption

• ↓ cholesterol biosynthesis

• ↑ bile acid/cholesterol excretion

11. What is the purpose of cholesterol biosynthesis?

To produce cholesterol needed for:

• Cell membranes

• Steroid hormones

• Vitamin D

• Bile acids

Most tissues can synthesize cholesterol, with the liver being the major source.

12. Where does cholesterol synthesis occur and what is its carbon source?

• Primarily cytosolic/ER-associated

• All carbons ultimately come from acetyl-CoA.

• Mitochondrial acetyl-CoA reaches the cytosol through the citrate transport system.

13. What are the major stages of cholesterol biosynthesis?

1. Acetyl-CoA → HMG-CoA

2. HMG-CoA → mevalonate

3. Mevalonate → activated 5-carbon isoprene units

4. Isoprene units → larger intermediates → squalene

5. Squalene → cholesterol

You do not need to memorize every intermediate.

14. What is the first reaction of cholesterol synthesis?

2 acetyl-CoA → acetoacetyl-CoA

• Enzyme: thiolase

15. What is the next important reaction leading to HMG-CoA?

Acetoacetyl-CoA + acetyl-CoA → HMG-CoA

• Enzyme: cytosolic HMG-CoA synthase

16. What is the rate-limiting and major regulatory reaction of cholesterol synthesis?

HMG-CoA → mevalonate

• Enzyme: HMG-CoA reductase

• Requires 2 NADPH

• Occurs at the ER membrane

• Irreversible

• Major regulatory point of cholesterol synthesis.

⭐ HMG-CoA reductase = enzyme you absolutely need to know.

17. What is the key intermediate produced after HMG-CoA reductase?

Mevalonate → activated 5-carbon isoprene units, including isopentenyl pyrophosphate (IPP).

IPP provides the activated carbon units used to build cholesterol.

18. What is the major pharmacologic target in cholesterol biosynthesis?

HMG-CoA reductase

• Statins inhibit HMG-CoA reductase

• ↓ cholesterol synthesis

• This contributes to lowering plasma LDL cholesterol.

19. What is the overall effect of high intracellular cholesterol on cholesterol synthesis?

High cholesterol causes:

• ↓ HMG-CoA reductase transcription

• ↑ HMG-CoA reductase degradation

• ↓ HMG-CoA reductase activity

Therefore:

High cholesterol → ↓ cholesterol synthesis

20. How does SREBP regulate cholesterol synthesis?

Low intracellular cholesterol:

• SCAP transports SREBP from ER → Golgi.

• SREBP is cleaved.

• Active SREBP enters nucleus.

• ↑ transcription of HMG-CoA reductase and other cholesterol-homeostasis genes.

21. What happens to SREBP when intracellular cholesterol is high?

• Cholesterol binds SCAP.

• SCAP retains the SREBP-SCAP complex in the ER.

• SREBP cannot be activated.

• ↓ transcription of cholesterol biosynthesis genes.

High cholesterol → SREBP OFF → HMG-CoA reductase ↓

22. What is the easiest way to remember SREBP regulation?

Intracellular cholesterol	SREBP	HMG-CoA reductase
Low	Activated → nucleus	↑
High	Retained in ER	↓

Low cholesterol → make more cholesterol.

23. How does high cholesterol regulate HMG-CoA reductase after transcription?

High cholesterol promotes:

• HMG-CoA reductase mRNA degradation

• HMG-CoA reductase protein degradation by proteasomes

So cholesterol decreases the enzyme at both the mRNA and protein levels.

24. How does phosphorylation affect HMG-CoA reductase?

• Phosphorylated HMG-CoA reductase = inactive

• Dephosphorylated HMG-CoA reductase = active

This is a major energy/hormonal control mechanism.

25. How does AMPK regulate HMG-CoA reductase?

Low energy → AMP ↑ → AMPK ↑ → HMG-CoA reductase phosphorylation → inactive

Therefore:

Low energy → ↓ cholesterol synthesis

26. How do insulin and glucagon affect HMG-CoA reductase?

• Insulin → phosphatase → dephosphorylation → HMG-CoA reductase active → ↑ cholesterol synthesis

• Glucagon → kinase/AMPK signaling → phosphorylation → HMG-CoA reductase inactive → ↓ cholesterol synthesis

Think:

Fed/insulin = synthesis ON
Fasting/glucagon = synthesis OFF

27. What other factors regulate HMG-CoA reductase according to the lecture?

Increase activity:

• Insulin

• Thyroid hormone

Decrease activity:

• Glucagon

• High cholesterol/sterols

• Chylomicron remnants

• LDL

• Statins

28. What are the major levels at which cholesterol synthesis is regulated?

HMG-CoA reductase is controlled by:

1. Transcription — SREBP/SCAP

2. mRNA degradation

3. Protein degradation

4. Phosphorylation/dephosphorylation

5. Pharmacologic inhibition — statins

This is a very testable summary card.

29. How is intracellular cholesterol esterified for storage?

ACAT (acyl-CoA:cholesterol acyltransferase) transfers a fatty acid from fatty acyl-CoA to cholesterol.

→ cholesterol ester for intracellular storage

30. How does LCAT differ from ACAT?

ACAT

• Intracellular

• Uses fatty acyl-CoA

• Produces cholesterol esters for cellular storage

LCAT

• Plasma enzyme associated with HDL

• Transfers a fatty acid from phosphatidylcholine (lecithin) to cholesterol

• Produces cholesterol ester for HDL-mediated transport

⭐ Don't confuse ACAT vs LCAT.

31. What is the purpose of bile acids/bile salts?

They:

• Are synthesized from cholesterol

• Act as detergents that emulsify dietary lipids

• Facilitate lipid digestion and absorption

• Provide an important route for cholesterol elimination

32. Where are bile acids synthesized, stored, and recycled?

• Synthesized: liver

• Stored/concentrated: gallbladder

• Released: intestine

• >95% reabsorbed: terminal ileum

• Returned to liver through the portal circulation

This recycling is the enterohepatic circulation.

33. What is the rate-limiting step of bile acid synthesis?

Cholesterol → 7α-hydroxycholesterol

• Enzyme: cholesterol 7α-hydroxylase

• This is the rate-limiting enzyme of bile acid synthesis.

⭐ Compare:

• Cholesterol synthesis: HMG-CoA reductase

• Bile acid synthesis: 7α-hydroxylase

34. How are cholesterol and bile acids structurally changed during bile acid synthesis?

Cholesterol 27 carbons → bile acids 24 carbons by:

• Hydroxylation of the steroid nucleus

• Removal of 3 carbons from the side chain

• Addition of a carboxyl group at C24

• Reduction of the cholesterol double bond

These changes increase solubility.

35. What are primary, secondary, and conjugated bile acids, and why do they matter clinically?

Primary bile acids: synthesized in liver

• Cholic acid

• Chenodeoxycholic acid

Secondary bile acids: produced by intestinal bacteria

• Deoxycholic acid

• Lithocholic acid

Conjugated bile acids: primary bile acids conjugated with:

• Glycine

• Taurine

Conjugation lowers pKa → more ionized at intestinal pH → better detergents.

36. How can bile acid sequestrants lower cholesterol?

Drugs such as cholestyramine bind bile acids in the intestine → prevent their reabsorption → ↑ fecal bile acid loss.

The liver must use more cholesterol to synthesize replacement bile acids:

↑ bile acid excretion → ↑ cholesterol consumption → ↓ hepatic cholesterol

37. How can dietary fiber lower cholesterol through bile acid metabolism?

Certain fibers decrease bile acid reabsorption → increase fecal bile acid loss → increase hepatic conversion of cholesterol into bile acids.

38. Why can liver disease cause increased bile acids in the blood?

Normally, the liver efficiently extracts bile acids returning through the portal circulation.

Liver dysfunction → impaired extraction → ↑ bile acids in blood.

39. What causes cholesterol gallstones (cholelithiasis)?

Gallstones can form when bile becomes relatively supersaturated with hydrophobic cholesterol compared with its solubilizing components:

• Cholesterol

• Phospholipids

• Bile salts

Imbalance → cholesterol precipitates → gallstones.

40. What are the major ways to lower plasma cholesterol, and what is the mechanism of each?

Strategy	Mechanism
Ezetimibe	↓ intestinal cholesterol absorption via NPC1L1 inhibition
Statins	↓ HMG-CoA reductase → ↓ cholesterol synthesis
Bile acid sequestrants	↑ fecal bile acid loss → ↑ hepatic cholesterol use
Plant sterols/fiber	↓ intestinal cholesterol/bile acid absorption

🔥 The 10 things I would absolutely memorize

• HMG-CoA reductase = rate-limiting enzyme of cholesterol synthesis

• HMG-CoA reductase uses 2 NADPH

• Statins inhibit HMG-CoA reductase

• Low cholesterol → SREBP activated → ↑ HMG-CoA reductase transcription

• High cholesterol → SREBP retained in ER → ↓ synthesis

• AMP/AMPK phosphorylates HMG-CoA reductase → OFF

• Insulin dephosphorylates HMG-CoA reductase → ON

• 7α-hydroxylase = rate-limiting enzyme of bile acid synthesis

• Primary = liver; secondary = intestinal bacteria

• >95% bile acids undergo enterohepatic recycling; fecal loss is an important route of cholesterol elimination