Cholesterol Metabolism

Question 1

Major sources of liver cholesterol

Answer 1

• Dietary: taken up in chylomicron remnants
• Extrahepatic tissues: taken up into HDL
• De novor synthesis by liver

Question 2

Major routes by which cholesterol leaves the liver

Answer 2

• Secretion of VLDL
• Free cholesterol secreted in bile
• Conversion to bile acids/salts

Question 3

Cholesterol structure

Answer 3

• Four planar hydrocarbon rings: steroid nucleus

- Most cholesterol in the plasma is esterified to a fatty acid at carbon 3 (more hydrophobic).

Question 4

Structural importance of cholesterol

Answer 4

• Component of cell membranes
• Increases mechanical strength
• Decreases permeability and fluidity

Question 5

Polar portion of cholesterol

Answer 5

• Hydroxyl group

- Small size of polar group allows for a large amount of flipping of cholesterol

Question 6

Sterols

Answer 6

• Cholesterol is the major form of sterols in animal tissue

- Four fused hydrocarbon rings and 8-10 carbons in the hydrocarbon tail attached to C17, and a hydroxyl at C3

Question 7

Sitosterolemia

Answer 7

• Rare inherited autosomal recessive plant sterol storage disease
• Mutations in ABCG5 and ABCG8 genes which encode ABC transpoorters: sterolin 1 and sterolin 2
• Diminished pumping of plant sterols back into intestine
• Increased phytosterols in blood and tissues
• Tuberous xanthomas
• Propensity toward premature coronary atherosclerosis

Question 8

Cells that synthesize cholesterol include

Answer 8

• Virtually all cells except RBCs in humans

- Majority by liver, intestines, adrenal cortex, and reproductive tissues

Question 9

Carbon source for cholesterols

Answer 9

-Acetyl CoA

Question 10

Reducing equivalent for cholesterol synthesis

Answer 10

-NADPH

Question 11

Cholesterol synthesis location

Answer 11

• Cytoplasmic surface of smooth ER

- Requires ER membrane and cytosolic enzymes

Question 12

Step 1 in cholesterol synthesis

Answer 12

• Acetyl CoA to HMG CoA: 2 carbons to 6 carbons
• 2 Acetyl CoA condense with loss of one CoA to form Acetoacetyl CoA (Thiolase enzyme)
• A third acetyl CoA molecule added by HMG CoA synthase (Cytosolic form) to form HMG CoA

Question 13

Step 2 in cholesterol synthesis

Answer 13

• HMG CoA to Mevalonate: KEY REGULATORY STEP
• Catalyzed by HMG CoA Reductase: Integral membrane protein of smooth ER with catalytic domain facing cytoplasm
• Excess cholesterol inhibits HMG CoA Reductase
• 2 NADPH required as reducing agent
• CoA released: makes it irreverisble

Question 14

Mevalonate to cholesterol steps

Answer 14

1. Mevalonate to 5-pyrophosphomevalonate: 2 steps that transfer phosphate from ATP. Makes more water soluble.
2. IPP formed by decarboxylation. Requires ATP.
3. IPP isomerized to DPP: Isomerase
4. IPP and DPP condense to form ten carbon GPP: Transferase
5. Second molecule of IPP condenses with GPP to form 15 carbon FPP: Transferase
6. Two FPP combine, release pyrophosphate, and reduced. Formed 30-C Squalene (total of 18 ATP used to make)
7. Squalene to sterol lanosterol by ER associated enzymes using oxygen and NADPH
8. Lanosterol to cholesterol. Multi step. ER associated process

Question 15

HMG CoA Reductase Regulation- Transcriptional level

Answer 15

• Catalyzes HMG CoA to Mevalonate
• Regulation under control of SREBP-2 which binds SRE and is associated with SCAP.
• Low cholesterol: SREPB-2-SCAP complex moves to golgi and stimulates cleavage of SREBP resulting in an active transcription factor
• TF enters the nucleus and stimulates expression of HMG CoA reductase mRNA.
• High cholesterol: binds to sterol sensing domain of SCAP, and prevents the complex from moving to the Golgi.

Question 16

HMG CoA Reductase Regulation- Post translational control

Answer 16

• Enzyme degradation
• High cholesterol binds to sterol sensing domain of reductase itself
• Trigger degradation of HMG CoA reductase and therefore decreased cholesterol biosynthesis

Question 17

HMG CoA Reductase Regulation- Phosphorylation/dephosphorylation

Answer 17

• Phosphorylated: inactive
• Dephosphorylated: active
• ATP low, AMP high, inactive, decreased cholesterol synthesis

Question 18

HMG CoA Reductase Regulation-Hormonal

Answer 18

• Insulin and thyroxine upregulate

- Glucagon and glucocorticoids down regulate

Question 19

Statin drugs

Answer 19

• Structual analogs of HMG
• Competitive inhibitors of HMG CoA Reductase
• Lower plasma levels of cholesterol (prevent synthesis)

Question 20

Degradation of cholesterol

Answer 20

• Sterol nucleus eliminated from body by conversion to bile acids and bile salts
• Small amount excreted in feces or secretion into bile
• Some modified by bacteria before excretion
• Primary products made: isomers of coprostanol and cholestanol

Question 21

Bile acids

Answer 21

• Steroid nucleus ring with two or three hydroxyl groups
• Half are protonated and half are deprotonated in the intestine
• Polar face and nonpolar face

Question 22

Two most common primary bile acids

Answer 22

• Cholic acid: Three hydroxyl groups

- Chenodeoxycholic acid: Two hydroxyl groups

Question 23

Bile acid synthesis

Answer 23

• OH groups added to sterol ring
• Double bond reduced
• Hydrocarbon chain shortened and carboxyl group introduced
• Rate limiting step: addition of hydroxyl group at carbon 7 of cholesterol: Cholesterol 7-alpha-hydroxylase (down regulated by bile acids)

Question 24

Conjugated bile salts

Answer 24

• Bile acids conjugated to serine or taurine before leaving the liver
• Glycocholic and glycochenocholic acid and taurocholic and taurochenocholic acids formed.
• Better detergents than bile acids
• Only conjugated forms found in bile

Question 25

Intestinal flora role with bile salts

Answer 25

• Bacteria in intestine can remove glycine and taurine from conjugated bile salts
• Can also remove hydroxyl group producing secondary bile acids

Question 26

Secondary bile acids

Answer 26

• Deoxycholic acid

- Lithocholic acid