Cholesterol Metabolism

Question 1

Cholesterol structure and functions

Answer 1

. 27-carbon sterol 
. 4 member ring structure 
. Hydroxyl group at C3 (ester)
. Double-bond (C5/C6)
. Hydrocarbon tail (C17)
. Synthesized from acetate 
. Essential function in animal cell membrane 
. Raw material for steroids and bile acid synthesis

Question 2

Sources of cellular cholesterol

Answer 2

.de novo in liver

. Dietary (LDL)

Question 3

Cholesterol functions

Answer 3

. Cell membrane fluidity
. Bile acid synthesis
. Steroid hormone synthesis

Question 4

Cholesterol biosynthesis

Answer 4

. Acetyl CoA -> HMG CoA -> mevalonate (rate-limiting) -> activated isprenoids ->cholesterol
. In cytosol in liver

Question 5

Synthesis of HMG CoA from acetyl CoA

Answer 5

. 2 acetyl CoA -> acetoacetyl CoA + acetyl CoA -> HMG CoA + CoA
. (3 acetyl CoA total used)
. 1st rxn through thiolase
. 2nd rxn through HMG-CoA synthase

Question 6

HMG CoA reduced to mevalonate

Answer 6

. Rate limiting step

. HMG CoA + 2NADPH + 2H -> mevalonic acid + CoA + 2NADP via HMG CoA reductase

Question 7

Intermediates of cholesterol synthesis participate in what other pathways?

Answer 7

. Protein prenylation

. Vit D synthesis (from 7- dehydrocholesterol)

Question 8

Covalent modification of HMG CoA reductase

Answer 8

. Pos: insulin: dephosphorylates HMG-CoA reductase activating it
. Neg: glucagon, AMP: AMPK phosphorylates it deactivating it

Question 9

Regulation of expression of gene coding for HMGR when cholesterol is high

Answer 9

. Cholesterol binds to sterol-sensing domain (SSD) of SCAP protein
. SCAP promotes localization of SREBP-2 transcription factor to the ER as part of INSIG-SCAP-SREBP-2 complex
. Means there is little gene expression of HMGR

Question 10

regulation of gene expression of HMGR when cholesterol is low

Answer 10

. No cholesterol binding to SCAP causes SCAP-SREBP-2 to dissociate from NSIG and translocate to golgi
. Proteolytic processing produces active SREBP-2 transcription factor
. Active SREBP-2 migrates to nucleus to promote transcription for gene coding for HMG-CoA and LDL receptor

Question 11

Sterol-induced proteolytic degradation of HMGR

Answer 11

. Binding of lanosterol to SSD of HMGR promotes its assoc. w/ NSIG in ER, ubiquination, and proteasomal degradation

Question 12

Statins

Answer 12

. Competitively inhibit HMGR leading to dec. in intracellular conc. Of cholesterol
. Transcription of gene coding for LDLL receptor inc.
. # of LDL receptors in cell surface inc. causing dec. LDL in blood and more uptake into cell

Question 13

Bile synthesis from cholesterol

Answer 13

. Cholesterol + O2+NADPH -> 7alpha-hydroxycholesterol + H2O+NADP -> Cholyl CoA + glycine (or taurine) -> glycocholic acid + CoA
. Rate limiting step: cholesterol 7alpha-hydroxylase

Question 14

Regulation of 7alpha-hydroxylase

Answer 14

. High intracellular cholesterol inc. gene expression for enzyme
. High bile salts dec. gene expression for enzyme

Question 15

Bile salt degradation

Answer 15

. Lower portion of SI distal to site of lipid absorption/digestion
. Bacterial metabolism causes deconjugation of some primary bile salts to release primary bile acids and then C7 hydroxyl group is removed converted it to secondary bile acid
. Mixture of primary, secondary bile acids w/ Gly and taurine and primary bile salts is reabsorbed in ileum and transported to liver via portal vein
. Primary and secondary bile acids are reconjugated and reused in fat digestion and absorption
. 98-99% bile acids/salts recycled

Question 16

Cholelithiasis

Answer 16

. Gall stones
. Lack of sufficient bile salts in bile causes cholesterol to precipitate leading to stones
. Causes: blockage of hepatobiliary system, hepatic dysfunction, impaired reabsorption of bile salts from intestine

Question 17

HMG CoA synthesis reaction differences between pathways

Answer 17

. Cholesterol and ketones have different enzymes in different locations for the same reaction sequence

Question 18

Mevalonate to cholesterol reactions

Answer 18

. Mevalonate (6C) -> isopentyl pyrophosphate (5C) with ATP and decarboxylation reaction
. Two 5C intermediates make 10C
. 10C plus 5C intermediate -> 15 C intermediate
. Two 15 C intermediates make 30C intermediate (squalene)
. Squalene becomes sterolized to form lanosterol
. Lanosterol sterolized again to form 27C cholesterol

Question 19

Protein prenylation

Answer 19

. FPP and GPP addition on to proteins makes them more hydrophobic
. Important for membrane-assoc. proteins
. Anchors proteins to membrane

Question 20

SREBP 1c is used in what lipid biosynthesis?

Answer 20

Fatty acids

Question 21

SREBP-1a is used in what lipid biosynthesis

Answer 21

Fatty acid and cholesterol

Question 22

How much bile acid/salts secreted each day?

Answer 22

15-30 g