Unit 6 - Cholesterol Metabolism I; Synthesis of Cholesterol, Bile Salts, and Acids

Question 1

overall cholesterol structure

Answer 1

4 planar hydrocarbon rings (A-D) of steroid nucleus

• has 8 carbon hydrocarbon attached to C17 of D-ring
• hydroxyl group attached to C3 of A ring
• DB between C5/6 of B right
• most cholesterol in plasma is esterified to FA at C3, and this cholesteryl ester is even more hydrophobic than cholesterol

Question 2

how much animal VS plant sterols are absorbed from diet?

Answer 2

40% of cholesterol

5% of phytosterols

Question 3

what is beta-sitosterol?

Answer 3

plant sterol

Question 4

what happens to plant sterols and excess cholesterol upon entering enterocytes?

Answer 4

actively transported back into intestinal lumen by 2 members of ATP binding cassette (ABC) family of transporters

Question 5

what is sitosterolemia?

Answer 5

autosomal recessive condition where sterol transporters ABCG5 and ABCG8 are defective
-accumulated beta-sitosterol and cholesterol in enterocytes enter blood stream to cause increased cardiovascular morbidity

Question 6

where is cholesterol made? where do the carbons come from? reducing equivalents? energy?

Answer 6

virtually all cells except RBCs in humans

• majority in liver, intestines, adrenal cortex, and reproductive tissues
• like FAS, all carbons from ACoA, while NADPH is reducing equivalents
• E from hydrolysis of thioester bond of ACoA and terminal Pi of ATP
• this happens on cytoplasmic surface of SER, and needs ER membrane and cytosolic enzymes

Question 7

does the HMG-CoA synthase for cholesterol work in the cytosol or the mitochondria?

Answer 7

the cytosol

Question 8

what is the key regulatory step for cholesterol synthesis?

Answer 8

HMG CoA reductase (integrated in SER facing cytoplasm)

• converts HMG CoA to mevalonate
• needs 2 NADPH, and CoA is released so irreversible
• inhibited by cholesterol

Question 9

what is isopentenyl pyrophosphate (IPP)

Answer 9

made in 8-step pathway for mevalonate to cholesterol

-precursor for isoprenoids (dolichol, CoQ, vit K, cholesterol)

Question 10

what are the 8 steps from mevalonate to cholesterol?

Answer 10

1. phosphorylation via 2 kinases (takes 2 ATP) = 6C
2. decarboxylation (takes 1 ATP, releases CO2) = 5C
3. isomerization = 5C
4. transferase (adds 1 IPP from 2) = 10C
5. transferase (adds 1 IPP from 2) = 15C
6. squalene synthase (adds 1 FPP from 5, needs 1 NADPH, releases 2 PPi and NADP+) = 30C
7. squalene monoxygenase (needs NADPH + O2, releases H2O and NADP) = 30C sterol
8. multistep reaction in ER = 27C cholesterol

Question 11

what is smith-lemli-opitz syndrome?

Answer 11

an autosomal recessive disorder of cholesterol biosynthesis

• caused by partial deficiency in 7-dehydrocholesterol-7-reductase that reduces DB in 7-DHC to convert to cholesterol
• one of the several multisystem embryonic malformation syndromes associated with impaired cholesterol synthesis

Question 12

how is HMG-CoA reductase regulated? proteins involved, areas involved, difference if low/high cholesterol, etc.

Answer 12

under control of transcription factor SREBP-2 (sterol regulatory element binding PRO 2) that binds cis acting SRE (sterol regulatory element)

• inactive SREBP-2 is integral ER membrane PRO that associates with ER PRO SCAP (SREBP cleavage activating PRO)
• when cholesterol levels are low, SREBP-2-SCAP complex moves to Golgi to cleave SREBP to soluble fragment (activated SREBP transcription factors)
• -SREBP transcription factor enters nucleus, binds SRE, and stimulates HMG CoA reductase mRNA and enzyme expression
• when cholesterol levels are high, it binds to SCAP, which binds to more ER PRO insigs (insulin induced gene products) that anchor SREBP-2-SCAP to ER membrane, so can’t go to Golgi and synthesis decreases

Question 13

how is HMG-CoA reductase degraded?

Answer 13

when cholesterol levels are high, they bind to sterol-sensing domain of HMG-CoA reductase
-causes binding of reductase to insigs (as if SCAP was activated), and triggers ubiquitination and proteosomal degradation of enzyme, leading to reduced cholesterol biosynthesis

Question 14

how does phosphorylation affect HMG-CoA reductase?

Answer 14

phosphorylated = inactive (AMP-activated protein kinase; if AMP is high and ATP is low)

dephosphorylated = active (phosphoprotein phosphatase; if AMP is low and ATP is high)

Question 15

how do hormones affect HMG-CoA reductase?

Answer 15

insulin and thyroxine upregulate expression

glucagon and glucocorticoids downregulate expression

Question 16

how do statin drugs work?

Answer 16

structural analogues of HMG, but with bulky hydrophobic groups
-serve as competitive inhibitors of HMG CoA reductase to lower cholesterol levels

Question 17

how is cholesterol degraded?

Answer 17

ring structure not metabolized to CO2 and water in humans, but sterol nucleus is eliminated by conversion to bile acids and bile salts

• small percentage of cholesterol in feces or bile
• -some is mmodified by intestinal bacteria before excretion

Question 18

what are the primary products of cholesterol degredation?

Answer 18

isomers of coprostanol and cholestanol (reduced forms of cholesterol)
-along with cholesterol, these make up the majority of neutral fecal sterols

Question 19

bile acid structure and what this means pH-wise

Answer 19

have steroid nucleus ring structure with 2-3 hydroxyl group sand a hydrocarbon side chain with terminal carboxyl group

• carboxyl group has pKa of 6 (matches duodenal pH) so 50% are protonated (bile acids) and 50% unprotonated (bile salts)
• OH groups are below plane of sterol ring (alpha); methyl groups are above (beta) so have polar and nonpolar faces

Question 20

what are the 2 most common “primary” bile acids?

Answer 20

cholic acid (triol) and chenodeoxycholic acid (diol)

Question 21

where does bile acid synthesis occur? what happens?

Answer 21

multistep process that involves multiple organelles in hepatic cells
-OH groups are added to ring, DB is reduced, and hydrocarbon chain is shortened by 3 carbons (has carboxyl group)

Question 22

what is the rate limiting step of bile acid synthesis?

Answer 22

cholesterol-7-alpha-hydroxylase; adds hydroxyl group at C7 of cholesterol to convert to 7-alpha-hydroxycholesterol
-downregulated by bile acids

Question 23

how do glycine or taurine conjugate bile acids?

Answer 23

before bile acids leave the liver

• amide bond forms between carboxyl group of bile acid and amino group of glycine (carboxyl) or taurine (sulfate) in 3:1 ratio
• lowers pKa, so bile salts are fully ionized at alkaline pH of bile, so better detergents

Question 24

what forms of bile salts are in bile?

Answer 24

only the conjugated forms

Question 25

what do intestinal flora do to bile salts?

Answer 25

remove glycine and taurine from conjugated form

-also can remove hydroxyl group from carbon 7 to make secondary bile acids

Question 26

enterohepatic circulation of bile salts

Answer 26

0. 5 g/day of primary and secondary bile salts (<3%) are lost in feces, and compensated for by liver synthesis
   - the rest can be reabsorbed in terminal ileum by Na+-bile salt cotransporter, and returned to blood via different transpoter
   - albumin binds and transports bile salts in blood
   - hepatocytes take bile salts from blood with an isoform of Na+-bile cotransporter