Cholesterol, bile Flashcards
What are the functions of cholesterol?
How is it stored in the body?
stored as cholesterolester
functions:
- important part of cellular membranes
- precursor of steroid hormones
- precursor of bile acids

How much cholesterol can be found in the body?
Where?
in total 140g (70kg human)
- 120g in membranes
- 10g in blood plasma
rest… ?
How much cholesterol is supplied and lost daily?
supply:
- 0.5g synthesized in liver
- 0.5g synthesized in extrahepatic tissue
- 0.5g dietary intake
loss:
- 0.5g synthesized to bile salts
- 0.5g free cholesterol in bile
- 0.5g desquamation of epithelium
Where can cholesterol be found in the membrane?
hydrophilic OH- group at aqueous interface,
remainder within lipid bilayer leaflet
- btw phospholipids
- esp. abundant in lipid-rafts and caveolae
What are the 5 steps of cholesterol synthesis?
- mevalonate formed from acetyl-CoA
- active isoprene, isopentenyl PP formed by loss of CO2
- condensates to squalene
- cyclizes to parent steroid lanosterol
- formation of cholesterol
How much energy and acetyl-CoA is needed to produce 1 molecule of cholesterole?
How many enzymes are involved?
- 18 acetyl-CoA
- 36 ATP
- 20 NADPH/H+
- 21 enzymes
Describe the stepwise production of mevalonate.
Where does it happen?
Enzymes + reactions.
first 2 steps of ketogenesis, BUT: in cytosol
-
thiolase
2acetyl-CoA → acetoacetyl-CoA + CoA-SH -
HMG-CoA synthase
… + acetyl-CoA → HMG-CoA + CoA-SH -
HMG-CoA reductase
… + 2NADPH/H+ → CoA-SH + 2NADP+ + mevalonate

What is special about HMG-CoA reductase?
Where can it be found?
catalyzes rate-limiting step of cholesterol synthesis
→ anchored to membrane of ER
(considered as enzyme of ER)
How is isopentenyl PP formed?
Anything important?
Structure.
mevalonate phosphorylated sequentially, after decarboxylation → isopentenyl PP
3 ATP used, 1 CO2 formed

How is squalene formed?
Structure.
formed from 6 isopentenyl PP
- isopentenyl PP isomerizes
- condensates w/ another isopentenyl PP → geranyl PP (10C)
- condensates w/ another isopentenyl PP → farnesyl PP (15C)
- 2 farnesyl PP → squalene (30C), 1NADPH/H+ used
Farnesyl (<u><strong>F</strong></u>ifteen)

How is lanosterol formed?
Where does it happen?
Structure.
- squalene converted to squalene 2,3-epoxide in the ER
- ring closure to form steroid nucleus, lanosterol

How is cholesterol formed?
Where does it happen?
Structure.
lanosterol undergoes changes in steroid nucleus + side chain → cholesterol
happens on ER membrane

Explain the general mechanism of cholesterol homeostasis.
cholesterol concentration more or less constant bc regulated via synthesis/excretion
-
↑ cholesterol → ↓ transcription of
- HMG-CoA synthase
- HMG-CoA reductase
- prenyltransferase
- genes coding LDL receptor
- ↓ cholesterol → ↑ transcription
What is the function of SRBEP?
Describe its structure.
3 isoenzymes: SREBP-1a, -1c, 2
→ SREBP-2 causes transcription of enzymes involved in cholesterol synthesis
3 domains:
- N-terminal: transcription factor, helix-loop-helix (HLH)
- C terminal: regulatory function, binding to SCAP
- luminal domain: incorporated in ER membrane

Which 2 enzymes show cholesterol sensor domains?
- HMG-CoA reductase: produces mevalonate, catalyzes committed step of chol. synthesis
- SCAP: SREBP cleavage-activating protein, bound to ER membrane, interacts w/ C-terminal of SRBEP-2
How is the activity of HMG-CoA reductase regulated?
Why is it so important?
catalyzes committed step of cholesterol synthesis, regulated by
- [cholesterol]
- oxysterol binding
- reversible phosphorylation
- competitive inhibition
Explain how HMG-CoA reductase is regulated by the cholesterol concentration.
- ↑ [cholesterol]: binding to SCAP → SRBEP-2/SCAP complex binds to insig → complex remains attached to ER membrane
- ↓ [cholesterol]: SRBEP-2/SCAP complex able to translocate to Golgi → interaction w/ proteases S1P, S2P
Explain the function of S1P and S2P.
↓ [cholesterol] → SREBP-2/SCAP complex translocated to Golgi membrane
- S1P: cleaves luminal domain of SRBEP-2, but still bound in Golgi membrane
- S2P: releases HLH domain (N-terminal of SREBP-2), translocates into nucleus → transcription of coded genes
⇒ ↓ [cholesterol] → ↑ HMG-CoA reductase expression
⇒ ↑ cholesterol synthesis

Explain how HMG-CoA reductase is regulated by oxysterols.
able to bind to HMG-CoA reductase
→ target for ubiquitination → ↓ [HMG-CoA reductase]
⇒ ↓ cholesterol synthesis
Explain how HMG-CoA reductase is regulated by reversible phosphorylation.
phosphorylated (= inactivated) by AMPK in response to
- glucagon, glucocorticoids via direct phosphorylation, and phosphorylation of AMPKK
- AMP binding to AMPK
dephosphorylated (= activated) by phosphatase in response to
- insulin
- thyroid hormones
→ inactivated when E deficit in cell
⇒ ↓ cholesterol synthesis
Explain how HMG-CoA reductase is regulated by competitive inhibition.
Clinical importance?
mevalonate = competitive inhibitor (product inhibition)
→ statins structurally similar to mevalonate, very high affinity
⇒ completely stop cholesterol synthesis
How are cholesterol, resp. cholesterolesters taken up from the intestinal lumen into the body?
- hydrolysis of CE to cholesterol in lumen
- uptake of free cholesterol through Niemann-Pick C1-like protein 1 (NPC1L1)
- resynthesis to CEs by ACAT 2
→ CE packed into chylomicrons

What happens w/ sterols taken up via NPC1L1?
sorted in endosome, then excreted again via ABCG5 and 8

What is the function of ACAT?
Reaction.
Distinguish btw ACAT 1 and ACAT 2.
acyl-CoA:cholesterol acyltransferase
produces cholesterol esters for intracellular storage of cholesterol
acyl-CoA + cholesterol ⇔ CoA-SH + cholesterol ester
- ACAT 1: in macrophages
- ACAT 2: in intestine, liver
