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
Which 2 mechanisms are intracellulary activated by cholesterol?
- decr. cholesterol synthesis via SREBP-2 pathway
- activates ACAT allosterically → stored as cholesterolester
What is the function of the LDL receptor?
Which group of drugs regulate its expression?
determines LDL level in blood
↑ [LDL receptor] → ↓ blood cholesterol level
induced by statins
NOTE: statins also inhibit HMG-CoA reductase comp.
What is the function of PCSK9?
proprotein convertase subtilisin/kexin type 9
regulates LDL receptor recycling by targeting it for degradation
⇒ ↓ LDL uptake → ↑ blood cholesterol level
What does IDOL do?
inducible degrader of the LDL receptor
ubiquitin ligase that is induced by intracellular cholesterol
⇒ degradation of LDL receptor → ↑ blood cholesterol level
How is cholesterol excreted?
converted to bile acids in liver (b/c otherwise cholesterol would be non-water soluble), then secreted into duodenum, eventually parts of it excreted in feces
NOTE: majority is reabsorbed in ileum = enterohepatic circulation
Distinguish btw primary and secondary bile acids.
primary bile acids are synthesized from cholesterol in the liver, then conjugated, secreted into duodenum and metabolized to secondary bile acids by intestinal bacteria
- cholic acid → deoxycholic acid
- chenodeoxycholic acid → lithocholic acid
What are bile salts?
since bile is usually slightly alkaline (pH 7.6 - 8.4), bile acid and their conjugates are assumed to be in salt form
As an overview..
Which coenzymes and type of enzymes are required for bile acid synthesis?
- coenzymes: NADPH, NADH, CoA
- enzymes: hydroxylases (CYP 450 dependent), dehydrogenases, isomerases
Which enzyme catalyzes the first step of bile synthesis?
Reaction + cofactor.
Why is it especially important?
common step of both primary bile acids
7α-hydroxylase (CYP7A1)
cholesterol + NADPH + O2 →
NADP+ + 7α-dehydroxycholesterol
NOTE: requires vit C as cofactor
important b/c principal regulatory step
List the remaining steps of bile acid synthesis, once 7α-hydroxycholesterol is formed.
- sevel steps to produce primary bile acids:
- cholic acid
- chenodeoxycholic acid
- are conjugated w/ taurine or glycine to form
- tauro-/glycocholic acid
- tauro-/glycochenodeoxycholic acid
- are deconjugated and 7α-dehydroxylated to form secondary bile acids:
- deoxycholic acid
- lithocholic acid
List the hydroxylation sites of both primary and secondary bile acids
-
cholic acid: 3α, 7α, 12α
→ deoxycholic acid: 3α, 12α -
chenodeoxycholic acid: 3α, 7α
→ lithocholic acid: 3α
What are the 2 functions of bile acids?
- incr. the activity of pancreatic lipase
- incr. the surface are__a of emulsified lipid droplets by forming mixed bile acid-phospholipid micelles
Which phospholipid is especially abundant in mixed micelles?
phosphatidylcholine (= lecithin)
How is 7α dehydroxylase regulated?
- inhibited by bile acids
- transcription downregulated by SHP
- transcription upregulated by LXR
Which 2 nuclear receptors play a role in the regulation of bile acid (ergo cholesterol) levels?
- LXR = liver X receptor
- FXR = farnesoid X receptor
= metabolite receptors
Explain the function of LXR.
Overall effect.
liver X receptor
- when oxysterols (formed when high levels of cholesterol present) bind to LXR
- LXR forms heterodimer w/ RXR
-
incr. transcription of:
- CYP7A1
- ABCA1
- IDOL
⇒ incr. bile acid synthesis, cholesterol efflux and decr. the expression of LDL-Rs to prevent cholesterol overload of cells
Explain the function of the FXR.
Overall effect?
farnesoid X receptor
when bile acids accumulate (i.e. due to cholestasis), incr. transcription of
- PCSK9
- SHP
- BSEP, MRP2: canalicular transporters
⇒ incr. bile excretion and degradation of LDL-R (+ SHP effects)
What is the function of SHP?
small heterodimer protein
- enhanced transcription of SHP if FXR is active in response to binding of bile acids
- binds to transcription factors, forming a heterodimer + blocking their acivity
- causes decr. expression of__ CYP7A1
→ decr. bile acid synthesis
