hydroxymethylglutaryl-coenzyme a (hmg-coa) is the precursor for cholesterol synthesis. hmg-coa is...
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بيوشيمي عموميگروه علوم دامي دانشکده کشاورزي
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CholesterolSynthesis
Hydroxymethylglutaryl-coenzyme A (HMG-CoA) is the precursor for cholesterol synthesis.
HMG-CoA is also an intermediate on the pathway for synthesis of ketone bodies from acetyl-CoA.
The enzymes for ketone body production are located in the mitochondrial matrix.
HMG-CoA destined for cholesterol synthesis is made by equivalent, but different, enzymes in the cytosol.
CH2 C CH2 C
OH O
SCoA
CH3
C
O
O
hydroxymethylglutaryl-CoA
HMG-CoA is formed by condensation of acetyl-CoA & acetoacetyl-CoA, catalyzed by HMG-CoA Synthase.
HMG-CoA Reductase catalyzes production of mevalonate from HMG-CoA.
H3C C CH2 C
O O
SCoA
H3C C
O
SCoA
HSCoA
CH2 C CH2 C
OH O
SCoA
CH3
C
O
O
H2O acetoacetyl-CoA
hydroxymethylglutaryl-CoA
acetyl-CoA HMG-CoA Synthase
The carboxyl of HMG that is in ester linkage to the CoA thiol is reduced to an aldehyde, and then to an alcohol.
NADPH serves as reductant in the 2-step reaction.
Mevaldehyde is thought to be an active site intermediate, following the first reduction and release of CoA.
+ HSCoA
H2CC
CH3HO
CH2
CO O
C SCoA
O
H2CC
CH3HO
CH2
CO O
H2C OH
2NADP+
2NADPH
HMG-CoA
mevalonate
HMG-CoAReductase
HMG-CoA Reductase is an integral protein of endoplasmic reticulum membranes.
The catalytic domain of this enzyme remains active following cleavage from the transmembrane portion of the enzyme.
The HMG-CoA Reductase reaction, in which mevalonate is formed from HMG-CoA, is rate-limiting for cholesterol synthesis.
This enzyme is highly regulated and the target of pharmaceutical intervention.
Mevalonate is phosphorylated by 2 sequential Pi transfers
from ATP, yielding the pyrophosphate derivative.
ATP-dependent decarboxylation, with dehydration, yields isopentenyl pyrophosphate.
H2CC
CH3HO
CH2
C O O
CH2 OH
H2C
C
CH2 CH2 O P O P O
O
O
O
O
CH3
H2CC
CH3HO
CH2
C O O
CH2 O P O P O
O
O
O
O
CO2
ATP
ADP + Pi
2 ATP
2 ADP
mevalonate
5-pyrophosphomevalonate
(2 steps)
isopentenyl pyrophosphate
Isopentenyl pyrophosphate is the first of several compounds in the pathway that are referred to as isoprenoids, by reference to the compound isoprene.
isoprene
H2CC
CCH2
CH3
H
is o p e n te n y l p y ro p h o s p h a te
H 2 CC
CH 2
H 2C
C H 3
O P
O
O
O P O
O
O
Isopentenyl Pyrophosphate Isomerase inter-converts isopentenyl pyrophosphate & dimethylallyl pyrophosphate.
Mechanism: protonation followed by deprotonation.
H2C
C
CH2 CH2 O P O P O
O
O
O
O
CH3
H3C
C
CH CH2 O P O P O
O
O
O
O
CH3
isopentenyl pyrophosphate
dimethylallyl pyrophosphate
Prenyl Transferase catalyzes head-to-tail condensations:
Dimethylallyl pyrophosphate & isopentenyl pyrophosphate react to form geranyl pyrophosphate.
Condensation with another isopentenyl pyrophosphate yields farnesyl pyrophosphate.
Each condensation reaction is thought to involve a reactive carbocation formed as PPi is eliminated.
Condensation Reactions
CH2 CH2 O P O P O
O
O
O
O
CH CH2 O P O P O
O
O
O
O
CH2C
CH3
CH3C
CH3
CH CH2CH3C
CH3
CH CH2 O P O P O
O
O
O
O
CCH2
CH3
PP i
CH2 CH2 O P O P O
O
O
O
O
CH2C
CH3
CH CH2CH3C
CH3
CH CH2CCH2
CH3
PP i
CH CH2 O P O P O
O
O
O
O
CCH2
CH3
dimethylallyl pyrophosphate
isopentenyl pyrophosphate
isopentenyl pyrophosphate
geranyl pyrophosphate
farnesyl pyrophosphate
Each condensation involves a carbocation formed as PPi is eliminated.
Squalene Synthase: Head-to-head condensation of 2 farnesyl pyrophosphate, with reduction by NADPH, yields squalene.
CH CH2CH3C
CH3
CH CH2CCH2
CH3
CH CH2 O P O P O
O
O
O
O
CCH2
CH3
2
O
NADP+
O2 H2O
HO
H+
NADPH
NADP+ + 2 PP i
NADPH
2 farnesyl pyrophosphate
squalene 2,3-oxidosqualene lanosterol
Squaline epoxidase catalyzes conversion of squalene to 2,3-oxidosqualene.
This mixed function oxidation requires NADPH as reductant & O2 as oxidant. One O atom is incorporated into substrate (as the epoxide) & the other O is reduced to water.
O
NADP+
O2 H2O
HO
H+NADPH
squalene 2,3-oxidosqualene lanosterol
Structural studies of a related bacterial enzyme have confirmed that the substrate binds at the active site in a conformation that permits cyclization with only modest changes in position as the reaction proceeds.
The product is the sterol lanosterol.
O HO
H+
2,3-oxidosqualene lanosterol
Squalene Oxidocyclase catalyzes a series of electron shifts, initiated by protonation of the epoxide, resulting in cyclization.
Conversion of lanosterol to cholesterol involves 19 reactions, catalyzed by enzymes in ER membranes.
Additional modifications yield the various steroid hormones or vitamin D.
Many of the reactions involved in converting lanosterol to cholesterol and other steroids are catalyzed by members of the cytochrome P450 enzyme superfamily.
H O H O
lan o ste ro l ch o leste ro l
1 9 s tep s