بيوشيمي عموميگروه علوم دامي دانشکده کشاورزي

دانشگاه ياسوجدکتر مختار خواجوی

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