CHOLESTEROL SYNTHESIS & REGULATION

Cholesterol is constituent of all cell membrane.

Necessary for synthesis of steroid hormones, bile salts and Vitamin D.

Carried as ester & free cholesterol in the lipoproteins in plasma.

Normal level <200mg/dl

Abnormality of cholesterol metabolism may lead to cardiovascular disease and heart attacks.

Importance of cholesterol

Cholesterol is required to build and maintain cell membranes.

Regulates membrane fluidity over a wide range of temperatures

Aids in the manufacture of bile salts..

Precursor for the synthesis of vitamin D and steroid hormones

StructureCholesterol is an acyclic compound which includes:

a) Perhydro cyclopentano phenanthrene nucleus with 4 fused rings

b) OH group at C3

c) One double bond between C5 and C6

d) 8 member hydrocarbon chain attached to D ring in position 17.

e) CH3 group attached at position 10 and another CH3 group at position 13.

CHOLESTEROL STRUCTURE

Sources of cholesterol

Diet (300mg)

Denovo synthesis (700)

Biosynthesis of cholesterolMajor site of synthesis is liver (Denovo synthesis).

Other sites are adrenal cortex, testis, ovaries and intestine.

Liver is responsible for 80% of endogenous cholesterol synthesis.

Enzymes involved are partly located in endoplasmic reticulum and partly in cytoplasm

The process has five major steps:

(1) Acetyl - CoA’s are converted to 3-hydroxy-3-methyl glutaryl-CoA (HMG-CoA). HMG-CoA is converted to mevalonate

(3) Mevalonate is converted to isopentenyl pyrophosphate (IPP), with the concomitant loss of CO2

(4) IPP is converted to squalene

(5) Squalene is converted to cholesterol.

C5 + C5 = C10 (2x)C10 + C5 = C15 (2x)C15 + C15 = C30

5C

10C

15C

30C30C27C

First Series of reaction

The first steps involve the synthesis of the important intermediate MEVALONIC ACID from acetyl-CoA and acetoacetyl-CoA, in two enzymatic steps.

The first enzyme, hydroxymethyl-glutaryl (HMG)-CoA synthase.

The second enzyme HMG-CoA reductase is the rate-limiting step in the overall synthesis of sterols.

Enzymes are membrane -bound and are located in the endoplasmic reticulum.

Second series of reaction

The next sequence of reactions involves phosphorylation of mevalonic acid by mevalonate kinase to form the5-monophosphate ester.

Followed by a further phosphorylation to yield an unstable pyrophosphate, which is rapidly decarboxylated to produce 5-ISOPENTENYL PYROPHOSPHATE (IPP).

An isomerase converts part of IPP to 3,3-dimethylallyl pyrophosphate.

Third series of reaction

IPP and 3,3-dimethylallyl pyrophosphate condense readily with the elimination of pyrophosphoric acid to form GERANYLPYROPHOSPHATE.

This reacts with another molecule of 5-isopentenyl pyrophosphate to produce FARNESYL PYROPHOSPHATE.

Two molecules of farnesyl pyrophosphate, condenseto yield PRESQUALENE PYROPHOSPHATE.

The last is reduced by squalene synthase and NADPH to produce a further key intermediate SQUALENE.

Squalene is first oxidized by squalene mono oxygenase to SQUALENE 2,3 EPOXIDE.

Squalene 2,3-epoxide undergoes cyclization catalyzed by squalene epoxide lanosterol-cyclase to form the first steroidal intermediate LANOSTEROL.

In this remarkable reaction, there is a series of 1,2-methyl group and hydride shifts along the chain of the squalene molecule to bring about the formation of the four rings.

Finally, lanosterol is converted to cholesterol by multiple reactions that involve the removal of three methyl groups, hydrogenation of the double bond in the side-chain, and a shift of the double bond from position 8,9 to 5,6 in ring B.

Fate of cholesterol

Cholesterol can be exported and transported to other tissues in the form of lipoprotein complexes for uptake via LDL receptors

Converted to bile acids (500mg) and secreted into the intestines.

Converted to neutral sterols, coprostanol and cholestano (l500mg)

Products formed “Vitamin D and Steroid hormones”

Cell membrane

12α-hydroxylase

Vit C

7α-hydroxylase

CATABOLISM OF CHOLESTEROL:SYNTHESIS OF BILE ACIDS

Regulation of cholesterol synthesisRegulatory enzyme is HMG – CoA reductase (HMGR).

Has a feed back regulation by cholesterol.

Amount of dietary cholesterol determines the cholesterol synthesis.

Short term regulation is by covalent modification of the enzyme.

Insulin and T3 increase activity of (HMGR).

Cortisol and glucagon decreases the activity.

Regulation of HMGR activity is the primary means for controlling the level of cholesterol biosynthesis.

The enzyme is controlled by three distinct mechanisms:

control of gene expression.

rate of enzyme degradation.

phosphorylation-dephosphorylation.

The first two control mechanisms are exerted by cholesterol itself.

Cholesterol acts as a feed-back inhibitor of pre-existing HMGR and induces rapid turn-over of the enzyme.

In addition, when cholesterol is in excess, the amount of mRNA for HMGR is reduced as a result of decreased expression of the gene.

Regulation of HMGR through covalent modification occurs as a result of phosphorylation and dephosphorylation.

Active form – dephosphorylated form.

Inactive form – phosphorylated form.

HMGR is phosphorylated by AMP-regulated protein kinase (AMPRK).

AMPRK itself is activated via phosphorylation.

The phosphorylation of AMPRK is catalyzed by kinase kinase.

Glucagon and epinephrine negatively affect cholesterol biosynthesis by increasing the activity of the inhibitor of phosphoprotein phosphatase inhibitor-1 (PPI-1).

Insulin stimulates the removal of phosphates and, thereby, activates HMG-CoA reductaseactivity.

Modes of lowering cholesterolDietary restriction.

Vegetable oils and PUFA.

Moderation in fat intake.

Green leafy vegetables.

Avoiding sucrose.

Exercise.

Keep DM & HT under control.

Hypolipidemic drugs.

Dietary restriction:

Eggs and meat contain high cholesterol

Vegetable oils and PUFA:

Sunflower oil and fish oils contain PUFA for esterification and final excretion of cholesterol.

Omega-3 FA from fish oils reduce LDL

Green leafy vegetables:

As they have high fiber content, they increase motility and reduce reabsorption of bile salts

Vegetables also contain plant sterol SITOSTEROL which decreases absorption of cholesterol

Hypolipidemic drugs:

Bile acid-binding resins decrease reabsorption of bile acids. E.g. Cholestyramine & Cholestipol

HMG CoA reductase inhibitor (STATINS) are recently used. E.g. Lovastatin and Simvostatin.

Nicotinic acid inhibits lipolysis and also lowers plasma cholesterol levels.