Download CHOLESTEROL SYNTHESIS

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
Structure
„ Cholesterol 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 cholesterol
„ Major 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-3methyl 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
27C
30C
30C
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 the
5-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, condense
to 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 lanosterolcyclase 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
CATABOLISM OF CHOLESTEROL:SYNTHESIS OF BILE ACIDS
Vit C
7α-hydroxylase
12α-hydroxylase
Regulation of cholesterol synthesis
„ Regulatory 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 reductase
activity.
Modes of lowering cholesterol
„ Dietary 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.