cholesterol sythesis and regulation

1. Synthesis of cholesterol
Maria gul
Lecturer
jips

2. CHOLESTEROLINTRODUCTION
Cholesterol is the major sterol in the animaltissues.
Cholesterol is present in tissues and in plasma either
as free cholesterol or as a storage form, combined
with a long-chain fatty acid as cholesteryl ester.
Inplasma, both forms are transported in lipoproteins
Plasma low-density lipoprotein (LDL) is the vehicle of
uptake of cholesterol and cholesteryl ester into many
tissues.
Free cholesterol is removed from tissues by plasma
high-density lipoprotein (HDL)and transported to
the liver, where it is eliminated from the bodyeither
unchanged or after conversion to bile acids in the
process known as reverse cholesterol transport . 2
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3. STRUCTUREOFCHOLESTEROL
The structure of cholesterol consists of four fused
rings (The rings in steroids are denoted by theletters
A,B,C,and D.),with the carbons numbered in the
sequence, and an eight numbered, and branched
hydrocarbon chain attached to the Dring.
Cholesterol contains two angular methyl groups: the
C-19 methyl group is attached to C-10, and the C-18
methyl group is attached to C-13.
The C-18 and C-19 methyl groups of cholesterollie
above the plane containing the four rings.
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4. STRUCTUREOFCHOLESTEROL(CONTD.)
Steroids with 8to 10 carbon atoms in the side chain
and an alcohol hydroxyl group at C-3 are classified as
sterols. Muchof the plasma cholesterol is in the
esterified form (with a fatty acid attached at carbon3),
which makes the structure even more hydrophobic.
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5. FUNCTIONSOFCHOLESTEROL
Cholesterol is the most abundant sterol in humans and
performs a number of essential functions. Forexample-
It is a major constituent of the plasma membrane and of
plasma lipoproteins.
It is a precursor of bilesalts,
It is a precursor of steroid hormones that include
adrenocortical hormones, sex hormones, placental
hormones etc
Also a precursor of vitamin D,cardiac glycosides,
Sitosterol of the plant kingdom, and some alkaloids.
It is required for the nerve transmission. Cholesterol is
widely distributed in all cells of the body butparticularly
abundant in nervous tissue.
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6. SOURCESOFCHOLESTEROL
Cholesterol is derived from
diet
denovosynthesis and
fromthe hydrolysis ofcholesteryl esters.
Alittle more than half the cholesterol of the body
arises by synthesis (about 700 mg/d), and the
remainder is provided by the averagediet.
The liver and intestine account for approximately
10% each of total synthesis inhumans.
Virtually all tissues containing nucleated cells are
capable of cholesterol synthesis, which occurs in the
endoplasmicreticulumandthe cytosol. 6
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7. STEPSOFSYNTHESISOFCHOLESTEROL
AcetylcoAactsasaprecursor of cholesterol.
All 27 carbon atoms of cholesterol are derived from
acetyl CoAin a three-stage synthetic process
Stageoneis the synthesisofIsopentenyl
pyrophosphate,anactivated isopreneunitthatis
the key buildingblockof cholesterol.
Stagetwois the condensationofsixmoleculesof
Isopentenylpyrophosphatetoform Squalene.
Instagethree,Squalenecyclizesinanastounding
reaction andthe tetracyclic productis
subsequentlyconvertedinto cholesterol.
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8. STAGE1 OFCHOLESTEROLSYNTHESIS
The first stage in the synthesis of cholesterol is the
formation of Isopentenyl pyrophosphate from acetyl
CoA.
This set of reactions, which takes place in the cytosol,
starts with the formation of 3-hydroxy-3-
methylglutaryl CoA(HMGCoA) from acetylCoA.
Initially, two molecules of acetyl-CoA condense to
form Acetoacetyl-CoA catalyzed by cytosolic thiolase.
Acetoacetyl-CoA condenses with a further molecule of
acetyl-CoA catalyzed by HMG-CoAsynthase to form
HMG-CoA,that is reduced to mevalonate by NADPH
catalyzed by HMG-CoAreductase. 8
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9. The synthesis of
mevalonate is the
committed step in
cholesterol formation.
The enzyme catalyzing
this irreversible step,
3-hydroxy-3-
methylglutaryl CoA
reductase (HMG-CoA
reductase), is an
important control site in
cholesterol biosynthesis,
and is the site of action of
the most effective class of
cholesterol-lowering
drugs, the HMG-CoA
reductase inhibitors
(statins). 9
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10. STAGE 1OFCHOLESTEROL SYNTHESIS
(CONTD.)
Mevalonate is converted into 3-isopentenyl
pyrophosphatein three consecutive reactions requiring
ATP.
Decarboxylation yields Isopentenyl pyrophosphate,
an activated isoprene unit that is a key buildingblock
for many important biomolecules.
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11. STAGE -2 OFCHOLESTEROLSYNTHESIS
Synthesisof Squalene
Squalene (C30) is synthesized from six molecules of
Isopentenyl Pyrophosphate (C5) and the reaction
sequence is-
C5 C10 C15 C30
This stage in the synthesis of cholesterol starts with
the isomerization of isopentenyl pyrophosphateto
dimethylallyl pyrophosphate.
Isopentenyl diphosphate is isomerized by a shift of
the double bond to form dimethylallyldiphosphate,
then condensed with another molecule ofisopentenyl
diphosphate to form the ten-carbon intermediate
geranyl diphosphate. 11
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13. STAGE -2 OFCHOLESTEROL SYNTHESIS
(CONTD.)
Afurther condensation with isopentenyl
diphosphate forms farnesyldiphosphate.Two
molecules of farnesyl diphosphate condense at the
diphosphate end to form squalene.
Initially, inorganic pyrophosphate is eliminated,
forming presqualene diphosphate, which is then
reduced by NADPH with elimination of a further
inorganic pyrophosphate molecule.
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14. STAGE-3- FORMATION OFCHOLESTEROL
FROM SQUALENE
Squalene can fold into a
structure that closely
resembles the steroid
nucleus .
Before ring closure occurs,
squalene is converted to
squalene 2,3-epoxide by a
mixed-function oxidase inthe
endoplasmic reticulum,
squaleneepoxidase.
The methyl group on C14is
transferred to C13and that on
C8to C14as cyclization occurs,
catalyzed by oxidosqualene:
lanosterol cyclase.
Thenewly formedcyclized
structure is Lanosterol
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15. STAGE-3- FORMATION OFCHOLESTEROL
FROM SQUALENE(CONTD.)
The formation of cholesterol from
lanosteroltakes place in the
membranes of the endoplasmic
reticulum and involves changes inthe
steroid nucleus and side chain .
The methyl groups on C14and C4are
removed to form 14-desmethyl
lanosterol and then zymosterol. The
double bond at C8–C9is subsequently
moved to C5–C6in two steps, forming
desmosterol.
Finally, the double bond of the side
chain is reduced, producingcholes
1
t
5
erol.
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16. REGULATION OFCHOLESTEROLBIOSYNTHESIS
Regulation of cholesterol synthesis is exerted near the
beginning of the pathway, at the HMG-CoAreductase
step.
Following mechanisms are involved at the regulatory
step-
Competitive inhibition
Feed back inhibition
Covalent modification(Role of hormones)
Sterol mediated regulation of transcription
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17. REGULATION OFCHOLESTEROLBIOSYNTHESIS
Competitive inhibition
Statins (Lovastatin,
Mevastatin, Atorva Statin
etc.) are the reversible
competitive inhibitors of
HMGCo A reductase.
They are used to decrease
plasma cholesterol levels in
patients of
hypercholesterolemia.
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18. REGULATION OFCHOLESTEROLBIOSYNTHESIS
Feedback inhibition
HMGCo Areductase is
inhibited by Mevalonateand
Cholesterol.
Mevalonate is the immediate
product of HMGCo Areductase
catalyzed reaction whereas
Cholesterol is the ultimate
product of the reaction
pathway.
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19. REGULATIONOFCHOLESTEROLBIOSYNTHESIS
Covalentmodification(Roleof hormones)
Phosphorylation decreases the activity of the
reductase.
Glucagon favors formation of the inactive
(phosphorylated form) form, hence decreases the
rate of cholesterol synthesis
Incontrast ,insulin favors formation of the
active(dephosphorylated )form of HMGCo A
reductase and results in an increase in the rate of
cholesterol synthesis
Cholesterol synthesis ceases when the ATPlevel is
low
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20. REGULATIONOFCHOLESTEROLBIOSYNTHESIS
Sterolmediatedregulation of transcription
The synthesis of cholesterol is also regulated by the
amount of cholesterol taken up by the cells during
lipoprotein metabolism.
Chylomicron remnants internalized by liver cells, and
low density lipoproteins internalized by liver cells
and peripheral tissues provide cholesterol which
causes a decrease in the transcription of HMGCoA
reductase gene, leading to a decrease in cholesterol
synthesis.
The rate of synthesisof reductase mRNAis controlled
bythe sterol regulatory element bindingprotein
(SREBP).
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21. REGULATIONOFCHOLESTEROLBIOSYNTHESIS
Sterolmediated regulation of transcription
This transcription factor binds to a short DNA sequence
called the sterol regulatory element (SRE) on the 5 side of
the reductase gene.
Inits inactive state, the SREBPis anchored to the
endoplasmic reticulum or nuclear membrane.
When cholesterol levels fall, the protein is released
The released protein migrates to the nucleus and binds
the SREof the HMG-CoAreductase gene, to enhance
transcription.
When cholesterol levels rise, the proteolytic release of
the SREBP is blocked, and the SREBP in the nucleus is
rapidly degraded
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22. TRANSPORTOFCHOLESTEROL
Cholesterol is transported in plasma in lipoproteins, and
in humans the highest proportion is found in LDL.
Cholesteryl ester in the diet is hydrolyzed to cholesterol,
which is then absorbed by the intestine together with
dietary unesterified cholesterol and other lipids.
With cholesterol synthesized in the intestines, it is then
incorporated into chylomicrons.
Ninety-five percent of the chylomicron cholesterol is
delivered to the liver in chylomicron remnants,
Most of the cholesterol secreted by the liver in VLDLis
retained during the formation of IDLand ultimately LDL,
which is taken up by the LDLreceptor in liver and extra
hepatic tissues.
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23. UPTAKEOFLDLCHOLESTEROL
The LDLs(containing cholesteryl esters) are taken up
by cells by a process known as receptor-mediated
endocytosis.
The LDLreceptor mediates this endocytosis and is
important to cholesterol metabolism.
After LDLbinding to the LDLreceptor, the ligand-
receptor complexes cluster on the plasma membrane
in coated pits, which then invaginate forming coated
vesicles.
These coated vesicles are internalized and clathrin,
the protein composing the lattice in membrane
coated pits, is removed.
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24. UPTAKEOFLDLCHOLESTEROL
These vesicles are now called endosomes and these
endosomes fuse with the lysosome.
The LDLreceptor–containing membrane buds off and
is recycled to the plasma membrane.
Fusion of the lysosome and endosome releases
lysosomal proteases that degrade the apoproteins
into amino acids.
Lysosomal enzymes also hydrolyze the cholesteryl
esters to free cholesterol and fattyacids.
The free cholesterol is released into the cell’s
cytoplasm, and this free cholesterol is then available
to be used by thecell.
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25. UPTAKEOFLDLCHOLESTEROL
Excess cholesterol is reesterified by acyl-CoA:
cholesterol acyltransferase (ACAT),which uses fatty
acyl-CoA as the source of activated fatty acid.
Free cholesterol affects cholesterol metabolism by
inhibiting cholesterol biosynthesis.
Cholesterol inhibits the enzyme hydroxy-
methylglutaryl-CoA reductase (HMG-CoAreductase),
which catalyzes an early rate-limiting step in
cholesterol biosynthesis.
Inaddition, free cholesterol inhibits the synthesis of
the LDLreceptor, thus limiting the amount of LDLs
that are taken up by the cell.
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26. UPTAKEOFLDLCHOLESTEROL
Receptor mediated endocytosis of LDL
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27. VARIATIONOFSERUMCHOLESTEROL
LEVELS
The normal serum cholesterol concentration ranges
between 150- 220mg/dl
Highcholesterol concentration is foundin-
Diabetes mellitus
Nephrotic syndrome
Obstructive jaundice
Familial hypercholesterolemia
Biliary cirrhosis
Hypothyroidism
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28. VARIATION OFSERUM CHOLESTEROL
LEVELS
Hypocholesterolemia-Low serum cholesterol
concentration is observed in-
Hyperthyroidism
Malnutrition
Malabsorption
Anemia
Physiologically lower levels are found inchildren
Persons on cholesterol lowering drugs
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29. HYPERCHOLESTEROLEMIA AND THE
CONSEQUENCES
Atherosclerosis is characterized
by the deposition of cholesterol
and cholesteryl ester from the
plasma lipoproteins into the
artery wall.
Diseases in which prolonged
elevated levels of VLDL,IDL,
chylomicron remnants, or LDL
occur in the blood are often
accompanied by premature or
more severe atherosclerosis.
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30. HYPOLIPIDEMIC DRUGS
Statins- The statins act as competitive inhibitors of the
enzyme HMG-CoAreductase.
Fibratessuch as Clofibrateand gemfibrozilact mainly
to lower plasma triacylglycerols by decreasing the
secretion of triacylglycerol and cholesterol-containing
VLDLby the liver.
Ezetimibe-ezetimibe, reduces blood cholesterol levels
by inhibiting the absorption of cholesterol by the
intestine
BileAcidSequestrants (Resins)-Bileacid sequestrants
bind bile acids in the intestine and promote their
excretion in the stool. Tomaintain the bile acid pool size,
the liver diverts cholesterol to bile acidsynthesis
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31. HYPOLIPIDEMIC DRUGS
Bileacidsequestrants(contd.)-Thedecreased hepatic
intracellular cholesterol content results in up regulation
of the LDLreceptor and enhanced LDLclearance from the
plasma. Bile acid sequestrants, including
Cholestyramine,Colestipol,and colesevelam.
Omega3 FattyAcids(FishOils)-The most widely used
n-3 PUFAsfor the treatment of hyperlipidemia are the
two active molecules in fish oil: Eicosapentaenoic acid
(EPA)and Docosahexaenoic acid (DHA).
Niacininhibits the release of free fatty acids from adipose
tissue which leads to a decrease of free fattyacids
entering the liver and decreased VLDLsynthesis in the
liver.
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32. ROLE OFDIET IN REGULATINGCHOLESTEROL
LEVELS
Polyunsaturated fatty acids have a cholesterol
lowering effect
There is the up-regulation of LDLreceptors by poly-
and monounsaturated as compared with saturated
fatty acids, causing an increase in the catabolic rate of
LDL,the main atherogenic lipoprotein.
Inaddition, saturated fatty acids cause the formation
of smaller VLDLparticles that contain relatively more
cholesterol, and they are utilized by extra hepatic
tissues at a slower rate than are larger particles and
thus may be regarded as atherogenic.
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33. LIFESTYLEAND THESERUM CHOLESTEROL
LEVELS
Additional factors considered to play a part in coronary
heart disease include high blood pressure, smoking,
male gender, obesity (particularly abdominal obesity)
and lack of exercise
Premenopausal women appear to be protected against
many of these deleterious factors, and this is thoughtto
be related to the beneficial effects ofestrogen.
There is an association between moderate alcohol
consumption and a lower incidence of coronary heart
disease. This may be due to elevation of HDL
concentrations resulting from increased synthesis of
apo A-I
Regular exercise lowers plasma LDLbut raises HDL.
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