Cholesterol is found exclusively in animals, hence it is often called as animal sterol. The total body content of cholesterol in an adult man weighing 70 kg is about 140 g i.e., around 2 g/kg body weight. The level of cholesterol in blood is related to the development of atherosclerosis & MI.

1. Cholesterol Biosynthesis
Rajendra Dev Bhatt, PhD Scholar
Asst. Professor
Clinical Biochemistry & Laboratory Medicine
Fellow: Translational Research (2018-2022) in CVD in Nepal, NHLBI & NIH,
USA

2.  Cholesterol is found exclusively in animals,
hence it isoften called as animal sterol.
 Thetotal body content of cholesterol in an
adult man weighing 70 kg is about 140 g i.e.,
around 2g/kg body weight.
The level of cholesterol in blood is related to the
development of atherosclerosis & MI.
Cholesterol

3.  All steroids have
cyclopentanoperhydrophenanthrene ring system.
 Cholesterol isamphipathic in nature.
 It possessesboth hydrophilic & hydrophobic
regions in the structure.
 All carbon atoms of cholesterol are derived
from acetyl CoA.

4. Cholesterol Structure
• It has a molecular weight of 386 Da and contains 27 carbon
atoms, of which 17 are incorporated into four fused rings (the
perhydro cyclopentano-phenanthrene nucleus), two are in
angular methyl groups attached at the junctions of rings AB
and CD and eight are in the peripheral side chain.
• Cholesterol is almost entirely composed of carbon and
hydrogen atoms; there is a solitary hydroxyl group attached
to carbon atom 3.
• It is also almost saturated, having just one double bond
between carbon atoms 5 and 6.

5. Cholesterol: Structure
Cholesterol
Perhydrocyclopentanophenanthrene

7. Sites:
 Major sites are liver, adrenal cortex, testes,
ovaries & intestine.
 All nucleated cells can synthesize
cholesterol, including arterial wall.
Location:
 The enzymes involved in the synthesis of
cholesterol are partly located in endoplasmic
reticulum &partly in cytoplasm.

8.  Acetyl CoAprovides all carbon atoms.
 Reducing equivalents are supplied by NADPH.
 ATPprovidesenergy.
For production of one molecule of cholesterol….
 18moles of acetyl CoA
 36moles of ATP
 16moles of NADPH are required.
Requirements

9. Five stages.
 Synthesis of HMG CoA(β-hydroxy β-
methylglutaryl CoA )
 Formation of Mevalonate
 Production of isoprenoid units
 Synthesis of squalene
 Conversion of squalene to cholesterol.

10. The acetyl-CoA utilized for cholesterol biosynthesis is derived from an oxidation
reaction (e.g., fatty acids or pyruvate) in the mitochondria and is transported to the
cytoplasm by the same process as that described for fatty acid synthesis

11.  Two moles of acetyl CoA condense to
form acetoacetyl CoA.
 Another molecule of acetyl CoA is
then added to produce HMG
CoA.
 These reactions are similar to
that of ketone body synthesis.
1. Synthesis of HMG CoA

12.  Thetwo pathways are distinct.
 Ketone bodies are produced in mitochondria
while cholesterol synthesis occursin cytosol.
 There exist two pools of HMG CoA in the
cell.

13.  Two isoenzymes of HMG CoA synthase are
known.
 The cytosomal enzyme is involved in
cholesterol synthesis whereas the
mitochondrial HMG CoA synthase
participates in ketone body formation.

14.  HMG CoA reductase is the
rate limiting enzyme in
cholesterol biosynthesis.
 This enzyme is present in
endoplasmic reticulum & catalyses
the reduction of HMG CoA to
mevalonate.
 The reducing equivalents are
suppplied by NADPH.
2. Formation of Mevalonate

15.  In a three step reaction catalyzed by
kinases, mevalonate isconverted to 3-
phospho 5-pyrophosphomevalonate which
on decarboxylation forms isopentenyl
pyrophosphate (lPP).
 It isisomerizes in to
dimethylallylpyrophosphate (DPP).
 IPP&DPPare 5-carbon isoprenoid units.
3. Production of Isoprenoid

18.  IPP&DPPcondense to produce a 10-
carbon geranyl pyrophosphate (GPP).
 Another molecule of IPPcondenses with GPP
to form a 15-carbon farnesyl pyrophosphate
(FPP).
 Two units of farnesyl pyrophosphate unite &
get reduced to produce a 30-carbon squalene.
4. Synthesis of Squalene

19.  Conversion of squalene to cholesterol:
 Squalene undergoes oxidation by epoxidase,
using molecular oxygen & NADPH to form
squalene epoxide.
 Cyclase converts it to 30carbon lanosterol.
 It is the first steroid compound
synthesized.
5.Cyclization

20.  The formation of cholesterol from
lanosterol is a multistep process with a
series of about 19 enzymatic reactions.
 Most important reactions:
 Reducing the carbon atoms from 30to 27.
 Removal of 2 methyl groups from C4 & C14
to produce zymosterol.
5. Cutting Size

21.  Shift of double bond from C8to C5
 Reduction in the double bond present
between C24and C25.
 Theenzymes (about 19)involved in the
Conversion of lanosterol to cholesterol are
associated with endoplasmic reticulum.

22.  14-desmethyl lanosterol, zymosterol, cholestadienol
& desmosterol are among the intermediates in the
cholesterol biosynthesis.
 The penultimate product is 7- dehydrocholesterol
which, on reduction, finally yields cholesterol.

24.  HMG CoAreductase israte-limiting enzyme.
HMG CoA reductase is found in association with
endoplasmic reticulum & is subjected to different
metabolic controls.
 Regulation at transcription:
Long-term regulation involves regulation of
transcription of gene for HMG CoA reductase.
Regulation of Cholesterol Synthesis

25.  Sufficient cholesterol is present in the cell,
transcription of the gene for HMG CoA
reductase is suppressed & cellular synthesis of
cholesterol is decreased.
 Cholesterol in diet is low, synthesis is
increased.
 Cholesterol regulates the expression of HMG
CoAreductase gene &LDLreceptor gene.

26.  A specific recognition sequence known as sterol
regulatory element (SRE)ispresent in DNA.
 S R E binding by sterol regulatory element binding
protein (SREBP) is essential for the transcription of
these genes.
 When cholesterol levels are high, the S R E B P
remains as inactive precursor.

27.  SREBP cleavage activator protein (SCAP), is an
intracellular cholesterol sensor.
 When cholesterol levels are less, SCAP escorts
SREBPto Golgi bodies.
 Two Golgi proteases - site 1protease 1& 2 (S1P& S2P)
sequentially cleave the SREBP to a protein which
binds to SRE & activates transcription of HMG CoA
reductase gene.

28.  Covalent modification:
HMG CoA reductase is inhibited by
phosphorylation, catalyzed by AMP- dependent
protein kinase (which also regulates fatty acid
synthesis & catabolism).
 Dephosphorylation by protein
phosphatase 1makes it active.
Short Term Regulation

30.  Low serum cholesterol concentration is observed
in Hyperthyroidism, Malnutrition,
Malabsorption, Anemia, Physiologically lower
levels are found in children
 Hypercholesterolemia in hypothyroidism is
mainly due to a reduction in low-density
lipoprotein (LDL) receptor activity.
 Enhanced Cholesterol Conversion to Bile Acids
Hormonal Regulation

31.  Cortisol & glucagon decreases the activity of
HMG CoAreductase.
 HMG CoA reductase activity is inhibited by
bile acids.
 Fasting is also reduces the activity of HMG
CoAreductase.
Hormonal Regulation….

33.  Lovastatin &other statin group drugs are
competitive inhibitors of HMG CoA reductase.
 These drugs are used to reduce the
cholesterol levels in blood.
Drugs

35.  The total body cholesterol content varies from 130-
150g.
 LDL transports cholesterol from liver to
peripheral tissues.
 HDLtransports cholesterol from tissuesto liver.
 Cells of extrahepatic tissues take up
cholesterol from LDL.

36.  The free cholesterol released within the cell has
following fates:
 Incorporated into cell membranes.
 Metabolized to steroid hormones, especially
in adrinal cortex & gonads
 Esterified with saturated fatty acids & stored
in cells.

37.  The enzyme ACAT (acyl cholesterol acyl
transferase) helps in this reaction.
 Esterified with LCAT & incorporated into HDL,
trnasported &finally excreted through liver.

38.  Average diet contains about 300 mg of cholesterol
per day.
 Body synthesizes about 700mg/day.
 About 500mgof cholesterol isexcreted through bile.
 Some of this partly reabsorbed from intestine.
 Remaining is converted to bile acids, which are excreted
in the bile as bile salts.
 By conversion into bile acids and bile salts- excreted in
the feces
Excretion of Cholesterol

39. The cholesterol serves as the precursor for the
following biosynthetic pathways, EXCEPT
a) Bile acid synthesis
b) Steroid hormone synthesis
c) Aldosterone synthesis
d) Thyroid hormone synthesis
d. Thyroid hormone synthesis

40. Which of the following statement is NOT
true?
A. Cholesterol is amphipathic in nature
B. Starvation inhibits synthesis of cholesterol
C. T4 increases cholesterol synthesis
D. In the stage 5, carbon chain become short
from 30 to 27
T4 increases cholesterol synthesis

41. Which enzyme is termed as the rate
limiting enzyme of cholesterol synthesis?
A. Thiolase
B. HMG-CoA reductase
C. HMG-CoA carboxylase
D. none of the above
HMG-CoA reductase

42. THANK YOU