2. The learner will be able to:
• Draw the structure of cholesterol and other steroid compounds
in the body
• Describe the biosynthesis of cholesterol
• Explain the regulation of cholesterol synthesis
• Classify the lipoproteins based on their composition
• Describe the metabolism of chylomicrons, very low density lipoproteins,
low-density lipoproteins and high-density lipoproteins
• Highlight the importance of free fatty acids and lipoproteins in plasma
• Enumerate the compounds formed from cholesterol
• Outline the formation of bile acids and bile salts and list their functions
Specific Learning Objectives
3. Competencies covered
BI-4.1 Describe and discuss main classes of lipids
(cholesterol and hormonal steroids) relevant to
human system and their major functions
BI-4.3 Explain the regulation of lipoprotein metabolism
and associated disorders
BI-11.9 Estimation of serum total cholesterol and HDL
cholesterol
BI-11.10 Estimation of triglycerides
4. Cholesterol is widely distributed in animal tissues. It is absent
in prokaryotes. In plants, cholesterol is absent, but other plant sterols
are present. In bacteria and plants, compounds similar to steroids
exist, known as hopanoids.
Cholesterol
5. The level of cholesterol in blood is related to the development
of atherosclerosis and myocardial infarction.
Abnormality of cholesterol metabolism may lead to cardiovascular
accidents and heart attacks.
Clinical Significance of Cholesterol
6. 1. Cell membranes: Cholesterol is a component of
membranes.
2. Nerve conduction: Cholesterol has an insulating effect onnerve
fibers.
3. Bile acids and bile salts are derived from cholesterol. Bile salts are
important for fat absorption.
4. Steroid hormones: Glucocorticoids, androgens and estrogens are
from cholesterol.
5. Vitamin D3 is from 7-dehydrocholesterol.
6. Esterification: The OH group of cholesterol is esterified to fatty
acids to form cholesterol esters. This esterification occurs in the
body by transfer of a PUFA moiety by lecithin-cholesterol acyl-
transferase.
Functions of cholesterol
7. Salient features of steroids
Name
of steroid
Total no of
carbon atoms
No of carbon
atoms in side
chain
Importance
Cholesterol 27 8 Most important
animal sterol
Bile acids 24 5 Emulsifying agents
Glucocorti- coids and
Mineralo- corticoids
21 2 Influences Metabolism
as well as fluid and
electrolyte balance
Testosterone 19 - Male sex hormones
Estrogens 18 - Female sex hormones
9. Cholesterol has a total of 27 carbon atoms.
One hydroxyl group at third position which is characteristic
of all sterols. The OH group is beta oriented, projecting above the
plane of ring.
There is a double bond between carbon atoms 5 and 6.
Further, there is an eight carbon side chain, beta-oriented attached to
17th carbon
10. The acetyl-CoA is provided by the ATP-citrate lyase reaction
as in the case of fatty acid synthesis. Two molecules of
acetyl-CoA condense to form acetoacetyl-CoA catalyzed by
cytoplasmic acetoacetyl-CoA synthase
Biosynthesis of Cholesterol
11. A third molecule of acetyl-CoA condenses with
acetoacetyl-CoA to form beta-hydroxy-beta-methylglutaryl
CoA (HMGCoA).
The enzyme is HMG-CoA synthase
Step 2: Production of HMG-CoA
12. The reduction of HMG-CoA to mevalonate is catalyzed by
HMG-CoA reductase. It is a microsomal enzyme. It uses 2
molecules of NADPH.
Step 3: The Committed Step
13. Mevalonate is phosphorylated to 3-phospho-5-pyrophospho
mevalonate. This then undergoes decarboxylation to give rise
to isopentenyl pyrophosphate, a 5 carbon unit. This requires 3
molecules of ATP
Isopentenyl pyrophosphte
Step 4: Production of 5 Carbon Unit
14. Six 5-carbon units are condensed to form a 30 carbon
compound, Squalene. In summary:
IPP(5C) + DMAPP(5C) ® GPP(10C) + IPP(5) ®
FPP(15C) + FPP(15C) ® Squalene (30C)
Step 6: Cyclization
Squalene is a straight line structure. Then squalene undergoes
oxidation by epoxidase, using molecular oxygen and NADPH to form
squalene epoxide. A cyclase converts it to 30C lanosterol. It is the
first steroid compound synthesized.
Step 5: Condensation of 5-Carbon Units
15. From Lanosterol, the 3 additional methyl groups on carbon atoms
4 and 14 are removed to produce zymosterol.
Then the double bond migrates from 8-9 position to 5-6 position,
when desmosterol is formed.
Finally, the double bond in theside chain (between carbon 24-25) is
reduced by NADPH when cholesterol is formed.
Step 7: Cutting to size
18. Regulation at transcription: Long-term regulation involves
regulation of transcription of the gene for HMG-CoA reductase by
suppression.
Cholesterol regulates the expression of HMG-CoA reductase gene.
Covalent modification: Short-term regulation is by covalent
modification of the enzyme. Cyclic AMP mediated cascade
phosphorylates the enzyme which is inactive. Dephosphorylated form
is active.
Regulation of Cholesterol Synthesis
19. Further, Insulin and thyroxine increase the activity of HMG-CoA
reductase.
Cortisol and glucagon decreases its activity.
Drugs: Lovastatin and other “statin” group of drugs are competitive
inhibitors of HMG-CoA reductase. So, they are used in clinical
practice to reduce the cholesterol level in blood.
Regulation of Cholesterol Synthesis
23. The liver has a major role in controlling the plasma levels of
LDL cholesterol.
1. Liver synthesizes cholesterol
2. Liver removes cholesterol from lipoprotein remnants.
3. Liver is the only organ that can excrete cholesterol through bile.
4. Liver converts cholesterol to bile acids.
Liver and Cholesterol
24. Analyte Normal value
Total plasma lipids 400-600 mg/dl
Total cholesterol 140-200 mg/dl
HDL cholesterol, male 30-60 mg/dl
HDL cholesterol, female 35-75 mg/dl
LDL cholesterol, 30-39 yrs 80-130 mg/dl
Triglycerides, male 50-150 mg/dl
Triglycerides, female 40-150 mg/dl
Phospholipids 150-200 mg/dl
Free fatty acids (FFA) (NEFA) 10-20 mg/dl
Plasma lipid profile (normal values)
25. 1. Chylomicrons: Contains apoprotein B-48.
2. Very low-density lipoproteins (VLDL). Main apoprotein is
B-100.
3. Intermediate density lipoproteins (IDL)
4. Low-density lipoproteins (LDL). Major apoprotein in LDL is
B-100.
5. High-density lipoproteins (HDL). Major apoprotein in HDL is
apo A.
Free fatty acids (FFA) are complexed with albumin. (FFAs are not
generally included in the classification of lipoproteins, because they
are loosely bound).
Classification of Lipoproteins
29. Characteristics of different classes of lipoproteins
Chylomicron VLDL IDL LDL HDL FFA (*)
Density g/ml <0.95 0.95-1.006 1.006-
1.019
1.019-
1.063
1.063-1.121 1.28-1.3
Diameter (nm) 500 70 30 25 15 -
Compo-sition
Protein 2 10 20 20 30-60 99
TAG 80 50 30 10 10 0
Phospho-lipid 10 20 20 20 20-30 0
Chole-sterol 8 20 30 50 10-30 0
FFA 0 0 0 0 0 1
Apoproteins A, B-48, C-II,
E
B-100, C-
II, E
B-100, E B-100 A-I, C, E Albumin
Transport
function
TAG
from gut to
muscle
TAG
from liver
to muscle
Cholesterol
from liver to
peripheral
tissues
Cholesterol from
peripheral tissues
to liver
FFA
from fat depot to
muscle
and liver
30. Characteristics of apoproteins and their functions
Apoprotein Component ofFunctions Mol. wt. Site of
production
apo A-I HDL-2 Activation of LCAT;
ligand for HDL
receptor; Anti-
atherogenic
28,000 Intestine; liver
apo A-lI HDL-3 Inhibits LCAT;
stimulates lipase
17,000 Intestine; liver
apo B-100 LDL; VLDL Binds LDL receptor 550,000 Liver
apo B-48 Chylomicrons 48% size of B-100 250,000 Intestine
apo C-l Chylo-
microns; VLDL
Activation of LCAT 7,000 Liver
31. Characteristics of apoproteins and their functions, continued
Apo-protein Compo-nent of Functions Mol. wt. Site of
production
apo C-ll do Activates extrahepatic
lipoprotein lipase in vessel
walls; clearance
chylomicrons and VLDL
9,000 Liver
apo C-lll do Inhibits lipoprotein lipase;
antiatherogenic
8,500 Liver
apo E LDL;
VLDL; chylo
Arginine rich; ligand for
hepatic uptake
30,000 Liver
apo Lp (a) Lp (a) Attached to B-100;
impairs fibrinolysis; highly
atherogenic
Liver
32. Apo B-48 and apo B-100 are produced from the same gene. In liver, the
mRNA is translated as B-100. But in intestine, a stop codon is generated in
the middle, and a short protein is produced in intestine (B48). Apo B-48 is
only 48% of the size of B-100.
33. • Synthesis of Chylomicrons
• Chylomicrons are formed in the intestinal mucosal cells,
and secreted into the lacteals of lymphatic system.
• They are rich in triglyceride.
• If lipemic serum is kept overnight in the refrigerator, chylomicrons
rise as a creamy layer to the top, leaving the subnatant clear.
• When the chylomicrons are synthesized by the intestinal mucosa,
they contain only apo-B-48 and apo-A.
• Apo-C and apo-E are added from HDL in blood during transport,
Chylomicrons
35. • Main sites of metabolism of chylomicrons are adipose tissue and
skeletal muscle.
• The enzyme lipoprotein lipase (LpL) is located at the endothelial
layer of capillaries of adipose tissue, muscles and heart; but not in
liver.
• Apo-C-II present in the chylomicrons activates LpL.
• LpL hydrolyzes triglycerides present in chylomicrons into fatty
acids and glycerol.
• Muscle and adipose tissue cells take up liberated fatty acids.
Metabolism of Chylomicrons
36. • As the TAG content is progressively decreased, the chylomicrons
shrink in size.
• These remnants containing apo-B-48 and apo-E are taken up by
hepatic cells by receptor mediated endocytosis.
• Apo-E binds the hepatic receptors.
Liver Takes up Chylomicron Remnants
38. • Chylomicrons are the transport form of dietary triglycerides from
intestines to the adipose tissue for storage; and to muscle or heart
for their energy needs.
Function of Chylomicrons
39. • Synthesis of VLDL
• Triacylglycerol synthesized in liver is incorporated into
VLDL along with hepatic cholesterol.
• Apo-B-100 is the major lipoprotein present in VLDL when it is
secreted.
Very Low Density Lipoproteins
40. • When they reach the peripheral tissues, apo-C-II activates LpL
which liberates fatty acids that are taken up by adipose tissue and
muscle.
• The remnant is now designated as IDL (intermediate density
lipoprotein) and contains less of TAG and more of cholesterol.
• The major fraction of IDL further loses triglyceride, so as to be
converted to LDL (low density lipoprotein).
• This conversion of VLDL to IDL and then to LDL is referred to
as lipoprotein cascade pathway.
• A fraction of IDL is taken up by the hepatic receptors.
• VLDL carries endogenous TAG from liver to peripheral tissues
for energy needs.
Metabolism of VLDL
41. • Low density lipoproteins (LDL) transports cholesterol from liver
to peripheral tissues.
• The only apoprotein present in LDL is apo-B-100.
• Most of the LDL particles are derived from VLDL.
• The half-life of LDL in blood is about 2 days.
Low Density Lipoproteins
42. • The LDL is taken up by peripheral tissues by receptor mediated
endocytosis.
• LDL receptors are present on all cells but most abundant in hepatic
cells.
• LDL receptors are located in specialized regions called clathrin-
coated pits.
• Binding of LDL to the receptor is by apo-B-100.
• When the apo-B-100 binds to the apo-B-100 receptor, the receptor-
LDL complex is internalized by endocytosis.
Metabolism of LDL and LDL Receptors
43. • The endosome vesicle thus formed fuses with lysosomes.
• The LDL particle, along with apoproteins and cholesterol
ester are hydrolyzed to form free cholesterol.
• The free receptors can now return to the membrane surface to bind
further LDL molecules.
• The free cholesterol is either incorporated into plasma membranes
or esterified (by ACAT) and stored within the cell.
• The excess cholesterol tends to be deposited within the arteries,
leading to atherosclerosis.
44. • About 75% of the plasma cholesterol is incorporated into the LDL
particles.
• LDL transports cholesterol from liver to the peripheral tissues.
• The cholesterol thus liberated in the cell has three major fates:
i. It is used for the synthesis of other steroids like steroid
hormones.
ii. Cholesterol may be incorporated into the membranes.
iii. Cholesterol may be esterified to a MUFA by acyl cholesterol
acyl transferase (ACAT) for storage.
• The cellular content of cholesterol regulates further endogenous
synthesis of cholesterol by regulating HMG-CoA reductase.
Function of LDL
46. • The LDL concentration in blood has positive correlation with
incidence of cardiovascular diseases.
• A fraction of cholesterol is taken up by macrophages.
• Increased levels of LDL or oxidation of LDL increases uptake of
cholesterol by macrophages.
• LDL infiltrates through arterial walls, and are taken up by
macrophages.
• This is the starting event of atherosclerosis leading to myocardial
infarction.
LDL and Clinical Applications
47. • When macrophages are filled with cholesterol, foam cells are
formed.
• They get deposited in the subendothelial space and leads to the
formation of atheromatous plaque.
• This results in increased chances of thrombosis and coronary artery
disease.
48. • Since LDL-cholesterol is thus deposited in tissues, the LDL
(low density lipoprotein) variety is called “bad cholesterol”
in common parlance.
• Insulin and tri-iodothyronine (T3) increase the binding of LDL to
liver cells.
• This explains the hypercholesterolemia seen in diabetes and
hypothyroidism.
• Defects in LDL receptor synthesis leads to familial
hypercholesterolemia.
49. • Lp(a) is very strongly associated with myocardial infarction and
is sometimes called the “little rascal”.
• Lp(a), when present, is attached to apo-B-100 by a disulfide
bond. In 40% population, there is no detectable level of Lp(a) in
serum.
• In 20% of population, the Lp(a) concentration in blood is more
than 30 mg/dL; and these persons are susceptible for heart attack
at a younger age.
• Lp(a) is associated with heart attacks at the age of 30 or 40 years.
Lipoprotein (a)
50. • Indians have a higher level of Lp(a) than Western populations.
• Lp(a) has significant homology with plasminogen.
• So, it interferes with plasminogen activation and impairs
fibrinolysis.
• This leads to unopposed intravascular thrombosis and possible
myocardial infarction.
51. Apo-A constituent of HDL.
This "A" is written in capital
letters.
It is anti-atherogenic.
Lp(a) constituent of LDL.
This "a" is written in small
letters.
Highly atherogenic
Apo-A and Lp(a) are different
52. Mechanism of action of Lp(a)
Lp(a) competitively inhibits plasminogen
activation; and so inhibits fibrinolysis.
54. • High density lipoproteins (HDL) transport cholesterol from
peripheral tissues to the liver.
• The major apoprotein in HDL is Apo-A-I.
High Density Lipoprotein
55. • The intestinal cells synthesize components of HDL and release
into blood.
• The nascent HDL in plasma are discoid in shape.
• The free cholesterol derived from peripheral tissue cells are taken
up by the HDL.
• The apo-A-l of HDL activates LCAT (lecithin cholesterol acyl
transferase) present in the plasma.
• The LCAT then binds to the HDL disk.
Metabolism of HDL
56. • Lecithin is a component of phospholipid bilayer of the HDL disk.
• The second carbon of lecithin contains one molecule of
polyunsaturated fatty acid (PUFA).
• It is transferred to the third hydroxyl group of cholesterol to form
cholesterol ester.
• The esterified cholesterol which is more hydrophobic, moves into
the interior of the HDL disk.
• This reaction continues; till HDL becomes spherical and a lot of
cholesterol esters are formed.
57. • Transport of cholesterol from peripheral tissue to liver, then
excreted through bile
• HDL in serum is inversely related to myocardial infarction.
• “Anti-atherogenic” or “protective” “good cholesterol”
High Density Lipoprotein (HDL)
• HDL level below 35 mg/dl increases the risk
• HDL level above 60 mg/dl gives protection from coronary artery
diseases
HDL activates LCAT (lecithin cholesterol acyl transferase)
60. • Mature HDL spheres are taken up by liver cells by apo-A-l
mediated receptor mechanism.
• Hepatic lipase hydrolyzes HDL phospholipid and TAG.
• Cholesterol esters are released into liver cells.
• The cholesterol that reaches the liver is used for synthesis of bile
acids or excreted as such in bile.
• The cholesterol ester from HDL is transferred to VLDL, IDL and
LDL by a Cholesterol Ester Transfer Protein (CETP).
61. i. HDL is the main transport form of cholesterol from
peripheral tissue to liver, which is later excreted through
bile.
• This is called reverse cholesterol transport by HDL.
ii. The only excretory route of cholesterol from the body is the
bile.
iii. Excretion of cholesterol needs prior esterification with PUFA.
• Thus PUFA will help in lowering of cholesterol in the body, and so
PUFA is antiatherogenic.
Functions of HDL
62. • The level of HDL in serum is inversely related to the incidence of
myocardial infarction.
• As it is “antiatherogenic” or “protective” in nature, HDL is
known as “good cholesterol” in common parlance.
• It is convenient to remember that "H" in HDL stands for
"Healthy".
• HDL level below 35 mg/dL increases the risk, while level above
60 mg/dL protects the person from coronary artery diseases.
Clinical Significance of HDL
63. Clinical Significance of HDL
HDL can bind to the antioxidant enzyme paraoxonase (PON1).
Since PON1 can inhibit lipid oxidation and macrophage foam cell
formation, the measurement of PON1 activity of HDL particles is a
better biomarker of its atheroprotective effect.
Myeloperoxidase (MPO) is another enzyme which can bind
HDL at the same site as PON1. In dyslipidemic patients, MPO
levels are higher than normals. Hence the reciprocal modulation of
the activity of these two enzymes associated with HDL particles will
decide the antioxidant and anti-inflammatory properties of these
particles.
The accumulation of cholesterol in beta cells of
Langerhans causes perturbations in glucose metabolism, and
reduction in insulin secretion. If cholesterol accumulation is too
much, It can cause beta-cell destruction.
68. Lipoproteins and Their Fate
Lipo-protein Site of
production
Major lipid trans-
ported
Major
apoproteins
Function
Chylo-microns Intestine Triacylglycerol B-48,
Apo A-1, CII
and E
from HDL
Dietary lipids to peripheral tissues
and liver (TAG,
cholesterol)
VLDL Liver Triacyl-glycerol B-100, CII
and E from
HDL
Endogenous TAG from liver to
peripheral tissues
LDL From VLDL Chole-sterol B-100 Cholesterol from liver to peripheral
tissues
HDL Inte-stine,
liver
Chole-sterol;
Phos-
pholipids
A1, CII, E Cholesterol from peripheral tissues
to liver (RCT)
69. • It is also known as nonesterified fatty acids (NEFA).
• It is complexed with albumin in plasma.
• The FFA is derived from lipolysis of triglyceride stored in
adipose tissue by hormone-sensitive lipase.
• Free fatty acids may be either long chain saturated or unsaturated
fatty acids.
• The FFA molecules are transported to peripheral tissues in
combination with albumin.
Free Fatty Acid (FFA)
70. • In the tissue cells, FFA-albumin complex is dissociated.
• Then FFA is taken into the cell.
• During starvation, about 40–50% energy requirement of the body is
met by oxidation of FFA.
• Blood level of FFA is very low in the fully fed condition, high in
the starved state, and very high in uncontrolled diabetes mellitus.
71. • Bile acids are synthesized in the liver from cholesterol.
• They contain 24 carbon atoms.
• All of them have an alpha-oriented (projecting below the plane
of ring) hydroxyl group at position 7.
Formation of Bile Acids
73. 1. Cholesterol hydroxylated at 3/7/12 positions
2. Removal of 3-carbon unit, to make it 24 C
3. Conjugation with glycine
4. Secretion into intestinal canal
5. In the intestine, deconjugation and removal of hydroxyl groups.
74. • One hydroxyl group is added by the enzyme 7-alphahydroxylase.
• This is the rate-limiting step.
• A third OH group is added at 12th carbon in the case of Cholic
acid.
• Chenodeoxycholic acid, another primary bile acid has only two
hydroxyl groups at positions 3 and 7.
• Ring B is reduced in all cases.
Hydroxylation Reactions
75. • Cleavage takes place at 24 C, with removal of propionic
acid (3 carbon) unit.
• The primary bile acids are now conjugated with either glycine or
taurine to form bile acids.
• The major conjugated bile acid is glycocholic acid.
• Conjugation adds more polar groups and increases the efficiency
of bile acids as surfactants.
• The conjugated bile acids are excreted through the bile.
• In the bile they exist as bile salts (sodium or potassium salts of
conjugated bile acids).
Formation of Bile Salts
76. • Intestinal bacteria deconjugate the primary bile acids.
• Then bile acids are partly converted to secondary bile acids by
removal of the alpha hydroxyl group at position 7.
• Cholic acid is thus converted to deoxycholic acid and
chenodeoxycholic acid to lithocholic acid.
Secondary Bile Acids/Bile Salts
78. • They facilitate the digestion of lipids.
• They can form molecular aggregates called micelle which bring
about the absorption of lipids.
• Bile salt micelle also plays an important role in keeping the
cholesterol in solution.
Functions of Bile Salts
79. • Of the total bile salts reaching the intestine (15–30 g/day)
only a very small fraction, about 300–500 mg/day is
excreted through feces.
• The rest is reabsorbed from ileum, reaches liver and re-excreted
through bile.
• This is referred to as the enterohepatic circulation.
• When bile acid binding resin (cholestyramine) is given, the
reabsorption of bile acids is inhibited.
• Hence more cholesterol gets converted to bile acids and cholesterol
is decreased.
Enterohepatic Circulation of Bile Salts
80. • It is the chief secretion of liver, the largest gland in the body.
• Daily volume of secretion is about 500 mL.
• The secreted bile is stored in the gallbladder and released on
demand.
• The pH of bile in hepatic duct is 7.8, and in gallbladder is 7.4.
• An enzyme present in bile is alkaline phosphatase.
Bile
81. • Choleretics are substances which stimulate the secretion of bile
by the liver.
• Cholagogues stimulate the release of bile from the gallbladder.
• The most important choleretics are bile salts, the hormone
secretin and vagal stimulation.
• Cholecystokinin is the most powerful cholagogue.
• The release of cholecystokinin itself is stimulated by fatty acids
and amino acids in duodenum.
82. i. The alkaline pH of the bile serves to neutralize the acidity
of the gastric juice.
ii. The bile salts are efficient surfactants and detergents.
iii. Bile is the only route of excretion for bilirubin, the end product
of heme catabolism.
iv. It serves to excrete cholesterol, thus regulating the body
cholesterol pool.
v. Bile serves as the medium of excretion for several drugs, which
are detoxified by the liver.
Functions of Bile
