The third major class of lipids
 The three cyclohexane rings (A, B, C)
are connected to a cyclopentane ring
(D).
 Significantly different structure from
other lipids
 Not necessarily esters
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Cholesterol is the most important and most
abundant steroid in the human body
 Necessary for human life
 Serves as a plasma membrane in all animal
cells
 Serves as raw material for the synthesis of
other steroids
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› Such as sex and adrenocorticoid hormones
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Exists in both free form and esterified with
fatty acids.
Cholesterol levels in the body are
regulated by the liver.
 In a dynamic state in the body,
circulating in the blood
 Cholesterol is hydrophobic, so it needs a
water-soluble carrier to circulate in the
blood.
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Lipoproteins transport cholesterol
 Most contain a core of hydrophobic
lipid molecules, surrounded by a shell of
hydrophilic molecules.
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› Such as proteins and phospholipids
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High-density lipoprotein (HDL)
› (“good cholesterol”)
› Consists of about 33% protein and about 30% cholesterol
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Low-density lipoprotein (LDL)
› (“bad cholesterol”)
› Consists of 25% protein and 50% cholesterol
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Very-low-density lipoprotein (VLDL)
› Mostly carries triglycerides (fats) synthesized by the liver
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Chylomicrons
› Carry dietary lipids synthesized in the intestines
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Transport of cholesterol from the liver starts out
as a large VLDL particle.
The core of a VLDL contains triglycerides and
cholesterol esters (largely cholesteryl linoeate.
And is surrounded by polar phospholipids and
proteins.
Fat is removed from the VLDL particles and the
diameter decreases, and only cholesteryl esters are
present.
Because fat is removed the density increases and
the VLDL particle becomes a LDL particle.
LDL’s stay in plasma for 2.5 days.
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LDLs carry cholesterol to specific cells located in
areas known as coated pits.
After the apoB-100 proteins binds specifically with
the molecules in the coated pits, the LDL is taken
inside the cell, where enzymes break down the
lipoprotein, liberating free cholesterol.
LDL receptors must be sufficient in number, or
cholesterol with build up in the blood; even with a
low intake of dietary cholesterol.
Cholesterol levels are determined by both genetics
and diet.
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HDL transports cholesterol peripheral tissues to
the liver, transferring cholesterol to LDL.
Free cholesterols are converted to cholesteryl
esters in HDL.
For the synthesis of bile acids and steroid
hormones the esterified cholesterols are delivered
to the liver.
Differing from LDL, HDL is bonded to the liver
cell surface and transfers its cholesteryl esters.
The HDL will reenter the circulation due to its
depleted lipid content.
High levels of HDL in the blood are desirable.
Cholesterol, like all lipids, is insoluble in water.
 Plaque-like deposits may form on the inner
surfaces of the arteries, if cholesterol levels are
elevated in the blood serum.
 If the diameter of the blood vessels is decreased,
blood flow will also decrease. This condition is
known as atherosclerosis. This can lead to heart
attacks, stroke, and organ failure.
 Blockage may deprive cells of oxygen, ceasing
function. The death of heart muscles is called
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myocardial infarction.
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Most cholesterol is transported by LDL
LDL concentration in the blood plasma drops, when
LDL is removed from circulation.
When the concentration of cholesterol is high in the
cells, the synthesis of the LDL receptor is suppressed,
just as when the cholesterol concentration is low, the
synthesis of LDL receptors increases.
Familial hypercholesterolemia is a disease contracted
when there is a lack of LDL receptors.
High LDL content means high cholesterol content in
the plasma.
High LDL content and low HDL content is a warning
for atherosclerosis.
The amount of cholesterol synthesized by the liver
is controlled by the serum cholesterol level.
 When serum cholesterol is high, synthesis is at a
low and visa versa.
 Low cholesterol diets can reduce serum levels but
a number of drugs such as Lipitor and Zocor can
inhibit levels of cholesterol in the liver.
 These drugs block the synthesis of cholesterol
inside the cells.
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It is desired to have high levels of HDL and low
levels of LDL.
HDL lipoproteins carry cholesterol from plaques
deposited in the arteries to the liver.
HDL levels can be increased by exercises and
weight loss.