Download Principles of BIOCHEMISTRY

LIPID
METABOLISM:
CHOLESTEROL
METABOLISM
Functions of Cholesterol
• a precursor of steroid
hormones (progesterone,
testosterone, estradiol,
cortisol, etc.)
• a precursor of bile acids
• a precursor of vitamin D
• important component of many mammalian
membranes (modulates the fluidity)
Sources of Cholesterol
• from the diet
• can be synthesized de novo (about 800 mg of
cholesterol per day)
- in the liver (major site)
- in the intestine
• Liver-derived and
dietary cholesterol are
both delivered to body
cells by lipoproteins
Synthesis of Cholesterol
Three stages of cholesterol biosynthesis
1. Synthesis of isopentenyl pyrophosphate, that is
the key building block of cholesterol, from acetyl
CoA
2. Condensation of six molecules of isopentenyl
pyrophosphate to form squalene
3. Squalene cyclizes and the tetracyclic product is
converted into cholesterol
Acetyl CoA (C2)
Isopentenyl pyrophosphate (C5)
Squalene (C30)
Cholesterol (C27)
A. Stage 1:
Acetyl CoA to Isopentenyl Pyrophosphate
• All carbons of cholesterol come from cytosolic
acetyl CoA (transported from mitochondria via
citrate transport system)
• Sequential condensation of three molecules
of acetyl CoA
Two molecules of
acetyl CoA
condense to form
acetoacetyl CoA.
Enzyme – thiolase.
Acetoacetyl CoA reacts with acetyl CoA and
water to give 3-hydroxy-3-methylglutaryl
CoA (HMG-CoA) and CoA.
Enzyme:
HMG-CoA synthase
In cytoplasm 3-Hydroxy-3-methylglutaryl CoA is reduced to
mevalonate.
Enzyme: HMG-CoA
reductase
In mitochondria 3Hydroxy-3-methylglutaryl
CoA is cleaved to acetyl
CoA and acetoacetate.
Enzyme: HMG-CoA lyase.
HMG-CoA reductase
• HMG-CoA reductase is an integral membrane protein in
the endoplasmic reticulum
• Primary site for regulating cholesterol synthesis
• Cholesterol-lowering statin drugs (e.g. Lovastatin) inhibit
HMG-CoA reductase
Lovastatin
resembles
mevalonate
Mevalonate is converted into 3-isopentenyl
pyrophosphate in three consecutive reactions
requiring ATP and decarboxylation.
Isopentenyl pyrophosphate is a key building block for
cholesterol and many other important biomolecules.
B.Stage 2:
Isopentenyl Pyrophosphate to Squalene
Isopentenyl pyrophosphate is isomerized to
dimethylallyl pyrophosphate.
C5 units isopentenyl pyrophosphate react with C5
units dimethylallyl pyrophosphate to yield C10
compound geranyl pyrophosphate
C10 compound geranyl pyrophosphate reacts with C5
units isopentenyl pyrophosphate and C15 compound is
formed - farnesyl pyrophosphate.
Reductive tail-to-tail condensation of two molecules of
farnesyl pyrophosphate results in the formation of
C30 compound squalene
C. Stage 3:
Squalene to Cholesterol
Squalene activated by
conversion into squalene
epoxide.
Squalene epoxide is cyclized
to lanosterol.
Lanosterol is converted into cholesterol in a multistep
process.
THE REGULATION OF
CHOLESTEROL BIOSYNTHESIS
Regulatory enzyme - 3-hydroxy-3-methylglutaryl
CoA reductase.
Tetrameric
enzyme.
NADPH coenzyme
HMG CoA reductase is controlled in multiple ways:
1. The rate of synthesis of reductase mRNA is controlled
by the sterol regulatory element binding protein (SREBP).
When cholesterol levels fall this protein migrates to the
nucleus and enhance transcription.
2. The rate of translation of reductase mRNA is inhibited
by cholesterol
3. The degradation of the reductase is controlled.
The increase of cholesterol concentration makes the enzyme
more susceptible to proteolysis.
4. Phosphorylation decreases the activity of the reductase.
Enzyme is switched off by an AMP-activated protein kinase.
Thus, cholesterol synthesis ceases when the ATP level is
low.
Products of Cholesterol Metabolism
ATHEROSCLEROSIS
The desirable
level of
cholesterol in
blood plasma:
< 200 mg/dl
(< 5 mmol/l)
For a
healthy
person, the
LDL/HDL
ratio is 3.5
KETONE BODIES
The entry of acetyl CoA into the citric acid cycle
depends on the availability of oxaloacetate.
The concentration of oxaloacetate is lowered if
carbohydrate is unavailable (starvation) or improperly
utilized (diabetes).
Oxaloacetate is
normally formed from
pyruvate by pyruvate
carboxylase
(anaplerotic reaction).
Fats burn in the flame
of carbohydrates.
In fasting or diabetes the gluconeogenesis is activated
and oxaloacetate is consumed in this pathway.
Fatty acids are oxidized producing excess of acetyl CoA
which is converted to ketone bodies:
b-Hydroxybutyrate
Acetoacetate
Acetone
Ketone bodies are synthesized
in liver mitochondria and
exported to different organs.
Ketone bodies are fuel
molecules (can fuel brain and
other cells during starvation)
A. Synthesis of ketone bodies
Two molecules
of acetyl CoA
condense to
form
acetoacetyl CoA.
Enzyme –
thiolase.
Acetoacetyl
CoA reacts
with acetyl
CoA and water
to give 3hydroxy-3methylglutaryl
CoA (HMGCoA) and CoA.
Enzyme:
HMG-CoA
synthase
3-Hydroxy-3methylglutaryl
CoA is then
cleaved to
acetyl CoA and
acetoacetate.
Enzyme:
HMG-CoA lyase.
3-Hydroxybutyrate is
formed by the reduction of
acetoacetate by
3-hydroxybutyrate
dehydrogenase.
Acetoacetate also
undergoes a slow,
spontaneous
decarboxylation to
acetone.
The odor of acetone may
be detected in the breath
of a person who has a high
level of acetoacetate in
the blood.
B. Ketone bodies are a major fuel
in some tissues
Ketone bodies diffuse from the liver
mitochondria into the blood and are transported
to peripheral tissues.
Ketone bodies are important molecules in energy
metabolism.
Heart muscle and the renal cortex use
acetoacetate in preference to glucose in
physiological conditions.
The brain adapts to the utilization of
acetoacetate during starvation and diabetes.
3-Hydroxybutyrate is oxidized to produce
acetoacetate as well as NADH for use in
oxidative phosphorylation.
3-hydroxybutyrate
dehydrogenase
Acetoacetate is activated
by the transfer of CoA
from succinyl CoA in a
reaction catalyzed by a
specific CoA transferase.
Acetoacetyl CoA is cleaved
by thiolase to yield two
molecules of acetyl CoA
(enter the citric acid
cycle).
CoA transferase is present
in all tissues except liver.
Ketone bodies are a watersoluble, transportable
form of acetyl units
KETOSIS
The absence of insulin in diabetes mellitus
 liver cannot absorb glucose
 inhibition of glycolysis
 activation of
gluconeogenesis
 deficit of oxaloacetate
 activation of fatty
acid mobilization by
adipose tissue
 large amounts of acetyl CoA which can
not be utilized in Krebs cycle
 large amounts of ketone bodies (moderately strong acids)
 severe acidosis (ketosis)
Impairment of the tissue function, most importantly
in the central nervous system