Principles of BIOCHEMISTRY

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LIPID METABOLISM
CHOLESTEROL METABOLISM
2
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)

3
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

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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
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A. Stage 1
Acetyl CoA to Isopentenyl Pyrophosphate

• All carbons of cholesterol come from cytosolic
  acetyl CoA (transported from mitochondria via
  citrate transport system)

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• Sequential condensation of three molecules of
  acetyl CoA

Two molecules of acetyl CoA condense to form
acetoacetyl CoA. Enzyme thiolase.
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Acetoacetyl CoA reacts with acetyl CoA and water
to give 3-hydroxy-3-methylglutaryl CoA (HMG-CoA)
and CoA. Enzyme HMG-CoA synthase
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In cytoplasm 3-Hydroxy-3-methylglutaryl CoA is
reduced to mevalonate.
Enzyme HMG-CoA reductase
In mitochondria 3-Hydroxy-3-methylglutaryl CoA is
cleaved to acetyl CoA and acetoacetate. Enzyme
HMG-CoA lyase.
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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
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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.
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• Stage 2
• Isopentenyl Pyrophosphate to Squalene

Isopentenyl pyrophosphate is isomerized to
dimethylallyl pyrophosphate.
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C5 units isopentenyl pyrophosphate react with C5
units dimethylallyl pyrophosphate to yield C10
compound geranyl pyrophosphate
13
C10 compound geranyl pyrophosphate reacts with C5
units isopentenyl pyrophosphate and C15 compound
is formed - farnesyl pyrophosphate.
14
Reductive tail-to-tail condensation of two
molecules of farnesyl pyrophosphate results in
the formation of C30 compound squalene
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C. Stage 3
Squalene to Cholesterol
Squalene activated by conversion into squalene
epoxide. Squalene epoxide is cyclized to
lanosterol.
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Lanosterol is converted into cholesterol in a
multistep process.
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THE REGULATION OF CHOLESTEROL BIOSYNTHESIS
Regulatory enzyme - 3-hydroxy-3-methylglutaryl
CoA reductase.
Tetrameric enzyme. NADPH - coenzyme
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HMG CoA reductase is controlled in multiple ways

• 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.
• The rate of translation of reductase mRNA is
  inhibited by cholesterol
• 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.

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Products of Cholesterol Metabolism
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ATHEROSCLEROSIS
The desirable level of cholesterol in blood
plasma lt 200 mg/dl (lt 5 mmol/l)
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For a healthy person, the LDL/HDL ratio is 3.5
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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.
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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-HydroxybutyrateAcetoacetateAcetone
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)
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A. Synthesis of ketone bodies
Two molecules of acetyl CoA condense to form
acetoacetyl CoA. Enzyme thiolase.
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Acetoacetyl CoA reacts with acetyl CoA and water
to give 3-hydroxy-3-methylglutaryl CoA
(HMG-CoA) and CoA. Enzyme HMG-CoA synthase
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3-Hydroxy-3-methylglutaryl CoA is then cleaved to
acetyl CoA and acetoacetate. Enzyme
HMG-CoA lyase.
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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.
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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.
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3-Hydroxybutyrate is oxidized to produce
acetoacetate as well as NADH for use in oxidative
phosphorylation.
30
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 water-soluble, transportable
form of acetyl units
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KETOSIS
The absence of insulin in diabetes mellitus

• liver cannot absorb glucose
• inhibition of glycolysis
• activation of gluconeogenesis

• activation of fatty acid mobilization by adipose
  tissue

• deficit of oxaloacetate

• 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