LIPID METABOLISM: CHOLESTEROL METABOLISM. Functions of Cholesterol a precursor of steroid hormones (progesterone, testosterone, estradiol, cortisol, etc.)

LIPID LIPID METABOLISM: METABOLISM:

CHOLESTEROL CHOLESTEROL METABOLISMMETABOLISM

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

Acetyl CoA (C2) Isopentenyl pyrophosphate (C5)

Squalene (C30) Cholesterol (C27)

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

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

A. Stage 1: Acetyl CoA to Isopentenyl

Pyrophosphate

•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.

In mitochondria 3-Hydroxy-3-methylglutaryl CoA is cleaved to acetyl CoA and acetoacetate.

Enzyme: HMG-CoA lyase.

Enzyme: 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

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

SqualeneIsopentenyl 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

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.

KETONE BODIES

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:

-HydroxybutyrateAcetoacetateAcetone

Ketone bodies are fuel molecules (can fuel brain and other cells during starvation)

Ketone bodies are synthesized in liver mitochondria and exported to different organs.

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 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) and CoA.

Enzyme: HMG-CoA synthase

3-Hydroxy-3-methylglutaryl 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 water-soluble, transportable form of acetyl units

Impairment of the tissue function, most importantly in the central nervous system

KETOSISThe 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)