CHOLESTEROL METABOLISM

ODUGBEMI A. I.

BCH 342

METABOLISM OF MACROMOLECULES

Textbooks

• BIOCHEMISTRY, Garrett & Grisham 4th Ed.

• BIOCHEMISTRY, Campbell & Farrell 7th Ed.

• BIOCHEMISTRY, Berg, Tymoczko & Stryer 7th Ed.

• BIOCHEMISTRY, Donald Voet & Judith Voet 4th Ed.

• HARPER’S ILLUSTRATED BIOCHEMISTRY 26th Ed.

• COLOR ATLAS OF BIOCHEMISTRY, Koolman & Roehm 2nd Ed.

Cholesterol is present in tissues and in plasma either as free cholesterol or as a storage form, combined with a long-chain fatty acid as cholesteryl ester.

It is a vital constituent of cell membranes and the precursor of steroid hormone, bile salts and vitamin D.

As a typical product of animal metabolism, cholesterol occurs in foods of animal origin such as egg yolk, meat, liver, and brain. Plasma low-density lipoprotein (LDL) is the vehicle of uptake of cholesterol and cholesteryl ester into many tissues.

It is clearly essential to life, yet its deposition in arteries has been associated with cardiovascular disease and stroke, two leading causes of death.

In a healthy organism, an intricate balance is maintained between the biosynthesis, utilization, and transport of cholesterol, keeping its harmful deposition to a minimum.

Cholesterol is Vital

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Cholesterol Biosynthesis

Cholesterol is synthesized in many tissues from acetyl-CoA.

The biosynthesis of cholesterol may be divided into five steps:

1. Synthesis of mevalonate occurs from acetylCoA

2. Isoprenoid units are formed from mevalonate by loss of CO2.

3. Six isoprenoid units condense to form squalene.

4. Squalene cyclizes to give rise to the parent steroid, lanosterol.

5. Cholesterol is formed from lanosterol

The CoA thioester group of HMG-CoA is reduced to an alcohol in an NADPH-dependent four-electron reduction catalyzed by HMG-CoA reductase, yielding mevalonate.

HMG-CoA reductase mediates the rate-determining stepof cholesterol biosynthesis and is the most elaborately regulated enzyme of this pathway

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The new OH group is phosphorylated by mevalonate-5-phosphotransferase

The phosphate group is converted to a pyrophosphate by phosphomevalonate kinase.

The molecule is decarboxylated and the resulting alcohol dehydrated by pyrophosphomevalonatedecarboxylase.

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Isopentenyl pyrophosphate

isomerase

Isoprenoid Intermediates

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Squalene Formation by the Condensation of Six Isoprenoids

The pathway involves two head-to-tail condensations catalyzed by prenyltransferase and a head-to-head condensation catalyzed by squalene synthase.

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Squalene cyclizes to give Lanosterol

Oxidosqualene cyclase (alternatively, lanosterolsynthase) converts this epoxide to lanosterol, the sterol precursor of cholesterol.

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Cholesterol Is Synthesized from Lanosterol

Cholesterol synthesis from Lanosterol is a 19-step process. It involves an oxidation and loss of three methyl groups.

The first methyl group is removed as formate and the other two are eliminated as CO2

in reactions that all require NADPH and O2. The enzymes involved in this process are embedded in the ER membrane.

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Regulation of Cholesterol Synthesis by HMG-CoA Reductase.

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Cholesterol Is Transported in Lipoprotein Complexes

Cholesterol synthesized by the liver is either converted to bile salts for use in the digestive process or esterified by acylCoA:cholesterol acyltransferase (ACAT) to form cholesteryl esters which are secreted into the bloodstream as part of the lipoprotein complexes called very low density lipoproteins (VLDL).

As the VLDL circulate, their component triacylglycerol and most types of their apolipoproteins are removed in the capillaries of muscle and adipose tissues, sequentially converting the VLDL to intermediate density lipoproteins (IDL) and then to low-density lipoproteins (LDL).

Peripheral tissues normally obtain most of their exogenous cholesterol from LDL by receptor mediated endocytosis.

Inside the cell, cholesteryl esters are hydrolyzed by a lysosomal lipase to free cholesterol, which is either incorporated into cell membranes or re-esterified by ACAT for storage as cholesteryl ester droplets.

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LDL receptor-mediated endocytosis in mammalian cells

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Dietary cholesterol, cholesteryl esters, and triacylglycerols are transported in the blood by intestinally synthesized lipoprotein complexes called chylomicrons.

After removal of their triacylglycerols at the peripheral tissues, the resulting chylomicron remnants bind to specific liver cell remnant receptors and are taken up by receptor-mediated endocytosis in a manner similar to that of LDL.

In the liver, dietary cholesterol is either used in bile salt biosynthesis or packaged into VLDL for export.

Liver and peripheral tissues therefore have two ways of obtaining cholesterol:1. They may either synthesize it from acetyl-CoA by the de novo pathway2. They may obtain it from the bloodstream by receptor-mediated endocytosis

While LDL transports cholesterol from the liver, cholesterol is transported back to the liver by high-density lipoproteins (HDL).

Surplus cholesterol is disposed of by the liver as bile salts, thereby protecting the body from an overaccumulation of this water-insoluble substance.

Cholesterol Is Transported in Lipoprotein Complexes

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Cellular Cholesterol Supply Is Maintained by Three (3) Ways

1. By regulating the activity of HMG-CoA reductase, the enzyme catalyzing the rate-limiting step in the de novo cholesterol biosynthesis pathway. This is accomplished in two ways: (i) Short-term regulation of the enzyme’s catalytic activity by competitive inhibition and covalent modification involving reversible phosphorylation. (ii) Long-term regulation of the enzyme’s concentration by modulating its rates of synthesis and degradation.

2. By regulating the rate of LDL receptor synthesis, and therefore the rate of cholesterol uptake. High intracellular concentrations of cholesterol suppress LDL receptor synthesis, whereas low cholesterol concentrations stimulate it.

3. By regulating the rate of esterification and hence the removal of free cholesterol. ACAT, the enzyme that catalyzes intracellular cholesterol esterification, is regulated by reversible phosphorylation and by long-term control.

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LDL Receptor Activity Controls Cholesterol Homeostasis

The serum concentration of LDL therefore depends on the rate at which liver removes LDL from the circulation, which, in turn, depends on the number of functioning LDL receptors on the liver cell surface.

High blood cholesterol (hypercholesterolemia), which results from the overproduction and/or underutilization of LDL, is known to be caused by either of two metabolic irregularities:1. Familial Hypercholesterolemia (FH)2. Consumption of a high cholesterol diet

FH is a dominant genetic defect that results in a deficiency of functional LDL receptors Homozygotes for this disorder lack functional LDL receptors, so their cells cannot absorb LDL by receptor-mediated endocytosis and it leads to increase in plasma LDL.

The long-term ingestion of a high-cholesterol diet has an effect similar, although not as extreme, as FH. Excessive dietary cholesterol enters the liver cells in chylomicron remnants and represses the synthesis of LDL receptor protein. The resulting insufficiency of LDL receptors on the liver cell surface has consequences similar to those of FH.

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Managing Hypercholesterolemia

Two strategies for reversing these conditions (besides maintaining a low cholesterol diet) are being used in humans:1. Ingestion of anion exchange resins that bind bile salts, thereby preventing their intestinal

absorption (resins are insoluble in water). Bile salts, which are derived from cholesterol, are normally efficiently recycled by the

liver. Elimination of resin-bound bile salts in the feces forces the liver to convert more cholesterol to bile salts than otherwise.

The consequent decrease in the serum cholesterol concentration induces synthesis of LDL receptors (but not in FH homozygotes).

Unfortunately, decreased serum cholesterol level also induces the synthesis of HMG-CoA reductase, which increases the rate of cholesterol biosynthesis. Ingestion of bile salt–binding resins such as cholestyramine (sold as Questran) therefore provides only a 15 to 20% drop in serum cholesterol levels.

2. Treatment with competitive inhibitors of HMG-CoA reductase. These include the fungal derivatives lovastatin (also called mevinolin and sold as Mevacor), pravastatin (Pravachol), and simvastatin (Zocor) as well as the synthetic inhibitor atorvastatin (Lipitor), compounds that are collectively known as statins. These show a serum cholesterol decrease of 40–50%

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Utilization of Cholesterol

Cholesterol is the precursor of steroid hormones and bile salts.

Steroid hormones mediate a wide variety of vital physiological functions

Steroid hormones are grouped into five categories, progestins, glucocorticoids, mineralocorticoids, androgens, and estrogens.

Simplified scheme of steroid biosynthesis. Theenzymes involved are (1) the cholesterol side chain cleavage enzyme, (2) steroid C17 hydroxylase, (3) steroid C17, C20 lyase, (4) steroid C21 hydroxylase, (5) steroid 11β-hydroxylase, (6) steroid C18 hydroxylase, (7) 18-hydroxysteroid oxidase, and (8) aromatase.

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The quantitatively most important pathway for the excretion of cholesterol in mammals is the formation of bile acids.

The major bile acids, cholate and chenodeoxycholate, are synthesized in the liver and secreted as their glycine or taurine conjugates, which are known as bile salts, into the gallbladder.

They are secreted into the small intestine, where they act as emulsifying agents in the digestion and absorption of fats and fat soluble vitamins.

An efficient recycling system allows the bile salts to reenter the bloodstream and return to the liver for reuse several times each day.

Bile salts normally escape this recycling system and this is the body’s only route for cholesterol excretion.

Cholesterols are Excreted via Bile Salts

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