Cholesterol Metabolism

 The chemical and biochemical aspects of cholesterol regarding structure, distribution and biological functions in human body. structure, distribution and biological functions in human body.  The main steps of synthesis of cholesterol with special reference to the rate-limiting step. to the rate-limiting step.  The regulation of cholesterol synthesis.  The excretion of cholesterol.  Main causes of hypercholesterolemia with reference to biochemical bases of treatment. biochemical bases of treatment. Objectives of the Lecture

Sterols Sterols : are steroids with 8-10 carbon atoms in the side chain at C-17 & OH at C-3 Sterols are found in animals & plant Cholesterol: is the major sterol in animal tissues Plant sterols: as are poorly absorbed by humans, it blocks the absorption of dietary cholesterol Dietary intake of plant steroid esters (trans fatty acid – free margarine ) helps in reduction of plasma cholesterol Sterols

membrane structure the steroid hormonesbile acids Cholesterol is an extremely important biological molecule that has roles in membrane structure as well as being a precursor for the synthesis of the steroid hormones, bile acids & Vitamin D3 dietary cholesterol synthesized de novo Both dietary cholesterol and that synthesized de novo are transported through the circulation in lipoprotein particles.lipoprotein particles CHOLESTEROL

must be tightly regulated The synthesis and utilization of cholesterol must be tightly regulated in order to prevent over-accumulation and abnormal deposition within the body atherosclerosis Such deposition, eventually leading to atherosclerosis, is the leading contributory factor in diseases of the coronary arteries. CHOLESTEROL CHOLESTEROL cont.

Mostplasma cholesterol hydropobic Most plasma cholesterol is in an esterified form (with fatty acid attached to C-3), which is more hydropobic than free cholesterol. not found in membranes Cholesteryl esters (CE) are not found in membranes CE are normally present in low levels in most cells CE are normally present in low levels in most cells Cholesterol & CE must be transported in association with protein in lipoproteins or solubilized by phospholipids & bile salts in the bile Cholesterol & CE must be transported in association with protein in lipoproteins or solubilized by phospholipids & bile salts in the bile (as cholesterol & CE are hydrophobic) (as cholesterol & CE are hydrophobic) Cholesterol & cholesterol esters

Structure of cholesterol & cholesterol ester

Liver Cholesterol (sources & fate)

PLASMA CHOLESTEROL Plasma cholesterol level is 150 – 250 mg/dl (average 175 mg/dl) Types Types: 30% 30% of plasma cholesterol are free 70% 70% are esterified with polyunsaturated fatty acids

all tissues especially: Cholesterol synthesis by all tissues especially: liver, intestine, adrenal cortex & reproductive tissues cytoplasm It occurs in the cytoplasm with enzymes in both the cytosol and the membrane of the endoplasmic reticulum acetyl CoA molecules Cholesterol is synthesized from acetyl CoA molecules begins with the transport of acetyl-CoA from the mitochondria to the cytosol Synthesis begins with the transport of acetyl-CoA from the mitochondria to the cytosol Biosynthesis of Cholesterol

citrate oxaloacetate acetyl-CoA ATP-citrate lyase reaction In the cytoplasm, citrate is converted to oxaloacetate & acetyl-CoA by the ATP-citrate lyase reaction. Transport of acetyl CoA from mitochondria to cytosol

Biosynthesis of Cholesterol Biosynthesis of Cholesterol cont. HMG CoA 3 Acetyl CoA molecules First two reactions of cholesterol synthesis Thiolase enzyme HMG CoA synthase

In the liver, two isoenzymes of HMG CoA synthase are available: 1- Cytosolic enzyme: for cholesterol synthesis 2- Mitochondrial enzyme: for ketone bodies synthesis Biosynthesis of Cholesterol cont.

Third step of cholesterol synthesis: is the formation of mevalonic acid by the enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) (HMG-CoA reductase) (Requires 2 NADPH as coenzymes) This step is the rate limiting step of cholesterol synthesis Biosynthesis of Cholesterol Biosynthesis of Cholesterol cont.

6C 5C 10C 15C 27C

The cellular supply of cholesterol is maintained at a steady level by three distinct mechanisms: HMG CoA reductase 1. Regulation of HMG CoA reductase activity & levels excess intracellular free cholesterol 2. Regulation of excess intracellular free cholesterol through the activity of acyl-CoA:cholesterol acyltransferase (ACAT) plasma cholesterol levels 3. Regulation of plasma cholesterol levels via LDL receptor-mediated uptake & HDL-mediated reverse transport (in liver). Regulation of Cholesterol Synthesis

Regulation of HMGCoA reductase: 1.Sterol-dependent regulation of gene expression: Low cholesterol level activates a transcription factor leading to increased HMG CO reductase Low cholesterol level activates a transcription factor leading to increased HMG CO reductase synthesis – increased cholesterol synthesis synthesis – increased cholesterol synthesis 2. Enzyme degradation by cholesterol ↑ Cholesterol decreases the stability of HMG CoA reductase resulting in its rapid degradation ↑ Cholesterol decreases the stability of HMG CoA reductase resulting in its rapid degradation 3.Sterol-independent phosphorylation/dephosphorylation AMP (i.e. decrease ATP availability) causes phosphorylation of HMG CoA reductase causing its AMP (i.e. decrease ATP availability) causes phosphorylation of HMG CoA reductase causing its inactivation (with decrease cholesterol synthesis) inactivation (with decrease cholesterol synthesis) 4.Hormonal regulation Insulin causes upregulation of expression of the HMG CoA reductase gene leading to increase Insulin causes upregulation of expression of the HMG CoA reductase gene leading to increase cholesterol synthesis cholesterol synthesis 5.Inhibition by statin drugs Regulation of Cholesterol Synthesis cont.

transcription of mRNA transcription of mRNA SREBP is proteolyticaly cleaved mRNA - ↑Cholesterol also decreases the stability of HMG CoA ptn the stability of HMG CoA ptn & mRNA & mRNA Insulin favors upregulation of the expression of HMG CoA reductase gene expression of HMG CoA reductase gene Statin drugs reversible reversible competitive inhibitors (structural analogs) SRE in DNA

Cholesterol Excretion & Degradation Ring of sterol can’t be metabolized to CO 2 & H 2 O in humans Ring of sterol can’t be metabolized to CO 2 & H 2 O in humans Bile acids Bile Juice Neutral sterols in stool Cholesterol Bacterial Reduction to coprostanol & cholestanol intestine Bile Juice Excreted as such Convertedto Converted to intestine

Hypercholesterolemia  It is the increase of plasma cholesterol above 250 mg/dl. atherosclerosiscoronary heart diseases (CHDs), heart attacks & stroke  Hypercholesterolemia is associated with atherosclerosis, coronary heart diseases (CHDs), heart attacks & stroke Causes Causes: 1- Excessive consumption of diet rich in cholesterol, fats specially saturated FA or carbohydrates or carbohydrates 2- Diabetes mellitus (DM) 3- Hypothyroidism 3- Hypothyroidism: due to decreased conversion of cholesterol to bile acids Obstructive jaundice 4- Obstructive jaundice: no excretion of cholesterol or bile salts in bile 5- Familial hypercholesterolemia

Treatment of Hypercholesterolemia 1- Diet: -Decrease intake of carbohydrate, saturated fatty acids & cholesterol -Increase intake of mono- & polyunsaturated fatty acids -Increase intake of fibers-rich diet 2- Hypocholesterolemic drugs: Statin drugs e.g.atorvastatin (Lipitor) and simvastatin (Zocor) : Statin drugs e.g.atorvastatin (Lipitor) and simvastatin (Zocor) : Statin drugs are competitive inhibitors of HMG CoA reductase resulting in Statin drugs are competitive inhibitors of HMG CoA reductase resulting in inhibition of cholesterol synthesis inhibition of cholesterol synthesis Cholestyramine Cholestyramine Binds to bile acids in the GIT & prevents their reabsorption & promote their Binds to bile acids in the GIT & prevents their reabsorption & promote their excretion. excretion. Reduced bile acids will relieve inhibition on bile acids synthesis in the liver & Reduced bile acids will relieve inhibition on bile acids synthesis in the liver & thus diverting more cholesterol to be converted to bile acids synthesis thus diverting more cholesterol to be converted to bile acids synthesis