Lecture 36 Cholesterol and Steroids PDF

Cholesterol and Steroids Lecture 36  Learning Outcomes At the end of this lecture, students should be able to: Explain the main phases of cholesterol biosynthesis Explain regulation of cholesterol synthesis Describe steroids and bile acids biosynthesis Explain...

Cholesterol and Steroids Lecture 36  Learning Outcomes At the end of this lecture, students should be able to: Explain the main phases of cholesterol biosynthesis Explain regulation of cholesterol synthesis Describe steroids and bile acids biosynthesis Explain Cyt P450 function and activity List main cholesterol derivatives  Cholesterol Structure & Properties 27 carbon atoms Four fused rings Peripheral side chain Only one double bond (C=C) Properties – Low solubility in water Only 30% in free form Majority of cholesterol is esterified through the hydroxyl group of long chain fatty acids Cholesteryl esters even less soluble – While it has a “bad reputation”, it is essential to all animal life – It is a precursor to many important biomolecules; including steroid hormones, bile salts, and Vitamin D  Four Stages of Cholesterol Synthesis 1) Three acetate condense to form mevalonate 2) Mevalonate converts to phosphorylated 5-C isoprene 3) Six isoprenes polymerize to form the 30-C linear squalene 4) Squalene cyclizes to form the four rings that are modified to produce cholesterol  Step 1: Formation of Mevalonate from Acetyl-CoA 2 Acetyl-CoAs →Acetyoacetyl-CoA – Catalyzed by acetyl-CoA acyl transferase Acetyl-CoA + Acetoacetyl-CoA → - hydroxyl--methylglutaryl-CoA (HMG-CoA) – Catalyzed by HMG-CoA synthase NOT the mitochondrial HMG-CoA synthase used in ketone body formation HMG-CoA + 2 NADPH →mevalonate – Catalyzed by HMG-CoA reductase – Rate-limiting step and point of regulation Step 2: Conversion of Mevalonate to Two Activated Isoprenes 3 PO43− transferred stepwise from ATP to mevalonate Decarboxylation and hydrolysis creates a diphosphorylated 5-C product (isoprene) with a double bond Isomerization to a second isoprene The two “activated” isoprene units are 3-isopentyl pyrophosphate and dimethylallylpyrophosphate Only reaction uses ATP. Three ATP molecules are used to create each of the six activated isoprenes required to construct squalene, for a total cost of 18 ATP molecules  Step 3: Six Activated Isoprene Units Condense to Form Squalene The two isoprenes join head-to-tail, displacing one set of diphosphates → forms geranyl pyrophopshate Geranyl pyrophosphate joins to another isopentenyl pyrophosphate → forms 15-C farnesyl pyrophosphate Two farnesyl pyrophosphates join head- to-head to form phosphate- free squalene  Step 4: Conversion of Squalene to Four-Ring Steroid Nucleus Squalene monooxygenase adds one oxygen to the end of the squalene chain → forms squalene 2,3-epoxide The cyclization product in animals is lanosterol, which converts to cholesterol  Fates of Cholesterol After Synthesis In vertebrates, most cholesterol synthesized in the liver, then exported. - They are exported as bile acids, biliary cholesterol or cholesteryl esters - Some of cholesterol is used in liver for membrane synthesis Other tissues use cholesterol as a precursor for steroid hormones and other molecules such as vitamin D Fates of Cholesterol After Synthesis  Cholsteryl esters are more nonpolar than cholesterol Contain a fatty acid esterified to the oxygen – Comes from a fatty acyl-CoA – Makes the cholesterol more hydrophobic, unable to enter membranes Transported in lipoproteins to other tissues or stored in liver – Surface is made of protein (apolipoprotein) and a phospholipid monolayer – Interior contains cholesterol, TAGs, cholesteryl esters ACAT is activated by cholesterol  HMG-CoA reductase is most active when dephosphorylated Covalent modification provides short-term regulation. 1) AMP-dependent protein kinase - AMP activates AMPK - AMPK phosphorylates and inhibits HMG CoA 2) Glucagon, epinephrine - cascades lead to HMG CoA phosphorylation and thus reduces its activity 3) Insulin - cascades lead to dephosphorylation thus increases activity  Statin drugs inhibit HMG-CoA reductase to lower cholesterol Statins resemble mevalonate → competitive inhibitors of HMG- CoA reductase Lowers serum cholesterol by tens of percent Side-effects include muscle weakness, coenzyme Q depletion Also reported to improve circulation, stabilize plaques by removing cholesterol from them, reduce vascular inflammation Regulation of Cholesterol Metabolism Oxysterol is a metabolite of cholesterol  There are several classes of cholesterol-derived steroids Adrenal gland- synthesized steroids: – Mineralcorticoids – Glucocorticoids Gonad-synthesized steroids (testes and ovaries): – Progesterone, testosterone, estrogens  Steroid Hormones Synthesis In mitochondria of steroidogenic tissues: placenta, ovaries, testes, and adrenal cortex How Steroids Are Made from Cholesterol Takes place in mitochondria Conversion of cholesterol to pregnenolone 1. Two adjacent carbons are hydroxylated by mixed-function oxidase (NADPH and cytochrome P450) on the side chain on C-17 of the D ring 2. Cleavage of the “side chain” by desmolase Hydroxylation of substrates makes them more water soluble. Easier for transport in blood and easier to excretion by urine  Cytochrome P450s Found associated with the inner mitochondrial and microsomal membranes. In steroidogenic tissues (placenta, ovaries, testes, and adrenal cortex): P450 is used to hydroxylate intermediates in the conversion of cholesterol to steroid hormones In liver: P450 is used to hydroxylate intermediates in: - Conversion of cholesterol to bile acid. - Hydroxylation of cholecalciferol to 25-hydroxycholecalciferol (vitamin D3) - Hydroxylation is important for activation or inactivation of foreign compounds and drug and thus change its level of toxicity and solubility In kideny: P450 is used to hydroxylate vitamin D3 to its biologically active 1,25-di hydroxylated form  Activated Isoprenes Have Many Alternative Fates Functions of isopentenyl pyrophosphate: 1. Activated precursor in biosynthesis of many biomolecules: cholesterol, vitamins A, E, and K, ubiquinone 2. Prenylation: anchoring of proteins to the inner surface of cellular membrnae by covalent attachment to an isoprenoid;