Lecture 7 cholesterol steroids

Lecture 7 cholesterol steroids - BergTymoczkoStryer...

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Biochemistry Sixth Edition Chapter 26 The Biosynthesis of Membrane Lipids  and Steroids Copyright © 2007 by W. H. Freeman and Company   Berg • Tymoczko • Stryer
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Three common types of membrane lipids 1. Phospholipids Major class of membrane lipids 2. Glycolipids sugar containing lipids 3. Cholesterol Lipid based on a steroid nucleus
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Cholesterol Present in all animal membranes Functions to regulate membrane fluidity A precursor of steroid hormones, bile salts, and vitamin D
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All 27 carbon atoms of cholesterol are derived from acetyl CoA Feeding experiments by Konrad Bloch in the 1940s with labeled acetate Nobel Prize in 1964
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Three stage synthetic process for formation of cholesterol Synthesis of isopentenyl pyrophosphate , activated isoprene unit Condensation of six molecules of isopentenyl pyrophosphate orm squalene Squalene cyclizes and is converted to cholesterol (19 steps) Key building block of cholesterol
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Cholesterol Synthesis Stage 1: Formation of isopentenyl pyrosphosphate from acetyl CoA 1. Acetoacetyl CoA and Acetyl CoA condense to form HMG-CoA 2. HMG-CoA is reduced to mevalonate 3. Mevalonate is converted to isopentenyl pyrophosphate 1 2 3
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1. Acetoacetyl CoA and Acetyl CoA condense to form HMG-CoA
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. HMG-CoA is reduced to mevalonate Committed step in cholesterol formation: Catalyzed by HMG CoA ketone bodies Enzyme : HMG-CoA reductase
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Mevalonate is converted to isopentenyl pyrophosphate Activated Isoprene unit Key Building block in cholesterol biosynthesis
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Cholesterol biosynthesis Stage 2: Condensation of six molecules of isopentenyl pyrophosphate to form squalene Reaction sequence: C5 C10 C15 C30
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Stage 2: Formation of squalene step 1: Isomerization reaction Versatility of isoprene is based on its reactions between its two isomers Condensation Mechanism
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Stage 2: Formation of squalene step 2 : 2 isomeric C5 units condense to form a C10 unit (geranyl pyrophosphate) step 3 : C10 condenses with C5 To form a C15 unit (farnesyl pyrophosphate) step 4 : 2xC15 condense to form squalene (C30) 2 3 4 Up to an including squalene all intermediates are linear
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Cholesterol Biosynthesis Stage 3: Squalene cyclizes to lanosterol which is converted to cholesterol activated carbocation
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Conversion of Lanosterol to Cholesterol 19 steps: Removal of 3 methyl groups Reduction of a double bond with NADPH Migration of the other double bond Acetyl CoA mevalonate isoprene farnesyl squalene cholesterol 2C 6C 5C 15C 30C 27C ALL carbon atoms are derived from acetyl CoA
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Regulation of body cholesterol Cholesterol synthesis is regulated by HMG CoA reductase Regulation of HMG CoA Reductase: 1. Rate of synthesis of reductase mRNA 2. Rate of translation of reductase mRNA 3. Degradation of the reductase protein 4. Activity of the protein is controlled by phosphorylation Controlling the rate of transcription of reductase mRNA, translation
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This note was uploaded on 01/16/2012 for the course BIOLCHEM 415 taught by Professor Michaeluhler during the Fall '06 term at University of Michigan.

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Lecture 7 cholesterol steroids - BergTymoczkoStryer...

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