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metabolism_lecture_12

metabolism_lecture_12 - Chapter 21 Lipid biosynthesis Bio s...

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Chapter 21: Lipid biosynthesis Bio s ynthe s is  o f fatty ac ids Bio s ynthe s is  o f triac ylg lyc e ro ls mammalian fatty acid synthase
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Biological functions of lipids Storage of energy Constituents of cellular membranes Anchors for membrane proteins Cofactors for enzymes Signaling molecules Pigments Detergents Transporters Antioxidants
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Catabolism and anabolism of fatty acids proceed via different pathways Catabolism: - produces  acetyl-CoA - intermediates are bound as a  thioester to Coenzyme A - e -  from  β -oxidation transferred to  FADH 2 and NADH - location:  mitochondria in animals, glyoxisomes and peroxisomes in plants Anabolism: - requires  malonyl-CoA and acetyl-CoA - intermediates are bound as a  thioester to the acyl-carrier protein (ACP) - e -  for reducing steps provided by  NADPH - location:  cytosol in animals, chloroplast in plants
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Fatty acid synthesis  fatty acids are synthesized in repeated cycles, adding  one acetate unit at a time  acetate units (acetyl-CoA) are activated for this condensation by carboxylation to     malonyl-CoA,  CO 2  is released again in the condensation step      ( drives this endergonic reaction )  each cycle involves reduction of a  carbonyl   to a methylene  carbon by  NADPH (     -     ) 1 4 2 1 st reduction carbonyl -> hydroxyl 2 nd reduction alkene -> alkane trans- 2 -acyl β -hydroxy- acyl 4 enzyme- catalyzed steps: condensation dehydration β -keto-acyl
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Synthesis of malonyl-CoA 1)  Bicarbonate is activated through  phosphorylation and transferred  to biotin by the biotin carboxylase 2) The  arm-swing  moves carboxybiotin  to the transcarboxylase site Terminal methyl of acetyl-CoA probably  deprotonates -> resonance- stabilized carbanion. Carbanion picks up the carboxylate moiety. ACC P i Malonyl-CoA is synthesized from acetyl-CoA + CO by  acetyl-CoA carboxylase (ACC) ACC is a bifunctional enzyme with 3 subunits: - Biotin carrier protein (biotin as prosthetic group) - Biotin carboxylase - Transcarboxylase (cp. pyruvate carboxylase) (=carboxyphosphate) = committed step
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Fatty acid synthesis The growing  fatty acid chain  (or  acetyl group  in the 1 st  cycle)  is initially attached via a thioester  to a cysteine  –SH of the synthase The  malonyl group  is bound via a  thioester to the  phosphopanteteine  prosthetic group  of the  acyl carrier protein (ACP)   ACP ->  Both thiols must be charged  for the      condensation reaction to occur     (in the very first step,  acetyl from acetyl-CoA  is        transferred to acyl carrier protein and then passed       to the Cys of fatty acid synthase, acyl carrier protein        is then charged with 
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