metabolism_lecture_12 - Chapter 21: Lipid biosynthesis Bio...

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Unformatted text preview: Chapter 21: Lipid biosynthesis Bio s ynthe s is o f fa tty a c ids Bio s ynthe s is o f tria c ylg lyc e ro ls mammalian fatty acid synthase Biological functions of lipids Storage of energy Constituents of cellular membranes Anchors for membrane proteins Cofactors for enzymes Signaling molecules Pigments Detergents Transporters Antioxidants 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 -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 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 -hydroxy- acyl 4 enzyme- catalyzed steps: condensation dehydration -keto-acyl 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 2 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 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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metabolism_lecture_12 - Chapter 21: Lipid biosynthesis Bio...

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