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369Lecture14_F11 - 1 CH369 Fall 2011 Lecture 14 Ch 14 The...

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+ Lecture 14: Ch 14 The Citric Acid Cycle CH369 Fall 2011 1
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+ TCA Centrality Citric acid cycle (TCA cycle; Krebs cycle) is a central pathway Multiple pathways feed into and from it Amphibolic Relates to both catabolism and anabolism Cyclic metabolic pathway Glycolysis; gluconeogenesis; pentose phosphate are linear We’ll look at the significance of this later 2
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+ Transition Step Pyruvate dehydrogenase complex Reaction: pyruvate + CoA + NAD + àฏ acetyl-CoA + CO 2 + NADH Oxidative carboxylation of pyruvate Transfer of acetyl unit to coenzyme A (CoA) Traffic director to control flow of 2-C units from glycolysis into TCA cycle Complex of 3 enzymes and several coenzymes Located in mitochondrion Pyruvate (glycolysis) must move from cytosol to mitochondrion 3
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+ Pyruvate Dehydrogenase Reactions 4 Step 1 Catalyzed by E1 (pyruvate dehydrogenase) Pyruvate decarboxylated Involves thiamine pyrophosphate (TPP) cofactor Thiazolium ring C attacks carbonyl C of pyruvate CO 2 leaves to form hydroxyethyl-TPP Stabilized by N + in ring
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+ Pyruvate Dehydrogenase Reactions 5 Step 2 Hydroxyethyl group transferred to lipoamide prosthetic group in E2 E2 = Dihydrolipoyl transacetylase Breaks disulfide bond of lipoamide TPP regenerated Hydroxyethyl oxidized to acetyl
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+ Pyruvate Dehydrogenase Reactions Step 3 E2 transfers acetyl group to CoA Lipoamide now fully reduced Energy released in oxidation of hydroxyethyl to acetyl is conserved in thioester bond of acetyl-CoA 6
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+ Pyruvate Dehydrogenase Reactions Step 4 E3 reoxidizes lipoamide group of E2 E3 = Dihydrolipoyl dehydrogenase Cys-Cys disulfide bond in enzyme is reduced 7
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