Question

The citric acid cycle is one of the major pathways to generate energy from sugars by oxidizing them. The cycle also generates NADH and FADH2, which are reduction equivalents that are needed in the synthesis of compounds for the body.

1. For each step in the citric acid cycle, determine which reaction occurs, then show an example for that type of reaction that you have seen before. (5 pts)

2. Propose a mechanism for the reaction that is catalyzed by citrate synthase. (3 pts)

3. Compare and contrast the reaction conditions that citrate synthase uses with the reaction conditions you have learned for that type of reaction. What does the enzyme have to do so that the reaction occurs at all? (2 pts)

Answer 1

Ans: Reaction of the cytic acid cycle;

Acetyl-CoA Claisen condensation: methyl group of acetyl-CoA converted to methylene in citrate. CH3-C-S-COA H2O Oxaloacetate citrate synthase Dehydrogenation: oxidation of -OH completes oxidation sequence; generates carbonyl positioned to facilitate Claisen condensation in next step. COA-SH Citrate CH2-C00 HO-C-C00 CH2-COO- O=c-coo 1 CH2-C00- Dehydration/rehydration: --OH group of citrate repositioned in isocitrate to set up decarboxylation in next step. malate dehydrogenase Citric acid cycle aconitase H2O Malate Hydration: çoo addition of HO-CH water across double bond CH, introduces COO -OH group for next oxidation fumarase CH-C00- C-C00 cis-Aconitate 0-COO H20 step. (3) NADH 20 aconitase (Rehydration) HO ÇOO Fumarate CH HC Coo- FADH succinate dehydrogenase Dehydrogenation: introduction of double bond initiates methylene oxidation sequence. CH-C00- H-C-C00 HO-C-H lsocitrate COO- isocitrate Oxidative dehydrogenase decarboxylation: -OH group oxidized to carbonyl, which in CH-COO- CO2 turn facilitates a-ketoglutarate decarboxylation by dehydrogenase CH stabilizing carbanion complex formed on adjacent carbon. coo COA-SH a-ketoglutarate CO2 CH2-C00 CH2 COO- succinyl-CoA synthetase Succinate CH2-C00- CH, C-S-COA -COA COA-SH GTP (ATP) GDP (ADP) + P Succinyl-CoA Substrate-level phosphorylation: energy of thioester conserved in phosphoanhydride bond of GTP or ATP. Oxidative decarboxylation: pyruvate-dehydrogenase-like mechanism; dependent on carbonyl on adjacent carbon.

CH2 - 0 S-CoA Acetyl-CoA H2O COA-SH + → citrate synthase CH HO-¢-C00- CH2-C00 Citrate O=c-C00 CH2-C0o- Oxaloacetate