Question

Identify the step in tricarboxylic acid cycle, which does not involve oxidation of substrate.

• A. Malic acid→ Oxaloacetic acid
• B. Succinic acid → Malic acid
• C. Succinyl-CoA→Succinic acid
• D. Isocitrate→α-ketoglutaric acid

Answer 1

The Correct Option is C

The tricarboxylic acid (TCA) cycle, also known as the citric acid cycle or Krebs cycle, involves a series of enzymatic reactions that lead to the oxidation of acetyl-CoA to carbon dioxide and the reduction of NAD⁺ and FAD to NADH and FADH₂. These reduced cofactors are later used in the electron transport chain to generate ATP. However, not all steps in the cycle involve oxidation reactions. Let's analyze the options:

Malic acid → Oxaloacetic acid: This step involves the oxidation of malate to oxaloacetate by the enzyme malate dehydrogenase, reducing NAD⁺ to NADH.

Succinic acid → Malic acid: This transformation does not involve any oxidation or reduction; succinate is hydrated to malate.

Succinyl-CoA → Succinic acid: This step involves the conversion of succinyl-CoA to succinate by the enzyme succinyl-CoA synthetase. It is a substrate-level phosphorylation step where CoA is released and GTP (or ATP) is generated from GDP (or ADP) without any participation in direct oxidation of the substrate.

Isocitrate → α-ketoglutaric acid: This step involves the oxidation of isocitrate to α-ketoglutarate by isocitrate dehydrogenase, reducing NAD⁺ to NADH while releasing CO₂.

Based on the above analysis, the step Succinyl-CoA → Succinic acid is the one in the TCA cycle that does not involve the oxidation of the substrate. It involves the conversion of succinyl-CoA to succinate with the release of CoA and the formation of GTP or ATP via substrate-level phosphorylation.