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, is a sequence of enzymatic reactions that oxidize acetyl-CoA to carbon dioxide. This process also reduces NAD⁺ and FAD to NADH and FADH₂, respectively. These reduced cofactors are subsequently utilized in the electron transport chain for ATP generation. However, not all stages of the cycle involve oxidation. Let's examine the given options:

Malic acid → Oxaloacetic acid: This conversion is an oxidation of malate to oxaloacetate, catalyzed by malate dehydrogenase, which reduces NAD⁺ to NADH.

Succinic acid → Malic acid: This transformation does not involve oxidation or reduction; succinate undergoes hydration to form malate.

Succinyl-CoA → Succinic acid: The enzyme succinyl-CoA synthetase facilitates the conversion of succinyl-CoA to succinate. This is a substrate-level phosphorylation step where CoA is released, and GTP (or ATP) is produced from GDP (or ADP) without direct substrate oxidation.

Isocitrate → α-ketoglutaric acid: Isocitrate dehydrogenase catalyzes the oxidation of isocitrate to α-ketoglutarate, reducing NAD⁺ to NADH and releasing CO₂.

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