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

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Question

Identify the step in tricarboxylic acid cycle, which does not involve oxidation of substrate.
(1) Malic acid → Oxaloacetic acid
(2) Succinic acid → Malic acid
(3) Succinyl-CoA → Succinic acid
(4) Isocitrate → α-ketoglutaric acid

Accepted Answer

Correct Answer: C — Succinyl-CoA → Succinic acidStep-by-step Solution:The TCA cycle (Krebs cycle) involves several oxidation steps where NAD⁺ or FAD are reduced. Let us examine each step:Malic acid → Oxaloacetic acid: Catalyzed by malate dehydrogenase, involves oxidation of malate to oxaloacetate with reduction of NAD⁺ to NADH. Involves oxidation.Succinic acid → Malic acid: This is actually a two-step process: succinate to fumarate (oxidation by succinate dehydrogenase, FAD→FADH₂), then fumarate to malate (hydration, no oxidation). But the overall conversion from succinic acid to malic acid involves oxidation. However, the direct step in the cycle from succinate to fumarate is oxidative. The option likely refers to succinate → fumarate? But given answer key, C is correct.Succinyl-CoA → Succinic acid: Catalyzed by succinyl-CoA synthetase (succinate thiokinase). This is a substrate-level phosphorylation step (GDP + Pi → GTP) and does not involve oxidation; no electron transfer to NAD⁺ or FAD. Hence, no oxidation of substrate.Isocitrate → α-ketoglutaric acid: Catalyzed by isocitrate dehydrogenase, involves oxidation (NAD⁺ → NADH).Thus, the step that does not involve oxidation is succinyl-CoA to succinic acid.Key Concepts: TCA cycle, oxidation vs substrate-level phosphorylation, succinyl-CoA synthetase reaction, role of NAD⁺ and FAD.

Identify the step in tricarboxylic acid cycle, which does not involve oxidation of substrate. (1) Malic acid → Oxaloacetic acid (2) Succinic acid → Malic acid (3) Succinyl-CoA → Succinic acid (4) Isocitrate → α-ketoglutaric acid

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Identify the step in tricarboxylic acid cycle, which does not involve oxidation of substrate. (1) Malic acid → Oxaloacetic acid (2) Succinic acid → Malic acid (3) Succinyl-CoA → Succinic acid (4) Isocitrate → α-ketoglutaric acid

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