Question: 127. 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 \rightarrow \alpha-ketoglutaric acid
Answer: Option (3)
Explanation:
In the tricarboxylic acid cycle, most steps involve oxidation of substrates where electrons are transferred to \mathrm{NAD}^{+} or \mathrm{FAD}.
The conversion of malic acid to oxaloacetic acid involves oxidation with the reduction of \mathrm{NAD}^{+} to \mathrm{NADH}.
The conversion of succinic acid to malic acid includes oxidation of succinic acid to fumaric acid with reduction of \mathrm{FAD} to \mathrm{FADH}_{2}.
The conversion of isocitrate to \alpha-ketoglutaric acid is an oxidative decarboxylation step producing \mathrm{NADH}.
The conversion of succinyl-CoA to succinic acid is a substrate-level phosphorylation step.
It involves the cleavage of a high-energy thioester bond and formation of \mathrm{GTP}
or \mathrm{ATP}, without oxidation or reduction of the substrate.
Therefore, the step that does not involve oxidation of substrate is succinyl-CoA to succinic acid.