Question

In which of the following steps of Krebs cycle, $C{O}_2$ is evolved?

• A. Isocitric acid $\to$ oxalosuccinic acid
• B. Oxalosuccinic acid $\to$ $\alpha$-ketoglutaric acid
• C. Succinic acid $\to$ fumaric acid
• D. Malic acid $\to$ oxaloacetic acid

Answer 1

The correct option is B Oxalosuccinic acid $\to$ $\alpha$-ketoglutaric acid

Isocitrate dehydrogenase catalyzes oxidative decarboxylation of isocitrate to form alpha-ketoglutarate. $M{n}^{2+}$ in the active site interacts with the carbonyl group of the intermediate oxalosuccinate, which is formed transiently but does not leave the binding site until decarboxylation converts it to alpha-ketoglutarate. $M{n}^{2+}$ also stabilizes the enol formed transiently by decarboxylation. There are two different forms of isocitrate dehydrogenase in all cells, one requiring NAD as an electron acceptor and the other requiring NADP.

So, the correct answer is option B.