Question

Which of the following is not an intermediate in Krebs cycle?

• A. Acetic acid
• B. Succinyl coenzyme A
• C. Malic acid
• D. Citric acid

Answer 1

The correct option is A Acetic acid

The TCA (Krebs) cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid. The reaction is catalysed by the enzyme citrate synthase and a molecule of CoA is released. Citrate is then isomerised to isocitrate. It is followed by two successive steps of decarboxylation, leading to the formation of alpha ketoglutaric acid and then succinyl CoA. During the conversion of succinyl CoA to succinic acid, a molecule of GTP is synthesised. Succinic acid is converted to fumaric acid, fumaric to malic acid and finally malate to oxaloacetic acid. Thus oxaloacetate is regenerated for another round of Krebs cycle. There are three points in the cycle where $NA{D}^{+}$ is reduced to $NADH+$ $H^{+}$ and one point where FAD is reduced to FADH${}_2$. Acetic acid is not an intermediate of Krebs cycle.

Thus, the correct answer is option A.