Question

Final decarboxylation in Krebs cycle occur between

• A. citric acid and 𝝰-ketoglutaric acid
• B. 𝝰-ketoglutaric acid and succinic acid
• C. succinic acid and malic acid
• D. malic acid and oxalo acetic acid

Answer 1

The correct option is B 𝝰-ketoglutaric acid and succinic acid

Decarboxylation refers to the removal of carboxyl group from a molecule by the evolution of ${\mathrm{CO}}_2$. In the 4th step of Krebs cycle there is oxidative decarboxylation of 𝝰-ketoglutarate. It is the 3rd oxidative decarboxylation process apart from the linking reaction and step 3 of Krebs cycle. In this step, 𝝰-ketoglutarate is both dehydrogenated (with the help of ${\mathrm{NAD}}^{+}$) and decarboxylated by an enzyme complex 𝝰-ketoglutarate dehydrogenase.

The product combines with CoA to form succinyl CoA. NADH (reduced nicotinamide adenine dinucleotide) is produced. This is the final decarboxylation process. No further decarboxylation step occurs during Krebs cycle.

In the subsequent step i.e in the 5th step of Krebs cycle, conversion of succinyl CoA to succinic acid (also known as succinate) occurs. Succinyl CoA is acted upon by an enzyme synthetase to form succinate (4 carbon compound). The reaction releases sufficient energy to form ATP (in plants) or GTP (in animals). GTP can form ATP through a coupled reaction.