Question

In the TCA cycle, isomerisation of citrate is followed by how many decarboxylation(s) to form succinic acid?

• A. 2
• B. 3
• C. 1
• D. 4

Answer 1

The correct option is A 2

Isomers are molecules with the same molecular formula but different structural formula. The arrangement of atoms in space is different in both the isomers, though they share the same formula. The process of conversion of a molecule into its isomer, is known as isomerization.

In the 3rd step of Krebs cycle also known as TCA (tricarboxylic acid) cycle or CAC (citric acid cycle) there is formation of 𝝰-ketoglutarate. Isocitrate undergoes oxidative decarboxylation in the presence of enzyme isocitrate dehydrogenase and ${\mathrm{Mn}}^{2+}$.

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 where the first oxidative decarboxylation process takes place. This is the final decarboxylation process. No further decarboxylation step occurs during Krebs cycle.

A molecule of pyruvic acid that enters the mitochondria is completely oxidised to form 3 ${\mathrm{CO}}_2$ molecules in one pre- Krebs cycle decarboxylation (during the Link Reaction) and 2 Krebs cycle decarboxylation (steps 3 and 4).