Question

In the TCA cycle, the conversion of succinyl Co-A to succinic acid requires

• A. Acetyl Co-A + GTP + Pi.
• B. Acetyl Co-A + GDP + Pi.
• C. Co-A + GTP + Pi.
• D. GDP + Pi.

Answer 1

The correct option is D GDP + Pi.

Conversion of Succinyl Co-A to succinate has a strongly negative free energy of hydrolysis of its thioester bond (ΔG°' = -36 kJ/mol). In the next step of the citric acid cycle, the energy released in the breakage of this bond is used to drive the synthesis of a phosphoanhydride bond in either GTP or ATP, and succinate is also formed in this process.

The enzyme that catalyzes this reversible reaction is called either succinyl Co-A synthetase or succinic thiokinase; both names indicate the participation of a nucleoside triphosphate in the reaction.