Question

In step 4 of the citric acid cycle, the reduction of NAD+ to NADH is coupled to the generation of CO2 and the formation of a high-energy thioester bond. What molecule provides the sulfhydryl group necessary to form the thioester bond?

a) FAD
b) Coenzyme A (CoA)
c) ATP
d) NADH

Answer 1

Coenzyme A (CoA) provides the sulfhydryl group necessary to form the thioester bond in step 4 of the citric acid cycle.

Step-by-step explanation:

The molecule that provides the sulfhydryl group necessary to form the thioester bond in step 4 of the citric acid cycle is Coenzyme A (CoA). In this step, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule to form a six-carbon citrate. During this process, CO2 is generated and NAD+ is reduced to NADH. Coenzyme A provides the sulfhydryl group to form the high-energy thioester bond with the acetate in Ac-S-CoA.