Question

Write with structural formulas and cofactors the reaction from the citric acid cycle, where a high energy thioester bond is broken.

Answer 1

The citric acid cycle has eight steps. In the first step and fifth step high energy thio-ester bond breaks and gives respective product.

First step: the first reaction of the citric acid cycle is the formation of citrate. Condensation of acetyl-CoA with oxaloacetate by citrate synthase catalyst, produce citrate. The hydrolysis of the high energy thioester intermediate makes the reaction highly exergonic. Large negative free energy change associated with the citrate synthase reaction is essential to the operation of the cycle, because of the very low concentration of oxaloacetate normally present.

Fifth step is the conversion of succinyl-CoA to succinate. Like acetyl-CoA succinyl-CoA has a strongly negative free energy of hydrolysis of its thioester bond. Energy released in this bond breakage is used to drive the synthesis of a phosphoanhydride bond in either GTP or ATP.