Which enzyme is responsible for substrate-level phosphorylation in the citric acid cycle?

Substrate-level phosphorylation in the citric acid cycle is catalyzed by succinyl CoA synthetase. This enzyme facilitates the conversion of succinyl-CoA to succinate, during which a molecule of GDP (or ADP) is phosphorylated to form GTP (or ATP) directly. This process does not involve the electron transport chain, distinguishing it as a key aspect of substrate-level phosphorylation.

In terms of its function within the cycle, succinyl CoA synthetase links the breakdown of succinyl-CoA, a high-energy thioester compound, to the generation of GTP (or ATP) by harnessing the energy released from the cleavage of the thioester bond. This step is significant because it represents one of the few instances in the citric acid cycle where energy is captured directly in the form of a nucleotide triphosphate, highlighting the cycle’s role not only in catabolism but also in energy production.