What type of substrate-level phosphorylation occurs in the Citric Acid Cycle?

Substrate-level phosphorylation in the Citric Acid Cycle specifically refers to the direct production of a high-energy molecule, such as GTP or ATP, from a substrate without the involvement of the electron transport chain or oxidative phosphorylation. This process occurs during the conversion of succinyl-CoA to succinate.

In this step of the cycle, succinyl-CoA undergoes hydrolysis, which releases energy that is then harnessed to phosphorylate GDP (or ADP) to GTP (or ATP). This reaction is unique because it captures energy in a way that directly generates a high-energy phosphate molecule, making it a key example of substrate-level phosphorylation.

Other processes in the cycle, such as the production of NADH or the transformation of citric acid to alpha-ketoglutarate, involve redox reactions or transformations of molecular structures but do not constitute substrate-level phosphorylation since they do not directly produce ATP or GTP from a substrate. Similarly, while the conversion of succinate to fumarate is an important reaction in the cycle, it doesn't involve the generation of ATP or GTP directly and therefore isn't an example of substrate-level phosphorylation.