The enzyme-catalyzed formation of ATP by direct transfer of a phosphate group to ADP from an intermediate substrate in catabolism is called

• A. Glycolysis
• B. Substrate-level phosphorylation
• C. Oxidative phosphorylation
• D. Fermentation

Answer: (B)

Direct phosphate transfer from a high-energy metabolic intermediate to ADP to form ATP is substrate-level phosphorylation. This enzymatic step occurs in pathways like glycolysis—where intermediates such as 1,3-bisphosphoglycerate or phosphoenolpyruvate donate a phosphate to ADP to generate ATP (and give rise to products like 3-phosphoglycerate or pyruvate)—and in the TCA cycle with succinyl-CoA synthetase producing GTP/ATP. This mechanism is distinct from oxidative phosphorylation, which uses ATP synthase driven by a proton gradient established by the electron transport chain. Fermentation can yield ATP via substrate-level phosphorylation during glycolysis, but the defining mechanism described here is substrate-level phosphorylation.