Substrate-level phosphorylations occur when a "high-energy" phosphate containing molecule transfers phosphate to ADP (or GDP) in a chemical reaction to form ATP (GTP). One example of this type occurs in the citric acid cycle (Figure 14.3):

Succinyl-CoA + Pi + GDP <=> Succinate + GTP + CoASH ( = -2.9 kJ/mol)

This reaction is catalyzed by succinyl-CoA synthetase. Note, however, that most of the metabolic energy from the citric acid cycle is realized by transfer of electrons from oxidation of the intermediates to the electron transport system .