The reaction catalyzed is:
ATP + AMP ⇔ 2 ADP
The equilibrium constant varies with condition, but it is close to 1. Thus, ΔG for this reaction is close to zero. In muscle from a variety of species of vertebrates and invertebrates, the concentration of ATP is typically 7-10 times that of ADP, and usually greater than 100 times that of AMP. The rate of oxidative phosphorylation is controlled by the availability of ADP. Thus, the mitochondrion attempts to keep ATP levels high due to the combined action of adenylate kinase and the controls on oxidative phosphorylation.