Adenylate kinase

Overview
Adenylate kinase (also known as ADK or myokinase) is a phosphotransferase enzyme that catalyzes the interconversion of adenine nucleotides, and plays an important role in cellular energy homeostasis (see the "Biological homeostasis" section of "Homeostasis"). The reaction catalyzed is:

2 ADP ⇔ ATP + AMP

The equilibrium constant in the direction written is 0.44. Thus, the ΔGo for this reaction is close to zero. In muscle of 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.