The adenosine monophosphate (AMP)-activated protein kinase (AMPK) regulates whole-body and cellular energy balance in response to energy demand and supply. AMPK is an alpha beta gamma heterotrimer activated by decreasing concentrations of adenosine triphosphate (ATP) and increasing AMP concentrations. AMPK activation depends on phosphorylation of the alpha catalytic subunit on threonine-172 (Thr(172)) by kinases LKB1 or CaMKK beta, and this is promoted by AMP binding to the gamma subunit. AMP sustains activity by inhibiting dephosphorylation of alpha-Thr(172), whereas ATP promotes dephosphorylation. Adenosine diphosphate (ADP), like AMP, bound to gamma sites 1 and 3 and stimulated alpha-Thr(172) phosphorylation. However, in contrast to AMP, ADP did not directly activate phosphorylated AMPK. In this way, both ADP/ATP and AMP/ATP ratios contribute to AMPK regulation.