The heterotrimeric AMP-activated protein kinase (AMPK) plays a key role in regulating cellular energy metabolism; in response to a fall in intracellular ATP levels it activates energy producing pathways and inhibits energy consuming processes1. AMPK has been implicated in a number of diseases related to energy metabolism including type 2 diabetes, obesity and, most recently, cancer 2,3,4,5,6. AMPK is converted from an inactive to catalytically competent form by phosphorylation of the activation loop within the kinase domain7; AMP binding to the γ regulatory domain promotes phosphorylation by the upstream kinase8, protects the enzyme against dephosphorylation as well as causing allosteric activation9. We show here that ADP binding to just one of the two exchangeable AXP binding sites on the regulatory domain protects the enzyme from dephosphorylation, although it does not lead to allosteric activation. Our studies show that active AMPK displays significantly tighter binding to ADP than to Mg.ATP, explaining how the enzyme is regulated under physiological conditions where the concentration of Mg.ATP is higher than that of ADP and much higher than that of AMP. We have determined the crystal structure of an active AMPK complex. It shows how the activation loop of the kinase domain is stabilized by the regulatory domain and how the kinase linker region interacts with the regulatory nucleotide binding site that mediates protection against dephosphorylation. From our biochemical and structural data we develop a model for how the energy status of a cell regulates AMPK activity (Supplementary Fig. 1).

Maintaining sufficient levels of ATP (the immediate source of cellular energy) is essential for the proper functioning of all living cells. As a consequence, cells require mechanisms to balance energy demand with supply. In eukaryotic cells the AMP-activated protein kinase (AMPK) cascade has an important role in this homeostasis. AMPK is activated by a fall in ATP (concomitant with a rise in ADP and AMP), which leads to the activation of catabolic pathways and the inhibition of anabolic pathways. Here we review the role of AMPK as an energy sensor and consider the recent finding that ADP, as well as AMP, causes activation of mammalian AMPK. We also review recent progress in structural studies on phosphorylated AMPK that provides a mechanism for the regulation of AMPK in which AMP and ADP protect it against dephosphorylation. Finally, we briefly survey some of the outstanding questions concerning the regulation of AMPK.