The cellular energy store of ATP (adenosine triphosphate) can be considered similar to a battery that needs to maintain its charge (see Hardie); cells sense changes in the abundance of ATP relative to its metabolites ADP (adenosine diphosphate) and AMP (adenosine monophosphate) and activate the heterotrimeric AMP-activated protein kinase (AMPK) when the “battery” needs recharging. AMPK activity requires phosphorylation of a threonine residue (Thr172) in the α subunit, and this site undergoes dynamic phosphorylation and dephosphorylation. AMP protects the kinase from dephosphorylation and also allosterically stimulates the activity of the kinase. The AMPK γ subunit has four nucleotide binding sites: Site 2 is unoccupied, site 4 is always occupied by AMP, and Mg-ATP and AMP compete for sites 1 and 3. Xiao et al. performed biochemical analysis and x-ray crystallography to determine how adenosine nucleotides regulated AMPK activity. Their analysis showed that, in addition to AMP, ADP also protected the enzyme from dephosphorylation and that Mg-ATP and ATP did not exhibit this protective effect. Sites 1 and 3 had different affinities for nucleotides, with both sites showing stronger affinities for AMP or ADP relative to Mg-ATP. NADH bound to AMPK at the higher affinity site (site 1), and both NADH and ADP prevented the allosteric activation of AMPK by AMP, suggesting that site 1 is the site responsible for the allosteric effects of AMP. NADH did not prevent ADP from protecting AMPK from dephosphorylation, which is consistent with site 3 serving this function. Structural analysis, supported by biochemical analysis of point mutants, provided a molecular basis for the protective effect of AMP or ADP binding to site 3 of the γ subunit. Analysis of structures reported here along with previously reported structures suggested that binding of either of these nucleotides to site 3 increased the interaction between the catalytic subunit and the regulatory β subunit and further predicted that binding of Mg-ATP to this site would disrupt this conformation. In addition to providing a detailed mechanistic view of AMPK regulation, this work suggests that cells can finely tune AMPK activity such that under mild energetic stress, ADP stimulates the activity by preventing dephosphorylation of AMPK, but under severe stress when AMP concentrations rise, AMP binding not only protects the enzyme from dephosphorylation but also further increases its activity allosterically.
B. Xiao, M. J. Sanders, E. Underwood, R. Heath, F. V. Mayer, D. Carmena, C. Jing, P. A. Walker, J. F. Eccleston, L. F. Haire, P. Saiu, S. A. Howell, R. Aasland, S. R. Martin, D. Carling, S. J. Gamblin, Structure of mammalian AMPK and its regulation by ADP. Nature 472, 230–233 (2011). [PubMed]
D. G. Hardie, How cells sense energy. Nature 472, 176–177 (2011). [PubMed]