AMP-activated protein kinase (AMPK) regulates cellular metabolism in response to the availability of energy, and is therefore a target for type-2 diabetes treatment.
AMPK senses changes in the ratio of AMP/ATP by binding both species in a competitive manner. Thus, an increase in the concentration of AMP activates AMPK, resulting in the phosphorylation and differential regulation of a series of downstream targets that control anabolic and catabolic pathways. Whilst most studies have focused on understanding the regulation of AMPK by AMP and Mg-ATP, we have shown that ADP is also a physiological activator of AMPK.
We have solved a number of high-resolution crystal structures of mammalian AMPK in complex with nucleotides. Structural and solution studies reveal that two sites on the regulatory domain bind AMP, ADP or Mg-ATP, whereas a third site contains a tightly bound AMP that does not exchange. The phosphate groups of AMP/ADP/ATP lie in a groove on the surface of the regulatory fragment, which is lined with basic residues, many of which are associated with disease-causing mutations. Our binding studies indicate that Mg-ATP binds more weakly to these exchangeable sites, providing an explanation as to how micromolar concentrations of AMP and ADP can compete with millimolar concentrations of Mg-ATP.
AMPK has emerged as an attractive therapeutic target for treating metabolic disorders and a number of direct AMPK activators have been developed. We solved the crystal structure of full length AMPK in complex with these activators. Activator 991 modulates the enzyme by binding to AMPK at an interface formed between the kinase domain and the carbohydrate-binding module. The activator binding site is separate from the nucleotide binding site and raises the possibility that these activators may be mimicking a natural metabolite. Further studies are ongoing to fully understand the molecular basis of cellular energy regulation of AMP-activated protein kinase.