Mechanism of AMP activation
AMPK α-, β-, and γ-subunits are all necessary for full activity. Active AMPK requires phosphorylation of Thr172 (p-T172) in the α-subunit, and the main upstream kinase, LKB1, which phosphorylates T172 AMPKα is constitutively active. Thus, a key regulatory step of AMPK activity the rate by which different phosphatases dephosphorylate p-T172 AMPKα. ATP, ADP and AMP all bind to the γ-subunit, and binding of both ADP and AMP, but not ATP suppresses the dephosphorylation of p-T172 AMPKα and increases AMPK activity. Binding of AMP, but not ADP or ATP, also increases allosterically the activity of AMPK.
AMPK acts acutely by phosphorylating targets proteins/enzymes thereby regulating their activities. Chronic activation of AMPK also causes more long-term changes in gene expression of key proteins/enzymes involved in multiple cellular functions, which is beneficial for intended chronic treatment of metabolic and vascular diseases by AMPK activators.
Indirect and Direct AMPK activators
AMPK can be indirectly activated by a large number of drugs and xenobiotics which act as inhibitors of metabolism and/or mitochondrial ATP synthesis, thereby by increasing cellular ADP/AMP. Such compounds, as exemplified by Phenformin and less so by Metformin, can cause life-threatening lactacidos. Thus, the development of AMPK activators has been aimed at identifying compounds which suppress the dephosphorylation of p-T172 AMPKα and/or allosterically activate AMPK without causing ATP depletion.