AMP-activated protein kinase (AMPK) is highly conserved from yeast to plants and animals and plays a key role in the regulation of energy homeostasis. AMPK is a heterodimeric protein serine/threonine kinase that is composed of alpha- (catalytic) and beta/gamma- (regulatory) subunits. AMPK is an important energy-sensing enzyme that monitors cellular energy status. This catalytic subunit contains a classical serine/threonine protein kinase domain close to the N-terminal. Free catalytic alpha subunits are usually inactive due to the presence of an autoinhibitory domain that is located in the center of this subunit. When the alpha subunit forms a functional complex with the beta and gamma subunits, this autoinhibitory function no longer blocks the catalytic function. In response to cellular metabolic stresses, AMPK is activated in an AMP-dependent manner by targeted phosphorylation of Thr172 in the alpha subunit by the upstream kinase like LKB1, calmodulin-dependent protein kinase kinases and transforming growth factor-beta-activated kinase-1. Activated AMPK phosphorylates many metabolic enzymes to stimulate catabolic pathways, such as ketogenesis, and inhibit anabolic pathways, such as protein synthesis. Recent studies also suggest the important role of AMPK in regulating chemotaxis and cytokine secretion in immune cells.