AMPK (5'-AMP-activated protein kinase) is a heterotrimeric complex comprising a catalytic ? subunit and regulatory ? and ? subunits. It protects cells from stresses that cause ATP depletion by switching off ATP-consuming bio-synthetic pathways. AMPK is activated by high AMP and low ATP through a mechanism involving allosteric regulation, promotion of phosphorylation by an upstream protein kinase known as AMPK kinase, and inhibition of dephosphorylation. Activated AMPK can phosphorylate and regulate in vivo hydroxy-methylglutaryl-CoA reductase and acetyl-CoA carboxylase, which are key regulatory enzymes of sterol synthesis and fatty acid synthesis, respectively. AMPK?1 (5'-AMP-activated protein kinase catalytic subunit alpha-1), also known as PRKAA1, is a 559 amino acid protein that belongs to the CAMK Ser/Thr protein kinase family and protein kinase superfamily. Highly phosphorylated, AMPK?1 exists as two alternatively spliced isoforms and is encoded by a gene that maps to human chromosome 5p13.1.