5' AMP-activated protein kinase or AMPK or 5' adenosine monophosphate-activated protein kinase is an enzyme (EC 2.7.11.31) that plays a role in cellular energy homeostasis. It belongs to a highly conserved eukaryotic protein family and its orthologues are SNF1 and SnRK1 in yeast and plants, respectively. It consists of three proteins (subunits) that together make a functional enzyme, conserved from yeast to humans. It is expressed in a number of tissues, including the liver, brain, and skeletal muscle. The net effect of AMPK activation is stimulation of hepatic fatty acid oxidation, ketogenesis, stimulation of skeletal muscle fatty acid oxidation and glucose uptake, inhibition of cholesterol synthesis, lipogenesis, and triglyceride synthesis, inhibition of adipocyte lipolysis and lipogenesis, and modulation of insulin secretion by pancreatic beta-cells.

It should not be confused with cyclic AMP-activated protein kinase (protein kinase A).

AMPK is a protein complex that is formed by α, β, and γ subunits. Each of these three subunits takes on a specific role in both the stability and activity of AMPK. Specifically, the γ subunit includes four particular Cystathionine beta synthase (CBS) domains giving AMPK its ability to sensitively detect shifts in the AMP:ATP ratio. The four CBS domains create two binding sites for AMP commonly referred to as Bateman domains. Binding of one AMP to a Bateman domain cooperatively increases the binding affinity of the second AMP to the other Bateman domain. As AMP binds both Bateman domains the γ subunit undergoes a conformational change which exposes the catalytic domain found on the α subunit. It is in this catalytic domain where AMPK becomes activated when phosphorylation takes place at threonine-172 by an upstream AMPK kinase (AMPKK). The α, β, and γ subunits can also be found in different isoforms: the γ subunit can exist as either the γ1, γ2 or γ3 isoform; the β subunit can exist as either the β1 or β2 isoform; and the α subunit can exist as either the α1 or α2 isoform. Although the most common isoforms expressed in most cells are the α1, β1, and γ1 isoforms, it has been demonstrated that the α2, β2, γ2, and γ3 isoforms are also expressed in cardiac and skeletal muscle.

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