AMPKs

The AMP-activated protein kinase (AMPK) acts as a sensor of cellular energy status. AMPK exists as heterotrimeric complexes comprising a catalytic α subunit and regulatory β and γ subunits. In mammals, each of these subunits is encoded by multiple genes and at least 12 possible combinations of subunit isoforms are possible. The α subunits (α1, α2) contain the kinase domain at the N-terminus followed by a C-terminal region that is required for formation of the αβγ complex. The β subunits (β1, β2) contain short, variable N-terminal regions followed by two more highly conserved regions. The first is now recognized to be a glycogen-binding domain (related to N-isoamylase domains that are found in enzymes that metabolize the α1->6 branches in α1->4 linked glucans such as glycogen) that causes AMPK to associate with glycogen particles inside the cell. The C-terminal conserved region is required for the formation of the αβγ complex. The γ subunit isoforms (γ1, γ2, γ3) contain variable N-terminal regions of unknown function, followed by four tandem repeats of a sequence termed a CBS motif. These motifs, which also occur in a small number of other proteins, act in pairs to form two domains that bind the regulatory nucleotides, AMP and ATP, in a mutually exclusive manner. Binding of AMP to the two sites is highly co-operative. Mutations within the AMP binding sites of the γ2 and γ3 isoforms cause glycogen storage disorders in cardiac muscle in humans and skeletal muscle in pigs, respectively.

The AMPK system is activated by cellular stresses that cause a drop in the cellular ATP:ADP ratio either by interfering with ATP synthesis (e.g. metabolic poisons, hypoxia, glucose starvation) or by increasing ATP consumption (e.g. contraction in muscle). An increase in the cellular ADP:ATP ratio is amplified into a much larger increase in AMP:ATP by adenylate kinase. AMP binds to the two sites on the γ subunit (an effect antagonized by high ATP). This promotes phosphorylation within the α subunit by the upstream kinase, which is essential for AMPK activity. The major form of the upstream kinase is a complex between the tumor suppressor LKB1, and two accessory subunits, STRAD and MO25. AMP binding also allosterically activates the phosphorylated AMPK complex. Dissociation of AMP both reverses the allosteric activation and also promotes dephosphorylation to switch the kinase off again.

Once activated, AMPK switches on catabolic processes that generate ATP, such as the uptake and oxidation of glucose and fatty acids. It also switches off processes that consume ATP that are not essential for the short-term survival of the cell. This includes the biosynthesis of fatty acids, cholesterol, glycogen and protein. AMPK switches off protein biosynthesis and cell growth in part by down-regulating the TOR (target-of-rapamycin) pathway. AMPK causes both short-term effects via direct phosphorylation of metabolic enzymes, and longer-term effects by modulating gene expression.

AMPK is a prime target for drugs aimed at treatment of obesity and Type 2 diabetes. It can be activated in intact cells and in vivo using the nucleoside 5-aminoimidazole-4-carboxamide riboside (AICAR), which is taken up by cells and converted to the equivalent monophosphorylated nucleotide, ZMP, which mimics all of the effects of AMP. AMPK is also activated in intact cells and/or in vivo by two major classes of anti-diabetic drugs, i.e. the biguanides (metformin and phenformin) and the thiazolidinediones (rosiglitazone and pioglitazone). These drugs appear to act indirectly on AMPK, possibly via inhibition of the respiratory chain, and it remains uncertain to what extent their therapeutic benefits are mediated by AMPK.

The Table below contains accepted modulators and additional information. For a list of additional products, see the "Similar Products" section below.

 

Isoforms α1 α2 β1 β2
Molecular Weight
(kDa)
62.8 kDa 62.3 kDa 30.1 kDa 30.3 kDa
Structural Data 550 aa 552 aa 269 aa
Myristoylation
272 aa
Myristoylation
Species Human Human Human Human
Domain
Organization
Kinase domain
Complex formation
Kinase domain
Complex formation
Glycogen binding
Complex formation
Glycogen binding
Complex formation
Phosphorylation
Sites
Thr172 (by LKB1, CaMKKs)
Thr258
Ser485
Thr172 (by LKB1, CaMKKs)
Thr258
Ser491
Ser24/S25
Ser96
Ser101
Ser108
Ser182
Not Known
Tissue
Distribution
Ubiquitous Muscle
Liver
Ubiquitous Skeletal/cardiac
Muscle
Others
Subcellular
Localization
Cytoplasmic Nuclear
Cytoplasmic
Extranuclear
Nuclear
Cytoplasm
Binding Partners/
Associated Proteins
β and γ subunits β and γ subunits α and γ subunits
glycogen
α and γ subunits
glycogen
Upstream
Activators
LKB1/STRAD/MO25
Calmodulin-dependent
protein kinases (CaMKKs)
LKB1/STRAD/MO25
Calmodulin-dependent
protein kinases (CaMKKs)
Not Known
Not Known
Downstream
Activation
Not Known
Not Known
Not Known
Not Known
Activatorsa AICAR (A9978)
Metformin (D150959)
Rosiglitazone (R2408)
Pioglitazone (E6910)
AICAR (A9978)
Metformin (D150959)
Rosiglitazone (R2408)
Pioglitazone (E6910)
Not Known
Not Known
Inhibitorsb Compound C2 Compound C2 Not Known
Not Known
Selective
Activators
Not Known
Not Known
Not Known
Not Known
Physiological
Function
Catalytic Catalytic Glycogen-binding Glycogen-binding
Disease
Relevance
Not Known
Mouse KO:
Insulin resistant
Glucose intolerant
Not Known
Not Known

 

 

Isoforms γ1 γ2 γ3
Molecular Weight
(kDa)
37.6 kDa 63.1 kDa 51.5 kDa
Structural Data 331 aa 569 aa
Myristoylation
464 aa
Species Human Human Human
Domain
Organization
AMP/ATP-binding (two sites) AMP/ATP-binding (two sites) AMP/ATP-binding (two sites)
Phosphorylation
Sites
Not Known
Not Known
Not Known
Tissue
Distribution
Ubiquitous Skeletal/cardiac
Muscle
Others
Skeletal muscle
Subcellular
Localization
Cytoplasm Cytoplasm Cytoplasm
Binding Partners/
Associated Proteins
α and β subunits α and β subunits α and β subunits
Upstream
Activators
Not Known
Not Known
Not Known
Downstream
Activation
Not Known
Not Known
Not Known
Activatorsa Not Known
Not Known
Not Known
Inhibitorsb Not Known
Not Known
Not Known
Selective
Activators
Not Known
Not Known
Not Known
Physiological
Function
AMP/ATP-binding AMP/ATP-binding AMP/ATP-binding
Disease
Relevance
Not Known
Mutations:
Cardiac glycogen increased,
Cardiac arrhythmias
Mutations (pig):
Skeletal muscle glycogen increased

 

Footnotes

a) These activators only work in intact cells and require an intact αβγ complex

b) Compound C may inhibit the isolated kinase domain of the α subunit but has only been tested on the intact αβγ complex; see Zhou, et al., J. Clin. Invest., 108, 1167-1174 (2001).

 

Similar Products


     

References