AMP-activated Protein Kinase - Activation

Activation

Triggering the activation of AMPK can be carried out provided that two conditions are met. First, the γ subunit of AMPK must undergo a conformational change so as to expose the active site (Thr-172) on the α subunit. The conformational change of the γ subunit of AMPK can be accomplished under increased concentrations of AMP. Increased concentrations of AMP will give rise to the conformational change on the γ subunit of AMPK as two AMP bind the two Bateman domains located on that subunit. It is this conformational change brought about by increased concentrations of AMP that exposes the active site (Thr-172) on the α subunit. This critical role of AMP is further substantiated in experiments that demonstrate AMPK activation via an AMP analogue 5-amino-4-imidazolecarboxamide ribotide (ZMP) which is derived from the familiar 5-amino-4-imidazolecarboxamide riboside (AICAR). The second condition that must be met is the phosphorylation and consequent activation of AMPK on its activating loop at Thr-172 of the α subunit brought about by an upstream kinase (AMPKK). The complex formed between LKB1 (STK 11), mouse protein 25 (MO25), and the pseudokinase STE-related adaptor protein (STRAD) has of late been identified as the major upstream kinase responsible for phosphorylation of AMPK on its activating loop at Thr-172. Although AMPK must be phosphorylated by the LKB1/MO25/STRAD complex, it can also be regulated by allosteric modulators which directly increase general AMPK activity and modify AMPK to make it a better substrate for AMPKK and a worse substrate for phosphatases. It has recently been found that 3-phosphoglycerate (glycolysis intermediate) acts to further pronounce AMPK activation via AMPKK.

Muscle contraction is the main method carried out by the body that can provide the conditions mentioned above needed for AMPK activation. As muscles contract, ATP is hydrolyzed, forming ADP. ADP then helps to replenish cellular ATP by donating a phosphate group to another ADP, forming an ATP and an AMP. As more AMP is produced during muscle contraction, the AMP:ATP ratio dramatically increases, leading to the allosteric activation of AMPK. This fact is further authenticated with studies, such as those sited above, that used electrical stimuli as a means to contract muscle to facilitate AMPK activation. For over a decade it has been known that calmodulin-dependent protein kinase kinase-beta (CaMKKbeta) can phosphorylate and thereby activate AMPK, but it was not the main AMPKK in liver. Richter et al. found that CaMKK inhibitors strongly inhibited AMPK phosphorylation in mouse soleus and EDL muscles after 2 minutes of contraction, but much less as time of contraction increased. CaMKK inhibitors had no effect on 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR) phosphorylation and activation of AMPK. AICAR is taken into the cell and converted to ZMP, an AMP analog that has been shown to activate AMPK. Recent LKB1 knockout studies have shown that without LKB1, electrical and AICAR stimulation of muscle results in very little phosphorylation of AMPK and of ACC, providing evidence that LKB1-STRAD-MO25 is the major AMPKK in muscle.