The eukaryotic cell enzyme 5' adenosine monophosphate-activated protein kinase, or AMPK, utilizes AMP for homeostatic energy processes during times of high cellular energy expenditure, such as exercise. Since ATP cleavage, and corresponding phosphorylation reactions, are utilized in various processes throughout the body as a source of energy, ATP production is necessary to further create energy for those mammalian cells. AMPK, as a cellular energy sensor, is activated by decreasing levels of ATP, which is naturally accompanied by increasing levels of ADP and AMP.