But cAMP (and AMPK) are important in cellular energy homeostatsis (which I think means it is required to break down glucogen for energy). So maybe it is the answer to its own ATP requirement??? Wiki actually says something I do not understand -- apparently there are different AMPK's... 5' AMP-activated protein kinase or AMPK or 5' adenosine monophosphate-activated protein kinase is an enzyme that plays a role in cellular energy homeostasis. The net effect of AMPK activation is stimulation of hepatic fatty acid oxidation and ketogenesis, inhibition of cholesterol synthesis, lipogenesis, and triglyceride synthesis, inhibition of adipocyte lipolysis and lipogenesis, stimulation of skeletal muscle fatty acid oxidation and muscle glucose uptake, and modulation of insulin secretion by pancreatic beta-cells. It should not be confused with cyclic AMP-activated protein kinase (protein kinase A), which, although being of similar nature, may have opposite effects. And even worse follows...idk how but CBS enzyme is involved here (who knew it was used anywhere but methylation and I am CBS +/+!). Also it sounds like AMPK makes you gain weight whereas cAMP makes you lose weight (which makes no sense to me because cAMP always activates AMPK, so they would go hand-in-hand you'd think?) So here is the rest of this passage from Wiki: AMPK acts as a metabolic master switch regulating several intracellular systems including the cellular uptake of glucose, the β-oxidation of fatty acids and the biogenesis of glucose transporter 4 (GLUT4) and mitochondria.[not in citation given][not in citation given] The energy-sensing capability of AMPK can be attributed to its ability to detect and react to fluctuations in the AMP:ATP ratio that take place during rest and exercise (muscle stimulation). During muscle stimulation, AMP increases while ATP decreases, which changes AMPK into a good substrate for activation via an upstream kinase complex, AMPKK, or better, where binding of AMP renders activated AMPK that is phosphorylated at Thr-172 a worse substrate for protein phosphatase 2Calpha. AMPKK is a complex of three proteins, STE-related adaptor (STRAD), mouse protein 25 (MO25), and LKB1 (a serine/threonine kinase). During a bout of exercise, AMPK activity increases while the muscle cell experiences metabolic stress brought about by an extreme cellular demand for ATP. Upon activation, AMPK increases cellular energy levels by inhibiting anabolic energy consuming pathways (fatty acid synthesis, protein synthesis, etc.) and stimulating energy producing, catabolic pathways (fatty acid oxidation, glucose transport, etc.). A recent JBC paper on mice at Johns Hopkins has shown that when the activity of brain AMPK was pharmacologically inhibited, the mice ate less and lost weight. When AMPK activity was pharmacologically raised (AICAR see below) the mice ate more and gained weight. Research in Britain has shown that the appetite-stimulating hormone ghrelin also affects AMPK levels. The antidiabetic drug metformin (Glucophage) acts by stimulating AMPK, leading to reduced glucose production in the liver and reduced insulin resistance in the muscle. (Metformin usually causes weight loss and reduced appetite, not weight gain and increased appetite, which is opposite of what might be expected given the Johns Hopkins mouse study results. ) Recent research  has implicated overproduction of AMPK in the genesis of Alzheimer's disease. This has raised theoretical concern over the safety of Metformin. A number of recent studies suggest that the botanical alkaloid berberine, also activates AMPK and glucose transport in muscles.