cAMP is a second messenger molecule, used for intracellular signal transduction. It is made from ATP by adenylate cyclase. Its main purpose is the activation of protein kinases; it is also used to regulate the passage of Ca2+ through ion channels, a mechanism used in smelling by mammals.
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cAMP synthesis and decomposition
Adenylate cyclase is located at the cell's membranes. It is activated by the hormones glucagon and adrenaline. These hormones can't go through the cell membrane, so all their activity is mediated by cAMP. Liver's adenylate cyclase reacts more strongly to glucagon, and muscle's adenylic cyclase reacts more strongly to adrenaline.
cAMP's decomposition into adenosine monophosphate is catalyzed by the phosphodiesterase enzyme. This enzyme is inhibited by caffeine, the stimulatory efffect of this drug being the result of the raised cAMP levels that it causes.
Protein kinase activation
In the absence of cAMP, a protein kinase is inactive and exists as a tetramer, consisting of two catalytic and two regulatory units (C2R2), with the regulatory units blocking the catalytic centers of the catalytic units. When a G protein activates the adenylate cyclase, cAMP is produced. It binds to specific locations on the regulatory units of the protein kinase, causing them to dissociate from the tetramer, thus activating the catalytic units so they can perform their function.