C O N T E N T S
Cyclic adenosine monophosphate (cAMP, cyclic AMP or 3'-5'-cyclic adenosine monophosphate) is a molecule that is important in many biological processes; it is derived from adenosine triphosphate (ATP). cAMP is a second messenger, used for intracellular signal transduction, such as transferring the effects of hormones like glucagon and adrenaline, which cannot get through the cell membrane. Its main purpose is the activation of protein kinases; it is also used to regulate the passage of Ca2+ through ion channels.
cAMP synthesis and decomposition
cAMP is synthesised from ATP by adenylate cyclase. Adenylate cyclase is located at the cell membranes. It is activated by the hormones glucagon and adrenaline and by G protein. Liver adenylate cyclase responds more strongly to glucagon, and muscle adenylate cyclase responds more strongly to adrenaline.
cAMP decomposition into AMP is catalyzed by the enzyme phosphodiesterase. This enzyme is inhibited by high concentrations of caffeine, so it is possible that the stimulatory effect of this drug is the result of the raised cAMP levels that it causes (However it seems the concentrations required for caffeine to be effective are very high and a more likely explanation for the drug's effects involve the adenosine molecule).
Protein kinase activation
Cyclic AMP is involved in some protein kinases. For example, PKA (protein kinase A, also known as cAMP-dependent protein kinase) is normally inactive as a tetrameric holoenzyme, consisting of 2 catalytic and 2 regulatory units (C2R2), with the regulatory units blocking the catalytic centers of the catalytic units.
Cyclic AMP binds to specific locations on the regulatory units of the protein kinase, and causes dissociation between the regulatory and catalytic subunits, thus activating the catalytic units and enabling them to phosphorylate substrate proteins.
Glycogen decomposition regulation
cAMP controls many biological processes, including glycogen decomposition into glucose (glycogenolysis), and lipolysis.
Role of cAMP in bacteria
In bacteria, the level of cAMP varies depending on the medium used for growth. In particular, cAMP is low when glucose is the carbon source. This occurs through inhibition of the cAMP-producing enzyme, adenylate cyclase, as a side effect of glucose transport into the cell. The transcription factor CRP (or CAP) forms a complex with cAMP and thereby is activated to bind to DNA. CRP-cAMP increases expression of a large number of genes, including some encoding enzymes that can supply energy independent of glucose.
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