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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). Molecular Formula: C10H12N5O6P Molecular Weight: 329.21 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, cAMP receptor protein (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. An example of cAMP's function is the positive regulation of the lac operon. In low glucose concenration, cAMP accumluates, it binds to the allosteric site on CRP, a transcription activator protein. The protein assumes its active shape and binds to a specific site beside the lac promoter, this make it easier for RNA polymerase to bind to the adjacent promoter to start transcription of the lac operon, therefore increasing the rate of lac operon transcription. In high glucose concentration, cAMP concentration decrease, and the CRP disengage from the lac operon. Role of cAMP in Dictyostelium discoideum The chemotactic movements of the cells are organized by periodic waves of cAMP that propagate through the cell. The waves are the result of a regulated production and secretion of extracellular cAMP and a spontaneous biological oscillator that initiates the waves at centers of territories. Role of cAMP in human carcinoma Some research has suggested that a deregulation of cAMP pathways and an aberrant activation of cAMP-controlled genes is linked to the growth of some cancers. American Association for Cancer Research (cAMP-responsive Genes and Tumor Progression) American Association for Cancer Research (cAMP Dysregulation and Melonoma) American Association for Cancer Research (cAMP-binding Proteins' Presence in Tumors) See also | |||||||||
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