Cyclic AMP is synthesized from ATP by the enzyme adenylate cyclase. The trigger for this reaction is thought to be the binding of an extracellular ligand with a membrane-bound receptor. Examples of ligands include adrenaline, glucagon, parathyroid hormone, antidiuretic hormone and thyroid stimulating hormone. Examples of suitable receptors include beta-adrenoceptors and H2 receptors.
On binding of a given ligand with its specific receptor, the complex then activates a nucleotide regulatory protein on the inside of the plasma membrane. The latter molecule is activated and binds to guanine triphosphate, GTP. The regulatory protein-GTP complex diffuses to and activates the enzyme adenylate cyclase that is bound to the inside of the plasma membrane. Adenylate cyclase produces cAMP.
cAMP passes into the cell to bind to and activate cAMP-dependent protein kinase enzymes. The latter phosphorylate and so activate further proteins.
Cyclic AMP is degraded by the enzyme phosphodiesterase.
Both adenylate cyclase and phosphodiesterase are the target for drugs to modulate cAMP levels. Pathologically, the production of cAMP may be targeted in cholera: cholera toxin binds to the nucleotide regulatory protein-GTP complex to prevent hydrolysis. This results in unrestrained cAMP production with subsequent overactive secretion.
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