Cyclic AMP

Cyclic AMP

Brennen Hodge
1/23/09



Cyclic AMP



Ligands, including substrates, inhibitors, activators, and neurotransmitters, are constantly being transported throughout the extracellular fluid. When one of these numerous hydrophilic molecules finds its proper receptor site on the plasma membrane of target cells, it binds by intermolecular forces, such as ionic bonding or hydrogen bonding. This binding of ligands (which are called the first messengers) to receptor sites initiate a series of events which leads to the production of second messengers within the cell. The second messengers then prompt a series of molecular interactions that alter the physiological state of the cell. This entire process is called signal transduction.
Currently, there are four recognized second messenger systems in cells. The main one that will be discussed is cyclic adenosine monophosphate, or cAMP. Cyclic AMP is a nucleotide formed from cytosolic ATP by the enzyme adenylyl cyclase (Second messenger, 2009). This process begins when a ligand binds to a surface receptor; the receptor-ligand complex associates with the Gs protein (stimulatory guanine nucleotide-binding protein), causing the displacement of GDP (guanosine diphosphate) by GTP (guanosine triphosphate) and the dissociation of the Gs alpha-GTP complex. Then the GTP-Gs alpha complex   binds to and activates adenylyl cyclase, which synthesizes cyclic AMP. When the ligand leaves the receptor, activation is ended and the GTP is hydrolyzed to GDP by the GTPase activity of the Gs alpha subunit, and the Gs alpha dissociates from the adenylyl cyclase. Then the adenylyl cyclase reverts back to the inactive form, the Gs alpha re-associates with the Gs beta gamma complex, and the cAMP molecules in the cytosol are hydrolyzed to AMP by the phosphodiesterase (Bowen, 1998). This process is difficult to imagine so this illustration sums it up clearly.

(Cell Communication)

The G proteins associated with this process respond quickly...