Coupling Of Receptors To Intracellular Messengers

The cAMP System The PLC System

The cAMP System The PLC System

BOX 2 Coupling of cell surface receptors to PKA and PKC. This illustration indicates the connecting pathway between the receptor and the downstream effects of the protein kinases. In both pathways the receptor is linked though G protein interactions with the second messenger producing enzymes that modulate PKA and PKC, which in turn effects phosphorylation of a variety of substrates. See text for full explanation.

BOX 2 Coupling of cell surface receptors to PKA and PKC. This illustration indicates the connecting pathway between the receptor and the downstream effects of the protein kinases. In both pathways the receptor is linked though G protein interactions with the second messenger producing enzymes that modulate PKA and PKC, which in turn effects phosphorylation of a variety of substrates. See text for full explanation.

Why does this elaborate machinery exist? Why not have the receptor directly interact with the second-messenger-generating enzyme? One answer to this question lies in the kinetics of the GTP hydrolysis reaction. On the kinetic scale at which enzymes typically operate, G proteins are notably slow. This means that once the receptor stimulates the exchange of GTP onto the G protein, the G protein will stay in the GTP-bound, active conformation for a relatively extended period of time, on the order of a few minutes. However, after GTP is bound, the G protein quickly dissociates from the ligand/receptor complex. The ligand-occupied receptor can then proceed to activate additional G protein molecules. The net effect of the G protein involvement, then, is both an amplification and a prolongation of the neurotransmitter-stimulated event. Overall, the cell trades energy (in the form of GTP hydrolysis) for the benefit of augmented signal transduction across the membrane.

Two well-characterized second-messenger-generating enzymes are activated by G proteins: adenylyl cyclase and phospholipase C. Adenylyl cyclase converts ATP to adeno-sine 3', 5'-cyclic monophosphate, using magnesium as a cofactor. cAMP is known to activate two downstream effectors: the cAMP-dependent protein kinase (PKA) and certain types of cyclic nucleotide-gated ion channels. The activity of phospholipase C results in the production of two different second messengers. Phospholipase C

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