Coupling Of Receptors To Intracellular Messengers

Neurotransmitters that act by binding to receptors on the cell surface can elicit a wide variety of biochemical effects inside a neuron. How does a signal at the cell's surface manage to alter the activities of enzymatic processes intracellularly? In many cases, this is achieved by virtue of the neuro-transmitter receptor coupling to and altering the catalytic activity of second-messenger-generating enzymes on the inner leaflet of the cell membrane. The second messengers thus produced typically activate downstream protein kinases, which are enzymes that regulate the activity of a wide variety of intracellular proteins by attaching phosphate groups to (phosphorylating) specific amino acids in the protein's sequence.

Typically, coupling of the receptor to the effector enzyme is mediated by G proteins, or guanine nucleotide binding proteins. G proteins are themselves enzymes that bind Guanosine triphosphate (GTP) and hydrolyze it to Guanosine diphosphate (GDP). Receptors activate G proteins by causing an allosteric change in the protein that causes an exchange of GTP onto the protein, replacing the GDP that is there in the inactive state. The GTP-bound version of the G protein is active and interacts with second-messenger-generating enzymes (or ion channels in some cases), greatly increasing their catalytic rate and producing elevations of the intracellular levels of the second messengers. After the G protein has hydrolyzed GTP to GDP, the G protein relaxes back to its inactive state, where it can remain unstimulated or once again be activated by the neurotransmitter/receptor complex (see figure).

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