G proteins

The G proteins, trimers having α-, β-, and γ-subunits, are therefore called as α- subunit binds the GTP. The Binding of neurotransmitter to GPCRs activates their related G proteins that may do one or both of the following:

- They might interact directly with the ion channels causing them to open or close.
- They might interact with the enzymes, for illustration, adenylyl cyclase and phospholipase C, to switch on or off the second messenger cascades which regulate ion channels and other proteins by phosphorylation.

The cycle of events through which a G protein couples GPCR activation to the second messenger modulation is as shown in figure.

 
Figure: Coupling of the metabotropic receptors to second messenger systems by G proteins. Here N=neurotransmitter; R=receptor; and E=enzyme. The Asterisks point out the activated states of receptor or enzyme.

The binding of transmitter permits the receptor and G protein to couple. The GDP leaves the α-subunit in replace for GTP. In its GTP-bound form the G protein dissociate into individual α and β/γ-subunits. α-subunit activates the second messenger enzyme. The α-subunit has an intrinsic GTPase activity which cleaves the terminal phosphodiester bond in the GTP converting it into the GDP. In its GDP-bound form the α-subunit uncouples from the enzyme that reverts to its basal activity. The one significance of this cycle is that it amplifies the effect of the small signal. The Single transmitter-binding event outcome in various cycles of G-protein shuttling among the receptor and enzyme. Moreover the enzyme will have time to catalyze the synthesis of hundreds of second messenger molecules before it is switched off by the hydrolysis of the G-protein-bound GTP.

There are numerous distinct G proteins, differing largely in their α-subunits. The GS and GI interact with adenylyl cyclase, Gq with the phospholipase C. In spite of this multiplicity, the G proteins serve as a point for convergence of signals impinging on a neuron as many receptors talk to a few second messenger systems.

        G-protein-linked receptors for chosen transmitters, altogether with the second messenger systems they are coupled to.