Marcel

GPCRs have 7 transmembrane (TM) helices; the intracellular loops that connect these helices form the G-protein binding domain. Binding of ligand to GPCR causes changes in 3 and 6 TM helices, which effects conformation of G-protein-interacting intracellular loops of the receptor and activates the G-protein binding site, which are previously masked. Binding of agonist to a stimulatory receptor induces coupling of heterotrimeric G-protein, and induces GDP release from GDP-Gas protein (Fig. 13). GTP binding to Gas protein leads to the dissociation of Gas-GTP subunit from the GPy complex and activation of downstream effectors by both Ga-GTP and free GPy subunits. Sixteen a subunits have been

Figure 13 G-protein regulatory cycle showing that the activated receptor interacts with heterotrimeric G-protein complex and induces GDP release from the G-protein. The G-protein activation leads to GTP-binding to the Ga-subunit. The Py subunits of heterotrim-eric G-proteins enhance receptor interaction with the a subunit. Free GPy formed after Ga-GTP dissociation from GPy is an activator of many effectors. The Ga-GTP after GTPase action forms Ga-GDP, which on reassociation with GPy forms inactive G-protein complex.

Figure 13 G-protein regulatory cycle showing that the activated receptor interacts with heterotrimeric G-protein complex and induces GDP release from the G-protein. The G-protein activation leads to GTP-binding to the Ga-subunit. The Py subunits of heterotrim-eric G-proteins enhance receptor interaction with the a subunit. Free GPy formed after Ga-GTP dissociation from GPy is an activator of many effectors. The Ga-GTP after GTPase action forms Ga-GDP, which on reassociation with GPy forms inactive G-protein complex.

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