FIGURE 15-25 Hormone-induced activation and inhibition of adenylyl cyclase in adipose cells. Ligand binding to G_αs-coupled receptors causes activation of adenylyl cyclase, whereas ligand binding to G_αi-coupled receptors causes inhibition of the enzyme. The G_βγ subunit in stimulatory and in inhibitory G proteins is identical; the G_α subunits and their corresponding receptors differ. Ligand-stimulated formation of active G_α·GTP complexes occurs by the same mechanism in both G_αs and G_αi proteins (see Figure 15-14). However, G_αs·GTP and G_αi·GTP interact differently with adenylyl cyclase, so that one stimulates and the other inhibits its catalytic activity. See A. G. Gilman, 1984, Cell 36:577.