FIGURE 15-16 Structure of the β₂-adrenergic receptor in the inactive and active states and with its associated heterotrimeric G protein, G_s. (a) The three-dimensional structure of the β₂-adrenergic receptor bound to an antagonist (not shown) and thus in the inactive state. Placed next to it are the three-dimensional structures of the subunits of the heterotrimeric G protein G_s: G_αs (dark purple), G_β (light purple), and G_γ (pink), showing the inability of the resting receptor to bind to and activate G_αs. (b) The overall structure of the active receptor complex shows the adrenergic receptor bound to an agonist (black and red spheres) and engaged in extensive interactions with a segment of G_αs.

[Data from S. Rasmussen et al., 2011, Nature 476:549, PDB ID 3sn6; and V. Cherezov et al., 2007, Science 318:1258, PDB ID 2rh1.]