Figure 1.

Minimal reactions for a GPCR molecule with two active states. The two active conformational A states are indicated by red and blue circles, with the basal B state as a gray square. (For the print version, the red and blue colors are replaced by shades of gray.) The central rectangular box presents the GPCR molecules in conformational equilibrium in the absence of agonists or transfer molecules. Above the rectangle are presented reactions involving binding by agonists, G-proteins, and β-arrestins. The agonists are indicated by turquoise or orange diamonds that preferentially bind to the A_blue or A_red states, respectively. All of these molecules can bind to both the A_blue and A_red states (as well as the B state) according to the principle of nonexclusive binding, but for simplicity, only binding to the A state with the highest affinity is shown. Similarly, for the two classes of agonists depicted, binding to the less favored state is not shown. Antagonists may also bind to each state, but are only shown for the B state. Allosteric modulators (not shown, but see Fig. 2D) can also bind to each state. The conformational equilibrium constants are indicated by L_blue and L_red for the interactions of the respective states with the B state. The constant M_A defines the equilibrium between the A_blue and A_red states. Other constants are defined in Table 1. To see this figure in color, go online.