Figure 6. A hypothetical [Greek small letter beta]-receptor exists in equilibrium between the R*(active, productive) and the R (inactive, nonproductive) forms, representing two conformations of the [Greek small letter beta]-receptor-G-protein-coupled-adenylyl cyclase enzyme system. The extent to which the equilibrium is perturbed depends on the relative affinity of a drug for the R* or the R conformations. A full agonist is sufficiently selective that it will drive the receptor completely to the active state (R*). A partial agonist is structurally analogous but exhibits only slightly greater affinity for R* than for R, meaning that even at saturating concentrations, the magnitude of the observed effect will be less. An antagonist binds with equal affinity to both conformations and does not alter the activation equilibrium while occupying receptor sites.