Figure 2.

Typical two-state receptor model with assumption of different binding affinities for the inactive and active forms of the receptor (R and R*, respectively). Schematics (A) and a simplified illustration of the corresponding processes (B) are shown. The ligand can bind to the inactive form of the receptor (R) and contribute to its activation (induced fit, conformational induction) or bind to the active form (R*) and lock it preferentially in that conformation (conformational selection). The case corresponding to the minimal two-state theory (no constitutive activity, i.e., ligand-free receptor has no active form) is highlighted with a light blue background.