Fig. 2.
Two regimes of a two-state receptor. Representative energy-level diagrams for a single two-state receptor as a function of ligand concentration. The four possible states of the receptor are shown in Inset. Red curves (on) and blue curves (off) correspond to active and inactive configurations of the receptor, and the superscripts refer to no ligand bound (0) and ligand bound (L). Dotted lines, energy levels of the unbound receptor; solid curves, ligand-bound receptor; arrows, ligand concentrations when lowest free-energy states cross. (a) Regime I : In the absence of a ligand, the on-state free energy is above the off-state free energy; crossing occurs at [L] = Kdoff. (b) Regime II: In the absence of a ligand, the on-state free energy is below the off-state free energy; crossing occurs at [L] = Kdoff exp(Eoff − Eon).