text: The original Ternary complex model was used to describe ligand, receptor, and G-protein interactions.  It uses equilibrium dissociation constants for the interactions between the receptor and each ligand (K for ligand A; K for ligand B), as well as a cooperativity factor (α) that denotes the mutual effect of the two ligands on each other's affinity for the receptor. An α > 1.0 refers to positive allosteric modulation, an α < 1.0 refers to negative allosteric modulation, and an α = 1.0 means that binding of either ligand to the receptor does not alter the affinity of the other ligand for the receptor (i.e., a neutral modulator). Further, the α parameter can be added as a subtle but highly useful extension to the ATCM in order to include effects of an allosteric modulator on the efficacy (as distinct from the affinity) of another ligand that binds the receptor, such as the orthosteric agonist. Some ligands can reduce the efficacy but increase the affinity of the orthosteric agonist for the receptor.
  Although it is a simple assumption that the proportional amount of an active receptor state should correlate with the biological response, the experimental evidence for receptor overexpression and spare receptors suggests that the calculation of the net change in the active receptor state is a much better measure for response than is the fractional or proportional change. This is demonstrated by the effects of agonist/ antagonist combinations on the desensitization of receptors. This is also demonstrated by receptors that are activated by overexpression, since this requires a change between R and R* that is difficult to understand in terms of a proportional rather than a net change, and for the molecular model that fits with the mathematical model.