Energetics, RR, and magnitude of the control response for a fully efficacious agonist co-applied with a positive allosteric modulator. (A) The graph shows free energy change provided by the modulator (ΔG_M) that is required to triple the control response at different magnitudes of the control response (EC_agonist). For a control response equal to or greater than one-third of maximal, the energy cannot be calculated, as that would require a probability of being active ≥1. The calculations were done using Eq. 5. (B) The relationship between the RR and EC_agonist was calculated (Eq. 6), assuming the modulator contributes −1 kcal/mol of free energy change toward gating.