Coupling schemes to explain the opposite shifts in the activation of the voltage sensors and the pore. (A) Scheme I shows a canonical allosteric scheme showing voltage sensor activation (intrinsic equilibrium constant: $KV0$⁠) and the pore opening (intrinsic equilibrium constant: $KP0$⁠) being energetically connected via an interaction in the doubly activated state (where the voltage sensor is activated and the pore is open), represented by θ. Scheme II is a noncanonical counterpart of Scheme I, where the two structural units are interacting in both the doubly activated and the doubly resting state (i.e., the states where both the conformation are in “like” conformations). Schemes III and IV are obligatory analogues of Schemes I and II, respectively, where the conformational state with a resting voltage sensor and an open pore (V_RP_O) is eliminated. (B and C) The variation of occupancy of the activated state of the voltage sensor (⁠$PVA$ at −50 mV) and the open state of the pore (⁠$PPO$ at −20 mV) due to change in the different model parameters in Scheme I. The values for the simulations are taken from Muroi et al. (2010). Changing all the model parameters one at a time affects the two probability estimates similarly, which suggests that an opposite effect on the voltage sensor and the pore cannot be simply explained by Scheme I. The colored bars alongside the correspondingly colored curves depict the range of parameter values for which $PVA$ is practically constant but $PPO$ changes significantly. Such an effect would manifest in increased separation between the two curves. (D and E) The similar curves (as in B and C) obtained for Scheme II show that only when θ is changed do $PVA$ and $PPO$ change in an opposite way. For all other parameters, the two probability terms change in the same direction. The same observation will hold true for the obligatory schemes (III and IV); i.e., Scheme III will not yield a situation when the two probability measures are affected in opposite ways due to alteration of a single thermodynamic parameter, whereas in Scheme IV, the opposite shifts can be accomplished by altering θ.