Left, Ion occupancy in the model channel relative to that at the outermost wells ( or ) as a function of voltage at the indicated locations. Right, Shape of the I-V relation at steady state. At zero voltage, the relative occupancy is 1 for all cases because our model assumes no binding affinity when no voltage gradient is present. The curves on the left panels were calculated using
and those on the right panels using
under symmetrical ionic conditions. The number of barriers is 3 in this model. This has been determined to describe the conduction properties of mutants in our previous study (Paulino et al., 2017). is the voltage-dependent rate constant of the outermost barrier. The rate constant of the innermost barrier is defined as and that of the middle barrier as where both fitted parameters and are independent of voltage. ’s are the probabilities of occupying the ith wells (from to ). is a proportionality factor that has a dimension of volume and may be interpreted as the hypothetical volume for outermost well at the channel entrance. is the membrane potential, is the valence of the ion, and , and have their usual meanings.