Average normalized amplitude of deactivating K ⁺ -currents (“tail” currents, I _TAIL ) plotted against the activating membrane potential (V _ACT ). (A) From oocytes expressing WT K_V7.2, the amplitude of I_TAIL was normalized by its maximum peak current value in each individual recording. Then, the normalized I_TAIL amplitudes were averaged and plotted as a function of V_ACT. The plots from recordings at pH_EXT ranged from 6.0 to 8.0. (B) Equivalent I_TAIL–V_ACT plots were generated from the recording of K⁺-current in oocytes expressing the mutant R198H. The I_TAIL–V_ACT plot shifted about −40 mV at pH_EXT 8.0 compared with pH_EXT 6.0. (C) A double Fermi–Dirac equation was fitted to I_TAIL–V_ACT plots from each experiment (see Materials and methods) and the weighted half-maximum of the I_TAIL–V_ACT curves (V_1/2) and plotted as a function of pH_EXT. To quantify the pH_EXT-dependence of V_1/2, the V_1/2–pH_EXT plots were fitted with a linear regression, yielding values of 2 ± 1 (mean ± SE, n = 7) and −17 ± 4 (n = 7), for WT and R198H, respectively. (D and E) Other two parameters derived from the Fermi–Dirac distribution were plotted against pH_EXT, namely the relative weight of the most negative component of the distribution (D) and the apparent charges associated with each component (E).