Model of hERG voltage sensor relaxation. (A) Markov gating scheme used to model hERG voltage sensor gating at pH 7.4 and pH 6.5 (top). Details are provided in the text, and transition rates are described in Table 4. The transition highlighted red represents that which was modified to simulate voltage sensor behavior at pH 6.5. Simulated on-gating currents using the model along with predicted on- and off-gating currents are shown for pH 7.4 (lower left) and pH 6.5 (lower right). Boltzmann fits yielded V_1/2 values of −52.8 mV and −73.0 mV for Q_on-V and Q_off-V, respectively, at pH 7.4 (mode shift = −20.2 mV). At pH 6.5, equivalent values were −47.9 mV and −54.7 mV (mode shift = −6.8 mV). The salient features of gating currents at pH 6.5 could be modeled by accelerating the rate of de-relaxation. (B) Cartoon representation of proposed reconfigurations of S2, S3, and S4 voltage sensor transmembrane domains in the resting, activated, and relaxed state. K1 and K6 refer to K525 and K538, respectively. R1–R5 describe R528–R537. Putative reconfigurations associated with entry into the relaxed state are shown, which may involve stabilizing interactions of D509 with outer S4 positive charges either directly or via a water molecule network. Neutralization of D509 or external protonation is predicted to destabilize these interactions and consequently the relaxed state.