Figure 7.

Diffusion model calculations. (A) Schematic representation of the proton depletion zone generated by an outward proton current through an Hv1 channel. (B) Calculations using Eq. 5 of the extent of the depletion of protons as a function of the radial coordinate, r, for different values of the bulk proton concentration, c_∞, as indicated; the value of q = 14 fA is the same for all curves as well as D_H = 7,000 µm²/s. (C) Calculation of the effect of the presence of buffer. The black curve was calculated with D_H = 7,000 µm²/s and Eq. 5. The effect of the presence of buffer is illustrated by the dotted line, which was calculated with Eq. 6. The values of the constants are c_∞ = 3.16 µM, [B] = 100 mM, a = 1 nm, k_b = 10¹⁰ M⁻¹s⁻¹, and D_H = 7,000 µm²/s. Data are presented as molar proton concentration (left axis) and corresponding pH (right axis). (D) Experimental (red) and simulated (black) currents in response to a 100-mV depolarization. Simulated current trace was generated taking into account a pH change during current development of 1 pH unit, which changes the single-channel current from 5 fA to 1.4 fA. The current is given by Eq. 7 in the form $I\left(H,t\right)=N·\left(1-e^{-t/\tau }\right)\left[5-\left(5-1.4\right)\left(1-e^{-t/\tau }\right)\right]$, with τ = 200 ms and N as an arbitrary whole number chosen to match the size of the experimental current.