Zn²⁺ sensitivity is altered by mutation of H140/H^2.40 and H193/H^3.71 to Ala. (A) Representative currents elicited by a voltage step protocol (−40 to +100 mV in 20-mV increments followed by a step to −90 mV) indicated in the diagram in the absence (Control) and presence of Zn²⁺ at the indicated concentrations are shown for a cell expressing WT human Hv1. (B) Normalized G_STEP-V relations are shown for the representative cell expressing WT Hv1, H140A, H193A, and H140A-H193A shown above. Colored lines represent fits of the mean data to a Boltzmann function: WT Hv1 (black line, V_0.5 = 61.9 mV, dx = 23.6), H140A (red line, V_0.5 = 78.1 mV, dx = 20.6), H193A (green line, V_0.5 = 71.4 mV, dx = 19.0), and H140A-H193A (blue line, V_0.5 = 73.3 mV, dx = 25.8). (C) Normalized Zn²⁺ concentration responses are shown for the representative cell expressing WT Hv1 (black squares), H140A (red circles), H193A (green triangles), and H140A-H193A (blue triangles) shown above. Data points represent means ± SEM, and colored lines represent fits of the mean data to a Hill function: WT Hv1 (black line, IC₅₀ = 3.6 µM, n_H = 0.8), H140A (red line, IC₅₀ = 39.0 µM, n_H = 0.7), H193A (green line, IC₅₀ = 82.2 µM, n_H = 1.0), and H140A-H193A (blue line, IC₅₀ = 1.743 mM, n_H = 1.5).