Figure 7.

Plot of the initial velocity v₀ for Fe²⁺ arrival at the ferroxidase center versus total Fe²⁺concentration. The initial velocity v₀ is given by v_o (μM/s) = α·k_obs·(I_o-I_∞) where the values of k_obs(s^-1) and I_o- I_∞ are from curve fitting of each kinetic trace in Figure 6. Error bars are from the propagated errors in k_obs and I_o-I_∞. α is the proportionality constant between the maximal concentration of Fe²⁺ bound at the ferroxidase centers and the maximal degree of fluorescence quenching, I_o-I_∞, namely α = 6μM Fe²⁺/0.54 = 11.1 μM (Supporting Information). Fitting of the unweighted data (red line) according to eq 3 with the concentration of channels [C]_o = 4 μM gives K_C = (7.0 ± 0.7) ×10⁴ M^-1 and k_d = 216 ± 12 s^-1 (95% confidence level).⁴⁸