Model fitting. (A) BK WT data (mean ± SEM) were fitted with the HA kinetic model (Horrigan and Aldrich, 2002) as is described in Materials and methods (final parameter values were L₀ = 5.5 × 10⁻⁶, z_L = 0.33 e₀, J₀ = 0.021, z_J = 0.58 e₀, k_d = 9.9 µM, C = 5.6, D = 19.6, E = 24.2). (B) F380A data (mean ± SEM) were fitted with the HA kinetic model. Effects of mutation on the gating can be explained by a decrease in the allosteric parameter between calcium binding and opening (C = 5.6; P < 0.01), the allosteric parameter between the voltage sensor activation and opening (D = 11.7; P < 0.01), the allosteric parameter between the calcium binding and the voltage sensor activation (E = 5.8; P < 0.01), and a large decrease in the open–closed intrinsic equilibrium constant (L₀ = 3.3 × 10⁻⁹; P < 0.01). Those parameters whose change was significant are colored in red. (C) F380W data (mean ± SEM) were fitted with the HA kinetic model. Effects of mutation on the gating can be explained by a decrease in the allosteric parameter between calcium binding and opening (C = 2.6; P < 0.01), the allosteric parameter between the voltage sensor activation and opening (D = 2.3; P < 0.01), and a large increase in the open–closed intrinsic equilibrium constant (L₀ = 4.2 × 10⁻²; P < 0.01). Those parameters whose change was significant are colored in blue. (D) Experimental V_h (mean ± SEM) of WT BK, F380A, and F380W versus the predicted V_h to different internal Ca²⁺ concentration using the HA allosteric model with the parameters obtained in A–C.