Fig. 4.

(A) Dependence of ${\mathrm{EC}}_{50}^2$ on the free [Mg²⁺] in the solution at fixed pH values of 6, 7 and 8. Symbols are measured EC₅₀ values as listed in Table 1. Solid lines are linear fits with m and A as fitting parameters, consistent with the model Eq. (14). (B) Dependence of m on the free H⁺ concentration in the solution. Symbols are m values obtained from Fig. 4(A). Solid line is the proposed model fit (Eq. (13B)) that assumes EMg²⁺, EH⁺Mg²⁺ and E2H⁺Mg²⁺ as the three magnesium related SERCA species; dashed line is an alternate model fit that is identical to the proposed model except EMg²⁺ and EH⁺Mg²⁺ are assumed as the two magnesium related species.