Figure 5. pH dependence of the ion-exchange reactions for the N81A mutant.

(A) Na⁺ coordination at S_mid, as originally suggested by the X-ray data of NCX_Mj. (B) The initial rates (t = 5 sec) of the Na⁺/Ca²⁺ and Ca²⁺/Ca²⁺ exchange reactions were measured as in the experiments shown in Figs 3 and 4, in an assay medium with 500 μM ⁴⁵CaCl₂. The intravesicular Na⁺ and Ca²⁺ values were 160 mM and 250 μM. The pH of the assay medium was controlled with a 20 mM MES/MOPS/Tris buffer.