Figure 7. Proposed mechanism underlying the catalytic preorganization for Ca²⁺ -bound species in the NCX_Mj antiporter.

(A) Inverted pair residues (shown in matching colors) contributing to the ion-transport activities, bidirectional ion movements, and local backbone dynamics at key functional residues are represented as sticks. (B) On the basis of the available experimental data, it is proposed that the backbone carbonyls of T209 and T50 and side chains of E54 and E213 bind Ca²⁺ in the ground state, where N73 and N232 form a hydrogen bond with T209 and T50, respectively (Fig. 1D). According to the available data, the side chains of E54, E213, D240, S51, T209, and S77 stabilize Ca²⁺ in the transition state, where ∆G_b = RTlnK_m; ∆G^≠_app = −RTln(k_cat/K_m); ∆G^≠_cat = −RTlnk_cat.