Figure 1.

Proposed kinetic mechanism of ATP (MgATP²⁻) and P_i (HPO₄²⁻) exchange via the ATP-Mg/P_i carrier (APC). The mechanism is ordered bi-bi for heteroexchange and random bi-bi for homoexchange of ATP and P_i. It is assumed that the APC has two binding sites: one exposed to the external side and the other to the matrix side. Once both the binding sites are occupied, the APC undergoes conformational change to translocate the bound substrates. E1 is the unbound state of the APC to which either of the external substrates can bind first for homoexchanges, but only external ATP can bind first for heteroexchange. E2 state is analogous to E1 with the exception that matrix substrate can bind first. K_T and K_P represent the binding affinity of the APC for ATP and P_i, respectively; k_E is the rate of futile exchange between the unbound states E1 and E2, k_T is the translocation rate when ATP is exchanged for P_i, and k_H is the translocation rate for homoexchanges. Here, T_x and P_x represent matrix ATP and P_i, whereas T_e and P_e represent external ATP and P_i.