Figure 1.

(A, B) Proposed kinetic mechanism of Na⁺-dependent Ca²⁺ efflux from mitochondria via Na⁺-Ca²⁺ antiporter with a presumed 3Na⁺:1Ca²⁺ stoichiometry. The antiporter E has three binding sites for Na⁺ and one binding site for Ca²⁺ facing either side of the IMM. In one process, three Na⁺ ions from cytoplasmic side first cooperatively bind to the unbound antiporter E in three consecutive steps to form the complex E3Na_e⁺. Then one Ca²⁺ ion from the matrix side binds to the complex E3Na_e⁺ to form the complex Ca_x²⁺E3Na_e⁺. In another process, one Ca²⁺ ion from the matrix side first binds to the unbound antiporter E to form the complex Ca_x²⁺E. Then three Na⁺ ions from the cytoplasmic side cooperatively bind to the complex Ca_x²⁺E in three consecutive steps to form the complex Ca_x²⁺E3Na_e⁺. The complex Ca_x²⁺E3Na_e⁺ then undergoes conformational changes to form the complex 3Na_x⁺ECa_e²⁺. The complex 3Na_x⁺ECa_e²⁺ then undergoes the reverse processes, where it dissociates in two distinct processes to form three Na⁺ ions in the matrix side and one Ca²⁺ ion in the cytoplasmic side, in addition to the unbound antiporter E. K′_{Ne, p}, K′_{Nx, p}, K′_Ce, and K′_Cx are the apparent dissociation constants associated with the binding of external and internal Na⁺ and Ca²⁺ to the antiporter. The 3Na⁺:1Ca²⁺ exchange via the interconversion mechanism Ca_x²⁺E3Na_e⁺ ↔ 3Na_x⁺ECa_e²⁺ is limited by the forward and reverse rate constants k_a and k_b, which depend on ΔΨ.