F-type ATP synthases have two large domains, a membrane integrated F_o portion (subunits a, b₂ and c₁₀ for the E. coli enzyme) which is involved in proton transport and a hydrophilic F₁ portion (subunits a₃b₃gde) which contains the ‘nucleotide’ (ATP, ADP), P_i, and Mg²⁺ binding sites. The nucleotide binding and dissociation steps at the three catalytic b-subunits of the F₁ portion are coupled to proton translocation through the F_o portion.