F-type ATP synthases have two large domains, a membrane integrated Fo 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), Pi, 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 Fo portion.