Fig. 1: Structure of F_oF₁-ATP synthase and rotation mechanism of F_o with a different number of the c-/a-subunits.

a Bacterial ATP synthase (from thermophilic Bacillus PS3) consists of F₁ (α₃β₃γδε) and F_o (ab₂c₁₀) motors. As F₁ has three catalytic sites, three ATP molecules are synthesized per turn of the rotor subunits (γεc₁₀) against the stator subunits (α₃β₃δab₂) during ATP synthesis. b Models of proton translocation through F_o coupled with the rotation of the c-ring. The highly conserved arginine residues of the a-subunit and glutamate (or aspartate) residues of c-subunits are depicted with pink and black open circles, respectively. Protons are depicted as light blue circles. In the models of c₁₀-ring/single a-subunit and c₁₅-ring/single a-subunit, 10 and 15 protons, equal to the number of the c-subunits, are transferred in one turn, respectively. Conversely, in the c₁₀-ring and triple a-subunits model, a total of 30 protons, equal to the number of the c-subunits multiplied by the number of the a-subunits, are transferred in one turn.