The F1Fo ATP synthase is the commonest chemi-mechanical motor found in nature. The Fo motor is an integral energy membrane protein complex that propels converts transmembrane chemical gradients into the rotary mechanical motion of the γ-subunit during ATP synthesis. The F1 motor is a peripheral membrane protein complex connected to the Fo motor. The F1 motor employs the Fo-propelled rotation to drive the sequential condensation of ADP with Pi at its three catalytic sites. ATP syntase can also be run in reverse as the chemical energy of ATP is employed to generate mechanical motion or to pump protons against a chemical potential.
The ATPase-driven rotation of the γ-subunit utilizes sequential conformational changes of each of three catalytic sites on the β-subunits. Because there are three catalytic sites on the enzyme, each ATPase event induces a 120 degree rotation of the γ-subunit. The conformational changes facilitate ATP synthesis by altering the dissociation constant of ATP relative to ADP. The conformations of the β-subunits are staggered such that all three conformations are present at any particular moment. Structural asymmetry of the catalytic sites plus their differences in affinity for ATP occur only when the nucleotide is bound as a complex with the Mg2+ cofactor, indicating that ATP synthesis depends on changes in metal ligands. Calcium, which can bind to two more ligands than can Mg2+, is an effective cofactor of the ATPase reaction. However, Ca2+ activity does not pump a transmembrane proton gradient, implying that Ca2+ couples with the protein at positions that couple ATP hydrolysis to the generation of rotational torque on the γ-subunit.