Rotational catalysis by F₁-ATPase

F₁-ATPase, a water-soluble portion of ATP synthase, is a fully reversible rotary molecular machine in which a central γ subunit rotates inside a cylinder made of three α and three β subunits alternately arranged. This motor rotates counterclockwise by hydrolyzing ATP in three catalytic sites but synthesizes ATP when forced to rotate clockwise by an external force. Single-molecule studies have revealed how the chemical reactions that occur in the three catalytic sites are coupled to mechanical rotation.