ATP synthase is regulated so as to prevent futile hydrolysis of ATP when the transmembrane proton electrochemical gradient, Delta mu(H)(+), falls. Mitochondria and chloroplasts have different mechanisms for inhibition of ATP synthase: by binding an inhibitor protein, and by stabilization of the ADP-inhibited state by making an intramolecular disulphide bond, respectively. The recently determined structure of bovine F-1-ATPase is locked in a conformation that probably represents the ADP-inhibited state of the enzyme.