Energy Transduction by the Two Molecular Motors of the F1Fo ATP Synthase

The F1Fo ATP synthase has nearly universal importance as the major source of ATP among all life forms. These molecular motors couple the energy provided by a transmembrane proton gradient to the production of ATP from ADP and phosphate. The intrinsic membrane complex of ab2c10–15 subunits, known as Fo, functions as a proton channel via a Brownian ratchet mechanism and the F1 peripheral membrane complex of α3β3γδe subunits contains one site for ATP synthesis/hydrolysis per αβ heterodimer. When F1 is purified from Fo and the membrane, it retains the ability to hydrolyze ATP. The ring of three αβ heterodimers form the stator around the γ-subunit rotor that rotates in response to ATP hydrolysis activity producing a torque of 61 pN nm. Rotation occurs via the alternating site mechanism in which ATP binds to one site, while product release occurs at another site. It uses the non-equilibrium transmembrane electrochemical proton gradient derived from the oxidation of metabolites or light during photosynthesis to drive the reaction ADP + Pi ↔ ATP + H2O away from equilibrium, and thereby maintains high cellular concentrations of ATP. Under some conditions, the enzyme can catalyze ATPase-driven proton pumping in the reverse direction across the membrane. However, the enzymes from mitochondria and chloroplasts employ mechanisms to minimize this reverse reaction.