Received May 5, 2001
The suggestions from the author's group over the past 25 years for how steps in catalysis by ATP synthase occur are reviewed. Whether rapid ATP hydrolysis requires the binding of ATP to a second site (bi-site activation) or to a second and third site (tri-site activation) is considered. Present evidence is regarded as strongly favoring bi-site activation. Presence of nucleotides at three sites during rapid ATP hydrolysis can be largely accounted for by the retention of ADP formed and/or by the rebinding of ADP formed. Menz, Leslie and Walker ((2001) FEBS Lett., 494, 11-14) recently attained an X-ray structure of a partially closed enzyme form that binds ADP better than ATP. This accomplishment and other considerations form the base for a revised reaction sequence. Three types of catalytic sites are suggested, similar to those proposed before the X-ray data became available. During net ATP synthesis a partially closed site readily binds ADP and Pi but not ATP. At a closed site, tightly bound ADP and Pi are reversibly converted to tightly bound ATP. ATP is released from a partially closed site that can readily bind ATP or ADP. ATP hydrolysis when protonmotive force is low or lacking occurs simply by reversal of all steps with the opposite rotation of the gamma subunit. Each type of site can exist in various conformations or forms as they are interconverted during a 120° rotation. The conformational changes with the ATP synthase, including the vital change when bound ADP and Pi are converted to bound ATP, are correlated with those that occur in enzyme catalysis in general, as illustrated by recent studies of Rose with fumarase. The betaE structure of Walker's group is regarded as an unlikely, or only quite transient, intermediate. Other X-ray structures are regarded as closely resembling but not identical with certain forms participating in catalysis. Correlation of the suggested reaction scheme with other present information is considered.
KEY WORDS: adenosine triphosphate, ATP, ATP synthase, catalysis, binding change