Control of ATP hydrolysis by ADP bound at the catalytic site of chloroplast ATP synthase as related to protonmotive force and Mg2+

The activation of the ATP synthesis and hydrolysis capacity of isolated chloroplast membranes by protonmotive force is known to be associated with the release of tightly bound ADP from the ATP synthase. The data support the view that the activation requires only those structural changes occurring in the steady-state reaction mechanism. The trapping of ADP released during light activation or the chelation of Mg2+ with EDTA effectively reduces the rate of decay of the ATPase activity. When the release of tightly bound ADP and Mg2+ is promoted by light activation, followed by immediate dilution and washing to retard the rebinding of the ADP and Mg2+ released, the ATPase activity remains high in the dark long after the protonmotive force has disappeared. After the addition of ADP and Mg2+ the decay of the ATPase activity has the same characteristics as those of the unwashed chloroplast membrane. The results are interpreted as indicating that both Mg2+ and ADP must be present prior to exposure to MgATP for the ATPase to be inhibited. However, in contrast to the isolated chloroplast ATPase, the steady-state activity of the membrane-bound ATPase is not inhibited by excess Mg2+. The replacement of [3H]ADP from catalytic sites during hydrolysis of unlabeled ATP or during photophosphorylation with unlabeled ADP occurs as anticipated if Mg2+ and ADP bound at one catalytic site without Pi block catalysis by all three enzyme sites. The inhibited form induced by Mg2+ and ADP may occur only under laboratory conditions and not have an in vivo role