Modulation of the chloroplast ATPase by tight binding of nucleotides

Inactivation of the chloroplast ATPase upon tight nucleotide binding was studied with several adenine nucleotide analogs. Compared with ADP, the other nucleoside diphosphates were less effective in the follwing order: IDP > ε{lunate}-ADP > 1-oxido-ADP > GDP. The nucleotide analogs compete with ADP for binding to the tight nucleotide-binding site(s) on the ATPase and also prevent further inactivation by ADP. AdoPP[NH]P also causes inactivation but has a lower affinity than ADP. [³H]GDP binds tightly to the ATPase, but the resulting enzyme-GDP complex is more readily dissociable than the enzyme-ADP complex. Although both nucleotides appear to bind to the same site, the catalytic and binding properties of the coresponding nucletide-enzyme complexes differ. Binding of GDP also decreases the rate and extent of the sontaneous decay of the activated enzyme. PP_i decreases the rate of inacivation caused by ADP and also the level of tigthly buond ADP. Based on these results, we suggest that two different confomations of the ATPase exist which contain tigthly bound ADP. The active conformation is conveted to the inactive conformation in the absence of a continued supply of energy by illumination or ATP hydrolysis.