The Effects of Octylglucoside on the Interactions of Chloroplast Coupling Factor 1 (CF1) with Adenine Nucleotides

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Abstract
The effects of octylglucoside (OcGlc) micelles, which stimulate a Mg-specific ATPase activity in chloroplast coupling factor 1 [CF1, isolated from spinach or lettuce] [Pick, U. and Bassilian, S. (1982)], on the interactions of the enzyme with adenine nucleotides were studied. OcGlc specifically accelerates the binding and the release of ADP but not of ATP or adenosine 5''[.beta.,.gamma.-imido]triphosphate (AdoPP[NH]P) from the tight-sites. The binding affinity for ADP and for ATP is only slightly decreased (2-fold) by the detergent. ATP competitively inhibits the binding of ADP and vice versa in the presence or absence of OcGlc. OcGlc-induced inactivation of CF1-ATPase is correlated with the release of bound nucleotides. In the absence of medium nucleotides ADP .cntdot. CF1 is rapidly inactivated while ATP .cntdot. CF1 and AdoPP[NH]P .cntdot. CF1 are slowly inactivated by OcGlc in parallel with the release of bound nucleotide. In contrast, low concentrations of either ATP or ADP in the medium effectively protect against OcGlc inactivation while AdoPP[NH]P, whose binding to CF1 is inhibited by OcGlc, is ineffective even at millimolar concentrations. The occupancy of the tight-sites may protect the enzyme against OcGlc-induced inactivation. Mg ions specifically inhibit the release of bound ADP and the OcGlc-induced inactivation of CF1. High concentrations of medium ATP and ADP (K50 = 100 .mu.M) also inhibit the OcGlc-induced release of bound nucleotides in an EDTA medium. In contrast, in the absence of OcGlc, medium ADP and ATP accelerate the release of bound adenine nucleotides. Mg-ATP in the presence of OcGlc stimulates the release of bound ADP from CF1. Bound ATP is neither released nor hydrolyzed at the tight-sites under these conditions where medium ATP is rapidly hydrolyzed. Mg-ADP stimulates the release of bound ADP only in the presence of Pi or of phosphate analogs, e.g. arsenate, pyrophosphate or selenate. ATP and ADP apparently bind to the same tight-sites, but OcGlc activation specifically accelerates the exchange of bound ADP at the site. CF1 may contain low affinity adenine nucleotide binding sites which may be the catalytical sites and which influence the tight-sites by cooperative interactions. Mg-ATP in the presence of OcGlc probably induces a conformational change at the catalytical site which accelerates the release of ADP from the tight-site. The implications of these results to the role of adenine nucleotides in the regulation and mechanism of ATP hydrolysis by CF1 are discussed.