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

The effects of octylglucoside (OcGlc) micelles, which stimulate a Mg‐specific ATPase activity in chloroplast coupling factor 1 [Pick, U. and Bassilian, S. (1982) Biochemistry, 21 , 6144–6152], on the interactions of the enzyme with adenine nucleotides have been studied.1 OcGle specifically accelerates the binding and the release of ADP but not of ATP or adenosine 5′[β, γ‐imido]triphosphate (Ado PP [NH] P ) from the tight‐sites. The binding affinity for ADP and for ATP is only slightly decreased (twofold) by the detergent. ATP competitively inhibits the binding of ADP and vice versa in the presence or absence of OcGlc. 2 OcGle‐induced inactivation of CF 1 ‐ATPase is correlated with the release of bound nucleotides. In the absence of medium nucleotides ADP · CF 1 is rapidly inactivated while ATP · CF 1 and Ado PP [NH] P · CF 1 are slowly inactivated by OcGle 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 Ado PP [NH] P , whose binding to CF 1 is inhibited by OcGle, is ineffective even at millimolar concentrations. The results suggest that the occupancy of the tight‐sites protects the enzyme against OcGlc‐induced inactivation. 3 Mg ions specifically inhibit the release of bound ADP and the OcGlc‐induced inactivation of CF 1 . High concentrations of medium ATP and ADP ( K 50 = 100 μM) 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. 4 Mg‐ATP in the presence of OcGlc stimulates the release of bound ADP from CF 1 . 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 inorganic phosphate or of phosphate analogs, e.g. arsenate, pyrophosphate or selenate. 5 It is suggested that: (a) ATP and ADP bind to the same tight‐sites, but OcGlc activation specifically accelerates the exchange of bound ADP at the site. (b) CF 1 contains low affinity adenine nucleotide binding sites which may be the catalytical sites and which influence the tight‐sites by cooperative interactions. (c) Mg‐ATP in the presence of OcGlc 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 CF 1 are discussed.