Activation of the H +-ATP synthases of a thermophilic cyanobacterium and chloroplasts — a comparative study

Abstract

The activation requirements of the ATP synthases of the thermophilic cyanobacterium Synechococcus 6716, studied in coupled membrane vesicles and in the isolated or reconstituted complex, and of the ATP synthase of spinach chloroplasts were compared. It was found that methanol, heat treatment or dithiothreitol did not activate ATP hydrolysis in Synechococcus 6716. In contrast to the chloroplast enzyme, activation could only be accomplished with sulfite, octyl glucoside, a proton electrochemical potential difference and trypsin. The lack of activation by dithiothreitol, heat and methanol in the cyanobacterial ATP synthase can be explained by the absence of three cysteine residues in the regulatory γ subunit of the F 1 part. The threshold value of the proton electrochemical potential difference at which ATP synthesis occurs at low G p was about 9.5 kJ mol −1 for the cyanobacterial ATP synthase. This is similar to the threshold value of the reduced form of the enzyme in chloroplasts. With cyanobacterial membrane vesicles, an H + / ATP stoichiometry slightly exceeding 4 was obtained in ATP hydrolysis as well as in ATP synthesis, measured as a function of an artificially applied proton electrochemical potential difference. These findings are discussed in terms of a single structural difference between the cyanobacterial and the chloroplast enzyme. When comparing the enzyme of Synechococcus 6716 with that of chloroplasts, our results indicate that the difference in activation requirements of both ATP synthases resides in a different arrangement of the γ and e subunits.