On the activation mechanism of the H+‐ATP synthase and unusual thermodynamic properties in the alkalophilic cyanobacterium Spirulina platensis

The activation requirements and thermodynamic characteristics of ATP synthase from the alkalophilic cyanobacterium Spirulina platensis were studied in coupled membrane vesicles. Activation by methanol increased the Vmax, while the Km for MgATP was unaffected (0.7 mM). We propose that in Sp. platensis, as in chloroplasts, the activating effect of methanol is based on perturbation of the γ‐ɛ subunit interaction. Light‐driven ATP synthesis by membrane vesicles of Sp. platensis was stimulated by dithiotreitol. The characteristics of the activation of the ATP synthase by the proton electrochemical potential difference (H+) were analyzed on the basis of the uncoupled rates of ATP hydrolysis as a function of a previously applied proton gradient. Two values of H+, at which 50% of the enzyme is active, were found; 13–14 kJ·mol−1 for untreated membrane vesicles, and 4–8 kJ·mol−1 for light‐treated and dithiotreitol‐treated membrane vesicles. These values are lower than the corresponding values for the oxidized and reduced forms, respectively, of the chloroplast enzyme. Although no bulk proton gradient could be observed, membrane vesicles of Sp. platensis were able to maintain an equilibrium phosphate potential (Gp) of 40–43.5 kJ·mol−1, comparable to values found for Synechococcus 6716 and Anabaena 7120 membrane vesicles. Acid/base‐transition experiments showed that the thermodynamic threshold, H+, for ATP synthesis, catalyzed by light‐treated and dithiotreitol‐treated Spirulina membrane vesicles, was less than 5 kJ·mol−1. The activation characteristics and the low thermodynamic threshold allow ATP synthesis to occur at low H+ values. The findings are discussed, both with repect to differences and similarities with the enzymes from chloroplasts and other cyanobacteria, and with respect to the alkalophilic properties of Sp. platensis.