Some unique characteristics of thylakoid unisite ATPase

Under unisite conditions (ratio of ATP to chloroplast coupling factor (CF0CF1), approximately 1:2.8), spinach thylakoid ATPase depends on prior reductive activation of CF1, just as multisite ATPase does, and is sensitive to removal of CF1 by EDTA. Faster rates in room light than in semidarkness and up to 80% inhibition by uncouplers only in room light indicate a strong effect of protonmotive force, which can be provided by room light. In addition, unisite ATPase is inhibited by azide as long as some ADP is bound to the CF1. Several differences were found between unisite and multisite ATPase. 1) The unisite activities of both membrane-bound and free enzyme were stimulated up to 3-fold by 4 mM free MgCl2 (a strong inhibitor of multisite ATPase). 2) Thylakoid unisite ATPase was inhibited by sulfite (50% inhibition at 5 mM), a powerful activator of multisite ATPase. This inhibition is attributed to a nonspecific ionic strength effect. 3) Unisite ATPase was inhibited by trypsin treatment, which increases multisite ATPase severalfold. 4) The pH profile of thylakoid unisite ATPase is somewhat different from that of multisite. 5) Alkylation of Cys-89 of the gamma subunit by N-ethylmaleimide did not affect the unisite activity, but inhibited multisite activity more than 90%.