Inhibition of chloroplast CF1-ATPase by vanadate

Inhibition of ATPase activity by vanadate, having K 1 2 of 0.5 mM, was demonstrated in the CF1-ATPase. The Ca2+-dependent ATPase activity of the isolated enzyme was inhibited in an allosteric manner by vanadate with a Hill coefficient of 3.19 ± 0.6. Vanadate also inhibited ATPase and Pi—ATP exchange activities of the chloroplast membrane-bound enzyme. Using 51V NMR it was demonstrated that ATP caused partial release of about 1.87 equivalents while ADP caused additional binding of approximately 1.46 equivalents of vanadate, when added to a solution containing CF1 equilibrated with vanadate. The relevance of these results to a possible involvement of a pentacovalent phosphate as transition state intermediate in the hydrolysis of ATP by CF1-ATPase is discussed.