Oxidation-reduction properties of the regulatory site of spinach phosphoribulokinase

The oxidation-reduction midpoint potential (E-m) of the regulatory disulfide, formed between Cys16 and Cys55, of spinach chloroplast phosphoribulokinase has been determined both for the wild-type enzyme and for a C244S-C250S double mutant, using enzymatic activity to monitor the oxidation-reduction state of the regulatory disulfide. At pH 7.0, E-m values for the two-electron reduction of the regulatory disulfide of -295 +/- 10 and -290 +/- 10 mV were measured for the wild-type and mutant, respectively. In contrast to the dependence of activity on ambient potential (E-h) observed for the wild-type enzyme and the double mutant, which both followed the Nernst equation for a two-electron process, high and constant activity was exhibited by a C16S-C244S-C250 triple mutant of the enzyme at all E-h values tested. E-m values for the wild-type enzyme were also measured at pH values of 6.7, 7.5, 7.7, and 8.2 and the E-m vs pH data in this region give a good fit to a straight line with a elope of -60 mV/pH unit. (C) 1998 Academic Press.