The oxidized form of spinach chloroplast fructose-1,6-bisphosphatase is devoid of activity under the H+and Mg2+concentrations which prevail in the illuminated chloroplast stroma. In contrast, the reductive activation of this enzyme form results in activity under these conditions. In this paper we demonstrate that preincubation of the oxidized enzyme in the presence of Ca2+/fructose-1,6-bisphosphate gives rise to an active enzyme under these same conditions. The activation appears to be fast at pH 8 and the final level of activity is roughly of the same order of magnitude as that of the enzyme reductively activated by thioredoxin. The ph/activity profile of the activated oxidized enzyme also resembles that of the reduced enzyme. © 1990.
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