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Phosphoribulokinase (ATP:d-ribulose-5-phosphate 1-phosphotransferase, EC 2.7.1.19) was purified from the cyanobacterium Chlorogloeopsis fritschii. The enzyme had a molecular weight of 230000, as determined by gel filtration, and a pH optimum of 8·6. Divalent cations were essential for activity, maximal activity being supported by Mg2+, while Mn2+, Ca2+ and Co2+ were less effective. AMP, ADP, phosphoenolpyruvate, aspartate and malate inhibited enzyme activity completely at 1 mm. No effects on phosphoribulokinase activity were observed with NAD, NADH, NADP or NADPH at up to 10 m m or with glyoxylate at up to 20 mm. The enzyme was activated in semi-purified extracts by the addition of dithiothreitol and reduced glutathione. SDS-PAGE of SDS-dissociated enzyme revealed only one polypeptide band of molecular weight 40000. This suggests that C. fritschii phosphoribulokinase is a hexamer consisting of six subunits of identical size.
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