Purification of Ribulose 5-phosphate Kinase and Minor Polypeptides of Pyrenoid from the Green Alga Bryopsis maxima

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Ribulose 5-phosphate (Ru5P) kinase (ATP:D-ribulose 5-phosphate 1-phosphotransferase; EC 2.7.1.19), an enzyme in the reductive pentose phosphate cycle, was purified from the green alga Bryopsis maxima and its activity and peptide composition were studied. The specific activity of purified Ru5P kinase was 20μmole RuBP formed (mg protein)^<-1> min^<-1> corresponding to a 490-fold purification from the supernatant of chloroplasts. The K_m values of Ru5P kinase for ATP and Ru5P were 69μM and 330μM, respectively. The molecular size of Ru5P kinase was estimated as 90 kDa by gel filtration and that of its polypeptide as 41 kDa by SDS-polyacrylamide gel electrophoresis. A small portion of the Ru5P kinase was found in a large molecular state (500 kDa) which was considered to be an inactive form of the enzyme. Ru5P kinase activity has been reported in the pyrenoid of Eremosphaera viridis as well as ribulose 1,5-bisphosphate carboxylase-oxygenase (RuBisCO) and ribose 5-phosphate isomerase activity (Holdsworth 1971). In Bryopsis maxima, among the pyrenoid polypeptides other than that of RuBisCO, we found a polypeptide of 42 kDa, similar to that of Ru5P kinase in molecular size and ratio to RuBisCO. A peptide map of the 42 kDa pyrenoid polypeptide, however, showed that it differed from that of Ru5P kinase. In conclusion, Ru5P kinase may be not involved in the pyrenoid of this alga.
日本植物生理学会の論文