Purification and Some Properties of Chlorella fusca Ribulose 1,5-Diphosphate Carboxylase

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Abstract
Ribulose 1,5-diphosphate carboxylase has been purified from extracts of autotrophically grown Chlorella fusca by ammonium sulfate precipitation and centrifugation on a linear sucrose density gradient. The enzyme was homogeneous by the criterion of polyacrylamide gel electrophoresis. The molecular weight of the enzyme was 530,000, and it was composed of two types of subunit of molecular weight 53,000 and 14,000. Ribulose 1,5-diphosphate, CO2, and Mg2+ had Michaelis constant values of 15 μm, 0.3 mm, and 0.37 mm, respectively. At high bicarbonate concentration (17 mm and 50 mm), 6-phosphogluconate inhibited the enzyme, the inhibition being noncompetitive with respect to ribulose 1,5-diphosphate (Ki 0.065 mm), whereas at low bicarbonate concentration (1 mm), 6-phosphogluconate activated the enzyme. Oxygen was a competitive inhibitor with respect to CO2, suggesting the enzyme also functions as an oxygenase. This was confirmed by direct assay, a 1: 1 stoichiometry between ribulose 1,5-diphosphate consumed and O2 uptake being observed.