Abstract
The effects of sugar acids on the kinetics and the protein conformation of ribulose 1, 5-bisphosphate carboxylase/oxygenase (RuBisCO) were examined to differentiate the ligand-binding modes of the catalytic sites. D-Glucarate, L-glucarate, and mucate inhibited the carboxylase activity of RuBisCO competitively with respect to ribulose 1, 5-bisphosphate. Their inhibition constants were 2. 5 to 8. 5 mM. The sugar acids induced such a change of the protein conformation of activated RuBisCO as is detected with a hydrophobic fluorescence probe. However, the change was not concerned with hysteresis of the enzyme. The sugar acids bound to the catalytic sites retarded the release of activator CO2 from the sites in the absence of CO2. These characteristics of the sugar acids were quite similar to those of 6-phosphogluconate, but not of fructose bisphosphate or 3-phosphoglycerate. It is proposed that sugar phosphates that bind to the catalytic sites of RuBisCO could be divided into two groups on the basis of the modes of the binding to the sites.
