Species variation in kinetic properties of ribulose 1,5-bisphosphate carboxylase/oxygenase

Abstract

Several kinetic parameters of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase from different species were measured and compared. The CO 2 O 2 specificity ( V cK o V oK c ) was found to be about 80 in the enzymes from several C 3 species and two C 4 species. Specificity values of 58 and 70, respectively, were found in enzymes from the C 4 plants Setaria italica and Sorghum bicolor. Two enzymes from cyanobacteria had values of about 50. Substitution of Mn 2+ for Mg 2+ reduced the CO 2 O 2 specificity by a factor of about 20 for all enzymes except that of Rhodospirillum rubrum, which was reduced by a factor of 10. Values for K Mg 2+(apparent) measured at 102 μ m CO 2 were found to vary by a factor of 8 between different RuBP carboxylase/oxygenase enzymes. Enzymes with high K Mg 2+(apparent) values generally had high Michaelis constants for CO 2. The rate of CO 2 Mg 2+ activation was inhibited by RuBP in all enzymes, although the concentration of RuBP required to inhibit activation in the enzyme from the cyanobacterium Aphanizomenon flos-aquae was increased by an order of magnitude compared to other higher plant structural-type enzymes. The wide variation found in the kinetic properties of RuBP carboxylase/oxygenase isolated from diverse species appears to be determined in part by past evolutionary pressures and the present physiocochemical environment in which the enzyme functions.