Enzymic Properties of Ribulose-1,5-bisphosphate Carboxylase/Oxygenase Purified from Rice Leaves

Abstract

The enzymic properties of ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase purified from rice (Oryza sativa L.) leaves were studied. Rice RuBPcarboxylase, activated by preincubation with CO(2) and Mg(2+) like other higher plant carboxylases, had an activation equilibrium constant (K(c)K(Mg)) of 1.90 x 10(5) to 2.41 x 10(5) micromolar(2) (pH 8.2 and 25 degrees C). Kinetic parameters of carboxylation and oxygenation catalyzed by the completely activated enzyme were examined at 25 degrees C and the respective optimal pHs. The K(m)(CO(2)), K(m)(RuBP), and V(max) values for carboxylation were 8 micromolar, 31 micromolar, and 1.79 units milligram(-1), respectively. The K(m)(O(2)), K(m)(RuBP), and V(max) values for oxygenation were 370 micromolar, 29 micromolar, and 0.60 units milligram(-1), respectively.Comparison of rice leaf RuBP carboxylase with other C(3) plant carboxylases showed that it had a relatively high affinity for CO(2) but the lowest catalytic turnover number (V(max)) among the species examined.