The rate constant for the reaction of CO2 with enzymebound ribulose 1,5-bisphosphate in vivo

Abstract

Abstract The partial reactions during carboxylation of ribulose-1,5-bisphosphate (RuBP) were studied. A method is proposed for the in vivo estimation of the rate constant for the reaction of CO2 with the ene-diol of RuBP formed in the active site of RuBP carboxylase/ oxygenase (Rubisco). The method involves a pretreatment of the leaves, in the light, with different concentrations of CO2 in nitrogen, followed by 6 s exposure to CO2-free air. Then the leaves are exposed to 14CO2 and the kinetics of 14C uptake are determined after 0.2–0.4 s. It is assumed that the mechanism is a regular bimolecular reaction with the rate linearly dependent on the concentrations of substrates. A value for the rate constant of 0.06 μM−1 s−1 was obtained, independent of the CO2 concentration during pretreatment. However, high pretreatment-[CO2] decreased the concentration of the competent RuBP-binding sites.