Regulation of Soybean Net Photosynthetic CO2 Fixation by the Interaction of CO2, O2, and Ribulose 1,5-Diphosphate Carboxylase

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Abstract
Kinetic properties of soybean net photosynthetic CO2 fixation and of the carboxylase and oxygenase activities of purified soybean (Glycine max [L.] Merr.) ribulose 1, 5-diphosphate carboxylase (EC 4.1.1.39) were examined as functions of temperature, CO2 concentration, and O2 concentration. With leaves, O2 inhibition of net photosynthetic CO2 fixation increased when the ambient leaf temperature was increased. The increased inhibition of CO2 fixation at higher temperatures was caused by a reduced affinity of the leaf for CO2 and an increased affinity of the leaf for O2. With purified ribulose 1,5-diphosphate carboxylase, O2 inhibition of CO2 incorporation and the ratio of oxygenase activity to carboxylase activity increased with increased temperature. The increased O2 sensitivity of the enzyme at higher temperature was caused by a reduced affinity of the enzyme for CO2 and a slightly increased affinity of the enzyme for O2. The similarity of the effect of temperature on the affinity of intact leaves and of ribulose 1,5-diphosphate carboxylase for CO2 and O2 provides further evidence that the carboxylase regulates the O2 response of photosynthetic CO2 fixation in soybean leaves. Based on results reported here and in the literature, a scheme outlining the stoichiometry between CO2 and O2 fixation in vivo is proposed.