Temperature Dependence of the Enzymic Carboxylation and Oxygenation of Ribulose 1,5-Bisphosphate in Relation to Effects of Temperature on Photosynthesis

Abstract

Carboxylase and oxygenase activities of ribulose bisphosphate carboxylase purified from wheat were measured over the temperature range 5.degree.-35.degree. C either at constant O2 and CO2 concentrations or where the O2 and CO2 simulated the concentrations in water equilibrated at each temperature with the same gaseous phase. At constant CO2 (14 .mu.M) and O2 (0.34 mM), the oxygenase to carboxylase ratio remained constant at 0.21 between 5 and 25.degree. C but increased to 0.26 at 35.degree. C. At O2 and CO2 concentrations near those expected in water equilibrated with air (21% [vol/vol]O2) containing 300 .mu.l/l CO2 at the various temperatures, the ratio of oxygenase to carboxylase activity increased 2.2-fold between 15.degree. and 35.degree. C. At CO2 and O2 concentrations expected in water in equilibrium with subatmospheric concentrations of CO2 in air (21% [vol/vol] O2, 210 .mu.l/l CO2), the oxygenase to carboxylase ratio increased from 0.25 at 10.degree. C to 0.56 at 35.degree. C. Between 20.degree. and 30.degree. C, the apparent Q10 value for the oxygenase reaction was 1.78 and that for the carboxylase was 1.26. Hence, the different responses of photosynthesis and photorespiration to temperature are due more to changes in the relative solubilities of CO2 and O2 (the solubility ratio) than to changes in kinetic parameters of the reactions catalyzed by ribulose bisphosphate carboxylase.