Temperature dependence of the enzymic carboxylation and oxygenation of ribulose 1,5-bisphosphate in relation to effects of temperature on photosynthesis

Carboxylase and oxygenase activities of ribulose bisphosphate carboxylase purified from wheat were measured over the temperature range 5 to 35°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 micromolar) and O₂ (0.34 millimolar), the oxygenase to carboxylase ratio remained constant at 0.21 between 5 and 25°C but increased to 0.26 at 35°C. At O₂ and CO₂ concentrations near those expected in water equilibrated with air (21% v/v] O₂) containing 300 μl/l CO₂ at the various temperatures, the ratio of oxygenase to carboxylase activity increased 2.2-fold between 15 and 35°C. At CO₂ and O₂ concentrations expected in water in equilibrium with subatmospheric concentrations of CO₂ in air (21% v/v] O₂, 210 μl/l CO₂), the oxygenase to carboxylase ratio increased from 0.25 at 10°C to 0.56 at 35°C. Between 20 and 30°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.