EFFECTS OF LOW-TEMPERATURE ACCLIMATION OF WINTER RYE ON CATALYTIC PROPERTIES OF ITS RIBULOSE BISPHOSPHATE CARBOXYLASE-OXYGENASE

A comparison was made of the kinetics of the carboxylation reaction of bicarbonate-magnesium-activated ribulose biphosphate carboxylase-oxygenase purified from cold-hardened and unhardened winter rye (Secale cereale L. cv. Puma). The activity of the (NH4)2SO4-precipitated enzyme from hardened plants was stable at -20 degrees C for a month, whereas the form from unhardened plants was reversibly cold inactivated. The KmCO2 of the unhardened form increased more rapidly with decreasing pH below 8.2, but the estimated pKa of chemical groups associated with the active site was not affected by the cold hardening. The temperature dependencies of the KmCO2 of the two forms of the enzyme crossed at 10 degrees C with the effect that the catalysis of carboxylation by ribulose biphosphate carboxylase-oxygenase from Puma rye was most efficient in the temperature range to which the plants had been adapted.