Cooperative Effects of Light and Temperature on the Activity of Phosphoenolpyruvate Carboxylase from Amaranthus paniculatus L.

The phosphoenolpyruvate carboxylase of Amaranthus paniculatus shows in vitro optimum affinity (S(0.5)) to phosphoenolpyruvate at a relatively high temperature (about 35 degrees C); even in the presence of activators, it functions efficiently only above 25 to 27 degrees C. At lower temperatures, a steep increase of activity with temperature is observed, due to the high activation energy for the catalyzed reaction. The same behavior in vivo could amplify the photoactivation of the enzyme to a large extent, since the night/day transition is soon followed by a considerable rise in leaf temperature.