EFFECTS OF TEMPERATURE ON THE ACTIVITY OF PHOSPHOENOLPYRUVATE CARBOXYLASE AND ON THE CONTROL OF CO2 FIXATION IN BRYOPHYLLUM-FEDTSCHENKOI

The phosphorylation state and the malate sensitivity of phosphoenolpyr uvate carboxylase (PEPCase, EC 4.1.1.31) in Bryophyllum fedtschenkoi H amet et Perrier are altered by changes in the ambient temperature. The se effects, in turn alter the in-vivo activity of the enzyme. Low temp erature (3 degrees C or less), stabilizes the phosphorylated form of t he enzyme, while high temperature (30 degrees C) promotes its dephosph orylation. The catalytic activity of the phosphorylated and dephosphor ylated forms of PEPCase increases with temperature, but the apparent K -i values for malate of both forms of the enzyme decrease. Results of experiments with detached leaves maintained in darkness in normal air indicate that the changes in malate sensitivity and phosphorylation st ate of PEPCase with temperature are of physiological significance. Whe n the phosphorylated form of PEPCase is stabilized by reducing the tem perature of leaves 9 h after transfer to constant darkness at 15 degre es C, a prolonged period of CO2 fixation follows. When leaves are main tained in constant darkness at 15 degrees C until CO2 output reaches a low steady-state level and the PEPCase is dephosphorylated, reducing the temperature to 3 degrees C results in a further period of CO2 fixa tion even though the phosphorylation state of PEPCase does not change.