PHOSPHOENOLPYRUVATE CARBOXYLASE FROM PENNYWORT (UMBILICUS, RUPESTRIS) - CHANGES IN PROPERTIES AFTER EXPOSURE TO WATER-STRESS

U. rupestris (pennywort) switches from C3 photosynthesis to an incomplete form of crassulacean acid metabolism (referred to as CAM-idling) when exposed to water stress (drought). This switch is accompanied by an increase in the activity of phosphoenolpyruvate carboxylase. This enzyme also shows several changes in properties, including a marked decrease in sensitivity to acid pH, a lower Km for phosphoenolpyruvate, very much decreased sensitivity to the allosteric inhibitor malate, and increased responsiveness to the allosteric effector glucose 6-phosphate. The MW of the enzyme remains unchanged, at .apprx. 185,000. These changes in properties of phosphoenolpyruvate carboxylase are discussed in relation to the roles of the enzyme in C3 and in CAM plants.