Phosphate Activates the Phosphoenolpyruvate Carboxylase from the C4 Plant Amaranthus viridis L.

Phosphoenolpyruvate carboxylase from Amaranthus viridis leaves was activated by inorganic orthophosphate in a concentration- and pH-dependent manner. Maximal activation at pH 7.0 was achieved at phosphate concentrations above 20 mM, and a positive cooperativity was observed for the binding of the anion at this pH. At pH 8.0 the maximum of activity was achieved at 10 mM phosphate; higher concentrations reduced the activation. KM for phosphoenolpyruvate-Mg at pH 7.0 was lowered by phosphate in all concentrations tested up to 30 mM. While at pH 8.0 the KM values were lower than that of the control up to 10 mM phosphate; higher anion concentrations raised the minimum value of KM at this pH. VMAX increased at pH 7.0, and remained unchanged at pH 8.0. A KA value of 0.41 mM was calculated for phosphate at the alkaline pH. The phosphate analogue arsenate also behaved as an activating agent, while other anions (e.g. nitrate, nitrite, sulfate, tetraborate) were ineffective. The phosphate-activated enzyme was shown to be insensitive to glucose-6-phosphate, but was inhibited by l-malate to the same extent as the control.