Plant Physiol, May 2000, Vol. 123, pp. 149-160

Physiological Implications of the Kinetics of Maize Leaf Phosphoenolpyruvate Carboxylase¹

Departamento de Bioquímica, Facultad de Química, Universidad Nacional Autónoma de México, México D.F., 04510, México

It has been a common practice to assay phosphoenolpyruvate carboxylase (PEPC) under high, nonphysiological concentrations of Mg²⁺ and bicarbonate. We have performed kinetic studies on the enzyme from maize (Zea mays) leaves at near physiological levels of free Mg²⁺ (0.4 mM) and bicarbonate (0.1 mM), and found that both the nonphosphorylated and phosphorylated enzymes exhibited a high degree of cooperativity in the binding of phosphoenolpyruvate, a much lower affinity for this substrate and for activators, and a greater affinity for malate than at high concentrations of these ions. Inhibition of the phosphorylated enzyme by malate was overcome by glycine or alanine but not by glucose-6-phosphate, either in the absence or presence of high concentrations of glycerol, a compatible solute. Alanine caused significant activation at physiological concentrations, suggesting a pivotal role for this amino acid in regulating maize leaf PEPC activity. Our results showed that the maximum enzyme activity attainable in vivo would be less than 50% of that attainable in vitro under optimum conditions. Therefore, the high levels of PEPC protein in the cytosol of C₄ mesophyll cells might be an adaptation for sustaining the steady-state rate of flux through the photosynthetic CO₂ assimilation pathway despite the limitations imposed by the PEPC kinetic properties and the conditions of its environment.