Physiological implications of the kinetics of maize leaf phosphoenolpyruvate carboxylase.
It has been a common practice to assay phosphoenolpyruvate carboxylase (PEPC) under high, nonphysiological concentrations of Mg(2+) and bicarbonate. We have performed kinetic studies on the enzyme from maize (Zea mays) leaves at near physiological levels of free Mg(2+) (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 C(4) mesophyll cells might be an adaptation for sustaining the steady-state rate of flux through the photosynthetic CO(2) assimilation pathway despite the limitations imposed by the PEPC kinetic properties and the conditions of its environment.
Tovar-Méndez
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A
Departamento de Bioquímica, Facultad de Química, Universidad Nacional Autónoma de México.
Mújica-Jiménez
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C
Muñoz-Clares
R A
RA
D004789:Enzyme Activation; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008293:Malates / Q000494:pharmacology; D010730:Phosphoenolpyruvate Carboxylase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D018515:Plant Leaves / Q000201:enzymology; D003313:Zea mays / Q000201:enzymology*
D008293:Malates; C030298:malic acid; D010730:Phosphoenolpyruvate Carboxylase; D008274:Magnesium