Modulation by bicarbonate of catalytic and regulatory properties of C₄ phosphoenolpyruvate carboxylase from Amaranthus hypochondriacus: desensitization to malate and glucose 6-phosphate and sensitization to Mg²⁺

Abstract

The catalytic and regulatory properties of phosphoenolpyruvate (PEP) carboxylase (PEPC) are modulated remarkably by the increase in the level of bicarbonate in the assay medium. The activity of PEPC increased by two-fold as the concentration of bicarbonate was raised from 0.05 to 10 mM. During this state, there was only marginal effect on K_m for PEP, while the affinity of PEPC to Mg²⁺ increased by >2 fold. In contrast, the sensitivity of PEPC to malate decreased with increasing concentration of HCO₃^-. Similarly, the stimulation by glucose 6-phosphate (G-6-P) at optimal concentration (10 mM) of HCO₃^- was much less than that at suboptimal concentration (0.05 mM). K₁ for malate increased by about 3 fold and K_a for G-6-P rised by fourfold as bicarbonate concentration was rised from 0.05 to 10 mM. These results suggest that HCO₃^- desensitizes PEPC to both malate and G-6-P. Further, these changes were manifested in both dark- as well as light-forms of the enzyme. Similar results were obtained with PEPC in leaf extracts or in purified form. We therefore propose that bicarbonate-induced changes are independent of phospho-rylation and possibly through a significant change in the conformation of the enzyme. This is the first detailed report indicating marked modulation of regulatory and catalytic properties of PEPC by bicarbonate, one of its substrate.