Testing the kinetic tradeoff between bicarbonate versus phosphoenolpyruvate affinity and glucose-6 phosphate response of phosphoenolpyruvate carboxylase from two C4 grasses.

Abstract

Phosphoenolpyruvate (PEP) carboxylase (PEPC) has an anaplerotic role in central plant metabolism but also initiates the carbon concentrating mechanism during C₄ photosynthesis. The C₄ PEPC has different binding affinities (K_m) for PEP (K_0.5PEP) and HCO₃^- (K_0.5HCO3), and allosteric regulation by glucose-6-phosphate (G6-P) compared to non-photosynthetic isoforms. These differences are linked to specific changes in amino acids within PEPC. For example, region II (residues 302-433, Zea mays numbering) has been identified as important for G6-P regulation and within this region residue 353 may be conserved in C₄ PEPC enzymes. Additionally, residue 780 influences the C₄ PEPC kinetic properties and may interact with region II as well as residue 353 to influence G6-P regulation. We test the hypothesis that variation within region II, including residue 353, and their interactions with residue 780 influence the kinetic and allosteric regulation by G6-P of two C₄ PEPC isozymes from two C₄ grasses. The data does not support a kinetic tradeoff between K_0.5HCO3 and K_0.5PEP in these PEPC isozymes. Additionally, these enzymes had different response to G6-P that was only partially attributed to region II, residue 353 and residue 780. This data provides new insights into factors influencing the kinetic variation of C₄ PEPC isozymes.