Allosteric Regulation of the Photosynthetic C4 Isoenzyme of Phosphoenolpyruvate Carboxylase: A Comparative Study Between Enzymes from Monocot and Dicot Plants

Abstract

In the present study, we have investigated the complex allosteric regulation of the non-phosphorylated forms of the photosynthetic phosphoenolpyruvate carboxylase isoenzymes (PEPC-C4) from amaranth (AhPEPC-C4) and maize (ZmPEPC-C4) leaves. Previous studies showed that glycine (Gly) only activates PEPC-C4 from monocot plants, as maize, but not from dicot plants, as amaranth. Our initial velocity data confirm this, in spite that AhPEPC-C4 binds Gly with much higher affinity than ZmPEPC-C4. In AhPEPC-C4, the lack of Gly activation is overcome mainly by its higher affinity for the substrate phosphoenolpyruvate and its lower affinity for the inhibitor malate compared with ZmPEPC-C4. We have also explored the structural determinants of the differences in Gly activation by performing multiple alignments between the known monocot and dicot PEPC-C4 sequences and by modeling, in both the AhPEPC-C4 and ZmPEPC-C4 isoenzymes, the three-dimensional structure of the loop proposed as the Gly binding site, which was not observed in the crystal structure of the maize enzyme due to its high flexibility. The models suggest that conserved lysyl and aspartyl residues are important for binding to the activator molecule, and that a nearby non-conserved residue may be responsible for differences between the amaranth and maize enzymes in the loop conformation, which would account for the poorer affinity for Gly of the maize enzyme as well as for its higher degree of activation.