The interplay between substrate and allosteric effectors of the photosynthetic C4 isoenzyme of phosphoenolpyruvate carboxylase from monocot and dicot plants

Abstract

Phosphoenolpyruvate carboxylase (PEPC) catalyzes the irreversible formation of oxaloacetate and Pi from phosphoenolpyruvate (PEP) and bicarbonate. This reaction constitutes the first step in the assimilation pathway of atmospheric CO2 in C4 plants, such as Zea mays, monocot, and Amaranthus hypochondriacus, dicot. The maize and amaranth photosynthetic PEPC isoenzymes (ZmPEPC‐C4 and AhPEPC‐C4 respectively) are activated by glucose‐6‐phosphate (Glc6P) and inhibited by malate and aspartate, but only ZmPEPC‐C4 is regulated by glycine (Gly), which is its main activator under physiological conditions. In this work we have explored the interplay between substrate and allosteric effectors in the two enzymes by initial velocity studies. At pH 7.3, 0.4 mM free Mg2+, and 0.1 mM bicarbonate, we found that: 1) The affinities for PEP and malate of AhPEPC‐C4, measured as Ks and Ki respectively, are higher than those of ZmPEPC‐C4, which result in similar I50 values for malate in both enzymes at physiological PEP concentrations. 2) Glc6P is an atypical activator because, unexpectedly, increases the inhibition by malate in both enzymes. 3) As expected, Gly counteracts the inhibition of ZmPEPC‐C4 by malate, which indicates the physiological relevance of the regulation by Gly of this enzyme. Why the activation by Gly of AhPEPC‐C4 is physiologically unnecessary remains to be explained.Supported by PAPIIT‐UNAM IN216911 grant.