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The interplay between substrate and allosteric effectors of the photosynthetic C4 isoenzyme of phosphoenolpyruvate carboxylase from monocot and dicot plants
Author(s) -
GuemezToro Rodrigo,
MuñozClares Rosario A.
Publication year - 2013
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.27.1_supplement.lb199
Subject(s) - phosphoenolpyruvate carboxylase , biochemistry , malic enzyme , allosteric regulation , phosphoenolpyruvate carboxykinase , enzyme , photosynthesis , chemistry , malate dehydrogenase , isozyme , pyruvate carboxylase , c4 photosynthesis , bicarbonate , malic acid , biology , dehydrogenase , citric acid , organic chemistry
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 CO 2 in C4 plants, such as Zea mays , monocot, and Amaranthus hypochondriacus , dicot. The maize and amaranth photosynthetic PEPC isoenzymes ( Zm PEPC‐C4 and Ah PEPC‐C4 respectively) are activated by glucose‐6‐phosphate (Glc6P) and inhibited by malate and aspartate, but only Zm PEPC‐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 Mg 2+ , and 0.1 mM bicarbonate, we found that: 1) The affinities for PEP and malate of Ah PEPC‐C4, measured as K s and K i respectively, are higher than those of Zm PEPC‐C4, which result in similar I 50 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 Zm PEPC‐C4 by malate, which indicates the physiological relevance of the regulation by Gly of this enzyme. Why the activation by Gly of Ah PEPC‐C4 is physiologically unnecessary remains to be explained. Supported by PAPIIT‐UNAM IN216911 grant.