Kinetic Properties of Phosphoenolpyruvate Carboxylase from C3, C4, and C3-C4 Intermediate Species of Flaveria (Asteraceae)

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Abstract
Flaveria cronquistii (C3), F. chloraefolia (C3-C4), F. floridana (C3-C4), F. pubescens (C3-C4), F. anomala (C3-C4), F. linearis (C3-C4), F. brownii (C4), F. palmeri (C4), F. trinervia (C4) and F. australasica (C4), comprising 10 out of the 21 known species of the genus Flaveria (Ateraceae), were included in a comparative study of the kinetic and regulatory properties of green leaf phosphoenolpyruvate (PEP) carboxylase. At least three kinetically distinct enzyme-forms were identified on the basis of their affinities for PEP and the degree of allosterism with respect to this substrate. The kinetic properties of PEP carboxylase of most of the species seemingly were modified in vivo depending on the growth conditions of the plants. Km(PEPfree)-values of the enzyme from the five C3-C4 intermediate species ranged from 6 micromolar (F. chloraefolia, low light-grown) to 38 micromolar (F. pubescens, high light-grown). In contrast, the Km for PEP of PEP carboxylase from the C3 species F. cronquistii (13 micromolar) apparently was not influenced by growth conditions. The response of the enzyme from the C3 and C3-C4 species was hyperbolic in all cases. A second isoform with a lower affinity for PEP (88-100 micromolar), but also hyperbolic kinetics was found in the C4 species F. brownsii, whereas in the three other C4 species examined a PEP carboxylase with a still lower affinity for PEP (187-221 micromolar) and sigmoidal kinetics was present. These isozyme-related kinetic data were supported by analyses of the elution behavior of the enzyme during anion-exchange chromatography on DEAE-Trisacryl M. The results are discussed with respect to the evolution of C4 photosynthesis in the Flaveria genus.