Photosynthetic Characteristics of C3–C4 Intermediate Flaveria Species II. Kinetic Properties of Phosphoenolpyruvate Carboxylase from C3, C4 and C3–C4 Intermediate Species

The kinetic properties of phosphoenolpyruvate (PEP) carboxylase have been studied among several Flaveria species: the C3 species F. cronquistii, the C3–C4 species F. pubescens and F. linearis, and the C4 species F. trinervia. At either pH 7 or 8, the maximum activities (in μmol.mg Chl−1.h−1) for F. pubescens and linearis (187–513) were intermediate to those of the C3 species (12–19) and the C4 species (2,182–2,627). The response curves of velocity versus PEP concentration were hyperbolic for the C3 and C3–C4 species at either pH 7 or 8 while they were sigmoidal for the C4 species at pH 7 and hyperbolic at pH 8. The Km values for PEP determined from reciprocal plots were lowest in the C3 species, and of intermediate value in the C3–C4 species compared to the K′ values of the C4 species determined from Hill plots at either pH 7 or 8. Glucose-6-phosphate (G6P) decreased the Km values for PEP at both pH 7 and 8 in the C3 and C3–C4 species. In the C4 species, G6P decreased the K′ values at pH 8 but increased the K′ values at pH 7. In all cases, G6P had its effect by influencing the activity at limiting PEP concentrations with little or no effect on the maximum activity. At pH 8 and limiting concentrations of PEP the degree of stimulation of the activity by G6P was greatest in the C4 species, intermediate in F. linearis, a C3–C4 species, and lowest in the C3 species. In several respects, the PEP carboxylases of the C3–C4 Flaveria species have properties intermediate to those of the C3 and C4 species.