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Comparative Characterization of Phosphoenolpyruvate Carboxylase in C₃, C₄, and C₃-C₄ Intermediate Panicum Species ¹

Department of Biochemistry, University of Georgia, Athens, Georgia 30602

Various properties of phosphoenolpyruvate carboxylases were compared in leaf preparations from C₃-C₄ intermediate, C₃, and C₄Panicum species. Values of V_max in micromoles per milligram chlorophyll per hour at pH 8.3 were 57 to 75 for the enzyme from Panicum milioides, Panicum schenckii, and Panicum decipiens (all C₃-C₄). The values for Panicum laxum (C₃) and Panicum prionitis (C₄) were 20 to 40 and 952 to 1374, respectively. The V_max values did not change at pH 7.3 except for the C₄ value, which increased about 24%. At pH 8.3, the phosphoenolpyruvate carboxylases from C₃ and C₃-C₄ species had slightly higher K_m HCO₃^– and lower K^' phosphoenolpyruvate values than did the C₄ enzyme. With each species at pH 7.3, all K^' phosphoenolpyruvate values were 2- to 4-fold greater.

The enzyme from all species was inhibited 85 to 90% by 1 millimolar malate at rate-limiting phosphoenolpyruvate and Mg²⁺ levels. With low levels of malate, 0.2 millimolar, the rate curve with respect to phosphoenolpyruvate was distinctly sigmoidal, and the inhibition was not eliminated at 5 millimolar phosphoenolpyruvate.

Malate at 10 millimolar protected all phosphoenolpyruvate carboxylases from inactivation at 55 C at pH 5.5, but not at pH 8.3. Aspartate did not protect well. When incubated at 37 C at pH 8.3 without phosphoenolpyruvate, but with HCO₃^–, the enzyme from several C₄ grasses lost 92 to 98% of the initial activity after 4 minutes, whereas the enzymes from C₃ and C₃-C₄Panicum species retained 60 to 70% of their activities. In contrast, 5 millimolar phosphoenolpyruvate stabilized the enzyme at 37 C in all plant extracts.

The phosphoenolpyruvate carboxylase from C₃-C₄ intermediate Panicum species has properties most similar to the enzyme from C₃Panicum species. The higher leaf activity of the enzyme from the intermediate plants than from C₃ species is not due to any unusual property assayed other than a higher V_max.