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

Various properties of phosphoenolpyruvate carboxylases were compared in leaf preparations from C(3)-C(4) intermediate, C(3), and C(4)Panicum 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 C(3)-C(4)). The values for Panicum laxum (C(3)) and Panicum prionitis (C(4)) were 20 to 40 and 952 to 1374, respectively. The V(max) values did not change at pH 7.3 except for the C(4) value, which increased about 24%. At pH 8.3, the phosphoenolpyruvate carboxylases from C(3) and C(3)-C(4) species had slightly higher K(m) HCO(3) (-) and lower K(') phosphoenolpyruvate values than did the C(4) 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 Mg(2+) 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 HCO(3) (-), the enzyme from several C(4) grasses lost 92 to 98% of the initial activity after 4 minutes, whereas the enzymes from C(3) and C(3)-C(4)Panicum 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 C(3)-C(4) intermediate Panicum species has properties most similar to the enzyme from C(3)Panicum species. The higher leaf activity of the enzyme from the intermediate plants than from C(3) species is not due to any unusual property assayed other than a higher V(max.).