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

Abstract

Various properties of phosphoenolpyruvate carboxylases were compared in leaf preparations from C3-C4 intermediate, C3 and C4 Panicum spp. Values of Vmax in .mu.M/mg chlorophyll per h at pH 8.3 were 57-75 for the enzyme from P. milioides, P. schenckii and P. decipiens (all C3-C4). The values for P. laxum (C3) and P. prionitis (C4) were 20-40 and 952-1374, respectively. The Vmax values did not change at pH 7.3 except for the C4 value, which increased .apprx. 24%. At pH 8.3, the phosphoenolpyruvate carboxylases from C3 and C3-C4 species had slightly higher Km 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 85-90% by 1 mM malate at rate-limiting phosphoenolpyruvate and Mg2+ levels. With low levels of malate, 0.2 mM, the rate curve with respect to phosphoenolpyruvate was distinctly sigmoidal and the inhibition was not eliminated at 5 mM phosphoenolpyruvate. Malate at 10 mM protected all phosphoenolpyruvate carboxylases from inactivation at 55.degree. C at pH 5.5, but not at pH 8.3. Aspartate did not protect well. When incubated at 37.degree. C at pH 8.3 without phosphoenolpyruvate but with HCO3-, the enzyme from several C4 grasses lost 92-98% of the initial activity after 4 min; the enzymes from C3 and C3-C4 Panicum spp. retained 60-70% of their activities. In contrast, 5 mM phosphoenolpyruvate stabilized the enzyme at 37.degree. in all plant extracts. The phosphoenolpyruvate carboxylase from C3-C4 intermediate Panicum spp. has properties most similar to the enzyme from C3 Panicum spp. The higher leaf activity of the enzyme from the intermediate plants than from C3 species is apparently due to a higher Vmax.