Heat inactivation of leaf phosphoenolpyruvate carboxylase: Protection by aspartate and malate in C4 plants.

The activity of phosphoenolpyruvate (PEP) carboxylase EC 4.1.1.31 in leaf extracts of Eleusine indica L. Gaertn., a C4 plant, exhibited a temperature optimum of 35-37° C with a complete loss of activity at 50° C. However, the enzyme was protected effectively from heat inactivation up to 55° C by L-aspartate. Activation energies (Ea) for the enzyme in the presence of aspartate were 2.5 times lower than that of the control enzyme. Arrhenius plots of PEP carboxylase activity (±aspartate) showed a break in the slope around 17-20° C with a 3-fold increase in the Ea below the break. The discontinuity in the slopes was abolished by treating the enzyme extracts with Triton X-100, suggesting that PEP carboxylase in C4 plants is associated with lipid and may be a membrane bound enzyme. Depending upon the species, the major C4 acid formed during photosynthesis (malate or aspartate) was found to be more protective than the minor C4 acid against the heat inactivation of their PEP carboxylase. Oxaloacetate, the reaction product, was less effective compared to malate or aspartate. Several allosteric inhibitors of PEP carboxylase were found to be moderately to highly effective in protecting the C4 enzyme while its activators showed no significant effect. PEP carboxylase from C3 species was not protected from thermal inactivation by the C4 acids. The physiological significance of these results is discussed in relation to the high temperature tolerance of C4 plants.