Dramatic difference in the responses of phosphoenolpyruvate carboxylase to temperature in leaves of C3 and C4 plants

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Abstract
Temperature caused phenomenal modulation of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) in leaf discs of Amaranthus hypochondriacus (NAD‐ME type C4 species), compared to the pattern in Pisum sativum (a C3 plant). The optimal incubation temperature for PEPC in A. hypochondriacus (C4) was 45 °C compared to 30 °C in P. sativum (C3). A. hypochondriacus (C4) lost nearly 70% of PEPC activity on exposure to a low temperature of 15 °C, compared to only about a 35% loss in the case of P. sativum (C3). Thus, the C4 enzyme was less sensitive to supra‐ optimal temperature and more sensitive to sub‐ optimal temperature than that of the C3 species. As the temperature was raised from 15 °C to 50 °C, there was a sharp decrease in malate sensitivity of PEPC. The extent of such a decrease in C4 plants (45%) was more than that in C3 species (30%). The maintenance of high enzyme activity at warm temperatures, together with a sharp decrease in the malate sensitivity of PEPC was also noticed in other C4 plants. The temperature‐induced changes in PEPC of both A. hypochondriacus (C4) and P. sativum (C3) were reversible to a large extent. There was no difference in the extent of phosphorylation of PEPC in leaves of A. hypochondriacus on exposure to varying temperatures, unlike the marked increase in the phosphorylation of enzyme on illumination of the leaves. These results demonstrate that (i) there are marked differences in the temperature sensitivity of PEPC in C3 and C4 plants, (ii) the temperature induced changes are reversible, and (iii) these changes are not related to the phosphorylation state of the enzyme. The inclusion of PEG‐6000, during the assay, dampened the modulation by temperature of malate sensitivity of PEPC in A. hypochondriacus. It is suggested that the variation in temperature may cause significant conformational changes in C4‐PEPC.