Photosynthetic Phosphoenolpyruvate Carboxylases

Abstract

A detailed comparison of green leaf phosphoenolpyruvate carboxylases from the C4-species Atriplex spongiosa and the C3-species Atriplex hastata revealed significant physical and kinetic differences. The two alloenzymes can be separated by anion exchange chromatography but have comparable molecular weights (350,000). Maximal velocity estimates were 38.0 and 1.48 micromoles per minute per milligram of chlorophyll for the carboxylases of A. spongiosa and A. hastata, respectively. Km phosphoenolpyruvate estimates were 0.49 and 0.08 mm for the C4 A. spongiosa and C3 A. hastata and the Km Mg estimates were 0.33 mm for the C4 species and 0.017 mm for the C3 species. The activity of the phosphoenolpyruvate carboxylase of A. spongiosa is more sensitive to chloride and phosphate than the phosphoenolpyruvate carboxylase of A. hastata, but both are equally sensitive to Mg chelating substances such as ATP, ADP, and citrate if assayed at their respective Km Mg values. A survey of the phosphoenolpyruvate carboxylases from 18 C4 and C3 species resulted in mean maximal velocity estimates of 29.0 ± 13.2 and 1.50 ± 0.57 micromoles per minute per milligram of chlorophyll for the C4 species and C3 species, respectively. Km phosphoenolpyruvate estimates were 0.59 ± 0.35 mm and 0.14 ± 0.07 mm for the C4 and C3, and Km Mg estimates were 0.50 ± 0.30 and 0.097 ± 0.057 mm for C4 and C3. All differences between means were significant at the 0.01 confidence level, supporting our hypothesis that the phosphoenolpyruvate carboxylase alloenzymes of C4 and C3 plants are functionally different and are associated with different photosynthetic roles. Both function in the photosynthetic production of C4 acids, the phosphoenolpyruvate carboxylase of C4 species largely producing malate or aspartate (or both) as a photosynthetic intermediate and the phosphoenolpyruvate carboxylase of C3 species producing malate or aspartate (or both) as a photosynthetic product.