The biochemistry of Rubisco in Flaveria

C₄ plants have been reported to have Rubiscos with higher maximum carboxylation rates (kcatCO₂) and Michaelis-Menten constants (K_m) for CO₂ (K_c) than the enzyme from C ₃ species, but variation in other kinetic parameters between the two photosynthetic pathways has not been extensively examined. The CO ₂/O₂ specificity (S_C/O), kcatCO₂, K_c, and the K_m for O₂ (K_o) and RuBP (K_m-RuBP), were measured at 25°C, in Rubisco purified from 16 species of Flaveria (Asteraceae). Our analysis included two C₃ species of Flaveria, four C₄ species, and ten C₃-C ₄ or C₄-like species, in addition to other C₄ (Zea mays and Amaranthus edulis) and C₃ (Spinacea oleracea and Chenopodium album) plants. The S_C/O of the C₄ Flaveria species was about 77 mol mol^-1, which was approximately 5% lower than the corresponding value in the C₃ species. For Rubisco from the C₄ Flaverias kcatCO₂ and K_c were 23% and 45% higher, respectively, than for Rubisco from the C₃ plants. Interestingly, it was found that the K_o for Rubisco from the C ₄ species F. bidentis and F. trinervia were similar to the C ₃ Flaveria Rubiscos (∼650 μM) while the K_o for Rubisco in the C₄ species F. kochiana, F. australasica, Z. mays, and A. edulis was reduced more than 2-fold. There were no pathway-related differences in K_m-RuBP. In the C₃-C₄ species kcatCO₂ and K_c were generally similar to the C₃ Rubiscos, but the K_o values were more variable. The typical negative relationships were observed between S_C/O and both kcatCO₂ and K_c, and a strongly positive relationship was observed between kcatCO₂ and Kc. However, the statistical significance of these relationships was influenced by the phylogenetic relatedness of the species.