The biochemistry of Rubisco in Flaveria

C-4 plants have been reported to have Rubiscos with higher maximum carboxylation rates (kcat(CO2)) 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 CO2/O-2 specificity (S-C/O), kcat(CO2), K-c, and the K-m for O-2 (K-o) and RuBP (Km-RuBP), were measured at 25 degrees C, in Rubisco purified from 16 species of Flaveria (Asteraceae). Our analysis included two C-3 species of Flaveria, four C-4 species, and ten C-3-C-4 or C-4-like species, in addition to other C-4 (Zea mays and Amaranthus edulis) and C-3 (Spinacea oleracea and Chenopodium album) plants. The S-C/O of the C-4 Flaveria species was about 77 mol mol 21, which was approximately 5% lower than the corresponding value in the C-3 species. For Rubisco from the C-4 Flaverias kcat(CO2) and K-c were 23% and 45% higher, respectively, than for Rubisco from the C-3 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 (similar to 650 mu M) while the K-o for Rubisco in the C-4 species F. kochiana, F. australasica, Z. mays, and A. edulis was reduced more than 2-fold. There were no pathway-related differences in Km-RuBP. In the C-3-C-4 species kcat(CO2) and Kc were generally similar to the C-3 Rubiscos, but the K-o values were more variable. The typical negative relationships were observed between S-C/O and both kcat(CO2) and K-c, and a strongly positive relationship was observed between kcat(CO2) and K-c. However, the statistical significance of these relationships was influenced by the phylogenetic relatedness of the species.