The biochemistry of Rubisco in Flaveria

Abstract

C4 plants have been reported to have Rubiscos with higher maximum carboxylation rates (kcatCO2) and Michaelis-Menten constants (Km) for CO2 (Kc) 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 (SC/O), kcatCO2, Kc, and the Km for O2 (Ko) and RuBP (Km-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 SC/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 Kc were 23% and 45% higher, respectively, than for Rubisco from the C3 plants. Interestingly, it was found that the Ko for Rubisco from the C 4 species F. bidentis and F. trinervia were similar to the C 3 Flaveria Rubiscos (∼650 μM) while the Ko 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 Km-RuBP. In the C3-C4 species kcatCO2 and Kc were generally similar to the C3 Rubiscos, but the Ko values were more variable. The typical negative relationships were observed between SC/O and both kcatCO2 and Kc, 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. Show more