Figure 7: Comparison of ancestral and extant RuBisCO.

(a) Specificity factor (τ) versus carboxylation rate (V_c) for characterized RuBisCOs. The specificity factor is measured as the ratio of the catalytic efficiency of carboxylation to oxygenation, thus specificity factor=(V_cK_o)/(V_oK_c). The best-fit curve for extant RuBisCOs (black line) is as previously described³². (Form 1A (brown open diamonds); Form 1B Cyanobacteria (blue open squares); eukaryotic non-green algae (purple filled diamonds); eukaryotic green algae (green filled triangles); C4 plants (green filled circles); C3 plants (green filled squares); ancestral (red filled circles); point mutant or chimeric (grey filled squares)). Values and error bars are summarized in Supplementary Table 3, and based on ^{30,32,33,34,35}. (b) Model of selective pressures pushing properties of RuBisCO towards the hypothesized protein landscape optimum—upper limits of the kinetic parameters—represented by the best-fit curve (black line). Coloured regions correspond to part a: ancestral (red), plants (green), eukaryotic non-green algae (purple) and cyanobacteria (blue). The grey shaded area represents the theoretical enzyme space that is biophysically infeasible for RuBisCO.