(A) Kinetic data assembled for ≈300 rubiscos from diverse organisms show that there is limited variation (less than one order of magnitude) in CO2 specificity (SC/O) and maximum carboxylation rate (kcat,C) among the Form I rubiscos found in all photoautotrophs and all bacteria harboring carboxysome CCMs (Flamholz et al., 2019). Moreover, SC/O and kcat,C appear to trade-off with each other. Although this relationship is not strict, rubiscos with high kcat,C values also typically have lower SC/O(Savir et al., 2010; Tcherkez et al., 2006). As carboxylation and oxygenation reactions occur at the same active site, elevated CO2 will both increase the carboxylation rate (until it reaches kcat,C) and also inhibit oxygenation by exclusion of oxygen from the active site. As it relies only on the well-founded assumption that catalysis with CO2 and O2 substrates are mutually exclusive, this mechanism should function for any rubisco. Panels B-D depict this effect for three distinct rubiscos, which are highlighted with black borders in (A). Panels give carboxylation (light green), oxygenation (red) and net carboxylation (dark green) rates as a function of the aqueous CO2 concentration at ambient O2 levels (270 uM at 25 ℃). All curves were calculated using standard kinetic equations for rubisco. Net carboxylation was calculated as the carboxylation rate less ½ the oxygenation rate, which presumes a plant-type photorespiratory pathway that loses one CO2 for every two oxygenation reactions. (B) Bacterial Form II rubiscos are typically found in organisms living in low O2 environments and, accordingly, display low CO2 specificities (SC/O ≈ 10) and relatively high maximum carboxylation rates (kcat,C ≈10–20 s−1, Davidi et al., 2020). As such, Form II rubiscos do not perform well in ambient CO2 and O2 concentrations. (C) C3 plants like spinach do not have CCMs. Furthermore, the CO2 concentration inside the leaf is typically measured to be lower than ambient due to a balance of stomatal conductance and CO2 fixation by rubisco itself (Caemmerer and Evans, 1991). Accordingly, C3 plant rubiscos display high CO2 specificities (SC/O ≈ 100), modest kcat,C ≈ 3 s−1, and perform well at ambient and sub-ambient CO2 levels, displaying relatively little oxygenation and, consequently, net carboxylation rates that are similar to the total carboxylation rate. (D) Rubiscos found in bacteria with a carboxysome CCM typically have relatively low CO2 specificities (SC/O ≈ 50) and fast maximum carboxylation rates relative to other Form I rubiscos (kcat,C ≈ 10 s−1). In general, rubiscos from organisms bearing CCMs (whether bacteria, algae, or plants) tend to have lower CO2 specificities and higher kcat,C than enzymes from related organisms without CCMs (Iñiguez et al., 2020; Savir et al., 2010). The carboxysomal rubisco from S. elongatus PCC 7942 performs worse than a typical C3 plant rubisco in ambient air, but much better in the elevated CO2 environment we presume is maintained by the carboxysome CCM. The aqueous CO2 and O2 concentrations were calculated assuming Henry’s law equilibrium at 25 ℃. Notably, changes in temperature will affect CO2 and O2 solubility (Milo and Phillips, 2015; Sander, 2015) and rubisco kinetics, most notably decreasing CO2-specificity at elevated temperatures (Boyd et al., 2019; Sage et al., 2012).