Fig. 2.

Carboxysome and condensate function are dependent on proton and RuBP permeabilities. Rubisco carboxylation turnover (Top), compartment pH (Middle), and the ratio of Rubisco compartment CO₂ to external CO₂ (Bottom) are dependent upon the permeability of the compartment to protons (Left) and RuBP (Right). Shown are modeled responses for free Rubisco (pink lines), a Rubisco condensate (blue lines), a large (1 × 10⁻⁶ m radius) carboxysome (purple lines), and a small (5 × 10⁻⁸ m radius) carboxysome (purple dashed lines) at subsaturating substrate concentrations (1 mM HCO₃⁻ [0.01 mM CO₂] and either 35, 50, 87, or 1,300 μM RuBP for the free enzyme, condensate, small carboxysome, and large carboxysome, respectively; see Fig. 3 and SI Appendix, Fig. S4). Open circles represent the values obtained for typical permeabilities used in the model (Table 1). Data were generated using the COPASI (66) model run in parameter scan mode, achieving steady-state values over the range of proton and RuBP permeabilities indicated for the Rubisco compartment. For all cases, CA activity was only present within the Rubisco compartment. Data presented are for the tobacco Rubisco with parameters listed in Table 2.