Figure 2.

The bacterial carbon dioxide concentrating mechanism. a: The bacterial CCM starts with transport of inorganic carbon into the cell as HCO₃⁻. Equilibrium between HCO₃⁻ and CO₂ is not reached due to a lack of carbonic anhydrase in the cytoplasm. HCO₃⁻ is converted to CO₂ by carboxysomal carbonic anhydrase within the microcompartment lumen. The protein shell of the carboxysome impedes CO₂ diffusion. Consequently, CO₂ can accumulate in the immediate vicinity of RuBisCO while diffusive loss of CO₂ through the cell membrane is minimized. Elevated CO₂ proximal to RuBisCO increases carbon fixation and suppresses photorespiration (a nonproductive process in which O₂ replaces CO₂ as a substrate for RuBisCO). b: A preliminary atomic model of the carboxysome shell based on crystal structures of the component shell proteins.⁽¹²⁾ The positively charged pores through the sheet of hexagonal proteins are indicated. These have been postulated to enhance the diffusion of negatively charged molecules such as bicarbonate (thick arrow) across the shell, compared to uncharged molecules such as CO₂ and O₂ (thin arrows).