Selective Permeability of Carboxysome Shell Pores to Anionic Molecules

Carboxysomes are closed polyhedral cellular microcompartments that increase the efficiency of carbon fixation in autotrophic bacteria. Carboxysome shells consist of small proteins that form hexameric units with semipermeable central pores containing binding sites for anions. This feature is thought to selectively allow access to RuBisCO enzymes inside the carboxysome by HCO₃ ^- (the dominant form of CO₂ in the aqueous solution at pH 7.4) but not O₂, which leads to a nonproductive reaction. To test this hypothesis, here we use molecular dynamics simulations to characterize the energetics and permeability of CO₂, O₂, and HCO₃ ^- through the central pores of two different shell proteins, namely, CsoS1A of α-carboxysome and CcmK4 of β-carboxysome shells. We find that the central pores are in fact selectively permeable to anions such as HCO₃ ^-, as predicted by the model.