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Selective Permeability of Carboxysome Shell Pores to Anionic Molecules

By Paween Mahinthichaichan, Dylan M Morris, Yi Wang, Grant J Jensen, Emad Tajkhorshid

Posted 11 Jul 2018
bioRxiv DOI: 10.1101/367714 (published DOI: 10.1021/acs.jpcb.8b06822)

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 semi-permeable 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 non-productive 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.

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