TCB Publications - Abstract

Paween Mahinthichaichan, Dylan M. Morris, Yi Wang, Grant J. Jensen, and Emad Tajkhorshid. Selective permeability of carboxysome shell pores to anionic molecules. Journal of Physical Chemistry B, 2018.

MAHI2018C-ET 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 HCO$_{3}^{-}$ (the dominant form of CO$_2$ in the aqueous solution at pH 7.4) but not O$_2$, which leads to a non-productive reaction. To test this hypothesis, here we use molecular dynamics simulations to characterize the energetics and permeability of CO$_2$, O$_2$, and HCO$_{3}^{-}$ through the central pores of two different shell proteins, namely, CsoS1A of $\alpha$-carboxysome and CcmK4 of $\beta$-carboxysome shells. We find that the central pores are in fact selectively permeable to anions such as HCO$_{3}^{-}$, as predicted by the model.

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