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J Am Chem Soc. 2012 Jun 27;134(25):10419-27. doi: 10.1021/ja300129x. Epub 2012 Jun 12.

Ammonium transporters achieve charge transfer by fragmenting their substrate.

Author information

1
Department of Chemistry and Biochemistry and Centre for Research in Molecular Modeling (CERMM), Concordia University, 7141 Sherbrooke Street West, Montréal, Québec H4B 1R6, Canada.

Abstract

Proteins of the Amt/MEP family facilitate ammonium transport across the membranes of plants, fungi, and bacteria and are essential for growth in nitrogen-poor environments. Some are known to facilitate the diffusion of the neutral NH(3), while others, notably in plants, transport the positively charged NH(4)(+). On the basis of the structural data for AmtB from Escherichia coli , we illustrate the mechanism by which proteins from the Amt family can sustain electrogenic transport. Free energy calculations show that NH(4)(+) is stable in the AmtB pore, reaching a binding site from which it can spontaneously transfer a proton to a pore-lining histidine residue (His168). The substrate diffuses down the pore in the form of NH(3), while the excess proton is cotransported through a highly conserved hydrogen-bonded His168-His318 pair. This constitutes a novel permeation mechanism that confers to the histidine dyad an essential mechanistic role that was so far unknown.

PMID:
22631217
DOI:
10.1021/ja300129x
[Indexed for MEDLINE]

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