title: Ammonium recruitment and ammonia transport by E. coli ammonia channel AmtB.
abstract: To investigate substrate recruitment and transport across the Escherichia coli Ammonia transporter B (AmtB) protein, we performed molecular dynamics simulations of the AmtB trimer. We have identified residues important in recruitment of ammonium and intraluminal binding sites selective of ammonium, which provide a means of cation selectivity. Our results indicate that A162 guides translocation of an extraluminal ammonium into the pore lumen. We propose a mechanism for transporting the intraluminally recruited proton back to periplasm. Our mechanism conforms to net transport of ammonia and can explain why ammonia conduction is lost upon mutation of the conserved residue D160. We unify previous suggestions of D160 having either a structural or an ammonium binding function. Finally, our simulations show that the channel lumen is hydrated from the cytoplasmic side via the formation of single file water, while the F107/F215 stack at the inner-most part of the periplasmic vestibule constitutes a hydrophobic filter preventing AmtB from conducting water.
authors.affiliation: MEMPHYS Center for Biomembrane Physics, Department of Chemistry, Technical University of Denmark, Lyngby, Denmark.
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Nygaard
authors.forename: Carme
authors.initials: C
authors.lastname: Rovira
authors.forename: Günther H
authors.initials: GH
authors.lastname: Peters
authors.forename: Morten Ø
authors.initials: MØ
authors.lastname: Jensen
mesh_terms: D000641:Ammonia / Q000378:metabolism*; D001665:Binding Sites; D001693:Biological Transport, Active; D027682:Cation Transport Proteins / Q000737:chemistry* / Q000502:physiology; D003198:Computer Simulation; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000502:physiology; D057927:Hydrophobic and Hydrophilic Interactions; D008958:Models, Molecular; D011485:Protein Binding; D011522:Protons; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D014867:Water / Q000737:chemistry
chemical_list: C111096:AmtB protein, E coli; D027682:Cation Transport Proteins; D029968:Escherichia coli Proteins; D011522:Protons; D000644:Quaternary Ammonium Compounds; D014867:Water; D000641:Ammonia