title: Ionization states of residues in OmpF and mutants: effects of dielectric constant and interactions between residues.
abstract: To understand ion permeation, one must assign correct ionization states to titratable amino acid residues in protein channels. We report on the effects of physical and methodological assumptions in calculating the protonation states at neutral bulk pH of titratable residues lining the lumen of the native Escherichia coli OmpF channel, and five mutants. We systematically considered a wide range of assumed protein dielectric constants and all plausible combinations of protonation states for electrostatically interacting side chains, and three different levels of accounting for solute shielding: 1), full nonlinear Poisson-Boltzmann; 2), linearized Poisson-Boltzmann; and 3), neglect of solute shielding. For this system we found it acceptable to neglect solute shielding, a result we postulate to be generalizable to narrow lumens of other protein channels. For the large majority of residues, the protonation state at neutral bulk pH was found to be independent of the assumed dielectric constant of the protein, and unambiguously determined by the calculation; for native OmpF only Asp-127 has a protonation state that is sensitive to the assumed protein dielectric constant. Our results are significant for understanding two published experimental observations: the structure of the narrow part of the channel, and the ionic selectivity of OmpF mutants.
authors.affiliation: Center for Biophysics and Computational Biology, National Center for Supercomputing Applications, University of Illinois at Urbana Champaign, Urbana, Illinois 61801, USA.
authors.forename: Sameer
authors.initials: S
authors.lastname: Varma
authors.forename: Eric
authors.initials: E
authors.lastname: Jakobsson
mesh_terms: D019943:Amino Acid Substitution; D000596:Amino Acids / Q000737:chemistry*; D001665:Binding Sites; D003198:Computer Simulation; D004553:Electric Conductivity; D004563:Electrochemistry / Q000379:methods; D015640:Ion Channel Gating; D007477:Ions / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D009154:Mutation; D018272:Porins / Q000737:chemistry*; D016062:Porosity; D011485:Protein Binding; D011487:Protein Conformation; D013329:Structure-Activity Relationship
chemical_list: D000596:Amino Acids; D007477:Ions; C109237:OmpF protein; D018272:Porins