title: Lactose permease H+-lactose symporter: mechanical switch or Brownian ratchet?
abstract: Lactose permease structure is deemed consistent with a mechanical switch device for H(+)-coupled symport. Because the crystallography-assigned docking position of thiodigalactoside (TDG) does not make close contact with several amino acids essential for symport; the switch model requires allosteric interactions between the proton and sugar binding sites. The docking program, Autodock 3 reveals other lactose-docking sites. An alternative cotransport mechanism is proposed where His-322 imidazolium, positioned in the central pore equidistant (5-7 A) between six charged amino acids, Arg-302 and Lys-319 opposing Glu-269, Glu-325, Asp-237, and Asp-240, transfers a proton transiently to an H-bonded lactose hydroxyl group. Protonated lactose and its dissociation product H(3)O+ are repelled by reprotonated His-322 and drift in the electrostatic field toward the cytosol. This Brownian ratchet model, unlike the conventional carrier model, accounts for diminished symport by H322N mutant; how H322 mutants become uniporters; why exchanging Lys-319 with Asp-240 paradoxically inactivates symport; how some multiple mutants become revertant transporters; the raised export rate and affinity toward lactose of uncoupled mutants; the altered specificity toward lactose, melibiose, and galactose of some mutants, and the proton dissociation rate of H322 being 100-fold faster than the symport turnover rate.
authors.affiliation: King's College London, Physiology Division, Franklin-Wilkins Building, London, United Kingdom. richard.naftalin@kcl.ac.uk
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Naftalin
authors.forename: Nicholas
authors.initials: N
authors.lastname: Green
authors.forename: Philip
authors.initials: P
authors.lastname: Cunningham
mesh_terms: D001665:Binding Sites; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D004058:Diffusion; D004789:Enzyme Activation; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000648:ultrastructure*; D007785:Lactose / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D020409:Molecular Motor Proteins / Q000737:chemistry* / Q000648:ultrastructure*; D009004:Monosaccharide Transport Proteins / Q000737:chemistry* / Q000648:ultrastructure*; D011485:Protein Binding; D011487:Protein Conformation; D011522:Protons; D013379:Substrate Specificity; D027981:Symporters / Q000737:chemistry* / Q000648:ultrastructure*; D014867:Water / Q000737:chemistry
chemical_list: D029968:Escherichia coli Proteins; C050221:LacY protein, E coli; D020409:Molecular Motor Proteins; D009004:Monosaccharide Transport Proteins; D011522:Protons; D027981:Symporters; D014867:Water; D007785:Lactose