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Lactose Permease H⁺-Lactose Symporter: Mechanical Switch or Brownian Ratchet?

Richard J. Naftalin ^*, Nicholas Green  and Philip Cunningham 

^* King's College London, Physiology Division, Franklin-Wilkins Building, London, United Kingdom; Department of Chemistry, Central Chemistry Laboratory, Oxford, United Kingdom; and King's College London, Bioinformatics, Franklin-Wilkins Building, London, United Kingdom

Correspondence: Address reprint requests to R. J. Naftalin, King's College London, Physiology Division, Franklin-Wilkins Bldg., London SE1 9NH, UK. Tel.: 0207-848-4646; Fax: 0207-848-4500; E-mail: richard.naftalin{at}kcl.ac.uk.

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 Å) 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 H₃O⁺ 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.