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

¹ King's College London
² Oxford University

^* To whom correspondence should be addressed. E-mail: richard.naftalin{at}kcl.ac.uk.

Submitted on November 13, 2006
Revised on December 26, 2006
Accepted on 24 January 2007

	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 His322 imidazolium, positioned in the central pore equidistant (5-7 Å) between six charged amino acids, Arg302, Lys319 opposing Glu269, Glu325, Asp237 and Asp240, transfers a proton transiently to an H-bonded lactose hydroxyl group. Protonated lactose and its dissociation product H₃O⁺ are repelled by reprotonated His322 and drift in the electrostatic field towards 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 Lys319 with Asp240 paradoxically inactivates symport; how some multiple mutants become revertant transporters; the raised export rate and affinity towards lactose of uncoupled mutants; the altered specificity towards lactose, melibiose and galactose of some mutants and the proton dissociation rate of H322 being 100-fold faster than the symport turnover rate.

Key Words: Brownian ratchet, Lac permease,, cotransport, electrostatic potential, molecular docking