Lactose-H(OH) transport system of Escherichia coli Multistate gated pore model based on half-sites stoichiometry for high-affinity substrate binding in a symmetrical dimer |
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Authors: | Frank J Lombardi |
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Institution: | Laboratory of Nuclear Medicine and Radiation Biology, 900 Veteran Avenue, Los Angeles, CA 90024 U.S.A. |
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Abstract: | A model is proposed for the d-galactoside-H+(−OH) transporter of Escherichia coli that accounts for essentially all the experimental observations established for this system to date. In this model, the functional unit is postulated to be a dimer (consisting of two copies of lacY-specified polypeptide) which spans the membrane with a 2-fold symmetry axis in the membrane plane (Lancaster, J.R. (1978) J. Theor. Biol. 75, 35–50). The functional dimer is assumed to possess a single pore flanked by an inner gate (gi) and an outer gate (go) and encompassing two oppositely oriented galactoside binding sites, designated m and μ. When go is open and gi is closed under non-energized conditions, binding site m adopts a configuration defined as State A (i.e., moA) exhibiting high affinity toward Class Ga galactosides (thiodigalactoside, melibiose, α-p-nitrophenylgalactoside) but low affinity for Class Gb galactosides (lactose, β-o-nitrophenylgalactoside, β-isopropylthiogalactoside), whereas binding site μ adopts State B (i.e., μoB) displaying relatively high affinity toward Class Gb galactosides but comparatively low affinity for Class Ga galactosides; further, each moA : μoB dimer contains one thiol group whose reaction with N-ethylmaleimide inactivates the transporter unless blocked by galactoside binding at site moA, while the second homologous thiol of the dimer is unreactive toward thiol reagents. Translocation of the moA : μoB dimer involves closing of go followed by opening of gi, and causes the two thiols (as well as sites m and μ) to interchange roles in a symmetrical fashion: moA : μoB ↔ miB : μiA. In the presence of a substantial (negative) transmembrane Δμ~H+, the m : μ dimer is postulated to undergo an electrogenic protein conformational change to a second form, *(m : μ), in which both sites m and μ possess low affinity toward internal Class Gb substrates; galactoside transport in both m : μ and *(m : μ) is assumed to be coupled to H+-symport (−OH-antiport) with a stoichiometry of approximately 1 : 1. Finally, five characteristic predictions of the half-sites model are outlined for further tests of its validity. |
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Keywords: | Galactoside-H+(OH− ) Gated pore model Half-site stoichiometry Substrate binding Lactose transport (E coli) |
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