Lactose Permease and the Alternating Access Mechanism

Biochemistry. 2011 Nov 15;50(45):9684-93. doi: 10.1021/bi2014294. Epub 2011 Oct 19.


Crystal structures of the lactose permease of Escherichia coli (LacY) reveal 12, mostly irregular transmembrane α-helices surrounding a large cavity open to the cytoplasm and a tightly sealed periplasmic side (inward-facing conformation) with the sugar-binding site at the apex of the cavity and inaccessible from the periplasm. However, LacY is highly dynamic, and binding of a galactopyranoside causes closing of the inward-facing cavity with opening of a complementary outward-facing cavity. Therefore, the coupled, electrogenic translocation of a sugar and a proton across the cytoplasmic membrane via LacY very likely involves a global conformational change that allows alternating access of sugar- and H(+)-binding sites to either side of the membrane. Here the various biochemical and biophysical approaches that provide strong support for the alternating access mechanism are reviewed. Evidence is also presented indicating that opening of the periplasmic cavity is probably the limiting step for binding and perhaps transport.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Alkylation
  • Binding Sites
  • Cross-Linking Reagents
  • Crystallography, X-Ray
  • Escherichia coli / chemistry
  • Escherichia coli / genetics
  • Escherichia coli / metabolism
  • Escherichia coli Proteins / chemistry*
  • Escherichia coli Proteins / genetics
  • Escherichia coli Proteins / metabolism*
  • Fluorescence Resonance Energy Transfer
  • Galactosides / metabolism
  • Models, Molecular
  • Molecular Dynamics Simulation
  • Monosaccharide Transport Proteins / chemistry*
  • Monosaccharide Transport Proteins / genetics
  • Monosaccharide Transport Proteins / metabolism*
  • Mutagenesis, Site-Directed
  • Protein Conformation
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Recombinant Proteins / metabolism
  • Spectrometry, Fluorescence
  • Static Electricity
  • Symporters / chemistry*
  • Symporters / genetics
  • Symporters / metabolism*
  • Tryptophan / chemistry


  • Cross-Linking Reagents
  • Escherichia coli Proteins
  • Galactosides
  • LacY protein, E coli
  • Monosaccharide Transport Proteins
  • Recombinant Proteins
  • Symporters
  • Tryptophan