title: Lactose permease and the alternating access mechanism.
abstract: 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.
authors: Department of Physiology and Department of Microbiology, University of California, Los Angeles, California 90095, United States.
authors: Irina
authors: I
authors: Smirnova
authors: Vladimir
authors: V
authors: Kasho
authors: H Ronald
authors: HR
authors: Kaback
mesh_terms: D000478:Alkylation; D001665:Binding Sites; D003432:Cross-Linking Reagents; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D031541:Fluorescence Resonance Energy Transfer; D005697:Galactosides / Q000378:metabolism; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D009004:Monosaccharide Transport Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D055672:Static Electricity; D027981:Symporters / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D014364:Tryptophan / Q000737:chemistry
chemical_list: D003432:Cross-Linking Reagents; D029968:Escherichia coli Proteins; D005697:Galactosides; C050221:LacY protein, E coli; D009004:Monosaccharide Transport Proteins; D011994:Recombinant Proteins; D027981:Symporters; D014364:Tryptophan