title: Energetics of ligand-induced conformational flexibility in the lactose permease of Escherichia coli.
abstract: Isothermal titration calorimetry has been applied to characterize the thermodynamics of ligand binding to wild-type lactose permease (LacY) and a mutant (C154G) that strongly favors an inward facing conformation. The affinity of wild-type or mutant LacY for ligand and the change in free energy (DeltaG) upon binding are similar. However, with the wild type, the change in free energy upon binding is due primarily to an increase in the entropic free energy component (TDeltaS), whereas in marked contrast, an increase in enthalpy (DeltaH) is responsible for DeltaG in the mutant. Thus, wild-type LacY behaves as if there are multiple ligand-bound conformational states, whereas the mutant is severely restricted. The findings also indicate that the structure of the mutant represents a conformational intermediate in the overall transport cycle.
authors.affiliation: Department of Physiology and Microbiology, Molecular Biology Institute, University of California Los Angeles, Los Angeles, California 90095, USA.
authors.forename: Yiling
authors.initials: Y
authors.lastname: Nie
authors.forename: Irina
authors.initials: I
authors.lastname: Smirnova
authors.forename: Vladimir
authors.initials: V
authors.lastname: Kasho
authors.forename: H Ronald
authors.initials: HR
authors.lastname: Kaback
mesh_terms: D002151:Calorimetry; D004926:Escherichia coli / Q000201:enzymology*; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000502:physiology*; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D009004:Monosaccharide Transport Proteins / Q000737:chemistry* / Q000502:physiology*; D010957:Plasmids / Q000378:metabolism; D018583:Pliability; D011485:Protein Binding; D011487:Protein Conformation; D027981:Symporters / Q000737:chemistry* / Q000502:physiology*; D013696:Temperature; D013816:Thermodynamics
chemical_list: D029968:Escherichia coli Proteins; C050221:LacY protein, E coli; D008024:Ligands; D009004:Monosaccharide Transport Proteins; D027981:Symporters