title: NMR dynamic studies suggest that allosteric activation regulates ligand binding in chicken liver bile acid-binding protein.
abstract: Apo chicken liver bile acid-binding protein has been structurally characterized by NMR. The dynamic behavior of the protein in its apo- and holo-forms, complexed with chenodeoxycholate, has been determined via (15)N relaxation and steady state heteronuclear (15)N((1)H) nuclear Overhauser effect measurements. The dynamic parameters were obtained at two pH values (5.6 and 7.0) for the apoprotein and at pH 7.0 for the holoprotein, using the model free approach. Relaxation studies, performed at three different magnetic fields, revealed a substantial conformational flexibility on the microsecond to millisecond time scales, mainly localized in the C-terminal face of the beta-barrel. The observed dynamics are primarily caused by the protonation/deprotonation of a buried histidine residue, His(98), located on this flexible face. A network of polar buried side chains, defining a spine going from the E to J strand, is likely to provide the long range connectivity needed to communicate motion from His(98) to the EF loop region. NMR data are accompanied by molecular dynamics simulations, suggesting that His(98) protonation equilibrium is the triggering event for the modulation of a functionally important motion, i.e. the opening/closing at the protein open end, whereas ligand binding stabilizes one of the preexisting conformations (the open form). The results presented here, complemented with an analysis of proteins belonging to the intracellular lipid-binding protein family, are consistent with a model of allosteric activation governing the binding mechanism. The functional role of this mechanism is thoroughly discussed within the framework of the mechanism for the enterohepatic circulation of bile acids.
authors.affiliation: Laboratorio NMR, ISMAC, Consiglio Nazionale delle Ricerche, via Bassini 15, 20133 Milano, Italy.
authors.forename: Laura
authors.initials: L
authors.lastname: Ragona
authors.forename: Maddalena
authors.initials: M
authors.lastname: Catalano
authors.forename: Marianna
authors.initials: M
authors.lastname: Luppi
authors.forename: Daniel
authors.initials: D
authors.lastname: Cicero
authors.forename: Tommaso
authors.initials: T
authors.lastname: Eliseo
authors.forename: Jefferson
authors.initials: J
authors.lastname: Foote
authors.forename: Federico
authors.initials: F
authors.lastname: Fogolari
authors.forename: Lucia
authors.initials: L
authors.lastname: Zetta
authors.forename: Henriette
authors.initials: H
authors.lastname: Molinari
mesh_terms: D000494:Allosteric Regulation; D000595:Amino Acid Sequence; D000818:Animals; D001053:Apolipoproteins / Q000737:chemistry / Q000378:metabolism; D001647:Bile Acids and Salts / Q000378:metabolism; D001665:Binding Sites; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism*; D002645:Chickens; D008024:Ligands; D008099:Liver / Q000201:enzymology; D008562:Membrane Glycoproteins / Q000737:chemistry* / Q000378:metabolism*; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D011485:Protein Binding; D011487:Protein Conformation
chemical_list: D001053:Apolipoproteins; D001647:Bile Acids and Salts; D002352:Carrier Proteins; D008024:Ligands; D008562:Membrane Glycoproteins; C037643:bile acid binding proteins