NMR dynamic studies suggest that allosteric activation regulates ligand binding in chicken liver bile acid-binding protein.
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.
Ragona
Laura
L
Laboratorio NMR, ISMAC, Consiglio Nazionale delle Ricerche, via Bassini 15, 20133 Milano, Italy.
Catalano
Maddalena
M
Luppi
Marianna
M
Cicero
Daniel
D
Eliseo
Tommaso
T
Foote
Jefferson
J
Fogolari
Federico
F
Zetta
Lucia
L
Molinari
Henriette
H
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
D001053:Apolipoproteins; D001647:Bile Acids and Salts; D002352:Carrier Proteins; D008024:Ligands; D008562:Membrane Glycoproteins; C037643:bile acid binding proteins