title: Temperature dependence of backbone dynamics in human ileal bile acid-binding protein: implications for the mechanism of ligand binding.
abstract: Human ileal bile acid-binding protein (I-BABP), a member of the family of intracellular lipid binding proteins plays a key role in the cellular trafficking and metabolic regulation of bile salts. The protein has two internal and, according to a recent study, an additional superficial binding site and binds di- and trihydroxy bile salts with positive cooperativity and a high degree of site-selectivity. Previously, in the apo form, we have identified an extensive network of conformational fluctuations on the millisecond time scale, which cease upon ligation. Additionally, ligand binding at room temperature was found to be accompanied by a slight rigidification of picosecond-nanosecond (ps-ns) backbone flexibility. In the current study, temperature-dependent (15)N NMR spin relaxation measurements were used to gain more insight into the role of dynamics in human I-BABP-bile salt recognition. According to our analysis, residues sensing a conformational exchange in the apo state can be grouped into two clusters with slightly different exchange rates. The entropy-enthalpy compensation observed for both clusters suggests a disorder-order transition between a ground and a sparsely populated higher energy state in the absence of ligands. Analysis of the faster, ps-ns motion of (15)N-(1)H bond vectors indicates an unusual nonlinear temperature-dependence for both ligation states. Intriguingly, while bile salt binding results in a more uniform response to temperature change throughout the protein, the temperature derivative of the generalized order parameter shows different responses to temperature increase for the two forms of the protein in the investigated temperature range. Analysis of both slow and fast motions in human I-BABP indicates largely different energy landscapes for the apo and holo states suggesting that optimization of binding interactions might be achieved by altering the dynamic behavior of specific segments in the protein.
authors: Institute of Organic Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences , 2 Magyar tudósok körútja, H-1117 Budapest, Hungary.
authors: Gergő
authors: G
authors: Horváth
authors: Orsolya
authors: O
authors: Egyed
authors: Orsolya
authors: O
authors: Toke
mesh_terms: D001059:Apoproteins / Q000737:chemistry / Q000378:metabolism; D001647:Bile Acids and Salts / Q000378:metabolism*; D001665:Binding Sites; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism*; D050556:Fatty Acid-Binding Proteins / Q000737:chemistry* / Q000378:metabolism*; D005768:Gastrointestinal Hormones / Q000737:chemistry* / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008024:Ligands; D008562:Membrane Glycoproteins / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D013816:Thermodynamics
chemical_list: D001059:Apoproteins; D001647:Bile Acids and Salts; D002352:Carrier Proteins; D050556:Fatty Acid-Binding Proteins; D005768:Gastrointestinal Hormones; D008024:Ligands; D008562:Membrane Glycoproteins; C037643:bile acid binding proteins; C057275:fatty acid-binding protein 6