trans-Parinaric acid as a versatile spectroscopic label to study ligand binding properties of bovine β-lactoglobulin

β-Lactoglobulin (BLG) like other lipocalins can be modified by mutagenesis to re-direct its ligand binding properties. Local site-directed mutagenesis was used to change the geometry of the BLG ligand binding pocket and therefore change BLG ligand preferences. The presented studies are focused on previously described mutants L39Y, I56F, L58F, F105L, and M107L and two new BLG variants, L39K and F105A, and their interactions with local anesthetic drug tetracaine. Binding of tetracaine to BLG mutants was investigated by X-ray crystallography. Structural analysis revealed that for tetracaine binding, the shape of the binding pocket seems to be a more important factor than the substitutions influencing the number of interactions. Analyzed BLG mutants can be classified according to their binding properties to variants: capable of binding tetracaine in the β-barrel (L58F, M107L); capable of accommodating tetracaine on the protein surface (I56F) and unable to bind tetracaine (F105L). Variants L39K, L39Y, and F105A, had a binding pocket blocked by endogenous fatty acids. The new tetracaine binding site was found in the I56F variant. The site localized on the surface near Arg124 and Trp19 was previously predicted by in silico studies and was confirmed in the crystal structure.

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Conformational flexibility and ligand binding properties of ovine β-lactoglobulin

Acta Biochimica Polonica ◽

10.18388/abp.2019_2883 ◽

2019 ◽

Cited By ~ 2

Author(s):

Joanna Loch ◽

Piotr Bonarek ◽

Krzysztof Lewiński

Keyword(s):

Ligand Binding ◽

Crystalline Phase ◽

Sodium Dodecyl ◽

Binding Pocket ◽

Conformational Flexibility ◽

Intramolecular Interactions ◽

Polar Group ◽

Binding Properties ◽

Asymmetric Dimer ◽

Β Lactoglobulin

Ovine β‑lactoglobulin was characterized by spectroscopic (CD), calorimetric (ITC) and X-ray structural studies. The structure of ovine β‑lactoglobulin complex with decanol showed that tight packing of molecules in the crystalline phase enforces a distortion of protein flexible loops resulting in the formation of an asymmetric dimer. The loops surrounding β-barrel in ovine lactoglobulin possessed the same conformational flexibility as observed previously in other lactoglobulins and the change of their conformation regulates the access to the binding pocket. The structure of asymmetric dimer revealed a new region in β-barrel where ligand polar group can be located. These findings indicated protein adaptability to ligand dimensions and inter- and intramolecular interactions in the crystalline phase. Calorimetric and crystallographic studies provided the experimental evidence that ovine lactoglobulin is able to bind aliphatic ligands. Thermodynamic parameters of sodium dodecyl sulfate binding determined by ITC at pH 7.5 had Ka, ΔH, TΔS and ΔG values similar to those observed for bovine and caprine protein indicating the same mechanism of ligand binding.

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