Bovine β-lactoglobulin and its variants:: A three-dimensional structural perspective

Genetic variants of bovine beta-lactoglobulin have a significant impact on the processing properties of cow's milk. The three-dimensional structures of three variants, A, B, and C, have been determined by X-ray crystallography to see whether differences in physical properties could be traced to differences in structure. The proteins were crystallized, in the same crystal form (Y), at a pH of 7.3-7.6. The structure of the A variant was solved by multiple isomorphous replacement and the B and C variants by molecular replacement, using the A variant structure. All three structures were refined at resolutions of 1.8-2.0 Angstrom. The three variants have highly similar three-dimensional structures. The core of the molecule, which comprises an 8-stranded beta-barrel, is essentially unchanged. The single free Cys residue, Cys 121, also has an identical environment in each variant, buried between a 3-turn alpha-helix and the beta-barrel. The Gly64Asp mutation (B-->A) occurs on a flexible external loop and has no impact on 3D structure. The Ala118Val mutation (B-->A), on the other hand, causes local internal readjustments which may affect the dimer interface; some of the differences between A and B variants may thus be traced to differences in aggregation. The Gln59His mutant (B-->C) appears to have as its main effect a change of stabilizing interactions on the surface, converting a hydrogen bond to an ion pair.