Structural mechanism and physiological significance of betalactoglobulin unfolding at solid and liquid interfaces

Betalactoglobulin (BLG) represents a major allergens in bovine milk. BLG is used alone or in combination with other proteins for the preparation of food emulsions, and interacts with the solid surfaces in processing equipment. From a structural standpoint, changes in the pattern of hydrophobic interaction is very diverse in these two cases. Adhesion to a hydrophobic solid surface in the absence of mechanical denaturation implies structural changes very different from those occurring when hydrophobic regions of the BLG structure are exposed by means of mechanical denaturation and end up penetrating a liquid apolar phase. Here, spectroscopic and limited proteolysis approaches were used – together with the accessibility of “reporter” amino acid side chains - to compare structural features of BLG adsorbed on “liquid” oil microdroplets and on “solid” styrene nanoparticles. Immunoreactvity of the two systems was also studied, along with that of the products of simulated digestion of the various BLG states. Structural changes were largest in the emulsion system, whereas the regions of BLG involved in ligand binding were only marginally affected in the solids-adsorbed protein. In both cases, there was a marked alteration in immunoreactivity (with a notable increase in emulsions), and in the pattern of proteolysis. Proteolytic products obtained from solids-adsorbed BLG remained bound to the styrene surface and retained their immunoreactivity, at contrast with those from emulsions, that were different in nature/reactivity also from those obtained from native or temperature-denatured BLG. These findings highlight the significance of structural issues in the field of food protein allergens.