Metal ions modulate thermal aggregation of beta-lactoglobulin: A joint chemical and physical characterization

Abstract

Molecular basis of the role played by Cu2+ and Zn2+ ions during the thermal aggregation processes of beta-lactoglobulin (BLG) was studied by using a joint application of different techniques. In particular, Raman spectroscopy was very useful in identifying the different effects caused by the two metals at molecular level (i.e. changes in His protonation state, disulfides bridge conformation, and micro-environment of aromatic residues), evidencing the primary importance of the protein charge distribution during the aggregation process. Both metal ions are able to act on this factor and favor the protein aggregation, but Zn2+ is able to alter the natural conformational state of BLG, causing a slight unfolding, whereas Cu2+ ions play a role only during the thermal treatment. Thus, Zn2+ ions favor the formation of bigger aggregates and branched fibril-like structures, whereas for Cu2+ ions a greater number of cross-beta structures during thermal incubation and finally, fibrillar structures. The aggregation process occurs in two phases, as suggested by the measurements on the time evolution of the BLG aggregates: the first one is characterized by a partial unfolding of the protein and aggregate growth, forming oligomers and protofibrils, whereas the second one is characterized by further supramolecular assembly, leading to the formation of fibrils.