Mechanism of co-aggregation in a protein mixture with small additives

Co-aggregation plays an important role in processing protein-rich food materials under heterogeneous conditions. The main cause of co-aggregation is an electrostatic attraction between oppositely charged molecules. This study investigated thermal aggregation of β-lactoglobulin (BLG) (pI=5.1) and lysozyme (LYZ) (pI=10.7) as a model for the heterogeneous conditions of a protein solution. BLG and LYZ were more aggregated in the mixture than in the single solutions. Co-aggregation of the BLG-LYZ mixture was not observed below 60°C at which temperature BLG and LYZ retained their native structures. Adding sugars, salts, or amino acids to the BLG-LYZ mixture during the heat treatment revealed the co-aggregation process as follows. (i) All additives tested suppressed both the nucleation and growth of aggregates. (ii) Salts affected nucleation stage to the same degree, except arginine hydrochloride (Arg). (iii) Arg specifically suppressed both nucleation and growth of aggregates. These results indicate that co-aggregation in a protein mixture is more sensitive to the partial unfolding of proteins than that in a single protein solution, due to the presence of electrostatic attraction between different molecules. These results provide new insight into protein aggregation as well as the molecular mechanism of additives under heterogeneous conditions.