Molecular basis of thermal aggregation of bovine β-lactoglobulin A
On heating, bovine β-lactoglobulin A (β-lg A) dimers dissociate and then aggregate. The extent of aggregation of β-lg A after a heat–quench treatment was studied quantitatively using photon correlation spectroscopy (PCS); circular dichroism (CD) spectroscopy was also carried out on heated solutions of β-lg A to examine changes in the secondary and tertiary structure of the protein. In pH 7.0 solution the protein underwent a change in tertiary structure at 67 °C; by contrast, the secondary structure, containing ca. 50%β-sheet, showed very little change up to 90 °C. PCS showed that the extent of aggregation of β-lg A depended on pH and buffer concentration; under some conditions it was observed that there was a temperature above which the rate of aggregation decreased. The pH-dependence of the temperature of onset of aggregation correlated with the conformational stability of β-lg A. Detailed examination of the light scattering data suggested initial aggregation of the protein molecules to form linear, rod-like particles. It is proposed that, at later stages in the aggregation process, the rods themselves aggregate and start to form a network.