The conformation and aggregation of bovine β‐casein A. I. Molecular aspects of thermal aggregation

Abstract

Abstract The conformation of β‐casein A in the monomeric and thermally aggregated states has been investigated by a range of techniques. β‐Casein exists as a monomer in solution at 4°C and at concentrations up to at least 3 g/dl. The molecule is flexible and exhibits a lot of segmental motion, but its secondary structure is not wholly random coil; about one‐third of the polypeptide chain is ordered and the likely locations of these regions are discussed. The radius of gyration, representing the time‐average distribution of the flexible chain, is 46 Å. Increasing temperature leads to aggregation of the β‐casein molecules. The degree of association is very sensitive to experimental conditions, and under our conditions a 14‐mer exists at 20°C. The aggregate is spherical with a radius of about 100 Å. The interior of the aggregate is relatively disordered, and the β‐casein molecules remain in a largely flexible, hydrated conformation. The volume restriction of the protein molecules which occurs on association leads to some immobilization of the hydrophobic C‐terminal region, which is packed toward the center of the aggregate.