Interaction Properties of Doubly Phosphorylated β-Casein, a Major Component of the Human Milk Caseins

Doubly phosphorylated β-casein constitutes nearly 30% of the total human β-caseins and is thus one of the major components of that fraction. The properties and mode of association of doubly phosphorylated β-casein are therefore important determinants of the structure and function of the human casein micelle. Doubly phosphorylated β-casein has an absorbency of 6.2 and a partial specific volume of .74. The protein precipitated at room temperature when 10 mM Ca²⁺ was added but produced a clear solution in 1 M NaCl. Equilibrium dialysis produced an average of 2.06 major Ca²⁺-binding sites at 37°C with a dissociation constant of 12.1 × 10^-4 M. The monomer at 20°C was calculated to have a solvation of 2.1 g of H₂O/g of protein and an axial ratio of 6.8, suggesting a prolate ellipsoid of about 11 by 2 nm. At high ionic strength, evidence exists for a spherical structure with a molecular weight of 2.25 × 10⁶. This structure would represent a polymer of about 90 monomers with a radius of 14.8 nm and a solvation of 1.93 g of H₂O/g of protein. This association behavior is similar to that of other phosphorylated human β-caseins but differs from the nonphosphorylated form. It changes when both Ca²⁺ and inorganic orthophosphate are present.