A model for the formation and structure of casein micelles from subunits of variable composition

Sedimentation coefficients were determined for association products in systems containing SH-κ-casein, SH-κ-casein-α_s1- or β-caseins (1:1, by weight) and SH-κ-casein-α_s1-casein-β-casein (2:1:1, by weight) under a variety of environmental conditions. As the concentration of NaCl is increased or, as the concentration of CaCl₂ is increased at 0.05 M NaCl, the system containing SH-κ-casein alone acts quite differently from those which also contain the Ca²⁺-sensitive caseins. It is concluded that electrostatic as well as hydrophobic attractions are important between SH-κ-casein monomers. Viscosity and sedimentation measurements combined indicate that nearly spherical polymers containing 25-30 monomers each and with radii of approx. 10 nm are formed in all systems. It is considered that these polymers are micelle subunits in which the non-polar portion of each monomer is oriented radially inward while the charged acidic peptides of the Ca²⁺-sensitive caseins and the hydrophilic, carbohydrate-containing portion of κ-casein are near the surface. Asymmetric distribution of κ-casein in a micelle subunit results in hydrophilic and hydrophobic areas on the subunit surface. In this situation, aggregation through hydrophobic interactions forms a porous micelle. Micelle growth is limited by the eventual concentration, at the micelle surface, of subunits rich in κ-casein. This model successfully accounts for the properties of equilibrium and nonequilibrium micelle systems, for the destabilization of the micelle by dephosphorylation of Ca²⁺-sensitive caseins or by high concentrations of NaCl or CaCl₂ and for the events associated with clotting by rennin action.