Hydrophobic bonding, casein micelles

Although the submicellar model of the casein micelle readily explains many of the principal features and physicochemical reactions undergone by the micelles and has been widely supported, it has never enjoyed unanimous support and two alternative models have been proposed recently. Visser (1992) proposed that the micelles are spherical conglomerates of individual casein molecules randomly aggregated and held together partly by salt bridges in the form of amorphous calcium phosphate and partly by other forces, e.g. hydrophobic bonds, with a surface layer of K-casein. Holt (1992, 1994) depicted the casein micelle as a tangled web of flexible casein... 

Like as2-casein, /e-casein has two disulfide bonds which can form cross-links with /3-lactoglobulin. The N-terminal two-thirds of the molecule is hydrophobic and contains the two disulfide bonds. The C-termi-nal end is hydrophilic, polar, and charged. It varies in the number of attached carbohydrate moieties and has only one phosphate group. These characteristics make /c-casein ideal for the surface of casein micelles, where it is most often found. It is not susceptible to calcium ion binding, as the other caseins are, and when present on the surface of micelles, it protects the other caseins from calcium (McMahon and Brown 1984A Swaisgood 1982). 

The gel structure can be controlled via changes in the hydrophobicity of the micelle surface. A decrease in hydrophobicity is possible, e. g., by heating milk (90 °C/10 min). Covalent bonding of denatured p-lactoglobulin to KT-casein (cf. 10.1.3.5) occurs, burying hydrophobic groups. 

No cysteine residues are found for alpha(sl) and P-caseins do. If any S-S bonds occur within the micelle, they are not the driving force for stabilization. Caseins are among the most hydrophobic proteins, and there is some evidence to suggest that they play a role in the stability of the micelle. It must be remembered that hydrophobic interactions are very temperature sensitive. 

Thus, K-casein in its native stabilizing role exists, probably as small disulphide linked polymers, bound to the micellar surface (the ill-defined boundary between the hydrophobic interior of the micelle and the aqueous phase). C-terminal polypeptides (61 residues) of the protein project from the surface into the solution. In this position, the macropeptide moiety of the protein is conformationally free (H), constrained only by its interactions with its neighbours (J ), and the bond 105-106 of the protein is held in a particularly advantageous position for attack by enzymes such as chymosin ( ). The importance of this will be apparent when emulsions stabilized by K-casein are being discussed. The enzymic action has a relatively small but detectable effect on the hydrodynamic diameters of the particles, and a large effect on their electrophoretic mobilities, which decrease by between one-third and one-half, depending on the solution conditions ( ). 

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