Casein monomer subunits

This paper draws heavily upon the "Nomenclature Committee Report" ( 1) as well as several recent comprehensive reports that have considered the primary structure and conformation of the casein monomer subunits and how they are assembled into submicel-lar aggregates and casein micelles (2, 3). These basic relationships were utilized to develop additional projections relating to the conformation and functional properties of the major milk proteins, e.g., commercial caseinates and whey protein concentrates in food applications. 

Primary Structure of the Major Casein Monomer Subunits. 

The major casein monomer subunits have random coil conformation that facilitates strong protein-protein interaction via hydrophobic and ionic bonding. The unique amphiphilic structure, which arises from separately clustered hydrophobic and negatively charged (acidic and ester phosphate) amino acid residues along the polypeptide chain, makes them susceptible to pH and Ca ion concentration effects. This amphiphilic nature is probably responsible for the excellent surfactant properties of commercial caseinate in a variety of food applications. 

Casein monomers or small polymers — caseinate subunits + calcium phosphate — casein micelles... 

It will be noted that K-casein is the only monomer subunit that contains a disulfide group, and this is undoubtedly responsible for its ability to self aggregate as well as to interact with g-lactoglobulin during heat processing of milk. 

K-casein also contains two Cys residues per monomer subunit and is thus capable of interacting with the whey proteins, e.g., mainly g-lactoglobulin, via the disulfide interchange mechanism at temperatures at or above 65°C. This latter phenomenon is believed to be important in providing colloidal stability to the milk casein micelle system, as well as to the whey proteins, in high temperature processed milk products. It has also been postulated that this latter interaction with g-lactoglobulin may alter the availability of K-casein in the micelle, and thus has a detrimental effect upon the cheese making properties of milk (4). 

Recently, Slattery and Evard (171) proposed a model for the formation and structure of casein micelles from studies devoted to association products of the purified caseins. They proposed that the micelle is composed of polymer subunits, each 20 nm in diameter. In the micellar subunits the nonpolar portion of each monomer is oriented radially inward, whereas the charged acidic peptides of the Ca2+-sensitive caseins and the hydrophilic carbohydrate-containing portion of K-casein are near the surface. Asymmetric distribution of K-casein in a micelle subunit results in hydrophilic and hydrophobic areas on the subunit surface. In this situation, aggregation through hydrophobic interaction forms a porous micelle (Figure 10). Micelle growth is limited by the eventual concentration, at the micelle surface, of subunits rich in K-casein. 

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