Casein micelles subunit

Figure 3. SANS intensity for wet pellets of whole casein micelles made with (a) 96% D20,4% H20 (b) 74% D20, 26% H20 (c) 41% D20, 59% H20. Casein concn. approx. 250 mg/ml. Calculated intensities for models with subunits in close packing are (—) for 74% D20 and Nl=2, N2=l, N3=2, D=168 A.

When casein micelles are dissociated, spherical particles are observed with a size similar to the scale of the substructure. Moreover, the number of spherical particles formed by dissociation appears to correspond roughly to the number of substructural elements in the micelle. In electron micrographs of mammary gland secretory cells, some of the Golgi vesicles contain particles of a size similar to that of the particles formed by dissociation of micelles, whereas others contain larger particles. Buchheim and Welsch (1973) proposed that the smaller particles are not small micelles but subunits that are to be assembled into full-sized micelles. The envisaged sequence of assembly is as follows ... 

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

This sequence of events has its parallel in the model of casein micelle structure proposed by Schmidt (1982) and Walstra (1990) in which small calcium phosphate particles link together discrete spherical protein 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. 

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). 

The major caseins exist in milk as highly structured, spherical aggregates, consisting of 450 to 10,000 subunits (3), commonly referred to as micelles. The important physico-chemical properties of the micelles are summarized in Table 3. Casein micelles are synthesized in vivo by biochemically controlled processes., which have not been totally characterized (5). Even... 

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. 

The model of Slattery and Evard (171) explains many of the properties of micelles, including the events associated with clotting by rennin action. For example, it explains the crenated surface of micelles observed on electron micrographs (179), the subunit structure of casein micelles (180), the porous nature of micelles (177, 178) allowing syneresis by continued action of rennin on interior subunits, and the more or less random distribution of the caseins in the micelle (170,177, 178,183). 

The size of particles ranges from 20 to 600 nm (2), Casein micelles are mainly responsible for the high sensitivity of milk to physical, chemical and enzymatic treatments (3). Many models have been proposed to explain the structure of the micelle (4). However, the nature of the interactions between caseins which lead to micelle formation remains unclear. One recently proposed model presents the micelle as an aggregate of subunits ( 5) basis of this model, calcium... 

Fig. 10.7. Schematic model of a casein micelle (a) a subunit consisting of agi-, p-, y-, K--caseins, (b) Micelle made of subunits bound by calcium phosphate bridges (according to Webb, 1974)...

Figure 3.19 Model for the structure of a casein micelle. Casein protein subunits are linked by colloidal calcium phosphate to produce a raspberry-like structure...

At the same time, the hydrophobic portion of the K-casein interacts with the calcium as- and /3-caseinates to form coat subunits which interact strongly with the calcium 8-caseinate in the core. The micelles are pictured as solid, crenated spheres (Figure 7) whereby the available K-casein for the formation of coat subunits determines the average size of the micelles. 

Figure 10. Milk micelle model of Slattery and Evard (171) containing ca. 40 subunits each comprised of c-, a8-, and (3-caseins.

From this model it is evident that K-casein on the surface would be readily available to rennin. The release of the macropeptide by rennin would result in the subsequent reduction in charge and hydrophilicity on the surface. That rennin-treated micelles do not aggregate in the cold is evidence of the fact that hydrophobic interaction is mainly responsible for clotting. The K-casein areas exposed to rennin action would result in hydrophobic patches separated by one subunit diameter (20 nm), whereby the micelles could cohere to form threads and fibrils characteristic of coagulation. 

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