Casein genetic variant

Amino acid composition. The approximate amino acid composition of the main caseins is shown in Table 4.4. Amino acid substitutions in the principal genetic variants can be deduced from the primary structures (Figures 4.9-4.12). Four features of the amino acid profile are noteworthy ... 

Figure 4.9 Amino acid sequence of bovine atl -casein, showing the amino acid substitutions or deletions in the principal genetic variants (from Swaisgood, 1992).

The proteins of milk fall into several classes of polypeptide chains. These have been delineated most completely in bovine milk, and a system of nonmenclature has been developed for them (Chapter 3 Eigel et al. 1984). One group, called caseins, consists of four kinds of polypeptides asr, as2-. and 3-, and k- with some genetic variants, post translational modifications, and products of proteolysis. Almost all of the caseins are associated with calcium and phosphate in micelles 20-300 fim in diameter (see Chapter 9). The other milk proteins, called whey proteins, are a diverse group including /3-lactoglobulin, a-lactalbumin, blood serum albumin, and immunoglobulins (Chapter 3). Almost all... 

Caseins. The /3-casein family consists of one major component with at least seven genetic variants and eight minor components which are proteolytic fragments of the major component. The relative mobility of the major /3-casein in alkaline urea gel electrophoresis is less than that of the a82-caseins. 

The ADSA Protein Nomenclature Committee (Eigel et al. 1984) recommends that these minor K-caseins be identified temporarily according to their genetic variant and numbered consecutively in their order of increasing relative electrophoretic mobility in alkaline urea gels in the presence of mercaptoethanol (Yaguchi et al 1968) as K-casein A-l or K-casein B-l, etc. 

Electrophoretic Methods. Several electrophoretic procedures have been developed to fractionate or purify the various caseins (McKenzie 1971C Thompson 1971 Whitney 1977). Wake and Baldwin (1961) fractionated whole casein by zone electrophoresis on cellulose powder in 7 M urea and 0.02 ionic strength sodium phosphate buffer at pH 7 and 5°C. Payens and co-workers employed several somewhat different electrophoretic conditions for the fractionation and purification of the caseins on cellulose columns (Payens 1961 Schmidt and Payens 1963 Schmidt 1967). Three fractions, as-, k-, and /3-caseins, were separated at pH 7.5 and 30°C with 4.6 M urea-barbiturate buffer. The purification of asi-casein and the separation of the genetic variants of K-casein were accomplished by altering the electrophoretic conditions. Manson (1965) fractionated acid casein on a starch gel column stabilized by a density gradient at 25 °C. 

Aschaffenburg, R., Sen, A. and Thompson, M. P. 1968. Genetic variants of casein in Indian and African zebu cattle. Comp. Biochem. Physiol. 25, 177-184. 

Casein micelle proteins are primarily a8i-, as2-, /3-, and -caseins in approximate proportions 3 .8 3 1. asi-Casein has eight or nine phosphate groups, depending on the genetic variant. aS2-Casein is the most hydrophilic of the caseins. It has two disulfide bonds which, by severe heat treatment, can be caused to interact with those of /3-lactoglobulin. It also has 10 to 13 phosphate groups and is very sensitive to the calcium ion concentration (Kinsella 1984 Swaisgood 1982). 

Genetic variants and protein characterization Enzymatic digest a-S 1 -casein, /3-lactoglobulin, thionins, gliadins, and glutenins... 

At least five genetic variants of asi-casein have been identified, but in Western breeds of cattle the B variant is predominant. The protein comprises a single polypeptide chain of known amino acid sequence (Fig. 2) which is found in milk as a major and a minor fraction. The major fraction, previously designated aSi-casein, contains 8 mol P/ mol (aSi-CN-8P), whereas the minor component (previously aSo-cas-ein) has an additional phosphorylated serine residue at position 41... 

Four genetic variants have been identified, with variant A the most common in Western commercial breeds. The complete amino acid sequence of as2-CN A-l IP is shown in Fig. 3. Although the sites of phosphorylation are not all identified, it is thought that aS2-casein exists, at least predominantly, in four phosphorylated forms containing 10-13 mol P/mol. 

A total of seven genetic variants of p-casein are known, of which three (the A1, A2, and A3 variants) predominate in Western breeds of cattle. The complete amino acid sequence of P-CN A2-5P is shown in Fig. 4. 

---