Casein amino 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).

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. 

K-Casein has been isolated and studied in some detail (2). It is the only casein component containing carbohydrate. However, the K-casein is heterogeneous with regard to the carbohydrate content some molecules contain carbohydrate, and others do not (151), In addition, K-casein exists as polymorphic genetic variants involving amino acid substitutions in the primary sequence (152). The primary structure of K-casein has been determined (153). 

Much has been written about the structure and properties of casein. Casein contains about 1% by weight of P and consists mainly of four phosphoproteins, which are designated as Oji, 0 2, P and k. Smaller quantities of other phosphoproteins (e.g. y casein -3%) are present, but some of these at least are breakdown products of the main varieties. Each of the four main varieties has a characteristic amino acid sequence (Chapter 10.2). All four varieties have genetic variants, however, and both the... 

The main components of the casein fraction in bovine milk are 0(5-caseins. Bovine milk contains phosphoprotein -casein and a52 Casein, both occurring in four genetic variants, identified by the letters A, B, C and D, which differ somewhat in their primary structure. The most common variant is B, which is a polypeptide chain composed of 199 amino acids (molecular weight 23.6 kDa, p/= 4.92-5.35). aj-Caseins comprise from 44 to 55% of the total caseins and ajj-casein comprises approximately 80% of the total aj-caseins. The polypeptide chain contains eight phosphoserine residues located mainly at positions 43-80, which makes this part of the molecule polar. Non-polar side chains of amino adds are located in positions 100-199. ajj-Casein forms insoluble calcium salts in the presence of calcium ions. Fragments of aji-casein are considered to be. X-casein. a52-Casein has a similar structure (molecular weight 25.2 kDa), but is less sensitive to the presence of calcium ions. 

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