Casein amino acid sequence

Parker, F., Migliore-Samour, D., Floc h, F., Zerial, A., Werner, G.H., Jolles, J., Casaretto, M., Zahn, H., and Jolles, P. 1984. Immunostimulating hexapeptide from human casein, amino acid sequence, synthesis and biological properties. Eur. J. Biochem. 145, 677-682. 

The elemental composition of casein is not greatly different from that of wool (Table 10.13). The approximate amino acid compositions of each component phosphoprotein are listed in Table 10.14, their relative proportions in Table 10.12, and the casein amino acid sequences are indicated in Figures 10.22 through 10.25. These sequences are subject to minor variations particularly between animal species. Casein has numerous non-food applications (Chapter 12.17). 

I I Phosphorylated residues FIGURE 10.22 Bovine asl-casein—amino acid sequence. 

FIGURE 10.23 Bovine as2 casein—amino acid sequence. Phosphorylation may also occur at Ser 31 andThr... 

Thr-Val-GIn-Val-Thr-Ser-Thr-Ala-Val-OH FIGURE 10.24 Bovine K-casein—amino acid sequence. 

Greenberg, R., Groves, M.L., and Dower, H.J. (1984) Human and casein amino acid sequence and identification of phosphorylation J. Biol. Chem., 259, 5132-5138. 

The most abundant milk protein is casein, of which there are several different kinds, usually designated a-, (1-, and K-casein. The different caseins relate to small differences in their amino acid sequences. Casein micelles in milk have diameters less than 300 nm. Disruption of the casein micelles occurs during the preparation of cheese. Lactic acid increases the acidity of the milk until the micelles crosslink and a curd develops. The liquid portion, known as whey, containing water, lactose and some protein, is removed. Addition of the enzyme rennet (chymosin) speeds up the process by hydrolysing a specific peptide bond in K-casein. This opens up the casein and encourages further cross-linking. 

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).

Figure 10.1 Amino acid sequence of K-casein, showing the principal chymosin cleavage site (J.) oligosaccharides are attached at some or all of the threonine residues shown in italics.

Brignon, G., Ribadeau-Dumas, B., Mercier, J.-C. Pelissier, J. P. and Das, B. C. 1977. Complete amino acid sequence of bovine as2-casein. FEBS Lett. 76, 274-279 (French). 

About 20% of milk protein is soluble in the aqueous phase of milk. These serum proteins are primarily a mixture of /3-lactoglobulin, a-lactalbumin, bovine serum albumin, and immunoglobulins. Each of these globular proteins has a unique set of characteristics as a result of its amino acid sequence (Swaisgood 1982). As a group, they are more heat sensitive and less calcium sensitive than caseins (Kinsella 1984). Some of these characteristics (Table 11.1) cause large differences in susceptibility to denaturation (de Wit and Klarenbeek 1984). 

Minamiura, N., Matsumera, Y., Fukumoro, J. and Yamamoto, T. 1972. Bitter peptides in cow milk casein digests with amino acid sequence of a bitter peptide. Agr. Biol Chem. 36, 588-595. 

Caseins were once defined as the phosphoproteins that precipitate from milk at pH 4.6 but are now defined by their amino acid sequences. In the elucidation of the primary structures of the caseins a French group has taken a prominent part (Ribadeau Dumas et al., 1975). The four polypeptide chains of bovine casein are the aSi-, as2-, P-, and K-caseins. Of these, the two as-caseins and K-casein are each phosphorylated to different extents and K-casein also exists in a number of glycosylated forms. 

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. 

The y-casein fractions and proteose peptone fractions 5, 8-slow, and 8-fast have amino acid sequences corresponding to parts of the sequence of p-casein (Fig. 1). Their occurrence in milk could be due to partial gene duplication but identical fractions can be produced... 

Several authors report coexisting clinical soy allergies in 5%-50% of patients with cows milk allergies (NDA Opinion 2004). It is unclear whether soy allergy represents a de novo sensitization or a cross-reaction of a soy protein component with caseins from milk (Rozenfeld et al. 2002). However, due to homology in the amino acid sequences of soybean and cow s milk allergens of 50%-70%, cross-reactivity is likely (Wilson et al. 2005). 

One point of interest emerges from these experiments, namely, that ovalbumin is the second protein from which phosphoserine has been isolated. As in the case of the dipeptide, SerP.Glu, isolated from casein (38,77) ovalbumin also contains an amino acid sequence with phosphoserine adjacent to a dicarboxylic acid. Moreover, the close association of this amino acid with aspartic and glutamic acids, isoleucine and alanine in ovalbumin (22, 68) and casein (78-80) suggests the existence of a systematically recurring sequence in phosphoproteins. 

The adsorption of proteins at fluid interfaces is a key step in the stabilization of numerous food and non-food foams and emulsions.1 Our general goal is to relate the amino acid sequence of proteins to their surface properties, e. g. to the equation of state or other structural and thermodynamic properties. To improve this understanding, the effect of guanidine hydrochloride (Gu HC1) on /1-casein adsorption is evaluated in the framework of the block-copolymer model for the adsorption of this protein. At first the main features of the model are presented, and then the effect of Gu HC1 is interpreted using the previously introduced concepts. 

Jolles (20) has presented an intriguing discussion of the similarities between the clotting of casein and fibrin including amino acid sequence similarities between the key proteins, -casein and fibrinogen. 

Seven of the eight phosphate groups of a8i-casein are contained in a 30-amino acid sequence between residues 46 and 75. This same region contains 10 of the total 26 Asp and Glu residues of the molecule and only one basic amino acid residue (Lys-58) and three hydrophobic residues (Ile-65, Ile-71, and Val-72). Thus, the region of the molecule is highly negatively charged and hydrophilic in character. There is a total of 16 serine and 5 threonine residues in a8i-casein, variant B. 

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