Casein primary structure

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 ... 

Primary structure. The primary structures of the four caseins of bovine milk are shown in Figures 4.9-4.12. The sequences of some non-bovine caseins have been established also. 

Originally, the caseins were defined as those phosphoproteins which precipitate from raw skim milk upon acidification to pH 4.6 at 20°C, and the individual families were identified by alkaline urea gel electrophoresis (Whitney et al 1976). With the resolution of their primary structure, it became possible to classify them according to their chemical structure, rather than on the basis of an operational definition. When one does this, it is apparent that not all of the caseins contain phosphorus (Table 3.1) some are also found in the acid whey after removal of the precipitated caseins. 

The primary structure of asl-casein B-8P, the major component of this variant, is shown in Figure 3.1. The additional phosphorylated serine residue in the minor component, a9i-casein B-9P, occurs at position 41 (Manson et al. 1976,1977). The B-variant consists of 199 amino acid... 

Figure 3.1. Primary Structure of Bos asi-casein B-8P. (From Mercier et al. 1971 Grosclaude et al. 1973 Eigel et al. 1984. Reprinted with permission of the American Dairy Science Association.)...

Peptides extracted from casein with N, N-dimethyl formamide have complex electrophoretic patterns identical to those of the fraction first prepared by Long and co-workers and called X-casein (El-Negoumy 1973). These peptides are identical electrophoretically to those released by the action of plasmin, which is present in fresh raw milk, upon asr casein (Aimutis and Eigel 1982). Two of these peptides have tryptic peptide maps and molecular weights identical to those of a pair of the peptides produced by plasmin degradation of asl-casein. These peptides appear to be fragments of a8l-casein which are present in milk as the result of plasmin proteolysis. More definitive information on their primary structure is needed before nomenclature for these fragments can be established. 

Based on the primary structure of the major /3-casein component,... 

Figure 3.4. Primary structure of Bos K-casein B-1P. As indicated, the A variant has a threonine residue at position 136 and an asparic acid residue at position 148. The arrow indicates the point of attack of rennin (MacDonald and Thomas 1970 Polzhofer 1972). Reprinted with permission of the American Dairy Science Association. (From Mercier et al. 1973 Eigel et al. 1984. Reprinted with permission of the American Dairy Science Association.)...

Studies of the structure and conformation of the as2-caseins have been limited. From their primary structure, one can conclude that they are... 

The configuration and association of the minor /3-caseins have not been investigated to any extent, but from their primary structure one can conclude that /3-casein X-4P-(f 1-28), -(f 1-105), and -(f 1-107) and /3-casein X-lP-(f 29-105) and -(f 29-107) are more hydrophilic than the main /3-casein component and that /3-casein X-lP-(f 29-209) and /3-casein-(f 106-209) and -(f 108-209) will be more hydrophobic. The hy-drophobicities of the last three caseins are the highest of all the caseins, with calculated Bigelow hydrophobicities of 1386 to 1511 (Swais-good 1973). Therefore, we would expect these to demonstrate a strong temperature-dependent association /3-casein B-(f 106-209) and -(108— 209) are insoluble at pH 8.0 and 25°C but soluble at 3°C, while 0-casein A-(f 108-209) is soluble at both temperatures. Sedimentation studies... 

At pH 12, the disulfide and noncovalent bonds are both broken, and the monomer with a sedimentation constant of 1.45 Svedberg units is released. From frictional ratios, the monomer appears to exist as a coil with a diameter of 16 A and a length of 150 A. Analysis of the primary structure of K-casein (Loucheux-Lefebvre et al. 1978) suggests considerable secondary structure in the monomer. 23% a-helix, 31% /3-sheets, and 24% 0-turns. In contrast, other investigators, using several different approaches, obtained a-helix contents ranging from 0 to 20.8% (Bloomfield and Mead 1975). Circular dichroism spectra on the monomer indicated 14 and 31% for a-helix and / -sheet, respectively (Loucheux-Lefebvre et al 1978). An earlier study of the optical rotatory dispersion of the K-casein monomer yielded values for the a-helix content ranging from 2 to 16% (Herskovits 1966). 

Brignon, G., Ribadeau-Dumas, B. and Mercier, J.-C. 1976. Primary elements of the primary structure of a,2-bovine casein FEBS Lett. 71, 111-116 (French). 

Grosclaude, F., Mahe, M.-F., Mercier, J.-C. and Ribadeau-Dumas, B. 1973. Primary structure of asi-casein and 0-casein. Correction. Eur. J. Bichem. 40, 323-324 (French). 

Jolles, J., Schoentgen, F Alais, C., Fiat, A.-M. and Joll6s, P 1972A. Studies on the primary structure of cow K-casein. Structural features of para-K-casein N-terminal sequence of K-casein-glycopeptides studied with a sequencer. Helv. Chim. Acta 55, 2872-2883. 

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. 

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. 

The four major casein fractions contain subfractions and genetic polymorphs (Table 1), each of which possesses a unique primary structure and associated physico-chemical properties (1). 

For example, a - casein contains one major subfraction (a -Cn) plus several minor components. The four genetic variantssof < si-casein have similar isoionic points, which would be expected from their similar primary structures. The 3-caseins, K-caseins, and "whole -caseins" have also been fractionated and their subfractions characterized (Table 1). The isoionic points for all of these casein subunits range from about 4.9 to 6.0 and their molecular weights range from about 11,500 to 24,000. 

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