Lactoglobulin genetic variant

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... 

Extensive studies have been made of the structure and conformation of /3-lactoglobulin. In the pH range from 5.2 to 7.5, all genetic variants of /3-lactoglobulin investigated have been shown to exist primarily as... 

A number of titration studies have been performed on the genetic variants of /3-lactoglobulin (Tanford 1962 Basch and Timasheff 1967 Brignon et al. 1969). The titration curves as illustrated in Figure 3.22 were reversible between the acid endpoint and pH 9.7. The maximum acid-binding capacity observed in genetic variants A, B, and C indicates 20 cationic groups per monomer compared to 21 from the pri-... 

Basch, J. J. and Timasheff, S. N. 1967. Hydrogen ion equilibria of the genetic variants of /3-lactoglobulin. Arch. Biochem. Biophys. 118, 37-47. 

Brignon, G. and Ribadeau-Dumas, B. 1973. The location in the peptide chain of bovine /3-lactoglobulin of the Glu/Gln substitution differentiating the genetic variants B and D. FEBS Lett. 33, 73-76 (French). 

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). 

Heat denaturation of protein solutions is normally retarded by concentration. Concentration of milk to total solids levels of 9, 28 and 44% decreases apparent denaturation by 40, 60, and 80% (Whitney 1977). Individual proteins are affected differently by concentration, a-lactal-bumin being denatured more easily as solids are increased and both A and B genetic variants of /3-lactoglobulin being denatured less easily (Hillier et al 1979). 

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

Titration curves of pure /8-lactoglobulins A and B have also been determined (Tanford and Nozaki, 1959). The two genetic variants differ in isoionic point, but they possess the same maximum positive charge (Fig. 

Lactoglobulin (/3-LG) is a very abundant protein found in the milk of mammals (McKenzie, 1971 Liberatori, 1977). The protein has been studied for decades and is considered one of the classical markers for milk proteins. It has been shown by a number of investigators that bovine /3-LG can form a complex with retinol. However, the exact in vivo function of the protein is still not known. The monomer of bovine /3-LG has a molecular weight of 18,000, corresponding to a chain of 162 amino acid residues. There are two genetic variants, commonly known as /3-LG A and )8-LG B (Braunitzer et al., 1973). The differences between the two variants are located at two positions where an Asp and a Val in variant A are substituted by a Gly and by an Ala in form B (McKenzie et al., 1972). Crystal forms of /3-LG A, /3-LG B, and /3-LG A-retinol complex have been obtained. The structures have been determined for the apo structure at 2.5 A (Papiz et al., 1986) and for a retinol complex (Monaco et al., 1987). 

The globular protein /i-lactoglobulin generally is a mixture of two genetic variants, A and B. One difference is that A has Asp at position 64, whereas B has Gly. How could this affect the solubility profile as given in Figure 7.12 ... 

The spectra of other genetic variants, )S-lactoglobulins B and C, were also found by TimashdT and Susi (1966) to be identical with the data for lactoglobulin A, suggesting very similar secondary structures. 

FIGURE 10.27 3-Lactoglobulin-amino acid sequence. May be replaced by other amino acids in genetic variants. Few, if any residues are phosphorylated. 

Lactoglobulin occurs in genetic variants A, B and C of the Jersey dairy cattle breed, and variant D of the Montbeliarde dairy cow. Two other Aor and Bor variants of Australian drought master cows are identical to variants A and B apart from the carbohydrate content. 

Grosclaude, F., Mahe, M.-F., Mercier, J.-C., Bonnemarie, J. and Tessier, J. H. 1976A. Polymorphism of the milk proteins of Nepalese bovines. I. The Yak and biochemical characterization of two new variants 0-lactoglobulin D (Yaki and usi-casein E. Ann. Genet. Sel. Anim. 8, 461-479 (French). 

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