192 DAIRY CHEMISTRY AND BIOCHEMISTRY/I-Lg also binds free fatty acids and thus it stimulates lipolysis (lipases are
inhibited by free fatty acids); perhaps this is its physiological function. BSA
also binds hydrophobic molecules, including fatty acids; perhaps BSA serves
a similar function to p-lg in those species lacking D-lg.
4.7.8 Denaturation
Denaturation of whey proteins is of major technological significance and
will be discussed in Chapter 9.4.8 or-Lactalburnin
a-Lactalbumin (a-la) represents about 20% of the proteins of bovine whey
(3.5% of total milk protein); it is the principal protein in human milk. It is
a small protein with a molecular mass of c. 14kDa. Recent reviews of the
literature on this protein include Kronman (1989) and Brew and Grobler
(1992).
4.8. I Amino acid composition
The amino acid composition is shown in Table 4.4. a-La is relatively rich in
tryptophan (four residues per mole). It is also rich in sulphur (1.9?40) which
is present in cystine (four intramolecular disulphides per mole) and me-
thionine; it contains no cysteine (sulphydryl groups). The principal a-la’s
contain no phosphorus or carbohydrate, although some minor forms may
contain either or both. The isoionic point is c. pH 4.8 and minimum
solubility in 0.5 M NaCl is also at pH 4.8.
4.8.2 Genetic variants
The milk of Western cattle contains only r-la B but Zebu and Droughtmas-
ter cattle secrete two variants, A and B. a-La A contains no arginine, the
one Arg residue of a-la B being replaced by glutamic acid.
4.8.3 Primary structure
The primary structure of a-la is shown in Figure 4.25. There is considerable
homology between the sequence of a-la and lysozymes from many sources.
The primary structures of r-la and chicken egg white lysozyme are very
similar. Out of a total of 123 residues in r-la, 54 are identical to correspond-
ing residues in lysozyme and a further 23 residues are structurally similar
(e.g. Ser/Thr, Asp/Glu).