Dairy Chemistry And Biochemistry

MILK PROTEINS 193

1 I Are Y (Variant B)

H. Glu-Gln-Leu-Thr-Lys-Cys-Glu-Val-F'he- -Glu-Leu-Lys- Asp-Leu-Lys-Gly-Tyr-Gl y-Gly-

Gln (Variant A)

21

Val-Ser-Leu-Pro-Glu-Trp-Val-Cys-Thr-Thr-Phe-His-Thr-Ser-G1y-Tyr-Asp-Thr-Glu-Ala-

Ile-Val-G1n-Asn-Asn-Asp-Ser-Thr-Glu-Tyr-Gly-Leu-Phe-Gln-Ile-Asn-Asn-Lys-Ile-Trp-

Cy~-Lys-Asp-Asp-Gln-Asn-Pro-His-Ser-Asn-I~e-Cys-Asn-Ile-~er-Cys-Asp-Lys-P~e- 1

Leu-Asp-Asp-Asp-Leu-Thr-Asp-Asp-Ile-Met-

-nrI Ile-Asn-Tyr-Trp-Leu-Ala-His-Lys-Ala-Leu-Cys-Ser-Glu-Lys-Leu-Asp-Gln-Trp-Leu-Cys- I

121 123

Glu-Lys-Leu. OH

I-

Figure 4.25 Amino acid sequence of a-lactalbumin showing intramolecular disulphide bonds

(-) and amino acid substitutions in genetic polymorphs (from Brew and Grobler, 1992).

4.8.4 Secondary and tertiary structure
cc-La is a compact globular protein, which exists in solution as a prolate
ellipsoid with dimensions of 2.5 x 3.7 x 3.2nm. It consists of 26% cc-helix,
14% p-structure and 60% unordered structure. The metal binding and
molecular conformational properties of r-la were discussed in detail by
Kronman (1989). The tertiary structure of a-la is very similar to that of
lysozyme. It has been difficult to crystallize bovine a-la in a form suitable
for X-ray crystallography but work on the detailed structure is at an
advanced stage (Brew and Grobler, 1992).

4.8.5 Quaternary structure

@-La associates under a variety of environmental conditions but the associ-
ation process has not been well studied.

4.8.6 Other species

a-La has been isolated from several species, including the cow, sheep, goat,
sow, human, buffalo, rat and guinea-pig. Some minor interspecies differences
in the amino acid sequence and properties have been reported. The milks of
sea mammals contain very little or no a-la.