Dairy Chemistry And Biochemistry

MILK PROTEINS^189

4.7.3 Primary structure

The amino acid sequence of bovine p-lg, consisting of 162 residues per

monomer, is shown in Figure 4.22.

4.7.4 Secondary structure

p-Lg is a highly structured protein: optical rotary dispersion and circular

dichroism measurements show that in the pH range 2-6, p-lg consists of

10- 15% a-helix, 43% P-sheet and 47% unordered structure, including

p-turns.

4.7.5 Tertiary structure

The tertiary structure of p-lg has been studied in considerable detail using

X-ray crystallography. It has a very compact globular structure in which the

,&sheets occur in a p-barrel-type structure or calyx (Figure 4.23). Each

monomer exists almost as a sphere with a diameter of about 3.6 nm.

1

H.Leu-Ile-Val-Thr-G1n-Thr-Met-Lys-Gly-Leu-Asp-Ile-Gln-Lys-Val-Ala-Gly-Thr-Trp-Tyr

21

Ser-Leu-Ala-Met-AIa-Ala-Ser-Asp-Ile-Ser-Leu-Leu-Asp-Ala-Gln-Ser-Ala-Pro-Leu-Arg

41 Glu (Variants A, B, C) Gln (Variant A,BI

Val-Tyr-Val-Glu- -Leu-Lys-Pro-Thr-Pro-Glu-GIy-Asp-Leu-Glu-Ile-Leu-Leu- -Lys-

Gln (Variant D) His (Variant C)

61Wariant A) Asp I

Trp-Glu-Asn- -Glu-Cys-Ala-G1n-Lys-Lys-Ile-Ile-Ala-Glu-Lys-Thr-Lys-I1e-Pro-Ala-

(Variant B, C) Gly

81

Val-Phe-Lys-Ile-Asp-Ala-Leu-Asn-Glu-Asn-Lys-Val-Leu-VaI-Leu-Asp-Thr-Asp-Tyr-Lys-

...................................................

101

Lys-Tyr-Leu-Leu-Phe-Cjrs-Met-Glu-Asn-Ser-Ala-Glu-Pro-Glu-Gln-Ser-Leu- -Cys-Gln-

(Variant A) Val ; SH

...................... (Variant B, C) Ala

15;

SH

Cys-Leu-Val-Arg-Thr-Pro-GIu-Val-Asp-Asp-Glu-Ala-Leu-Glu-Lys-Phe-Asp-Lys-Ala-Leu

141

Lys-Ala-Leu-Pro-Met-His-11e-Arg-Leu-Ser-Phe-Asn-Pro-Thr-Gln-Leu-GI-Glu-Gln-Cys-

161 162

His-Ile. OH

L

Figure 4.22 Amino acid sequence of bovine B-lactoglobulin, showing amino acid substitutions
in genetic polymorphs and the intramolecular disulphide bonds (-, - - -) (from Swaisgood,
1982).