Dairy Chemistry And Biochemistry

MILK PROTEINS 167

1

H.Arg-Clu-Leu-Glu-Glu-Leu-Asn.Val-Pro-Gly.Glu-1le-Val-Glu~SerP.LeuSerP-SerP-SerP-Glu-

SerP Glu

(Variants A, B)

21

Clu-Ser-Ile-Thr.Arg-Ile- Asn-Lys

41

Thr-Clu-Asp-Glu-Leu-Gln-Asp-Lys~Ile-His-Pro-Phe-Ala-Gln-Thr-Gln-Ser-Leu-Val-Tyr~

Pro (Variants A’ A’)

His (Variants C, A” and B)

61

Pro-Phe-Pro-Gly.Pro-Ile- -Asn-Ser-Le~.Pro-Gln-Asn-lle-Pro-Pro-Leu-Thr-Gln-Thr

81

Pro-Val-Val-Val-Pro-Pro~Phe-Leu-Gln-Pro-Glu-Val-Met-Gly.Val-Ser-Lys-Val-Lys-Glu-

121 Ser Variants A c’@~~~~~

Glu- .~ln-Ser-Le~-~r-Leu.Thr-Asp-Val-Glu-Asn-Leu.His-Leu-Pro-Leu.Pr~Leu-Le~-

Arg (Variant B)

1 I1

Cln-Ser-Trp-Met-His-Cln-Pro-His-GIn-Pro-Leu-Pro-Pro-~r-Val-Met-Phe-Pro-Pro-Gln.

161

Ser-Val-Leu-Ser~Leu-Ser-Gln-Ser-Lys-Val-Leu-Pro~Val-Pro-Gln~Lys-Ala-Val~Pro-Tyr-

181

Pro-Cln-Arg-Asp-Met-Pro-Ile-GIn-Ala-Phe-Leu-Leu-Tyr-CIn-GIu-Pro-Val-Leu-Gly-Pro-

(^201 209)
Val-Arg-Cly-Pro-Phe-Pro-Ile-Ile-Val.OH
Figure 4.11 Amino acid sequence of bovine /?-casein, showing the amino acid substitutions in
the genetic variants and the principal plasmin cleavage sites (V) (from Swaisgood, 1992).

4. The caseins, especially us,-casein, are rich in lysine, an essential amino
   acid in which many plant proteins are deficient. Consequently, casein and
   skim-milk powder are very good nutritional supplements for cereal
   proteins which are deficient in lysine. Owing to the high lysine content,
   casein and products containing it may undergo extensive non-enzymatic
   Maillard browning on heating in the presence of reducing sugars (Chap-
   ter 2).
   At pH values on the acid side of their isoelectric point, proteins carry
   a net positive charge and react with anionic dyes (e.g. amido black or
   orange G), forming an insoluble protein-dye complex. This is the
   principle of the rapid dye-binding methods for quantifying proteins in
   milk and milk products and for visualizing protein bands in gel elec-
   trophoretograms; dye-binding is normally performed at pH 2.5-3.5.