2009_Book_FoodChemistry

10.1 Milk 505
Table 10.9. Amino acid sequences a of genetic variants of bovine milk proteins
Protein Variant Frequency b Positions of the substitutions
14–26 53 59 192
α s1 -Casein A s are lacking
(199 AS) B w Ala Glu Glu
C i Gly
D s ThrP
E s Lys Gly
α s2 -Casein 33 47 50–58 130
(207 AS) A Glu Ala Thr
B
C Gly Thr Ile
D
lacking
18 35 36 37 67 106 122
β-Casein A 1 His
(209 AS) A 2 w, i SerP SerP Glu Glu Pro His Ser
A 3
Gln
B s His Arg
C s Ser Lys His
D s Lys
E s Lys
κ-Casein 97 136 148 155
(169 AS) A x Thr Asp Ser
B w, i Arg Ile Ala
C
His
E
Gly
10
α-Lactalbumin A i Gln
(123 AS) B w Arg
β-Lactoglobulin 45 50 59 64 78 118 130 158
(162 AS) A x Asp Val
B w, i Glu Pro Gln Gly Ile Ala Asp Gln
C
His
D
Gln
E
Gly
F Ser Tyr Gly
G Met Gly
a cf. Table 10.8.
b w: predominant in the western world (Bos taurus), i: predominant in India (Bos indicus, Bos grunniens), s: rare,
x: not predominant, but not rare.
Normally, κ-casein occurs as a trimer or as
a higher oligomer in which the formation of
disulfide bonds is probably involved. The protein
contains varying amounts of carbohydrates (average
values: 1% galactose, 1.2% galactosamine,
2.4% N-acetyl neuramic acid) that are bound
to the peptide chain through Thr-131, 133, 135
or (in variant A) 136. κ-Casein is separated
electrophoretically into various components that
have the same composition of amino acids, but
differ in their carbohydrate moiety, e. g., per protein
molecule they contain 0–3 moles N-acetyl
neuramic acid, 0–4 moles galatose and 0–3 moles
galactosamine. Three different glycosyl residues
could be isolated, one of which has the structure
shown in Formula 10.3.