Dairy Chemistry And Biochemistry

MILK PROTEINS 171

I Positive charge

I %I - Casein

Negative charge

c- Casein

L\Y 1

(^1) (10 a, 100 120 140 I 10 110 I 200
P 40
Residue number
Figure 4.15 Ten residue sequences of bovine caseins with a charge density of 0.5 or greater at
pH 6.6 (from Swaisgood, 1992).
species, found very little homology. Although the sequences of /3-caseins
from cow, sheep, mouse, rat, rabbit and human could be aligned readily,
very little homology was evident between all six species (Figure 4.16): the
only long homologous sequence was the signal peptide, the two N-terminal
residues of the mature protein and the sequence SSEE (residues 18-21 of
the mature protein, which is the principal phosphorylation site). The
sequence of the signal peptides of asl- and tc-caseins also show a high degree
of interspecies homology but several long insertions were required to obtain
even a moderate degree of alignment of the sequences of the mature
proteins.
Casein phosphorus. Milk contains about 900mg phosphorus l-l, which
occurs in five types of phosphate-containing compounds, as will be dis-
cussed in Chapter 5:
0 inorganic: soluble and colloidal phosphates;
0 organic: phospholipids, casein and sugar phosphates, nucleotides (ATP,
Whole casein contains about 0.85% phosphorus; aSl-, /3- and Ic-caseins
contain 1.1,0.6 and 0.16% P, respectively; on a molar basis, asl-, tls2-, /I- and
UTP, etc.).