Dairy Chemistry And Biochemistry

MILK PROTEINS 175

Table 4.5 Variability of bovine K-casein with respect to sugars and phosphate

N-acetyl-

N-acetyl- neuraminic

Fraction Galactose galactosamine acid Phosphate

B- 1 0 0 0 1

B-2 1 1 1 1

B-3 1 1 2 1

B-4 0 0 0 2

B-5 2 2 3 1

1

B-6 0 0 O(4)

B-7 3 3 6

B-8 4 4 8 1

B-9 5 5 10 1

3(1)

responsible for the solubility of the glycomacropeptides in 12% TCA (see

Chapter 10). Although the sugars increase the hydrophilicity of casein, they

are not responsible for the micelle-stabilizing properties of Ic-casein, the

carbohydrate-free form being as effective in this respect as the glycosylated

forms.

4.5.2 Secondary and tertiary structures
Physical methods, such as optical rotary dispersion and circular dichroism,
indicate that the caseins have relatively little secondary or tertiary structure,
probably due to the presence of high levels of proline residues, especially in
p-casein, which disrupt r-helices and P-sheets. However, theoretical calcula-
tions (Kumosinski, Brown and Farrell, 1993a, b; Kumosinski and Farrell,
1994) indicate that while a,,-casein has little or-helix, it probably contains
some P-sheets and p-turns. The C-terminal half of a,,-casein probably has
a globular conformation (i.e. a compact structure containing some or-helix
and P-sheet) while the N-terminal region probably forms a randomly
structured hydrophilic tail. Theoretical calculations suggest that p-casein
could have 10% of its residues in a-helices, 17% in /?-sheets and 70% in
unordered structures. Ic-Casein appears to be the most highly structured of
the caseins, perhaps with 23% of its residues in or-helices, 31% in /3-sheets
and 24% in p-turns. Energy-minimized models of asl-, p- and Ic-caseins are
shown in Figure 4.18a-c. Holt and Sawyer (1993) coined the term ‘rheomor-
phic’ to describe the caseins as proteins with an open, flexible, mobile
conformation in order to avoid using the ‘demeaning’ term, ‘random coil’.
The lack of secondary and tertiary structures is probably significant for
the following reasons:

1. The caseins are readily susceptible to proteolysis, in contrast to globular
   proteins, e.g. whey proteins, which are usually very resistant in their