Dairy Chemistry And Biochemistry

MILK PROTEINS 173

Ic-caseins contain 8(9), 10-13, 5(4) and 1(2,3) moles P per mole. The

phosphorus is very important:

0 nutritionally, per se, and because it can bind large amounts of Ca2+, ZnZ+

0 it increases the solubility of caseins;

0 it probably contributes to the high heat stability of casein; and

0 it is significant in the coagulation of rennet-altered casein during the

secondary phase of rennet action (Chapter 10).

The phosphorus is covalently bound to the protein and is removed only

by very severe heat treatments, high pH or some phosphatases. The

phosphate is esterified mainly to serine (possibly a little to threonine) as a

monoester:

and probably other polyvalent metals;

0

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Ser-0-P-OH

0-

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Phosphorylation occurs in the Golgi membranes of the mammary cell,

catalysed by two serine-specific casein kinases. Only certain serines are

phosphorylated; the principal recognition site is Ser/Thr.X.Y, where Y is a

glutamyl and occasionally an aspartyl residue; once a serine residue has

been phosphorylated, SerP can serve as a recognition site. X may be any

amino acid but a basic or a very bulky residue may reduce the extent of

phosphorylation. However, not all serine residues in a suitable sequence are

phosphorylated, suggesting that there may be a further topological require-

ment, e.g. a surface location in the protein conformation.

Casein carbohydrate. asl-, aS2- and b-caseins contain no carbohydrate but
K-casein contains about 5%, consisting of N-acetylneuraminic acid (sialic
acid), galactose and N-acetylgalactosamine. The carbohydrate exists as tri-
or tetrasaccharides, located toward the C-terminal of the molecule, attached
through an 0-threonyl linkage, mainly to Thr,,, of K-casein (Figure 4.17).
The number of oligosaccharides per K-casein molecule varies from 0 to 4.
The variability of glycosylation results in at least nine, and probably 10,
molecular forms of K-casein (Table 4.5). The K-casein in colostrum is even
more highly glycosylated; more sugars are present and the structures are
more complex and uncertain.
The carbohydrate is attached to the (g1yco)macropeptides which are
produced from K-caseins on hydrolysis by rennets. The carbohydrate be-
stows on x-casein quite high solubility and hydrophilicity. It is also