Casein Interactions: Does the Chemistry Really Matter? - Food Colloids: Self-Assembly and Material Science - page 162

For both of these models, an important question is this - how many casein
molecules interact with the calcium phosphate nanocluster? Although Holt first
estimated 13 a figure of 5 casein molecules, in more recent and substantially
more detailed descriptions of his model he proposed 11,14,15 that around 48
casein molecules surround every calcium phosphate nanocluster in the casein
micelle. This estimate was based on the belief that the in vitro entity stabilized
by a preparation of b-casein phosphopeptide would be identical to the micro-
crystalline nodes in the casein micelle. Arguments against this assumption have
been put forward elsewhere 16 and will not be repeated here. Instead we propose
a simple method of estimating the number of phosphoserine cluster motifs
interacting with the nanocluster, based on micellar composition and reasonable
assumptions on motif numbers and sizes, and their role in controlling nano-
cluster size. Primarily, the phosphoserine cluster is seen as initiating the growth
of the nanocluster, but it is also seen as terminating that growth by capping the
facets of the growing microcrystallite. These capping motifs are not envisaged
as adsorbing or coating the surface; rather, like the initiating cluster, their
phosphate groups are considered to be integrated into, and forming part of,
that surface layer.
Consideration of the casein sequences (Figure 5) indicates that there can be a
maximum of 3 cluster motifs along a s2 -casein, 2 along a s1 -casein, 1 on b-casein
and none on k -casein. Figure 6(a) illustrates schematically the presence of a
phosphoserine cluster on a calcium-sensitive casein. Bovine casein is generally
accepted to consist of 40% a s1 , 40% b, 10% a s2 and 10% k. 17 The weight

α S1 - casein (8P)

- SerP 46 -Glu- SerP 48 -Thr-Glu- - SerP 64 -Ile- SerP 66 -SerP 67 -SerP 68 -Glu-Glu-

- SerP 75 -Val-Glu- - SerP 115 -Ala-Glu-

α S2 - casein (11 or 12P)

- SerP 8 - SerP 9 - SerP 10 -Glu-Glu- - SerP 16 -Gln-Glu-
- SerP 56 - SerP 57 - SerP 58 -Glu-Glu- SerP 61 -Ala-Glu- - SerP 129 -Thr- SerP 131 -Glu-Glu-

- SerP 143 -Thr-Glu-

β -casein (5P)

- SerP 15 -Leu- SerP 17 - SerP 18 - SerP 19 -Glu-Glu- - SerP 35 -Glu-Glu-

K-casein (1P)

- SerP 149 -Pro-Glu-

Figure 5 Positions of phosphorylated residues (bold typeface) in the bovine caseins. Also
indicated are the positions on the C-terminal side of each SerP group of the
negatively charged residues (exclusively glutamic acid here) required by the
casein kinase template
Source: from ref. 8.