Chemistry Reference
In-Depth Information
the caseins in the absence of phosphoseryl clusters. Most molecules of
-
casein contain only one phosphoserine residue, rarely two or three, and all
singlets located in the hydrophilic C-terminal region. The caseins are there-
fore sensitive to coagulation or precipitation by Ca. Horne and Dalgleish
(1980) demonstrated that the logarithm of this critical coagulation time is a
linear function of Q
2
,whereQ is the net negative charge on the protein,
taking into account the binding of Ca to the casein. This linear correlation
was also maintained when protein charge was changed following chemical
modification of charged residues along the protein chain (Horne, 1979,
1983; Horne and Moir, 1984).
9.6.2.
Casein Micelle Formation
The caseins are sensitive to precipitation due to the presence of approxi-
mately 30 mM Ca in milk. However, a key biological purpose of milk is to
provide high concentrations of the essential Ca and phosphate required for
the growth of the newborn mammal. So how are these two conflicting factors
resolved? The solution involves casein micelle formation, the formation of an
insoluble CCP phase within the micelles and the requirement for one of the
caseins (usually
-casein) to be insensitive to Ca and provide stability against
Ca-induced precipitation to the other caseins. The details of how this occurs
have been the subject of much debate and intensive study. For a discussion of
the various casein micelle models, the reader is referred to various reviews
(Farrell, 1973; Slattery, 1976; Rollema, 1992; De Kruif and Holt, 2003;
Farrell et al., 2006; Qi, 2007). We will focus our explanations on the dual-
binding approach for micelle formation as described by Horne (1998, 2002,
2006, 2008).
In the dual-binding model (Figure 9.4), micellar assembly and growth
take place by a polymerization process involving two distinct forms of bond-
ing/interactions, namely association through clustering of hydrophobic
regions/patches of the caseins and second, linking of several phosphopeptides
into the Ca phosphate nanoclusters. Central to the model is the concept that
bond formation is facilitated, and hence micellar integrity and stability are
maintained, by a local excess of hydrophobic attraction over electrostatic
repulsion (otherwise if the repulsive interactions were too large, little associa-
tion of casein would occur and micelle formation would not be observed in
milk). It should be noted that there are quite different ranges for these
interaction components. Compared to hydrophobic interactions, electro-
static repulsion is a long-range force. Clustering of charged groups in specific
regions of the protein molecule means that electric dipole moments may be
