Caseins calcium phosphate interactions

Mizuno, R., and Lucey, J.A. (2005). Effects of emulsifying salts on the turbidity and calcium phosphate-protein interactions in casein micelles. /. Dairy Sci. 88,3070-3078. 

These different casein monomers combine with calcium phosphate to form discrete particles on the nano-size scale. The phosphoserines of the caseins are seemingly clustered for the purpose of linking within the micelle to putative calcium phosphate microcrystallites, also known as nanoclusters (Holt, 1992 Home, 1998, 2002, 2003, 2006 Holt et al., 2003 Home et al., 2007). Structural evidence for the existence of such nanoclusters has come from neutron and X-ray scattering (de Kruif and Holt, 2003 Holt et al., 2003 Pignon et al., 2004 Marchin et al., 2007). The presence of nanoclusters allows native casein micelles to be effective natural suppliers of essential calcium salts in the human diet in a readily assimilated functional form. Protein-nanocluster interactions are the central concept of the cross-linking mechanism in Holt s model of casein micellar assembly (Holt et al., 2003 de Kruif and Holt, 2003). Any analogy with conventional soap-like micelles is considered to be... 

We have seen earlier in this chapter how the self-assembly of casein systems is sensitively affected by temperature. Another thermodynamic variable that can affect protein-protein interactions in aqueous media is the hydrostatic pressure. Static high-pressure treatment causes the disintegration of casein micelles due to the dismption of internal hydro-phobic interactions and the dissociation of colloidal calcium phosphate. This phenomenon has been used to modify the gelation ability of casein without acidification as a consequence of exposure of hydrophobic parts of the casein molecules into the aqueous medium from the interior of the native casein micelles (Dickinson, 2006). High-pressure treatment leads to a reduction in the casein concentration required for gelation under neutral conditions, especially in the presence of cosolutes such as sucrose (Abbasi and Dickinson, 2001, 2002, 2004 Keenan et al., 2001). 

Lowering the pH of milk to 4.6 solubilizes colloidal calcium phosphate. This removes its neutralizing effect, allowing electrostatic interactions between micelles. Under these conditions, micelles coagulate and precipitate from solution. Kudo (1980C) showed that release of whey proteins and K-casein from casein micelle surfaces as the pH is increased from 6.2 to 7.2 allows micelles to stick together and precipitate from solution. 

The subject matter of this section has been treated, for the most part, in some earlier reviews (Swaisgood, 1982 Schmidt, 1982 Pay-ens and Vreeman, 1982 Farrell and Thompson, 1988). The coverage here is highly selective, reviewing mainly the more recent findings. However, because of their relevance to our understanding of the structure and stability of native casein micelles, studies dealing with the interactions of the caseins with calcium phosphate are considered more fully. 

Their calculation takes the premise that the casein proteins do not change their titration behavior when bound to the colloidal calcium phosphate, an assumption that appears unreasonable if the phos-phoseryl residues are involved in the interaction. An alternative calculation which allows the casein phosphoseryl groups to titrate when the calcium phosphate is removed predicts, from the same titration data, that over half the orthophosphate groups in the micelles are protonated (Holt et al., 1989a). 

Gagnaire, V., Pierre, A., Molle, D., and Leonil, J. 1996. Phosphopeptides interacting with colloidal calcium phosphate isolated by tryptic hydrolysis of bovine casein micelles. J. Dairy Res. 63, 405-422. 

Since milk is the vehicle by which the infant mammal obtains its calcium and phosphorus requirements for growth, the fluid must contain large quantities of both elements. Both solid and solution phases coexist, but the solid phase is maintained in a colloidal state by interaction with protein and the nature of the solid state is of considerable interest.347 In the 3fP solution-state spectrum of whole cow s milk the inorganic phosphate time is broad on adding EDTA this narrows and increases in intensity. This is consistent with the dissolution of the solid phase and reduction of the interaction of the phosphate with either the solid phase and/or the protein.346 The temperature dependence of31P signal intensity has been measured in simulated milk ultrafiltrate solution (SMUF), a colloidal calcium phosphate-free milk (CCP-free) and a casein micelle suspension (CMS).345... 

A better knowledge of casein micelle structure is a prerequiste for further characterization of casein derivatives. One way to study the structure of the micelle is by inducing controlled dissociation. The analysis of resultant Fractions provides information on the initial state of aggregation. Removal of calcium phosphate has been used to dissociate the micelle (6). The resultant complexes have been separated by chromatography, and their composition, and average size evaluated (7,8,9). However, the nature of interactions leading to their formation is still obscure. In the present work, we removed calcium to induce dissociation of the micelle, but afterwards, we paid attention to the interactive properties of the isolated complexes. ... 

Reported studies on native casein micelles have indicated that P NMR spectroscopy was useful to determine the nature of phosphate molecules as phosphoserins and inorganic calcium phosphate" . Quantitative assessment of the various micellar components was also probed by Rasmussen et al (1997). Nevertheless, in the case of cheeses which are highly-hydrated and heterogeneous samples, investigations by high-field NMR appear as a true challenge. The NMR technique implies some evident technical limitations as for instance, the sample preparation (introduction in NMR tubes or rotor), but NMR is also dependant on the intrinsic sample properties such as heterogeneity and multi-phase liquid/solid nature which induce some susceptibility effects and the presence of anisotropic interactions. The feasibility of the P NMR spectroscopy to study cheeses has been first probed on the milk powder, main component of these dairy products. 

Casein micelle proteins are primarily a8i-, as2-, /3-, and -caseins in approximate proportions 3 .8 3 1. asi-Casein has eight or nine phosphate groups, depending on the genetic variant. aS2-Casein is the most hydrophilic of the caseins. It has two disulfide bonds which, by severe heat treatment, can be caused to interact with those of /3-lactoglobulin. It also has 10 to 13 phosphate groups and is very sensitive to the calcium ion concentration (Kinsella 1984 Swaisgood 1982). 

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