Casein self association

Figure 6.3 Representation of casein self-association structures according to die simple copolymer model (a) p-casein, (b) asi-casein. Reproduced from Home (1998) with permission.

An additional example of how enzymes can affect the physical properties of purified proteins relates to the enzymic dephosphorylation of /3-casein (4). Bovine /3-casein contains five phosphate groups per monomer as phosphoseryl residues. Purified /3-casein from bovine milk was dephosphorylated by a phosphoprotein phosphatase. Both the native /3-casein and 65% dephosphorylated /3-casein self-associated to form polymers at 35°C. However, the dephosphorylated /3-casein had a larger sedimentation coefficient (S35 22.5) than that of the native /3-casein (S35 18.2). Also the sedimentation pattern of the dephosphorylated /3-casein was more polydisperse than the hypersharp pattern of the native /3-casein. These properties were accentuated with 95% dephosphorylated /3-casein. The decrease in the negative charge caused by the loss of phosphate apparently favors the self-association of /3-casein resulting from hydrophobic bonding. 

Euston, S.R., Home, D.S. (2005). Simulating the self-association of caseins. Food Hydrocolloids, 19, 379-386. 

Payens, T.A.J., Brinkhuis, J.A., van Markwijk, B.W. (1969). Self-association in nonideal systems. Combined light scattering and sedimentation measurements in p-casein solutions. Biochimica etBiophysica Acta, 175, 434 137. 

In milk powders, the caseins are the principal water sorbants at low and intermediate values of aw. The water sorption characteristics of the caseins are influenced by their micellar state, their tendency towards self-association, their degree of phosphorylation and their ability to swell. Sorption isotherms for casein micelles and sodium caseinate (Figure 7.14) are generally sigmoidal. However, isotherms of sodium caseinate show a marked increase at aw between 0.75 and 0.95. This has been attributed to the... 

Yoshikawa, M., Tamaki, M., Sugimoto, E., Chiba, H. Effect of dephosphorylation on the self-association and the precipitation of P-Casein. Agric. Biol. Chem. 38, 2051 (1974)... 

The weak physical forces that hold together self-assembled nanoparticles are, of course, susceptible to disruption under the influence of thermodynamic and/or mechanical stresses. Hence some workers have investigated ways to reinforce nanoscale structures via covalent bonding. For instance, improved stability of protein nanoparticles, in particular, casein micelles, can be achieved by enzymatic cross-linking with the enzyme transglutaminase, which forms bonds between protein-bound glutamine and lysine residues. By this means native casein micelles can be converted from semi-reversible association colloids into permanent nanogel particles (Huppertz and de Kruif, 2008). 

Milk is a natural colloidal dispersion that contains casein micelles, self-assembled protein associates with a diameter of about 200 nm [20]. The casein micelles are protected against flocculation by an assembly of dense hairs (often called a brush ) at their surfaces. Polymer brushes can thus provide steric stabilization of colloids. For millennia, man used the fact that milk flocculates and gels when it is acidified, as in yogurt production. Below pH = 5 macroscopic flocculation of the casein micelles in milk is observed [21]. This means that the interactions between casein micelles change from repulsive to attractive. The explanation is that acidification leads to collapse of the casein brushes [22]. In cheese-making the steric stabilization is removed by enzymes, which induce gelation into cheese curd. 

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