K-Caseins

Casein refers to a family of proteins, namely, ttgi-, 0. 2-, p-, and k-caseins (Table 5.1). Digestion of a- and p-caseins leads to production of peptides that may bind to opioid receptors that exist in the nervous, endocrine, immime, or gastrointestinal system (Kampa et ah, 1996 Meisel, 2004). These compoimds may modulate absorption processes in the gut and can potentially affect gastrointestinal fimction through transit... 

The most abundant milk protein is casein, of which there are several different kinds, usually designated a-, (1-, and K-casein. The different caseins relate to small differences in their amino acid sequences. Casein micelles in milk have diameters less than 300 nm. Disruption of the casein micelles occurs during the preparation of cheese. Lactic acid increases the acidity of the milk until the micelles crosslink and a curd develops. The liquid portion, known as whey, containing water, lactose and some protein, is removed. Addition of the enzyme rennet (chymosin) speeds up the process by hydrolysing a specific peptide bond in K-casein. This opens up the casein and encourages further cross-linking. 

Products of Limited K-Casein Proteolysis Book 2 XXI Intern. Dairy Congr. Moscow,1982, ppl61. 

The casein responsible for colloidal stabilization of the casein micelle is K-casein. This glycoprotein has a molecular weight of 19 kDa and is composed of 169 amino acids (Swaisgood, 2003). K-Casein is special in being the only casein component that is insensitive to calcium ions (up to concentrations of 400 mM). 

The essence of the shell model is that the association constant of the first step is very much smaller than the association constants of all subsequent steps. It was assumed by de Kruif et al. (2002) that, since there is a considerable similarity in various properties of p- and K-caseins, then the k-casein micelle can also be described using the shell model. Nevertheless, Vreeman et al. (1981) had previously suggested that the structure of the... 

K-casein aggregates in aqueous medium are more complicated than those of p-casein, being composed of star-like sub-micelles, where each submicelle contains nine K-casein chains and the total degree of association may reach about 140 (Thum el al., 1987a). Payens and Vreeman (1982) used sedimentation measurements to infer a critical micelle concentration for K-casein of 0.5 mg/ml. 

Dickinson, E., Whyman, R.H., Dalgleish, D.G. (1988). Colloidal properties of model oil-in-water food emulsions stabilized separately by oisi-casein, p-casein and K-casein. In Dickinson, E. (Ed.). Food Emulsions and Foams, London Royal Society of Chemistry, pp. 40-51. 

Holt, C., Sawyer, L. (1993). Caseins as rheomorphic proteins interpretation of the primary and secondary structures of the otsi-, p- and K-caseins. Journal of the Chemical Society, Faraday Transactions, 89, 2683-2692. 

Thurn, A., Burchard, W., Niki, R. (1987a). Structure of casein micelles. I. Small-angle neutron scattering and light scattering from p- and K-casein. Colloid and Polymer Science, 265, 653-666. 

Vreeman, H.J., Brinkhuis, J.A., van der Spek, C.A. (1981). Some association properties of bovine SH-K-casein. Biophysical Chemistry, 14, 185-193. 

Snoeren, T.H., Payens, T.A.J., Jeunink, J., Both, P. (1975). Electrostatic interaction between K-carrageenan and K-casein. Milchwissenschaft, 30, 393-396. 

Casein is low in sulphur (0.8%) while the whey proteins are relatively rich (1.7%). Differences in sulphur content become more apparent if one considers the levels of individual sulphur-containing amino acids. The sulphur of casein is present mainly in methionine, with low concentrations of cysteine and cystine in fact the principal caseins contain only methionine. The whey proteins contain significant amounts of both cysteine and cystine in addition to methionine and these amino acids are responsible, in part, for many of the changes which occur in milk on heating, e.g. cooked flavour, increased rennet coagulation time (due to interaction between /Mactoglobulin and K-casein) and improved heat stability of milk pre-heated prior to sterilization. 

Casein may be coagulated and recovered as rennet casein by treatment of milk with selected proteinases (rennets). However, one of the caseins, K-casein, is hydrolysed during renneting and therefore the properties of rennet casein differ fundamentally from those of acid casein. Rennet casein, which contains the colloidal calcium phosphate of milk, is insoluble in water at pH 7 but can be dissolved by adding calcium sequestering agents, usually citrates or polyphosphates. It has desirable functional properties for certain food applications, e.g. in the production of cheese analogues. 

Disulphide bonding. The two principal caseins, asl and / , contain no cysteine or cystine but the two minor caseins, as2 and k, each contains two cysteines per mole which normally exist as intermolecular disulphide bonds. Under non-reducing conditions, ocs2-casein exists as a disulphide-linked dimer (previously known as as5 casein) while K-casein exists as a series of disulphide-linked molecules ranging from dimers to decamers. 

Variations in the degree of glycosylation. K-Casein is the only one of the principal milk proteins which is normally glycosylated but, as discussed on... 

All the caseins have a very high proline content 17, 10, 35 and 20 Pro residues per mole of asl-, as2-, / - and K-caseins, respectively (out of a total of 199,207,209 and 169 residues, respectively). Such high levels of proline... 

Figure 4.12 Amino acid sequence of bovine k-casein, showing the amino acid substitutions in genetic polymorphs A and B and the chymosin cleavage site, Sites of post-translational phosphorylation or glycosylation are italicized (from Swaisgood, 1992).

Figure 4.13 Distribution of charged residues (pH 6-7), proline ( ) and cysteine (S) in as 1 aS2-, / - and K-caseins. a, Location of oligosaccharide moieties and b, chymosin cleavage site in K-casein (from Walstra and Jenness, 1984).

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