K-Caseins in milk

The appealing feature of this model is the occurrence of K-casein on the surface of the micelle where it would be available for the action of proteolytic enzymes. Presumably, proteases such as rennin would cleave the exposed macropeptide from the K-casein on the micellar surface yielding patches of p-K-casein on the surface of the micelle. After a certain portion of the coat units is modified, adjacent micelles would cohere, leading to the coagulation of the milk. Approximately 30% of the K-casein in milk is so-called "soluble K-casein (176) and is also... 

MiraUes, B., Rothbaner, V., Manso, M., Amigo, 1., Krause, I., and Ramos, M., Improved method for the simnltaneons determination of whey proteins, caseins, and para-k-casein in milk and dairy prodncts by capillary electrophoresis. J. Chromatogr. A, 915, 225, 2001. 

Addition of K-casein to milk increases stability in the pH range of the HCT minimum. 

Most current models put K-casein on the outer casein micelle surface (Heth and Swaisgood 1982 McMahon and Brown 1984A Shahani 1974). This allows the possibility that heat-induced coagulation of milk is the result of serum proteins interacting with K-casein on the micelle surface and with each other to interconnect micelles. The observation that chymosin cannot release macropeptides from K-casein in heated milk (Morrissey 1969 Shalabi and Wheelock 1976,1977) suggests that... 

Chabance, B., Jolles, P., Izquierdo, C., Mazoyer, E., Francoual, C., Drouet, L., and Fiat, A.-M. 1995. Characterization of an antithrombotic peptide from K-casein in newborn plasma after milk ingestion. Br. J. Nutr. 73, 583-590. 

We chose to prepare 14C-methyl-K-casein (M-k-C) as a tracer because of the important role of K-casein in stabilizing casein micelles (8) and because K-casein is known to participate in heat-induced interactions with whey proteins, thereby influencing the heat stability of milk (9). The reductive methylation radiolabeling procedure used low concentrations of reagents (10) and resulted in M-k-C containing approximately 1 fiinol of 14C-methyl groups for every micromole of protein monomer (about 3 /xCi/mg). When tracer M-k-C was added to skim milk, and trichloroacetic acid was added to a concentration of 2%, about 1% of the radioactivity remained soluble. After clotting of the milk with excess... 

K-casein also contains two Cys residues per monomer subunit and is thus capable of interacting with the whey proteins, e.g., mainly g-lactoglobulin, via the disulfide interchange mechanism at temperatures at or above 65°C. This latter phenomenon is believed to be important in providing colloidal stability to the milk casein micelle system, as well as to the whey proteins, in high temperature processed milk products. It has also been postulated that this latter interaction with g-lactoglobulin may alter the availability of K-casein in the micelle, and thus has a detrimental effect upon the cheese making properties of milk (4). 

Reduction of microorganisms in milk before cheese production must be achieved in such a way that the functionality of the milk proteins is not affected. Heat-resistant spores, such as Clostridium tyrobutyricum and C. sporogenes can cause severe spoilage of the cheese by late fermentation that can result in the production of H2 and CO2 gases, and unpleasant smelling fermentation products [102]. Although heat sterilization reduces these spores, because of the heat-induced complexation between 3-Lg and K-casein, UHT milk normally does not form a rennet gel and consequently could not be used efficiently for cheesemaking [87]. 

The residues Phe and Met are not intrinsically essential for chymosin action. Replacement of Phe by Phe (NO2) or cyclohexylamine reduces k J K 3- and 50-fold, respectively (Visser et al, 1977). Oxidation of Metioe reduces kcJKm 10-fold but substitution of norleucine for Met increases this ratio 3-fold. Neither porcine nor human K-casein possesses a Phe-Met bond [both have a Phe-Ile bond at this position (Brignon et al, 1985 Chobert et al, 1976 Fiat et al, 1977)], yet both are readily hydrolyzed by calf chymosin, although more slowly than bovine K-casein in contrast, porcine milk is coagulated more effectively than bovine milk by porcine chymosin (Foltmann, 1987). Thus, the sequence around the Phe-Met bond, rather than the bond itself, contains the important determinants for hydrolysis. The particularly important residues are Ser jo4, the hydrophobic residues Leuio3 and Ileiog, at least one of the three histidines (residues 98, 100, or 102), some or all of the four prolines (residues 99, 101, 109, and 110), and Lysiii. 

Chatchatee P, Jarvinen KM, Baedina L, Vila L, Beyer K, Sampson HA (2001). Identification of IgE and IgG binding epitopes on 3- and K-casein in cow s milk allergic patients. Clin. Exp. Allergy, 31 1256-1262. 

Hydrolysis of the casein in milk liberates a tetrapeptide, H2N-Tyr-Pro-Phe-Pro(OH), whose moderate analgesic activity is exerted on the mu, but not on the delta, receptor (Chang et al., 1981). The (apparent) hormone, dynorphin acts on the kappa (k) receptor. 

Caseins in milk do not occur as monomers, but are aggregated into casein complexes and spherical particles called micelles (see Section 7.6.3.2.1). Caseins P and y can easily associate into polymeric structures at about 20 °C, while at temperatures <8 °C they dissociate back to the monomers. The aggregation of molecules of a -, P- and K-caseins into micelles occurs at temperatures >5°C. Molecules of a -, P- and K-caseins are first arranged into particles... 

These protein forming micelles, namely casein is the major protein fraction in bovine milk (about 80 % of the total milk protein). Several components may be identified, namely Og and 0 2-caseins, p-casein and K-casein. A protolytic breakdown product of p-casein is y-casein. Similar to ovalbumin, caseins are phosphoproteins. Large spherical casein micelles are formed by association of a -, p- and K-casein in the presence of free phosphate and calcium ions. The molecules are held together by electrostatic and hydrophobic interactions. The Ug- and p-caseins are surrounded by the flexible hydrophilic K-casein which forms the surface layer of the micelle. The high negative charge of the K-casein prevents collapse of the micelle by electrostatic repulsion. The micelle diameter varies between 50 and 300 run. 

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. 

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