Casein components

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). 

Because they occur as large aggregates, micelles, most (90-95%) of the casein in milk is sedimented by centrifugation at 100000 g for 1 h. Sedimentation is more complete at higher (30-37°C) than at low (2°C) temperature, at which some of the casein components dissociate from the micelles and are non-sedimentable. Casein prepared by centrifugation contains its original level of colloidal calcium phosphate and can be redispersed as micelles with properties essentially similar to the original micelles. 

Based on the primary structure of the major /3-casein component,... 

The configuration and association of the minor /3-caseins have not been investigated to any extent, but from their primary structure one can conclude that /3-casein X-4P-(f 1-28), -(f 1-105), and -(f 1-107) and /3-casein X-lP-(f 29-105) and -(f 29-107) are more hydrophilic than the main /3-casein component and that /3-casein X-lP-(f 29-209) and /3-casein-(f 106-209) and -(f 108-209) will be more hydrophobic. The hy-drophobicities of the last three caseins are the highest of all the caseins, with calculated Bigelow hydrophobicities of 1386 to 1511 (Swais-good 1973). Therefore, we would expect these to demonstrate a strong temperature-dependent association /3-casein B-(f 106-209) and -(108— 209) are insoluble at pH 8.0 and 25°C but soluble at 3°C, while 0-casein A-(f 108-209) is soluble at both temperatures. Sedimentation studies... 

While salt or buffer gradients at constant pH were employed for elution in all of the above procedures, Vreeman et al. (1977) demonstrated an improved separation of the K-casein components on DEAE-... 

Klostermeyer (1975) used an activated thiol-Sepharose 4B column with Tris-HCl buffer containing dithiothreitol to separate the K-and aa2-caseins from the aai-and /3-caseins in whole casein. More recently, Creamer and Matheson (1981) studied the fractionation of casein by hydrophobic interaction chromatography on octyl- or phenyl-Sephar-ose CL-4B columns. The whole casein was adsorbed onto the column from dilute phosphate buffers. A gradient of 0 to 40% ethylene glycol followed by 6 M urea was employed to desorb the protein. Optimum separation was obtained with an increasing urea gradient. Under all conditions, the major /3-casein component was eluted more readily than the asi-casein in spite of its higher hydrophobicity. 

Pujolle, J., Ribadeau-Dumas, B., Gamier, J. and Pion, R. 1966. A study of kappa-casein components. I. Preparation. Evidence of a common C-terminal sequence. Biochem. Biophys. Res. Commun. 25, 285-290. 

Doi, H., Ibuki, F. and Kanamori, M. 1979. Interactions of x-casein components with (island ds2-caseins. Agri. BioL Chem. 43, 1301-1308. 

Preparing a 1% casein solution is somewhat tricky because dried casein preparations are typically only sparingly soluble in neutral aqueous systems. This method, which uses alkaline conditions to prepare a casein sol, should work for most purposes. The casein substrate solutions used for peptidase assays are often more accurately described as substrate suspensions, since these preparations are typically not true solutions—the casein component being of a colloidal nature. 

Yoshikawa, M., Sasakai, R., and Chiba, H. 1981. Effect of chemical phosphorylation of bovine casein components on the properties related to casein micelle formation. Agric. Biol. Chem. 45,... 

The arrangements of the various casein components in the casein micelles are not known with certainty. However, a number of models have been proposed for the micelle structure, and this matter is currently die subject of much debate (144). 

The major casein components differ in their behavior toward the calcium ion (145). The aB group of caseins are very susceptible to precipitation by the calcium ion. Beta casein undergoes a temperature-dependent precipitation in the presence of calcium ion, being soluble at 4°C and insoluble at 35°C. -Casein, discovered by Waugh and von Hippel (146) in 1956, is resistant to precipitation in the presence of calcium ion, and when mixed with appropriate portions of s- and -casein, it forms complexes or micelles with these caseins and stabilizes... 

K-Casein has been isolated and studied in some detail (2). It is the only casein component containing carbohydrate. However, the K-casein is heterogeneous with regard to the carbohydrate content some molecules contain carbohydrate, and others do not (151), In addition, K-casein exists as polymorphic genetic variants involving amino acid substitutions in the primary sequence (152). The primary structure of K-casein has been determined (153). 

In the absence of calcium ions but in the presence of other casein components the normally insoluble K-casein is apparently stabilized by the calcium-sensitive caseins (157). Thus calcium ions are required to coagulate whole casein after treatment with rennin. Therefore in the native milk system the micelle-stabilizing power of K-casein is specifically destroyed by rennin, and in the presence of calcium ions in milk a coagulum is formed (2). This offers a dramatic example of how the functionality of an entire protein system can be altered by specific proteolytic action on a component of that system. 

The proteases currently used as coagulants in the cheese industry are known to proteolyze the various casein components. Rennin, porcine pepsin, and proteases from Endothia parasitica, Mucor miehei, and Mucor pusillus readily attack 8-> / -, and K-caseins (195, 199). 

Kaminogawa, S., Yamauchi, K., Miyazawa, S., and Koga, Y. (1980). Degradation of casein components by acid protease of bovine milk. J. Dairy Sci 63, 701-704. 

Sodium caseinate Free casein components Highly soluble and very surface active Emulsions and foam products... 

Rennet casein is somewhat different from acid-precipitated casein and it is a relatively minor industrial product. It is obtained from skim milk by the action of the enzyme rennin (chymosin) around pH 6.6. The enzyme splits up the casein component and this destabilises the milk micelles. This is followed by aggregation and precipitation of calcium caseinate along with the calcium phosphate which is present. 

Phosphopeptides can be separated on an ion-exchange column. Mixtures of phosphoserine, phosphothreonine and phosphotyrosine, for example, and mixtures of other bio materials such as inositol phosphates can be successfully estimated. Casein components can be separated and estimated by liquid chromatography [46-50]. 

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