The Role of (alpha)s1-Casein in the formation of the Casein Micelle

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Title: The Role of (alpha)s1-Casein in the formation of the Casein Micelle
Author: Chen, Chinchi
Abstract: Caseins are the major protein components of milk. Bovine caseins have been the most thoroughly studied and serve as the principal area of research for the dairy industry. Bovine caseins are composed of several major polypeptide families, (alpha)s1-, (alpha)s1-, (beta)- and K-caseins. The (alpha)s1-caseins constitute 30.6% of bovine milk protein. The distribution of polar and apolar amino acids within each (alpha)s1 -casein molecule tends to be asymmetric, yielding protein molecules with hydrophobic and hydrophilic domains that give the caseins excellent functional properties. Moreover, the phosphoserine groups of (alpha)s1 -casein associate with calcium phosphate complexes in milk to form loosely ordered aggregates, termed casein micelles. This conformational feature renders (alpha)s1 -casein with great capacity fkor the delivery of a highly nutritious solution of protein, carbohydrate, phosphate, calcium and other minerals. In this study, we have investigated the physical properties of (alpha)s1casein under different pHs and/or various concentration of calcium. The fluorescence studies suggested that a decrease in pH and an increase of calcium concentration switch the surrounding environment of Trp 164 from hydrophilic to hydrophobic. The results, obtained from the studies of viscosity, phase separation and isoelectric point calculations, suggested that phosphorylated (alpha)s1 -casein forms micelles at a pH of -5.5. This is consistent with the pH where milk micelles form and; therefore, lends evidence to the hypothesis that the main component of the micelle framework in milk is (alpha)s1 -casein. In conjunction and our results with the predicted secondary structure, we proposed that, at a pH -5.5, the negative charges in the hydrophobic domain of (alpha)s1 -casein are neutralized and the electrostatic repulsion of the hydrophilic domain is reduced. Therefore, the protein-protein interaction is promoted and micelle formation is favored.
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Date: 1997-08

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