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J. Dairy Sci. 2009. 92:1338-1353. doi:10.3168/jds.2008-1711
© 2009 American Dairy Science Association ®
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Review of the chemistry of {alpha}S2-casein and the generation of a homologous molecular model to explain its properties1

H. M. Farrell, Jr.*,2, E. L. Malin*, E. M. Brown* and A. Mora-Gutierrez{dagger}

* Dairy Processing and Products Research Unit, USDA, Agricultural Research Service, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, PA 19038
{dagger} Cooperative Agricultural Research Center, Prairie View A&M University, Prairie View, TX 77446

2 Corresponding author: harsuefar{at}aol.com

{alpha}S2-Casein ({alpha}S2-CN) comprises up to 10% of the casein fraction in bovine milk. The role of {alpha}S2-CN in casein micelles has not been studied in detail in part because of a lack of structural information on the molecule. Interest in the utilization of this molecule in dairy products and nutrition has been renewed by work in 3 areas: biological activity via potentially biologically active peptides, functionality in cheeses and products, and nutrition in terms of calcium uptake. To help clarify the behavior of {alpha}S2-CN in its structure-function relationships in milk and its possible applications in dairy products, this paper reviews the chemistry of the protein and presents a working 3-dimensional molecular model for this casein. The model was produced by threading the backbone sequence of the protein onto a homologous protein: chloride intracellular channel protein-4. Overall, the model is in good agreement with experimental data for the protein, although the amount of helix may be over-predicted. The model, however, offers a unique view of the highly positive C-terminal portion of the molecule as a surface-accessible area. This region may be the site for interactions with {kappa}-carrageenan, phosphate, and other anions. In addition, most of the physiologically active peptides isolated from {alpha}S2-CN occur in this region. This structure should be viewed as a working model that can be changed as more precise experimental data are obtained.

Key Words: casein structure • casein molecular model • protein functionality • value-added milk product






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