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Title: IMPLICATION OF C-TERMINAL DELETION ON THE STRUCTURE AND STABILITY OF BOVINE B-CASEIN

Author
item Qi, Phoebe
item Wickham, Edward
item Piotrowski, Edwin
item Fagerquist, Clifton - Keith
item Farrell Jr, Harold

Submitted to: Protein Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/18/2005
Publication Date: 11/10/2005
Citation: Qi, P.X., Wickham, E.D., Piotrowski, E.G., Fagerquist, C.K., Farrell Jr, H.M. 2005. Implication of c-terminal deletion on the structure and stability of bovine b-casein. Protein Journal. 24(7-8):431-444.

Interpretive Summary: Dairy products are the major source of calcium in the average American diet. However, milk consumption has declined in favor of non-calcium rich beverages. This demonstrates the need for new dairy based beverages. The major proteins of milk, called caseins, are complexed with calcium and phosphorus into small packages called micelles. When rennet, cultures, and or acid are added to milk, changes occur on the surfaces of these micelles that cause the milk to change from liquid to a semi-solid state; this is a product similar to cottage cheese and is called caseinate. Raw whole caseinate has a number of desirable properties, but it is possible to break this material down into smaller particles. Selected fractions of the product may be even more valuable. To gain better insight into the use of caseins, the properties of sub-fractions of casein have been studied. Using previously generated computer models, it was possible to predict and to isolate a new enzymatically derived casein fraction. This fraction may yield a soluble milk-based calcium-carrying molecule suitable for use in clear sports beverages. This information will aid dairy processors who are searching for new methods of including dairy ingredients in nutritional dairy-based beverage products.

Technical Abstract: The properties of bovine Beta-casein with its C-terminal 20 residues removed by chymosin digestion, 1-192 fragment (f1-192), were examined and compared to the parent protein (Beta-casein). The f1-192 molecule could not form a complex with the hydrophobic probe ANS; this convincingly illustrates that it may possess many characteristics resembling some native globular proteins. Analytical ultracentrifugation results indicated a negligible degree of self-association in f1-192 compared to whole Beta-casein in the temperature range of 2 - 37 degrees C. Analysis of the CD data revealed little changes in the overall secondary structural content in f1-192 relative to the parent, i.e. 5-10% alpha-helix, 31-35% turns, and 25-29% extended sheet from 5 to 70 degrees C. The temperature dependence of the CD spectra of f1-192 showed two distinctive conformational transitions, at 12 and 34-36 degrees C, relative to four, 10, 33, 40 and 78 degrees in its parent protein. The transition at 12 degrees C may actually represent a general conformational change or cold denaturation in much similar fashion as the 10 degrees C transition in the native protein. The transition at about 5 degrees C is more likely the reflection of hydrophobic changes in the core of f1-192. Results obtained in this work suggest possible secondary structural disruption of Beta-casein upon its C-terminal deletion. Moreover, it has been demonstrated conclusively that the C-terminal tail peptide (f193-209) is essential for the self-association and ANS binding. These results may provide further insights into an alternative role in a natural cellular function for Beta-casein.