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item Qi, Phoebe

Submitted to: American Chemical Society National Meeting
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/12/2005
Publication Date: 2/25/2006
Citation: Qi, P.X.. 2006. Importance of c-terminal region of bovine b-casein. American Chemical Society Symposium Series 935. p.71-92.

Interpretive Summary: Milk and dairy products provide a major source of calcium in our diet. It is still not fully understood, though, how calcium, in the form of calcium phosphate, remains suspended in milk. At the concentrations present in milk, calcium phosphate should precipitate, but a specific class of milk proteins, the caseins, prevents this from occurring. Caseins hold the calcium phosphate tightly enough to keep it suspended in milk, but not so tight as to prevent its controlled release during digestion of milk. To better understand how the caseins regulate calcium phosphate binding or releasing, we studied B-casein, one of the major calcium-carrying proteins. Previous theories on calcium carrying by caseins proposed that the process was random and did not depend on the caseins doing anything specific. Our studies now show that the B-casein molecule has a specific shape and this shape determines its calcium carrying function and its ability to interact with other caseins. These studies not only show clearly how the calcium is carried and released, but also suggest that portions of the molecule could be prepared and used as nutritional additives to bind and release other vital minerals.

Technical Abstract: Bovine B-casein (B-CN) with the C-terminal truncated by chymosin digestion, B-CN-(f1-192), was examined using circular dichroism (CD) under various temperature and solvent conditions. The effect of C-terminal deletion on the structure and stability of the parent protein, B-casein (B-CN), is analyzed and discussed. Analytical ultracentrifugation results showed reduced degree of self-association in B-CN-(f1-192) compared to whole B-casein. CONTIN/LL analysis of the CD data revealed clear changes in the overall secondary structural content in B-CN-(f1-192) in the temperature range studied (5 to 70 deg C). Results obtained in this work suggest significant secondary structure disruption of B-casein upon its C-terminal deletion. It has been shown that 80% trifluoroethanol (TFE) at low temperatures (plus/minus 25 deg C) can effectively induce a-helical content in B-CN-(f1-192), to the same level as that of B-CN. In addition, sodium dodecyl sulfate (SDS) at greater than micellar concentration is capable of refolding B-CN-(f1-192). Temperature dependence of these induced structures was also studied. Furthermore, binding experiments with the common hydrophobic probe-8-Anilino-1-naphthalene sulfonate (ANS) illustrated that B-CN-(f1-192) is ineffective in forming a complex with ANS. These results clearly demonstrated that the tail peptide, B-CN-(f193-209) is important in maintaining the hydrophobic core of B-CN.