Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/29/2005
Publication Date: 7/14/2005
Citation: Malin, E., Brown, E., Wickham, E., Farrell, H. 2005. Contributions of terminal peptides to the associative behavior of the alpha-S1-casein. Journal of Dairy Science. 88(7):2318-23-28.
Interpretive Summary: The properties of dairy products depend upon the structure of the casein micelles, but micelle formation is not well understood. Computer modeling of a fragment from the N-terminal of alpha-s1 casein showed that it can form a strong link with an identical fragment and that two unbroken alpha-s1 casein chains can interact in the same way. In similar studies, a much larger fragment from the C-terminal of alpha-s1 casein also formed links with an identical fragment. Comparison of the structural stabilities of the N- and C-terminal peptides suggests that casein micelle formation in the mammary gland may begin with the linking of two alpha-s1 casein chains at the C-terminals, followed by stabilization involving additional alpha-s1 casein interchain links at the N-terminals, as well as binding with calcium and phosphate. This framework would provide structural stabilization of the micelles in milk and dairy products.
Technical Abstract: The N- and C-terminal segments of bovine alpha-s1 casein-B (f1-23 and f136-196) were characterized under conditions that promoted or inhibited self-association to determine the relative contributions of each fragment to the interaction of alpha-s1 casein with itself or with other caseins. In earlier studies of f1-23, nuclear magnetic resonance (NMR) data and circular dichroism (CD) spectra showed that its conformation was thermostable between 10 and 60 degrees C. In contrast, NMR studies of f136-196 inidicated temperature sensitivity between 10 and 60 degrees C, as did near-UV and far-UV CD data, suggestinig a molten globule-like structure at higher temperatures. To compare the effects of temperature on conformational attributes of alpha-s1 casein and its terminal peptides, additional CD studies were conducted over a broader temperature range (10 to 70 degrees C). The far-UV CD spectra indicated little temperature sensitivity for alpha-s1 casein and the N-terminal peptide remained thermostable. During molecular dynamics simulations, the N-terminal peptide conformation did not change significantly, but the conformation of the C-terminal peptide (f136-196) was dramatically altered. These changes are correlated with the thermal instability observed by both CD and NMR in f136-196. Analytical ultracentrifugation studies of the self-association reactions of genetic variants A, B, and C of alpha-s1 casein showed that at 37 degrees C the associative state is primarily dimeric; the amounts of higher order polymers significantly decreased when temperature was increased from 20 to 37 degrees C. In all three genetic variants, the C-terminal portion of the whole molecule showed thermal instability with respect to aggregation to higher polymers, confirming the predictions of CD data and molecular dynamics simulations. The temperature dependency of these conformational changes suggests a possible function for alpha-s1 casein in facilitating casein-casein interactions in casein micelle formation.