Submitted to: Journal of Protein Chemistry
Publication Type: Abstract Only
Publication Acceptance Date: 7/15/2004
Publication Date: 8/1/2004
Citation: Qi, P.X., Wickham, E.D., Farrell, H.M.,Jr. 2004. Thermal and alkaline denaturation of bovine-b casein. (abstract). The Protein Journal. 1572-3887:/04/0800-0389/0. P. 389-402. Interpretive Summary: The unique biological functions of milk proteins arise directly from their three-dimensional shape, which in turn determines the properties of products made from them. B-casein, one of the major calcium carrying proteins in milk, was studied to better understand its function in milk and products made from milk. Previous theories proposed that B-casein had a random or 'spaghetti'-like shape and as a result, interactions with other milk proteins were also nonspecific. 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 in milk. Our results not only show clearly how B-casein molecule functions as an effective calcium carrier, but also suggest that portions of the molecule could be prepared and used as nutritional additives to transport other vital minerals.
Technical Abstract: Thermal and chemical-induced unfolding of bovine B-casein at neutral pH (6.75) and low ionic strength (I=50 mM) was studied. Contrary to globular proteins, B-casein appears to adopt more compact conformation at elevated temperature, which provides further evidence to support our previous hypothesis of cold denaturation. Far- and near-UV circular dichroism (CD) spectra were used to study the unique unfolding behavior of B-casein. Intrinsic fluorescence of single Trp (position 143) was monitored as a function of temperature and denaturant concentration to gain better insight on the molecular response on a tertiary level. We have shown in this work that urea and guanidine hydrochloride (GdmCl) disrupt secondary structure of B-casein through different molecular mechanisms. Temperature and denaturant effects on the tertiary conformation of B-casein were significantly different. Urea denaturation of Trp-143 fluorescence suggested a mostly but not completely solvent exposed environment for that portion of the molecule. We hypothesize that B-casein exists as a largely but not entirely disordered conformation under physiological conditions (25oC and neutral pH). At lower temperature (below 10oC), it appears to be even more flexible. At elevated temperature (above 37oC), however, B-casein is likely to assume more compact conformation, leading to self-association and eventually the formation of micelles with other caseins.