MICELLE STABILITY - KAPPA-CASEIN STRUCTURE AND FUNCTION

The stability of the casein micelle is dependent on the presence of ka ppa-casein (CN) on the surface of the micelle where it functions as an interface between the hydrophobic caseins of the micelle interior and the aqueous environment. kappa-Casein is also involved in thiol-catal yzed disulfide interchange reactions with the whey proteins during hea t treatments and, after rennet cleavage, in the facilitation of micell e coagulation. These functions of kappa-CN are regulated by the three- dimensional structure of the protein on the micelle surface. The usual means of determining structure are not available for kappa-CN because this protein is strongly self-associating and has never been crystall ized. Instead, algorithms were used to predict selected secondary stru ctures and circular dichroism spectroscopy on kappa-CN and the macrope ptide released by chymosin. Three peptides were synthesized to cover t he chymosin-sensitive site (His(98)-Lys(111)), the region in the macro peptide that could be helical (Pro(130)-Ile(153)), and the region betw een. Nuclear magnetic resonance spectroscopy showed that the peptide H is(98)-Lys(111) was probably a beta-strand with tight turns at each en d. This hypothesis was confirmed by a study of the molecular dynamics showing that the C variant of kappa-CN interacted less strongly with c hymosin; consequently, the slow renneting time of milk that contains t his protein was explainable. Both circular dichroism and nuclear magne tic resonance indicated that the peptide Pro(130)-Ile(153) was probabl y helical under normal physiological conditions. A preliminary study u sing nuclear magnetic resonance showed that the intervening peptide ha d no discernible secondary structure. Consequently, most of the beta-s heet structure of kappa-CN is likely in the para-kappa-CN region.