Micelle Stability: κ-Casein Structure and Function

Abstract

The stability of the casein micelle is dependent on the presence of kappa-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-catalyzed disulfide interchange reactions with the whey proteins during heat treatments and, after rennet cleavage, in the facilitation of micelle 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 crystallized. Instead, algorithms were used to predict selected secondary structures and circular dichroism spectroscopy on kappa-CN and the macropeptide released by chymosin. Three peptides were synthesized to cover the chymosin-sensitive site (His98-Lys111), the region in the macropeptide that could be helical (Pro130-Ile153), and the region between. Nuclear magnetic resonance spectroscopy showed that the peptide His98-Lys111 was probably a beta-strand with tight turns at each end. This hypothesis was confirmed by a study of the molecular dynamics showing that the C variant of kappa-CN interacted less strongly with chymosin; consequently, the slow renneting time of milk that contains this protein was explainable. Both circular dichroism and nuclear magnetic resonance indicated that the peptide Pro130-Ile153 was probably helical under normal physiological conditions. A preliminary study using nuclear magnetic resonance showed that the intervening peptide had no discernible secondary structure. Consequently, most of the beta-sheet structure of kappa-CN is likely in the para-kappa-CN region.

Allergenicity of Milk Proteins

“…κ-CN is most likely more structured than αs-and β-CN and contains specific disulfide bonds [53][54]. The β-CN can rearrange on heating [55]. κ-CN is sensitive to proteolysis and is hydrolyzed by chymosin to produce para-κ-CN and caseinomacropeptide in the cheese-making process.…”

“…Allergenic potential sequences remain in cooked cheeses. κ-CN is essential to the stability of CN micelles [55]. …”

Comparison of the chaperon activity of glycerol and α-casein on amyloid formation of κ-casein in the presence of glycine and arginine

“…Fibril formation in κ-casein is induced by quaternary structural destabilization caused by the reduction of intermolecular disulfide linkages. The resulting monomer appears to be highly susceptible to temperature-dependent fibril formation (Creamer 1998;Thorn et al 2005).…”

“…κ-Casein on the surface of the micelle functions as an interface between the hydrophobic caseins of the micelle interior and the aqueous environment. It is also crucially important in stabilizing the micelle (Eigel et al 1984;Walsta & Jenness 1984;Creamer 1991Creamer , 1998. κ-Casein normally has cross exchanges through S-S bonds between 3 to 8 monomer subunits.…”

RecombinantLactococcus lactisfails to secrete bovine chymosine

“…Chymosin releases a hydrophilic kcasein glycopeptide, and the insoluble para-k-casein peptide aggregates with other caseins to form an insoluble clot via a Ca 2C dependent process. 2,3 Chymosin is one of the most important enzymes in the cheese-making process due to its high specificity for casein. 4,5 Commercial preparations of calf rennet contain two forms of chymosin, denominated chymosin A and B. Chymosin A slightly exceeds chymosin B in proteolytic activity, whereas chymosin B is more stable at low pH.…”