Micelle stability: kappa-casein structure and function.
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.
Creamer
L K
LK
Food Science Section, New Zealand Dairy Research Institute, Palmerston North, New Zealand.
Plowman
J E
JE
Liddell
M J
MJ
Smith
M H
MH
Hill
J P
JP
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002364:Caseins / Q000737:chemistry* / Q000378:metabolism*; D012085:Chymosin / Q000378:metabolism; D004355:Drug Stability; D006801:Humans; D009682:Magnetic Resonance Spectroscopy; D008823:Micelles; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary
D002364:Caseins; D008823:Micelles; D012085:Chymosin