Micelle Stability: -Casein Structure and Function

¹ Food Science Section, New Zealand Dairy Research Institute, Palmerston North, New Zealand
² Wool Research Organisation of New Zealand, Christchurch, New Zealand
³ Chemistry Department, James Cook University, Cairns, Queensland, Australia

The stability of the casein micelle is dependent on the presence of -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. -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 -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 -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 -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 ß-strand with tight turns at each end. This hypothesis was confirmed by a study of the molecular dynamics showing that the C variant of -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 ß-sheet structure of -CN is likely in the para--CN region.

Key Words: casein micelle • -casein structure • glycomacropeptide structure

Submitted on June 23, 1997
Accepted on March 16, 1998

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