Chymosin-Catalyzed Hydrolysis of Glycosylated and Nonglycosylated Bovine κ-Casein Adsorbed on Latex Particles

Abstract

Bovine κ-casein A has been fractionated into its nonglycosylated and variously glycosylated forms. The nonglycosylated fraction and a highly glycosylated fraction were allowed to adsorb to negatively charged polystyrene latex particles, and the hydrodynamic thickness of the protein layers was determined by photon correlation spectrometry. After the coated latex was washed, the proteolytic enzyme chymosin, which cleaves κ-casein at a specific peptide bond, was added. The diameter of the coated particles decreased due to the release of the negatively charged caseinomacropeptide part of the protein, leaving the hydrophobic para-κ-casein still adsorbed to the latex. In the presence of calcium ions, this decrease in diameter was followed by an increase as the latex particles aggregated. The rate of the aggregation was dependent on the concentration of calcium ions, with the aggregation of the nonglycosylated κ-casein-coated latex being significantly more sensitive. Since the caseinomacropeptide portion of the protein molecule is the part that contains the glycosylation sites, it would appear that the difference is due to formation of calcium bridges between the unhydrolyzed molecules remaining on the latex surface. The results are discussed in terms of the suitability of this system as a model for the chymosin-catalyzed aggregation of casein micelles in milk, the first step in the cheesemaking process.