Constitutive expression, purification and characterization of bovine prochymosin in Pichia pastoris GS115.

摘要

Chymosin can specifically break down the Phe105-Met106 peptide bond of milk kappa-casein to form insoluble para- kappa-casein, resulting in milk coagulation, a process that is used in making cheese. In this study, in order to obtain an alternative milk coagulant which is safe and efficient, and simultaneously can produce cheese with a good taste, bovine prochymosin B was chosen and constitutively expressed to a high level in Pichia pastoris. The recombinant chymosin was expressed mainly as a secretory form, and it exhibited milk-clotting activity. It was purified by ammonium sulfate fractionation, anion exchange, followed by cation exchange chromatography. A final yield of 24.2% was obtained for the purified enzyme, which appeared as a single band in SDS-PAGE having a molecular mass of approximate 36 kDa. Proteolysis assay showed that it specifically hydrolyzed kappa-casein. It was stable at 25-50 degrees C and had optimal activity at 37 degrees C and pH 4.0. The activity of the recombinant chymosin was activated by cations such as Mn2+, Fe3+, Mg2+ and Na+, but inhibited by K+, Co2+, Zn2+, Ni2+, and to a lesser extent by Cu2+. These results suggested that recombinant bovine chymosin is an acid milk coagulant, and it could be considered as a safe and efficient enzyme suitable for use in cheese production