Constitutive expression,purification and characterization of bovine prochymosin in <Emphasis Type="Italic">Pichia pastoris</Emphasis> GS115

Chymosin can specifically break down the Phe105–Met106 peptide bond of milk κ-casein to form insoluble para-κ-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 κ-casein. It was stable at 25–50°C and had optimal activity at 37°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.