Genetic Engineering and Protein Engineering on Chymosin and Mucor Rennin

Abstract

Chymosin (calf rennin), an enzyme obtained from the calf stomach has been used as a milk-coagulant in the cheese industry. Chymosin, a member of the aspartic proteinases, cleaves at a specific position (Phel05-Metl06) of κ-casein, as a result of which, milk micelles are destabilized, leading to the clotting of milk. Chymosin is characterized by its high milk-clotting activity and very weak proteolytic activity. Recent success in X-ray crystallographic analysis has revealed its bilobal structure composed of two topologically similar domains rich in β-structures [1]. At their junction is located the substrate-binding cleft and at the bottom two catalytic aspartyl residues, Asp32 and Asp215, are contained. Despite a wide variety of catalytic properties in many aspects, members of the aspartic proteinases possess well-conserved tertiary structure and well-conserved amino acid sequences covering the two catalytic aspartyl residues.

Chymosin and Mucor pusillus rennin are aspartic proteinases and important as milk-coagulants in the cheese industry. A system for production of chymosin in Escherichia coli cells and its refolding into the active form was established. A Saccharomyces cerevisiae system for production of Mucor rennin was also established. Mucor rennin was efficiently excreted by yeast as a heavily glycosylated form. Glycosylation affected both the secretion and the enzyme properties. By the use of the secretion-signal of Mucor rennin, pro-urokinase and human growth hormone (hGH) were excreted by yeast. Generation of a Lys-Arg linker (a KEX2-recognition sequence) between the preprosequence and the hGH-coding sequence led to extracellular production of mature hGH. Protein engineering on Mucor rennin for the purpose of its practical improvement as a milk-coagulant is also described.

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