Enhancement of oxidative stability of the subtilisin nattokinase by site-directed mutagenesis expressed in Escherichia coli

Nattokinase (subtilisin NAT, NK) is a bacterial serine protease with strong fibrinolytic activity and it is a potent cardiovascular drug. In medical and commercial applications, however, it is susceptible to chemical oxidation, and subsequent inactivation or denaturation. Here we show that the oxidative stability of NK was substantially increased by optimizing the amino acid residues Thr220 and Met222, which were in the vicinity of the catalytic residue Ser221 of the enzyme. Two nonoxidative amino acids (Ser and Ala) were introduced at these sites using site-directed mutagenesis. Active enzymes were successfully expressed in Escherichia coli with periplasmic secretion and enzymes were purified to homogeneity. The purified enzymes were analyzed with respect to oxidative stability, kinetic parameters, fibrinolytic activity and thermal stability. M222A mutant was found to have a greatly increased oxidative stability compared with wild-type enzyme and it was resistant to inactivation by more than 1 M H2O2, whereas the wild-type enzyme was inactivated by 0.1 M H2O2 (t1/2 ∼ 11.6 min). The other mutant (T220S) also showed an obvious increase in antioxidative ability. Molecular dynamic simulations on wild-type and T220S mutant proteins suggested that a hydrogen bond was formed between Ser220 and Asn155, and the spatial structure of Met222 was changed compared with the wild-type. The present study demonstrates the feasibility of improving oxidative stability of NK by site-directed mutagenesis and shows successful protein engineering cases to improve stability of NK as a potent therapeutic agent.