A cold-adapted protease subtilisin was successfully isolated by evolutionary engineering based on sequential in vitro random mutagenesis and an improved method of screening (H. Kano, S. Taguchi, and H. Momose, Appl. Microbiol. Biotechnol. 47:46–51, 1997). The mutant subtilisin, termed m-63, exhibited a catalytic efficiency (expressed as the kcat/Km value) 100% higher than that of the wild type at 10°C when N-succinyl-l-Ala-l-Ala-l-Pro-l-Phe-p-nitroanilide was used as a synthetic substrate. This cold adaptation was achieved with three mutations, Val to Ile at position 72 (V72I), Ala to Thr at position 92 (A92T), and Gly to Asp at position 131 (G131D), and it was found that an increase in substrate affinity (i.e., a decreased Km value) was mostly responsible for the increased activity. Analysis of kinetic parameters revealed that the V72I mutation contributed negatively to the activity but that the other two mutations, A92T and G131D, overcame the negative contribution to confer the 100% increase in activity. Besides suppression of the activity-negative mutation (V72I) by A92T and G131D, suppression of structural stability was observed in measurements of activity retention at 60°C and circular dichroism spectra at 10°C.
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