Improvement of the Thermostability of Xylanase by N-terminus Replacement

The hybrid xylanase TB was constructed by the substitution of the N-terminus segment of the Streptomyces olivaceoviridis xylanase XYNB with the corresponding region of Thermomonospora fusca xylanase TfxA. The hybrid gene tb, encoding the TB, was correctly expressed in E. coli BL21 and Pichia pastoris GS115. The TB was purified and its enzymatic properties were determined. The results revealed that the optimal temperature and optimal pH of TB were at 70 °C and 6.0, which have been obviously improved compared with those of XYNB. The thermostability of the TB was all about six-fold of XYNB' s after incubating the properly diluted enzyme solutions at 80 °C and 90 °C for 3 min, respectively. The pH stability of TB was 5-9 (pH range in which the relative activity was above 50 %), which was narrower than that of XYNB. Still, TB remains a high specific activity, as does XYNB. Analysis of a homology modeling and sequence similarity were used to reveal the factors influencing the enzymatic properties of TB and the relationship between structure and function of xylanase was discussed.