Improvement of thermostability of fungal xylanase by using site-directed mutagenesis

Replacing several serine and threonine residues on the Ser/Thr surface of the xylanase from Aspergillus niger BCC14405 with four and five arginines effectively increases the thermostability of the enzyme. The modified enzymes showed 80% of maximal activity after incubating in xylan substrate for 2 h at 50 °C compared to only 15% activity for wild-type enzyme. The half-life of the mutated enzymes increased to 257 ± 16 and 285 ± 10 min for the four- and five-arginine mutants, respectively, compared to 14 ± 1 min for the wild-type enzyme. Thus, the arginine substitutions effectively increase stability by 18–20-fold. Kinetic parameters of the four-arginine-substitution enzyme were maintained at the level of the wild-type enzyme with the Km and Vmax values of 8.3 ± 0.1 mg ml−1 and 9556 ± 66 (n = 3) U mg−1 protein, respectively. The five-arginine-substitution enzyme showed only slight alteration in Km and Vmax with Km of 11.7 ± 1.7 mg ml−1 and Vmax of 8502 ± 65 U mg−1 protein, indicating lower substrate affinity and catalytic rate. Our study demonstrated that properly introduced arginine residues on the Ser/Thr surface of xylanase family 11 might be very effective in improvement of enzyme thermostability.