2School of Pharmaceutical Sciences, Jiangnan University, 1800 Lihu Road, 214122 Wuxi, China; fax: +086-510-8532-9042; E-mail: email@example.com
3Wuxi Medical School, Jiangnan University, 1800 Lihu Road, 214122 Wuxi, China; fax: +086-510-8532-9042; E-mail: firstname.lastname@example.org
# These authors contributed equally to this work.
* To whom correspondence should be addressed.
Received November 30, 2013; Revision received December 6, 2013
Based on the hyperthermostable family 11 xylanase (EvXyn11TS) gene sequence (EU591743), the gene Syxyn11 encoding a thermophilic xylanase SyXyn11 was synthesized with synonymous codons biasing towards Pichia pastoris. The homology alignment of primary structures among family 11 xylanases revealed that, at their N-termini, only SyXyn11 contains a disulfide bridge (Cys5–Cys32). This to some extent implied the significance of the disulfide bridge of SyXyn11 to its thermostability. To confirm the correlation between the N-terminal disulfide bridge and thermostability, a SyXyn11C5T-encoding gene, Syxyn11C5T, was constructed by mutating the Cys5 codon of Syxyn11 to Thr5. Then, the genes for the recombinant xylanases, reSyXyn11 and reSyXyn11C5T, were expressed in P. pastoris GS115, yielding xylanase activity of about 35 U per ml cell culture. Both xylanases were purified to homogeneity with specific activities of 363 and 344 U/mg, respectively. The temperature optimum and stability of reSyXyn11C5T decreased to 70 and 50°C from 85 and 80°C of reSyXyn11, respectively. There was no obvious change in pH characteristics.
KEY WORDS: xylanase, thermostability, disulfide bridge, computational prediction, site-directed mutagenesis