Two endoxylanases active and stable at alkaline pH from the newly isolated thermophilic fungus, Myceliophthora sp. IMI 387099.

Abstract

Two extra-cellular endoxylanases (Xyl Ia and Ib) were purified to homogeneity from the newly isolated thermophilic fungus, Myceliophthora sp. IMI 387099. Xyl Ia and Ib, having a molecular mass of approximately 53 kDa and pI of 5.2 and 4.8, respectively, were optimally active at 75 degrees C and at pH 6.0. They were stable at pH 9.2 at 60 degrees C for 2 h, but less stable at pH 6.0 and above 50 degrees C. Mg+2, Zn+2, Ca+2, Co+2 and DTT increased their activity by 1.5-3.0-folds, while SDS and NBS completely inhibited their activity. Both xylanases were active on pNPX and pNPC, but their activity on pNPC was three times higher than that on pNPX. Xyl Ia was more active than Xyl Ib on pNP-alpha-L-Arap, while the latter preferred pNP-alpha-L-Araf. Both xylanases showed two to four times higher activity on rye and wheat arabinoxylans than on birchwood xylan, but Xyl Ib was more active than Xyl Ia on oat spelt xylan. Wheat insoluble pentosan was a good substrate for Xyl Ia, while Xyl Ib preferred wheat soluble arabinoxylan. Xyl Ia had lower Km and higher kcat/Km ratios than Xyl Ib towards all three xylans tested. Both xylanases degraded X4-X6 in an endo-fashion and catalysed hydrolysis and trans-xylosylation reactions. HPLC and LC/MS analysis showed that Xyl Ia and Ib released the unsubstituted X2-X6 as well as mono and di-methyl glucuronic acid substituted X3 and X2 from arabinoxylans.