Purification and properties of an extracellular acidophilic endo-1,4-β-xylanase, naturally deleted in the “thumb”, from Penicillium occitanis Pol6

An extracellular, acidophilic endo-β-1,4-xylanase was purified to homogeneity from the culture filtrate of the filamentous fungus Penicillium occitanis Pol6, grown on oat spelt xylan. The purified enzyme (PoXyn3) was identified as a glycoprotein (carbohydrate content 10.83%) and showed a single band on SDS-PAGE with an apparent molecular weight of 22 kDa. The xylanase activity was optimal at pH 3.0 and 45 °C. The specific activity measured for oat spelt xylan was 358 U mg−1. The apparent KM and Vmax values were 14.13 mg ml−1 and 806.3 μmol min−1 ml−1, respectively, as measured on oat spelt xylan. PoXyn3 initially released larger xylooligosaccharides from oat spelt xylan. These oligosaccharides are principally hydrolyzed to xylobiose and xylotriose, which confirmed that the enzyme is an endoxylanase (EC 3.2.1.8). The genomic DNA and cDNA encoding this protein were cloned and sequenced. This PoXyn3 presents an open reading frame of 678 bp, interrupted by one intron of 87 bp and encoding for a mature protein of 197 amino acids and 21.73 kDa. Notably, based on homology modeling, the “thumb” of PoXyn3 was found to be naturally partially deleted in comparison with other homologous xylanases, and this deletion may confer resistance to some xylanase inhibitors.