Xylanase and β-xylosidase from Penicillium janczewskii  : Purification, characterization and hydrolysis of substrates

Background: Xylanases and β-D-xylosidases are the most important enzymes responsible for the degradation of xylan, the second main constituent of plant cell walls.
Results: In this study, the main extracellular xylanase (XYL I) and β-xylosidase (BXYL I) from the fungus Penicillium janczewskii were purified, characterized and applied for the hydrolysis of different substrates. Their molecular weights under denaturing and non-denaturing conditions were, respectively, 30.4 and 23.6 kDa for XYL I, and 100 and 200 kDa for BXYL I, indicating that the latter is homodimeric. XYL I is highly glycosylated (78%) with optimal activity in pH 6.0 at 65°C, while BXYL I presented lower sugar content (10.5%) and optimal activity in pH 5.0 at 75°C. The half-lives of XYL I at 55, 60 and 65°C were 125, 16 and 6 min, respectively. At 60°C, BXYL I retained almost 100% of the activity after 6 h. NH₄ ⁺, Na⁺, DTT and β-mercaptoethanol stimulated XYL I, while activation of BXYL I was not observed. Interestingly, XYL I was only partially inhibited by Hg²⁺, while BXYL I was completely inhibited. Xylobiose, xylotriose and larger xylooligosaccharides were the main products from xylan hydrolysis by XYL I. BXYL I hydrolyzed xylobiose and larger xylooligosaccharides with no activity against xylans.
Conclusion: The enzymes act synergistically in the degradation of xylans, and present industrial characteristics especially in relation to optimal activity at high temperatures, prolonged stability of BXYL I at 60°C, and stability of XYL I in wide pH range.

Keywords
Xylanolytic enzymes; Enzyme characterization; Enzyme purification; Xylan hydrolysis; Xylooligosaccharides hydrolysis