Purification and characterization of an extracellular xylanase produced by the endophytic fungus, <i>Aspergillus terreus</i>, grown in submerged fermentation

Abstract

Aspergillus terreus produced high levels of a thermotolerant extracellular xylanase and showed low cellulase activity when cultured at 30°C for 48 h, in liquid medium supplemented with wheat bran as carbon source. Xylanase was purified 45-fold to homogeneity with a recovery yield of 67% by carboxymethyl (CM)-cellulose chromatography. The enzyme, a glycoprotein with 33% of carbohydrate content, appeared as a single protein band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) gel with a molecular mass corresponding to 23 kDa. Optimal temperature and pH were 55°C and 4.5, respectively. The enzyme was thermotolerant at 45 and 50°C, with a half-life of 55 and 36 min, respectively. The K_m was calculated as 22 mg/ml and V_max as 625 mg/ml of protein using birchwood xylan as substrate. Metal ions, such as Ag⁺, Cu⁺², Fe⁺², Hg⁺ and Zn⁺² strongly inhibited xylanase, whereas K⁺ and Mn⁺² resulted in activation. Xylanase hydrolyzed birchwood xylan and oatspelt xylan, mostly yielding xylooligosaccharides, suggest that it is an endoxylanase (EC. 3.2.1.37).

Keywords: Aspergillus terreus, endoxylanase, thermostability