Catalytic properties of the endoxylanase I from Thermoascus aurantiacus

Endo-beta -1,4-xylanase I previously purified from Thermoascus aurantiacus solid state culture was further characterized. The enzyme had a molecular weight of 33 kDa by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and 31 kDa by gel filtration. Thin layer chromatography (TLC) analysis showed that endoxylanase liberates aldotetrauronic acid MeGlcA alpha -1,2-Xyl beta -1,4-Xyl beta -1,4-Xyl as the shortest acidic fragment from glucuronoxylan and an isomeric xylotriose (Xyl(3)) of the structure Xyl beta1-3Xyl beta1-4Xyl from rhodymenan. The enzyme performed ideally on O-acetyl-4-O-methyl-glucuronoxylan, Liberating large amounts of shea acetylated and non-acetylated fragments. Also, the enzyme was capable to hydrolyse arabinoxylan to arabinose (Arab), xylose (Xyl) and xylobiose (Xyl(2)). The enzyme degraded pNPX (4-nitrophenyl beta -D-xylopyranoside) by a complex reaction pathway that involved both hydrolysis and glycosyl transfer reactions. The enzyme tolerates the replacement of beta -xylopyranosyl units in several artificial substrates by beta -glucopyranosyl, alpha -L-arabinopyranosyl and alpha -L-arabinofuranosyl units and was active on pNPC (4-nitrophenyl beta -D-cellobioside), pNP-Arap (4-nitrophenyl alpha -L-arabinopyranoside) and pNPAraf (4-nitrophenyl alpha -L-arabinofuranoside). The enzyme also hydrolysed the 4-methylumbelliferyl glycosides of beta -D-xylobiose and beta -D-xylotriose at the agluconic linkage. The results suggested that the xylanase I from T. aurantiacus has catalytic properties similar to those belonging to family 10. (C) 2001 Elsevier Science B.V. All rights reserved.