Purification of a third distinct xylanase from the xylanolytic system of Trichoderma harzianum

Abstract

Three of the xylanases produced by Trichoderma harzianum E58 passed through a polysulfone membrane with molecular mass cut-off of 10 000 daltons, even though their molecular mass had been estimated to be 20 000, 22 000, and 29 000 daltons. The 22 000 dalton xylanase was purified to homogeneity from a preparation containing a mixture of 22 000 and 20 000 dalton xylanase using a combination of hydrophobic column chromatography and chromatofocusing. This enzyme has a pI of 8.5, a specific activity of 0.28 U/mg, a temperature optimum between 45 and 50 °C, a pH optimum between 4.5 and 5.0, and the ability to cleave xylotriose. It differs from the other two xylanases by having a lower pI, a lower specific activity, and a lower thermal tolerance. All three xylanases are highly specific for xylan hydrolysis and they do not cleave xylobiose or release arabinose substituents from arabinoxylan. Their amino acid compositions suggest that they are three distinct gene products. The three enzymes are major components of the xylanolytic system of T. harzianum, which consists of at least two other xylanases and two β-xylosidases which are responsible for the release of arabinose substituents and the hydrolysis of xylobiose.