The role of acetyl xylan esterase in the solubilization of xylan and enzymatic hydrolysis of wheat straw and giant reed

Junhua Zhang,Matti Siika-Aho,Liisa Viikari,Maija Tenkanen

doi:10.1186/1754-6834-4-60

Abstract

BackgroundDue to the complexity of lignocellulosic materials, a complete enzymatic hydrolysis into fermentable sugars requires a variety of cellulolytic and xylanolytic enzymes. Addition of xylanases has been shown to significantly improve the performance of cellulases and to increase cellulose hydrolysis by solubilizing xylans in lignocellulosic materials. The goal of this work was to investigate the effect of acetyl xylan esterase (AXE) originating from Trichoderma reesei on xylan solubilization and enzymatic hydrolysis of cellulose.ResultsThe solubilization of xylan in pretreated wheat straw and giant reed (Arundo donax) by xylanolytic enzymes and the impact of the sequential or simultaneous solubilization of xylan on the hydrolysis of cellulose by purified enzymes were investigated. The results showed that the removal of acetyl groups in xylan by AXE increased the accessibility of xylan to xylanase and improved the hydrolysis of xylan in pretreated wheat straw and giant reed. Solubilization of xylan led to an increased accessibility of cellulose to cellulases and thereby increased the hydrolysis extent of cellulose. A clear synergistic effect between cellulases and xylanolytic enzymes was observed. The highest hydrolysis yield of cellulose was obtained with a simultaneous use of cellulases, xylanase and AXE, indicating the presence of acetylated xylan within the cellulose matrix. Acetylated xylobiose and acetylated xylotriose were produced from xylan without AXE, as confirmed by atmospheric pressure matrix-assisted laser desorption/ionization ion trap mass spectrometry.ConclusionsThe results in this paper demonstrate that supplementation of xylanase with AXE enhances the solubilization of xylan to some extent and, consequently, increases the subsequent hydrolysis of cellulose. The highest hydrolysis yield was, however, obtained by simultaneous hydrolysis of xylan and cellulose, indicating a layered structure of cellulose and xylan chains in the cell wall substrate. AXE has an important role in the hydrolysis of lignocellulosic materials containing acetylated xylan.

Highlights

Due to the complexity of lignocellulosic materials, a complete enzymatic hydrolysis into fermentable sugars requires a variety of cellulolytic and xylanolytic enzymes
It has been reported that oligomers from xylobiose to xylopentaose were released from isolated xylans by XYL from Thermoascus aurantiacus during initial hydrolysis and xylose and xylobiose were the main products after prolonged hydrolysis [20]
The results indicated that xylan solubilization is important for the hydrolysis of xylan-containing lignocellulosic materials, which is in accordance with other published results [10,11,14]

Summary

Introduction

Due to the complexity of lignocellulosic materials, a complete enzymatic hydrolysis into fermentable sugars requires a variety of cellulolytic and xylanolytic enzymes. Addition of xylanases has been shown to significantly improve the performance of cellulases and to increase cellulose hydrolysis by solubilizing xylans in lignocellulosic materials. Plant cell walls consist of three major polymers: cellulose, hemicelluloses and lignin. Due to the complexity of lignocellulosic materials, a complete enzymatic hydrolysis into fermentable monosaccharides requires a variety of cellulolytic and xylanolytic enzymes. Given the diversity of xylan structures, a complete hydrolysis of xylan involves the synergistic action of main chain degrading enzymes, including endo-b-1,4-xylanases (EC 3.2.1.8) and b-D-xylosidases (EC 3.2.1.37), and side group cleaving enzymes, including a-L-arabinofuranosidases (EC 3.2.1.55), a-glucuronidases (EC 3.2.1.139), acetyl xylan esterases (AXEs) (EC 3.1.1.72), and feruloyl esterases (EC 3.1.1.73)

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