Production of xylooligosaccharides from enzymatic hydrolysis of xylan by the white-rot fungi Pleurotus

Abstract

The white-rot fungi basidiomycetes Pleurotus sp. BCCB068 and Pleurotus tailandia were used to degrade oat-spelt xylan under submerged fermentation over a period of 40 days. Activities of endo-1,4-β-xylanase and β-xylosidase and xylan degradation products were determined. Xylan degradation by Pleurotus sp. BCCB068 and P. tailandia reached 75.1% and 73.4%, respectively. The formation of xylooligosaccharides and the simple sugars xylose, arabinose, cellobiose, mannose, and maltose were observed for both strains. The xylan degradation exhibited by these Pleurotus strains indicates they have potential for use in biotechnological processes related to degradation of hemicellulose sources.

Introduction

Hemicelluloses are a heterogeneous group of functionally related branched heteropolymeric polysaccharides present in almost all plant cell walls, usually in association with cellulose. While cellulose is crystalline, and resistant to hydrolysis, hemicellulose has a random, amorphous structure that can be readily hydrolyzed by dilute acid or base as well as by numerous hemicellulase enzymes.

Agricultural residues, such as corn fiber, corn stover, wheat straw, rice straw, and sugarcane bagasse, contain about 20–40% hemicellulose. Hemicelluloses are the second most abundant polysaccarides in nature and xylans, e.g., xylan, xyloglucan, arabinoxylan, and glucuronoxylan, are the most abundant hemicelluloses (Saha, 2003, Tan et al., 2008, Wagschal et al., 2008). Xylans of many plant materials are heteropolysaccharides with homopolymeric backbone chains of β-1,4-linked xylose units, which can contain various branching substitutions including l-arabinose, d-glucose, d-galactose, d-mannose, d-glucuronic acid, 4-O-methyl glucuronic acid, d-galacturonic acid, ferulic acid, and acetic acid residues, and to a lesser extent l-rhamnose, l-fucose, and various O-methylated neutral sugars (Sun et al., 2000, Wagschal et al., 2008). The complete enzymatic degradation of these complex structures depends on different enzymes acting in synergy, including endo-β-1,4-xylanases, which hydrolyze β-1,4-bonds between d-xylose residues in the main chain producing xylooligosaccharides (XOSs), and β-d-xylosidases, which convert xylooligosaccharides to xylose (Den Haan and Van Zyl, 2003). Other enzymes are involved in the cleavage of side groups, including α-l-arabinofuranosidase and α-glucuronidase, as well as several esterases, which, in combination with the previously mentioned enzymes, result in the complete degradation of xylans (Uffen, 1997, Tenkanen et al., 1996).

Multiple endoxylanase isoenzymes are produced by microorganisms (fungi and bacteria) and may reflect the need to produce xylanases with strict and relaxed specificities, capable of acting on different substrates (Milagres et al., 2005). The fungal genera Trichoderma, Aspergillus, Fusarium, and Pichia are considered great producers of xylanases (Christakopoulos et al., 1996, Den Haan and Van Zyl, 2003, de Vries et al., 2000, Wong and Saddler, 1992, Adsul et al., 2005). White-rot fungi have also been shown to produce extracellular xylanases that act on a wide range of hemicellulosic materials and are also useful as food sources (Buswell and Chang, 1994) and metabolites of interest to the pharmaceutical, cosmetic, and food industries (Jong and Donovick, 1989, Qinnghe et al., 2004). White-rot basidiomycetes usually secrete large amounts of these enzymes to degrade lignocellulosic materials (Elisashvili et al., 2002). For example, Phanerochaete chrysosporium produces high levels of α-glucuronidase (Castanares et al., 1995), and Coriolus versicolor produces a complex xylanolytic combination of enzymes (El-Nasser et al., 1997). Xylanase is also produced by Cuninghamella subvermispora when growing on plant cell-wall polysaccharides or on wood chips (Sethuraman et al., 1998, Souza-Cruz et al., 2004) and studies suggest that xylanase is a key enzyme for hemicellulose degradation (Palaniswamy et al., 2008). In addition, the edible white-rot fungi Pleurotus ostreatus and Pleurotus sajor-caju and the tough and leathery Trametes versicolor (Reddy et al., 2003, Valasková and Baldrian, 2006) are also producers of lignocellulolytic enzymes. Furthermore, the species of the genus Pleurotus produce large amounts of biomass and branched extracellular polysaccharides (EPS) in submerged cultures that are mainly composed of β-1,3 and β-1,6 glucans forming a gel-forming mucilage or sheaths in fungi (Rosado et al., 2003, Santos-Neves et al., 2008). Glucose, followed by galactose, mannose, xylose, and arabinose, are the main constituents of these EPS.

Agro-industrial and food-processing wastes are available in large quantities all over the world, and they frequently become a source of pollution and a health hazard. The majority of these wastes contain cellulose (30–40%), hemicellulose (xylan, 20–40%), and lignin (20–30%) (Palaniswamy et al., 2008). The utilization of these wastes for the production of food additives, chemicals, and fuel requires hydrolysis of all the components. Xylanases and the microorganisms that produce them could be used in the management of these wastes to degrade xylan to renewable fuels and chemicals and for use in food, agrofiber, and the paper and pulp industries, reducing their environmental impact (Collins et al., 2002, Techapun et al., 2002, Palaniswamy et al., 2008). Oligosaccharides produced by the action of xylanases are used as functional food additives or alternative sweeteners with beneficial properties (Pellerin et al., 1991, Puls et al., 1988, Kim et al., 2006, Menezes and Durrant, 2008).

In the present study the white-rot fungi Pleurotus sp. BCCB068 and Pleurotus tailandia were evaluated with regard to their ability to produce xylanases and degrade oat-spelt xylan using submerged fermentation. The ability of these fungi to degrade xylan suggests they may be useful in later research to optimize the production of xylanolytic enzymes, producing XOS and xylose and other useful products from cellulosic wastes.