Utilization of lignocellulosic materials offers great potential to reduce our dependence on fossil fuels. The enzymatic hydrolysis step converting lignocellulosic materials into fermentable sugars is recognized as one of the major limiting steps in biomass-to-ethanol process due to the recalcitrant and complex structure of the lignocellulosic substrate and the relatively high cost of enzymes. The fermentable sugars are derived from cellulose and hemicelluloses in lignocellulosic materials. To convert cellulose into glucose, three major cellulase groups are required: endoglucanases, cellobiohydrolases and β-glucosidase, which synergistically hydrolyze cellulose . Xylans, the main hemicelluloses in hardwoods and annual plants, are closely associated with the cellulose fibrils, as well as lignin, and cover the fiber surfaces . After pretreatment, even low amounts of residual xylan can limit the extent and efficiency of cellulose hydrolysis by cellulases, but the limitation can be overcome by addition of xylanases that solubilize xylan in the substrates [3–5]. Thus, xylanases play an important role in efficient hydrolysis of xylan-containing lignocellulosic materials.
Xylans in annual plants consist of a linear backbone of β-(1 → 4)-D-xylopyranosyl residues, substituted by α-L-arabinofuranosyl units in the positions of 2-O and/or 3-O, by 4-O-methyl-glucopyranosyl uronic acid in the position of 2-O, and/or by acetyl groups in 2-O and/or 3-O . Furthermore, some of the arabinofuranosyl units may be esterified with ferulic or p-coumaric acids . Endo-1,4-xylanases cleave the internal β-1,4-glycosyl bonds in the xylan main chain and produce xylo-oligosaccharides as main products. In the hydrolysis of lignocellulosic materials, addition of xylanases has been shown to significantly improve the performance of cellulases and to increase the cellulose conversion [8–10].
Most fungal cellulases and some hemicellulases studied so far have a complex modular architecture comprising a catalytic domain (CD) connected usually to the non-catalytic carbohydrate-binding module (CBM) via a flexible linker rich in either proline, threonine, and/or serine residues . The CBMs are located either at the N- or C- terminal or both and are currently categorized into 64 defined families based on amino acid sequence similarities (http://www.cazy.org). Furthermore, these families have been categorized into three types based on their structure, function and ligand specificities: surface binding CBM (type A), glycan-chain binding CBM (type B) and small-sugar binding CBM (type C). Type A CBM includes members of families 1, 2a, 3, 5 and 10 and are recognized to bind on insoluble, highly crystalline cellulose and/or chitin .
It has been reported that CBMs play an important role in the improvement of enzymatic hydrolysis by cellulases [13–15]. Based on present data, the main contribution of the CBM to the enzymatic hydrolysis is the ability of CBM to target the catalytic domain to a specific substrate, thereby increasing the concentration of enzymes on the surface of the substrate. Recently, however, it has been shown that intact cellobiohydrolases and their core domains lacking CBM possess similar catalytic activity towards cellulose , and that cellobiohydrolases with and without CBM proceed along the cellulose chain with a similar speed .
The impacts of cellulose-binding modules in hemicellulases on the adsorption and hydrolysis of hemicelluloses have also been reported. Obviously, the close presence of hemicelluloses and cellulose in the substrates results in improved hydrolytic efficiency on hemicelluloses by enzymes containing a cellulose-binding module. Thus, the cellulose-binding module of Trichoderma reesei mannanase did not bind to mannan but increased the hydrolysis rate of insoluble mannan-cellulose complexes . Fusion of the mannanase from Aspergillus aculeatus with a family I CBM from A. niger cellobiohydrolase B also improved the hydrolysis of NaOH-pretreated softwood pulp . The adsorption and hydrolytic activity on insoluble xylan by the xylanases A and B from Clostridium stercorarium, was found to be increased by the presence of two family 6 and one family 9 cellulose-binding modules, respectively [20, 21]. Fusion of the family 6 CBM from C. stercorarium xylanase to a Bacillus halodurans xylanase also resulted to an increased adsorption on cellulose and insoluble xylan . However, the effect of the actual xylan binding modules on the adsorption and solubilization of xylans has received only limited attention. The two N-terminal family 22 CBMs from Thermotoga neapolitana xylanase A were found to bind on xylan but not on cellulose. The fusion of these CBMs with a family 10 xylanase from Bacillus halodurans increased the adsorption on insoluble xylan, and improved the hydrolytic efficiency of insoluble xylan but not of soluble xylan . It has also been reported that the family 2b xylan-binding domain 1 from Cellulomonas fimi xylanase D bound on xylan but not on cellulose  whereas the xylanase 11A from the same fungus was shown to contain two family 2b CBMs binding on both cellulose and xylan. The CBM2b-1was shown to bind specifically on xylan and the CBM2b-2 on both insoluble and soluble oat spelt xylan, but exhibited also weak affinity to insoluble cellulose . The family IIb CBM of xylanase from Streptomyces thermoviolaceus increased the catalytic activity of a xylanase from Thermotoga maritima on soluble xylan, but not on insoluble xylan .
The family 1 cellulose binding modules of Cel7A and Cel5A of T. reesei have been shown to be mainly responsible for the non-specific binding of the enzymes on lignins . The intact T. reesei Cel7A and Cel5A enzymes were found to bind more on isolated lignins than the corresponding core domains. The β-glucosidase from T. reesei lacking a CBM was, however, found to bind strongly on lignin-rich residues but much less on Avicel and steam pretreated spruce . Limited information on the adsorption properties of xylanases on different lignin containing materials is available.
The hydrolytic pattern of the core domain of the thermostable Xyn11 Nonomuraea flexuosa has been previously characterized on isolated xylans and lignocellulosic substrates . Based on the C-termini amino acid sequence similarities, the xylanase Xyn11A of N. flexuosa contains a family II CBM . In this work, the family II CBM of the Xyn11 from N. flexuosa was characterized with respect to its adsorption on insoluble xylan, lignin and pretreated wheat straw. The role of the CBM from N. flexuosa xylanase in the hydrolysis of isolated xylan was evaluated and the effect of CBM in xylanase from N. flexuosa on non-productive adsorption on lignin was investigated. The main objective of this work was to understand the impact of CBM from N. flexuosa xylanases in the total hydrolysis of lignocellulosic materials for platform sugars.