Purification and characterization of a family G/11 beta-xylanase from Streptomyces olivaceoviridis E-86

Structure-based substrate specificity analysis of GH11 xylanase from Streptomyces olivaceoviridis E-86

Although many xylanases have been studied, many of the characteristics of xylanases toward branches in xylan remain unclear. In this study, the substrate specificity of a GH11 xylanase from Streptomyces olivaceoviridis E-86 (SoXyn11B) was elucidated based on its three-dimensional structure. Subsite mapping suggests that SoXyn11B has seven subsites (four subsites on the - side and three subsites on the + side), and it is one longer than the GH10 xylanase from S. olivaceoviridis (SoXyn10A). SoXyn11B has no affinity for the subsites at either end of the scissile glycosidic bond, and the sugar-binding energy at subsite - 2 was the highest, followed by subsite + 2. These properties were very similar to those of SoXyn10A. In contrast, SoXyn11B produced different branched oligosaccharides from bagasse compared with those of SoXyn10A. These branched oligosaccharides were identified as O-β-D-xylopyranosyl-(1→4)-[O-α-L-arabinofuranosyl-(1→3)]-O-β-D-xylopyranosyl-(1→4)-β-D-xylopyranosyl-(1→4)-β-D-xylopyranose (Ara³Xyl₄) and O-β-D-xylopyranosyl-(1→4)-[O-4-O-methyl-α-D-glucuronopyranosyl-(l→2)]-β-D-xylopyranosyl-(1→4)-β-D-xylopyranosyl-(1→4)-β-D-xylopyranose (MeGlcA³Xyl₄) by nuclear magnetic resonance (NMR) and electrospray ionization mass spectrometry (ESI-MS) and confirmed by crystal structure analysis of SoXyn11B in complex with these branched xylooligosaccharides. SoXyn11B has a β-jerryroll fold structure, and the catalytic cleft is located on the inner β-sheet of the fold. The ligand-binding structures revealed seven subsites of SoXyn11B. The 2- and 3-hydroxy groups of xylose at the subsites + 3, + 2, and - 3 face outwards, and an arabinose or a glucuronic acid side chain can be linked to these positions. These subsite structures appear to cause the limited substrate specificity of SoXyn11B for branched xylooligosaccharides. KEY POINTS: • Crystal structure of family 11 β-xylanase from Streptomyces olivaceoviridis was determined. • Topology of substrate-binding cleft of family 11 β-xylanase from Streptomyces olivaceoviridis was characterized. • Mode of action of family 11 β-xylanase from Streptomyces olivaceoviridis for substitutions in xylan was elucidated.

Evaluation of Ammonia Pretreatment for Enzymatic Hydrolysis of Sugarcane Bagasse to Recover Xylooligosaccharides

Sugarcane bagasse is a useful biomass resource. In the present study, we examined the efficacy of ammonia pretreatment for selective release of hemicellulose from bagasse. Pretreatment of bagasse with aqueous ammonia resulted in significant loss of xylan. In contrast, pretreatment of bagasse with anhydrous ammonia resulted in almost no xylan loss. Aqueous ammonia or anhydrous ammonia-pretreated bagasse was then subjected to enzymatic digestion with a xylanase from the glycoside hydrolase (GH) family 10 or a xylanase from the GH family 11. The hydrolysis rate of xylan in bagasse pretreated with aqueous ammonia was approximately 50 %. In contrast, in the anhydrous ammonia-treated bagasse, xylan hydrolysis was > 80 %. These results suggested that anhydrous ammonia pretreatment would be an effective method for preparation of sugarcane bagasse for enzymatic hydrolysis to recover xylooligosaccharides.

Functional Characterization of the GH10 and GH11 Xylanases from Streptomyces olivaceoviridis E-86 Provide Insights into the Advantage of GH11 Xylanase in Catalyzing Biomass Degradation

We functionally characterized the GH10 xylanase (SoXyn10A) and the GH11 xylanase (SoXyn11B) derived from the actinomycete Streptomyces olivaceoviridis E-86. Each enzyme exhibited differences in the produced reducing power upon degradation of xylan substrates. SoXyn10A produced higher reducing power than SoXyn11B. Gel filtration of the hydrolysates generated by both enzymes revealed that the original substrate was completely decomposed. Enzyme mixtures of SoXyn10A and SoXyn11B produced the same level of reducing power as SoXyn10A alone. These observations were in good agreement with the composition of the hydrolysis products. The hydrolysis products derived from the incubation of soluble birchwood xylan with a mixture of SoXyn10A and SoXyn11B produced the same products as SoXyn10A alone with similar compositions. Furthermore, the addition of SoXyn10A following SoXyn11B-mediated digestion of xylan produced the same products as SoXyn10A alone with similar compositions. Thus, it was hypothesized that SoXyn10A could degrade xylans to a smaller size than SoXyn11B. In contrast to the soluble xylans as the substrate, the produced reducing power generated by both enzymes was not significantly different when pretreated milled bagasses were used as substrates. Quantification of the pentose content in the milled bagasse residues after the enzyme digestions revealed that SoXyn11B hydrolyzed xylans in pretreated milled bagasses much more efficiently than SoXyn10A. These data suggested that the GH10 xylanases can degrade soluble xylans smaller than the GH11 xylanases. However, the GH11 xylanases may be more efficient at catalyzing xylan degradation in natural environments (e.g. biomass) where xylans interact with celluloses and lignins.

Structural characterization of hemicellulose released from corn cob in continuous flow type hydrothermal reactor

Hydrothermal reaction is known to be one of the most efficient procedures to extract hemicelluloses from lignocellulosic biomass. We investigated the molecular structure of xylooligosaccharides released from corn cob in a continuous flow type hydrothermal reactor designed in our group. The fraction precipitable from the extract with four volumes of ethanol was examined by ¹H-NMR spectroscopy and MALDI-TOF MS before and after enzymatic treatment with different purified enzymes. The released water-soluble hemicellulose was found to correspond to a mixture of wide degree of polymerization range of acetylarabinoglucuronoxylan fragments (further as corn cob xylan abbreviated CX). Analysis of enzymatic hydrolyzates of CX with an acetylxylan esterase, GH3 β-xylosidase, GH10 and GH11 xylanases revealed that the main chain contains unsubstituted regions mixed with regions of xylopyranosyl residues partially acetylated and occasionally substituted by 4-O-methyl-d-glucuronic acid and arabinofuranose esterified with ferulic or coumaric acid. Single 2- and 3-O-acetylation was accompanied by 2,3-di-O-acetylation and 3-O-acetylation of Xylp residues substituted with MeGlcA. Most of the non-esterified arabinofuranose side residues were lost during the hydrodynamic process. Despite reduced branching, the acetylation and ferulic acid modification of pentose residues contribute to high yields and high solubility of the extracted CX. It is also shown that different enzyme treatments of CX may lead to various types of xylooligosaccharides of different biomedical potential.

4-O-Methyl Modifications of Glucuronic Acids in Xylans Are Indispensable for Substrate Discrimination by GH67 α-Glucuronidase from Bacillus halodurans C-125

A GH67 α-glucuronidase gene derived from Bacillus halodurans C-125 was expressed in E. coli to obtain a recombinant enzyme (BhGlcA67). Using the purified enzyme, the enzymatic properties and substrate specificities of the enzyme were investigated. BhGlcA67 showed maximum activity at pH 5.4 and 45 °C. When BhGlcA67 was incubated with birchwood, oat spelts, and cotton seed xylan, the enzyme did not release any glucuronic acid or 4-O-methyl-glucuronic acid from these substrates. BhGlcA67 acted only on 4-O-methyl-α-D-glucuronopyranosyl-(1→2)-β-D-xylopyranosyl-(1→4)-β-D-xylopyranosyl-(1→4)-β-D-xylopyranose (MeGlcA³Xyl₃), which has a glucuronic acid side chain with a 4-O-methyl group located at its non-reducing end, but did not on β-D-xylopyranosyl-(1→4)-[4-O-methyl-α-D-glucuronopyranosyl-(l→2)]-β-D-xylopyranosyl-(1→4)-β-D-xylopyranosyl-(1→4)-β-D-xylop- yranose (MeGlcA³Xyl₄) and α-D-glucuronopyranosyl-(l→2)-β-D-xylopyranosyl-(1→4)-β-D-xylopyranosyl-(1→4)-β-D-xylopyranose (GlcA³Xyl₃). The environment for recognizing the 4-O-methyl group of glucuronic acid was observed in all the crystal structures of reported GH67 glucuronidases, and the amino acids for discriminating the 4-O-methyl group of glucuronic acid were widely conserved in the primary sequences of the GH67 family, suggesting that the 4-O-methyl group is critical for the activities of the GH67 family.

Microbial xylanases and their industrial application in pulp and paper biobleaching: a review

Xylanases are hydrolytic enzymes which cleave the β-1, 4 backbone of the complex plant cell wall polysaccharide xylan. Xylan is the major hemicellulosic constituent found in soft and hard food. It is the next most abundant renewable polysaccharide after cellulose. Xylanases and associated debranching enzymes produced by a variety of microorganisms including bacteria, actinomycetes, yeast and fungi bring hydrolysis of hemicelluloses. Despite thorough knowledge of microbial xylanolytic systems, further studies are required to achieve a complete understanding of the mechanism of xylan degradation by xylanases produced by microorganisms and their promising use in pulp biobleaching. Cellulase-free xylanases are important in pulp biobleaching as alternatives to the use of toxic chlorinated compounds because of the environmental hazards and diseases caused by the release of the adsorbable organic halogens. In this review, we have focused on the studies of structural composition of xylan in plants, their classification, sources of xylanases, extremophilic xylanases, modes of fermentation for the production of xylanases, factors affecting xylanase production, statistical approaches such as Plackett Burman, Response Surface Methodology to enhance xylanase production, purification, characterization, molecular cloning and expression. Besides this, review has focused on the microbial enzyme complex involved in the complete breakdown of xylan and the studies on xylanase regulation and their potential industrial applications with special reference to pulp biobleaching, which is directly related to increasing pulp brightness and reduction in environmental pollution.

Assembly of xylanases into designer cellulosomes promotes efficient hydrolysis of the xylan component of a natural recalcitrant cellulosic substrate

In nature, the complex composition and structure of the plant cell wall pose a barrier to enzymatic degradation. Nevertheless, some anaerobic bacteria have evolved for this purpose an intriguing, highly efficient multienzyme complex, the cellulosome, which contains numerous cellulases and hemicellulases. The rod-like cellulose component of the plant cell wall is embedded in a colloidal blend of hemicelluloses, a major component of which is xylan. In order to enhance enzymatic degradation of the xylan component of a natural complex substrate (wheat straw) and to study the synergistic action among different xylanases, we have employed a variation of the designer cellulosome approach by fabricating a tetravalent complex that includes the three endoxylanases of Thermobifida fusca (Xyn10A, Xyn10B, and Xyn11A) and an Xyl43A β-xylosidase from the same bacterium. Here, we describe the conversion of Xyn10A and Xyl43A to the cellulosomal mode. The incorporation of the Xyl43A enzyme together with the three endoxylanases into a common designer cellulosome served to enhance the level of reducing sugars produced during wheat straw degradation. The enhanced synergistic action of the four xylanases reflected their immediate juxtaposition in the complex, and these tetravalent xylanolytic designer cellulosomes succeeded in degrading significant (~25%) levels of the total xylan component of the wheat straw substrate. The results suggest that the incorporation of xylanases into cellulosome complexes is advantageous for efficient decomposition of recalcitrant cellulosic substrates--a distinction previously reserved for cellulose-degrading enzymes.

IMPORTANCE

Xylanases are important enzymes for our society, due to their variety of industrial applications. Together with cellulases and other glycoside hydrolases, xylanases may also provide cost-effective conversion of plant-derived cellulosic biomass into soluble sugars en route to biofuels as an alternative to fossil fuels. Xylanases are commonly found in multienzyme cellulosome complexes, produced by anaerobic bacteria, which are considered to be among the most efficient systems for degradation of cellulosic biomass. Using a designer cellulosome approach, we have incorporated the entire xylanolytic system of the bacterium Thermobifida fusca into defined artificial cellulosome complexes. The combined action of these designer cellulosomes versus that of the wild-type free xylanase system was then compared. Our data demonstrated that xylanolytic designer cellulosomes displayed enhanced synergistic activities on a natural recalcitrant wheat straw substrate and could thus serve in the development of advanced systems for improved degradation of lignocellulosic material.

GH11 xylanases: Structure/function/properties relationships and applications

For technical, environmental and economical reasons, industrial demands for process-fitted enzymes have evolved drastically in the last decade. Therefore, continuous efforts are made in order to get insights into enzyme structure/function relationships to create improved biocatalysts. Xylanases are hemicellulolytic enzymes, which are responsible for the degradation of the heteroxylans constituting the lignocellulosic plant cell wall. Due to their variety, xylanases have been classified in glycoside hydrolase families GH5, GH8, GH10, GH11, GH30 and GH43 in the CAZy database. In this review, we focus on GH11 family, which is one of the best characterized GH families with bacterial and fungal members considered as true xylanases compared to the other families because of their high substrate specificity. Based on an exhaustive analysis of the sequences and 3D structures available so far, in relation with biochemical properties, we assess biochemical aspects of GH11 xylanases: structure, catalytic machinery, focus on their "thumb" loop of major importance in catalytic efficiency and substrate selectivity, inhibition, stability to pH and temperature. GH11 xylanases have for a long time been used as biotechnological tools in various industrial applications and represent in addition promising candidates for future other uses.

Thermostable recombinant xylanases from Nonomuraea flexuosa and Thermoascus aurantiacus show distinct properties in the hydrolysis of xylans and pretreated wheat straw

BACKGROUND

In the hydrolysis of lignocellulosic materials, thermostable enzymes decrease the amount of enzyme needed due to higher specific activity and elongate the hydrolysis time due to improved stability. For cost-efficient use of enzymes in large-scale industrial applications, high-level expression of enzymes in recombinant hosts is usually a prerequisite. The main aim of the present study was to compare the biochemical and hydrolytic properties of two thermostable recombinant glycosyl hydrolase families 10 and 11 (GH10 and GH11, respectively) xylanases with respect to their potential application in the hydrolysis of lignocellulosic substrates.

RESULTS

The xylanases from Nonomuraea flexuosa (Nf Xyn11A) and from Thermoascus aurantiacus (Ta Xyn10A) were purified by heat treatment and gel permeation chromatography. Ta Xyn10A exhibited higher hydrolytic efficiency than Nf Xyn11A toward birchwood glucuronoxylan, insoluble oat spelt arabinoxylan and hydrothermally pretreated wheat straw, and it produced more reducing sugars. Oligosaccharides from xylobiose to xylopentaose as well as higher degree of polymerization (DP) xylooligosaccharides (XOSs), but not xylose, were released during the initial hydrolysis of xylans by Nf Xyn11A, indicating its potential for the production of XOS. The mode of action of Nf Xyn11A and Ta Xyn10A on glucuronoxylan and arabinoxylan showed typical production patterns of endoxylanases belonging to GH11 and GH10, respectively.

CONCLUSIONS

Because of its high catalytic activity and good thermostability, T. aurantiacus xylanase shows great potential for applications aimed at total hydrolysis of lignocellulosic materials for platform sugars, whereas N. flexuosa xylanase shows more significant potential for the production of XOSs.

Variation of xylanosomal subunit composition of Streptomyces olivaceoviridis by nitrogen sources

Besides affecting the xylanases production, different nitrogen sources present in the media also caused changes in the xylanosomal subunit composition of Streptomyces olivaceoviridis E-86. Four xylanosome fractions, purified from the culture supernatant of S. olivaceoviridis E-86 grown on different nitrogen sources, exhibited high specificity towards different xylans and were composed of different subunits. Thus, S. olivaceoviridis E-86 regulates the expression of xylanase activity and varies the xylanosome composition according to the nitrogen sources possibly through the action of the secreted proteases.

Rational affinity purification of native Streptomyces family 10 xylanase

Xylanase SoXyn10A from Streptomyces olivaceoviridis E-86 comprises a family 10 catalytic module linked to a family 13 carbohydrate-binding module (SoCBM13). The SoCBM13 has a beta-trefoil structure, with binding sites in each subdomain (alpha, beta and gamma). Subdomain alpha, but not subdomains beta and gamma, binds tightly to lactose. It was, therefore, thought that immobilized lactose could be used for the affinity purification of SoXyn10A. Lactosyl-Sepharose was prepared and tested as an affinity matrix. SoXyn10A produced from the cloned xyn10A gene by Escherichia coli, and native SoXyn10A in culture supernatants from S. olivaceoviridis, were purified to homogeneity in a single step by affinity chromatography using this matrix. This simple purification of SoXyn10A makes the enzyme an attractive candidate for applications requiring xylanase. The CBM also has the potential for use as an affinity tag for the purification of other proteins.

Enhancement of the thermostability and hydrolytic activity of xylanase by random gene shuffling

The thermostability of Streptomyces lividans xylanase B (SlxB-cat) was significantly increased by the replacement of its N-terminal region with the corresponding region from Thermomonospora fusca xylanase A (TfxA-cat) without observing a decrease in enzyme activity. In spite of the significant similarity between the amino acid sequences of the two xylanases, their thermostabilities are quite different. To facilitate an understanding of the contribution of structure to the thermostability observed, chimaeric enzymes were constructed by random gene shuffling and the thermostable chimaeric enzymes were selected for further study. A comparative study of the chimaeric and parental enzymes indicated that the N-terminus of TfxA-cat contributed to the observed thermostability. However, too many substitutions decreased both the thermostability and the activity of the enzyme. The mutants with the most desirable characteristics, Stx15 and Stx18, exhibited significant thermostabilities at 70 degrees C with optimum temperatures which were 20 degrees C higher than that of SlxB-cat and equal to that of TfxA-cat. The ability of these two chimaeric enzymes to produce reducing sugar from xylan was enhanced in comparison with the parental enzymes. These results suggest that these chimaeric enzymes inherit both their thermostability from TfxA-cat and their increased reactivity from SlxB-cat. Our study also demonstrates that random shuffling between a mesophilic enzyme and its thermophilic counterpart represents a facile approach for the improvement of the thermostability of a mesophilic enzyme.