Distribution of uronic acids in xylans from various species of soft- and hardwood as determined by MALDI mass spectrometry

A novel substitution pattern in glucuronoarabinoxylans from woody bamboos

(1 → 4)-β-D-Xylans are the second most abundant plant biopolymers on Earth after cellulose. Although their structures have been extensively studied, and industrial applications have been found for them and their derivatives, they are still investigated due to the diversity of their structures and uses. In this work, hemicellulose fractions obtained previously with 1 M KOH from two species of woody bamboos, Phyllostachys aurea and Guadua chacoensis, were purified, and the structures of the glucuronoarabinoxylans (GAX) were studied by chemical and spectroscopic methods. In both cases, major amounts of α-L-arabinofuranose residues were linked to C3 of the xylose units of the backbone, and also α-D-glucuronic acid residues and their 4-O-methyl-derivatives were detected in minor quantities, linked to C2 of some xylose residues. Methylation analysis of the carboxyl-reduced derivative from GAX from P. aurea indicated the presence of terminal and 5-linked arabinofuranose units. NMR spectroscopy showed the presence of disaccharidic side chains of 5-O-α-l-arabinofuranosyl-L-arabinofuranose for the GAX from P. aurea, while for those of G. chacoensis, only single side chains were found. To the best of our knowledge, this disaccharide was not found before as side chain of xylans.

Woody plant cell walls: Fundamentals and utilization

Cell walls in plants, particularly forest trees, are the major carbon sink of the terrestrial ecosystem. Chemical and biosynthetic features of plant cell walls were revealed early on, focusing mostly on herbaceous model species. Recent developments in genomics, transcriptomics, epigenomics, transgenesis, and associated analytical techniques are enabling novel insights into formation of woody cell walls. Here, we review multilevel regulation of cell wall biosynthesis in forest tree species. We highlight current approaches to engineering cell walls as potential feedstock for materials and energy and survey reported field tests of such engineered transgenic trees. We outline opportunities and challenges in future research to better understand cell type biogenesis for more efficient wood cell wall modification and utilization for biomaterials or for enhanced carbon capture and storage.

Xylopentose production from crop residue employing xylanase enzyme

To produce xylo-oligosaccharides (XOS) from the agriculture waste, which included, green coconut and vegetable cocktail. The two pretreatment - hydrogen peroxide-acetic acid (HP-AC) and sodium hypochlorite-sodium hydroxide (SH-SH) - were used for this study. The optimal conditions for the pretreatment were 80 °C, 4.0 % NaClO, and 2 h, followed by 0.08 % NaOH, 55 °C, and 1 h. Further enzymatic hydrolysis of green coconut (GC) and vegetable cocktail (VC) were performed and found in case of GC, the best outcomes were observed. Different types of XOS were obtained from the treated biomass whereas a single type of XOS xylo-pentose was obtained in high quantity (96.44 % and 93.09 % from CG and VC respectively) with the production of other XOS < 2 %. This study presents a reasonably secure and economical method for turning secondary crop residue into XOS and fermentable sugars.

An Arabidopsis family GT106 glycosyltransferase is essential for xylan biosynthesis and secondary wall deposition

We have demonstrated that the Arabidopsis FRA9 (fragile fiber 9) gene is specifically expressed in secondary wall-forming cells and essential for the synthesis of the unique xylan reducing end sequence. Xylan is made of a linear chain of β-1,4-linked xylosyl (Xyl) residues that are often substituted with (methyl)glucuronic acid [(Me)GlcA] side chains and may be acetylated at O-2 and/or O-3. The reducing end of xylan from gymnosperms and dicots contains a unique tetrasaccharide sequence consisting of β-D-Xylp-(1 → 3)-α-L-Rhap-(1 → 2)-α-D-GalpA-(1 → 4)-D-Xylp, the synthesis of which requires four different glycosyltransferase activities. Genetic analysis in Arabidopsis thaliana has so far implicated three glycosyltransferase genes, FRA8 (fragile fiber 8), IRX8 (irregular xylem 8) and PARVUS, in the synthesis of this unique xylan reducing end sequence. Here, we report the essential role of FRA9, a member of glycosyltransferase family 106 (GT106), in the synthesis of this sequence. The expression of the FRA9 gene was shown to be induced by secondary wall master transcriptional regulators and specifically associated with secondary wall-forming cells, including xylem and fiber cells. T-DNA knockout mutation of the FRA9 gene caused impaired secondary cell wall thickening in leaf veins and a severe arrest of plant growth. RNA interference (RNAi) downregulation of FRA9 led to a significant reduction in secondary wall thickening of fibers, a deformation of xylem vessels and a decrease in xylan content. Structural analysis of xylanase-released xylooligomers revealed that RNAi downregulation of FRA9 resulted in a diminishment of the unique xylan reducing end sequence and complete methylation of xylan GlcA side chains, chemotypes reminiscent of those of the fra8, irx8 and parvus mutants. Furthermore, two FRA9 close homologs from Populus trichocarpa were found to be wood-associated functional orthologs of FRA9. Together, our findings uncover a member of the GT106 family as a new player involved in the synthesis of the unique reducing end sequence of xylan.

Outstanding questions on xylan biosynthesis

Xylan is the second most abundant polysaccharide in plant biomass. It is a crucial component of cell wall structure as well as a significant factor contributing to biomass recalcitrance. Xylan consists of a linear chain of β-1,4-linked xylosyl residues that are often substituted with glycosyl side chains, such as glucuronosyl/methylglucuronosyl and arabinofuranosyl residues, and acetylated at O-2 and/or O-3. Xylan from gymnosperms and dicots contains a unique reducing end tetrasaccharide sequence that is not detected in xylan from grasses, bryophytes and seedless vascular plants. Grass xylan is heavily decorated at O-3 with arabinofuranosyl residues that are frequently esterified with hydroxycinnamates. Genetic and biochemical studies have uncovered a number of genes involved in xylan backbone elongation and acetylation, xylan glycosyl substitutions and their modifications, and the synthesis of the unique xylan reducing end tetrasaccharide sequence, but some outstanding issues on the biosynthesis of xylan still remain unanswered. Here, we provide a brief overview of xylan structure and focus on discussion of the current understanding and open questions on xylan biosynthesis. Further elucidation of the biochemical mechanisms underlying xylan biosynthesis will not only shed new insights into cell wall biology but also provide molecular tools for genetic modification of biomass composition tailored for diverse end uses.

Independent recruitment of glycosyltransferase family 61 members for xylan substitutions in conifers

Several pine members of the gymnosperm-specific GT61 clades were demonstrated to be arabinosyltransferases and xylosyltransferases catalyzing the transfer of 2-O-Araf, 3-O-Araf and 2-O-Xyl side chains onto xylooligomer acceptors, indicating their possible involvement in Araf and Xyl substitutions of xylan in pine. Xylan in conifer wood is substituted at O-2 with methylglucuronic acid (MeGlcA) as well as at O-3 with arabinofuranose (Araf), which differs from xylan in dicot wood that is typically decorated with MeGlcA but not Araf. Currently, glycosyltransferases responsible for conifer xylan arabinosylation have not been identified. Here, we investigated the roles of pine glycosyltransferase family 61 (GT61) members in xylan substitutions. Biochemical characterization of four pine wood-associated GT61 members showed that they exhibited three distinct glycosyltransferase activities involved in xylan substitutions. Two of them catalyzed the addition of 2-O-α-Araf or 3-O-α-Araf side chains onto xylooligomer acceptors and thus were named Pinus taeda xylan 2-O-arabinosyltransferase 1 (PtX2AT1) and 3-O-arabinosyltransferase 1 (PtX3AT1), respectively. Two other pine GT61 members were found to be xylan 2-O-xylosyltransferases (PtXYXTs) adding 2-O-β-Xyl side chains onto xylooligomer acceptors. Furthermore, sequential reactions with PtX3AT1 and the PtGUX1 xylan glucuronyltransferase demonstrated that PtX3AT1 could efficiently arabinosylate glucuronic acid (GlcA)-substituted xylooligomers and likewise, PtGUX1 was able to add GlcA side chains onto 3-O-Araf-substituted xylooligomers. Phylogenetic analysis revealed that PtX2AT1, PtX3AT1 and PtXYXTs resided in three gymnosperm-specific GT61 clades that are separated from the grass-expanded GT61 clade harboring xylan 3-O-arabinosyltransferases and 2-O-xylosyltransferases, suggesting that they might have been recruited independently for xylan substitutions in gymnosperms. Together, our findings have established several pine GT61 members as xylan 2-O- and 3-O-arabinosyltransferases and 2-O-xylosyltransferases and they indicate that pine xylan might also be substituted with 2-O-Araf and 2-O-Xyl side chains.

Bacterial xylan utilization regulons: Systems for coupling depolymerization of methylglucuronoxylans with assimilation and metabolism

Bioconversion of lignocellulosic resources to fuels and chemicals offers an economically promising path to renewable energy. Technological challenges to achieving bioconversion include the development of cost-effective processes that render the cellulose and hemicellulose components of these resources to fermentable hexoses and pentoses. Natural bioprocessing of the hemicellulose fraction of lignocellulosic biomass requires depolymerization of methylglucuronoxylans. This depends upon the secretion of endoxylanases that release xylooligosaccharides and aldouronates. Physiological, biochemical and genetic studies with selected bacteria support a process in which a cell-anchored multimodular GH10 endoxylanase catalyzes the release of the hydrolysis products, aldotetrauronate, xylotriose, and xylobiose that are directly assimilated and metabolized. Gene clusters encoding intracellular enzymes, including α-glucuronidase, endo-xylanase, β-xylosidase, ABC transporter proteins, and transcriptional regulators are coordinately responsive to substrate induction or repression. The rapid rates of glucuronoxylan utilization and microbial growth, along with the absence of detectable products of depolymerization in the medium, indicate that assimilation and depolymerization are coupled processes. Genomic comparisons provide evidence that such systems occur in xylanolytic species in several genera, including Clostridium, Geobacillus, Paenibacillus, and Thermotoga. These systems offer promise, either in their native configurations or through gene transfer to other organisms, to develop biocatalysts for efficient production of fuels and chemicals from the hemicellulose fractions of lignocellulosic resources.

The coordinated action of glucuronoyl esterase and α-glucuronidase promotes the disassembly of lignin-carbohydrate complexes

Glucuronoxylans represent a significant fraction of woody biomass, and its decomposition is complicated by the presence of lignin–carbohydrate complexes (LCCs). Herein, LCCs from birchwood were used to investigate the potential coordinated action of a glucuronoyl esterase (TtCE15A) and two α‐glucuronidases (SdeAgu115A and AxyAgu115A). When supplementing α‐glucuronidase with equimolar quantities of TtCE15A, total MeGlcpA released after 72 h by SdeAgu115A and AxyAgu115A increased from 52% to 67%, and 61% to 95%, respectively. Based on the combined TtCE15A and AxyAgu115A activities, ~ 34% of MeGlcpA in the extracted birchwood glucuronoxylan was occupied as LCCs. Notably, insoluble LCC fractions reduced soluble α‐glucuronidase concentrations by up to 70%, whereas reduction in soluble TtCE15A was less than 30%, indicating different tendencies to adsorb onto the LCC substrate.

Cellulose Nanofibers from a Dutch Elm Disease-Resistant Ulmus minor Clone

The potential use of elm wood in lignocellulosic industries has been hindered by the Dutch elm disease (DED) pandemics, which have ravaged European and North American elm groves in the last century. However, the selection of DED-resistant cultivars paves the way for their use as feedstock in lignocellulosic biorefineries. Here, the production of cellulose nanofibers from the resistant Ulmus minor clone Ademuz was evaluated for the first time. Both mechanical (PFI refining) and chemical (TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation) pretreatments were assessed prior to microfluidization, observing not only easier fibrillation but also better optical and barrier properties for elm nanopapers compared to eucalyptus ones (used as reference). Furthermore, mechanically pretreated samples showed higher strength for elm nanopapers. Although lower nanofibrillation yields were obtained by mechanical pretreatment, nanofibers showed higher thermal, mechanical and barrier properties, compared to TEMPO-oxidized nanofibers. Furthermore, lignin-containing elm nanofibers presented the most promising characteristics, with slightly lower transparencies.

In-depth structural characterization of pentosan polysulfate sodium complex drug using orthogonal analytical tools

Rapid advances have been made in developing analytical technologies for characterization of highly heterogeneous active ingredients of complex drugs, such as pentosan polysulfate (PPS), active ingredient of the drug Elmiron®, approved by the Food and Drug Administration and marketed in the United States to treat interstitial cystitis. PPS sulfated polysaccharides comprise of a repeat unit of β(1-4)-D-xylopyranoses randomly substituted by 4-O-methyl-glucopyranosyluronic acid. To define the critical quality attributes (CQAs) of such a complex drug, it is critical to develop an approach that integrates data from orthogonal analytical methodologies. Here, we developed an approach integrating diverse analytical tools including gel permeation chromatography, LC/ESI-MS and NMR to measure CQAs of PPS. The proposed mathematical framework integrates the data from these diverse analytical methods as function of PPS chain length and building blocks. Our approach would facilitate in establishing a scientific foundation for comparative characterization of drug products with complex active ingredients.

Secondary cell wall biosynthesis

Contents Summary 1703 I. Introduction 1703 II. Cellulose biosynthesis 1705 III. Xylan biosynthesis 1709 IV. Glucomannan biosynthesis 1713 V. Lignin biosynthesis 1714 VI. Concluding remarks 1717 Acknowledgements 1717 References 1717 SUMMARY: Secondary walls are synthesized in specialized cells, such as tracheary elements and fibers, and their remarkable strength and rigidity provide strong mechanical support to the cells and the plant body. The main components of secondary walls are cellulose, xylan, glucomannan and lignin. Biochemical, molecular and genetic studies have led to the discovery of most of the genes involved in the biosynthesis of secondary wall components. Cellulose is synthesized by cellulose synthase complexes in the plasma membrane and the recent success of in vitro synthesis of cellulose microfibrils by a single recombinant cellulose synthase isoform reconstituted into proteoliposomes opens new doors to further investigate the structure and functions of cellulose synthase complexes. Most genes involved in the glycosyl backbone synthesis, glycosyl substitutions and acetylation of xylan and glucomannan have been genetically characterized and the biochemical properties of some of their encoded enzymes have been investigated. The genes and their encoded enzymes participating in monolignol biosynthesis and modification have been extensively studied both genetically and biochemically. A full understanding of how secondary wall components are synthesized will ultimately enable us to produce plants with custom-designed secondary wall composition tailored to diverse applications.

Designer biomass for next-generation biorefineries: leveraging recent insights into xylan structure and biosynthesis

Xylans are the most abundant noncellulosic polysaccharides in lignified secondary cell walls of woody dicots and in both primary and secondary cell walls of grasses. These polysaccharides, which comprise 20-35% of terrestrial biomass, present major challenges for the efficient microbial bioconversion of lignocellulosic feedstocks to fuels and other value-added products. Xylans play a significant role in the recalcitrance of biomass to degradation, and their bioconversion requires metabolic pathways that are distinct from those used to metabolize cellulose. In this review, we discuss the key differences in the structural features of xylans across diverse plant species, how these features affect their interactions with cellulose and lignin, and recent developments in understanding their biosynthesis. In particular, we focus on how the combined structural and biosynthetic knowledge can be used as a basis for biomass engineering aimed at developing crops that are better suited as feedstocks for the bioconversion industry.

Structural Investigation of Cell Wall Xylan Polysaccharides from the Leaves of Algerian Argania spinosa

Xylan-type polysaccharides were isolated from the leaves of Argania spinosa (L.) Skeels collected in the Tindouf area (southwestern Algeria). Xylan fractions were obtained by sequential alkaline extractions and purified on Sepharose CL-4B. The xylan structure was investigated by enzymatic hydrolysis with an endo-β(1→4)-xylanase followed by chromatography of the resulting fragments on Biogel P2, characterization by sugar analysis and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS ). The results show that the A. spinosa xylan is composed of a β-(1→4)-d-xylopyranose backbone substituted with 4-O-methyl-d-glucuronic acid and L-arabinose residues.

MALDI-TOF MS analysis of cellodextrins and xylo-oligosaccharides produced by hindgut homogenates of Reticulitermes santonensis

Hindgut homogenates of the termite Reticulitermes santonensis were incubated with carboxymethyl cellulose (CMC), crystalline celluloses or xylan substrates. Hydrolysates were analyzed with matrix-assisted laser desorption/ionization coupled to time-of-flight mass spectrometry (MALDI-TOF MS). The method was first set up using acid hydrolysis analysis to characterize non-enzymatic profiles. Commercial enzymes of Trichoderma reesei or T. longibrachiatum were also tested to validate the enzymatic hydrolysis analysis. For CMC hydrolysis, data processing and visual display were optimized to obtain comprehensive profiles and allow rapid comparison and evaluation of enzymatic selectivity, according to the number of substituents of each hydrolysis product. Oligosaccharides with degrees of polymerization (DPs) ranging from three to 12 were measured from CMC and the enzymatic selectivity was demonstrated. Neutral and acidic xylo-oligosaccharides with DPs ranging from three to 11 were measured from xylan substrate. These results are of interest for lignocellulose biomass valorization and demonstrated the potential of termites and their symbiotic microbiota as a source of interesting enzymes for oligosaccharides production.

GUX1 and GUX2 glucuronyltransferases decorate distinct domains of glucuronoxylan with different substitution patterns

Xylan comprises up to one-third of plant cell walls, and it influences the properties and processing of biomass. Glucuronoxylan in Arabidopsis is characterized by a linear β-(1,4)-linked backbone of xylosyl residues substituted by glucuronic acid and 4-O-methylglucuronic acid (collectively termed [Me]GlcA). The role of these substitutions remains unclear. GUX1 (glucuronic acid substitution of xylan 1) and GUX2, recently identified as glucuronyltransferases, are both required for substitution of the xylan backbone with [Me]GlcA. Here, we demonstrate clear differences in the pattern of [Me]GlcA substitution generated by each of these glucuronyltransferases. GUX1 decorates xylan with a preference for addition of [Me]GlcA at evenly spaced xylosyl residues. Intervals of eight or 10 residues dominate, but larger intervals are observed. GUX2, in contrast, produces more tightly clustered decorations with most frequent spacing of five, six or seven xylosyl residues, with no preference for odd or even spacing. Moreover, each of these GUX transferases substitutes a distinct domain of secondary cell wall xylan, which we call the major and minor domains. These major and minor xylan domains were not separable from each other by size or charge, a finding that suggests that they are tightly associated. The presence of both differently [Me]GlcA decorated domains may produce a xylan molecule that is heterogeneous in its properties. We speculate that the major and minor domains of xylan may be specialised, such as for interaction with cellulose or lignin. These findings have substantial implications for our understanding of xylan synthesis and structure, and for models of the molecular architecture of the lignocellulosic matrix of plant cell walls.

Mass spectrometry for characterizing plant cell wall polysaccharides

Mass spectrometry is a selective and powerful technique to obtain identification and structural information on compounds present in complex mixtures. Since it requires only small sample amount it is an excellent tool for researchers interested in detecting changes in composition of complex carbohydrates of plants. This mini-review gives an overview of common mass spectrometry techniques applied to the analysis of plant cell wall carbohydrates. It presents examples in which mass spectrometry has been used to elucidate the structure of oligosaccharides derived from hemicelluloses and pectins and illustrates how information on sequence, linkages, branching, and modifications are obtained from characteristic fragmentation patterns.

Purification and characterization of two thermostable xylanases from Malbranchea flava active under alkaline conditions

Two xylanases, MFX I and MFX II, from the thermophilic fungus Malbranchea flava MTCC 4889 with molecular masses of 25.2 and 30kDa and pIs of 4.5 and 3.7, respectively were purified to homogeneity. The xylanases were optimally active at pH 9.0 and at 60 degrees C, exhibited a half-life of 4h at 60 degrees C, and showed distinct mode of action and product profiles when applied to birchwood, oat spelt, and larchwood xylan, and to wheat and rye arabinoxylan. The xylanases were most active on larchwood xylan with K(m) values of 1.25 and 3.7mg/ml. K(cat)/K(m) values suggested that the xylanases preferentially hydrolyzed rye arabinoxylan. LC-MS/MS (liquid chromatography/mass spectrometry) analysis of tryptic digests of MFX I and MFX II revealed similarity with known fungal xylanases and suggests that that they belonged to the GH 11 and 10 glycosyl hydrolase super families, respectively. These xylanases can potentially be used in enzyme-assisted bleaching of the pulp derived from agro-residues, as well as production of xylooligosaccharides for pre-biotic functional food applications.

Genetic engineering of Enterobacter asburiae strain JDR-1 for efficient production of ethanol from hemicellulose hydrolysates

Dilute acid pretreatment is an established method for hydrolyzing the methylglucuronoxylans of hemicellulose to release fermentable xylose. In addition to xylose, this process releases the aldouronate methylglucuronoxylose, which cannot be metabolized by current ethanologenic biocatalysts. Enterobacter asburiae JDR-1, isolated from colonized wood, was found to efficiently ferment both methylglucuronoxylose and xylose in acid hydrolysates of sweet gum xylan, producing predominantly ethanol and acetate. Transformation of E. asburiae JDR-1 with pLOI555 or pLOI297, each containing the PET operon containing pyruvate decarboxylase (pdc) and alcohol dehydrogenase B (adhB) genes derived from Zymomonas mobilis, replaced mixed-acid fermentation with homoethanol fermentation. Deletion of the pyruvate formate lyase (pflB) gene further increased the ethanol yield, resulting in a stable E. asburiae E1(pLOI555) strain that efficiently utilized both xylose and methylglucuronoxylose in dilute acid hydrolysates of sweet gum xylan. Ethanol was produced from xylan hydrolysate by E. asburiae E1(pLOI555) with a yield that was 99% of the theoretical maximum yield and at a rate of 0.11 g ethanol/g (dry weight) cells/h, which was 1.57 times the yield and 1.48 times the rate obtained with the ethanologenic strain Escherichia coli KO11. This engineered derivative of E. asburiae JDR-1 that is able to ferment the predominant hexoses and pentoses derived from both hemicellulose and cellulose fractions is a promising subject for development as an ethanologenic biocatalyst for production of fuels and chemicals from agricultural residues and energy crops.

Structural characterization and cytotoxic properties of a 4-O-methylglucuronoxylan from castanea sativa. 2. Evidence of a structure-activity relationship

Xylans were purified from delignified holocellulose alkaline extracts of Castanea sativa (Spanish chestnut) and Argania spinosa (Argan tree) and their structures analyzed by means of GC of their per-trimethylsilylated methylglycoside derivatives and (1)H NMR spectroscopy. The structures deduced were characteristic of a 4-O-methylglucuronoxylan (MGX) and a homoxylan (HX), respectively, with degrees of polymerization ranging from 182 to 360. In the case of MGX, the regular or random distribution of 4-O-methylglucuronic acid along the xylosyl backbone--determined by MALDI mass spectrometry after autohydrolysis of the polysaccharide--varied and depended both on the botanical source from which they were extracted and on the xylan extraction procedure. The MGX also inhibited in different ways the proliferation as well as the migration and invasion capability of A431 human epidermoid carcinoma cells. These biological properties could be correlated with structural features including values of the degree of polymerization, 4-O-MeGlcA to xylose ratios, and distribution of 4-O-MeGlcA along the xylosyl backbone, giving evidence of a defined structure-activity relationship.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update covering the period 2001-2002

This review is the second update of the original review on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates that was published in 1999. It covers fundamental aspects of the technique as applied to carbohydrates, fragmentation of carbohydrates, studies of specific carbohydrate types such as those from plant cell walls and those attached to proteins and lipids, studies of glycosyl-transferases and glycosidases, and studies where MALDI has been used to monitor products of chemical synthesis. Use of the technique shows a steady annual increase at the expense of older techniques such as FAB. There is an increasing emphasis on its use for examination of biological systems rather than on studies of fundamental aspects and method development and this is reflected by much of the work on applications appearing in tabular form.

Characterization of XynC from Bacillus subtilis subsp. subtilis strain 168 and analysis of its role in depolymerization of glucuronoxylan

Secretion of xylanase activities by Bacillus subtilis 168 supports the development of this well-defined genetic system for conversion of methylglucuronoxylan (MeGAXn [where n represents the number of xylose residues]) in the hemicellulose component of lignocellulosics to biobased products. In addition to the characterized glycosyl hydrolase family 11 (GH 11) endoxylanase designated XynA, B. subtilis 168 secretes a second endoxylanase as the translated product of the ynfF gene. This sequence shows remarkable homology to the GH 5 endoxylanase secreted by strains of Erwinia chrysanthemi. To determine its properties and potential role in the depolymerization of MeGAXn, the ynfF gene was cloned and overexpressed to provide an endoxylanase, designated XynC, which was characterized with respect to substrate preference, kinetic properties, and product formation. With different sources of MeGAXn as the substrate, the specific activity increased with increasing methylglucuronosyl substitutions on the beta-1,4-xylan chain. With MeGAXn from sweetgum as a preferred substrate, XynC exhibited a Vmax of 59.9 units/mg XynC, a Km of 1.63 mg MeGAXn/ml, and a k(cat) of 2,635/minute at pH 6.0 and 37 degrees C. Matrix-assisted laser desorption ionization-time of flight mass spectrometry and 1H nuclear magnetic resonance data revealed that each hydrolysis product has a single glucuronosyl substitution penultimate to the reducing terminal xylose. This detailed analysis of XynC from B. subtilis 168 defines the unique depolymerization process catalyzed by the GH 5 endoxylanases. Based upon product analysis, B. subtilis 168 secretes both XynA and XynC. Expression of xynA was subject to MeGAXn induction; xynC expression was constitutive with growth on different substrates. Translation and secretion of both GH 11 and GH 5 endoxylanases by the fully sequenced and genetically malleable B. subtilis 168 recommends this bacterium for the introduction of genes required for the complete utilization of products of the enzyme-catalyzed depolymerization of MeGAXn. B. subtilis may serve as a model platform for development of gram-positive biocatalysts for conversion of lignocellulosic materials to renewable fuels and chemicals.

Electrospray tandem mass spectrometry of underivatised acetylated xylo-oligosaccharides

Acetylated neutral (Xyl(n)Ac(m)) and acidic xylo-oligosaccharides (Xyl(n)Ac(m)MeGlcA, and Xyl(n)Ac(m)MeGlcAHex) obtained by partial acid hydrolysis of Eucalyptus globulus wood glucuronoxylans and fractionated by preparative ligand exchange/size-exclusion chromatography were identified by electrospray ionisation mass spectrometry (ESI-MS). Low molecular weight acetylated xylo-oligosaccharides were studied by ESI-tandem mass spectrometry (MS/MS). All the acetylated xylo-oligosaccharides showed an abundant ion due to the neutral loss of 60 Da (CH(3)CO(2)H) in the MS/MS spectra. The presence of diacetylated xylo-oligosaccharides was confirmed by the ions formed by loss of two molecules of acetic acid. Furthermore, characteristic [Xyl(res)Ac(2)+Na](+) and [XylAc(2)+Na](+) ions, and ions due to loss of XylAc(2), indicate that both acetyl groups are located in the same Xyl residue. On the other hand, losses of Xyl(res)Ac and XylAc are also observed as well as [Xyl(res)Ac+Na](+) and [XylAc+Na](+) , indicating the location of both acetyl groups in different Xyl residues, in some cases even in adjacent xyloses. The MS/MS spectra of triacetylated xylo-oligosaccharides were complex due to the presence of different isobaric xylo-oligosaccharides containing the acetyl groups at different locations in the xylo-oligosaccharide backbone. In the MS/MS spectra of acidic xylo-oligosaccharides, the ion at m/z 387, [Xyl(res)AcMeGlcA+Na](+), indicates that the acetyl groups are preferentially linked to Xyl substituted with MeGlcA. However, acidic xylo-oligosaccharides with the acetyl and 4-O-methylglucuronic acid groups in different Xyl residues were also identified. In neutral and in acidic xylo-oligosaccharides several possible locations of the acetyl groups were identified, namely at terminal positions. In summary, ESI-MS/MS is shown to be a powerful tool for the characterisation of acetylated patterns in complex mixtures of oligosaccharides.

Cloning, functional expression and characterization of three Phanerochaete chrysosporium endo-1,4-beta-xylanases

Three Phanerochaete chrysosporium endo-1,4-beta-xylanase genes were cloned and expressed in Aspergillus niger. Two of these genes, xynA and xynC, encode family 10 glycoside hydrolases, while the third, xynB, codes for a family 11 glycoside hydrolase. All three xylanases possess a type I carbohydrate-binding domain connected to the catalytic domain by a linker region. The three xylanases were purified to homogeneity by weak anion or Avicell column chromatography and subsequently characterized. The XynA, XynB and XynC enzymes have molecular masses of 52, 30 and 50 kDa, respectively. Optimal activity was obtained at pH 4.5 and 70 degrees C with the family 10 xylanases and pH 4.5 and 60 degrees C with the family 11 xylanase. The measured Km when using birchwood xylan as the substrate was 3.71 +/- 0.69 mg/ml for XynA and XynC and was 9.96 +/- 1.45 mg/ml for XynB. Substrate specificity studies and the products released during the degradation of birchwood xylan suggest differences in catalytic properties between the two family 10 xylanases and the family 11 xylanase.

Differentiation of isomeric oligosaccharide structures by ESI tandem MS and GC–MS

A mixture of arabinoxylan oligosaccharides from wheat seedling was permethylated and analyzed by electrospray ion trap MS and GC-MS. The presence of isomeric structures differing in degree of branching and position of the branched residue along the xylose backbone was demonstrated for oligosaccharides with four and five monosaccharide residues. No isomeric structures were found for oligosaccharides with three monosaccharide residues. Linkage analysis by GC-MS reveals that xylose residues were substituted with single arabinoxyl residues at C-3.

Structural differentiation of uronosyl substitution patterns in acidic heteroxylans by electrospray tandem mass spectrometry

The structures of two oligomers of acidic xylo-oligosaccharides (XOS) of the same molecular weight (634 Da), Xyl(2)MeGlcAHex and Xyl(2)GlcA(2) were differentiated by electrospray tandem mass spectrometry (ESI-MS/MS). These oligomers were present in a mixture of XOS obtained by acid hydrolysis of heteroxylans extracted from Eucalyptus globulus wood (Xyl(2)MeGlcAHex) and Olea europaea olive fruit (Xyl(2)GlcA(2)). In the ESI-MS spectra of the XOS, ions at m/z 657 and 652 were observed and assigned to [M + Na](+) and [M + NH(4)](+), respectively. The ESI-MS/MS spectrum of [M + Na](+) ion of Xyl(2)MeGlcAHex showed the loss of Hex residue from the reducing end followed by the loss of MeGlcA moiety. Simultaneously, the loss of a Xyl residue from either the reducing or the non-reducing ends was detected. On the other hand, the fragmentation of Xyl(2)GlcA(2) occurs mainly by the loss of one and two GlcA residues or by the loss of the GlcAXyl moiety, due to the glycosidic bond cleavage between the two Xyl residues. Loss of one and two CO(2) molecules was only observed for this oligomer, where the GlcA are in vicinal Xyl residues. The ESI-MS/MS spectra of [M + NH(4)](+) of both oligomers showed the loss of NH(3), resulting in the protonated molecule, where the presence of ions assigned as protonated molecules of aldobiuronic acid residues, [MeGlcA - Xyl + H](+) and [GlcA - Xyl + H](+), are diagnostic ions of the presence of MeGlcA and GlcA moieties in XOS. Since these structures occur in small amounts in complex acidic XOS mixtures and are very difficult, if possible, to isolate, tandem mass spectrometry revealed to be a powerful tool for the characterization of these types of substitution patterns present in heteroxylans.

Isolation and characterization of water-soluble hemicelluloses from flax shive

Partially depolymerized, water-soluble hemicelluloses were solubilized from flax shive employing hydrothermal microwave treatment and thereafter subjected to ion-exchange chromatography, enzymatic purification and/or size-exclusion chromatography (SEC). The oligo- and polysaccharide fractions thus obtained were characterized with respect to molar mass, molar mass distribution, degree of polymerization (DP) and degree of substitution with acetyl moieties (DSAc) by employing SEC in combination with MALDI-TOF mass spectrometry. The major portion of the water-soluble flax hemicellulose consisted of an O-acetyl-4-O-methylglucuronoxylan exhibiting a DPp value (i.e., peak-average DP) of 28. When the DSAc for this O-acetyl-4-O-methylglucuronoxylan was calculated on the basis of the MALDI-MS spectra obtained without and following deacetylation, a value of 0.7 was obtained. In addition, an O-acetyl-glucomannan (DPp=9, DS=0.4) and minor quantities of small neutral O-acetyl-xylooligosaccharides were also isolated from the mixture of water-soluble hemicelluloses released from the flax shive by microwave treatment.

Positive and negative electrospray ionisation tandem mass spectrometry as a tool for structural characterisation of acid released oligosaccharides from olive pulp glucuronoxylans

Xylo-oligosaccharides with degrees of polymerisation 5-13, formed by partial acid hydrolysis from an extract representative of olive pulp glucuronoxylans (GX), were analysed by electrospray ionisation mass spectrometry (ESI-MS), both in positive and negative modes. The positive spectrum showed the presence of xylo-oligosaccharides in the mass range between m/z 500 and 1500 corresponding to singly [M+Na](+) charged ions of neutral (Xyl(7-9)) and acidic xylo-oligosaccharides (Xyl(5-9)MeGlcA), and doubly [M+2Na](2+) charged ions of Xyl(9-13) and Xyl(7-11)MeGlcA. Ammonium adducts [M+NH(4)](+) were also observed for Xyl(5-9)MeGlcA. The negative spectra showed the contribution of ions in the mass range between m/z 600 and 1400, ascribed to the deprotonated molecules [M-H](-) of Xyl(3-9)MeGlcA. Tandem mass spectrometry (MS/MS) of the major ions observed in the MS spectra was performed. The MS/MS spectra of the [M+Na](+) adducts showed the loss of MeGlcA residues as the major fragmentation pathway and glycosidic fragment ions of Xyl(n) and Xyl(n)MeGlcA structures. The MS/MS spectra of the [M+NH(4)](+) adducts suggests the occurrence of isomers of Xyl(5-9)MeGlcA oligosaccharides with the MeGlcA residue at the reducing end and at the non-reducing end of the molecules, although other structural isomers can also occur. Both glycosidic bond and cross-ring cleavages in the MS/MS spectra of the [M-H](-) ion suggest the occurrence of Xyl(3-9)MeGlcA with the substituting group at the reducing end position of the xylose backbone, as the main fragmentation ions. The results obtained by ESI-MS/MS, both in positive and negative modes, of Xyl(7-13)- and Xyl(5-11)MeGlcA, allow to identify fragmentation patterns of the structural isomers with MeGlcA linked to the terminal xylosyl residues of the oligosaccharides. The occurrence of these higher molecular weight oligosaccharides with a low substitution pattern allows to infer a scatter and random distribution of MeGlcA along the xylan backbone of olive pulp.

Characterization of water-soluble hemicelluloses from spruce and aspen employing SEC/MALDI mass spectroscopy

Partly depolymerized hemicelluloses isolated from wood chips of spruce and aspen employing microwave treatment were resolved using size-exclusion chromatography (SEC) into oligo- and polysaccharide fractions containing components with a narrow range of sizes, as determined by MALDI mass spectroscopy. The degree of substitution with acetyl moieties (DS) was also calculated on the basis of the MALDI-MS spectra obtained prior to and following deacetylation. For spruce hemicelluloses, the low molecular mass fraction contained small arabino-4-O-methylglucuronoxylan oligosaccharides, with DP values ranging from 4 to approximately 20, separated primarily on the basis of their charge density. The fraction eluted last consisted of an O-acetyl-(galacto)glucomannan polysaccharide of peak-average DP value (DP(p)) 14. The degree of substitution with acetyl groups (DS) decreased with decreasing DP, a value DS of 0.39 being obtained for the fraction with DP(p) 12. For the aspen hemicelluloses, the SEC fractions eluted first contained an acidic O-acetyl-4-O-methylglucuronoxylan polysaccharide with DP ranging from 10 to approximately 28 and an average DS of approximately 0.75. The fractions eluted last consisted of oligosaccharide mixtures composed primarily of small neutral O-acetyl-xylooligosaccharides (DP(p) 6, DS 0.41), together with minor quantities of an O-acetyl-glucomannan.