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PubMed2002

Influence of the transposition of the thermostabilizing domain of Clostridium thermocellum xylanase (XynX) on xylan binding and thermostabilization.

A xylanase gene, xynX, of Clostridium thermocellum had one thermostabilizing domain (TSD) between the signal peptide sequence and the catalytic domain (CD). The TSD of a truncated xylanase gene, xynX'(TSD-CD), was transpositioned from the N terminus to the C terminus of the CD by overlapping PCRs, and a modified product, xynX'(CD-TSD), was constructed. XynX'(TSD-CD) had a higher optimum temperature (70 degrees C versus 65 degrees C) and was more thermostable (residual activity of 68% versus 46% after a 20-min preincubation at 70 degrees C) than the one without the TSD, XynX'(CD). However, the domain-transpositioned enzyme, XynX'(CD-TSD), showed a lower optimum temperature (30 degrees C) and thermostability (20%) than XynX'(CD). Both XynX'(TSD-CD) and XynX'(CD-TSD) showed significantly higher binding capacity toward xylan than XynX'(CD), and the domain transposition did not cause any change in the binding ability. XynX'(TSD-CD) and XynX'(CD-TSD) also showed considerable binding to lichenan but not to carboxymethyl cellulose and laminarin. XynX'(TSD-CD) and XynX'(CD-TSD) had higher activities for insoluble xylan than XynX'(CD), while XynX'(CD) was more active against soluble xylan than XynX'(TSD-CD) and XynX'(CD-TSD). These results indicate that the TSD of XynX has dual functions, xylan binding and thermostabilization, and the domain should also be classified as a xylan-binding domain (XBD). The binding capacity of the XBD was not affected by domain transpositioning within the gene.

Clostridium↗
PubMed2004

Effect of xylan and lignin removal by batch and flowthrough pretreatment on the enzymatic digestibility of corn stover cellulose.

Compared with batch systems, flowthrough and countercurrent reactors have important potential advantages for pretreating cellulosic biomass, including higher hemicellulose sugar yields, enhanced cellulose digestibility, and reduced chemical additions. Unfortunately, they suffer from high water and energy use. To better understand these trade-offs, comparative data are reported on xylan and lignin removal and enzymatic digestibility of cellulose for corn stover pretreated in batch and flowthrough reactors over a range of flow rates between 160 degrees and 220 degrees C, with water only and also with 0.1 wt% sulfuric acid. Increasing flow with just water enhanced the xylan dissolution rate, more than doubled total lignin removal, and increased cellulose digestibility. Furthermore, adding dilute sulfuric acid increased the rate of xylan removal for both batch and flowthrough systems. Interestingly, adding acid also increased the lignin removal rate with flow, but less lignin was left in solution when acid was added in batch. Although the enzymatic hydrolysis of pretreated cellulose was related to xylan removal, as others have shown, the digestibility was much better for flowthrough compared with batch systems, for the same degree of xylan removal. Cellulose digestibility for flowthrough reactors was related to lignin removal as well. These results suggest that altering lignin also affects the enzymatic digestibility of corn stover.

Bioreactors↗
PubMed1993

Esterases of xylan-degrading microorganisms: production, properties, and significance.

This review focuses on the description of recently discovered esterase enzymes involved in xylan degradation (acetyl xylan, feruloyl, and p-coumaroyl esterases). The occurrence of these enzymes in various microorganisms, assays used for determination of their activity, induction and production on different substrates, interaction with other xylanolytic enzymes, mode of action, substrate specificity, and biochemical characteristics are presented. The nature of substrates on which acetyl xylan esterase, feruloyl, and p-coumaroyl esterase are active and their role in xylan hydrolysis is emphasized. The potential applications of xylan-debranching esterases are outlined and their significance to applied microbiology is discussed.

Acetylesterase↗
PubMed2005

Conformational analysis of xylan chains.

The present study provides a theoretical description of the different levels of structural organization that characterize the xylan polysaccharide in its native and hydrophobic lauroyl esterified forms. The goal of this study was to ascertain the role played by the hydroxyl or lauroyl side groups on the conformational flexibility of the xylan chain backbone. The results reported provide a detailed description of the low-energy conformers of the dimer segments, a complete characterization of the helical structures, an insight into the disordered state of the polysaccharide chains and an estimation of the cohesion of the amorphous solids. Esterification of xylan hydroxyl groups by lauric acid has a large effect on the conformational properties of the glycosidic bonds linking two repeat units. Both the location and the relative energies of the low energy areas of the potential energy surfaces strongly differ: extended and coiled conformations are preferred for the native and hydrophobic forms, respectively. Consequently, the predicted unperturbed polymer chain extension strongly depends on the structure, predicted Lp of the native xylan of 35 A compares favourably well with the experimental ones, this characteristic dramatically decreases to 9A for the hydrophobically modified chain. Curiously, only extended 2(1) and left-handed 3(1) helical structures are calculated stable for both polymers. The estimated cohesive parameters of amorphous bulks reveal that inter-chain interactions are stronger for the xylan chain than that for modified one, the former being stabilized by hydrogen bonds whereas hydrophobic interactions play a determinant role for the latter.

Carbohydrate Conformation↗
PubMed2002

Isolation and structure of D-xylans from pericarp seeds of Opuntia ficus-indica prickly pear fruits.

Xylans were isolated from the pericarp of prickly pear seeds of Opuntia ficus-indica (OFI) by alkaline extraction, fractionated by precipitation and purified. Six fractions were obtained and characterized by sugar analysis and NMR spectroscopy. They were assumed to be (4-O-methyl-D-glucurono)-D-xylans, with 4-O-alpha-D-glucopyranosyluronic acid groups linked at C-2 of a (1-->4)-beta-D-xylan. The sugar composition and the 1H and 13C NMR spectra showed that their chemical structures were very similar, but with different proportions of D-Xyl and 4-O-Me-D-GlcA. Our results showed that, on average, the water soluble xylans have one nonreducing terminal residue of 4-O-methyl-D-glucuronic acid for every 11 to 14 xylose units, whereas in the water non-soluble xylans, xylose units can varied from 18 to 65 residues for one nonreducing terminal residue of 4-O-methyl-D-glucuronic acid.

Magnetic Resonance Spectroscopy↗
PubMed2001

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

The distribution of 4-O-methylglucuronic acid residues along the polysaccharide chains of xylans isolated from birch, aspen, spruce, pine, and larch was studied here employing matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) analysis of the oligosaccharide mixtures obtained by partial acid hydrolysis. The hydrolyzates thus obtained were analyzed by MALDI-MS or by capillary electrophoresis as well as by size exclusion chromatography in combination with MALDI-MS. In the case of all softwood xylans examined, the major portion of the 4-O-methylglucuronic acid residues were distributed regularly on every seventh or eighth xylose residue, while a minor portion of these residues were attached to adjacent xylose residues in the beta-(1-->4)-D-xylopolysaccharide chains. In contrast, the 4-O-methylglucuronic acid residues in the hardwood xylans examined were found to be distributed irregularly within the xylan. Further support for these experimental findings with the xylans was obtained by simulation of the distribution patterns of the molecular masses of oligosaccharides arising from polysaccharides with the postulated distribution of uronic acid residues.

Carbohydrates↗
PubMed1985

Measurement of xylanase activity with insoluble xylan substrate.

Insoluble xylan was prepared from ground birch (Betula pubescens) pulp by alkali extraction and precipitation with ethanol. The only sugar detected after acid hydrolysis of the preparation was xylose. The insoluble xylan was used as substrate in a nephelometric assay to determine the xylanase (EC 3.2.1.8, 1,4-beta-D-xylan xylanohydrolase and EC 3.2.1.37, 1,4-beta-D-xylan xylohydrolase) activities of Aspergillus and Trichoderma enzymes. The nephelometric method is reliable in evaluating xylanase hydrolysis of insoluble xylan.

Aspergillus↗
PubMed2006

Differential recognition of plant cell walls by microbial xylan-specific carbohydrate-binding modules.

Glycoside hydrolases that degrade plant cell walls have complex molecular architectures in which one or more catalytic modules are appended to noncatalytic carbohydrate-binding modules (CBMs). CBMs promote binding to polysaccharides and potentiate enzymic hydrolysis. Although there are diverse sequence-based families of xylan-binding CBMs, these modules, in general, recognize both decorated and unsubstituted forms of the target polysaccharide, and thus the evolutionary rationale for this diversity is unclear. Using immunohistochemistry to interrogate the specificity of six xylan-binding CBMs for their target polysaccharides in cell walls has revealed considerable differences in the recognition of plant materials between these protein modules. Family 2b and 15 CBMs bind to xylan in secondary cell walls in a range of dicotyledon species, whereas family 4, 6, and 22 CBMs display a more limited capability to bind to secondary cell walls. A family 35 CBM, which displays more restricted ligand specificity against purified xylans than the other five protein modules, reveals a highly distinctive binding pattern to plant material including the recognition of primary cell walls of certain dicotyledons, a feature shared with CBM15. Differences in the specificity of the CBMs toward walls of wheat grain and maize coleoptiles were also evident. The variation in CBM specificity for ligands located in plant cell walls provides a biological rationale for the repertoire of structurally distinct xylan-binding CBMs present in nature, and points to the utility of these modules in probing the molecular architecture of cell walls.

Bacterial Proteins↗
PubMed2005

Distribution of cell-wall xylans in bryophytes and tracheophytes: new insights into basal interrelationships of land plants.

Xylans are known to be major cellulose-linking polysaccharides in secondary cell walls in higher plants. We used two monoclonal antibodies (LM10 and LM11) for a comparative immunocytochemical analysis of tissue and cell distribution of xylans in a number of taxa representative of all major tracheophyte and bryophyte lineages. The results show that xylans containing the epitopes recognized by LM10 and LM11 are ubiquitous components of secondary cell walls in vascular and mechanical tissues in all present-living tracheophytes. In contrast, among the three bryophyte lineages, LM11 binding was detected in specific cell-wall layers in pseudoelaters and spores in the sporophyte of hornworts, while no binding was observed with either antibody in the gametophyte or sporophyte of liverworts and mosses. The ubiquitous occurrence of xylans containing LM10 and LM11 epitopes in tracheophytes suggests that the appearance of these polysaccharides has been a pivotal event for the evolution of highly efficient vascular and mechanical tissues. LM11 binding in the sporophyte of hornworts, indicating the presence of relatively highly substituted xylans (possibly arabinoxylans), separates these from the other bryophytes and is consistent with recent molecular data indicating a sister relationship of the hornworts with tracheophytes.

Antibodies, Monoclonal↗
PubMed2005

Monoclonal antibodies to plant cell wall xylans and arabinoxylans.

Two rat monoclonal antibodies have been generated to plant cell wall (1-->4)-beta-D-xylans using a penta-1,4-xylanoside-containing neoglycoprotein as an immunogen. The monoclonal antibodies, designated LM10 and LM11, have different specificities to xylans in relation to the substitution of the xylan backbone as indicated by immunodot assays and competitive-inhibition ELISAs. LM10 is specific to unsubstituted or low-substituted xylans, whereas LM11 binds to wheat arabinoxylan in addition to unsubstituted xylans. Immunocytochemical analyses indicated the presence of both epitopes in secondary cell walls of xylem but differences in occurrence in other cell types.

Animals↗
PubMed1997

Degradation and utilization of xylans by the rumen anaerobe Prevotella bryantii (formerly P. ruminicola subsp. brevis) B(1)4.

Freshly harvested whole cells from cultures of P. bryantii B(1)4 grown with oat spelt xylan (OSX) as an energy source showed less than 25% of the enzyme activity against OSX, and less than 15% of the activity against birchwood xylan (BWX) and carboxymethylcellulose, that was detectable in sonicated cell preparations. This indicates that much of this hydrolytic activity is either periplasmic, membrane-associated or intracellular and may be concerned with the processing of transported oligosaccharides.P. bryantii B(1)4 cultures were able to utilise up to 45% and 51% of the total pentose present in OSX and BWX, respectively, after 24 h, but could utilize 84% of a water-soluble fraction of BWX. Analysis of the xylan left undegraded after incubation with P. bryantii showed that while xylose and arabinose were removed to a similar extent, uronic acids were utilized to a greater extent than xylose. Predigestion of xylans with two cloned xylanases from the cellulolytic rumen anaerobe Ruminococcus flavefaciens gave little increase in overall pentose utilization suggesting that external P. bryantii xylanases are as effective as the cloned R. flavefaciens enzymes in releasing products that can be utilised by P. bryantii cells. The xylanase system of P. bryantiiis able to efficiently utilise not only xylo-oligosaccharides but also larger water-soluble xylan fragments.

Journal Article↗
PubMed1983

Isolation and anticoagulant properties of a new sulfated xylan: comparison with heparin and a sodium pentosan polysulfate (SP-54).

Larchwood xylan was purified by diaminoethylaminoethyl (DEAE) cellulose chromatography, sulfated by heating with chlorosulfonic acid -pyridine complex and the sulfated polysaccharide was isolated by epichlorohydrin triethanolamine (ECTEOLA) cellulose chromatography as the sodium salt. It's molecular weight was approximately 25000 and the specific rotation was [alpha]D 20-70 degrees. Electrophoretic analysis of the sulfated xylan along with heparin and SP-54 using lithium acetate-agarose technique showed that the charge density of the xylan sulfate was similar to heparin and it moved as a single component in contrast to commercial heparin which resolved into two while SP-54 moved at the same rate as the marker dye. Anticoagulant properties of the sulfated xylan were compared with heparin and SP-54 by studying its effect on the activated partial thromboplastin time (APTT), prothrombin time (PT) and the thrombin time (TT) using pooled normal human plasma. The sulfated xylan was more effective than SP-54 in delaying coagulation by all the three measurements while it was less active than heparin in inhibiting APTT and PT.

Anticoagulants↗
PubMed2002

Comparative subcellular immunolocation of polypeptides associated with xylan and callose synthases in French bean (Phaseolus vulgaris) during secondary wall formation.

The Golgi apparatus of plant cells is thought to be the main site of synthesis of cell wall matrix polysaccharides and the terminal glycosylation of glycoproteins. Much of this evidence still depends on earlier biochemical studies employing subcellular fractionation. However acquiring pure Golgi membranes is still difficult and the question of spatial organisation of glycosyl transferases can be addressed by immunolocation of the enzymes. An antibody to a xylan synthase-associated polypeptide from French bean, the enzyme which synthesises the core polysaccharide for secondary wall xylan, has been raised and shown to inhibit its activity. Xylan is deposited in secondary thickenings and the xylan synthase was only detected in appreciable amounts in developing xylem cells. The location within the Golgi stack was observed throughout the dictyosomes. Some enzyme subunits were also detected in post-Golgi vesicles. A second antibody to a non-catalytic M(r) 65000 subunit of beta 1,3- glucan (callose) synthase was used for a comparative study. Although the bulk of this enzyme has been detected in previous studies at plasmamembrane-wall interfaces in sieve plates and stressed tissue, a Golgi-location can be observed in root tip meristematic cells during cell plate formation. The enzyme was present throughout the stacks. Callose was also immunolocated in a similar manner to xylan in secondary walls and thickenings and in pits in developing xylem. In these cells, the callose synthase was detected at the surface of the growing thickenings and the plasmamembrane within the pits.

Blotting, Western↗
PubMed1997

The beta-D-xylosidase of Trichoderma reesei is a multifunctional beta-D-xylan xylohydrolase.

An extracellular multifunctional beta-D-xylan xylohydrolase, previously described as beta-xylosidase, was purified from Trichoderma reesei RUT C-30 to physical homogeneity. The active enzyme was a 100 (+/-5) kDa glycosylated monomer that exhibited a pl of 4.7. Its activity was optimal at pH 4 and it was stable between pH 3 and 6. Its temperature-stability was moderate (70 degrees zero of activity remaining after 60 min at 50 degrees C) and optimal activity was observed at 60 degrees C. It is capable of hydrolysing beta-1.4-xylo-oligosaccharides [degree of polymerization (DP) 2-7], the apparent Vmax increasing with increasing chain length. The enzyme also attacked debranched beech-wood (Lenzing) xylan and 4-O-methylglucuronoxylan, forming xylose as the only end product. The K(m) for xylan was 0.7 g/l. For this reason we consider the enzyme to be a beta-D-xylan xylohydrolase. The enzyme also exhibits alpha-L-arabinofuranosidase activity on 4-nitrophenyl alpha-L-arabinofuranoside, and evidence is presented that this is not caused by an impurity in the enzyme preparation. The beta-D-xylan xylohydrolase exhibits glycosyltransferase activity with xylo-oligosaccharides and at high concentrations of 4-nitrophenyl beta-D-xylopyranoside (4-Nph-beta-Xyl). The enzyme hydrolyses beta-1, 4-linkages preferentially to beta-1,3-linkages, and beta-1,2-linked xylo-oligosaccharides are not hydrolysed at all. The enzyme liberates terminal beta-1,4-xylopyranose residues linked to a 2-O-substituted xylopyranose residue, but not that linked to a 3-O-substituted xylopyranose residue. The enzyme does not attack methyl, methyl 1-thio-benzyl or butyl l-thio-beta-D-xylopyranosides and 4-naphthyl, 2-naphthyl and phenyl beta-D-xylopyranosides.

Amino Acid Sequence↗
PubMed1991

Purification and Cooperative Activity of Enzymes Constituting the Xylan-Degrading System of Thermomonospora fusca.

The thermophilic actinomycete Thermomonospora fusca produced endoxylanase, alpha-arabinofuranosidase, beta-xylosidase, and acetyl esterase activities maximally during growth on xylan. Growth yields on glucose, xylose, or arabinose were comparable, but production of endoxylanase and beta-xylosidase was not induced on these substrates. The crude xylanase activity was thermostable and relatively resistant to end product inhibition by xylobiose and xylan hydrolysis products. Six proteins with xylanase activity were identified by zymogram analysis of isoelectric focusing gels, but only a 32-kDa protein exhibiting three isomeric forms could be purified by fast protein liquid chromatography. Endoglucanases were also identified in carboxymethylcellulose-grown cultures, and their distinction from endoxylanases was confirmed. alpha-Arabinofuranosidase activity was due to a single dimeric protein of 92 kDa, which was particularly resistant to end product inhibition by arabinose. Three bands of acetyl esterase activity were detected by zymogram analysis, and there was evidence that these mainly consisted of an intracellular 80-kDa protein secreted to yield active 40-kDa subunits in the culture supernatant. The acetyl esterases were found to be responsible for acetyl xylan esterase activity in T. fusca, in contrast to the distinction proposed in some other systems. The addition of purified betaxylosidase to endoxylanase increased the hydrolysis of xylan, probably by relieving end product inhibition. The enhanced saccharification of wheat straw caused by the addition of purified alpha-arabinofuranosidase to T. fusca endoxylanase suggested a truly synergistic relationship, in agreement with proposals that arabinose side groups on the xylan chain participate in cross-linking within the plant cell wall structure.

Journal Article↗
PubMed1999

Purification and properties of a xylan-binding endoxylanase from Alkaliphilic bacillus sp. strain K-1.

An alkaliphilic bacterium, Bacillus sp. strain K-1, produces extracellular xylanolytic enzymes such as xylanases, beta-xylosidase, arabinofuranosidase, and acetyl esterase when grown in xylan medium. One of the extracellular xylanases that is stable in an alkaline state was purified to homogeneity by affinity adsorption-desorption on insoluble xylan. The enzyme bound to insoluble xylan but not to crystalline cellulose. The molecular mass of the purified xylan-binding xylanase was estimated to be approximately 23 kDa. The enzyme was stable at alkaline pHs up to 12. The optimum temperature and optimum pH of the enzyme activity were 60 degrees C and 5.5, respectively. Metal ions such as Fe2+, Ca2+, and Mg2+ greatly increased the xylanase activity, whereas Mn2+ strongly inhibited it. We also demonstrated that the enzyme could hydrolyze the raw lignocellulosic substances effectively. The enzymatic products of xylan hydrolysis were a series of short-chain xylooligosaccharides, indicating that the enzyme was an endoxylanase.

Journal Article↗
PubMed1968

Mechanism of isolated hemicellulose and xylan degradation by cellulolytic rumen bacteria.

Although certain strains of cellulolytic rumen bacteria cannot utilize isolated hemicelluloses or xylan as a source of energy, all strains examined can degrade or solubilize these materials from an 80% ethyl alcohol insoluble to a soluble form. Centrifugation and washing of the cellobiose-grown bacterial cells did not affect the rate or extent of utilization or degradation or both. When the level of a nonutilizing culture inoculum (either normal or washed) was doubled, a corresponding increase in the initial rate of degradation was observed. With a nitrogen-free medium, utilization of xylan was almost completely inhibited for a utilizing strain, whereas degradation by either type of organism was not markedly affected. Cellobiose medium cell-free culture filtrates from a nonutilizing strain were able to degrade or solubilize xylan. The percentage of degradation increased with the volume of cell-free filtrate, and all activity was lost when the filtrate was boiled. No utilization (loss in total pentose) was observed with cell-free filtrates from utilizing or nonutilizing strains. The release of free hexose from insoluble cellulose by culture filtrates from a nonutilizing strain was very limited. On the other hand, carboxymethylcellulose (CMC-70L) and cellulodextrins were degraded to an 80% ethyl alcohol soluble form by filtrates from both types of organisms. Similar enzyme activity was obtained in cell-free culture filtrates from four additional strains of cellulolytic rumen bacteria (one xylan utilizer and three nonutilizers). When the assays were carried out aerobically, CMC-70L solubilization was reduced to a much greater extent than xylan or cellulodextrin solubilization. The enzyme or enzymes responsible for the degradation of hemicellulose by cellololytic rumen bacteria unable to utilize the hemicellulose as an energy source appear to be constitutive in nature, and this activity may be a nonspecific action of a beta-1, 4-glucosidase or -cellulase.

Animals↗
PubMed1966

Characterization of several bovine rumen bacteria isolated with a xylan medium.

Dehority, B. A. (Ohio Agricultural Research and Development Center, Wooster). Characterization of several bovine rumen bacteria isolated with a xylan medium. J. Bacteriol. 91:1724-1729. 1966.-Studies were conducted to characterize eight strains of bacteria isolated from bovine rumen contents, by use of a medium containing xylan as the only added carbohydrate source. Based on morphology, biochemical reactions, nutritional requirements, and fermentation products, five of the eight strains were identified as Butyrivibrio fibrisolvens. Many properties of the remaining three strains resembled Bacteroides ruminicola; however, propionic acid was consistently found as a fermentation product. When the type strains for B. ruminicola subsp. ruminicola and B. ruminicola subsp. brevis were compared with the present isolates, it was found that propionic acid was a normal fermentation product for the type strain B. ruminicola subsp. ruminicola when grown in a 40% rumen fluid-0.5% glucose broth. Production of propionic acid was markedly reduced for all strains when grown in a 20% rumen fluid-1% glucose broth. The three remaining strains were thus placed in the species B. ruminicola, and further classified into the subspecies ruminicola (one strain) and brevis (two strains) on the basis of their requirement for hemin. Although the type strain of B. ruminicola subsp. brevis did not produce propionic acid, both of the present isolates classified as this subspecies produced substantial amounts. One strain of B. ruminicola subsp. brevis had an absolute requirement for volatile fatty acids. Either isobutyric or dl-2-methylbutyric acid would satisfy this requirement, whereas isovaleric acid was ineffective. It is of interest that xylan-fermenting bacteria isolated from 10(-7) and 10(-8) dilutions of rumen contents by use of a xylan medium are similar to the xylan fermenters isolated at the same dilutions with a nonselective medium.

Animals↗