Wheat Arabinoxylan Research Articles

Rye (Secale cerealeL.) Arabinoxylans: A Critical Review

Abstract The arabinoxylans of rye are of considerable importance for rye processing, breadmaking quality and nutritional properties. We review their occurrence in the rye kernel, the variability in their structure and their physico-chemical, nutritional and technological properties, including oxidative gelation. Where appropriate, they are compared with wheat arabinoxylans. Finally, we emphasise that progress in the understanding of their functionality is to be expected on the basis of an understanding of the variability in their structure.

Evidence for a general role for non-catalytic thermostabilizing domains in xylanases from thermophilic bacteria

A genomic library of Clostridium thermocellum DNA constructed in lambda ZAPII was screened for xylanase-expressing clones. Cross-hybridization experiments revealed a new xylanase gene isolated from the gene library, which was designated xyn Y. The encoded enzyme, xylanase Y (XYLY), displayed features characteristic of an endo-beta1,4-xylanase: the enzyme rapidly hydrolysed oat spelt, wheat and rye arabinoxylans and was active against methyl-umbelliferyl-beta-D-cellobioside, but did not hydrolyse any cellulosic substrates. The pH and temperature optima of the enzyme were 6.8 and 75 degrees C respectively, and the recombinant XYLY, expressed by Escherichia coli had a maximum Mr of 116000. The nucleotide sequence of xyn Y contained an open reading frame of 3228 bp encoding a protein of predicted Mr 120 105. The encoded enzyme contained a typical N-terminal 26-residue signal peptide, followed by a 164 amino acid sequence, designated domain A, that was not essential for catalytic activity. Downstream of domain A was a 351-residue xylanase Family F catalytic domain, followed by a 180-residue sequence that exhibited 28% sequence identity with a thermostable domain of Thermoanaerobacterium saccharolyticum xylanase A. The C-terminal portion of XYLY comprised the 23-residue duplicated docking sequence found in all other C. thermocellum plant cell wall hydrolases that are constituents of the bacterium's multienzyme complex, termed the cellulosome, followed by a 286-residue domain which exhibited 32% sequence identity with the N-terminal region of C. thermocellum xylanase Z. The enzyme did not contain linker sequences found in other C. thermocellum plant cell wall hydrolases. Analysis of truncated forms of XYLY and hybrid proteins, comprising segments of XYLY fused to the E. coli maltose binding domain, confirmed that XYLY contained a central catalytic domain and an adjacent thermostable domain. The C-terminal domain did not bind to cellulose or xylan. Western blot analysis using antiserum raised against XYLY showed that the xylanase was located in the cellulosome and did not appear to be extensively glycosylated. The non-catalytic domains of XYLY are discussed in relation to the general stability of thermophilic xylanases.

Cereal arabinoxylans: advances in structure and physicochemical properties

Arabinoxylans constitute a major fraction of cereal cell wall polysaccharides. They consist of a linear β-(1 → 4) linked xylan backbone to which α- l-arabinofuranose units are attached as side residues via α-(1 → 3) and/or α-(1 → 2) linkages. Several structural models have been put forward based on enzymic degradation studies and structure elucidation of oligosaccharides by NMR, methylation, and periodate oxidation techniques. These tentative models present different substitution patterns of arabinoses along the xylan chain. Cereal arabinoxylans exhibit a great deal of structural heterogeneity with respect to ratio of Araf Xylp , substitution pattern of arabinoses, content of feruloyl groups and molecular size. The conformation and physicochemical properties (viscosity, gelation potential, intermolecular association) of arabinoxylans in aqueous solutions are dependent on the molecular features of these polysaccharides; specific structure-property relationships have been established in model and actual food systems. Wheat and rye arabinoxylans are important functional ingredients in baked products affecting the mechanical properties of dough, as well as the texture and other end-product quality characteristics.

Evidence for the presence of arabinoxylan hydrolysing enzymes in European wheat flours

Water extracts of flour samples prepared from six sound European wheat varieties hydrolyse p-nitrophenyl-β-d-xylopyranoside and p-nitrophenyl-α-l-arabino-furanoside and, in addition, release soluble, dyed fragments from azurine crosslinked xylan. Incubation of water soluble wheat arabinoxylan with water extracts of flour results in the release of low molecular weight fractions consisting mainly of arabinose and xylose and small proportions of oligosaccharides as detected by high performance anion exchange chromatography. Gel permeation profiles of the incubation mixtures show a clear breakdown of the arabinoxylan.

Effects of wheat pentosans on endogenous amino acid losses in chickens

AbstractThe effects of wheat pentosans on protein digestibility and endogenous amino acid losses in precision‐fed chickens was investigated using the homo‐arginine marker technique. Addition of pentosans equivalent to 15 g and 35 g of wheat arabinoxylans per kg diet significantly depressed (P < 005) the overall digestibility of amino acids and increased (P < 0‐05) endogenous losses of amino acids, but the inclusion of cellulose or polyethylene polymer (92 g per kg diet) had no measurable effects.

Xylanase B from Neocallimastix patriciarum contains a non-catalytic 455-residue linker sequence comprised of 57 repeats of an octapeptide

A Neocallimastix patriciarum cDNA library was screened for xylanase-expressing clones, which were distinct from the previously characterized N. patriciarum xynA cDNA encoding xylanase A. A single cDNA, designated xynB, which did not exhibit homology with xynA, was isolated. Northern-blot analysis of mRNA from Avicel-grown N. patriciarum showed that xynB hybridized to a 3.4 kb mRNA species. The nucleotide sequence of xynB revealed a single open reading frame of 2580 bp coding for a protein designated xylanase B (XYLB), of M(r) 88,066. The primary structure of XYLB was comprised of a 21-residue N-terminal signal peptide, followed by a 304-amino acid sequence that exhibited substantial homology with the catalytic domains of family F xylanases. The N-terminal domain was linked to a C-terminal 70-residue sequence by a putative linker region, comprising 12 tandem repeats of a sequence containing TLPG as the core sequence, followed by an octapeptide XSKTLPGG where X can be S, K or N, which was repeated in tandem 45 times. Truncated derivatives of xynB encoding the N-terminal 338 residues directed the synthesis of a functional xylanase, confirming that the region of XYLB, which exhibited homology with family F xylanases, constitutes the catalytic domain. To investigate the catalytic properties of XYLB, the catalytic domain was fused to the Escherichia coli maltose-binding protein, and the fusion protein purified by amylose affinity chromatography. The purified enzyme hydrolysed oat, rye and wheat arabinoxylan releasing primarily xylobiose, xylotriose and some xylose. The XYLB fusion did not cleave any cellulosic substrates. The data presented in this report suggest that the multiple xylanases of N. patriciarum arose, not through the duplication of a single gene, but by the transfer of distinct xylanase-encoding DNA sequences into the anaerobic fungus. The possible origin of the xynB gene is discussed.

Xylanase production inAspergillus nidulans: induction and carbon catabolite repression

The induction of the synthesis of extracellular xylanases was investigated in the fungus Aspergillus nidulans using a number of compounds, including xylans of different origin, monosaccharides, xylooligosaccharides and xylose derivatives. Certain xylans (wheat arabinoxylan, oat spelt xylan, birchwood xylan and 4-O-methyl-D-glucurono-D-xylan) were found to be the most powerful inducers. Also, xylooligosaccharides such as xylobiose, xylotriose and xylotetraose served as inducers, their efficiency being directly related to their chain length. Xylose, on the contrary, was not a true inducer. Of the three endo-β-(1,4)-xylanases secreted by A. nidulans, that of 24 kDa was not under carbon catabolite repression, whereas the other two, of 22 and 34 kDa, were under glucose repression mediated by the creA gene product.

The role of wheat non-starch polysaccharides in broiler nutrition

It has been well established over a number of years that the apparent metabolisable energy (AME) value of wheat is highly variable. In 1983 and 1987 in Australia two surveys indicated that approximately 25% of wheats have AME values lower than 13 MJ/kg.DM (range 10.4-15.9 MJ/kg.DM). Following recent studies it has been proposed that the soluble non-starch polysaccharide cell-wall components of wheat (mainly arabinoxylan with some G-glucan) have an anti-nitritive activity when wheats are present at high levels in broiler diets and are responsible for the low-AME wheat phenomenon. The main findings supporting this hypothesis are (1) wheat AME values are negatively correlated with soluble non-starch polysaccharide levels, (2) low level addition (30g/kg) of commercially available pur non-starch polysaccharides to broiler diets depresses the AME,of the diets, (3) degradation of the cell wall polysaccharides in situ by addition of glycanases to broiler diets raises AME values, and (4) addition of purified wheat arabinoxylan to broiler diets depresses the AME in a dose-dependant manner. The AME depression is a result of the inhibition of starch, lipid and proteindigestion in the fore-gut. This paper reviews the experiments and the data from the studies and discusses further aspects of the anti-nutritive activity of cereal polysaccharides in broiler diets. The possible role of the gut microflora in the growth depression observed when diets containing high levels of rye, barley and wheat are fed to broiler chickens is also examined.

Corrigendum - The role of wheat non-starch polysaccharides in broiler nutrition

It has been well established over a number of years that the apparent metabolisable energy (AME) value of wheat is highly variable. In 1983 and 1987 in Australia two surveys indicated that approximately 25% of wheats have AME values lower than 13 MJ/kg.DM (range 10.4-15.9 MJ/kg.DM). Following recent studies it has been proposed that the soluble non-starch polysaccharide cell-wall components of wheat (mainly arabinoxylan with some G-glucan) have an anti-nitritive activity when wheats are present at high levels in broiler diets and are responsible for the low-AME wheat phenomenon. The main findings supporting this hypothesis are (1) wheat AME values are negatively correlated with soluble non-starch polysaccharide levels, (2) low level addition (30g/kg) of commercially available pur non-starch polysaccharides to broiler diets depresses the AME,of the diets, (3) degradation of the cell wall polysaccharides in situ by addition of glycanases to broiler diets raises AME values, and (4) addition of purified wheat arabinoxylan to broiler diets depresses the AME in a dose-dependant manner. The AME depression is a result of the inhibition of starch, lipid and proteindigestion in the fore-gut. This paper reviews the experiments and the data from the studies and discusses further aspects of the anti-nutritive activity of cereal polysaccharides in broiler diets. The possible role of the gut microflora in the growth depression observed when diets containing high levels of rye, barley and wheat are fed to broiler chickens is also examined.

Conformational analysis of the .alpha.-L-arabinofuranosides present in wheat arabinoxylans from proton-proton coupling constants

The conformational behavior of α-L-arabinofuranosides present in wheat endosperm L-arabino-D-xylans has been investigated. To that purpose, oligosaccharides obtained by enzymatic digestion of a wheat endosperm L-arabino-D-xylan were submitted to z-filtered 1D homonuclear Hartmann-Hahn (HOHAHA) spectroscopy, and from the obtained vicinal proton-proton coupling constants, the ring conformation was deduced with the use of appropriate Karplus equations in conjunction with the concept of pseudorotation. The results revealed that α-L-arabinofuranosides linked to 0 3 of an internal O-3-mOnO- or O-2,3-disubstituted (1→4)-linked β-D-XylOpyranOSe residue are present in a conformational equilibrium of equal amounts of 4E-40T (N-type) and 12T-2E (S-type) conformers, whereas the α-L-arabinofuranoside linked to 02 of an internal 02,3-disubstituted (1→4)-linked β-D-xylopyranose residue is present in a conformational equilibrium of equal amounts of 43T-4E (N-type) and 1E-12T (S-type) conformers. Both N- and S-type conformers of the investigated alpha-L-arabinofuranosides contain a rigid C4-04-Cl-01 segment, assuming for 01 a pseudoaxial orientation and for the exocyclic CH20H group a pseudoequatorial orientation. From the proton-proton coupling constants the exocyclic rotamer populations could also be determined showing nearly equal preference for the g- and t rotamers.

Effect of molecular size on physical properties of wheat arabinoxylan

A water-soluble wheat arabinoxylan was fractionated by molecular sieve chromatography into five polysaccharide fractions with limiting viscosities of 8.5 (F1), 6.2 (F2), 4.3 (F3), 3.8 (F4), and 3.4 (F5) dL/g. Compositional, 13C NMR, and 1H NMR analyses revealed very little structural variation among these fractions. The flow behavior of arabinoxylan fractions in dilute and concentrated solutions showed two critical concentrations (c and c). Oxidative gelation of the arabinoxylan fractions, involving intermolecular cross-linking via feruloyl residues, was followed by dynamic rheometry. Almost linear relationships between the storage modulus (G') of the gel network and polymer concentration were found for all fractions. However, while G' values of F1 and F2 increased very rapidly with increasing polymer concentration, F3, F4, and F5 exhibited much less concentration dependence of G'. Positive linear relationships between rigidity (G') of cross-linked arabinoxylan gels and intrinsic viscosity [171 of arabinoxylan fractions were found. The effectiveness of cross-linking reactions, ne(exp)/ne(theor) (ratio of cross-link density experimentally determined to the theoretical value), increased with increasing molecular size of the arabinoxylan fractions. Fractions that formed networks with a high density of cross-linking (F1 and F2) were the most effective in their ability to entrap and hold water. The arabinoxylan fractions were also effective in stabilizing protein foams against thermal disruption in the order F1 > F2 > F3 > F4 > F5.

Influence of structure on the physicochemical properties of wheat arabinoxylan

A water-soluble arabinoxylan from wheat flour was purified and fractionated by a graded ammonium sulphate fractionation technique. The yields (based on the total amount of the material recovered) of the four polysaccharide fractions obtained at 60% (F60), 70% (F70), 80% (F80), and 95% (F95) saturation with (NH 4) 2SO 4 were 58·8%, 29·2%, 7·6%, and 4·4%, respectively. Monosaccharide, 1H-NMR, and 13C-NMR analyses revealed differences in structural characteristics among the fractions. F60 had the lowest arabinose:xylose ratio (0·58), followed by F70 (0·71), F80 (0·85), and F95 (0·88). The relative amount of Xyl p residues doubly substituted at O-2 and O-3 with Ara f increased substantially from F60 to F95, whereas the amount of unsubstituted and monosubstituted Xyl p decreased in the same order. The content of ferulic acid was much higher in F60 than in the remaining samples; F60 also showed the highest intrinsic viscosity (4·70 dl/g) followed by F70 (4·20 dl/g), F80 (3·16 dl/g), and F95 (1·90 dl/g). Calculated coil overlap parameter values ( c ∗[η]: 1·20-1·30 ) were much lower than those of other random coil polysaccharides, implying a rather stiff/extended chain conformation. Oxidative gelation of arabinoxylan fractions was probed by dynamic rheometry. Fraction F60 formed rigid gel networks, as assessed by the storage modulus ( G′) and tan δ ( G′' G′ ) values, while F70 and F80 exhibited poor gelling capacity. The arabinoxylan fractions were also effective in stabilizing protein foams against thermal disruption in the order of F60 ? F70 > F80.

Analysis of wheat arabinoxylans from a large-scale isolation

Dietary fibre is of interest because of its nutritional activity and its role in the aetiology of some diseases. Arabinoxylans form the major part of wheat dietary fibre and this paper describes the large-scale aqueous and alkaline (0·2 m NaOH) extractions of a wheat milling by-product which yielded approximately 450 g and 1200 g of arabinoxylan-rich materials, respectively. The structures of the arabinoxylans were investigated using acid hydrolysis, methylation analysis and 1- and 2D-NMR experiments. The polysaccharide is confirmed to be a β-(1–4)-xylan substituted with α- l-arabinose (arabinose:xylose ratio of approximately 0·60) substituted at O2 and O3. Methylation analysis and 1H-NMR data indicated that the arabinoxylans from the aqueous extraction were very similar in structure to those isolated by alkaline extraction. Both were similar to arabinoxylans isolated directly from wheat by similar laboratory scale procedures. Detailed 1H- 1H-NMR homonuclear and 1H- 13C-NMR heteronuclear correlation spectroscopy analysis of a partially depolymerized sample confirmed the structure of the arabinoxylans and allowed an almost complete assignment of resonances in both the 1H- and 13C-NMR spectra. It was concluded that the nutritional activities ascribed to these materials are also likely to occur when they are ingested as components of complete foods.

Purification and characterization of a (1,4)-?-d-arabinoxylan arabinofuranohydrolase from Aspergillus awamori

An enzyme able to split off arabinose sidechains from cereal arabinoxylans was isolated from a cell-free culture filtrate of Aspergillus awamori CMI 142717 containing milled oat straw as the carbon source. The enzyme was highly specific for arabinoxylans and, unlike other α-l-arabinofuranosidases reported in the literature, did not show any activity towards p-nitrophenyl α-l-arabinofuranoside, arabinans and arabinogalactans. This novel enzyme, which can be described as a (1,4)-β-d-arabinofuranohydrolase, had a molecular mass of 32 000 Da when determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and a specific activity of 22 units/mg on wheat arabinoxylan.