Structural characterization of (1→2)-β-xylose-(1→3)-α-arabinose-containing oligosaccharide products of extracted switchgrass (Panicum virgatum, L.) xylan after exhaustive enzymatic treatment with α-arabinofuranosidase and β-endo-xylanase

Identification of glycosyltransferases mediating 2-O-arabinopyranosyl and 2-O-galactosyl substitutions of glucuronosyl side chains of xylan

Xylan is one of the major hemicelluloses in plant cell walls and its xylosyl backbone is often decorated at O-2 with glucuronic acid (GlcA) and/or methylglucuronic acid (MeGlcA) residues. The GlcA/MeGlcA side chains may be further substituted with 2-O-arabinopyranose (Arap) or 2-O-galactopyranose (Gal) residues in some plant species, but the enzymes responsible for these substitutions remain unknown. During our endeavor to investigate the enzymatic activities of Arabidopsis MUR3-clade members of the GT47 glycosyltransferase family, we found that one of them was able to transfer Arap from UDP-Arap onto O-2 of GlcA side chains of xylan, and thus it was named xylan 2-O-arabinopyranosyltransferase 1 (AtXAPT1). The function of AtXAPT1 was verified in planta by its T-DNA knockout mutation showing a loss of the Arap substitution on xylan GlcA side chains. Further biochemical characterization of XAPT close homologs from other plant species demonstrated that while the poplar ones had the same catalytic activity as AtXAPT1, those from Eucalyptus, lemon-scented gum, sea apple, 'Ohi'a lehua, duckweed and purple yam were capable of catalyzing both 2-O-Arap and 2-O-Gal substitutions of xylan GlcA side chains albeit with differential activities. Sequential reactions with XAPTs and glucuronoxylan methyltransferase 3 (GXM3) showed that XAPTs acted poorly on MeGlcA side chains, whereas GXM3 could efficiently methylate arabinosylated or galactosylated GlcA side chains of xylan. Furthermore, molecular docking and site-directed mutagenesis analyses of Eucalyptus XAPT1 revealed critical roles of several amino acid residues at the putative active site in its activity. Together, these findings establish that XAPTs residing in the MUR3 clade of family GT47 are responsible for 2-O-arabinopyranosylation and 2-O-galactosylation of GlcA side chains of xylan.

Biochemical Characterization of Rice Xylan Biosynthetic Enzymes in Determining Xylan Chain Elongation and Substitutions

Grass xylan consists of a linear chain of β-1,4-linked xylosyl residues that often form domains substituted only with either arabinofuranose (Araf) or glucuronic acid (GlcA)/methylglucuronic acid (MeGlcA) residues, and it lacks the unique reducing end tetrasaccharide sequence found in dicot xylan. The mechanism of how grass xylan backbone elongation is initiated and how its distinctive substitution pattern is determined remains elusive. Here, we performed biochemical characterization of rice xylan biosynthetic enzymes, including xylan synthases, glucuronyltransferases and methyltransferases. Activity assays of rice xylan synthases demonstrated that they required short xylooligomers as acceptors for their activities. While rice xylan glucuronyltransferases effectively glucuronidated unsubstituted xylohexaose acceptors, they transferred little GlcA residues onto (Araf)-substituted xylohexaoses and rice xylan 3-O-arabinosyltransferase could not arabinosylate GlcA-substituted xylohexaoses, indicating that their intrinsic biochemical properties may contribute to the distinctive substitution patterns of rice xylan. In addition, we found that rice xylan methyltransferase exhibited a low substrate binding affinity, which may explain the partial GlcA methylation in rice xylan. Furthermore, immunolocalization of xylan in xylem cells of both rice and Arabidopsis showed that it was deposited together with cellulose in secondary walls without forming xylan-rich nanodomains. Together, our findings provide new insights into the biochemical mechanisms underlying xylan backbone elongation and substitutions in grass species.

Xylooligosaccharides from lignocellulosic biomass and their applications as nutraceuticals: a review on their production, purification, and characterization

Xylooligosaccharides (XOS) are considered a potent source of prebiotics for humans. The global prebiotic market is expanding in size, was valued at USD 6.05 billion in 2021, and is expected to grow at a 14.9% compound annual growth rate between 2022 and 2030, indicating a huge demand. These XOS are non-digestible pentose sugar oligomers comprising mainly xylose. Xylose is naturally present in the lignocellulosic biomass (LCB), fruits and vegetables. Apart from the prebiotic effect, these XOS have been reported to reduce blood cholesterol, possess antioxidant effects, increase calcium absorption, reduce colon cancer risk, and benefit diabetic patients. The primary use of XOS is reported in the feed industry followed by health, medical use, food and drinks. LCB mainly contains glucan, xylan and lignin. After glucan, xylan is the second-highest available sugar on the globe composed of xylose. Therefore, the xylan fraction of LCB has great significance in producing food, feed and energy. Glucan has been exploited for the commercial production of ethanol, xylitol, furfural, hydroxymethyl furfural and glucose. As of now, xylan has limited applications. Therefore, xylan can be exploited to convert to XOS. The production of XOS from LCB fraction not only helps to produce these at a very low price, but also helps in the reduction of greenhouse gases. Its use in food and drinks is increasing as it can be derived from the abundantly and cheaply available LCB. The article provides a review on the production, purification and characterization of XOS in view of their use as nutraceuticals. © 2024 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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.

Profiling of cool-season forage arabinoxylans via a validated HPAEC-PAD method

Cool-season pasture grasses contain arabinoxylans (AX) as their major cell wall hemicellulosic polysaccharide. AX structural differences may influence enzymatic degradability, but this relationship has not been fully explored in the AX from the vegetative tissues of cool-season forages, primarily because only limited AX structural characterization has been performed in pasture grasses. Structural profiling of forage AX is a necessary foundation for future work assessing enzymatic degradability and may also be useful for assessing forage quality and suitability for ruminant feed. The main objective of this study was to optimize and validate a high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) method for the simultaneous quantification of 10 endoxylanase-released xylooligosaccharides (XOS) and arabinoxylan oligosaccharides (AXOS) in cool-season forage cell wall material. The following analytical parameters were determined or optimized: chromatographic separation and retention time (RT), internal standard suitability, working concentration range (CR), limit of detection (LOD), limit of quantification (LOQ), relative response factor (RRF), and quadratic calibration curves. The developed method was used to profile the AX structure of four cool-season grasses commonly grown in pastures (timothy, Phleum pratense L.; perennial ryegrass, Lolium perenne L.; tall fescue, Schedonorus arundinaceus (Schreb.) Dumort.; and Kentucky bluegrass, Poa pratensis L.). In addition, the cell wall monosaccharide and ester-linked hydroxycinnamic acid contents were determined for each grass. The developed method revealed unique structural aspects of the AX structure of these forage grass samples that complemented the results of the cell wall monosaccharide analysis. For example, xylotriose, representing an unsubstituted portion of the AX polysaccharide backbone, was the most abundantly-released oligosaccharide in all the species. Perennial rye samples tended to have greater amounts of released oligosaccharides compared to the other species. This method is ideally suited to monitor structural changes of AX in forages as a result of plant breeding, pasture management, and fermentation of plant material.

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.

Identification of xylan arabinosyl 2-O-xylosyltransferases catalyzing the addition of 2-O-xylosyl residue onto arabinosyl side chains of xylan in grass species

Grass xylan, the major hemicellulose in both primary and secondary cell walls, is heavily decorated with α-1,3-linked arabinofuranosyl (Araf) residues that may be further substituted at O-2 with xylosyl (Xyl) or Araf residues. Although xylan 3-O-arabinosyltransferases (XATs) catalyzing 3-O-Araf addition onto xylan have been characterized, glycosyltransferases responsible for the transfer of 2-O-Xyl or 2-O-Araf onto 3-O-Araf residues of xylan to produce the Xyl-Araf and Araf-Araf disaccharide side chains remain to be identified. In this report, we showed that a rice GT61 member, named OsXAXT1 (xylan arabinosyl 2-O-xylosyltransferase 1) herein, was able to mediate the addition of Xyl-Araf disaccharide side chains onto xylan when heterologously co-expressed with OsXAT2 in the Arabidopsis gux1/2/3 (glucuronic acid substitution of xylan 1/2/3) triple mutant that lacks any glycosyl substitutions. Recombinant OsXAXT1 protein expressed in human embryonic kidney 293 cells exhibited a xylosyltransferase activity catalyzing the addition of Xyl from UDP-Xyl onto arabinosylated xylooligomers. Consistent with its function as a xylan arabinosyl 2-O-xylosyltransferase, CRISPR-Cas9-mediated mutations of the OsXAXT1 gene in transgenic rice plants resulted in a reduction in the level of Xyl-Araf disaccharide side chains in xylan. Furthermore, we revealed that XAXT1 close homologs from several other grass species, including switchgrass, maize, and Brachypodium, possessed the same functions as OsXAXT1, indicating functional conservation of XAXTs in grass species. Together, our findings establish that grass XAXTs are xylosyltransferases catalyzing Xyl transfer onto O-2 of Araf residues of xylan to form the Xyl-Araf disaccharide side chains, which furthers our understanding of genes involved in xylan biosynthesis.

Potential Valorization of Hazelnut Shells through Extraction, Purification and Structural Characterization of Prebiotic Compounds: A Critical Review

Hazelnuts are one of the most widely consumed nuts, but their production creates large quantities of by-products, especially shells, that could be upcycled into much more valuable products. Recent studies have shown that hazelnut shell hemicellulose is particularly rich in compounds that are potential precursors of xylooligosaccharides and arabino-xylooligosaccharides ((A)XOS), previously defined as emerging prebiotics very beneficial for human health. The production of these compounds on an industrial scale-up could have big consequences on the functional foods market. However, to produce (A)XOS from a lignocellulosic biomass, such as hazelnut shell, is not easy. Many methods for the extraction and the purification of these prebiotics have been developed, but they all have different efficiencies and consequences, including on the chemical structure of the obtained (A)XOS. The latter, in turn, is strongly correlated to the nutritional effects they have on health, which is why the optimization of the structural characterization process is also necessary. Therefore, this review aims to summarize the progress made by research in this field, so as to contribute to the exploitation of hazelnut waste streams through a circular economy approach, increasing the value of this biomass through the production of new functional ingredients.

Development of an oligosaccharide library to characterise the structural variation in glucuronoarabinoxylan in the cell walls of vegetative tissues in grasses

BACKGROUND

Grass glucuronoarabinoxylan (GAX) substitutions can inhibit enzymatic degradation and are involved in the interaction of xylan with cell wall cellulose and lignin, factors which contribute to the recalcitrance of biomass to saccharification. Therefore, identification of xylan characteristics central to biomass biorefining improvement is essential. However, the task of assessing biomass quality is complicated and is often hindered by the lack of a reference for a given crop.

RESULTS

In this study, we created a reference library, expressed in glucose units, of Miscanthus sinensis GAX stem and leaf oligosaccharides, using DNA sequencer-Assisted Saccharide analysis in high throughput (DASH), supported by liquid chromatography (LC), nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). Our analysis of a number of grass species highlighted variations in substitution type and frequency of stem and leaf GAX. In miscanthus, for example, the β-Xylp-(1 → 2)-α-Araf-(1 → 3) side chain is more abundant in leaf than stem.

CONCLUSIONS

The reference library allows fast identification and comparison of GAX structures from different plants and tissues. Ultimately, this reference library can be used in directing biomass selection and improving biorefining.

Hemicellulose characterization of deuterated switchgrass

This work describes the structural characterization of hemicellulose isolated from hydroponically grown switchgrass in H₂O medium (protiated) or 50% D₂O medium (deuterated) through compositional analysis, GPC, FTIR, ¹³C and ¹H/¹³C HSQC NMR. 4-O-methyl glucuronoarabinoxylan (GAX), the major hemicellulose in switchgrass isolated from deuterated switchgrass, had structural properties similar to hemicellulose isolated from protiated switchgrass. Both had comparable arabinose to xylose ratio (0.25) and molecular weight (47-50 kDa). Structural similarities show that deuterated switchgrass hemicellulose can be used as a model carbohydrate polymer in neutron scattering, or pharmaceutical studies due to their immunomodulatory activity and gastroprotective effects.

Sugar loss and enzyme inhibition due to oligosaccharide accumulation during high solids-loading enzymatic hydrolysis

BACKGROUND

Accumulation of recalcitrant oligosaccharides during high-solids loading enzymatic hydrolysis of cellulosic biomass reduces biofuel yields and increases processing costs for a cellulosic biorefinery. Recalcitrant oligosaccharides in AFEX-pretreated corn stover hydrolysate accumulate to the extent of about 18-25 % of the total soluble sugars in the hydrolysate and 12-18 % of the total polysaccharides in the inlet biomass (untreated), equivalent to a yield loss of about 7-9 kg of monomeric sugars per 100 kg of inlet dry biomass (untreated). These oligosaccharides represent a yield loss and also inhibit commercial hydrolytic enzymes, with both being serious bottlenecks for economical biofuel production from cellulosic biomass. Very little is understood about the nature of these oligomers and why they are recalcitrant to commercial enzymes. This work presents a robust method for separating recalcitrant oligosaccharides from high solid loading hydrolysate in gramme quantities. Composition analysis, recalcitrance study and enzyme inhibition study were performed to understand their chemical nature.

RESULTS

Oligosaccharide accumulation occurs during high solid loading enzymatic hydrolysis of corn stover (CS) irrespective of using different pretreated corn stover (dilute acid: DA, ionic liquids: IL, and ammonia fibre expansion: AFEX). The methodology for large-scale separation of recalcitrant oligosaccharides from 25 % solids-loading AFEX-corn stover hydrolysate using charcoal fractionation and size exclusion chromatography is reported for the first time. Oligosaccharides with higher degree of polymerization (DP) were recalcitrant towards commercial enzyme mixtures [Ctec2, Htec2 and Multifect pectinase (MP)] compared to lower DP oligosaccharides. Enzyme inhibition studies using processed substrates (Avicel and xylan) showed that low DP oligosaccharides also inhibit commercial enzymes. Addition of monomeric sugars to oligosaccharides increases the inhibitory effects of oligosaccharides on commercial enzymes.

CONCLUSION

The carbohydrate composition of the recalcitrant oligosaccharides, ratios of different DP oligomers and their distribution profiles were determined. Recalcitrance and enzyme inhibition studies help determine whether the commercial enzyme mixtures lack the enzyme activities required to completely de-polymerize the plant cell wall. Such studies clarify the reasons for oligosaccharide accumulation and contribute to strategies by which oligosaccharides can be converted into fermentable sugars and provide higher biofuel yields with less enzyme.