Phylogenetic, functional and structural characterization of a GH10 xylanase active at extreme conditions of temperature and alkalinity

Identification and characterization of endo-xylanases from families GH10 and GH11 sourced from marine thermal environments

Seaweed biomass is an underutilized resource that is rich in polysaccharides, including xylan. Seaweed polysaccharides could be used as a feedstock in industrial microbiology and and for production of prebiotic oligosaccharides and rare monosaccharides - processes that would benefit from the availability of robust enzymes that break down the seaweed polysaccharides. The present study aimed to identify genes encoding endo-xylanases in bacterial genomes and metagenomes sourced from marine thermal environments, and to characterize the respective enzymes. Twelve endo-xylanases were studied which displayed 59 % median maximal sequence similarity to characterized GH10 or GH11 enzymes. Overall, most of the enzymes functioned optimally at high temperatures, in the presence of salt, and at circumneutral pH. Eight enzymes functioned optimally at temperatures of 50°C or higher, and in the most extreme cases at 85°C to 95°C. Six enzymes retained activity after three-hour incubation at 60°C or higher. Ten enzymes displayed improved catalytic function in the presence of salt, and several retained high catalytic function at 10 % NaCl concentration. All the enzymes hydrolyzed xylan from diverse sources, including crude biomass. The study contributes to an increased understanding of the structural diversity of xylanases; it expands the availability of thermostable xylanases of marine origin; and contributes to increased valorization of seaweed biomass.

Channel-Directed Enzymatic Depolymerization within a Metal-Organic Framework

Controlled growth of metal-organic frameworks (MOFs) under mild conditions has enabled the production of hybrid biocomposites with potential applications in biocatalysis. While the structure and bioactivity of confined enzymes are retained, improving the mass transport across the porous architecture remains a challenge. Here, we report a biocompatible and scalable synthetic procedure of a phase-pure aluminum trimesate porous framework, MIL-110(Al), featuring accessible microporous channels. The method is compatible with the in situ encapsulation of enzymes via a Lewis acid-mediated mineralization, reaching high efficiencies, and with control over protein loading. Moreover, we demonstrate a favored channel-directed depolymerization in a model biocomposite, xylanase@MIL-110(Al), which successfully hydrolyzes the xylan polymer over consecutive cycles. This work emphasizes the possibility of improving the overall enzymatic performance in depolymerization reactions by using MOF-protective scaffolds featuring large accessible porosity.

Characterization of two GH10 enzymes with ability to hydrolyze pretreated Sorghum bicolor bagasse

In this study, we characterized two novel enzymes of the glycoside hydrolase family 10 (GH10), Xyl10 C and Xyl10E, identified in the termite gut microbiome. The activities of both enzymes were assayed using beechwood xylan, barley β-glucan, and pretreated Sorghum bicolor bagasse (SBB) as substrates. Both enzymes, assessed individually and in combination, showed activity on beechwood xylan and pretreated SBB, whereas Xyl10E also showed activity on barley β-glucan. The composition of pretreated SBB mainly consisted of xylose and arabinose content. Purified Xyl10 C showed optimum xylanase activity in the pH range 7.0-8.0 and at a temperature of 50-60 °C, while Xyl10E was active at a wider pH range (5.0-10.0) and at 50 °C. The residual activities of Xyl10 C and Xyl10E after 8 h of incubation at 40 °C were 85% and 70%, respectively. The enzymatic activity of Xyl10 C increased to 115% in the presence of 5 M NaCl, was only inhibited in the presence of 0.5% sodium dodecyl sulfate (SDS), and decreased with β-mercaptoethanol. The xylanase and glucanase activities of Xyl10E were inhibited only in the presence of MnSO4, NaCl, and SDS. The main hydrolysis enzymatic product of Xyl10 C and Xyl10E on pretreated SBB was xylobiose. In addition, the xylo-oligosaccharides produced by xylanase Xyl10E on pretreated SBB demonstrated promising antioxidant activity. Thus, the hydrolysis products using Xyl10E on pretreated SBB indicate potential for antioxidant activity and other valuable industrial applications. KEY POINTS: • Two novel GH10 xylanases from the termite gut microbiome were characterized. • Xylo-oligosaccharides obtained from sorghum bagasse exhibited antioxidant potential. • Both enzymes and their hydrolysis product have potential to add value to agro-waste.

Order matters: Methods for extracting cellulose from rice straw by coupling alkaline, ozone and enzymatic treatments

Rice straw is a widely produced residue that can be converted into value-added products. This work aimed at using greener processes combining mild alkali (A), ozone (O) and enzymatic (engineered xylanase) (E) treatments to extract cellulose and other value-added compounds from rice straw and to evaluate the effects of the order of the treatments. Solid (S) and liquid (L) fractions from the process were collected for physicochemical characterization. AOE treatment showed the best capacity to extract high purity cellulose and other valuable compounds. The lignin content was significantly decreased independently of the order of the treatments and, its content in the extract obtained after the AOE process was lower than the one obtained after the OAE process. Moreover, thermal stability of the samples increased after the enzymatic process, being higher in SAOE. The alkaline treatment increased the hemicellulose and polyphenol content (antioxidant activity) in the liquid fractions (LA and LOA). In contrast, the ozonized liquid fractions had lower polyphenol content. Therefore, alkali was fundamental in the process. In conclusion, the AOE strategy could be a more environmentally friendly method for extracting cellulose and other valuable compounds, which could be used to develop active materials in the future.

Use of a Novel Extremophilic Xylanase for an Environmentally Friendly Industrial Bleaching of Kraft Pulps

Xylanases can boost pulp bleachability in Elemental Chlorine Free (ECF) processes, but their industrial implementation for producing bleached kraft pulps is not straightforward. It requires enzymes to be active and stable at the extreme conditions of alkalinity and high temperature typical of this industrial process; most commercial enzymes are unable to withstand these conditions. In this work, a novel highly thermo and alkaline-tolerant xylanase from Pseudothermotoga thermarum was overproduced in E. coli and tested as a bleaching booster of hardwood kraft pulps to save chlorine dioxide (ClO2) during ECF bleaching. The extremozyme-stage (EXZ) was carried out at 90 °C and pH 10.5 and optimised at lab scale on an industrial oxygen-delignified eucalyptus pulp, enabling us to save 15% ClO2 to reach the mill brightness, and with no detrimental effect on paper properties. Then, the EXZ-assisted bleaching sequence was validated at pilot scale under industrial conditions, achieving 25% ClO2 savings and reducing the generation of organochlorinated compounds (AOX) by 18%, while maintaining pulp quality and papermaking properties. Technology reproducibility was confirmed with another industrial kraft pulp from a mix of hardwoods. The new enzymatic technology constitutes a realistic step towards environmentally friendly production of kraft pulps through industrial integration of biotechnology.

Engineering mesophilic GH11 xylanase from Cellulomonas flavigena by rational design of N-terminus substitution

Xylanase, a glycoside hydrolase, is widely used in the food, papermaking, and textile industries; however, most xylanases are inactive at high temperatures. In this study, a xylanase gene, CFXyl3, was cloned from Cellulomonas flavigena and expressed in Escherichia coli BL21 (DE3). To improve the thermostability of xylanase, four hybrid xylanases with enhanced thermostability (designated EcsXyl1–4) were engineered from CFXyl3, guided by primary and 3D structure analyses. The optimal temperature of CFXyl3 was improved by replacing its N-terminus with the corresponding area of SyXyn11P, a xylanase that belongs to the hyperthermostable GH11 family. The optimal temperatures of the hybrid xylanases EcsXyl1–4 were 60, 60, 65, and 85°C, respectively. The optimal temperature of EcsXyl4 was 30 C higher than that of CFXyl3 (55°C) and its melting temperature was 34.5°C higher than that of CFXyl3. After the hydrolysis of beechwood xylan, the main hydrolysates were xylotetraose, xylotriose, and xylobiose; thus, these hybrid xylanases could be applied to prebiotic xylooligosaccharide manufacturing.

Expression of an extremophilic xylanase in Nicotiana benthamiana and its use for the production of prebiotic xylooligosaccharides

A gene construct encoding a xylanase, which is active in extreme conditions of temperature and alkaline pH (90 °C, pH 10.5), has been transitorily expressed with high efficiency in Nicotiana benthamiana using a viral vector. The enzyme, targeted to the apoplast, accumulates in large amounts in plant tissues in as little as 7 days after inoculation, without detrimental effects on plant growth. The properties of the protein produced by the plant, in terms of resistance to temperature, pH, and enzymatic activity, are equivalent to those observed when Escherichia coli is used as a host. Purification of the plant-produced recombinant xylanase is facilitated by exporting the protein to the apoplastic space. The production of this xylanase by N. benthamiana, which avoids the hindrances derived from the use of E. coli, namely, intracellular production requiring subsequent purification, represents an important step for potential applications in the food industry in which more sustainable and green products are continuously demanded. As an example, the use of the enzyme producing prebiotic xylooligosdaccharides from xylan is here reported.

A thermostable and CBM2-linked GH10 xylanase from Thermobifida fusca for paper bleaching

Xylanases have the potential to be used as bio-deinking and bio-bleaching materials and their application will decrease the consumption of the chlorine-based chemicals currently used for this purpose. However, xylanases with specific properties could act effectively, such as having significant thermostability and alkali resistance, etc. In this study, we found that TfXyl10A, a xylanase from Thermobifida fusca, was greatly induced to transcript by microcrystalline cellulose (MCC) substrate. Biochemical characterization showed that TfXyl10A is optimally effective at temperature of 80 °C and pH of 9.0. After removing the carbohydrate-binding module (CBM) and linker regions, the optimum temperature of TfXyl10A-CD was reduced by 10°C (to 70°C), at which the enzyme’s temperature tolerance was also weakened. While truncating only the CBM domain (TfXyl10AdC) had no significant effect on its thermostability. Importantly, polysaccharide-binding experiment showed that the auxiliary domain CBM2 could specifically bind to cellulose substrates, which endowed xylanase TfXyl10A with the ability to degrade xylan surrounding cellulose. These results indicated that TfXyl10A might be an excellent candidate in bio-bleaching processes of paper industry. In addition, the features of active-site architecture of TfXyl10A in GH10 family were further analyzed. By mutating each residue at the -2 and -1 subsites to alanine, the binding force and enzyme activity of mutants were observably decreased. Interestingly, the mutant E51A, locating at the distal -3 subsite, exhibited 90% increase in relative activity compared with wild-type (WT) enzyme TfXyl10A-CD (the catalytic domain of TfXyl110A). This study explored the function of a GH10 xylanase containing a CBM2 domain and the contribution of amino acids in active-site architecture to catalytic activity. The results obtained provide guidance for the rational design of xylanases for industrial applications under high heat and alkali-based operating conditions, such as paper bleaching.

Recent advances in the enzymatic production and applications of xylooligosaccharides

The majority of lignocellulosic biomass on the planet originates from plant cell walls, which are complex structures build up mainly by cellulose, hemicellulose and lignin. The largest part of hemicellulose, xylan, is a polymer with a β-(1→4)-linked xylose residues backbone decorated with α-D-glucopyranosyl uronic acids and/or L-arabinofuranose residues. Xylan is the second most abundant biopolymer in nature, which can be sustainably and efficiently degraded into decorated and undecorated xylooligosaccharides (XOS) using combinations of thermochemical pretreatments and enzymatic hydrolyses, that have broad applications in the food, feed, pharmaceutical and cosmetic industries. Endo-xylanases from different complex carbohydrate-active enzyme (CAZyme) families can be used to cleave the backbone of arabino(glucurono)xylans and xylooligosaccharides and degrade them into short XOS. It has been shown that XOS with a low degree of polymerization have enhanced prebiotic effects conferring health benefits to humans and animals. In this review we describe recent advances in the enzymatic production of XOS from lignocellulosic biomass arabino- and glucuronoxylans and their applications as food and feed additives and health-promoting ingredients. Comparative advantages of xylanases from different CAZy families in XOS production are discussed and potential health benefits of different XOS are presented.