Towards enzymatic breakdown of complex plant xylan structures: State of the art

High-level expression and enzymatic properties of a novel thermostable xylanase with high arabinoxylan degradation ability from Chaetomium sp. suitable for beer mashing

A novel thermostable xylanase gene from Chaetomium sp. CQ31 was cloned and codon-optimized (CsXynBop). The deduced protein sequence of the gene shared the highest similarity of 75% with the glycoside hydrolase (GH) family 10 xylanase from Achaetomium sp. Xz-8. CsXynBop was over-expressed in Pichia pastoris GS115 by high-cell density fermentation, with the highest xylanase yield of 10,017 U/mL. The recombinant xylanase (CsXynBop) was purified to homogeneity and biochemically characterized. CsXynBop was optimally active at pH 6.5 and 85 °C, respectively, and stable over a broad pH range of 5.0-9.5 and up to 60 °C. The enzyme exhibited strict substrate specificity towards oat-spelt xylan (2, 489 U/mg), beechwood xylan (1522 U/mg), birchwood xylan (1067 U/mg), and showed relatively high activity towards arabinoxylan (1208 U/mg), but exhibited no activity on other tested polysaccharides. CsXynBop hydrolyzed different xylans to yield mainly xylooligosaccharides (XOSs) with degree of polymerization (DP) 2-5. The application of CsXynBop (200 U/g malt) in malt mashing substantially decreased the filtration time and viscosity of malt by 42.3% and 8.6%, respectively. These excellent characteristics of CsXynBop may make it a good candidate in beer industry.

High stabilization and hyperactivation of a Recombinant β-Xylosidase through Immobilization Strategies

Attainment of a stable and highly active β-xylosidase is of major importance for the efficient and cost-competitive hydrolysis of hemicellulose xylan, as well as for its industrial conversion into biofuels and biochemicals. Here, a recombinant β-xylosidase of the glycoside hydrolase family (GH43) from Bacillus subtilis was produced in Escherichia coli culture, purified, and subsequently immobilized on agarose and chitosan. Glutaraldehyde and glyoxyl groups were evaluated as activating agents to select the most efficient derivative. Multi-point immobilization on agarose led to an extraordinary thermal stability (half-lives 3604 and 164-fold higher than the free enzyme, at 50° and 35 °C, respectively). Even for chitosan activated with glutaraldehyde, a low-cost support, thermal stability of the immobilized enzyme was 326 and 12-fold higher than the free enzyme at 50° and 35°C, respectively. Immobilized enzymes showed no release of any subunit for the agarose-glyoxyl derivative, and only a few ones for the support activated with glutaraldehyde. Most remarkably, the enzyme kinetic behavior after immobilization increased up to 4-fold in relation to the free one. β-xylosidase, a tetrameric enzyme with four identical subunits, exists in equilibrium between the monomeric and oligomeric forms in solution. Depending on the pH of immobilization, the enzyme oligomerization can be favored, thus explaining the hyperactivation phenomenon. Both glyoxyl-agarose and chitosan-glutaraldehyde derivatives were used to catalyze corncob xylan hydrolysis, reaching 72 % conversion, representing a xylose productivity of around 20 g L^-1 h^-1. After ten 4h-cycles (pH 6.0, 35 °C), the xylan-to-xylose conversion remained approximately unchanged. Therefore, the immobilized β-xylosidases prepared in this work can be of great interest as biocatalysts in a biorefinery context.

High-Throughput Generation of Product Profiles for Arabinoxylan-Active Enzymes from Metagenomes

Metagenomics is an exciting alternative to seek carbohydrate-active enzymes from a range of sources. Typically, metagenomics reveals dozens of putative catalysts that require functional characterization for further application in industrial processes. High-throughput screening methods compatible with adequate natural substrates are crucial for an accurate functional elucidation of substrate preferences. Based on DNA sequencer-aided fluorophore-assisted carbohydrate electrophoresis (DSA-FACE) analysis of enzymatic-reaction products, we generated product profiles to consequently infer substrate cleavage positions, resulting in the generation of enzymatic-degradation maps. Product profiles were produced in high throughput for arabinoxylan (AX)-active enzymes belonging to the glycoside hydrolase families GH43 (subfamilies 2 [MG43₂], 7 [MG43₇], and 28 [MG43₂₈]) and GH8 (MG8) starting from 12 (arabino)xylo-oligosaccharides. These enzymes were discovered through functional metagenomic studies of feces from the North American beaver (Castor canadensis). This work shows how enzyme loading alters the product profiles of all enzymes studied and gives insight into AX degradation patterns, revealing sequential substrate preferences of AX-active enzymes.IMPORTANCE Arabinoxylan is mainly found in the hemicellulosic fractions of rice straw, corn cobs, and rice husk. Converting arabinoxylan into (arabino)xylo-oligosaccharides as added-value products that can be applied in food, feed, and cosmetics presents a sustainable and economic alternative for the biorefinery industries. Efficient and profitable AX degradation requires a set of enzymes with particular characteristics. Therefore, enzyme discovery and the study of substrate preferences are of utmost importance. Beavers, as consumers of woody biomass, are a promising source of a repertoire of enzymes able to deconstruct hemicelluloses into soluble oligosaccharides. High-throughput analysis of the oligosaccharide profiles produced by these enzymes will assist in the selection of the most appropriate enzymes for the biorefinery.

Balanced Xylan Acetylation is the Key Regulator of Plant Growth and Development, and Cell Wall Structure and for Industrial Utilization

Xylan is the most abundant hemicellulose, constitutes about 25–35% of the dry biomass of woody and lignified tissues, and occurs up to 50% in some cereal grains. The accurate degree and position of xylan acetylation is necessary for xylan function and for plant growth and development. The post synthetic acetylation of cell wall xylan, mainly regulated by Reduced Wall Acetylation (RWA), Trichome Birefringence-Like (TBL), and Altered Xyloglucan 9 (AXY9) genes, is essential for effective bonding of xylan with cellulose. Recent studies have proven that not only xylan acetylation but also its deacetylation is vital for various plant functions. Thus, the present review focuses on the latest advances in understanding xylan acetylation and deacetylation and explores their effects on plant growth and development. Baseline knowledge about precise regulation of xylan acetylation and deacetylation is pivotal to developing plant biomass better suited for second-generation liquid biofuel production.

An integrated approach to obtain xylo-oligosaccharides from sugarcane straw: From lab to pilot scale

Sugarcane straw (SS) is a widely available agricultural processing feedstock with the potential to produce 2nd generation bioethanol and bioproducts, in addition to the more conventional use for heat and/or electrical power generation. In this study, we investigated the operational parameters to maximize the production of xylo-oligosaccharides (XOS) using mild deacetylation, followed by hydrothermal pretreatment. From the laboratory to the pilot-scale, the optimized two-stage pretreatment promoted 81.5% and 70.5% hemicellulose solubilization and led to XOS yields up to 9.8% and 9.1% (w/w of initial straw), respectively. Moreover, different fungal xylanases were also tested to hydrolyze XOS into xylobiose (X2) and xylotriose (X3). GH10 from Aspergillus nidulans performed better than GH11 xylanases and the ratio of the desired products (X2 + X3) increased to 72% due to minimal monomeric sugar formation. Furthermore, a cellulose-rich fraction was obtained, which can be used in other high value-added applications, such as for the production of cello-oligomers.

S-Glycosides: synthesis of S-linked arabinoxylan oligosaccharides

S-Glycosides are important tools for the elucidation of specific protein-carbohydrate interactions and can significantly aid structural and functional studies of carbohydrate-active enzymes, as they are often inert or act as enzyme inhibitors. In this context, this work focuses on the introduction of an S-linkage into arabinoxylan oligosaccharides (AXs) in order to obtain a small collection of synthetic tools for the study of AXs degrading enzymes. The key step for the introduction of the S-glycosidic linkage involved anomeric thiol S-alkylation of an orthogonally protected l-arabinopyranoside triflate. The resulting S-linked disaccharide was subsequently employed in a series of glycosylation reactions to obtain a selectively protected tetrasaccharide. This could be further elaborated through chemoselective deprotection and glycosylation reactions to introduce branching l-arabinofuranosides.

Proteomic Characterization of Lignocellulolytic Enzymes Secreted by the Insect-Associated Fungus Daldinia decipiens oita, Isolated from a Forest in Northern Japan

Wood-devastating insects utilize their symbiotic microbes with lignocellulose-degrading abilities to extract energy from recalcitrant woods. It is well known that free-living lignocellulose-degrading fungi secrete various carbohydrate-active enzymes (CAZymes) to degrade plant cell wall components, mainly cellulose, hemicellulose, and lignin. However, CAZymes from insect-symbiotic fungi have not been well documented except for a few examples. In this study, an insect-associated fungus, Daldinia decipiens oita, was isolated as a potential symbiotic fungus of female Xiphydria albopicta captured from Hokkaido forest. This fungus was grown in seven different media containing a single carbon source, glucose, cellulose, xylan, mannan, pectin, poplar, or larch, and the secreted proteins were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). A total of 128 CAZymes, including domains of 92 glycoside hydrolases, 15 carbohydrate esterases, 5 polysaccharide lyases, 17 auxiliary activities, and 11 carbohydrate-binding modules, were identified, and these are involved in degradation of cellulose and hemicellulose but not lignin. Together with the results of polysaccharide-degrading activity measurements, we concluded that D. decipiens oita tightly regulates the expression of these CAZymes in response to the tested plant cell wall materials. Overall, this study described the detailed proteomic approach of a woodwasp-associated fungus and revealed that the new isolate, D. decipiens oita, secretes diverse CAZymes to efficiently degrade lignocellulose in the symbiotic environment.IMPORTANCE Recent studies show the potential impacts of insect symbiont microbes on biofuel application with regard to their degradation capability of a recalcitrant plant cell wall. In this study, we describe a novel fungal isolate, D. decipiens oita, as a single symbiotic fungus from the Xiphydria woodwasp found in the northern forests of Japan. Our detailed secretome analyses of D. decipiens oita, together with activity measurements, reveal that this insect-associated fungus exhibits high and broad activities for plant cell wall material degradation, suggesting potential applications within the biomass conversion industry for plant mass degradation.

Identification and characterization of a novel bacterial carbohydrate esterase from the bacterium Pantoea ananatis Sd-1 with potential for degradation of lignocellulose and pesticides

OBJECTIVE

Identification and characterization of a novel bacterial carbohydrate esterase (PaCes7) with application potential for lignocellulose and pesticide degradation.

RESULTS

PaCes7 was identified from the lignocellulolytic bacterium, Pantoea ananatis Sd-1 as a new carbohydrate esterase. Recombinant PaCes7 heterologously expressed in Escherichia coli showed a clear preference for esters with short-chain fatty acids and exhibited maximum activity towards α-naphthol acetate at 37 °C and pH 7.5. Purified PaCes7 exhibited its catalytic activity under mesophilic conditions and retained more than 40% activity below 30 °C. It displayed a relatively wide pH stability from pH 6-11. Furthermore, the enzyme was strongly resistant to Mg²⁺, Pb²⁺, and Co²⁺ and activated by K⁺ and Ca²⁺. Both P. ananatis Sd-1 and PaCes7 could degrade the pesticide carbaryl. Additionally, PaCes7 was shown to work in combination with cellulase and/or xylanase in rice straw degradation.

CONCLUSIONS

The data suggest that PaCes7 possesses promising biotechnological potential.

Isolation of four xylanases capable of hydrolyzing corn fiber xylan from Paenibacillus sp. H2C

Corn fibre xylan (CX) shows high resistance to enzymatic hydrolysis due to its densely decorated side chains. To find enzymes capable of hydrolyzing CX, we isolated a bacterial strain (named H2C) from soil, by enrichment culture using non-starch polysaccharides of corn as the sole carbon source. Analysis based on the 16S rRNA sequence placed strain H2C within genus Paenibacillus. Enzymes were purified from supernatant of culture broth of strain H2C based on solubilizing activities toward CX. Four enzymes, Xyn5A, Xyn10B, Xyn11A, and Xyn30A, were successfully identified, which belong to glycoside hydrolase (GH) families, 5, 10, 11, and 30, respectively. Phylogenetic analysis classified Xyn5A in subfamily 35 of GH family 5, a subfamily of unknown function. Their activities toward beechwood xylan and/or wheat arabinoxylan indicated that these enzymes are β-1,4-xylanases. They showed high solubilizing activities toward a feed material, corn dried distiller’s grains with solubles, compared to five previously characterized xylanases.

Abbreviations : CX: corn fibre xylan; DDGS: corn dried distiller’s grains with solubles

Enzymatic degradation of plant biomass and synthetic polymers

Plant biomass is an abundant renewable resource on Earth. Microorganisms harvest energy from plant material by means of complex enzymatic systems that efficiently degrade natural polymers. Intriguingly, microorganisms have evolved to exploit these ancient mechanisms to also decompose synthetic plastic polymers. In this Review, we summarize the mechanisms by which they decompose non-starch plant biomass and the six major types of synthetic plastics. We focus on the structural features of the enzymes that contribute to substrate recognition and then describe the catalytic mechanisms of polymer metabolism. An understanding of these natural biocatalysts is valuable if we are to exploit their potential for the degradation of synthetic polymers.

Cell Wall Acetylation in Hybrid Aspen Affects Field Performance, Foliar Phenolic Composition and Resistance to Biological Stress Factors in a Construct-Dependent Fashion

The production of biofuels and "green" chemicals from the lignocellulose of fast-growing hardwood species is hampered by extensive acetylation of xylan. Different strategies have been implemented to reduce xylan acetylation, resulting in transgenic plants that show good growth in the greenhouse, improved saccharification and fermentation, but the field performance of such plants has not yet been reported. The aim of this study was to evaluate the impact of reduced acetylation on field productivity and identify the best strategies for decreasing acetylation. Growth and biological stress data were evaluated for 18 hybrid aspen lines with 10-20% reductions in the cell wall acetyl content from a five year field experiment in Southern Sweden. The reduction in acetyl content was achieved either by suppressing the process of acetylation in the Golgi by reducing expression of REDUCED WALL ACETYLATION (RWA) genes, or by post-synthetic acetyl removal by fungal acetyl xylan esterases (AXEs) from two different families, CE1 and CE5, targeting them to cell walls. Transgene expression was regulated by either a constitutive promoter (35S) or a wood-specific promoter (WP). For the majority of transgenic lines, growth was either similar to that in WT and transgenic control (WP:GUS) plants, or slightly reduced. The slight reduction was observed in the AXE-expressing lines regulated by the 35S promoter, not those with the WP promoter which limits expression to cells developing secondary walls. Expressing AXEs regulated by the 35S promoter resulted in increased foliar arthropod chewing, and altered condensed tannins and salicinoid phenolic glucosides (SPGs) profiles. Greater growth inhibition was observed in the case of CE5 than with CE1 AXE, and it was associated with increased foliar necrosis and distinct SPG profiles, suggesting that CE5 AXE could be recognized by the pathogen-associated molecular pattern system. For each of three different constructs, there was a line with dwarfism and growth abnormalities, suggesting random genetic/epigenetic changes. This high frequency of dwarfism (17%) is suggestive of a link between acetyl metabolism and chromatin function. These data represent the first evaluation of acetyl-reduced plants from the field, indicating some possible pitfalls, and identifying the best strategies, when developing highly productive acetyl-reduced feedstocks.

A new synergistic relationship between xylan-active LPMO and xylobiohydrolase to tackle recalcitrant xylan

BACKGROUND

Hemicellulose accounts for a significant part of plant biomass, and still poses a barrier to the efficient saccharification of lignocellulose. The recalcitrant part of hemicellulose is a serious impediment to the action of cellulases, despite the use of xylanases in the cellulolytic cocktail mixtures. However, the complexity and variety of hemicelluloses in different plant materials require the use of highly specific enzymes for a complete breakdown. Over the last few years, new fungal enzymes with novel activities on hemicelluloses have emerged. In the present study, we explored the synergistic relationships of the xylan-active AA14 lytic polysaccharide monooxygenase (LPMO), PcAA14B, with the recently discovered glucuronoxylan-specific xylanase TtXyn30A, of the (sub)family GH30_7, displaying xylobiohydrolase activity, and with commercial cellobiohydrolases, on pretreated natural lignocellulosic substrates.

RESULTS

PcAA14B and TtXyn30A showed a strong synergistic interaction on the degradation of the recalcitrant part of xylan. PcAA14B was able to increase the release of xylobiose from TtXyn30A, showing a degree of synergism (DS) of 3.8 on birchwood cellulosic fibers, and up to 5.7 on pretreated beechwood substrates. The increase in activity was dose- and time- dependent. A screening study on beechwood materials pretreated with different methods showed that the effect of the PcAA14B-TtXyn30A synergism was more prominent on substrates with low hemicellulose content, indicating that PcAA14B is mainly active on the recalcitrant part of xylan, which is in close proximity to the underlying cellulose fibers. Simultaneous addition of both enzymes resulted in higher DS than sequential addition. Moreover, PcAA14B was found to enhance cellobiose release from cellobiohydrolases during hydrolysis of pretreated lignocellulosic substrates, as well as microcrystalline cellulose.

CONCLUSIONS

The results of the present study revealed a new synergistic relationship not only among two recently discovered xylan-active enzymes, the LPMO PcAA14B, and the GH30_7 glucuronoxylan-active xylobiohydrolase TtXyn30A, but also among PcAA14B and cellobiohydrolases. We hypothesize that PcAA14B creates free ends in the xylan polymer, which can be used as targets for the action of TtXyn30A. The results are of special importance for the design of next-generation enzymatic cocktails, able to efficiently remove hemicelluloses, allowing complete saccharification of cellulose in plant biomass.

Synthesis of Arabinoxylan Oligosaccharides by Preactivation-Based Iterative Glycosylations

A concise synthetic strategy has been developed for assembling densely substituted arabinoxylan oligosaccharides, which are valuable substrates for characterizing hemicellulose-degrading enzymes. The xylan backbone has been prepared by an iterative preactivation-based glycosylation approach with phenyl thioglycosides. The preactivation has been performed with in situ generated p-nitrobenzenesulfenyl triflate prior to addition of the acceptor. The glycosylation temperature was shown to have an important impact on the yield of the coupling. The arabinose substituents have been introduced in one high-yielding glycosylation with an N-phenyl trifluoroacetimidate donor. The strategy has been successfully employed for the synthesis of three heptasaccharides in seven steps and overall yields of 24-36% from the corresponding monosaccharide building blocks.

A mini review of xylanolytic enzymes with regards to their synergistic interactions during hetero-xylan degradation

This review examines the recent models describing the mode of action of various xylanolytic enzymes and how these enzymes can be applied (sequentially or simultaneously) with their distinctive roles in mind to achieve efficient xylan degradation. With respect to homeosynergy, synergism appears to be as a result of β-xylanase and/or oligosaccharide reducing-end β-xylanase liberating xylo-oligomers (XOS) that are preferred substrates of the processive β-xylosidase. With regards to hetero-synergism, two cross relationships appear to exist and seem to be the reason for synergism between the enzymes during xylan degradation. These cross relations are the debranching enzymes such as α-glucuronidase or side-chain cleaving enzymes such as carbohydrate esterases (CE) removing decorations that would have hindered back-bone-cleaving enzymes, while backbone-cleaving-enzymes liberate XOS that are preferred substrates of the debranching and side-chain-cleaving enzymes. This interaction is demonstrated by high yields in co-production of xylan substituents such as arabinose, glucuronic acid and ferulic acid, and XOS. Finally, lytic polysaccharide monooxygenases (LPMO) have also been implicated in boosting whole lignocellulosic biomass or insoluble xylan degradation by glycoside hydrolases (GH) by possibly disrupting entangled xylan residues. Since it has been observed that the same enzyme (same Enzyme Commission, EC, classification) from different GH or CE and/or AA families can display different synergistic interactions with other enzymes due to different substrate specificities and properties, in this review, we propose an approach of enzyme selection (and mode of application thereof) during xylan degradation, as this can improve the economic viability of the degradation of xylan for producing precursors of value added products.

Purification and characterization of an endo-xylanase from Trichoderma sp., with xylobiose as the main product from xylan hydrolysis

Fungal endo-β-1,4-xylanases (endo-xylanases) can hydrolyze xylan into xylooligosaccharides (XOS), and have potential biotechnological applications for the exploitation of natural renewable polysaccharides. In the current study, we aimed to screen and characterize an efficient fungal endo-xylanase from 100 natural humus-rich soil samples collected in Guizhou Province, China, using extracted sugarcane bagasse xylan (SBX) as the sole carbon source. Initially, 182 fungal isolates producing xylanases were selected, among which Trichoderma sp. strain TP3-36 was identified as showing the highest xylanase activity of 295 U/mL with xylobiose (X2) as the main product when beechwood xylan was used as substrate. Subsequently, a glycoside hydrolase family 11 endo-xylanase, TXyn11A, was purified from strain TP3-36, and its optimal pH and temperature for activity against beechwood xylan were identified to be 5.0 and 55 °C, respectively. TXyn11A was stable across a broad pH range (3.0-10.0), and exhibited strict substrate specificity, including xylan from beechwood, wheat, rye, and sugarcane bagasse, with K_m and V_max values of 5 mg/mL and 1250 μmol/mg min, respectively, toward beechwood xylan. Intriguingly, the main product obtained from hydrolysis of beechwood xylan by TXyn11A was xylobiose, whereas SBX hydrolysis resulted in both X2 and xylotriose. Overall, these characteristics of the endo-xylanase TXyn11A indicate several potential industrial applications.

GH30-7 Endoxylanase C from the Filamentous Fungus Talaromyces cellulolyticus

Glycoside hydrolase family 30 subfamily 7 (GH30-7) enzymes include various types of xylanases, such as glucuronoxylanase, endoxylanase, xylobiohydrolase, and reducing-end xylose-releasing exoxylanase. Here, we characterized the mode of action and gene expression of the GH30-7 endoxylanase from the cellulolytic fungus Talaromyces cellulolyticus (TcXyn30C). TcXyn30C has a modular structure consisting of a GH30-7 catalytic domain and a C-terminal cellulose binding module 1, whose cellulose-binding ability has been confirmed. Sequence alignment of GH30-7 xylanases exhibited that TcXyn30C has a conserved Phe residue at the position corresponding to a conserved Arg residue in GH30-7 glucuronoxylanases, which is required for the recognition of the 4-O-methyl-α-d-glucuronic acid (MeGlcA) substituent. TcXyn30C degraded both glucuronoxylan and arabinoxylan with similar kinetic constants and mainly produced linear xylooligosaccharides (XOSs) with 2 to 3 degrees of polymerization, in an endo manner. Notably, the hydrolysis of glucuronoxylan caused an accumulation of 2²-(MeGlcA)-xylobiose (U^4m2X). The production of this acidic XOS is likely to proceed via multistep reactions by putative glucuronoxylanase activity that produces 2²-(MeGlcA)-XOSs (X _n U^4m2X, n ≥ 0) in the initial stages of the hydrolysis and by specific release of U^4m2X from a mixture containing X _n U^4m2X. Our results suggest that the unique endoxylanase activity of TcXyn30C may be applicable to the production of linear and acidic XOSs. The gene xyn30C was located adjacent to the putative GH62 arabinofuranosidase gene (abf62C) in the T. cellulolyticus genome. The expression of both genes was induced by cellulose. The results suggest that TcXyn30C may be involved in xylan removal in the hydrolysis of lignocellulose by the T. cellulolyticus cellulolytic system.IMPORTANCE Xylooligosaccharides (XOSs), which are composed of xylose units with a β-1,4 linkage, have recently gained interest as prebiotics in the food and feed industry. Apart from linear XOSs, branched XOSs decorated with a substituent such as methyl glucuronic acid and arabinose also have potential applications. Endoxylanase is a promising tool in producing XOSs from xylan. The structural variety of XOSs generated depends on the substrate specificity of the enzyme as well as the distribution of the substituents in xylan. Thus, the exploration of endoxylanases with novel specificities is expected to be useful in the provision of a series of XOSs. In this study, the endoxylanase TcXyn30C from Talaromyces cellulolyticus was characterized as a unique glycoside hydrolase belonging to the family GH30-7, which specifically releases 2²-(4-O-methyl-α-d-glucuronosyl)-xylobiose from hardwood xylan. This study provides new insights into the production of linear and branched XOSs by GH30-7 endoxylanase.

Coupled chemistry kinetics demonstrate the utility of functionalized Sup35 amyloid nanofibrils in biocatalytic cascades

Concerns over the environment are a central driver for designing cell-free enzymatic cascade reactions that synthesize non-petrol-based commodity compounds. An often-suggested strategy that would demonstrate the economic competitiveness of this technology is recycling of valuable enzymes through their immobilization. For this purpose, amyloid nanofibrils are an ideal scaffold to realize chemistry-free covalent enzyme immobilization on a material that offers a large surface area. However, in most instances, only single enzyme-functionalized amyloid fibrils have so far been studied. To embark on the next stage, here we displayed xylanase A, β-xylosidase, and an aldose sugar dehydrogenase on Sup35(1-61) nanofibrils to convert beechwood xylan to xylonolactone. We characterized this enzymatic cascade by measuring the time-dependent accumulation of xylose, xylooligomers, and xylonolactone. Furthermore, we studied the effects of relative enzyme concentrations, pH, temperature, and agitation on product formation. Our investigations revealed that a modular cascade with a mixture of xylanase and β-xylosidase, followed by product removal and separate oxidation of xylose with the aldose sugar dehydrogenase, is more productive than an enzyme mix containing all of these enzymes together. Moreover, we found that the nanofibril-coupled enzymes do not lose activity compared with their native state. These findings provide proof of concept of the feasibility of functionalized Sup35(1-61) fibrils as a molecular scaffold for biocatalytic cascades consisting of reusable enzymes that can be used in biotechnology.

A carbohydrate-binding family 48 module enables feruloyl esterase action on polymeric arabinoxylan

Feruloyl esterases (EC 3.1.1.73), belonging to carbohydrate esterase family 1 (CE1), hydrolyze ester bonds between ferulic acid (FA) and arabinose moieties in arabinoxylans. Recently, some CE1 enzymes identified in metagenomics studies have been predicted to contain a family 48 carbohydrate-binding module (CBM48), a CBM family associated with starch binding. Two of these CE1s, wastewater treatment sludge (wts) Fae1A and wtsFae1B isolated from wastewater treatment surplus sludge, have a cognate CBM48 domain and are feruloyl esterases, and wtsFae1A binds arabinoxylan. Here, we show that wtsFae1B also binds to arabinoxylan and that neither binds starch. Surface plasmon resonance analysis revealed that wtsFae1B's K_d for xylohexaose is 14.8 μm and that it does not bind to starch mimics, β-cyclodextrin, or maltohexaose. Interestingly, in the absence of CBM48 domains, the CE1 regions from wtsFae1A and wtsFae1B did not bind arabinoxylan and were also unable to catalyze FA release from arabinoxylan. Pretreatment with a β-d-1,4-xylanase did enable CE1 domain-mediated FA release from arabinoxylan in the absence of CBM48, indicating that CBM48 is essential for the CE1 activity on the polysaccharide. Crystal structures of wtsFae1A (at 1.63 Å resolution) and wtsFae1B (1.98 Å) revealed that both are folded proteins comprising structurally-conserved hydrogen bonds that lock the CBM48 position relative to that of the CE1 domain. wtsFae1A docking indicated that both enzymes accommodate the arabinoxylan backbone in a cleft at the CE1-CBM48 domain interface. Binding at this cleft appears to enable CE1 activities on polymeric arabinoxylan, illustrating an unexpected and crucial role of CBM48 domains for accommodating arabinoxylan.

Cooperation of hydrolysis modes among xylanases reveals the mechanism of hemicellulose hydrolysis by Penicillium chrysogenum P33

Background

Xylanases randomly cleave the internal β-1,4-glycosidic bonds in the xylan backbone and are grouped into different families in the carbohydrate-active enzyme (CAZy) database. Although multiple xylanases are detected in single strains of many filamentous fungi, no study has been reported on the composition, synergistic effect, and mode of action in a complete set of xylanases secreted by the same microorganism.

Results

All three xylanases secreted by Penicillium chrysogenum P33 were expressed and characterized. The enzymes Xyl1 and Xyl3 belong to the GH10 family and Xyl3 contains a CBM1 domain at its C-terminal, whereas Xyl2 belongs to the GH11 family. The optimal temperature/pH values were 35 °C/6.0, 50 °C/5.0 and 55 °C/6.0 for Xyl1, Xyl2, and Xyl3, respectively. The three xylanases exhibited synergistic effects, with the maximum synergy observed between Xyl3 and Xyl2, which are from different families. The synergy between xylanases could also improve the hydrolysis of cellulase (C), with the maximum amount of reducing sugars (5.68 mg/mL) observed using the combination of C + Xyl2 + Xyl3. Although the enzymatic activity of Xyl1 toward xylan was low, it was shown to be capable of hydrolyzing xylooligosaccharides into xylose. Xyl2 was shown to hydrolyze xylan to long-chain xylooligosaccharides, whereas Xyl3 hydrolyzed xylan to xylooligosaccharides with a lower degree of polymerization.

Conclusions

Synergistic effect exists among different xylanases, and it was higher between xylanases from different families. The cooperation of hydrolysis modes comprised the primary mechanism for the observed synergy between different xylanases. This study demonstrated, for the first time, that the hydrolysates of GH11 xylanases can be further hydrolyzed by GH10 xylanases, but not vice versa.

Metagenomic discovery of feruloyl esterases from rumen microflora

Feruloyl esterases (FAEs) are a key group of enzymes that hydrolyze ferulic acids ester-linked to plant polysaccharides. The cow's rumen is a highly evolved ecosystem of complex microbial microflora capable of converting fibrous substances to energy. From direct cloning of the rumen microbial metagenome, we identified seven active phagemids conferring feruloyl esterase activity. The genomic inserts ranged from 1633 to 4143 bp, and the ORFs from 681 to 1359 bp. BLAST search reveals sequence homology to feruloyl esterases and esterases/lipases identified in anaerobes. The seven genes were expressed in Escherichia coli, and the proteins were purified to homogeneity. The FAEs were found to cover types B, C, and D in the feruloyl esterase classification system using model hydroxycinnamic acid esters. The release of ferulic acid (FA) catalyzed by these enzymes was established using natural substrates corn fiber (CF) and wheat insoluble arabinoxylan (WIA). Three of the enzymes were demonstrated to cleave diferulates and hence the capability to break down Araf-FA-FA-Araf cross-links. The wide variation in the sequence, activity, and substrate specificity observed in the FAEs discovered in this study is a confirming evidence that combined actions of a full range of FAE enzymes contribute to the high-efficiency fiber digestion in the rumen microbial ecosystem.

Biochemical characterization and low-resolution SAXS shape of a novel GH11 exo-1,4-β-xylanase identified in a microbial consortium

Biotechnologies that aim to produce renewable fuels, chemicals, and bioproducts from residual ligno(hemi)cellulosic biomass mostly rely on enzymatic depolymerization of plant cell walls (PCW). This process requires an arsenal of diverse enzymes, including xylanases, which synergistically act on the hemicellulose, reducing the long and complex xylan chains to oligomers and simple sugars. Thus, xylanases play a crucial role in PCW depolymerization. Until recently, the largest xylanase family, glycoside hydrolase family 11 (GH11) has been exclusively represented by endo-catalytic β-1,4- and β-1,3-xylanases. Analysis of a metatranscriptome library from a microbial lignocellulose community resulted in the identification of an unusual exo-acting GH11 β-1,4-xylanase (MetXyn11). Detailed characterization has been performed on recombinant MetXyn11 including determination of its low-resolution small-angle X-ray scattering (SAXS) molecular envelope in solution. Our results reveal that MetXyn11 is a monomeric globular enzyme that liberates xylobiose from heteroxylans as the only product. MetXyn11 has an optimal activity in a pH range from 6 to 9 and an optimal temperature of 50 °C. The enzyme maintained above 65% of its original activity in the pH range 5 to 6 after being incubated for 72 h at 50 °C. Addition of the enzyme to a commercial enzymatic cocktail (CelicCtec3) promoted a significant increase of enzymatic hydrolysis yields of hydrothermally pretreated sugarcane bagasse (16% after 24 h of hydrolysis).

Biochemical characterization of a novel exo-oligoxylanase from Paenibacillus barengoltzii suitable for monosaccharification from corncobs

BACKGROUND

Xylan is the major component of hemicelluloses, which are the second most abundant polysaccharides in nature, accounting for approximately one-third of all renewable organic carbon resources on earth. Efficient degradation of xylan is the prerequisite for biofuel production. Enzymatic degradation has been demonstrated to be more attractive due to low energy consumption and environmental friendliness, when compared with chemical degradation. Exo-xylanases, as a rate-limiting factor, play an important role in the xylose production. It is of great value to identify novel exo-xylanases for efficient bioconversion of xylan in biorefinery industry.

RESULTS

A novel glycoside hydrolase (GH) family 8 reducing-end xylose-releasing exo-oligoxylanase (Rex)-encoding gene (PbRex8) was cloned from Paenibacillus barengoltzii and heterogeneously expressed in Escherichia coli. The deduced amino acid sequence of PbRex8 shared the highest identity of 74% with a Rex from Bacillus halodurans. The recombinant enzyme (PbRex8) was purified and biochemically characterized. The optimal pH and temperature of PbRex8 were 5.5 and 55 °C, respectively. PbRex8 showed prominent activity on xylooligosaccharides (XOSs), and trace activity on xylan. It also exhibited β-1,3-1,4-glucanase and xylobiase activities. The enzyme efficiently converted corncob xylan to xylose coupled with a GH family 10 endo-xylanase, with a xylose yield of 83%. The crystal structure of PbRex8 was resolved at 1.88 Å. Structural comparison suggests that Arg67 can hydrogen-bond to xylose moieties in the -1 subsite, and Asn122 and Arg253 are close to xylose moieties in the -3 subsite, the hypotheses of which were further verified by mutation analysis. In addition, Trp205, Trp132, Tyr372, Tyr277 and Tyr369 in the grove of PbRex8 were found to involve in glucooligosaccharides interactions. This is the first report on a GH family 8 Rex from P. barengoltzii.

CONCLUSIONS

A novel reducing-end xylose-releasing exo-oligoxylanase suitable for xylose production from corncobs was identified, biochemically characterized and structurally elucidated. The properties of PbRex8 may make it an excellent candidate in biorefinery industries.

Biofuel production from straw hydrolysates: current achievements and perspectives

Straw is an agricultural residue of the production of e.g. cereals, rapeseed or sunflowers. It includes dried stalks, leaves, and empty ears and corncobs, which are separated from the grains during harvest. Straw is a promising lignocellulosic feedstock with a beneficial greenhouse gas balance for the production of biofuels and chemicals. Like all lignocellulosic materials, straw is recalcitrant and requires thermochemical and enzymatic pretreatment to enable access to the three major biopolymers of straw-the polysaccharides cellulose and hemicellulose and the polyaromatic compound lignin. Straw is used for commercial ethanol and biogas production. Considerable research has also been conducted to produce biobutanol, biodiesel and biochemicals from this raw material, but more research is required to establish them on a commercial scale. The major hindrance for launching industrial biofuel and chemicals' production from straw is the high cost necessitated by pretreatment of the material. Improvements of microbial strains, production and extraction technologies, as well as co-production of high-value compounds represent ways of establishing straw as feedstock for the production of biofuels, chemicals and food.

Dynamic and Functional Profiling of Xylan-Degrading Enzymes in Aspergillus Secretomes Using Activity-Based Probes

Plant polysaccharides represent a virtually unlimited feedstock for the generation of biofuels and other commodities. However, the extraordinary recalcitrance of plant polysaccharides toward breakdown necessitates a continued search for enzymes that degrade these materials efficiently under defined conditions. Activity-based protein profiling provides a route for the functional discovery of such enzymes in complex mixtures and under industrially relevant conditions. Here, we show the detection and identification of β-xylosidases and endo-β-1,4-xylanases in the secretomes of Aspergillus niger, by the use of chemical probes inspired by the β-glucosidase inhibitor cyclophellitol. Furthermore, we demonstrate the use of these activity-based probes (ABPs) to assess enzyme-substrate specificities, thermal stabilities, and other biotechnologically relevant parameters. Our experiments highlight the utility of ABPs as promising tools for the discovery of relevant enzymes useful for biomass breakdown.

Mode of Action of GH30-7 Reducing-End Xylose-Releasing Exoxylanase A (Xyn30A) from the Filamentous Fungus Talaromyces cellulolyticus

In this study, we characterized the mode of action of reducing-end xylose-releasing exoxylanase (Rex), which belongs to the glycoside hydrolase family 30-7 (GH30-7). GH30-7 Rex, isolated from the cellulolytic fungus Talaromyces cellulolyticus (Xyn30A), exists as a dimer. The purified Xyn30A released xylose from linear xylooligosaccharides (XOSs) 3 to 6 xylose units in length with similar kinetic constants. Hydrolysis of branched, borohydride-reduced, and p-nitrophenyl XOSs clarified that Xyn30A possesses a Rex activity. ¹H nuclear magnetic resonance (¹H NMR) analysis of xylotriose hydrolysate indicated that Xyn30A degraded XOSs via a retaining mechanism and without recognizing an anomeric structure at the reducing end. Hydrolysis of xylan by Xyn30A revealed that the enzyme continuously liberated both xylose and two types of acidic XOSs: 2²-(4-O-methyl-α-d-glucuronyl)-xylotriose (MeGlcA²Xyl₃) and 2²-(MeGlcA)-xylobiose (MeGlcA²Xyl₂). These acidic products were also detected during hydrolysis using a mixture of MeGlcA²Xyl _n (n = 2 to 14) as the substrate. This indicates that Xyn30A can release MeGlcA²Xyl _n (n = 2 and 3) in an exo manner. Comparison of subsites in Xyn30A and GH30-7 glucuronoxylanase using homology modeling suggested that the binding of the reducing-end residue at subsite +2 was partially prevented by a Gln residue conserved in GH30-7 Rex; additionally, the Arg residue at subsite -2b, which is conserved in glucuronoxylanase, was not found in Xyn30A. Our results lead us to propose that GH30-7 Rex plays a complementary role in hydrolysis of xylan by fungal cellulolytic systems.IMPORTANCE Endo- and exo-type xylanases depolymerize xylan and play crucial roles in the assimilation of xylan in bacteria and fungi. Exoxylanases release xylose from the reducing or nonreducing ends of xylooligosaccharides; this is generated by the activity of endoxylanases. β-Xylosidase, which hydrolyzes xylose residues on the nonreducing end of a substrate, is well studied. However, the function of reducing-end xylose-releasing exoxylanases (Rex), especially in fungal cellulolytic systems, remains unclear. This study revealed the mode of xylan hydrolysis by Rex from the cellulolytic fungus Talaromyces cellulolyticus (Xyn30A), which belongs to the glycoside hydrolase family 30-7 (GH30-7). A conserved residue related to Rex activity is found in the substrate-binding site of Xyn30A. These findings will enhance our understanding of the function of GH30-7 Rex in the cooperative hydrolysis of xylan by fungal enzymes.

Degradation profile of nixtamalized maize pericarp by the action of the microbial consortium PM-06

The nixtamalized maize pericarp (NMP) is a plentiful by-product of the tortilla industry and an important source of fermentable sugars. The aim of this study was to describe the degradation profile of NMP by the action of a consortium (PM-06) obtained from the native microbial community of this residue. The degradation was analyzed in terms of the changes in the community dynamics, production of enzymes (endo-xylanase and endo-cellulase), physicochemical parameters, and substrate chemical and microstructural characteristics, to understand the mechanisms behind the process. The consortium PM-06 degraded 86.8 ± 3.3% of NMP after 192 h of growth. Scanning electron microscopy images, and the composition and weight of the residual solids, showed that degradation was sequential starting with the consumption of hemicellulose. Xylanase was the highest enzyme activity produced, with a maximum value of 12.45 ± 0.03 U mL⁻¹. There were fluctuations in the pH during the NMP degradation, starting with the acidification of the culture media and finishing with a pH close to 8.5. The most abundant species in the consortium, at the moment of maximum degradation activity, were Aneurinibacillus migulanus, Paenibacillus macerans, Bacillus coagulans, Microbacterium sp. LCT-H2, and Bacillus thuringiensis. The diversity of PM-06 provided metabolic abilities that in combination helped to produce an efficient process. The consortium PM-06 generated a set of different tools that worked coordinated to increase the substrate availability through the solubilization of components and elimination of structural diffusion barriers. This is the first report about the degradation of NMP using a microbial consortium.

The xyl-doc gene cluster of Ruminiclostridium cellulolyticum encodes GH43- and GH62-α-l-arabinofuranosidases with complementary modes of action

BACKGROUND

The α-l-arabinofuranosidases (α-l-ABFs) are exoenzymes involved in the hydrolysis of α-l-arabinosyl linkages in plant cell wall polysaccharides. They play a crucial role in the degradation of arabinoxylan and arabinan and they are used in many biotechnological applications. Analysis of the genome of R. cellulolyticum showed that putative cellulosomal α-l-ABFs are exclusively encoded by the xyl-doc gene cluster, a large 32-kb gene cluster. Indeed, among the 14 Xyl-Doc enzymes encoded by this gene cluster, 6 are predicted to be α-l-ABFs belonging to the CAZyme families GH43 and GH62.

RESULTS

The biochemical characterization of these six Xyl-Doc enzymes revealed that four of them are α-l-ABFs. GH43₁₆-1229 (RcAbf43A) which belongs to the subfamily 16 of the GH43, encoded by the gene at locus Ccel_1229, has a low specific activity on natural substrates and can cleave off arabinose decorations located at arabinoxylan chain extremities. GH43₁₀-1233 (RcAbf43A_d2,3), the product of the gene at locus Ccel_1233, belonging to subfamily 10 of the GH43, can convert the double arabinose decorations present on arabinoxylan into single O2- or O3-linked decorations with high velocity (k _cat = 16.6 ± 0.6 s^-1). This enzyme acts in synergy with GH62-1234 (RcAbf62A_m2,3), the product of the gene at locus Ccel_1234, a GH62 α-l-ABF which hydrolyzes α-(1 → 3) or α-(1 → 2)-arabinosyl linkages present on polysaccharides and arabinoxylooligosaccharides monodecorated. Finally, a bifunctional enzyme, GH62-CE6-1240 (RcAbf62B_m2,3Axe6), encoded by the gene at locus Ccel_1240, which contains a GH62-α-l-ABF module and a carbohydrate esterase (CE6) module, catalyzes deacylation of plant cell wall polymers and cleavage of arabinosyl mono-substitutions. These enzymes are also active on arabinan, a component of the type I rhamnogalacturonan, showing their involvement in pectin degradation.

CONCLUSION

Arabinofuranosyl decorations on arabinoxylan and pectin strongly inhibit the action of xylan-degrading enzymes and pectinases. α-l-ABFs encoded by the xyl-doc gene cluster of R. cellulolyticum can remove all the decorations present in the backbone of arabinoxylan and arabinan, act synergistically, and, thus, play a crucial role in the degradation of plant cell wall polysaccharides.

Comparison of enzymatic activities and proteomic profiles of Butyrivibrio fibrisolvens grown on different carbon sources

Background

The rumen microbiota is one of the most complex consortia of anaerobes, involving archaea, bacteria, protozoa, fungi and phages. They are very effective at utilizing plant polysaccharides, especially cellulose and hemicelluloses. The most important hemicellulose decomposers are clustered with the genus Butyrivibrio. As the related species differ in their range of hydrolytic activities and substrate preferences, Butyrivibrio fibrisolvens was selected as one of the most effective isolates and thus suitable for proteomic studies on substrate comparisons in the extracellular fraction. The B. fibrisolvens genome is the biggest in the butyrivibria cluster and is focused on “environmental information processing” and “carbohydrate metabolism”.

Methods

The study of the effect of carbon source on B. fibrisolvens 3071 was based on cultures grown on four substrates: xylose, glucose, xylan, xylan with 25% glucose. The enzymatic activities were studied by spectrophotometric and zymogram methods. Proteomic study was based on genomics, 2D electrophoresis and nLC/MS (Bruker Daltonics) analysis.

Results

Extracellular β-endoxylanase as well as xylan β-xylosidase activities were induced with xylan. The presence of the xylan polymer induced hemicellulolytic enzymes and increased the protein fraction in the interval from 40 to 80 kDa. 2D electrophoresis with nLC/MS analysis of extracellular B. fibrisolvens 3071 proteins found 14 diverse proteins with significantly different expression on the tested substrates.

Conclusion

The comparison of four carbon sources resulted in the main significant changes in B. fibrisolvens proteome occurring outside the fibrolytic cluster of proteins. The affected proteins mainly belonged to the glycolysis and protein synthesis cluster.

Electronic supplementary material

The online version of this article (10.1186/s12953-019-0150-3) contains supplementary material, which is available to authorized users.

Novel xylanolytic triple domain enzyme targeted at feruloylated arabinoxylan degradation

A three catalytic domain multi-enzyme; a CE1 ferulic acid esterase, a GH62 α-l-arabinofuranosidase and a GH10 β-d-1,4-xylanase was identified in a metagenome obtained from wastewater treatment sludge. The capability of the CE1-GH62-GH10 multi-enzyme to degrade arabinoxylan was investigated to examine the hypothesis that CE1-GH62-GH10 would degrade arabinoxylan more efficiently than the corresponding equimolar mix of the individual enzymes. CE1-GH62-GH10 efficiently catalyzed the production of xylopyranose, xylobiose, xylotriose, arabinofuranose and ferulic acid (FA) when incubated with insoluble wheat arabinoxylan (WAX-I) (k_cat = 20.8 ± 2.6 s^-1). Surprisingly, in an equimolar mix of the individual enzymes a similar k_cat towards WAX-I was observed (k_cat = 17.3 ± 3.8 s^-1). Similarly, when assayed on complex plant biomass the activity was comparable between CE1-GH62-GH10 and an equimolar mix of the individual enzymes. This suggests that from a hydrolytic point of view a CE1-GH62-GH10 multi-enzyme is not an advantage. Determination of the melting temperatures for CE1-GH62-GH10 (71.0 ± 0.05 °C) and CE1 (69.9 ± 0.02), GH62 (65.7 ± 0.06) and GH10 (71 ± 0.05 °C) indicates that CE1 and GH62 are less stable as single domain enzymes. This conclusion was corroborated by the findings that CE1 lost ˜50% activity within 2 h, while GH62 retained ˜50% activity after 24 h, whereas CE1-GH62-GH10 and GH10 retained ˜50% activity for 72 h. GH62-GH10, when appended to each other, displayed a higher specificity constant (k_cat/K_m = 0.3 s^-1 mg^-1 ml) than the individual GH10 (k_cat/K_m = 0.12 s^-1 ± 0.02 mg^-1 ml) indicating a synergistic action between the two. Surprisingly, CE1-GH62, displayed a 2-fold lower k_cat towards WAX-I than GH62, which might be due to the presence of a putative carbohydrate binding module appended to CE1 at the N-terminal. Both CE1 and CE1-GH62 released insignificant amounts of FA from WAX-I, but FA was released from WAX-I when both CE1 and GH10 were present, which might be due to GH10 releasing soluble oligosaccharides that CE1 can utilize as substrate. CE1 also displayed activity towards solubilized 5-O-trans-feruloyl-α-l-Araf (k_cat = 36.35 s^-1). This suggests that CE1 preferably acts on soluble oligosaccharides.

Heterologous expression, purification and biochemical characterization of a new xylanase from Myceliophthora heterothallica F.2.1.4

Myceliophthora heterothallica is a thermophilic fungus potentially relevant for the production of enzymes involved in the degradation of plant biomass. A xylanase encoding gene of this species was identified by means of RT-PCR using primers designed based on a xylanase coding sequence (GH11) of the fungus M. thermophila. The obtained gene was ligated to the vector pET28a(+) and the construct was transformed into Escherichia coli cells. The recombinant xylanase (r-ec-XylMh) was heterologously expressed, and the highest activity was observed at 55 °C and pH 6. The enzyme stability was greater than 70% between pH 4.5 and 9.5 and the inclusion of glycerol (50%) resulted in a significant increase in thermostability. Under these conditions, the enzyme retained more than 50% residual activity when incubated at 65 °C for 1 h, and approximately 30% activity when incubated at 70 °C for the same period. The tested cations did not increase xylanolytic activity, and the enzyme indicated significant tolerance to several phenolic compounds after 24 h, as well as high specificity for xylan, with no activity for other substrates such as CMC (carboxymethylcellulose), Avicel, pNPX (p-nitrophenyl-β-D-xylopyranoside) and pNPA (p-nitrophenyl-α-L-arabinofuranoside), and is thus, of potential relevance in pulp bleaching.

Discovery of a Thermostable GH10 Xylanase with Broad Substrate Specificity from the Arctic Mid-Ocean Ridge Vent System

A two-domain GH10 xylanase-encoding gene (amor_gh10a) was discovered from a metagenomic data set, generated after in situ incubation of a lignocellulosic substrate in hot sediments on the sea floor of the Arctic Mid-Ocean Ridge (AMOR). AMOR_GH10A comprises a signal peptide, a carbohydrate-binding module belonging to a previously uncharacterized family, and a catalytic glycosyl hydrolase (GH10) domain. The enzyme shares the highest sequence identity (42%) with a hypothetical protein from a Verrucomicrobia bacterium, and its GH10 domain shares low identity (24 to 28%) with functionally characterized xylanases. Purified AMOR_GH10A showed thermophilic and halophilic properties and was active toward various xylans. Uniquely, the enzyme showed high activity toward amorphous cellulose, glucomannan, and xyloglucan and was more active toward cellopentaose than toward xylopentaose. Binding assays showed that the N-terminal domain of this broad-specificity GH10 binds strongly to amorphous cellulose, as well as to microcrystalline cellulose, birchwood glucuronoxylan, barley β-glucan, and konjac glucomannan, confirming its classification as a novel CBM (CBM85).IMPORTANCE Hot springs at the sea bottom harbor unique biodiversity and are a promising source of enzymes with interesting properties. We describe the functional characterization of a thermophilic and halophilic multidomain xylanase originating from the Arctic Mid-Ocean Ridge vent system, belonging to the well-studied family 10 of glycosyl hydrolases (GH10). This xylanase, AMOR_GH10A, has a surprisingly wide substrate range and is more active toward cellopentaose than toward xylopentaose. This substrate promiscuity is unique for the GH10 family and could prove useful in industrial applications. Emphasizing the versatility of AMOR_GH10A, its N-terminal domain binds to both xylans and glycans, while not showing significant sequence similarities to any known carbohydrate-binding module (CBM) in the CAZy database. Thus, this N-terminal domain lays the foundation for the new CBM85 family.

An Innovative Biocatalyst for Continuous 2G Ethanol Production from Xylo-Oligomers by Saccharomyces cerevisiae through Simultaneous Hydrolysis, Isomerization, and Fermentation (SHIF)

Many approaches have been considered aimed at ethanol production from the hemicellulosic fraction of biomass. However, the industrial implementation of this process has been hindered by some bottlenecks, one of the most important being the ease of contamination of the bioreactor by bacteria that metabolize xylose. This work focuses on overcoming this problem through the fermentation of xylulose (the xylose isomer) by native Saccharomyces cerevisiae using xylo-oligomers as substrate. A new concept of biocatalyst is proposed, containing xylanases and xylose isomerase (XI) covalently immobilized on chitosan, and co-encapsulated with industrial baker’s yeast in Ca-alginate gel spherical particles. Xylo-oligomers are hydrolyzed, xylose is isomerized, and finally xylulose is fermented to ethanol, all taking place simultaneously, in a process called simultaneous hydrolysis, isomerization, and fermentation (SHIF). Among several tested xylanases, Multifect CX XL A03139 was selected to compose the biocatalyst bead. Influences of pH, Ca2+, and Mg2+ concentrations on the isomerization step were assessed. Experiments of SHIF using birchwood xylan resulted in an ethanol yield of 0.39 g/g, (76% of the theoretical), selectivity of 3.12 gethanol/gxylitol, and ethanol productivity of 0.26 g/L/h.

Diversity and Biotechnological Potential of Xylan-Degrading Microorganisms from Orange Juice Processing Waste

The orange juice processing sector produces worldwide massive amounts of waste, which is characterized by high lignin, cellulose and hemicellulose content, and which exceeds 40% of the fruit’s dry weight (d.w.). In this work, the diversity and the biotechnological potential of xylan-degrading microbiota in orange juice processing waste were investigated through the implementation of an enrichment isolation strategy followed by enzyme assays for the determination of xylanolytic activities, and via next generation sequencing for microbial diversity identification. Intracellular rather than extracellular endo-1,4-β-xylanase activities were detected, indicating that peripheral cell-bound (surface) xylanases are involved in xylan hydrolysis by the examined microbial strains. Among the isolated microbial strains, bacterial isolates belonging to Pseudomonas psychrotolerans/P. oryzihabitans spectrum (99.9%/99.8% similarity, respectively) exhibited activities of 280 U/mg protein. In contrast, almost all microbial strains isolated exerted low extracellular 1,4-β-xylosidase activities (<5 U/mg protein), whereas no intracellular 1,4-β-xylosidase activities were detected for any of them. Illumina data showed the dominance of lactic and acetic acid bacteria and of the yeasts Hanseniaspora and Zygosaccharomyces. This is the first report on indigenous xylanolytic microbiota isolated from orange juice processing waste, possessing the biotechnological potential to serve as biocatalysts for citrus biomass valorization through the production of high-added value products and energy recovery.