Bioinformatics-based identification of GH12 endoxyloglucanases in citrus-pathogenic Penicillium spp.

Millions of tons of citrus peel waste are produced every year as a byproduct of the juice industry. Citrus peel is rich in pectin and xyloglucan, but while the pectin is extracted for use in the food industry, the xyloglucan is currently not valorized. To target hydrolytic degradation of citrus peel xyloglucan into oligosaccharides, we have used bioinformatics to identify three glycoside hydrolase 12 (GH12) endoxyloglucanases (EC 3.2.1.151) from the citrus fruit pathogens Penicillium italicum GL-Gan1 and Penicillium digitatum Pd1 and characterized them on xyloglucan obtained by alkaline extraction from citrus peel. The enzymes displayed pH-temperature optima of pH 4.6-5.3 and 35-37°C. PdGH12 from P. digitatum and PiGH12A from P. italicum share 84% sequence identity and displayed similar kinetics, although kcat was highest for PdGH12. In contrast, PiGH12B from P. italicum, which has the otherwise conserved Trp in subsite -4 replaced with a Tyr, displayed a 3 times higher KM and a 4 times lower kcat/KM than PiGH12A, but was the most thermostable enzyme of the three Penicillium-derived endoxyloglucanases. The benchmark enzyme AnGH12 from Aspergillus nidulans was more thermally stable and had a higher pH-temperature optimum than the enzymes from Penicillum spp. The difference in structure of the xyloglucan oligosaccharides extracted from citrus peel xyloglucan and tamarind xyloglucan by the new endoxyloglucanases was determined by LC-MS. The inclusion of citrus peel xyloglucan demonstrated that the endoxyloglucanases liberated fucosylated xyloglucan oligomers, implying that these enzymes have the potential to upgrade citrus peel residues to produce oligomers useful as intermediates or bioactive compounds.

Efficient activity screening of new glucuronoyl esterases using a pNP-based assay.

Glucuronoyl esterases (CE15, EC 3.1.1.117) catalyze the hydrolysis of ester bonds between lignin and carbohydrates in lignocellulose. They are widespread within fungi and bacteria, and are subjects to research interest due to their potential applicability in lignocellulose processing. Identifying new and relevant glucuronoyl esterase candidates is challenging because available model substrates poorly represent the natural substrate, which leads to inefficient screening for the activity. In this study, we demonstrate how fifteen novel, fungal, putative glucuronoyl esterases from family CE15 were expressed and screened for activity towards a commercially available, colorimetric assay based on the methyl-ester of 4-O-methyl-aldotriuronic acid linked to para-nitrophenol (methyl ester-UX-ß-pNP) and coupled with the activity of GH67 (α-glucuronidase) and GH43 (ß-xylosidase) activity. The assay provides easy means for accurately establishing activity and determining specific activity of glucuronoyl esterases. Out of the fifteen expressed CE15 proteins, seven are active and were purified to determine their specific activity. The seven active enzymes originate from Auricularia subglabra (3 proteins), Ganoderma sinensis (2 proteins) and Neocallimastix californiae (2 proteins). Among the CE15 proteins not active towards the screening substrate (methyl ester-UX-ß-pNP) were proteins originating from Schizophyllum commune, Podospora anserina, Trametes versicolor, and Coprinopsis cinerea. It is unexpected that CE15 proteins from such canonical lignocellulose degraders do not have the anticipated activity, and these observations call for deeper investigations.

Polyphenol Oxidase Products Are Priming Agents for LPMO Peroxygenase Activity.

Polyphenol oxidases catalyze the hydroxylation of monophenols to diphenols, which are reducing agents for lytic polysaccharide monooxygenases (LPMOs) in their degradation of cellulose. In particular, the polyphenol oxidase MtPPO7 from Myceliophthora thermophila converts lignocellulose-derived monophenols, and under the new perspective of the peroxygenase reaction catalyzed by LPMOs, we aim to differentiate the role of the catalytic products of MtPPO7 in priming and fueling of LPMO activity. Exemplified by the activity of MtPPO7 towards guaiacol and by using the benchmark LPMO NcAA9C from Neurospora crassa we show that MtPPO7 catalytic products provide the initial electron for the reduction of Cu(II) to Cu(I) but cannot provide the required reducing power for continuous fueling of the LPMO. The priming reaction is shown to occur with catalytic amounts of MtPPO7 products and those compounds do not generate substantial amounts of H2 O2 in situ to fuel the LPMO peroxygenase activity. Reducing agents with a low propensity to generate H2 O2 can provide the means for controlling the LPMO catalysis through exogenous H2 O2 and thereby minimize any enzyme inactivation.

Fungal feruloyl esterases can catalyze release of diferulic acids from complex arabinoxylan.

Feruloyl esterases (FAEs, EC 3.1.1.73) catalyze the hydrolytic cleavage of ester bonds between feruloyl and arabinosyl moieties in arabinoxylans. Recently, we discovered that two bacterial FAEs could catalyze release of diferulic acids (diFAs) from highly substituted, cross-linked corn bran arabinoxylan. Here, we show that several fungal FAEs, notably AnFae1 (Aspergillus niger), AoFae1 (A. oryzae), and MgFae1 (Magnaporthe oryzae (also known as M. grisae)) also catalyze liberation of diFAs from complex arabinoxylan. By comparing the enzyme kinetics of diFA release to feruloyl esterase activity of the enzymes on methyl- and arabinosyl-ferulate substrates we demonstrate that the diFA release activity cannot be predicted from the activity of the enzymes on these synthetic substrates. A detailed structure-function analysis, based on AlphaFold2 modeled enzyme structures and docking with the relevant di-feruloyl ligands, reveal how distinct differences in the active site topology and surroundings may explain the diFA releasing action of the enzymes. Interestingly, the analysis also unveils that the carbohydrate binding module of the MgFae1 may play a key role in the diFA releasing ability of this enzyme. The findings contribute further understanding of the function of FAEs in the deconstruction of complex arabinoxylans and provide new opportunities for enzyme assisted upgrading of complex bran arabinoxylans.

Bio-based surfactants: enzymatic functionalization and production from renewable resources.

Bio-based surfactants produced from renewable resources are increasing in market demand. In this review, we focus on enzymatic functionalization and coupling of carbohydrate-based heads to fatty aliphatic chains as tails for the synthesis of bio-based surfactants. We point to concrete examples of how transferase, lipase, and glycoside hydrolase-catalyzed esterification or glycoside formation can link a variety of mono- and oligosaccharides with fatty acids. Similarly, enzymatic reductive amination also leads to coupling. Another approach for surfactant synthesis is enzymatic carbohydrate functionalization before click chemistry coupling, and here LPMOs, oxidases, and dehydrogenases are relevant. C6 or C1-oxidizing activities are particularly important for converting nonionic surfactants into highly demanded anionic counterparts.

Feruloylated Arabinoxylan and Oligosaccharides: Chemistry, Nutritional Functions, and Options for Enzymatic Modification.

Cereal brans and grain endosperm cell walls are key dietary sources of different types of arabinoxylan. Arabinoxylan is the main group of hemicellulosic polysaccharides that are present in the cell walls of monocot grass crops and hence in cereal grains. The arabinoxylan polysaccharides consist of a backbone of ß-(1→4)-linked xylopyranosyl residues, which carry arabinofuranosyl moieties, hence the term arabinoxylan. Moreover, the xylopyranosyl residues can be acetylated or substituted by 4-O-methyl-d-glucuronic acid. The arabinofuranosyls may be esterified with a feruloyl group. Feruloylated arabinoxylo-oligosaccharides exert beneficial bioactivities via prebiotic, immunomodulatory, and/or antioxidant effects. New knowledge on microbial enzymes that catalyze specific structural modifications of arabinoxylans can help us understand how these complex fibers are converted in the gut and provide a foundation for the production of feruloylated arabinoxylo-oligosaccharides from brans or other cereal grain processing sidestreams as functional food ingredients. There is a gap between the structural knowledge, bioactivity data, and enzymology insight. Our goal with this review is to present an overview of the structures and bioactivities of feruloylated arabinoxylo-oligosaccharides and review the enzyme reactions that catalyze specific changes in differentially substituted arabinoxylans.

The structural basis of fungal glucuronoyl esterase activity on natural substrates.

Structural and functional studies were conducted of the glucuronoyl esterase (GE) from Cerrena unicolor (CuGE), an enzyme catalyzing cleavage of lignin-carbohydrate ester bonds. CuGE is an α/ß-hydrolase belonging to carbohydrate esterase family 15 (CE15). The enzyme is modular, comprised of a catalytic and a carbohydrate-binding domain. SAXS data show CuGE as an elongated rigid molecule where the two domains are connected by a rigid linker. Detailed structural information of the catalytic domain in its apo- and inactivated form and complexes with aldouronic acids reveal well-defined binding of the 4-O-methyl-a-D-glucuronoyl moiety, not influenced by the nature of the attached xylo-oligosaccharide. Structural and sequence comparisons within CE15 enzymes reveal two distinct structural subgroups. CuGE belongs to the group of fungal CE15-B enzymes with an open and flat substrate-binding site. The interactions between CuGE and its natural substrates are explained and rationalized by the structural results, microscale thermophoresis and isothermal calorimetry.

Crystal structure and substrate interactions of an unusual fungal non-CBM carrying GH26 endo-ß-mannanase from Yunnania penicillata.

Endo-ß(1 → 4)-mannanases (endomannanases) catalyse degradation of ß-mannans, an abundant class of plant polysaccharides. This study investigates structural features and substrate binding of YpenMan26A, a non-CBM carrying endomannanase from Yunnania penicillata. Structural and sequence comparisons to other fungal family GH26 endomannanases showed high sequence similarities and conserved binding residues, indicating that fungal GH26 endomannanases accommodate galactopyranosyl units in the -3 and -2 subsites. Two striking amino acid differences in the active site were found when the YpenMan26A structure was compared to a homology model of Wsp.Man26A from Westerdykella sp. and the sequences of nine other fungal GH26 endomannanases. Two YpenMan26A mutants, W110H and D37T, inspired by differences observed in Wsp.Man26A, produced a shift in how mannopentaose bound across the active site cleft and a decreased affinity for galactose in the -2 subsite, respectively, compared to YpenMan26A. YpenMan26A was moreover found to have a flexible surface loop in the position where PansMan26A from Podospora anserina has an α-helix (α9) which interacts with its family 35 CBM. Sequence alignment inferred that the core structure of fungal GH26 endomannanases differ depending on the natural presence of this type of CBM. These new findings have implications for selecting and optimising these enzymes for galactomannandegradation.

Laccase activity measurement by FTIR spectral fingerprinting.

Laccases (EC 1.10.3.2) are enzymes known for their ability to catalyze the oxidation of phenolic compounds using molecular oxygen as the final electron acceptor. Laccase activity is commonly determined by monitoring spectrophotometric changes (absorbance) of the product or substrate during the enzymatic reaction. Fourier Transform Infrared Spectroscopy (FTIR) is a fast and versatile technique where spectral evolution profiling, i.e. assessment of the spectral changes of both substrate and products during enzymatic conversion in real time, can be used to assess enzymatic activity when combined with multivariate data analysis. We employed FTIR to monitor enzymatic oxidation of monolignols (sinapyl, coniferyl and p-coumaryl alcohol), sinapic acid, and sinapic aldehyde by four different laccases: three fungal laccases from Trametes versicolor, Trametes villosa and Ganoderma lucidum, respectively, and one bacterial laccase from Meiothermus ruber. By coupling the FTIR measurements with Parallel Factor Analysis (PARAFAC) we established a quantitative assay for assessing laccase activity. By combining PARAFAC modelling with Principal Component Analysis we show the usefulness of this technology as a multivariate tool able to compare and distinguish different laccase reaction patterns. We also demonstrate how the FTIR approach can be used to create a reference system for laccase activity comparison based on a relatively low number of measurements. Such a reference system has potential to function as a high-throughput method for comparing reaction pattern similarities and differences between laccases and hereby identify new and interesting enzyme candidates in large sampling pools.

Antifungal activity of well-defined chito-oligosaccharide preparations against medically relevant yeasts.

Due to their antifungal activity, chitosan and its derivatives have potential to be used for treating yeast infections in humans. However, to be considered for use in human medicine, it is necessary to control and know the chemical composition of the compound, which is not always the case for polymeric chitosans. Here, we analyze the antifungal activity of a soluble and well-defined chito-oligosaccharide (CHOS) with an average polymerization degree (DPn) of 32 and fraction of acetylation (FA) of 0.15 (C32) on 52 medically relevant yeast strains. Minimal inhibitory concentrations (MIC) varied widely among yeast species, strains and isolates (from > 5000 to < 9.77 µg mL-1) and inhibition patterns showed a time- and dose-dependencies. The antifungal activity was predominantly fungicidal and was inversely proportional to the pH, being maximal at pH 4.5, the lowest tested pH. Furthermore, antifungal effects of CHOS fractions with varying average molecular weight indicated that those fractions with an intermediate degree of polymerization, i.e. DP 31 and 54, had the strongest inhibitory effects. Confocal imaging showed that C32 adsorbs to the cell surface, with subsequent cell disruption and accumulation of C32 in the cytoplasm. Thus, C32 has potential to be used as a therapy for fungal infections.

Multiple Reaction Monitoring for quantitative laccase kinetics by LC-MS.

Laccases (EC 1.10.3.2) are enzymes known for their ability to catalyse the oxidation of phenolic compounds using molecular oxygen as the final electron acceptor. Lignin is a natural phenylpropanoids biopolymer whose degradation in nature is thought to be aided by enzymatic oxidation by laccases. Laccase activity is often measured spectrophotometrically on compounds such as syringaldazine and ABTS which poorly relate to lignin. We employed natural phenolic hydroxycinnamates having different degree of methoxylations, p-coumaric, ferulic and sinapic acid, and a lignin model OH-dilignol compound as substrates to assess enzyme kinetics by HPLC-MS on two fungal laccases Trametes versicolor laccase, Tv and Ganoderma lucidum laccase, Gl. The method allowed accurate kinetic measurements and detailed insight into the product profiles of both laccases. Both Tv and Gl laccase are active on the hydroxycinnammates and show a preference for substrate with methoxylations. Product profiles were dominated by the presence of dimeric and trimeric species already after 10 minutes of reaction and similar profiles were obtained with the two laccases. This new HPLC-MS method is highly suitable and accurate as a new method for assaying laccase activity on genuine phenolic substrates, as well as a tool for examining laccase oxidation product profiles.

Structure and function of a broad-specificity chitin deacetylase from Aspergillus nidulans FGSC A4.

Enzymatic conversion of chitin, a ß-1,4 linked polymer of N-acetylglucosamine, is of major interest in areas varying from the biorefining of chitin-rich waste streams to understanding how medically relevant fungi remodel their chitin-containing cell walls. Although numerous chitinolytic enzymes have been studied in detail, relatively little is known about enzymes capable of deacetylating chitin. We describe the structural and functional characterization of a 237 residue deacetylase (AnCDA) from Aspergillus nidulans FGSC A4. AnCDA acts on chito-oligomers, crystalline chitin, chitosan, and acetylxylan, but not on peptidoglycan. The K m and k cat of AnCDA for the first deacetylation of penta-N-acetyl-chitopentaose are 72 µM and 1.4 s-1, respectively. Combining mass spectrometry and analyses of acetate release, it was shown that AnCDA catalyses mono-deacetylation of (GlcNAc)2 and full deacetylation of (GlcNAc)3-6 in a non-processive manner. Deacetylation of the reducing end sugar was much slower than deacetylation of the other sugars in chito-oligomers. These enzymatic characteristics are discussed in the light of the crystal structure of AnCDA, providing insight into how the chitin deacetylase may interact with its substrates. Interestingly, AnCDA activity on crystalline chitin was enhanced by a lytic polysaccharide monooxygenase that increases substrate accessibility by oxidative cleavage of the chitin chains.

A New Functional Classification of Glucuronoyl Esterases by Peptide Pattern Recognition.

Glucuronoyl esterases are a novel type of enzymes believed to catalyze the hydrolysis of ester linkages between lignin and glucuronoxylan in lignocellulosic biomass, linkages known as lignin carbohydrate complexes. These complexes contribute to the recalcitrance of lignocellulose. Glucuronoyl esterases are a part of the microbial machinery for lignocellulose degradation and coupling their role to the occurrence of lignin carbohydrate complexes in biomass is a desired research goal. Glucuronoyl esterases have been assigned to CAZymes family 15 of carbohydrate esterases, but only few examples of characterized enzymes exist and the exact activity is still uncertain. Here peptide pattern recognition is used as a bioinformatic tool to identify and group new CE15 proteins that are likely to have glucuronoyl esterase activity. 1024 CE15-like sequences were drawn from GenBank and grouped into 24 groups. Phylogenetic analysis of these groups made it possible to pinpoint groups of putative fungal and bacterial glucuronoyl esterases and their sequence variation. Moreover, a number of groups included previously undescribed CE15-like sequences that are distinct from the glucuronoyl esterases and may possibly have different esterase activity. Hence, the CE15 family is likely to comprise other enzyme functions than glucuronoyl esterase alone. Gene annotation in a variety of fungal and bacterial microorganisms showed that coprophilic fungi are rich and diverse sources of CE15 proteins. Combined with the lifestyle and habitat of coprophilic fungi, they are predicted to be excellent candidates for finding new glucuronoyl esterase genes.

Display of a ß-mannanase and a chitosanase on the cell surface of Lactobacillus plantarum towards the development of whole-cell biocatalysts.

BACKGROUND: Lactobacillus plantarum is considered as a potential cell factory because of its GRAS (generally recognized as safe) status and long history of use in food applications. Its possible applications include in situ delivery of proteins to a host, based on its ability to persist at mucosal surfaces of the human intestine, and the production of food-related enzymes. By displaying different enzymes on the surface of L. plantarum cells these could be used as whole-cell biocatalysts for the production of oligosaccharides. In this present study, we aimed to express and display a mannanase and a chitosanase on the cell surface of L. plantarum. RESULTS: ManB, a mannanase from Bacillus licheniformis DSM13, and CsnA, a chitosanase from Bacillus subtilis ATCC 23857 were fused to different anchoring motifs of L. plantarum for covalent attachment to the cell surface, either via an N-terminal lipoprotein anchor (Lp_1261) or a C-terminal cell wall anchor (Lp_2578), and the resulting fusion proteins were expressed in L. plantarum WCFS1. The localization of the recombinant proteins on the bacterial cell surface was confirmed by flow cytometry and immunofluorescence microscopy. The highest mannanase and chitosanase activities obtained for displaying L. plantarum cells were 890 U and 1360 U g dry cell weight, respectively. In reactions with chitosan and galactomannans, L. plantarum CsnA- and ManB-displaying cells produced chito- and manno-oligosaccharides, respectively, as analyzed by high performance anion exchange chromatography (HPAEC) and mass spectrometry (MS). Surface-displayed ManB is able to break down galactomannan (LBG) into smaller manno-oligosaccharides, which can support growth of L. plantarum. CONCLUSION: This study shows that mannanolytic and chitinolytic enzymes can be anchored to the cell surface of L. plantarum in active forms. L. plantarum chitosanase- and mannanase-displaying cells should be of interest for the production of potentially 'prebiotic' oligosaccharides. This approach, with the enzyme of interest being displayed on the cell surface of a food-grade organism, may also be applied in production processes relevant for food industry.

Bacterial translocation and in vivo assessment of intestinal barrier permeability in rainbow trout (Oncorhynchus mykiss) with and without soyabean meal-induced inflammation.

The primary aim of this experiment was to evaluate the intestinal barrier permeability in vivo in rainbow trout (Oncorhynchus mykiss) fed increasing levels of soyabean meal (SBM). The relationship between SBM-induced enteritis (SBMIE) and the permeability markers was also investigated. Our results showed that the mean score of morphological parameters was significantly higher as a result of 37·5 % SBM inclusion in the diet, while the scores of fish fed 25 % SBM or lower were not different from those of the fish meal-fed controls (P < 0·05). SBMIE was found in the distal intestine (DI) in 18 % of the fish (eleven of sixty): ten in the 37·5 % SBM-fed group and one in the 25 % SBM-fed group. Sugar markers in plasma showed large variation among individuals probably due to variation in feed intake. We found, however, a significant linear increase in the level of plasma d-lactate with increasing SBM inclusion level (P < 0·0001). Plasma concentration of endotoxin was not significantly different in groups with or without SBMIE. Some individual fish showed high values of endotoxin in blood, but the same individuals did not show any bacterial translocation. Plasma bacterial DNA was detected in 28 % of the fish with SBMIE, and 8 % of non-SBMIE fish (P = 0·07). Plasma concentration of d-lactate was significantly higher in fish with SBMIE (P < 0·0001). To conclude, SBMIE in the DI of rainbow trout was associated with an increase in bacterial translocation and plasma d-lactate concentration, suggesting that these permeability markers can be used to evaluate intestinal permeability in vivo.

A novel analytical method for d-glucosamine quantification and its application in the analysis of chitosan degradation by a minimal enzyme cocktail.

Enzymatic depolymerization of chitosan, a ß-(1,4)-linked polycationic polysaccharide composed of d-glucosamine (GlcN) and N-acetyl-d-glucosamine (GlcNAc) provides a possible route to the exploitation of chitin-rich biomass. Complete conversion of chitosan to mono-sugars requires the synergistic action of endo- and exo- chitosanases. In the present study we have developed an efficient and cost-effective chitosan-degrading enzyme cocktail containing only two enzymes, an endo-attacking bacterial chitosanase, ScCsn46A, from Streptomyces coelicolor, and an exo-attacking glucosamine specific ß-glucosaminidase, Tk-Glm, from the archaeon Thermococcus kodakarensis KOD1. Moreover, we developed a fast, reliable quantitative method for analysis of GlcN using high performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD). The sensitivity of this method is high and less than 50 pmol was easily detected, which is about 1000-fold better than the sensitivity of more commonly used detection methods based on refractive index. We also obtained qualitative insight into product development during the enzymatic degradation reaction by means of ElectroSpray Ionization-Mass Spectrometry (ESI-MS).

Comparison of fungal carbohydrate esterases of family CE16 on artificial and natural substrates.

The enzymatic conversion of acetylated hardwood glucuronoxylan to functional food oligomers, biochemicals or fermentable monomers requires besides glycoside hydrolases enzymes liberating acetic acid esterifying position 2 and/or 3 in xylopyranosyl (Xylp) residues. The 3-O-acetyl group at internal Xylp residues substituted by MeGlcA is the only acetyl group of hardwood acetylglucuronoxylan and its fragments not attacked by acetylxylan esterases of carbohydrate esterase (CE) families 1, 4, 5 and 6 and by hemicellulolytic acetyl esterases classified in CE family 16. Monoacetylated aldotetraouronic acid 3″-Ac(3)MeGlcA(3)Xyl3, generated from the polysaccharide by GH10 endoxylanases, appears to be one of the most resistant fragments. The presence of the two substituents on the non-reducing-end Xylp residue prevents liberation of MeGlcA by α-glucuronidase of family GH67 and blocks the action of acetylxylan esterases. The Ac(3)MeGlcA(3)Xyl3 was isolated from an enzymatic hydrolysate of birchwood acetylglucuronoxylan and characterized by (1)H NMR spectroscopy as a mixture of two positional isomers, 3″-Ac(3)MeGlcA(3)Xyl3 and 4″-Ac(3)MeGlcA(3)Xyl3, the latter being the result of acetyl group migration. The mixture was used as a substrate for three members of CE16 family of fungal origin. Trichoderma reesei CE16 esterase, inactive on polymeric substrate, deacetylated both isomers. Podospora anserina and Aspergillus niger esterases, active on acetylglucuronoxylan, deesterified effectively only the 4″-isomer. The results indicate catalytic diversity among CE16 enzymes, but also their common and unifying catalytic ability to exo-deacetylate positions 3 and 4 on non-reducing-end Xylp residues, which is an important step in plant hemicellulose saccharification.

Steam explosion pretreatment for enhancing biogas production of late harvested hay.

Grasslands are often abandoned due to lack of profitability. Extensively cultivating grassland for utilization in a biogas-based biorefinery concept could mend this problem. Efficient bioconversion of this lignocellulosic biomass requires a pretreatment step. In this study the effect of different steam explosion conditions on hay digestibility have been investigated. Increasing severity in the pretreatment induced degradation of the hemicellulose, which at the same time led to the production of inhibitors and formation of pseudo-lignin. Enzymatic hydrolysis showed that the maximum glucose yields were obtained under pretreatment at 220 °C for 15 min, while higher xylose yields were obtained at 175 °C for 10 min. Pretreatment of hay by steam explosion enhanced 15.9% the methane yield in comparison to the untreated hay. Results indicate that hay can be effectively converted to methane after steam explosion pretreatment.

Discovery of LPMO activity on hemicelluloses shows the importance of oxidative processes in plant cell wall degradation.

The recently discovered lytic polysaccharide monooxygenases (LPMOs) are known to carry out oxidative cleavage of glycoside bonds in chitin and cellulose, thus boosting the activity of well-known hydrolytic depolymerizing enzymes. Because biomass-degrading microorganisms tend to produce a plethora of LPMOs, and considering the complexity and copolymeric nature of the plant cell wall, it has been speculated that some LPMOs may act on other substrates, in particular the hemicelluloses that tether to cellulose microfibrils. We demonstrate that an LPMO from Neurospora crassa, NcLPMO9C, indeed degrades various hemicelluloses, in particular xyloglucan. This activity was discovered using a glycan microarray-based screening method for detection of substrate specificities of carbohydrate-active enzymes, and further explored using defined oligomeric hemicelluloses, isolated polymeric hemicelluloses and cell walls. Products generated by NcLPMO9C were analyzed using high performance anion exchange chromatography and multidimensional mass spectrometry. We show that NcLPMO9C generates oxidized products from a variety of substrates and that its product profile differs from those of hydrolytic enzymes acting on the same substrates. The enzyme particularly acts on the glucose backbone of xyloglucan, accepting various substitutions (xylose, galactose) in almost all positions. Because the attachment of xyloglucan to cellulose hampers depolymerization of the latter, it is possible that the beneficial effect of the LPMOs that are present in current commercial cellulase mixtures in part is due to hitherto undetected LPMO activities on recalcitrant hemicellulose structures.

A C4-oxidizing lytic polysaccharide monooxygenase cleaving both cellulose and cello-oligosaccharides.

Lignocellulosic biomass is a renewable resource that significantly can substitute fossil resources for the production of fuels, chemicals, and materials. Efficient saccharification of this biomass to fermentable sugars will be a key technology in future biorefineries. Traditionally, saccharification was thought to be accomplished by mixtures of hydrolytic enzymes. However, recently it has been shown that lytic polysaccharide monooxygenases (LPMOs) contribute to this process by catalyzing oxidative cleavage of insoluble polysaccharides utilizing a mechanism involving molecular oxygen and an electron donor. These enzymes thus represent novel tools for the saccharification of plant biomass. Most characterized LPMOs, including all reported bacterial LPMOs, form aldonic acids, i.e., products oxidized in the C1 position of the terminal sugar. Oxidation at other positions has been observed, and there has been some debate concerning the nature of this position (C4 or C6). In this study, we have characterized an LPMO from Neurospora crassa (NcLPMO9C; also known as NCU02916 and NcGH61-3). Remarkably, and in contrast to all previously characterized LPMOs, which are active only on polysaccharides, NcLPMO9C is able to cleave soluble cello-oligosaccharides as short as a tetramer, a property that allowed detailed product analysis. Using mass spectrometry and NMR, we show that the cello-oligosaccharide products released by this enzyme contain a C4 gemdiol/keto group at the nonreducing end.