Enzymatic cellulose oxidation is linked to lignin by long-range electron transfer

Active roles of lytic polysaccharide monooxygenases in human pathogenicity

Lytic polysaccharide monooxygenases (LPMOs) are redox enzymes widely studied for their involvement in microbial and fungal biomass degradation. The catalytic versatility of these enzymes is demonstrated by the recent discovery of LPMOs in arthropods, viruses, insects and ferns, where they fulfill diverse functions beyond biomass conversion. This mini-review puts a spotlight on a recently recognized aspect of LPMOs: their role in infectious processes in human pathogens. It discusses the occurrence and potential biological mechanisms of LPMOs associated with human pathogens and provides an outlook on future avenues in this emerging and exciting research field.

Synthesis and Characterization of Lignin-Silver Nanoparticles

Metal nanoparticle synthesis via environmentally friendly methods is gaining interest for their potential advantages over conventional physico-chemical approaches. Herein, we propose a robust green synthesis route for lignin-modified silver nanoparticles, utilizing the recovery of lignin as a renewable raw material and exploring its application in valuable areas. Through a systematic approach combining UV-Vis spectroscopy with AAS and DLS, we identified repeatable and scalable reaction conditions in an aqueous solution at pH 11 for homogeneous silver nanoparticles with high uniformity. The TEM median sizes ranged from 12 to 15 nm with circularity between 0.985 and 0.993. The silver nanoparticles yield exceeded 0.010 mol L-1, comparable with traditional physico-chemical methods, with a minimal loss of silver precursor ranging between 0.5 and 3.9%. Characterization by XRD and XPS revealed the presence of Ag-O bonding involving lignin functional groups on the pure face-centered cubic structure of metallic silver. Moreover, the lignin-modified silver nanoparticles generated a localized thermal effect upon near-infrared laser irradiation (808 nm), potentially allowing for targeted applications in the biomedical field. Our study showcases the potential of lignin as a renewable reducing and capping agent for silver nanoparticle synthesis, addressing some shortcomings of green synthesis approaches and contributing to the development of suitable nanomaterials.

The thin line between monooxygenases and peroxygenases. P450s, UPOs, MMOs, and LPMOs: A brick to bridge fields of expertise

Many scientific fields, although driven by similar purposes and dealing with similar technologies, often appear so isolated and far from each other that even the vocabularies to describe the very same phenomenon might differ. Concerning the vast field of biocatalysis, a special role is played by those redox enzymes that employ oxygen-based chemistry to unlock transformations otherwise possible only with metal-based catalysts. As such, greener chemical synthesis methods and environmentally-driven biotechnological approaches were enabled over the last decades by the use of several enzymes and ultimately resulted in the first industrial applications. Among what can be called today the environmental biorefinery sector, biomass transformation, greenhouse gas reduction, bio-gas/fuels production, bioremediation, as well as bulk or fine chemicals and even pharmaceuticals manufacturing are all examples of fields in which successful prototypes have been demonstrated employing redox enzymes. In this review we decided to focus on the most prominent enzymes (MMOs, LPMO, P450 and UPO) capable of overcoming the ∼100 kcal mol-1 barrier of inactivated CH bonds for the oxyfunctionalization of organic compounds. Harnessing the enormous potential that lies within these enzymes is of extreme value to develop sustainable industrial schemes and it is still deeply coveted by many within the aforementioned fields of application. Hence, the ambitious scope of this account is to bridge the current cutting-edge knowledge gathered upon each enzyme. By creating a broad comparison, scientists belonging to the different fields may find inspiration and might overcome obstacles already solved by the others. This work is organised in three major parts: a first section will be serving as an introduction to each one of the enzymes regarding their structural and activity diversity, whereas a second one will be encompassing the mechanistic aspects of their catalysis. In this regard, the machineries that lead to analogous catalytic outcomes are depicted, highlighting the major differences and similarities. Finally, a third section will be focusing on the elements that allow the oxyfunctionalization chemistry to occur by delivering redox equivalents to the enzyme by the action of diverse redox partners. Redox partners are often overlooked in comparison to the catalytic counterparts, yet they represent fundamental elements to better understand and further develop practical applications based on mono- and peroxygenases.

Effects of Lignin-Diverted Reductant with Polyphenol Oxidases on Cellulose Degradation by Wild and Mutant Types of Lytic Polysaccharide Monooxygenase

Establishing a multi-enzyme synergistic lignocellulosic biodegradation system using lytic polysaccharide monooxygenase (LPMO) and polyphenol oxidases is vital for efficiently utilizing plant biomass waste, ultimately benefiting the carbon cycle and promoting environmental protection. Single-residue mutations of LPMO can improve the efficiency of lignocellulosic biomass degradation. However, the activity of mutant-type LPMO in relation to lignin-diverted reducing agents has not been sufficiently explored. In this study, laccase and tyrosinase were initially investigated and their optimal conditions and impressive thermal stability were revealed, indicating their potential synergistic abilities with LPMO in lignocellulose biodegradation. When utilizing gallic acid as a reducing agent, the activities of LPMOs were increased by over 10%, which was particularly evident in mutant-type LPMOs after the addition of polyphenol oxidases. In particular, the combination of tyrosinase with either 4-hydroxy-3-methoxyphenylacetone or p-coumaric acid was shown to enhance the efficacy of LPMOs. Furthermore, the highest activity levels of wild-type LPMOs were observed with the addition of laccase and 3-methylcatechol. The similarities between wild and mutant LPMOs regarding their activities in lignin-diverted phenolic compounds and reducing agents are almost identical, suggesting that the single-residue mutation of LPMO does not have a detrimental effect on its performance. Above all, this study indicates that understanding the performance of both wild and mutant types of LPMOs in the presence of polyphenol oxidases and various reducing agents constitutes a key link in the industrialization of the multi-enzyme degradation of lignocellulose.

Assessing the role of redox partners in TthLPMO9G and its mutants: focus on H2O2 production and interaction with cellulose

BACKGROUND

The field of enzymology has been profoundly transformed by the discovery of lytic polysaccharide monooxygenases (LPMOs). LPMOs hold a unique role in the natural breakdown of recalcitrant polymers like cellulose and chitin. They are characterized by a "histidine brace" in their active site, known to operate via an O2/H2O2 mechanism and require an electron source for catalytic activity. Although significant research has been conducted in the field, the relationship between these enzymes, their electron donors, and H2O2 production remains complex and multifaceted.

RESULTS

This study examines TthLPMO9G activity, focusing on its interactions with various electron donors, H2O2, and cellulose substrate interactions. Moreover, the introduction of catalase effectively eliminates H2O2 interference, enabling an accurate evaluation of each donor's efficacy based on electron delivery to the LPMO active site. The introduction of catalase enhances TthLPMO9G's catalytic efficiency, leading to increased cellulose oxidation. The current study provides deeper insights into specific point mutations, illuminating the crucial role of the second coordination sphere histidine at position 140. Significantly, the H140A mutation not only impacted the enzyme's ability to oxidize cellulose, but also altered its interaction with H2O2. This change was manifested in the observed decrease in both oxidase and peroxidase activities. Furthermore, the S28A substitution, selected for potential engagement within the His1-electron donor-cellulose interaction triad, displayed electron donor-dependent alterations in cellulose product patterns.

CONCLUSION

The interaction of an LPMO with H2O2, electron donors, and cellulose substrate, alongside the impact of catalase, offers deep insights into the intricate interactions occurring at the molecular level within the enzyme. Through rational alterations and substitutions that affect both the first and second coordination spheres of the active site, this study illuminates the enzyme's function. These insights enhance our understanding of the enzyme's mechanisms, providing valuable guidance for future research and potential applications in enzymology and biochemistry.

Enhancement of rice husks saccharification through hydrolase preparation assisted by lytic polysaccharide monooxygenase

Rice husk is an abundant agricultural waste generated from rice production, but its application is limited. Considering its complex components, the rice husk was hydrolyzed by different enzymes to enhance its saccharification. In this study, saccharification of the rice husk by cellulase, xylosidase, and xylanase was first investigated. The synergistic effect of LPMO on the above hydrolases and different enzyme combinations in the saccharification process was then explored. Thereafter, the formulation of the enzyme cocktail and the degradation conditions were optimized to obtain the highest saccharification efficiency. The results showed that the optimum enzyme cocktail consists of Celluclast 1.5 L (83.3 mg/g substrate), the key enzymes in the saccharification process, worked with BpXyl (20 mg/g substrate), BpXyn11 (24 mg/g substrate), and R17L/N25G (4 mg/g substrate). The highest reducing sugar concentration (1.19 mg/mL) was obtained at pH 6.0 and 60 ℃ for 24 h. Fourier transform infrared spectroscopy and scanning electron microscopy were employed to characterize the structural changes in the rice husk after degradation. The results showed that the key chemical bonds in cellulose and hemicellulose were broken. This study illuminated the concept of saccharifying lignocellulose from rice husk using LPMO synergistically assisted combined-hydrolase including cellulase, xylosidase, and xylanase, and provided a theoretical basis for lignocellulose biodegradation.

Lytic polysaccharide monooxygenase synergized with lignin-degrading enzymes for efficient lignin degradation

Even though the discovery of lytic polysaccharide monooxygenases (LPMOs) has fundamentally shifted our understanding of biomass degradation, most of the current studies focused on their roles in carbohydrate oxidation. However, no study demonstrated if LPMO could directly participate to the process of lignin degradation in lignin-degrading microbes. This study showed that LPMO could synergize with lignin-degrading enzymes for efficient lignin degradation in white-rot fungi. The transcriptomics analysis of fungi Irpex lacteus and Dichomitus squalens during their lignocellulosic biomass degradation processes surprisingly highlighted that LPMOs co-regulated with lignin-degrading enzymes, indicating their more versatile roles in the redox network. Biochemical analysis further confirmed that the purified LPMO from I. lacteus CD2 could use diverse electron donors to produce H2O2, drive Fenton reaction, and synergize with manganese peroxidase for lignin oxidation. The results thus indicated that LPMO might uniquely leverage the redox network toward dynamic and efficient degradation of different cell wall components.

Functional characterization of a lytic polysaccharide monooxygenase from Schizophyllum commune that degrades non-crystalline substrates

Lytic polysaccharide monooxygenases (LPMOs) are mono-copper enzymes that use O2 or H2O2 to oxidatively cleave glycosidic bonds. LPMOs are prevalent in nature, and the functional variation among these enzymes is a topic of great interest. We present the functional characterization of one of the 22 putative AA9-type LPMOs from the fungus Schizophyllum commune, ScLPMO9A. The enzyme, expressed in Escherichia coli, showed C4-oxidative cleavage of amorphous cellulose and soluble cello-oligosaccharides. Activity on xyloglucan, mixed-linkage β-glucan, and glucomannan was also observed, and product profiles differed compared to the well-studied C4-oxidizing NcLPMO9C from Neurospora crassa. While NcLPMO9C is also active on more crystalline forms of cellulose, ScLPMO9A is not. Differences between the two enzymes were also revealed by nuclear magnetic resonance (NMR) titration studies showing that, in contrast to NcLPMO9C, ScLPMO9A has higher affinity for linear substrates compared to branched substrates. Studies of H2O2-fueled degradation of amorphous cellulose showed that ScLPMO9A catalyzes a fast and specific peroxygenase reaction that is at least two orders of magnitude faster than the apparent monooxygenase reaction. Together, these results show that ScLPMO9A is an efficient LPMO with a broad substrate range, which, rather than acting on cellulose, has evolved to act on amorphous and soluble glucans.

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.

Revisiting the AA14 family of lytic polysaccharide monooxygenases and their catalytic activity

Lytic polysaccharide monooxygenases (LPMOs) belonging to the AA14 family are believed to contribute to the enzymatic degradation of lignocellulosic biomass by specifically acting on xylan in recalcitrant cellulose-xylan complexes. Functional characterization of an AA14 LPMO from Trichoderma reesei, TrAA14A, and a re-evaluation of the properties of the previously described AA14 from Pycnoporus coccineus, PcoAA14A, showed that these proteins have oxidase and peroxidase activities that are common for LPMOs. However, we were not able to detect activity on cellulose-associated xylan or any other tested polysaccharide substrate, meaning that the substrate of these enzymes remains unknown. Next to raising questions regarding the true nature of AA14 LPMOs, the present data illustrate possible pitfalls in the functional characterization of these intriguing enzymes.

Production of recombinant lytic polysaccharide monooxygenases and evaluation effect of its addition into Aspergillus fumigatus var. niveus cocktail for sugarcane bagasse saccharification

Lignocellulosic biomass is a promising alternative for producing biofuels, despite its recalcitrant nature. There are microorganisms in nature capable of efficiently degrade biomass, such as the filamentous fungi. Among them, Aspergillus fumigatus var. niveus (AFUMN) has a wide variety of carbohydrate-active enzymes (CAZymes), especially hydrolases, but a low number of oxidative enzymes in its genome. To confirm the enzymatic profile of this fungus, this study analyzed the secretome of AFUMN cultured in sugarcane bagasse as the sole carbon source. As expected, the secretome showed a predominance of hydrolytic enzymes compared to oxidative activity. However, it is known that hydrolytic enzymes act in synergy with oxidative proteins to efficiently degrade cellulose polymer, such as the Lytic Polysaccharide Monooxygenases (LPMOs). Thus, three LPMOs from the fungus Thermothelomyces thermophilus (TtLPMO9D, TtLPMO9H, and TtLPMO9O) were selected, heterologous expressed in Aspergillus nidulans, purified, and used to supplement the AFUMN secretome to evaluate their effect on the saccharification of sugarcane bagasse. The saccharification assay was carried out using different concentrations of AFUMN secretome supplemented with recombinant T. thermophilus LPMOs, as well as ascorbic acid as reducing agent for oxidative enzymes. Through a statistic design created by Design-Expert software, we were able to analyze a possible cooperative effect between these components. The results indicated that, in general, the addition of TtLPMO9D and ascorbic acid did not favor the conversion process in this study, while TtLPMO9O had a highly significant cooperative effect in bagasse saccharification compared to the control using only AFUMN secretome.

Efficient Secretory Production of Lytic Polysaccharide Monooxygenase BaLPMO10 and Its Application in Plant Biomass Conversion

Lytic polysaccharide monooxygenases (LPMOs) can oxidatively break the glycosidic bonds of crystalline cellulose, providing more actionable sites for cellulase to facilitate the conversion of cellulose to cello-oligosaccharides, cellobiose and glucose. In this work, a bioinformatics analysis of BaLPMO10 revealed that it is a hydrophobic, stable and secreted protein. By optimizing the fermentation conditions, the highest protein secretion level was found at a IPTG concentration of 0.5 mM and 20 h of fermentation at 37 °C, with a yield of 20 mg/L and purity > 95%. The effect of metal ions on the enzyme activity of BaLPMO10 was measured, and it was found that 10 mM Ca²⁺ and Na⁺ increased the enzyme activity by 47.8% and 98.0%, respectively. However, DTT, EDTA and five organic reagents inhibited the enzyme activity of BaLPMO10. Finally, BaLPMO10 was applied in biomass conversion. The degradation of corn stover pretreated with different steam explosions was performed. BaLPMO10 and cellulase had the best synergistic degradation effect on corn stover pretreated at 200 °C for 12 min, improving reducing sugars by 9.2% compared to cellulase alone. BaLPMO10 was found to be the most efficient for ethylenediamine-pretreated Caragana korshinskii by degrading three different biomasses, increasing the content of reducing sugars by 40.5% compared to cellulase alone following co-degradation with cellulase for 48 h. The results of scanning electron microscopy revealed that BaLPMO10 disrupted the structure of Caragana korshinskii, making its surface coarse and poriferous, which increased the accessibility of other enzymes and thus promoted the process of conversion. These findings provide guidance for improving the efficiency of enzymatic digestion of lignocellulosic biomass.

Enzymatic hydrolysis cocktail optimization for the intensification of sugar extraction from sugarcane bagasse

Lignocellulosic biomasses have a very important role as a raw material to produce biofuels and biochemicals. However, a sustainable, efficient, and economically competitive process for the release of sugars from such materials has still not been achieved. In this work, the optimization of the enzymatic hydrolysis cocktail was evaluated as an approach to maximize sugar extraction from mildly pretreated sugarcane bagasse. Different additives and enzymes, including hydrogen peroxide (H₂O₂), laccase, hemicellulase and the surfactants Tween 80 and PEG4000 were added to a cellulolytic cocktail with the aim of improving biomass hydrolysis. An increase of 39 % and 46 % of glucose and xylose concentrations, respectively, compared to the control (when only the cellulolytic cocktail (20 or 35 FPU g^-1 dry mass), was obtained when H₂O₂ (0.24 mM) was added at the beginning of the hydrolysis. On the other hand, the addition of hemicellulase (81-162 μL g^-1 DM) increased the production of glucose up to 38 % and xylose up to 50 %. The findings of this study reveal that it is possible to increase the extraction of sugars from mildly pretreated lignocellulosic biomass by using an appropriate enzymatic cocktail supplemented with additives. This opens up new opportunities for the development of a more sustainable, efficient, and economically competitive process for biomass fractionation.

Investigating the role of AA9 LPMOs in enzymatic hydrolysis of differentially steam-pretreated spruce

BACKGROUND

To realize the full potential of softwood-based forest biorefineries, the bottlenecks of enzymatic saccharification of softwood need to be better understood. Here, we investigated the potential of lytic polysaccharide monooxygenases (LPMO9s) in softwood saccharification. Norway spruce was steam-pretreated at three different severities, leading to varying hemicellulose retention, lignin condensation, and cellulose ultrastructure. Hydrolyzability of the three substrates was assessed after pretreatment and after an additional knife-milling step, comparing the efficiency of cellulolytic Celluclast + Novozym 188 and LPMO-containing Cellic CTec2 cocktails. The role of Thermoascus aurantiacus TaLPMO9 in saccharification was assessed through time-course analysis of sugar release and accumulation of oxidized sugars, as well as wide-angle X-ray scattering analysis of cellulose ultrastructural changes.

RESULTS

Glucose yield was 6% (w/w) with the mildest pretreatment (steam pretreatment at 210 °C without catalyst) and 66% (w/w) with the harshest (steam pretreatment at 210 °C with 3%(w/w) SO₂) when using Celluclast + Novozym 188. Surprisingly, the yield was lower with all substrates when Cellic CTec2 was used. Therefore, the conditions for optimal LPMO activity were tested and it was found that enough O₂ was present over the headspace and that the reducing power of the lignin of all three substrates was sufficient for the LPMOs in Cellic CTec2 to be active. Supplementation of Celluclast + Novozym 188 with TaLPMO9 increased the conversion of glucan by 1.6-fold and xylan by 1.5-fold, which was evident primarily in the later stages of saccharification (24-72 h). Improved glucan conversion could be explained by drastically reduced cellulose crystallinity of spruce substrates upon TaLPMO9 supplementation.

CONCLUSION

Our study demonstrated that LPMO addition to hydrolytic enzymes improves the release of glucose and xylose from steam-pretreated softwood substrates. Furthermore, softwood lignin provides enough reducing power for LPMOs, irrespective of pretreatment severity. These results provided new insights into the potential role of LPMOs in saccharification of industrially relevant softwood substrates.

Visible light-exposed lignin facilitates cellulose solubilization by lytic polysaccharide monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) catalyze oxidative cleavage of crystalline polysaccharides such as cellulose and are crucial for the conversion of plant biomass in Nature and in industrial applications. Sunlight promotes microbial conversion of plant litter; this effect has been attributed to photochemical degradation of lignin, a major redox-active component of secondary plant cell walls that limits enzyme access to the cell wall carbohydrates. Here, we show that exposing lignin to visible light facilitates cellulose solubilization by promoting formation of H2O2 that fuels LPMO catalysis. Light-driven H2O2 formation is accompanied by oxidation of ring-conjugated olefins in the lignin, while LPMO-catalyzed oxidation of phenolic hydroxyls leads to the required priming reduction of the enzyme. The discovery that light-driven abiotic reactions in Nature can fuel H2O2-dependent redox enzymes involved in deconstructing lignocellulose may offer opportunities for bioprocessing and provides an enzymatic explanation for the known effect of visible light on biomass conversion.

Closing the Nutrient Loop-The New Approaches to Recovering Biomass Minerals during the Biorefinery Processes

The recovery of plant mineral nutrients from the bio-based value chains is essential for a sustainable, circular bioeconomy, wherein resources are (re)used sustainably. The widest used approach is to recover plant nutrients on the last stage of biomass utilization processes-e.g., from ash, wastewater, or anaerobic digestate. The best approach is to recover mineral nutrients from the initial stages of biomass biorefinery, especially during biomass pre-treatments. Our paper aims to evaluate the nutrient recovery solutions from a trans-sectorial perspective, including biomass processing and the agricultural use of recovered nutrients. Several solutions integrated with the biomass pre-treatment stage, such as leaching/bioleaching, recovery from pre-treatment neoteric solvents, ionic liquids (ILs), and deep eutectic solvents (DESs) or integrated with hydrothermal treatments are discussed. Reducing mineral contents on silicon, phosphorus, and nitrogen biomass before the core biorefinery processes improves processability and yield and reduces corrosion and fouling effects. The recovered minerals are used as bio-based fertilizers or as silica-based plant biostimulants, with economic and environmental benefits.

Analyzing Activities of Lytic Polysaccharide Monooxygenases by Liquid Chromatography and Mass Spectrometry

Lytic polysaccharide monooxygenases perform oxidative cleavage of glycosidic bonds in various polysaccharides. The majority of LMPOs studied so far possess activity on either cellulose or chitin and analysis of these activities is therefore the main focus of this review. Notably, however, the number of LPMOs that are active on other polysaccharides is increasing. The products generated by LPMOs from cellulose are either oxidized in the downstream end (at C1) or upstream end (at C4), or at both ends. These modifications only result in small structural changes, which makes both chromatographic separation and product identification by mass spectrometry challenging. The changes in physicochemical properties that are associated with oxidation need to be considered when choosing analytical approaches. C1 oxidation leads to a sugar that is no longer reducing but instead has an acidic functionality, whereas C4 oxidation leads to products that are inherently labile at high and low pH and that exist in a keto-gemdiol equilibrium that is strongly shifted towards the gemdiol in aqueous solutions. Partial degradation of C4-oxidized products leads to the formation of native products, which could explain why some authors claim to have observed glycoside hydrolase activity for LPMOs. Notably, apparent glycoside hydrolase activity may also be due to small amounts of contaminating glycoside hydrolases since these normally have much higher catalytic rates than LPMOs. The low catalytic turnover rates of LPMOs necessitate the use of sensitive product detection methods, which limits the analytical possibilities considerably. Modern liquid chromatography and mass spectrometry have become essential tools for evaluating LPMO activity and this chapter provides an overview of available methods together with a few novel tools. The methods described constitute a suite of techniques for analyzing oxidized carbohydrate products, which can be applied to LPMOs as well as other carbohydrate-active redox enzymes.

Lytic polysaccharide monooxygenase increases cellobiohydrolases activity by promoting decrystallization of cellulose surface

Efficient depolymerization of crystalline cellulose requires cooperation between multiple cellulolytic enzymes. Through biochemical approaches, molecular dynamics (MD) simulation, and single-molecule observations using high-speed atomic force microscopy (HS-AFM), we quantify and track synergistic activity for cellobiohydrolases (CBHs) with a lytic polysaccharide monooxygenase (LPMO) from Phanerochaete chrysosporium. Increasing concentrations of LPMO (AA9D) increased the activity of a glycoside hydrolase family 6 CBH, Cel6A, whereas the activity of a family 7 CBH (Cel7D) was enhanced only at lower concentrations of AA9D. MD simulation suggests that the result of AA9D action to produce chain breaks in crystalline cellulose can oxidatively disturb the crystalline surface by disrupting hydrogen bonds. HS-AFM observations showed that AA9D increased the number of Cel7D molecules moving on the substrate surface and increased the processivity of Cel7D, thereby increasing the depolymerization performance, suggesting that AA9D not only generates chain ends but also amorphizes the crystalline surface, thereby increasing the activity of CBHs.

Comparison of solid and liquid fractions of pretreated Norway spruce as reductants in LPMO-supported saccharification of cellulose

The role of lignin in enzymatic saccharification of cellulose involving lytic polysaccharide monooxygenase (LPMO) was investigated in experiments with the solid and liquid fractions of pretreated Norway spruce from a biorefinery demonstration plant using hydrothermal pretreatment and impregnation with sulfur dioxide. Pretreated biomass before and after enzymatic saccharification was characterized using HPAEC, HPLC, Py-GC/MS, 2D-HSQC NMR, FTIR, and SEM. Chemical characterization indicated that relatively harsh pretreatment conditions resulted in that the solid phase contained no or very little hemicellulose but considerable amounts of pseudo-lignin, and that the liquid phase contained a relatively high concentration (∼5 g/L) of lignin-derived phenolics. As judged from reactions continuously supplied with either air or nitrogen gas, lignin and lignin fragments from both the solid and the liquid phases efficiently served as reductants in LPMO-supported saccharification. When air was used to promote LPMO activity, the enzymatic conversion of cellulose after 72 h was 25% higher in reactions with pretreated solids and buffer, and 14% higher in reactions with pretreatment liquid and microcrystalline cellulose. Research in this area is useful for designing efficient saccharification steps in biochemical conversion of lignocellulosic biomass.

Transcriptome Analysis of Podoscypha petalodes Strain GGF6 Reveals the Diversity of Proteins Involved in Lignocellulose Degradation and Ligninolytic Function

The present study reports transcriptomic profiling of a Basidiomycota fungus, Podoscypha petalodes strain GGF6 belonging to the family Podoscyphaceae, isolated from the North-Western Himalayan ranges in Himachal Pradesh, India. Podoscypha petalodes strain GGF6 possesses significant biotechnological potential as it has been reported for endocellulase, laccase, and other lignocellulolytic enzymes under submerged fermentation conditions. The present study attempts to enhance our knowledge of its lignocellulolytic potential as no previous omics-based analysis is available for this white-rot fungus. The transcriptomic analysis of P. petalodes GGF6 reveals the presence of 280 CAZy proteins. Furthermore, bioprospecting transcriptome signatures in the fungi revealed a diverse array of proteins associated with cellulose, hemicellulose, pectin, and lignin degradation. Interestingly, two copper-dependent lytic polysaccharide monooxygenases (AA14) and one pyrroloquinolinequinone-dependent oxidoreductase (AA12) were also identified, which are known to help in the lignocellulosic plant biomass degradation. Overall, this transcriptome profiling-based study provides deeper molecular-level insights into this Basidiomycota fungi, P. petalodes, for its potential application in diverse biotechnological applications, not only in the biofuel industry but also in the environmental biodegradation of recalcitrant molecules.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12088-022-01037-6.

Capture of activated dioxygen intermediates at the copper-active site of a lytic polysaccharide monooxygenase

Metalloproteins perform a diverse array of redox-related reactions facilitated by the increased chemical functionality afforded by their metallocofactors. Lytic polysaccharide monooxygenases (LPMOs) are a class of copper-dependent enzymes that are responsible for the breakdown of recalcitrant polysaccharides via oxidative cleavage at the glycosidic bond. The activated copper-oxygen intermediates and their mechanism of formation remains to be established. Neutron protein crystallography which permits direct visualization of protonation states was used to investigate the initial steps of oxygen activation directly following active site copper reduction in Neurospora crassa LPMO9D. Herein, we cryo-trap an activated dioxygen intermediate in a mixture of superoxo and hydroperoxo states, and we identify the conserved second coordination shell residue His157 as the proton donor. Density functional theory calculations indicate that both superoxo and hydroperoxo active site states are stable. The hydroperoxo formed is potentially an early LPMO catalytic reaction intermediate or the first step in the mechanism of hydrogen peroxide formation in the absence of substrate. We observe that the N-terminal amino group of the copper coordinating His1 remains doubly protonated directly following molecular oxygen reduction by copper. Aided by molecular dynamics and mining minima free energy calculations we establish that the conserved second-shell His161 in MtPMO3* displays conformational flexibility in solution and that this flexibility is also observed, though to a lesser extent, in His157 of NcLPMO9D. The imidazolate form of His157 observed in our structure following oxygen intermediate protonation can be attributed to abolished His157 flexibility due steric hindrance in the crystal as well as the solvent-occluded active site environment due to crystal packing. A neutron crystal structure of NcLPMO9D at low pH further supports occlusion of the active site since His157 remains singly protonated even at acidic conditions.

Advances in lytic polysaccharide monooxygenases with the cellulose-degrading auxiliary activity family 9 to facilitate cellulose degradation for biorefinery

One crucial step in processing the recalcitrant lignocellulosic biomass is the fast hydrolysis of natural cellulose to fermentable sugars that can be subsequently converted to biofuels and bio-based chemicals. Recent studies have shown that lytic polysaccharide monooxygenase (LPMOs) with auxiliary activity family 9 (AA9) are capable of efficiently depolymerizing the crystalline cellulose via regioselective oxidation reaction. Intriguingly, the catalysis by AA9 LPMOs requires reductant to provide electrons, and lignin and its phenolic derivatives can be oxidized, releasing reductant to activate the reaction. The activity of AA9 LPMOs can be enhanced by in-situ generation of H₂O₂ in the presence of O₂. Although scientific understanding of these enzymes remains somewhat unknown or controversial, structure modifications on AA9 LPMOs through protein engineering have emerged in recent years, which are prerequisite for their extensive applications in the development of cellulase-mediated lignocellulosic biorefinery processes. In this review, we critically comment on advances in studies for AA9 LPMOs, i.e., characteristic of AA9 LPMOs catalysis, external electron donors to AA9 LPMOs, especially the role of the oxidization of lignin and its derivatives, and AA9 LPMOs protein engineering as well as their extensive applications in the bioprocessing of lignocellulosic biomass. Perspectives are also highlighted for addressing the challenges.

The crystal structure of CbpD clarifies substrate-specificity motifs in chitin-active lytic polysaccharide monooxygenases

The 3 Å resolution crystal structure of the Pseudomonas aeruginosa virulence factor CbpD both supports and challenges the current model of how lytic polysaccharide monooxygenases bind chitin and raises interesting possibilities about how type 2 secretion-system substrates may interact with the secretion machinery. This structure also demonstrates the utility of new, AI-powered, protein structure-prediction algorithms in making challenging structural targets tractable.

Pseudomonas aeruginosa secretes diverse proteins via its type 2 secretion system, including a 39 kDa chitin-binding protein, CbpD. CbpD has recently been shown to be a lytic polysaccharide monooxygenase active on chitin and to contribute substantially to virulence. To date, no structure of this virulence factor has been reported. Its first two domains are homologous to those found in the crystal structure of Vibrio cholerae GbpA, while the third domain is homologous to the NMR structure of the CBM73 domain of Cellvibrio japonicusCjLPMO10A. Here, the 3.0 Å resolution crystal structure of CbpD solved by molecular replacement is reported, which required ab initio models of each CbpD domain generated by the artificial intelligence deep-learning structure-prediction algorithm RoseTTAFold. The structure of CbpD confirms some previously reported substrate-specificity motifs among LPMOAA10s, while challenging the predictive power of others. Additionally, the structure of CbpD shows that post-translational modifications occur on the chitin-binding surface. Moreover, the structure raises interesting possibilities about how type 2 secretion-system substrates may interact with the secretion machinery and demonstrates the utility of new artificial intelligence protein structure-prediction algorithms in making challenging structural targets tractable.

Deletion of AA9 Lytic Polysaccharide Monooxygenases Impacts A. nidulans Secretome and Growth on Lignocellulose

Lytic polysaccharide monooxygenases (LPMOs) are oxidative enzymes found in viruses, archaea, and bacteria as well as eukaryotes, such as fungi, algae and insects, actively contributing to the degradation of different polysaccharides. In Aspergillus nidulans, LPMOs from family AA9 (AnLPMO9s), along with an AA3 cellobiose dehydrogenase (AnCDH1), are cosecreted upon growth on crystalline cellulose and lignocellulosic substrates, indicating their role in the degradation of plant cell wall components. Functional analysis revealed that three target LPMO9s (AnLPMO9C, AnLPMO9F and AnLPMO9G) correspond to cellulose-active enzymes with distinct regioselectivity and activity on cellulose with different proportions of crystalline and amorphous regions. AnLPMO9s deletion and overexpression studies corroborate functional data. The abundantly secreted AnLPMO9F is a major component of the extracellular cellulolytic system, while AnLPMO9G was less abundant and constantly secreted, and acts preferentially on crystalline regions of cellulose, uniquely displaying activity on highly crystalline algae cellulose. Single or double deletion of AnLPMO9s resulted in about 25% reduction in fungal growth on sugarcane straw but not on Avicel, demonstrating the contribution of LPMO9s for the saprophytic fungal lifestyle relies on the degradation of complex lignocellulosic substrates. Although the deletion of AnCDH1 slightly reduced the cellulolytic activity, it did not affect fungal growth indicating the existence of alternative electron donors to LPMOs. Additionally, double or triple knockouts of these enzymes had no accumulative deleterious effect on the cellulolytic activity nor on fungal growth, regardless of the deleted gene. Overexpression of AnLPMO9s in a cellulose-induced secretome background confirmed the importance and applicability of AnLPMO9G to improve lignocellulose saccharification. IMPORTANCE Fungal lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that boost plant biomass degradation in combination with glycoside hydrolases. Secretion of LPMO9s arsenal by Aspergillus nidulans is influenced by the substrate and time of induction. These findings along with the biochemical characterization of novel fungal LPMO9s have implications on our understanding of their concerted action, allowing rational engineering of fungal strains for biotechnological applications such as plant biomass degradation. Additionally, the role of oxidative players in fungal growth on plant biomass was evaluated by deletion and overexpression experiments using a model fungal system.

On the expansion of biological functions of lytic polysaccharide monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) constitute an enigmatic class of enzymes, the discovery of which has opened up a new arena of riveting research. LPMOs can oxidatively cleave the glycosidic bonds found in carbohydrate polymers enabling the depolymerisation of recalcitrant biomasses, such as cellulose or chitin. While most studies have so far mainly explored the role of LPMOs in a (plant) biomass conversion context, alternative roles and paradigms begin to emerge. In the present review, we propose a historical perspective of LPMO research providing a succinct overview of the major achievements of LPMO research over the past decade. This journey through LPMOs landscape leads us to dive into the emerging biological functions of LPMOs and LPMO-like proteins. We notably highlight roles in fungal and oomycete plant pathogenesis (e.g. potato late blight), but also in mutualistic/commensalism symbiosis (e.g. ectomycorrhizae). We further present the potential importance of LPMOs in other microbial pathogenesis including diseases caused by bacteria (e.g. pneumonia), fungi (e.g. human meningitis), oomycetes and viruses (e.g. entomopox), as well as in (micro)organism development (including several plant pests). Our assessment of the literature leads to the formulation of outstanding questions, promising for the coming years exciting research and discoveries on these moonlighting proteins.

Polymer ultrastructure governs AA9 lytic polysaccharide monooxygenases functionalization and deconstruction efficacy on cellulose nano-crystals

Lytic Polysaccharide MonoOxygenases display great variability towards cellulose ultrastructure while performing oxidative functionalization of the polymers. Aiming at employing AA9-LPMOs for isolation of cellulose nano-crystals (CNCs), the ratio between functionalization/crystalline degradation became a crucial parameter. Here are reported the constraints posed by the substrate ultrastructure on the activity of seven different AA9 LPMOs representative of various regioselectivity and substrate affinity: TtAA9E, TaAA9A, PcAA9D, MtAA9A, MtAA9D, MtAA9I-CBM and MtAA9J. The substrate crystallinity and dry matter loading greatly affected the seven AA9s in an enzyme-specific manner, impacting their efficiency for CNCs functionalization purposes. X-ray diffraction pattern analyses were used to assess the cracking efficacy of the enzymatic treatment to de-crystallize CNCs, revealing that those AA9s with minor efficiency in releasing oligosaccharides resulted instead the most disruptive towards the crystal lattice and in reducing the particle sizes. These non-catalytic effects were found comparable with the one caused by the expansin BsEXLX1 enzyme.

Protonation State of an Important Histidine from High Resolution Structures of Lytic Polysaccharide Monooxygenases

Lytic Polysaccharide Monooxygenases (LPMOs) oxidatively cleave recalcitrant polysaccharides. The mechanism involves (i) reduction of the Cu, (ii) polysaccharide binding, (iii) binding of different oxygen species, and (iv) glycosidic bond cleavage. However, the complete mechanism is poorly understood and may vary across different families and even within the same family. Here, we have investigated the protonation state of a secondary co-ordination sphere histidine, conserved across AA9 family LPMOs that has previously been proposed to be a potential proton donor. Partial unrestrained refinement of newly obtained higher resolution data for two AA9 LPMOs and re-refinement of four additional data sets deposited in the PDB were carried out, where the His was refined without restraints, followed by measurements of the His ring geometrical parameters. This allowed reliable assignment of the protonation state, as also validated by following the same procedure for the His brace, for which the protonation state is predictable. The study shows that this histidine is generally singly protonated at the Nε2 atom, which is close to the oxygen species binding site. Our results indicate robustness of the method. In view of this and other emerging evidence, a role as proton donor during catalysis is unlikely for this His.

Anammox Bacteria Are Potentially Involved in Anaerobic Ammonium Oxidation Coupled to Iron(III) Reduction in the Wastewater Treatment System

Anaerobic ammonium oxidation coupled to nitrite reduction (termed as Anammox) was demonstrated as an efficient pathway to remove nitrogen from a wastewater treatment system. Recently, anaerobic ammonium oxidation was also identified to be linked to iron(III) reduction (termed Feammox) with dinitrogen, nitrite, or nitrate as end-product, reporting to enhance nitrogen removal from the wastewater treatment system. However, little is known about the role of Anammox bacteria in the Feammox process. Here, slurry from wastewater reactor amended with ferrihydrite was employed to investigate activity of Anammox bacteria in the Feammox process using the ¹⁵N isotopic tracing technique combined with 16S rRNA gene amplicon sequencing. A significantly positive relationship between rates of ¹⁵N₂ production and iron(III) reduction indicated the occurrence of Feammox during incubation. Relative abundances of Anammox bacteria including Brocadia, Kuenenia, Jettenia, and unclassified Brocadiaceae were detected with low relative abundances, whereas Geobacteraceae dominated in the treatment throughout the incubation. ¹⁵N₂ production rates significantly positively correlated with relative abundances of Geobacter, unclassified Geobacteraceae, and Anammox bacteria, revealing their contribution to nitrogen generation via Feammox. Overall, these findings suggested Anammox bacteria or cooperation between Anammox bacteria and iron(III) reducers serves a potential role in Feammox process.

Unlocking the secret of lignin-enzyme interactions: Recent advances in developing state-of-the-art analytical techniques

Bioconversion of renewable lignocellulosics to produce liquid fuels and chemicals is one of the most effective ways to solve the problem of fossil resource shortage, energy security, and environmental challenges. Among the many biorefinery pathways, hydrolysis of lignocellulosics to fermentable monosaccharides by cellulase is arguably the most critical step of lignocellulose bioconversion. In the process of enzymatic hydrolysis, the direct physical contact between enzymes and cellulose is an essential prerequisite for the hydrolysis to occur. However, lignin is considered one of the most recalcitrant factors hindering the accessibility of cellulose by binding to cellulase unproductively, which reduces the saccharification rate and yield of sugars. This results in high costs for the saccharification of carbohydrates. The various interactions between enzymes and lignin have been explored from different perspectives in literature, and a basic lignin inhibition mechanism has been proposed. However, the exact interaction between lignin and enzyme as well as the recently reported promotion of some types of lignin on enzymatic hydrolysis is still unclear at the molecular level. Multiple analytical techniques have been developed, and fully unlocking the secret of lignin-enzyme interactions would require a continuous improvement of the currently available analytical techniques. This review summarizes the current commonly used advanced research analytical techniques for investigating the interaction between lignin and enzyme, including quartz crystal microbalance with dissipation (QCM-D), surface plasmon resonance (SPR), attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, atomic force microscopy (AFM), nuclear magnetic resonance (NMR) spectroscopy, fluorescence spectroscopy (FLS), and molecular dynamics (MD) simulations. Interdisciplinary integration of these analytical methods is pursued to provide new insight into the interactions between lignin and enzymes. This review will serve as a resource for future research seeking to develop new methodologies for a better understanding of the basic mechanism of lignin-enzyme binding during the critical hydrolysis process.

Clostridium thermocellum as a Promising Source of Genetic Material for Designer Cellulosomes: An Overview

Plant biomass-based biofuels have gradually substituted for conventional energy sources thanks to their obvious advantages, such as renewability, huge quantity, wide availability, economic feasibility, and sustainability. However, to make use of the large amount of carbon sources stored in the plant cell wall, robust cellulolytic microorganisms are highly demanded to efficiently disintegrate the recalcitrant intertwined cellulose fibers to release fermentable sugars for microbial conversion. The Gram-positive, thermophilic, cellulolytic bacterium Clostridium thermocellum possesses a cellulolytic multienzyme complex termed the cellulosome, which has been widely considered to be nature’s finest cellulolytic machinery, fascinating scientists as an auspicious source of saccharolytic enzymes for biomass-based biofuel production. Owing to the supra-modular characteristics of the C. thermocellum cellulosome architecture, the cellulosomal components, including cohesin, dockerin, scaffoldin protein, and the plentiful cellulolytic and hemicellulolytic enzymes have been widely used for constructing artificial cellulosomes for basic studies and industrial applications. In addition, as the well-known microbial workhorses are naïve to biomass deconstruction, several research groups have sought to transform them from non-cellulolytic microbes into consolidated bioprocessing-enabling microbes. This review aims to update and discuss the current progress in these mentioned issues, point out their limitations, and suggest some future directions.

Engineered tyrosinases with broadened bio-catalysis scope: immobilization using nanocarriers and applications

Enzyme immobilization is a widely used technology for creating more stable, active, and reusable biocatalysts. The immobilization process also improves the enzyme's operating efficiency in industrial applications. Various support matrices have been designed and developed to enhance the biocatalytic efficiency of immobilized enzymes. Given their unique physicochemical attributes, including substantial surface area, rigidity, semi-conductivity, high enzyme loading, hyper catalytic activity, and size-assisted optical properties, nanomaterials have emerged as fascinating matrices for enzyme immobilization. Tyrosinase is a copper-containing monooxygenase that catalyzes the o-hydroxylation of monophenols to catechols and o-quinones. This enzyme possesses a wide range of uses in the medical, biotechnological, and food sectors. This article summarizes an array of nanostructured materials as carrier matrices for tyrosinase immobilization. Following a detailed background overview, various nanomaterials, as immobilization support matrices, including carbon nanotubes (CNTs), carbon dots (CDs), carbon black (CB), nanofibers, Graphene nanocomposite, platinum nanoparticles, nano-sized magnetic particles, lignin nanoparticles, layered double hydroxide (LDH) nanomaterials, gold nanoparticles (AuNPs), and zinc oxide nanoparticles have been discussed. Next, applied perspectives have been spotlights with particular reference to environmental pollutant sensing, phenolic compounds detection, pharmaceutical, and food industry (e.g., cereal processing, dairy processing, and meat processing), along with other miscellaneous applications.

Oxidative Power: Tools for Assessing LPMO Activity on Cellulose

Lytic polysaccharide monooxygenases (LPMOs) have sparked a lot of research regarding their fascinating mode-of-action. Particularly, their boosting effect on top of the well-known cellulolytic enzymes in lignocellulosic hydrolysis makes them industrially relevant targets. As more characteristics of LPMO and its key role have been elucidated, the need for fast and reliable methods to assess its activity have become clear. Several aspects such as its co-substrates, electron donors, inhibiting factors, and the inhomogeneity of lignocellulose had to be considered during experimental design and data interpretation, as they can impact and often hamper outcomes. This review provides an overview of the currently available methods to measure LPMO activity, including their potential and limitations, and it is illustrated with practical examples.

Genome and Secretome Analysis of Staphylotrichum longicolleum DSM105789 Cultured on Agro-Residual and Chitinous Biomass

Staphylotrichum longicolleum FW57 (DSM105789) is a prolific chitinolytic fungus isolated from wood, with a chitinase activity of 0.11 ± 0.01 U/mg. We selected this strain for genome sequencing and annotation, and compiled its growth characteristics on four different chitinous substrates as well as two agro-industrial waste products. We found that the enzymatic mixture secreted by FW57 was not only able to digest pre-treated sugarcane bagasse, but also untreated sugarcane bagasse and maize leaves. The efficiency was comparable to a commercial enzymatic cocktail, highlighting the potential of the S. longicolleum enzyme mixture as an alternative pretreatment method. To further characterize the enzymes, which efficiently digested polymers such as cellulose, hemicellulose, pectin, starch, and lignin, we performed in-depth mass spectrometry-based secretome analysis using tryptic peptides from in-gel and in-solution digestions. Depending on the growth conditions, we were able to detect from 442 to 1092 proteins, which were annotated to identify from 134 to 224 putative carbohydrate-active enzymes (CAZymes) in five different families: glycoside hydrolases, auxiliary activities, carbohydrate esterases, polysaccharide lyases, glycosyl transferases, and proteins containing a carbohydrate-binding module, as well as combinations thereof. The FW57 enzyme mixture could be used to replace commercial enzyme cocktails for the digestion of agro-residual substrates.

Sugar oxidoreductases and LPMOs - two sides of the same polysaccharide degradation story?

Lytic polysaccharide monooxygenases (LPMOs) catalyze the oxidative cleavage of glycosidic bonds in recalcitrant polysaccharides such as chitin and cellulose and their discovery has revolutionized our understanding of enzymatic biomass conversion. The discovery of LPMOs raises interesting new questions regarding the roles of other oxidoreductases and abiotic redox processes in biomass conversion. LPMOs need reducing power and an oxygen co-substrate and biomass degrading ecosystems contain a multitude of redox enzymes that affect the availability of both. For example, biomass degrading fungi produce multiple sugar oxidoreductases whose biological functions so far have remained somewhat enigmatic. It is now conceivable that these redox enzymes, in particular H2O2-producing sugar oxidases, could play a role in fueling and controlling LPMO reactions. Here, we shortly review contemporary issues in the LPMO field, paying particular attention to the possible roles of sugar oxidoreductases.

Enhanced Fenton Reaction for Xenobiotic Compounds and Lignin Degradation Fueled by Quinone Redox Cycling by Lytic Polysaccharide Monooxygenases

The Fenton reaction is considered to be of great significance in the initial attack of lignocellulose in wood-decaying fungi. Quinone redox cycling is the main way to induce the Fenton reaction in fungi. We show that lytic polysaccharide monooxygenases (LPMOs), through LPMO-catalyzed oxidation of hydroquinone, can efficiently cooperate with glucose dehydrogenase (GDH) to achieve quinone redox cycling. The LPMO/GDH system can enhance Fe3+-reducing activity, H2O2 production, and hydroxyl radical generation, resulting in a fueled Fenton reaction. The system-generated hydroxyl radicals exhibited a strong capacity to decolorize different synthetic dyes and degrade lignin. Our results reveal a potentially critical connection between LPMOs and the Fenton reaction, suggesting that LPMOs could be involved in xenobiotic compound and lignin degradation in fungi. This new role of LPMOs may be exploited for application in biorefineries.

Microbial demethylation of lignin: Evidence of enzymes participating in the removal of methyl/methoxyl groups

Lignin is an abundant natural plant aromatic biopolymer containing various functional groups that can be exploited for activating lignin for potential commercial applications. Applications are hindered due to the presence of a high content of methyl/methoxyl groups that affects reactiveness. Various chemical and enzymatic approaches have been investigated to increase the functionality in transforming lignin. Among these is demethylation/demethoxylation, which increases the potential numbers of vicinal hydroxyl groups for applications as phenol-formaldehyde resins. Although the chemical route to lignin demethylation is well-studied, the biological route is still poorly explored. Bacteria and fungi have the ability to demethylate lignin and lignin-related compounds. Considering that appropriate microorganisms possess the biochemical machinery to demethylate lignin by cleaving O-methyl groups liberating methanol, and modify lignin by increasing the vicinal diol content that allows lignin to substitute for phenol in organic polymer syntheses. Certain bacteria through the actions of specific O-demethylases can modify various lignin-related compounds generating vicinal diols and liberating methanol or formaldehyde as end-products. The enzymes include: cytochrome P₄₅₀-aryl-O-demethylase, monooxygenase, veratrate 3-O-demethylase, DDVA O-demethylase (LigX; lignin-related biphenyl 5,5'-dehydrodivanillate (DDVA)), vanillate O-demethylase, syringate O-demethylase, and tetrahydrofolate-dependent-O-demethylase. Although, the fungal counterparts have not been investigated in depth as in bacteria, O-demethylases, nevertheless, have been reported in demethylating various lignin substrates providing evidence of a fungal enzyme system. Few fungi appear to have the ability to secrete O-demethylases. The fungi can mediate lignin demethylation enzymatically (laccase, lignin peroxidase, manganese peroxidase, O-demethylase), or non-enzymatically in brown-rot fungi through the Fenton reaction. This review discusses details on the aspects of microbial (bacterial and fungal) demethylation of lignins and lignin-model compounds and provides evidence of enzymes identified as specific O-demethylases involved in demethylation.

Colorimetric LPMO assay with direct implication for cellulolytic activity

BACKGROUND

Lytic polysaccharide monooxygenases (LPMOs) are important industrial enzymes known for their catalytic degradation of recalcitrant polymers such as cellulose or chitin. Their activity can be measured by lengthy HPLC methods, while high-throughput methods are less specific. A fast and specific LPMO assay would simplify screening for new or engineered LPMOs and accelerate biochemical characterization.

RESULTS

A novel LPMO activity assay was developed based on the production of the dye phenolphthalein (PHP) from its reduced counterpart (rPHP). The colour response of rPHP oxidisation catalysed by the cellulose-specific LPMO from Thermoascus aurantiacus (TaAA9A), was found to increase tenfold by adding dehydroascorbate (DHA) as a co-substrate. The assay using a combination of rPHP and DHA was tested on 12 different metallo-enzymes, but only the LPMOs catalysed this reaction. The assay was optimized for characterization of TaAA9A and showed a sensitivity of 15 nM after 30 min incubation. It followed apparent Michaelis-Menten kinetics with kcat = 0.09 s-1 and KM = 244 µM, and the assay was used to confirm stoichiometric copper-enzyme binding and enzyme unfolding at a temperature of approximately 60 °C. DHA, glutathione and fructose were found to enhance LPMO oxidation of rPHP and in the optimized assay conditions these co-substrates also enabled cellulose degradation.

CONCLUSIONS

This novel and specific LPMO assay can be carried out in a convenient microtiter plate format ready for high-throughput screening and enzyme characterization. DHA was the best co-substrate tested for oxidation of rPHP and this preference appears to be LPMO-specific. The identified co-substrates DHA and fructose are not normally considered as LPMO co-substrates but here they are shown to facilitate both oxidation of rPHP and degradation of cellulose. This is a rare example of a finding from a high-throughput assay that directly translate into enzyme activity on an insoluble substrate. The rPHP-based assay thus expands our understanding of LPMO catalysed reactions and has the potential to characterize LPMO activity in industrial settings, where usual co-substrates such as ascorbate and oxygen are depleted.

Do Lytic Polysaccharide Monooxygenases Aid in Plant Pathogenesis and Herbivory?

Lytic polysaccharide monooxygenases (LPMOs), copper-dependent enzymes mainly found in fungi, bacteria, and viruses, are responsible for enabling plant infection and degradation processes. Since their discovery 10 years ago, significant progress has been made in understanding the major role these enzymes play in biomass conversion. The recent discovery of additional LPMO families in fungi and oomycetes (AA16) as well as insects (AA15) strongly suggests that LPMOs might also be involved in biological processes such as overcoming plant defenses. In this review, we aim to give a comprehensive overview of the potential role of different LPMO families from the perspective of plant defense and their multiple implications in devising new strategies for achieving crop protection from plant pathogens and insect pests.

Enzyme Cascade Reaction Involving Lytic Polysaccharide Monooxygenase and Dye-Decolorizing Peroxidase for Chitosan Functionalization

In situ H₂O₂ generation systems are efficient for H₂O₂-dependent biocatalytic oxidation reactions. Here, we report that lytic polysaccharide monooxygenases (LPMOs), copper-dependent polysaccharide monooxygenases, can efficiently supply H₂O₂ in situ to dye-decolorizing peroxidases (DyPs) using substrate gallic acid (GA) for chitosan functionalization. The maximum grafting ratio induced by the cascade reaction was significantly higher than that achieved by a reaction with initial exogenous H₂O₂. The maximum grafting ratio was obtained with 12 g/L GA, 5.6 mg/L DyP, 20-30 mg/L LPMO, and pH 4.5-5.0. UV-vis, Fourier transform infrared (FT-IR), and nuclear magnetic resonance (¹H NMR) spectroscopy confirmed GA grafting onto chitosan. X-ray diffraction (XRD) analysis and thermogravimetric analysis (TGA) indicated that GA-chitosan conjugates had lower thermal stability and crystallinity than chitosan. The GA-chitosan conjugates had significantly higher antioxidant activity than chitosan. This study supplies a green and high-efficiency approach to achieve an enzymatic cascade reaction for chitosan functionalization and has potential applications in H₂O₂-dependent biocatalytic oxidation reactions.

Insights into the H2 O2 -driven catalytic mechanism of fungal lytic polysaccharide monooxygenases

Fungal lytic polysaccharide monooxygenases (LPMOs) depolymerise crystalline cellulose and hemicellulose, supporting the utilisation of lignocellulosic biomass as a feedstock for biorefinery and biomanufacturing processes. Recent investigations have shown that H₂O₂ is the most efficient cosubstrate for LPMOs. Understanding the reaction mechanism of LPMOs with H₂O₂ is therefore of importance for their use in biotechnological settings. Here, we have employed a variety of spectroscopic and biochemical approaches to probe the reaction of the fungal LPMO9C from N. crassa using H₂O₂ as a cosubstrate and xyloglucan as a polysaccharide substrate. We show that a single ‘priming’ electron transfer reaction from the cellobiose dehydrogenase partner protein supports up to 20 H₂O₂‐driven catalytic cycles of a fungal LPMO. Using rapid mixing stopped‐flow spectroscopy, alongside electron paramagnetic resonance and UV‐Vis spectroscopy, we reveal how H₂O₂ and xyloglucan interact with the enzyme and investigate transient species that form uncoupled pathways of NcLPMO9C. Our study shows how the H₂O₂ cosubstrate supports fungal LPMO catalysis and leaves the enzyme in the reduced Cu⁺ state following a single enzyme turnover, thus preventing the need for external protons and electrons from reducing agents or cellobiose dehydrogenase and supporting the binding of H₂O₂ for further catalytic steps. We observe that the presence of the substrate xyloglucan stabilises the Cu⁺ state of LPMOs, which may prevent the formation of uncoupled side reactions.

Comparative analysis of two recombinant LPMOs from Aspergillus fumigatus and their effects on sugarcane bagasse saccharification

Lytic polysaccharide monooxygenases (LPMOs) have been introduced into industrial cocktails used for biomass saccharification due to their capacity to boost enzymatic conversion of recalcitrant cellulose. The genome of the thermotolerant ascomycete Aspergillus fumigatus encodes 7 genes for LPMOs that belong to auxiliary activity family 9 (AA9). Here, we cloned, successfully expressed and performed biochemical evaluation of two CBM-less A. fumigatus LPMOs (AfAA9A and AfAA9B). A high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) analysis demonstrated that AfAA9A and AfAA9B are able to oxide cellulose at C1 and C1/C4 positions, respectively. Synergic effects of LPMOs (separately and in combination) with cellulases were investigated. Supplementation of Celluclast 1.5 L with a low concentration of AfAA9B improved in 20 % the saccharification of sugarcane bagasse pretreated by steam explosion (SEB), while AfAA9A did not improvethe saccharification. Analysis of the hydrolyzed biomass by confocal laser scanning microscopy (CLSM) showed the LPMOs are promoting lignin oxidation in the lignocellulosic material. This study complements the available results concerning the utilization of LPMOs in the enzymatic saccharification of lignocellulosic biomass.

Engineered LPMO Significantly Boosting Cellulase-Catalyzed Depolymerization of Cellulose

Lytic polysaccharide monooxygenases (LPMOs) play a crucial role in the enzymatic depolymerization of cellulose through oxidative cleavage of the glycosidic bond in the highly recalcitrant crystalline cellulose region. Improving the activity of LPMOs is of considerable importance for second-generation biorefinery. In this study, we identified a beneficial amino acid substitution (N526S) located in the cellulose binding module (CBM) of HcLPMO10 (LPMO of Hahella chejuensis) using directed evolution. The improved variant HcLPMO10 M1 (N526S) exhibits 2.1-fold higher activity for the H₂O₂ production, 2.7-fold higher oxidation activity, and 1.9-fold higher binding capacity toward cellulose compared with those of the wild type (WT). Furthermore, M1 shows 2.1-fold higher activity for degradation of crystalline cellulose in synergy with cellulase, compared to the WT. Structural analysis through molecular modeling and molecular dynamics (MD) simulation revealed that the substitution N526S located in the CBM likely stabilizes the cellulose binding surface and enhances the binding capacity of HcLPMO10 to cellulose, thereby enhancing enzyme activity. These findings demonstrate the important role of the CBM in the catalytic function of LPMO.

Characterization of an AA9 LPMO from Thielavia australiensis, TausLPMO9B, under industrially relevant lignocellulose saccharification conditions

BACKGROUND

The discovery of lytic polysaccharide monooxygenases (LPMO) has changed our perspective on enzymatic degradation of plant biomass. Through an oxidative mechanism, these enzymes are able to cleave and depolymerize various polysaccharides, acting not only on crystalline substrates such as chitin and cellulose, but also on other polysaccharides, such as xyloglucan, glucomannan and starch. Despite their widespread use, uncertainties related to substrate specificity and stereospecificity, the nature of the co-substrate, in-process stability, and the nature of the optimal reductant challenge their exploitation in biomass processing applications.

RESULTS

In this work, we studied the properties of a novel fungal LPMO from the thermophilic fungus Thielavia australiensis, TausLPMO9B. Heterologous expression of TausLPMO9B in Aspergillus niger yielded a glycosylated protein with a methylated N-terminal histidine showing LPMO activity. High sequence identity of the AA9 domain to that of MtLPMO9B (MYCTH_80312) from Myceliophthora thermophila (84%) indicated strictly C1-oxidizing activity on cellulose, which was confirmed experimentally by the analysis of products released from cellulose using HPAEC. The enzyme was stable and active at a pH ranging from 4 to 6, thus matching the conditions commonly used in industrial biomass processing, where a low pH (between 4 and 5) is used due to the pH-optima of commercial cellulases and a desire to limit microbial contamination.

CONCLUSION

While the oxidative cleavage of phosphoric acid swollen cellulose (PASC) by TausLPMO9B was boosted by the addition of H₂O₂ as a co-substrate, this effect was not observed during the saccharification of acid pretreated corn stover. This illustrates key differences between the lab-scale tests with artificial, lignin-free substrates and industrial settings with lignocellulosic biomass as substrate.

A fast and easy strategy for lytic polysaccharide monooxygenase-cleavable His6-Tag cloning, expression, and purification

Lytic polysaccharide monooxygenases (LPMOs) are industrially important enzymes able to enhance the enzymatic lignocellulose saccharification in synergism with classical glycoside hydrolases. Fungal LPMOs have been classified as AA9, AA11, and AA13-16 families showing a diverse specificity for substrates such as soluble and insoluble beta-glucans, chitin, starch, and xylan, besides cellulose. These enzymes are still not fully characterized, and for example this is testify by their mechanism of oxidation regularly reviewed multiple times in the last decade. Noteworthy is that despite the extremely large abundance in the entire Tree of Life, our structural and functional knowledge is based on a restricted pool of LPMO, and probably one of the main reason reside in the challenging posed by their heterologous expression. Notably, the lack of a simple cloning protocol that could be universally applied to LPMO, hinders the conversion of the ever-increasing available genomic information to actual new enzymes. Here, we provide an easy and fast protocol for cloning, expression, and purification of active LPMOs in the following architecture: natural signal peptide, LPMO enzyme, TEV protease site, and His₆-Tag. For this purpose, a commercial methanol inducible expression vector was initially modified to allow the LPMO expression containing the above characteristics. Gibson assembly, a one-step isothermal DNA assembly, was adopted for the direct assembly of intron-less or intron-containing genes and the modified expression vector. Moreover, His₆-tagged LPMO constructs can be submitted to TEV proteolysis for removal of the questionable C-terminal His₆-Tag, obtaining a close-to-native form of LPMO. We successfully applied this method to clone, express, and purify six LPMOs from AA9 family with different regioselectivities. The proposed protocol, provided as step-by-step, could be virtually applied in many laboratories thanks to the choice of popular and commons materials.

In-situ lignin drives lytic polysaccharide monooxygenases to enhance enzymatic saccharification

Recently low-molecular lignin was reported to activate lytic polysaccharide monooxygenases (LPMOs) to oxidize cellulose. However, whether lignin formed in cell wall can play the role as electron donor for LPMOs is still largely unknown due to the complex ultrastructure of lignocellulosic biomass. In this study, we presented a new strategy to elucidate in-situ lignin function in LPMOs reaction. A lignocellulosic mimicking model was used as substrate, which was equipped with a polysaccharide template of self-assembled bacterial cellulose film and synthesized lignin. Remarkably, it has been demonstrated that lignin polymer deposited on cellulose can reduce LPMOs in-situ for cellulose oxidation and then boost cellulose hydrolysis, and the cellulose conversion ratio of the mimicked lignocellulosic film was increased by 26.0%. More importantly, lignin in-situ might exceed the well-known reductant of ascorbic acid to drive LPMOs for cellulase enzymatic hydrolysis with equivalent cellulose oxidation efficiency and extremely lower H₂O₂ generation, avoiding the inactivation of enzymes. The maximum H₂O₂ yield from lignin-driven LPMO reaction was 75.8% lower than that from ascorbic acid-driven reaction. Therefore, by using the lignocellulosic mimicking model, we have elucidated the function of in-situ lignin in boosting enzymatic hydrolysis. Such understanding could significantly promote current utilization of LPMOs in lignocellulosic biorefinery.

Engineering lytic polysaccharide monooxygenases (LPMOs)

Lytic polysaccharide monooxygenases (LPMOs) are mono-copper enzymes that catalyze the hydroxylation of glycosidic bonds found in the most abundant and recalcitrant polysaccharides on Earth. Since their discovery in 2010, these enzymes have received extensive attention in both fundamental and applied research due to their remarkable oxidative power and synergistic interplay with hydrolytic enzymes. The harsh and unnatural conditions used in industrial enzymatic saccharification processes and the sensitivity of LPMOs for damage induced by reactive oxygen species call for enzyme engineering to develop LPMOs to become robust industrial biocatalysts. Other engineering targets include improved catalytic activity, adjusted substrate specificity and the introduction of completely new activities. Reaching these targets not only requires appropriate methods for measuring enzyme activity, but also requires in-depth knowledge of the active site and the reaction mechanism, which is yet to be achieved in the LPMO field. Here we describe what has been done in the LPMO engineering field so far. Furthermore, we address the difficulties involved in properly assessing LPMO functionality, which are due to common side reactions taking place in LPMO reactions and which complicate screening methods.

Oligosaccharide Binding and Thermostability of Two Related AA9 Lytic Polysaccharide Monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that cleave polysaccharide substrates oxidatively. First discovered because of their action on recalcitrant crystalline substrates (chitin and cellulose), a number of LPMOs are now reported to act on soluble substrates, including oligosaccharides. However, crystallographic complexes with oligosaccharides have been reported for only a single LPMO so far, an enzyme from the basidiomycete fungus Lentinus similis (LsAA9_A). Here we present a more detailed comparative study of LsAA9_A and an LPMO from the ascomycete fungus Collariella virescens (CvAA9_A) with which it shares 41.5% sequence identity. LsAA9_A is considerably more thermostable than CvAA9_A, and the structural basis for the difference has been investigated. We have compared the patterns of oligosaccharide cleavage and the patterns of binding in several new crystal structures explaining the basis for the product preferences of the two enzymes. Obtaining structural information about complexes of LPMOs with carbohydrates has proven to be very difficult in general judging from the structures reported in the literature thus far, and this can be attributed only partly to the low affinity for small substrates. We have thus evaluated the use of differential scanning fluorimetry as a guide to obtaining complex structures. Furthermore, an analysis of crystal packing of LPMOs and glycoside hydrolases corroborates the hypothesis that active site occlusion is a very significant problem for LPMO-substrate interaction analysis by crystallography, due to their relatively flat and extended substrate binding sites.