Activation of bacterial lytic polysaccharide monooxygenases with cellobiose dehydrogenase

Expanding the catalytic landscape of metalloenzymes with lytic polysaccharide monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) have an essential role in global carbon cycle, industrial biomass processing and microbial pathogenicity by catalysing the oxidative cleavage of recalcitrant polysaccharides. Despite initially being considered monooxygenases, experimental and theoretical studies show that LPMOs are essentially peroxygenases, using a single copper ion and H2O2 for C-H bond oxygenation. Here, we examine LPMO catalysis, emphasizing key studies that have shaped our comprehension of their function, and address side and competing reactions that have partially obscured our understanding. Then, we compare this novel copper-peroxygenase reaction with reactions catalysed by haem iron enzymes, highlighting the different chemistries at play. We conclude by addressing some open questions surrounding LPMO catalysis, including the importance of peroxygenase and monooxygenase reactions in biological contexts, how LPMOs modulate copper site reactivity and potential protective mechanisms against oxidative damage.

Current insights of factors interfering the stability of lytic polysaccharide monooxygenases

Cellulose and chitin are two of the most abundant biopolymers in nature, but they cannot be effectively utilized in industry due to their recalcitrance. This limitation was overcome by the advent of lytic polysaccharide monooxygenases (LPMOs), which promote the disruption of biopolymers through oxidative mechanism and provide a breakthrough in the action of hydrolytic enzymes. In the application of LPMOs to biomass degradation, the key to consistent and effective functioning lies in their stability. The efficient transformation of biomass resources using LPMOs depends on factors that interfere with their stability. This review discussed three aspects that affect LPMO stability: general external factors, structural factors, and factors in the enzyme-substrate reaction. It explains how these factors impact LPMO stability, discusses the resulting effects, and finally presents relevant measures and considerations, including potential resolutions. The review also provides suggestions for the application of LPMOs in polysaccharide degradation.

A novel self-purified auxiliary protein enhances the lichenase activity towards lichenan for biomass degradation

Due to the complex composition of lichenan, lichenase alone cannot always hydrolyze it efficiently. Carbohydrate-binding modules (CBMs) and lytic polysaccharide monooxygenases (LPMOs) have been confirmed to increase the hydrolysis efficiency of lichenases. However, their practical application was hampered by the complex and costly preparation procedure, as well as the poor stability of LPMOs. Herein, we discovered a novel and stable auxiliary protein named SCE to boost the hydrolysis efficiency. SCE was composed of SpyCatcher (SC) and elastin-like polypeptides (ELPs) and could be easily and cheaply prepared. Under the optimal conditions, the boosting degree for SCE/lichenase was 1.45, and the reducing sugar yield improved by nearly 45%. The results of high-performance liquid chromatography (HPLC) indicated that SCE had no influence on the hydrolysis pattern of lichenase. Through the experimental verification and bioinformatics analysis, we proposed the role of SCE in promoting the interaction between the lichenase and substrates. These findings endow SC with a novel function in binding to insoluble lichenan, paving the way for biomass degradation and biorefinery. KEY POINTS: • A novel self-purification auxiliary protein that could boost the hydrolysis efficiency of lichenase has been identified. • The protein is highly produced, simple to prepare, well stable, and does not require any external electron donor. • The novel function of SpyCatcher in binding to insoluble lichenan was first demonstrated.

Molecular Mechanism of Substrate Oxidation in Lytic Polysaccharide Monooxygenases: Insight from Theoretical Investigations

Lytic polysaccharide monooxygenases (LPMOs) are copper enzymes that today comprise a large enzyme superfamily, grouped into the distinct members AA9-AA17 (with AA12 exempted). The LPMOs have the potential to facilitate the upcycling of biomass waste products by boosting the breakdown of cellulose and other recalcitrant polysaccharides. The cellulose biopolymer is the main component of biomass waste and thus comprises a large, unexploited resource. The LPMOs work through a catalytic, oxidative reaction whose mechanism is still controversial. For instance, the nature of the intermediate performing the oxidative reaction is an open question, and the same holds for the employed co-substrate. Here we review theoretical investigations addressing these questions. The applied theoretical methods are usually based on quantum mechanics (QM), often combined with molecular mechanics (QM/MM). We discuss advantages and disadvantages of the employed theoretical methods and comment on the interplay between theoretical and experimental results.

Lytic polysaccharide monooxygenase - A new driving force for lignocellulosic biomass degradation

Lytic polysaccharide monooxygenases (LPMOs) can catalyze polysaccharides by oxidative cleavage of glycosidic bonds and have catalytic activity for cellulose, hemicellulose, chitin, starch and pectin, thus playing an important role in the biomass conversion of lignocellulose. The catalytic substrates of LPMOs are different and the specific catalytic mechanism has not been fully elucidated. Although there have been many studies related to LPMOs, few have actually been put into industrial biomass conversion, which poses a challenge for their expression, regulation and application. In this review, the origin, substrate specificity, structural features, and the relationship between structure and function of LPMOs are described. Additionally, the catalytic mechanism and electron donor of LPMOs and their heterologous expression and regulation are discussed. Finally, the synergistic degradation of biomass by LPMOs with other polysaccharide hydrolases is reviewed, and their current problems and future research directions are pointed out.

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.

Lytic polysaccharide monooxygenases as powerful tools in enzymatically assisted preparation of nano-scaled cellulose from lignocellulose: A review

Nanocellulose, either in the form of fibers or crystals, constitutes a renewable, biobased, biocompatible material with advantageous mechanical properties that can be isolated from lignocellulosic biomass. Enzyme-assisted isolation of nanocellulose is an attractive, environmentally friendly approach that leads to products of higher quality compared to their chemically prepared counterparts. Lytic polysaccharide monooxygenases (LPMOs) are enzymes that oxidatively cleave the β-1,4-glycosidic bond of polysaccharides upon activation of O₂ or H₂O₂ and presence of an electron donor. Their use for treatment of cellulose fibers towards the preparation of nano-scaled cellulose is related to the ability of LPMOs to create nicking points on the fiber surface, thus facilitating fiber disruption and separation. The aim of this review is to describe the mode of action of LPMOs on cellulose fibers towards the isolation of nanostructures, thus highlighting their great potential for the production of nanocellulose as a novel value added product from lignocellulose.

The impact of reductants on the catalytic efficiency of a lytic polysaccharide monooxygenase and the special role of dehydroascorbic acid

Monocopper lytic polysaccharide monooxygenases (LPMOs) catalyse oxidative cleavage of glycosidic bonds in a reductant-dependent reaction. Recent studies indicate that LPMOs, rather than being O2 -dependent monooxygenases, are H2 O2 -dependent peroxygenases. Here, we describe SscLPMO10B, a novel LPMO from the phytopathogenic bacterium Streptomyces scabies and address links between this enzyme's catalytic rate and in situ hydrogen peroxide production in the presence of ascorbic acid, gallic acid and l-cysteine. Studies of Avicel degradation showed a clear correlation between the catalytic rate of SscLPMO10B and the rate of H2 O2 generation in the reaction mixture. We also assessed the impact of oxidised ascorbic acid, dehydroascorbic acid (DHA), on LPMO activity, since DHA, which is not considered a reductant, was recently reported to drive LPMO reactions. Kinetic studies, combined with NMR analysis, showed that DHA is unstable and converts into multiple derivatives, some of which are redox active and can fuel the LPMO reaction by reducing the active site copper and promoting H2 O2 production. These results show that the apparent monooxygenase activity observed in SscLPMO10B reactions without exogenously added H2 O2 reflects a peroxygenase reaction.

Characterization of a lytic polysaccharide monooxygenase from Aspergillus fumigatus shows functional variation among family AA11 fungal LPMOs

The discovery of oxidative cleavage of recalcitrant polysaccharides by lytic polysaccharide monooxygenases (LPMOs) has affected the study and industrial application of enzymatic biomass processing. Despite being widespread in fungi, LPMOs belonging to the auxiliary activity (AA) family AA11 have been understudied. While these LPMOs are considered chitin active, some family members have little or no activity toward chitin, and the only available crystal structure of an AA11 LPMO lacks features found in bacterial chitin-active AA10 LPMOs. Here, we report structural and functional characteristics of a single-domain AA11 LPMO from Aspergillus fumigatus, AfAA11A. The crystal structure shows a substrate-binding surface with features resembling those of known chitin-active LPMOs. Indeed, despite the absence of a carbohydrate-binding module, AfAA11A has considerable affinity for α-chitin and, more so, β-chitin. AfAA11A is active toward both these chitin allomorphs and enhances chitin degradation by an endoacting chitinase, in particular for α-chitin. The catalytic activity of AfAA11A on chitin increases when supplying reactions with hydrogen peroxide, showing that, like LPMOs from other families, AfAA11A has peroxygenase activity. These results show that, in stark contrast to the previously characterized AfAA11B from the same organism, AfAA11A likely plays a role in fungal chitin turnover. Thus, members of the hitherto rather enigmatic family of AA11 LPMOs show considerable structural and functional differences and may have multiple roles in fungal physiology.

Kinetic Characterization of a Putatively Chitin-Active LPMO Reveals a Preference for Soluble Substrates and Absence of Monooxygenase Activity

Enzymes known as lytic polysaccharide monooxygenases (LPMOs) are recognized as important contributors to aerobic enzymatic degradation of recalcitrant polysaccharides such as chitin and cellulose. LPMOs are remarkably abundant in nature, with some fungal species possessing more than 50 LPMO genes, and the biological implications of this diversity remain enigmatic. For example, chitin-active LPMOs have been encountered in biological niches where chitin conversion does not seem to take place. We have carried out an in-depth kinetic characterization of a putatively chitin-active LPMO from Aspergillus fumigatus (AfAA11B), which, as we show here, has multiple unusual properties, such as a low redox potential and high oxidase activity. Furthermore, AfAA11B is hardly active on chitin, while being very active on soluble oligomers of N-acetylglucosamine. In the presence of chitotetraose, the enzyme can withstand considerable amounts of H₂O₂, which it uses to efficiently and stoichiometrically convert this substrate. The unique properties of AfAA11B allowed experiments showing that it is a strict peroxygenase and does not catalyze a monooxygenase reaction. This study shows that nature uses LPMOs for breaking glycosidic bonds in non-polymeric substrates in reactions that depend on H₂O₂. The quest for the true substrates of these enzymes, possibly carbohydrates in the cell wall of the fungus or its competitors, will be of major interest.

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.

C-type cytochrome-initiated reduction of bacterial lytic polysaccharide monooxygenases

The release of glucose from lignocellulosic waste for subsequent fermentation into biofuels holds promise for securing humankind's future energy needs. The discovery of a set of copper-dependent enzymes known as lytic polysaccharide monooxygenases (LPMOs) has galvanised new research in this area. LPMOs act by oxidatively introducing chain breaks into cellulose and other polysaccharides, boosting the ability of cellulases to act on the substrate. Although several proteins have been implicated as electron sources in fungal LPMO biochemistry, no equivalent bacterial LPMO electron donors have been previously identified, although the proteins Cbp2D and E from Cellvibrio japonicus have been implicated as potential candidates. Here we analyse a small c-type cytochrome (CjX183) present in Cellvibrio japonicus Cbp2D, and show that it can initiate bacterial Cu^II/I LPMO reduction and also activate LPMO-catalyzed cellulose-degradation. In the absence of cellulose, CjX183-driven reduction of the LPMO results in less H₂O₂ production from O₂, and correspondingly less oxidative damage to the enzyme than when ascorbate is used as the reducing agent. Significantly, using CjX183 as the activator maintained similar cellulase boosting levels relative to the use of an equivalent amount of ascorbate. Our results therefore add further evidence to the impact that the choice of electron source can have on LPMO action. Furthermore, the study of Cbp2D and other similar proteins may yet reveal new insight into the redox processes governing polysaccharide degradation in bacteria.

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.

Genomic and transcriptomic analysis of the thermophilic lignocellulose-degrading fungus Thielavia terrestris LPH172

BACKGROUND

Biomass-degrading enzymes with improved activity and stability can increase substrate saccharification and make biorefineries economically feasible. Filamentous fungi are a rich source of carbohydrate-active enzymes (CAZymes) for biomass degradation. The newly isolated LPH172 strain of the thermophilic Ascomycete Thielavia terrestris has been shown to possess high xylanase and cellulase activities and tolerate low pH and high temperatures. Here, we aimed to illuminate the lignocellulose-degrading machinery and novel carbohydrate-active enzymes in LPH172 in detail.

RESULTS

We sequenced and analyzed the 36.6-Mb genome and transcriptome of LPH172 during growth on glucose, cellulose, rice straw, and beechwood xylan. 10,128 predicted genes were found in total, which included 411 CAZy domains. Compared to other fungi, auxiliary activity (AA) domains were particularly enriched. A higher GC content was found in coding sequences compared to the overall genome, as well as a high GC3 content, which is hypothesized to contribute to thermophilicity. Primarily auxiliary activity (AA) family 9 lytic polysaccharide monooxygenase (LPMO) and glycoside hydrolase (GH) family 7 glucanase encoding genes were upregulated when LPH172 was cultivated on cellulosic substrates. Conventional hemicellulose encoding genes (GH10, GH11 and various CEs), as well as AA9 LPMOs, were upregulated when LPH172 was cultivated on xylan. The observed co-expression and co-upregulation of genes encoding AA9 LPMOs, other AA CAZymes, and (hemi)cellulases point to a complex and nuanced degradation strategy.

CONCLUSIONS

Our analysis of the genome and transcriptome of T. terrestris LPH172 elucidates the enzyme arsenal that the fungus uses to degrade lignocellulosic substrates. The study provides the basis for future characterization of potential new enzymes for industrial biomass saccharification.

Recent Advances in Screening Methods for the Functional Investigation of Lytic Polysaccharide Monooxygenases

Lytic polysaccharide monooxygenase (LPMO) is a newly discovered and widely studied enzyme in recent years. These enzymes play a key role in the depolymerization of sugar-based biopolymers (including cellulose, hemicellulose, chitin and starch), and have a positive significance for biomass conversion. LPMO is a copper-dependent enzyme that can oxidize and cleave glycosidic bonds in cellulose and other polysaccharides. Their mechanism of action depends on the correct coordination of copper ions in the active site. There are still difficulties in the analysis of LPMO activity, which often requires multiple methods to be used in concert. In this review, we discussed various LPMO activity analysis methods reported so far, including mature mass spectrometry, chromatography, labeling, and indirect measurements, and summarized the advantages, disadvantages and applicability of different methods.

Unraveling the roles of the reductant and free copper ions in LPMO kinetics

BACKGROUND

Lytic polysaccharide monooxygenases (LPMOs) are monocopper enzymes that catalyze oxidative depolymerization of industrially relevant crystalline polysaccharides, such as cellulose, in a reaction that depends on an electron donor and O2 or H2O2. While it is well known that LPMOs can utilize a wide variety of electron donors, the variation in reported efficiencies of various LPMO-reductant combinations remains largely unexplained.

RESULTS

In this study, we describe a novel two-domain cellulose-active family AA10 LPMO from a marine actinomycete, which we have used to look more closely at the effects of the reductant and copper ions on the LPMO reaction. Our results show that ascorbate-driven LPMO reactions are extremely sensitive to very low amounts (micromolar concentrations) of free copper because reduction of free Cu(II) ions by ascorbic acid leads to formation of H2O2, which speeds up the LPMO reaction. In contrast, the use of gallic acid yields steady reactions that are almost insensitive to the presence of free copper ions. Various experiments, including dose-response studies with the enzyme, showed that under typically used reaction conditions, the rate of the reaction is limited by LPMO-independent formation of H2O2 resulting from oxidation of the reductant.

CONCLUSION

The strong impact of low amounts of free copper on LPMO reactions with ascorbic acid and O2, i.e. the most commonly used conditions when assessing LPMO activity, likely explains reported variations in LPMO rates. The observed differences between ascorbic acid and gallic acid show a way of making LPMO reactions less copper-dependent and illustrate that reductant effects on LPMO action need to be interpreted with great caution. In clean reactions, with minimized generation of H2O2, the (O2-driven) LPMO reaction is exceedingly slow, compared to the much faster peroxygenase reaction that occurs when adding H2O2.

Chimeric Cellobiose Dehydrogenases Reveal the Function of Cytochrome Domain Mobility for the Electron Transfer to Lytic Polysaccharide Monooxygenase

The natural function of cellobiose dehydrogenase (CDH) to donate electrons from its catalytic flavodehydrogenase (DH) domain via its cytochrome (CYT) domain to lytic polysaccharide monooxygenase (LPMO) is an example of a highly efficient extracellular electron transfer chain. To investigate the function of the CYT domain movement in the two occurring electron transfer steps, two CDHs from the ascomycete Neurospora crassa (NcCDHIIA and NcCDHIIB) and five chimeric CDH enzymes created by domain swapping were studied in combination with the fungus’ own LPMOs (NcLPMO9C and NcLPMO9F). Kinetic and electrochemical methods and hydrogen/deuterium exchange mass spectrometry were used to study the domain movement, interaction, and electron transfer kinetics. Molecular docking provided insights into the protein–protein interface, the orientation of domains, and binding energies. We find that the first, interdomain electron transfer step from the catalytic site in the DH domain to the CYT domain depends on steric and electrostatic interface complementarity and the length of the protein linker between both domains but not on the redox potential difference between the FAD and heme b cofactors. After CYT reduction, a conformational change of CDH from its closed state to an open state allows the second, interprotein electron transfer (IPET) step from CYT to LPMO to occur by direct interaction of the b-type heme and the type-2 copper center. Chimeric CDH enzymes favor the open state and achieve higher IPET rates by exposing the heme b cofactor to LPMO. The IPET, which is influenced by interface complementarity and the heme b redox potential, is very efficient with bimolecular rates between 2.9 × 10⁵ and 1.1 × 10⁶ M^–1 s^–1.

Purified cellobiose dehydrogenase of Termitomyces sp. OE147 fuels cellulose degradation resulting in the release of reducing sugars

Termitomyces sp. OE 147 is one of the active cellulose degraders in the ecosphere and produces large amount of cellobiose dehydrogenase (CDH) and β-glucosidases when cultivated on cellulose. In order to investigate its effect on cellulose, a highly purified preparation of CDH was obtained from the culture supernatant of the fungus cultivated on cellulose. A combination of ultrafiltration, ion-exchange and gel-filtration chromatography was used to purify CDH by ∼172-fold to a high specific activity of ∼324 U/mg protein on lactose which was used for routine measurement of enzyme activity. The enzyme displayed a pH optimum of 5.0 and stability between pH 5.0 and 8.0 with maximum catalytic efficiency (k_cat/K_m) of 397 mM^-1 s^-1 on cellobiose. Incubation of microcrystalline cellulose with the purified CDH led to production of reducing sugars which was accelerated by the addition of FeCl₃ during the early stages of incubation. A mass spectrometric analysis revealed fragmentation products of cellulose which were concluded to be cellodextrins, sugars, and corresponding aldonic acids suggesting that CDH can release reducing sugars in the absence of externally added lytic polysaccharide monooxygenases. Polymerized products of glucose were also detected at low intensity.

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.

Enzymatic processing of lignocellulosic biomass: principles, recent advances and perspectives

Efficient saccharification of lignocellulosic biomass requires concerted development of a pretreatment method, an enzyme cocktail and an enzymatic process, all of which are adapted to the feedstock. Recent years have shown great progress in most aspects of the overall process. In particular, increased insights into the contributions of a wide variety of cellulolytic and hemicellulolytic enzymes have improved the enzymatic processing step and brought down costs. Here, we review major pretreatment technologies and different enzyme process setups and present an in-depth discussion of the various enzyme types that are currently in use. We pay ample attention to the role of the recently discovered lytic polysaccharide monooxygenases (LPMOs), which have led to renewed interest in the role of redox enzyme systems in lignocellulose processing. Better understanding of the interplay between the various enzyme types, as they may occur in a commercial enzyme cocktail, is likely key to further process improvements.

A novel AA10 from Paenibacillus curdlanolyticus and its synergistic action on crystalline and complex polysaccharides

Lytic polysaccharide monooxygenases (LPMOs) play an important role in the degradation of complex polysaccharides in lignocellulosic biomass. In the present study, we characterized a modular LPMO (PcAA10A), consisting of a family 10 auxiliary activity of LPMO (AA10) catalytic domain, and non-catalytic domains including a family 5 carbohydrate-binding module, two fibronectin type-3 domains, and a family 3 carbohydrate-binding module from Paenibacillus curdlanolyticus B-6, which was expressed in a recombinant Escherichia coli. Comparison of activities between full-length PcAA10A and the catalytic domain polypeptide (PcAA10A_CD) indicates that the non-catalytic domains are important for the deconstruction of crystalline cellulose and complex polysaccharides contained in untreated lignocellulosic biomass. Interestingly, PcAA10A_CD acted not only on cellulose and chitin, but also on xylan, mannan, and xylan and cellulose contained in lignocellulosic biomass, which has not been reported for the AA10 family. Mutation of the key residues, Trp51 located at subsite - 2 and Phe171 located at subsite +2, in the substrate-binding site of PcAA10A_CD revealed that these residues are substantially involved in broad substrate specificity toward cellulose, xylan, and mannan, albeit with a low effect toward chitin. Furthermore, PcAA10A had a boosting effect on untreated corn hull degradation by P. curdlanolyticus B-6 endo-xylanase Xyn10D and Clostridium thermocellum endo-glucanase Cel9A. These results suggest that PcAA10A is a unique LPMO capable of cleaving and enhancing lignocellulosic biomass degradation, making it a good candidate for biotechnological applications. KEY POINTS: • PcAA10A is a novel modular LPMO family 10 from Paenibacillus curdlanolyticus. • PcAA10A showed broad substrate specificity on β-1,4 glycosidic linkage substrates. • Non-catalytic domains are important for degrading complex polysaccharides. • PcAA10A is a unique LPMO capable of enhancing lignocellulosic biomass degradation.

Kinetic analysis of amino acid radicals formed in H2O2-driven CuI LPMO reoxidation implicates dominant homolytic reactivity

Lytic polysaccharide monooxygenases (LPMOs) have been proposed to react with both [Formula: see text] and [Formula: see text] as cosubstrates. In this study, the [Formula: see text] reaction with reduced Hypocrea jecorina LPMO9A (Cu^I-HjLPMO9A) is demonstrated to be 1,000-fold faster than the [Formula: see text] reaction while producing the same oxidized oligosaccharide products. Analysis of the reactivity in the absence of polysaccharide substrate by stopped-flow absorption and rapid freeze-quench (RFQ) electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) yields two intermediates corresponding to neutral tyrosyl and tryptophanyl radicals that are formed along minor reaction pathways. The dominant reaction pathway is characterized by RFQ EPR and kinetic modeling to directly produce Cu^II-HjLPMO9A and indicates homolytic O-O cleavage. Both optical intermediates exhibit magnetic exchange coupling with the Cu^II sites reflecting facile electron transfer (ET) pathways, which may be protective against uncoupled turnover or provide an ET pathway to the active site with substrate bound. The reactivities of nonnative organic peroxide cosubstrates effectively exclude the possibility of a ping-pong mechanism.

Protein Conformational Change Is Essential for Reductive Activation of Lytic Polysaccharide Monooxygenase by Cellobiose Dehydrogenase

Large-scale protein domain dynamics and electron transfer are often associated. However, as protein motions span a broad range of time and length scales, it is often challenging to identify and thus link functionally relevant dynamic changes to electron transfer in proteins. It is hypothesized that large-scale domain motions direct electrons through a FAD and a heme b cofactor of the fungal cellobiose dehydrogenase (CDH) enzymes to the type-II copper center (T2Cu) of the polysaccharide-degrading lytic polysaccharide monooxygenases (LPMOs). However, as of yet, domain motions in CDH have not been linked formally to enzyme-catalyzed electron transfer reactions. The detailed structural features of CDH, which govern the functional conformational landscapes of the enzyme, have only been partially resolved. Here, we use a combination of pressure, viscosity, ionic strength, and temperature perturbation stopped-flow studies to probe the conformational landscape associated with the electron transfer reactions of CDH. Through the use of molecular dynamics simulations, potentiometry, and stopped-flow spectroscopy, we investigated how a conserved Tyr99 residue plays a key role in shaping the conformational landscapes for both the interdomain electron transfer reactions of CDH (from FAD to heme) and the delivery of electrons from the reduced heme cofactor to the LPMO T2Cu. Our studies show how motions gate the electron transfer within CDH and from CDH to LPMO and illustrate the conformational landscape for interdomain and interprotein electron transfer in this extracellular fungal electron transfer chain.

Industrial Use of Cell Wall Degrading Enzymes: The Fine Line Between Production Strategy and Economic Feasibility

Cell Wall Degrading Enzymes (CWDEs) are a heterogeneous group of enzymes including glycosyl-hydrolases, oxidoreductases, lyases, and esterases. Microbes with degrading activities toward plant cell wall polysaccharides are the most relevant source of CWDEs for industrial applications. These organisms secrete a wide array of CWDEs in amounts strictly necessary for their own sustenance, nonetheless the production of CWDEs from wild type microbes can be increased at large-scale by using optimized fermentation strategies. In the last decades, advances in genetic engineering allowed the expression of recombinant CWDEs also in lab-domesticated organisms such as E. coli, yeasts and plants, dramatically increasing the available options for the large-scale production of CWDEs. The optimization of a CWDE-producing biofactory is a hard challenge that biotechnologists tackle by testing different expression strategies and expression-hosts. Although both the yield and production costs are critical factors to produce biomolecules at industrial scale, these parameters are often disregarded in basic research. This review presents the main characteristics and industrial applications of CWDEs directed toward the cell wall of plants, bacteria, fungi and microalgae. Different biofactories for CWDE expression are compared in order to highlight strengths and weaknesses of each production system and how these aspects impact the final enzyme cost and, consequently, the economic feasibility of using CWDEs for industrial applications.

Engineering of industrially important microorganisms for assimilation of cellulosic biomass: towards consolidated bioprocessing

Conversion of cellulosic biomass (non-edible plant material) to products such as chemical feedstocks and liquid fuels is a major goal of industrial biotechnology and an essential component of plans to move from an economy based on fossil carbon to one based on renewable materials. Many microorganisms can effectively degrade cellulosic biomass, but attempts to engineer this ability into industrially useful strains have met with limited success, suggesting an incomplete understanding of the process. The recent discovery and continuing study of enzymes involved in oxidative depolymerisation, as well as more detailed study of natural cellulose degradation processes, may offer a way forward.

Functional characterization of cellulose-degrading AA9 lytic polysaccharide monooxygenases and their potential exploitation

Cellulose-degrading auxiliary activity family 9 (AA9) lytic polysaccharide monooxygenases (LPMOs) are known to be widely distributed among filamentous fungi and participate in the degradation of lignocellulose via the oxidative cleavage of celluloses, cello-oligosaccharides, or hemicelluloses. AA9 LPMOs have been reported to have extensive interactions with not only cellulases but also oxidases. The addition of AA9 LPMOs can greatly reduce the amount of cellulase needed for saccharification and increase the yield of glucose. The discovery of AA9 LPMOs has greatly changed our understanding of how fungi degrade cellulose. In this review, apart from summarizing the recent discoveries related to their catalytic reaction, functional diversity, and practical applications, the stability, expression system, and protein engineering of AA9 LPMOs are reviewed for the first time. This review may provide a reference value to further broaden the substrate range of AA9 LPMOs, expand the scope of their practical applications, and realize their customization for industrial utilization.Key Points• The stability and expression system of AA9 LPMOs are reviewed for the first time.• The protein engineering of AA9 LPMOs is systematically summarized for the first time.• The latest research results on the catalytic mechanism of AA9 LPMOs are summarized.• The application of AA9 LPMOs and their relationship with other enzymes are reviewed.

Controlled depolymerization of cellulose by light-driven lytic polysaccharide oxygenases

Lytic polysaccharide (mono)oxygenases (LPMOs) perform oxidative cleavage of polysaccharides, and are key enzymes in biomass processing and the global carbon cycle. It has been shown that LPMO reactions may be driven by light, using photosynthetic pigments or photocatalysts, but the mechanism behind this highly attractive catalytic route remains unknown. Here, prompted by the discovery that LPMOs catalyze a peroxygenase reaction more efficiently than a monooxygenase reaction, we revisit these light-driven systems, using an LPMO from Streptomyces coelicolor (ScAA10C) as model cellulolytic enzyme. By using coupled enzymatic assays, we show that H₂O₂ is produced and necessary for efficient light-driven activity of ScAA10C. Importantly, this activity is achieved without addition of reducing agents and proportional to the light intensity. Overall, the results highlight the importance of controlling fluxes of reactive oxygen species in LPMO reactions and demonstrate the feasibility of light-driven, tunable enzymatic peroxygenation to degrade recalcitrant polysaccharides.

Structural Dynamics of Lytic Polysaccharide Monooxygenase during Catalysis

Lytic polysaccharide monooxygenases (LPMOs) are industrially important oxidoreductases employed in lignocellulose saccharification. Using advanced time-resolved mass spectrometric techniques, we elucidated the structural determinants for substrate-mediated stabilization of the fungal LPMO9C from Neurosporacrassa during catalysis. LPMOs require a reduction in the active-site copper for catalytic activity. We show that copper reduction in NcLPMO9C leads to structural rearrangements and compaction around the active site. However, longer exposure to the reducing agent ascorbic acid also initiated an uncoupling reaction of the bound oxygen species, leading to oxidative damage, partial unfolding, and even fragmentation of NcLPMO9C. Interestingly, no changes in the hydrogen/deuterium exchange rate were detected upon incubation of oxidized or reduced LPMO with crystalline cellulose, indicating that the LPMO-substrate interactions are mainly side-chain mediated and neither affect intraprotein hydrogen bonding nor induce significant shielding of the protein surface. On the other hand, we observed a protective effect of the substrate, which slowed down the autooxidative damage induced by the uncoupling reaction. These observations further complement the picture of structural changes during LPMO catalysis.

Improved spectrophotometric assay for lytic polysaccharide monooxygenase

BACKGROUND

The availability of a sensitive and robust activity assay is a prerequisite for efficient enzyme production, purification, and characterization. Here we report on a spectrophotometric assay for lytic polysaccharide monooxygenase (LPMO), which is an advancement of the previously published 2,6-dimethoxyphenol (2,6-DMP)-based LPMO assay. The new assay is based on hydrocoerulignone as substrate and hydrogen peroxide as cosubstrate and aims toward a higher sensitivity at acidic pH and a more reliable detection of LPMO in complex matrices like culture media.

RESULTS

An LPMO activity assay following the colorimetric oxidation of hydrocoerulignone to coerulignone was developed. This peroxidase activity of LPMO in the presence of hydrogen peroxide can be detected in various buffers between pH 4-8. By reducing the substrate and cosubstrate concentration, the assay has been optimized for minimal autoxidation and enzyme deactivation while maintaining sensitivity. Finally, the optimized and validated LPMO assay was used to follow the recombinant expression of an LPMO in Pichia pastoris and to screen for interfering substances in fermentation media suppressing the assayed reaction.

CONCLUSIONS

The biphenol hydrocoerulignone is a better substrate for LPMO than the monophenol 2,6-DMP, because of a ~ 30 times lower apparent K _M value and a 160 mV lower oxidation potential. This greatly increases the measured LPMO activity when using hydrocoerulignone instead of 2,6-DMP under otherwise similar assay conditions. The improved activity allows the adaptation of the LPMO assay toward a higher sensitivity, different buffers and pH values, more stable assay conditions or to overcome low concentrations of inhibiting substances. The developed assay protocol and optimization guidelines increase the adaptability and applicability of the hydrocoerulignone assay for the production, purification, and characterization of LPMOs.

Polysaccharide oxidation by lytic polysaccharide monooxygenase is enhanced by engineered cellobiose dehydrogenase

The catalytic function of lytic polysaccharide monooxygenases (LPMOs) to cleave and decrystallize recalcitrant polysaccharides put these enzymes in the spotlight of fundamental and applied research. Here we demonstrate that the demand of LPMO for an electron donor and an oxygen species as cosubstrate can be fulfilled by a single auxiliary enzyme: an engineered fungal cellobiose dehydrogenase (CDH) with increased oxidase activity. The engineered CDH was about 30 times more efficient in driving the LPMO reaction due to its 27 time increased production of H₂O₂ acting as a cosubstrate for LPMO. Transient kinetic measurements confirmed that intra‐ and intermolecular electron transfer rates of the engineered CDH were similar to the wild‐type CDH, meaning that the mutations had not compromised CDH’s role as an electron donor. These results support the notion of H₂O₂‐driven LPMO activity and shed new light on the role of CDH in activating LPMOs. Importantly, the results also demonstrate that the use of the engineered CDH results in fast and steady LPMO reactions with CDH‐generated H₂O₂ as a cosubstrate, which may provide new opportunities to employ LPMOs in biomass hydrolysis to generate fuels and chemicals.

Cellobiose dehydrogenase: Bioelectrochemical insights and applications

Cellobiose dehydrogenase (CDH) is a flavocytochrome with a history of bioelectrochemical research dating back to 1992. During the years, it has been shown to be capable of mediated electron transfer (MET) and direct electron transfer (DET) to a variety of electrodes. This versatility of CDH originates from the separation of the catalytic flavodehydrogenase domain and the electron transferring cytochrome domain. This uncoupling of the catalytic reaction from the electron transfer process allows the application of CDH on many different electrode materials and surfaces, where it shows robust DET. Recent X-ray diffraction and small angle scattering studies provided insights into the structure of CDH and its domain mobility, which can change between a closed-state and an open-state conformation. This structural information verifies the electron transfer mechanism of CDH that was initially established by bioelectrochemical methods. A combination of DET and MET experiments has been used to investigate the catalytic mechanism and the electron transfer process of CDH and to deduce a protein structure comprising of mobile domains. Even more, electrochemical methods have been used to study the redox potentials of the FAD and the haem b cofactors of CDH or the electron transfer rates. These electrochemical experiments, their results and the application of the characterised CDHs in biosensors, biofuel cells and biosupercapacitors are combined with biochemical and structural data to provide a thorough overview on CDH as versatile bioelectrocatalyst.

Methylation of the N-terminal histidine protects a lytic polysaccharide monooxygenase from auto-oxidative inactivation

The catalytically crucial N-terminal histidine (His1) of fungal lytic polysaccharide monooxygenases (LPMOs) is post-translationally modified to carry a methylation. The functional role of this methylation remains unknown. We have carried out an in-depth functional comparison of two variants of a family AA9 LPMO from Thermoascus aurantiacus (TaLPMO9A), one with, and one without the methylation on His1. Various activity assays showed that the two enzyme variants are identical in terms of substrate preferences, cleavage specificities and the ability to activate molecular oxygen. During the course of this work, new functional features of TaLPMO9A were discovered, in particular the ability to cleave xyloglucan, and these features were identical for both variants. Using a variety of techniques, we further found that methylation has minimal effects on the pK_a of His1, the affinity for copper and the redox potential of bound copper. The two LPMOs did, however, show clear differences in their resistance against oxidative damage. Studies with added hydrogen peroxide confirmed recent claims that low concentrations of H₂ O₂ boost LPMO activity, whereas excess H₂ O₂ leads to LPMO inactivation. The methylated variant of TaLPMO9A, produced in Aspergillus oryzae, was more resistant to excess H₂ O₂ and showed better process performance when using conditions that promote generation of reactive-oxygen species. LPMOs need to protect themselves from reactive oxygen species generated in their active sites and this study shows that methylation of the fully conserved N-terminal histidine provides such protection.

pH-Dependent Relationship between Catalytic Activity and Hydrogen Peroxide Production Shown via Characterization of a Lytic Polysaccharide Monooxygenase from Gloeophyllum trabeum

Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that perform oxidative cleavage of recalcitrant polysaccharides. We have purified and characterized a recombinant family AA9 LPMO, LPMO9B, from Gloeophyllum trabeum (GtLPMO9B) which is active on both cellulose and xyloglucan. Activity of the enzyme was tested in the presence of three different reductants: ascorbic acid, gallic acid, and 2,3-dihydroxybenzoic acid (2,3-DHBA). Under standard aerobic conditions typically used in LPMO experiments, the first two reductants could drive LPMO catalysis whereas 2,3-DHBA could not. In agreement with the recent discovery that H₂O₂ can drive LPMO catalysis, we show that gradual addition of H₂O₂ allowed LPMO activity at very low, substoichiometric (relative to products formed) reductant concentrations. Most importantly, we found that while 2,3-DHBA is not capable of driving the LPMO reaction under standard aerobic conditions, it can do so in the presence of externally added H₂O₂ At alkaline pH, 2,3-DHBA is able to drive the LPMO reaction without externally added H₂O₂, and this ability overlaps entirely the endogenous generation of H₂O₂ by GtLPMO9B-catalyzed oxidation of 2,3-DHBA. These findings support the notion that H₂O₂ is a cosubstrate of LPMOs and provide insight into how LPMO reactions depend on, and may be controlled by, the choice of pH and reductant.IMPORTANCE Lytic polysaccharide monooxygenases promote enzymatic depolymerization of lignocellulosic materials by microorganisms due to their ability to oxidatively cleave recalcitrant polysaccharides. The properties of these copper-dependent enzymes are currently of high scientific and industrial interest. We describe a previously uncharacterized fungal LPMO and show how reductants, which are needed to prime the LPMO by reducing Cu(II) to Cu(I) and to supply electrons during catalysis, affect enzyme efficiency and stability. The results support claims that H₂O₂ is a natural cosubstrate for LPMOs by demonstrating that when certain reductants are used, catalysis can be driven only by H₂O₂ and not by O₂ Furthermore, we show how auto-inactivation resulting from endogenous generation of H₂O₂ in the LPMO-reductant system may be prevented. Finally, we identified a reductant that leads to enzyme activation without any endogenous H₂O₂ generation, allowing for improved control of LPMO reactivity and providing a valuable tool for future LPMO research.

On the functional characterization of lytic polysaccharide monooxygenases (LPMOs)

Lytic polysaccharide monooxygenases (LPMOs) are abundant in nature and best known for their role in the enzymatic conversion of recalcitrant polysaccharides such as chitin and cellulose. LPMO activity requires an oxygen co-substrate, which was originally thought to be O₂, but which may also be H₂O₂. Functional characterization of LPMOs is not straightforward because typical reaction mixtures will promote side reactions, including auto-catalytic inactivation of the enzyme. For example, despite some recent progress, there is still limited insight into the kinetics of the LPMO reaction. Recent discoveries concerning the role of H₂O₂ in LPMO catalysis further complicate the picture. Here, we review commonly used methods for characterizing LPMOs, with focus on benefits and potential pitfalls, rather than on technical details. We conclude by pointing at a few key problems and potential misconceptions that should be taken into account when interpreting existing data and planning future experiments.

Chitin-Active Lytic Polysaccharide Monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that catalyze the cleavage of 1,4-glycosidic bonds various plant cell wall polysaccharides and chitin. In contrast to glycoside hydrolases, LPMOs are active on the crystalline regions of polysaccharides and thus synergize with hydrolytic enzymes. This synergism leads to an overall increase in the biomass-degradation activity of enzyme mixtures. Chitin-active LPMOs were discovered in 2010 and are currently classified in families AA10, AA11, and AA15 of the Carbohydrate-Active enZYmes database, which include LPMOs from bacteria, fungi, insects, and viruses. LPMOs have become important enzymes both industrially and scientifically and, in this chapter, we provide a brief introduction to chitin-active LPMOs including a summary of the 20+ chitin-active LPMOs that have been characterized so far. Then, we describe their structural features, catalytic mechanism, and appended carbohydrate modules. Finally, we show how chitin-active LPMOs can be used to perform chemo-enzymatic modification of chitin substrates.

Thermal unfolding and refolding of a lytic polysaccharide monooxygenase fromThermoascus aurantiacus

TaLPMO9A regains its catalytic power after a thermal unfolding and refolding cycle.

Kinetic insights into the role of the reductant in H2O2-driven degradation of chitin by a bacterial lytic polysaccharide monooxygenase

Lytic polysaccharide monooxygenases (LPMOs) are monocopper enzymes that catalyze oxidative cleavage of glycosidic bonds in polysaccharides in the presence of an external electron donor (reductant). In the classical O₂-driven monooxygenase reaction, the reductant is needed in stoichiometric amounts. In a recently discovered, more efficient H₂O₂-driven reaction, the reductant would be needed only for the initial reduction (priming) of the LPMO to its catalytically active Cu(I) form. However, the influence of the reductant on reducing the LPMO or on H₂O₂ production in the reaction remains undefined. Here, we conducted a detailed kinetic characterization to investigate how the reductant affects H₂O₂-driven degradation of ¹⁴C-labeled chitin by a bacterial LPMO, SmLPMO10A (formerly CBP21). Sensitive detection of ¹⁴C-labeled products and careful experimental set-ups enabled discrimination between the effects of the reductant on LPMO priming and other effects, in particular enzyme-independent production of H₂O₂ through reactions with O₂ When supplied with H₂O₂, SmLPMO10A catalyzed 18 oxidative cleavages per molecule of ascorbic acid, suggesting a "priming reduction" reaction. The dependence of initial rates of chitin degradation on reductant concentration followed hyperbolic saturation kinetics, and differences between the reductants were manifested in large variations in their half-saturating concentrations (K _mR ^app). Theoretical analyses revealed that K _mR ^app decreases with a decreasing rate of polysaccharide-independent LPMO reoxidation (by either O₂ or H₂O₂). We conclude that the efficiency of LPMO priming depends on the relative contributions of reductant reactivity, on the LPMO's polysaccharide monooxygenase/peroxygenase and reductant oxidase/peroxidase activities, and on reaction conditions, such as O₂, H₂O₂, and polysaccharide concentrations.

Oxidoreductases and Reactive Oxygen Species in Conversion of Lignocellulosic Biomass

Biomass constitutes an appealing alternative to fossil resources for the production of materials and energy. The abundance and attractiveness of vegetal biomass come along with challenges pertaining to the intricacy of its structure, evolved during billions of years to face and resist abiotic and biotic attacks. To achieve the daunting goal of plant cell wall decomposition, microorganisms have developed many (enzymatic) strategies, from which we seek inspiration to develop biotechnological processes. A major breakthrough in the field has been the discovery of enzymes today known as lytic polysaccharide monooxygenases (LPMOs), which, by catalyzing the oxidative cleavage of recalcitrant polysaccharides, allow canonical hydrolytic enzymes to depolymerize the biomass more efficiently. Very recently, it has been shown that LPMOs are not classical monooxygenases in that they can also use hydrogen peroxide (H2O2) as an oxidant. This discovery calls for a revision of our understanding of how lignocellulolytic enzymes are connected since H2O2 is produced and used by several of them. The first part of this review is dedicated to the LPMO paradigm, describing knowns, unknowns, and uncertainties. We then present different lignocellulolytic redox systems, enzymatic or not, that depend on fluxes of reactive oxygen species (ROS). Based on an assessment of these putatively interconnected systems, we suggest that fine-tuning of H2O2 levels and proximity between sites of H2O2 production and consumption are important for fungal biomass conversion. In the last part of this review, we discuss how our evolving understanding of redox processes involved in biomass depolymerization may translate into industrial applications.

Recent insights into lytic polysaccharide monooxygenases (LPMOs)

Lytic polysaccharide monooxygenases (LPMOs) are copper enzymes discovered within the last 10 years. By degrading recalcitrant substrates oxidatively, these enzymes are major contributors to the recycling of carbon in nature and are being used in the biorefinery industry. Recently, two new families of LPMOs have been defined and structurally characterized, AA14 and AA15, sharing many of previously found structural features. However, unlike most LPMOs to date, AA14 degrades xylan in the context of complex substrates, while AA15 is particularly interesting because they expand the presence of LPMOs from the predominantly microbial to the animal kingdom. The first two neutron crystallography structures have been determined, which, together with high-resolution room temperature X-ray structures, have putatively identified oxygen species at or near the active site of LPMOs. Many recent computational and experimental studies have also investigated the mechanism of action and substrate-binding mode of LPMOs. Perhaps, the most significant recent advance is the increasing structural and biochemical evidence, suggesting that LPMOs follow different mechanistic pathways with different substrates, co-substrates and reductants, by behaving as monooxygenases or peroxygenases with molecular oxygen or hydrogen peroxide as a co-substrate, respectively.

Leite AE, Costa-Filho AJ, Kumagai PS, Prade RA, Polikarpov I, Eijsink VGH. Functional characterization of a lytic polysaccharide monooxygenase from the thermophilic fungus Myceliophthora thermophila

Thermophilic fungi are a promising source of thermostable enzymes able to hydrolytically or oxidatively degrade plant cell wall components. Among these enzymes are lytic polysaccharide monooxygenases (LPMOs), enzymes capable of enhancing biomass hydrolysis through an oxidative mechanism. Myceliophthora thermophila (synonym Sporotrichum thermophile), an Ascomycete fungus, expresses and secretes over a dozen different LPMOs. In this study, we report the overexpression and biochemical study of a previously uncharacterized LPMO (MtLPMO9J) from M. thermophila M77 in Aspergillus nidulans. MtLPMO9J is a single-domain LPMO and has 63% sequence similarity with the catalytic domain of NcLPMO9C from Neurospora crassa. Biochemical characterization of MtLPMO9J revealed that it performs C4-oxidation and is active against cellulose, soluble cello-oligosaccharides and xyloglucan. Moreover, biophysical studies showed that MtLPMO9J is structurally stable at pH above 5 and at temperatures up to 50°C. Importantly, LC-MS analysis of the peptides after tryptic digestion of the recombinantly produced protein revealed not only the correct processing of the signal peptide and methylation of the N-terminal histidine, but also partial autoxidation of the catalytic center. This shows that redox conditions need to be controlled, not only during LPMO reactions but also during protein production, to protect LPMOs from oxidative damage.

The Pyrroloquinoline-Quinone-Dependent Pyranose Dehydrogenase from Coprinopsis cinerea Drives Lytic Polysaccharide Monooxygenase Action

Fungi secrete a set of glycoside hydrolases and oxidoreductases, including lytic polysaccharide monooxygenases (LPMOs), for the degradation of plant polysaccharides. LPMOs catalyze the oxidative cleavage of glycosidic bonds after activation by an external electron donor. So far, only flavin-dependent oxidoreductases (from the auxiliary activity [AA] family AA3) have been shown to activate LPMOs. Here, we present LPMO activation by a pyrroloquinoline-quinone (PQQ)-dependent pyranose dehydrogenase (PDH) from Coprinopsis cinerea, CcPDH, the founding member of the recently discovered auxiliary activity family AA12. CcPDH contains a C-terminal family 1 carbohydrate binding module (CBM1), an N-terminal family AA8 cytochrome domain, and a central AA12 dehydrogenase domain. We have studied the ability of full-length CcPDH and its truncated variants to drive catalysis by two Neurospora crassa LPMOs. The results show that CcPDH indeed can activate the C-1-oxidizing N. crassa LPMO 9F (NcLPMO9F) and the C-4-oxidizing Neurospora crassa LPMO 9C (NcLPMO9C), that this activation depends on the cytochrome domain, and that the dehydrogenase and the LPMO reactions are strongly coupled. The two tested CcPDH-LPMO systems showed quite different efficiencies, and this difference disappeared upon the addition of free PQQ acting as a diphenol/quinone redox mediator, showing that LPMOs differ when it comes to their direct interactions with the cytochrome domain. Surprisingly, removal of the CBM domain from CcPDH had a considerable negative impact on the efficiency of the CcPDH-LPMO systems, suggesting that electron transfer in the vicinity of the substrate is beneficial. CcPDH does not oxidize cello-oligosaccharides, which makes this enzyme a useful tool for studying cellulose-oxidizing LPMOs.IMPORTANCE Lytic polysaccharide monooxygenases (LPMOs) are currently receiving increasing attention because of their importance in degrading recalcitrant polysaccharides and their potential roles in biological processes, such as bacterial virulence. LPMO action requires an external electron donor, and fungi growing on biomass secrete various so-called glucose-methanol-choline (GMC) oxidoreductases, including cellobiose dehydrogenase, which can donate electrons to LPMOs. This paper describes how an enzyme not belonging to the GMC oxidoreductase family, CcPDH, can activate LPMOs, and it provides new insights into the activation process by (i) describing the roles of individual CcPDH domains (a dehydrogenase, a cytochrome, and a carbohydrate-binding domain), (ii) showing that the PDH and LPMO enzyme reactions are strongly coupled, (iii) demonstrating that LPMOs differ in terms of their efficiencies of activation by the same activator, and (iv) providing indications that electron transferring close to the substrate surface is beneficial for the overall efficiency of the CcPDH-LPMO system.