Enhanced in situ H2O2 production explains synergy between an LPMO with a cellulose-binding domain and a single-domain LPMO

Functional variation among LPMOs revealed by the inhibitory effects of cyanide and buffer ions

Enzymes known as lytic polysaccharide monooxygenases (LPMOs) are mono-copper polysaccharide-degrading peroxygenases that engage in several on- and off-pathway redox reactions involving O2 and H2O2. Herein, we show that the known metalloenzyme inhibitor cyanide inhibits reductive activation of LPMOs by binding to the LPMO-Cu(II) state and that the degree of inhibition depends on the concentrations of the polysaccharide substrate, the reductant and H2O2. Importantly, this analysis revealed differences between fungal NcAA9C and bacterial SmAA10A, which have different secondary copper coordination spheres. These differences were also highlighted by the observation that phosphate, a commonly used buffer ion, strongly inhibits NcAA9C while not affecting reactions with SmAA10A. The results provide insight into LPMO inhibition and catalysis and highlight pitfalls in the analysis thereof.

Plasma-Activated Water Boosts the Activity of Lytic Polysaccharide Monooxygenase and the Conversion of Chitin by a Chitinolytic Enzyme Cocktail

The plasma-activated water (PAW) containing numerous reactive species can facilitate chitin degradation. Given the intricate interplay between PAW treatment and the diverse activities of chitinolytic enzymes, further investigation is imperative for enhancing the chitin bioconversion efficiency. This study revealed that PAW-treated chitin exhibited improved degradability toward BtLPMO10A, endochitinases OfChtI, OfChtII-B4C1, and exochitinase OfChi-h. Furthermore, H2O2 in PAW boosted BtLPMO10A, whereas the soluble constituents in PAW generated during chitin pretreatment inhibited OfChi-h. Notably, this inhibition effect toward OfChi-h can be mitigated by the addition of β-N-acetylhexosaminidase. In the end, the synergy among the chitinolytic enzyme was also promoted by PAW pretreatment. On this as a basis, a chitin degradation strategy using a combination of PAW treatment and an enzyme cocktail was applied to degrade chitin, achieving a chitin conversion yield of 97% within 2 h. This strategy could also be applied to the degradation of other polysaccharides, such as cellulose.

Theoretical study of the in situ formation of H2O2 by lytic polysaccharide monooxygenases: the reaction mechanism depends on the type of reductant

Lytic polysaccharide monooxygenases (LPMOs) are a unique group of monocopper enzymes that exhibit remarkable ability to catalyze the oxidative cleavage of recalcitrant carbohydrate substrates, such as cellulose and chitin, by utilizing O2 or H2O2 as the oxygen source. One of the key challenges in understanding the catalytic mechanism of LPMOs lies in deciphering how they activate dioxygen using diverse reductants. To shed light on this intricate process, we conducted in-depth investigations using quantum mechanical/molecular mechanical (QM/MM) metadynamics simulations, molecular dynamics (MD) simulations, and density functional theory (DFT) calculations. Specifically, our study focuses on elucidating the in situ formation mechanism of H2O2 by LPMOs in the presence of cellobiose dehydrogenase (CDH), a proposed natural reductant of LPMOs. Our findings reveal a proton-coupled electron transfer (PCET) process in generating the Cu(ii)-hydroperoxide intermediate from the Cu(ii)-superoxide intermediate. Subsequently, a direct proton transfer to the proximal oxygen of Cu(ii)-hydroperoxide results in the formation of H2O2 and LPMO-Cu(ii). Notably, this mechanism significantly differs from the LPMO/ascorbate system, where two hydrogen atom transfer reactions are responsible for generating H2O2 and LPMO-Cu(i). Based on our simulations, we propose a catalytic mechanism of LPMO in the presence of CDH and the polysaccharide substrate, which involves competitive binding of the substrate and CDH to the reduced LPMOs. While the CDH-bound LPMOs can activate dioxygen to generate H2O2, the substrate-bound LPMOs can employ the H2O2 generated from the LPMO/CDH system to perform the peroxygenase reactions of the polysaccharide substrate. Our work not only provides valuable insights into the reductant-dependent mechanisms of O2 activation in LPMOs but also holds implications for understanding the functions of these enzymes in their natural environment.

A modular enzyme with combined hemicellulose-removing and LPMO activity increases cellulose accessibility in softwood

Because of the association with other complex polysaccharides, extracting and utilizing cellulose from lignocellulosic materials requires the combined action of a broad range of carbohydrate-active enzymes, including multiple glycoside hydrolases (GHs) and lytic polysaccharide monooxygenases (LPMOs). The interplay between these enzymes and the way in which Nature orchestrates their co-existence and combined action are topics of great scientific and industrial interest. To gain more insight into these issues, we have studied the lignocellulose-degrading abilities of an enzyme from Caldibacillus cellulovorans (CcLPMO10-Man5), comprising an LPMO domain, a GH5 mannanase domain and two family 3 carbohydrate-binding modules (CBM3). Using a natural softwood substrate, we show that this enzyme promotes cellulase activity, i.e., saccharification of cellulose, both by removing mannan covering the cellulose and by oxidatively breaking up the cellulose structure. Synergy with CcLPMO10-Man5 was most pronounced for two tested cellobiohydrolases, whereas effects were smaller for a tested endoglucanase, which is in line with the notion that cellobiohydrolases and LPMOs attack the same crystalline regions of the cellulose, whereas endoglucanases attack semi-crystalline and amorphous regions. Importantly, the LPMO domain of CcLPMO10-Man5 is incapable of accessing the softwood cellulose in absence of the mannanase domain. Considering that LPMOs not bound to a substrate are sensitive to autocatalytic inactivation, this intramolecular synergy provides a perfect rationale for the evolution of modular enzymes such as CcLPMO10-Man5. The intramolecular coupling of the LPMO with a mannanase and two CBMs ensures that the LPMO is directed to areas where mannans are removed and cellulose thus becomes available.

Calcium-binding site in AA10 LPMO from Vibrio cholerae suggests modulating effects during environmental survival and infection

Despite major efforts toward its eradication, cholera remains a major health threat and economic burden in many low- and middle-income countries. Between outbreaks, the bacterium responsible for the disease, Vibrio cholerae, survives in aquatic environmental reservoirs, where it commonly forms biofilms, for example, on zooplankton. N-acetyl glucosamine-binding protein A (GbpA) is an adhesin that binds to the chitinaceous surface of zooplankton and breaks its dense crystalline packing thanks to its lytic polysaccharide monooxygenase (LPMO) activity, which provides V. cholerae with nutrients. In addition, GbpA is an important colonization factor associated with bacterial pathogenicity, allowing the binding to mucins in the host intestine. Here, we report the discovery of a cation-binding site in proximity of the GbpA active site, which allows Ca2+, Mg2+, or K+ binding close to its carbohydrate-binding surface. In addition to the X-ray crystal structures of cation-LPMO complexes (to 1.5 Å resolution), we explored how the presence of ions affects the stability and activity of the protein. Calcium and magnesium ions were found to bind to GbpA specifically, with calcium ions - abundant in natural sources of chitin - having the strongest effect on protein stability. When the cation-binding site was rendered non-functional, a decrease in activity was observed, highlighting the importance of the structural elements stabilized by calcium. Our findings suggest a cation-binding site specific to GbpA and related LPMOs that may fine-tune binding and activity for its substrates during environmental survival and host infection.

pH-mediated manipulation of the histidine brace in LPMOs and generation of a tri-anionic variant, investigated by EPR, ENDOR, ESEEM and HYSCORE spectroscopy

Lytic Polysaccharide Monooxygenases (LPMOs) catalyze the oxidative depolymerization of polysaccharides at a monocopper active site, that is coordinated by the so-called histidine brace. In the past, this motif has sparked considerable interest, mostly due to its ability to generate and stabilize highly oxidizing intermediates during catalysis. We used a variety of advanced EPR techniques, including Electron Nuclear Double Resonance (ENDOR), Electron Spin Echo Envelope Modulation (ESEEM) and Hyperfine Sublevel Correlation (HYSCORE) spectroscopy in combination with isotopic labelling (15N, 2H) to characterize the active site of the bacterial LPMO SmAA10A over a wide pH range (pH 4.0-pH 12.5). At elevated pH values, several ligand modifications are observed, including changes in the H x O ligand coordination, but also regarding the protonation state of the histidine brace. At pH > 11.5, the deprotonation of the two remote nitrogen nuclei of the imidazole moieties and of the terminal amine is observed. These deprotonations are associated with major electronic changes, including increased σ-donor capabilities of the imidazolates and an overall reduced interaction of the deprotonated amine function. This observation highlights a potentially more significant role of the imidazole ligands, particularly for the stabilization of potent oxidants during turnover. The presented study demonstrates the application of advanced EPR techniques for a thorough characterization of the active site in LPMOs, which ultimately sets a foundation for and affords an outlook on future applications characterizing reaction intermediates.

Design of Plastic Binding Lytic Polysaccharide Monooxygenases via Modular Engineering

The worldwide accumulation of plastic waste in the environment, along with its lifespan of hundreds of years, represents a serious threat to ecosystems. Enzymatic recycling of plastic waste offers a promising solution, but the high chemical inertness and hydrophobicity of plastics pose several challenges to enzymes. In nature, lytic polysaccharide monooxygenases (LPMOs) can act at the surface of recalcitrant biopolymers, taking advantage of their solvent-exposed active sites and appended carbohydrate-binding modules (CBMs). LPMOs can disrupt the densely packed chains of polysaccharides (e.g., cellulose) by the oxidation of C-H bonds. Given the similarities between these natural and artificial polymers, we aimed here at promoting plastic-binding properties to LPMOs, by swapping their CBM with three natural, surface-active accessory modules displaying different amphipathic properties. The polymer binding capacity of the resulting LPMO chimeras was assessed on a library of synthetic polymers, including polyester, polyamide, and polyolefin substrates. We demonstrated that the plastic binding properties of these engineered LPMOs are polymer-dependent and can be tuned by playing on the nature of the accessory module and reaction conditions. Remarkably, we gained full binding for some chimera LPMOs with striking results for polyhydroxyalkanoates (PHA). In the long term perspective of harnessing the unique copper chemistry of LPMOs to degrade plastics, we also provided the first evidence of LPMO-dependent modification of the PHA polymer, as supported by enzyme assays, gel permeation chromatography, and scanning electron microscopy. Altogether, our study provides the first roadmap for engineering plastic-binding ability in LPMOs, constituting a crucial first step on the evolutionary path toward efficient interfacial catalysis of plastic-active enzymes.

The Effect of CBM1 and Linker on the Oxidase, Peroxidase and Monooxygenase Activities of AA9 LPMOs: Insight into Their Correlation with the Nature of Reductants and Crystallinity of Celluloses

This study explores the effect of carbohydrate-binding module 1 (CBM1) and the linker on the function of auxiliary activity 9 (AA9) lytic polysaccharide monooxygenases (LPMOs), with a particular focus on monooxygenase activity, using different crystallinity celluloses and electron donors. The tested C1/C4-oxidizing AA9 LPMOs exhibited higher oxidase and peroxidase activities compared to those of the C4-oxidizing AA9 LPMOs. While the presence of CBM1 promoted cellulose-binding affinity, it reduced the oxidase activity of modular AA9 LPMOs. The effect of CBM1 on peroxidase activity was variable and enzyme-specific. Its influence on monooxygenase activity was linked to the type of reductants and the crystallinity of celluloses. Overall, CBM1 improved the monooxygenase activity on high-, medium-, and low-crystallinity celluloses when ascorbic acid (AscA) was used as the electron donor. CBM1 also facilitated monooxygenase activity on high-crystallinity cellulose, but significantly inhibited monooxygenase activity on low-crystallinity cellulose when cellobiose dehydrogenase (CDH) was the electron donor. Linker truncation of NcLOMO9C enhanced the cellulose-binding affinity but decreased both the oxidase and peroxidase activities. Linker truncation also impacted the monooxygenase activity in both the AscA-AA9 LPMO and AfCDH-AA9 LPMO systems, though its effect was less pronounced compared to that of CBM1. This work provides new insights into the role of the reductant type and cellulose crystallinity in the functionality of CBM1 and the linker in AA9 LPMOs.

The influence of photosynthetic pigment chlorophyllin in light-driven LPMO system on the hydrolytic action of cellulases

It has been demonstrated that LPMO reactions can be driven by light, using the photosynthetic pigment chlorophyllin to achieve efficient oxidative degradation of cellulose. However, the effect of chlorophyllin on cellulases remains unclear. This study discovered that chlorophyllin does not affect the hydrolytic activity of cellulases under dark conditions. However, under light exposure, chlorophyllin-derived reactive oxygen species (ROS) exhibit a strong inhibitory effect on cellulases. These ROS primarily inhibit the hydrolytic action of endoglucanase II (Cel5A) and cellobiohydrolase II (Cel6A), while the action of cellobiohydrolase I and β-glucosidase remains unaffected. Scavenger studies revealed that singlet oxygen (1O₂) is the key inhibitory ROS responsible for the inhibition of Cel5A and Cel6A. The removal of 1O₂ by sodium azide effectively mitigates this inhibition, increasing the conversion yield of cellulose to glucose by 25.9 % when using the light-driven LPMO system in conjunction with cellulases. This study provides new insights into the role of chlorophyllin-derived 1O₂ in hindering hydrolytic action of cellulases and demonstrates the successful mitigation of this inhibition by sodium azide, thereby enhancing the cooperative degradation of cellulose to glucose by the light-driven LPMO system and cellulases.

The impact of the carbohydrate-binding module on how a lytic polysaccharide monooxygenase modifies cellulose fibers

BACKGROUND

In recent years, lytic polysaccharide monooxygenases (LPMOs) that oxidatively cleave cellulose have gained increasing attention in cellulose fiber modification. LPMOs are relatively small copper-dependent redox enzymes that occur as single domain proteins but may also contain an appended carbohydrate-binding module (CBM). Previous studies have indicated that the CBM "immobilizes" the LPMO on the substrate and thus leads to more localized oxidation of the fiber surface. Still, our understanding of how LPMOs and their CBMs modify cellulose fibers remains limited.

RESULTS

Here, we studied the impact of the CBM on the fiber-modifying properties of NcAA9C, a two-domain family AA9 LPMO from Neurospora crassa, using both biochemical methods as well as newly developed multistep fiber dissolution methods that allow mapping LPMO action across the fiber, from the fiber surface to the fiber core. The presence of the CBM in NcAA9C improved binding towards amorphous (PASC), natural (Cell I), and alkali-treated (Cell II) cellulose, and the CBM was essential for significant binding of the non-reduced LPMO to Cell I and Cell II. Substrate binding of the catalytic domain was promoted by reduction, allowing the truncated CBM-free NcAA9C to degrade Cell I and Cell II, albeit less efficiently and with more autocatalytic enzyme degradation compared to the full-length enzyme. The sequential dissolution analyses showed that cuts by the CBM-free enzyme are more evenly spread through the fiber compared to the CBM-containing full-length enzyme and showed that the truncated enzyme can penetrate deeper into the fiber, thus giving relatively more oxidation and cleavage in the fiber core.

CONCLUSIONS

These results demonstrate the capability of LPMOs to modify cellulose fibers from surface to core and reveal how variation in enzyme modularity can be used to generate varying cellulose-based materials. While the implications of these findings for LPMO-based cellulose fiber engineering remain to be explored, it is clear that the presence of a CBM is an important determinant of the three-dimensional distribution of oxidation sites in the fiber.

Sequence and structure analyses of lytic polysaccharide monooxygenases mined from metagenomic DNA of humus samples around white-rot fungi in Cuc Phuong tropical forest, Vietnam

Background

White-rot fungi and bacteria communities are unique ecosystems with different types of symbiotic interactions occurring during wood decomposition, such as cooperation, mutualism, nutritional competition, and antagonism. The role of chitin-active lytic polysaccharide monooxygenases (LPMOs) in these symbiotic interactions is the subject of this study.

Method

In this study, bioinformatics tools were used to analyze the sequence and structure of putative LPMOs mined by hidden Markov model (HMM) profiles from the bacterial metagenomic DNA database of collected humus samples around white-rot fungi in Cuc Phuong primary forest, Vietnam. Two genes encoding putative LPMOs were expressed in E. coli and purified for enzyme activity assay.

Result

Thirty-one full-length proteins annotated as putative LPMOs according to HMM profiles were confirmed by amino acid sequence comparison. The comparison results showed that although the amino acid sequences of the proteins were very different, they shared nine conserved amino acids, including two histidine and one phenylalanine that characterize the H1-Hx-Yz motif of the active site of bacterial LPMOs. Structural analysis of these proteins revealed that they are multidomain proteins with different functions. Prediction of the catalytic domain 3-D structure of these putative LPMOs using Alphafold2 showed that their spatial structures were very similar in shape, although their protein sequences were very different. The results of testing the activity of proteins GL0247266 and GL0183513 show that they are chitin-active LPMOs. Prediction of the 3-D structures of these two LPMOs using Alphafold2 showed that GL0247266 had five functional domains, while GL0183513 had four functional domains, two of which that were similar to the GbpA_2 and GbpA_3 domains of protein GbpA of Vibrio cholerae bacteria. The GbpA_2 - GbpA_3 complex was also detected in 11 other proteins. Based on the structural characteristics of functional domains, it is possible to hypothesize the role of chitin-active GbpA-like LPMOs in the relationship between fungal and bacterial communities coexisting on decomposing trees in primary forests.

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.

A frontier-orbital view of the initial steps of lytic polysaccharide monooxygenase reactions

Lytic polysaccharide monooxygenases (LPMOs) are copper enzymes that oxidatively cleave the strong C-H bonds in recalcitrant polysaccharide substrates, thereby playing a crucial role in biomass degradation. Recently, LPMOs have also been shown to be important for several pathogens. It is well established that the Cu(II) resting state of LPMOs is inactive, and the electronic structure of the active site needs to be altered to transform the enzyme into an active form. Whether this transformation occurs due to substrate binding or due to a unique priming reduction has remained speculative. Starting from four different crystal structures of the LPMO LsAA9A with well-defined oxidation states, we use a frontier molecular orbital approach to elucidate the initial steps of the LPMO reaction. We give an explanation for the requirement of the unique priming reduction and analyse electronic structure changes upon substrate binding. We further investigate how the presence of the substrate could facilitate an electron transfer from the copper active site to an H2O2 co-substrate. Our findings could help to control experimental LPMO reactions.

Enhancing enzymatic saccharification yields of cellulose at high solid loadings by combining different LPMO activities

BACKGROUND

The polysaccharides in lignocellulosic biomass hold potential for production of biofuels and biochemicals. However, achieving efficient conversion of this resource into fermentable sugars faces challenges, especially when operating at industrially relevant high solid loadings. While it is clear that combining classical hydrolytic enzymes and lytic polysaccharide monooxygenases (LPMOs) is necessary to achieve high saccharification yields, exactly how these enzymes synergize at high solid loadings remains unclear.

RESULTS

An LPMO-poor cellulase cocktail, Celluclast 1.5 L, was spiked with one or both of two fungal LPMOs from Thermothielavioides terrestris and Thermoascus aurantiacus, TtAA9E and TaAA9A, respectively, to assess their impact on cellulose saccharification efficiency at high dry matter loading, using Avicel and steam-exploded wheat straw as substrates. The results demonstrate that LPMOs can mitigate the reduction in saccharification efficiency associated with high dry matter contents. The positive effect of LPMO inclusion depends on the type of feedstock and the type of LPMO and increases with the increasing dry matter content and reaction time. Furthermore, our results show that chelating free copper, which may leak out of the active site of inactivated LPMOs during saccharification, with EDTA prevents side reactions with in situ generated H2O2 and the reductant (ascorbic acid).

CONCLUSIONS

This study shows that sustaining LPMO activity is vital for efficient cellulose solubilization at high substrate loadings. LPMO cleavage of cellulose at high dry matter loadings results in new chain ends and thus increased water accessibility leading to decrystallization of the substrate, all factors making the substrate more accessible to cellulase action. Additionally, this work highlights the importance of preventing LPMO inactivation and its potential detrimental impact on all enzymes in the reaction.

Lytic polysaccharide monooxygenase activity increases productive binding capacity of cellobiohydrolases on cellulose

Cellobiohydrolases are crucial for cellulose breakdown, but their efficiency on crystalline cellulose is hampered by limited access to single chain ends to initiate hydrolysis. As a result, they depend on enzymes like lytic polysaccharide monooxygenases (LPMOs), which directly target the crystalline cellulose surface. This study investigated how LPMO pretreatment affected the productive binding capacity of a Trichoderma longibrachiatum cellobiohydrolase, TlCBHI, on crystalline cellulose by applying an amperometric cellobiose dehydrogenase biosensor. After the 24-hour of LPMO pretreatment, the productive binding capacity of TlCBHI significantly increased in all reactions. However, with a shorter 5-hour LPMO pretreatment, minimal to no effect on productive binding capacity was observed. Of note, all LPMO reactions were inactivated around this time point. This delayed LPMO effect suggests that the improved binding capacity for cellulases does not directly result from cellulose chain cleavage by LPMOs but rather from the cellulose decrystallization following the oxidative cleavage.

The effect of linker conformation on performance and stability of a two-domain lytic polysaccharide monooxygenase

A considerable number of lytic polysaccharide monooxygenases (LPMOs) and other carbohydrate-active enzymes are modular, with catalytic domains being tethered to additional domains, such as carbohydrate-binding modules, by flexible linkers. While such linkers may affect the structure, function, and stability of the enzyme, their roles remain largely enigmatic, as do the reasons for natural variation in length and sequence. Here, we have explored linker functionality using the two-domain cellulose-active ScLPMO10C from Streptomyces coelicolor as a model system. In addition to investigating the WT enzyme, we engineered three linker variants to address the impact of both length and sequence and characterized these using small-angle X-ray scattering, NMR, molecular dynamics simulations, and functional assays. The resulting data revealed that, in the case of ScLPMO10C, linker length is the main determinant of linker conformation and enzyme performance. Both the WT and a serine-rich variant, which have the same linker length, demonstrated better performance compared with those with either a shorter linker or a longer linker. A highlight of our findings was the substantial thermostability observed in the serine-rich variant. Importantly, the linker affects thermal unfolding behavior and enzyme stability. In particular, unfolding studies show that the two domains unfold independently when mixed, whereas the full-length enzyme shows one cooperative unfolding transition, meaning that the impact of linkers in biomass-processing enzymes is more complex than mere structural tethering.

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.

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.

The "life-span" of lytic polysaccharide monooxygenases (LPMOs) correlates to the number of turnovers in the reductant peroxidase reaction

Lytic polysaccharide monooxygenases (LPMOs) are monocopper enzymes that degrade the insoluble crystalline polysaccharides cellulose and chitin. Besides the H2O2 cosubstrate, the cleavage of glycosidic bonds by LPMOs depends on the presence of a reductant needed to bring the enzyme into its reduced, catalytically active Cu(I) state. Reduced LPMOs that are not bound to substrate catalyze reductant peroxidase reactions, which may lead to oxidative damage and irreversible inactivation of the enzyme. However, the kinetics of this reaction remain largely unknown, as do possible variations between LPMOs belonging to different families. Here, we describe the kinetic characterization of two fungal family AA9 LPMOs, TrAA9A of Trichoderma reesei and NcAA9C of Neurospora crassa, and two bacterial AA10 LPMOs, ScAA10C of Streptomyces coelicolor and SmAA10A of Serratia marcescens. We found peroxidation of ascorbic acid and methyl-hydroquinone resulted in the same probability of LPMO inactivation (pi), suggesting that inactivation is independent of the nature of the reductant. We showed the fungal enzymes were clearly more resistant toward inactivation, having pi values of less than 0.01, whereas the pi for SmAA10A was an order of magnitude higher. However, the fungal enzymes also showed higher catalytic efficiencies (kcat/KM(H2O2)) for the reductant peroxidase reaction. This inverse linear correlation between the kcat/KM(H2O2) and pi suggests that, although having different life spans in terms of the number of turnovers in the reductant peroxidase reaction, LPMOs that are not bound to substrates have similar half-lives. These findings have not only potential biological but also industrial implications.

A Conserved Second Sphere Residue Tunes Copper Site Reactivity in Lytic Polysaccharide Monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) are powerful monocopper enzymes that can activate strong C-H bonds through a mechanism that remains largely unknown. Herein, we investigated the role of a conserved glutamine/glutamate in the second coordination sphere. Mutation of the Gln in NcAA9C to Glu, Asp, or Asn showed that the nature and distance of the headgroup to the copper fine-tune LPMO functionality and copper reactivity. The presence of Glu or Asp close to the copper lowered the reduction potential and decreased the ratio between the reduction and reoxidation rates by up to 500-fold. All mutants showed increased enzyme inactivation, likely due to changes in the confinement of radical intermediates, and displayed changes in a protective hole-hopping pathway. Electron paramagnetic resonance (EPR) and X-ray absorption spectroscopic (XAS) studies gave virtually identical results for all NcAA9C variants, showing that the mutations do not directly perturb the Cu(II) ligand field. DFT calculations indicated that the higher experimental reoxidation rate observed for the Glu mutant could be reconciled if this residue is protonated. Further, for the glutamic acid form, we identified a Cu(III)-hydroxide species formed in a single step on the H2O2 splitting path. This is in contrast to the Cu(II)-hydroxide and hydroxyl intermediates, which are predicted for the WT and the unprotonated glutamate variant. These results show that this second sphere residue is a crucial determinant of the catalytic functioning of the copper-binding histidine brace and provide insights that may help in understanding LPMOs and LPMO-inspired synthetic catalysts.

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.

AA16 Oxidoreductases Boost Cellulose-Active AA9 Lytic Polysaccharide Monooxygenases from Myceliophthora thermophila

Copper-dependent lytic polysaccharide monooxygenases (LPMOs) classified in Auxiliary Activity (AA) families are considered indispensable as synergistic partners for cellulolytic enzymes to saccharify recalcitrant lignocellulosic plant biomass. In this study, we characterized two fungal oxidoreductases from the new AA16 family. We found that MtAA16A from Myceliophthora thermophila and AnAA16A from Aspergillus nidulans did not catalyze the oxidative cleavage of oligo- and polysaccharides. Indeed, the MtAA16A crystal structure showed a fairly LPMO-typical histidine brace active site, but the cellulose-acting LPMO-typical flat aromatic surface parallel to the histidine brace region was lacking. Further, we showed that both AA16 proteins are able to oxidize low-molecular-weight reductants to produce H2O2. The oxidase activity of the AA16s substantially boosted cellulose degradation by four AA9 LPMOs from M. thermophila (MtLPMO9s) but not by three AA9 LPMOs from Neurospora crassa (NcLPMO9s). The interplay with MtLPMO9s is explained by the H2O2-producing capability of the AA16s, which, in the presence of cellulose, allows the MtLPMO9s to optimally drive their peroxygenase activity. Replacement of MtAA16A by glucose oxidase (AnGOX) with the same H2O2-producing activity could only achieve less than 50% of the boosting effect achieved by MtAA16A, and earlier MtLPMO9B inactivation (6 h) was observed. To explain these results, we hypothesized that the delivery of AA16-produced H2O2 to the MtLPMO9s is facilitated by protein-protein interaction. Our findings provide new insights into the functions of copper-dependent enzymes and contribute to a further understanding of the interplay of oxidative enzymes within fungal systems to degrade lignocellulose.

Structural and functional characterization of the catalytic domain of a cell-wall anchored bacterial lytic polysaccharide monooxygenase from Streptomyces coelicolor

Bacterial lytic polysaccharide monooxygenases (LPMOs) are known to oxidize the most abundant and recalcitrant polymers in Nature, namely cellulose and chitin. The genome of the model actinomycete Streptomyces coelicolor A3(2) encodes seven putative LPMOs, of which, upon phylogenetic analysis, four group with typical chitin-oxidizing LPMOs, two with typical cellulose-active LPMOs, and one which stands out by being part of a subclade of non-characterized enzymes. The latter enzyme, called ScLPMO10D, and most of the enzymes found in this subclade are unique, not only because of variation in the catalytic domain, but also as their C-terminus contains a cell wall sorting signal (CWSS), which flags the LPMO for covalent anchoring to the cell wall. Here, we have produced a truncated version of ScLPMO10D without the CWSS and determined its crystal structure, EPR spectrum, and various functional properties. While showing several structural and functional features typical for bacterial cellulose active LPMOs, ScLPMO10D is only active on chitin. Comparison with two known chitin-oxidizing LPMOs of different taxa revealed interesting functional differences related to copper reactivity. This study contributes to our understanding of the biological roles of LPMOs and provides a foundation for structural and functional comparison of phylogenetically distant LPMOs with similar substrate specificities.

A novel lytic polysaccharide monooxygenase from enrichment microbiota and its application for shrimp shell powder biodegradation

Lytic polysaccharide monooxygenases (LPMO) are expected to change the current status of chitin resource utilization. This study reports that targeted enrichment of the microbiota was performed with chitin by the selective gradient culture technique, and a novel LPMO (M2822) was identified from the enrichment microbiota metagenome. First, soil samples were screened based on soil bacterial species and chitinase biodiversity. Then gradient enrichment culture with different chitin concentrations was carried out. The efficiency of chitin powder degradation was increased by 10.67 times through enrichment, and chitin degradation species Chitiniphilus and Chitinolyticbacter were enriched significantly. A novel LPMO (M2822) was found in the metagenome of the enriched microbiota. Phylogenetic analysis showed that M2822 had a unique phylogenetic position in auxiliary activity (AA) 10 family. The analysis of enzymatic hydrolysate showed that M2822 had chitin activity. When M2822 synergized with commercial chitinase to degrade chitin, the yield of N-acetyl glycosamine was 83.6% higher than chitinase alone. The optimum temperature and pH for M2822 activity were 35°C and 6.0. The synergistic action of M2822 and chitin-degrading enzymes secreted by Chitiniphilus sp. LZ32 could efficiently hydrolyze shrimp shell powder. After 12 h of enzymatic hydrolysis, chitin oligosaccharides (COS) yield reached 4,724 μg/mL. To our knowledge, this work is the first study to mine chitin activity LPMO in the metagenome of enriched microbiota. The obtained M2822 showed application prospects in the efficient production of COS.

H2 O2 feeding enables LPMO-assisted cellulose saccharification during simultaneous fermentative production of lactic acid

Simultaneous saccharification and fermentation (SSF) is a well-known strategy for valorization of lignocellulosic biomass. Because the fermentation process typically is anaerobic, oxidative enzymes found in modern commercial cellulase cocktails, such as lytic polysaccharide monooxygenases (LPMOs), may be inhibited, limiting the overall efficiency of the enzymatic saccharification. Recent discoveries, however, have shown that LPMOs are active under anoxic conditions if they are provided with H₂ O₂ at low concentrations. In this study, we build on this concept and investigate the potential of using externally added H₂ O₂ to sustain oxidative cellulose depolymerization by LPMOs during an SSF process for lactic acid production. The results of bioreactor experiments with 100 g/L cellulose clearly show that continuous addition of small amounts of H₂ O₂ (at a rate of 80 µM/h) during SSF enables LPMO activity and improves lactic acid production. While further process optimization is needed, the present proof-of-concept results show that modern LPMO-containing cellulase cocktails such as Cellic CTec2 can be used in SSF setups, without sacrificing the LPMO activity in these cocktails.

Electrochemical characterization of a family AA10 LPMO and the impact of residues shaping the copper site on reactivity

Research on enzymes for lignocellulose biomass degradation has progressively increased in recent years due to the interest in taking advantage of this natural resource. Among these enzymes are the lytic polysaccharide monooxygenases (LPMOs) that oxidatively depolymerize crystalline cellulose using a reactive oxygen species generated in a reduced mono‑copper active site. The copper site comprises of a highly conserved histidine-brace, providing three equatorial nitrogen ligands, whereas less conserved residues close to the copper contribute to shaping and confining the site. The catalytic copper site is exposed to the solvent and to the crystalline substrates, and as so, the influence of the copper environment on LPMO properties, including the redox potential, is of great interest. In the current work, a direct electrochemical study of an LPMO (ScLPMO10C) was conducted allowing to retrieve kinetic and thermodynamic data associated with the redox transition in the catalytic centre. Moreover, two residues that do not bind to the copper but shape the copper sites were mutated, and the properties of the mutants were compared with those of the wild-type enzyme. The direct electrochemical studies, using cyclic voltammetry, yielded redox potentials in the +200 mV range, well in line with LPMO redox potentials determined by other methods. Interestingly, while the mutations hardly affected the formal redox potential of the enzyme, they drastically affected the reactivity of the copper site and enzyme functionality.

Looking at LPMO reactions through the lens of the HRP/Amplex Red assay

Lytic polysaccharide monooxygenases (LPMOs) are unique redox enzymes capable of disrupting the crystalline surfaces of industry-relevant recalcitrant polysaccharides, such as chitin and cellulose. Historically, LPMOs were thought to be slow enzymes relying on O2 as the co-substrate, but it is now clear that these enzymes prefer H2O2, allowing for fast depolymerization of polysaccharides through a peroxygenase reaction. Thus, quantifying H2O2 in LPMO reaction set-ups is of a great interest. The horseradish peroxidase (HRP)/Amplex Red (AR) assay is one of the most popular and accessible tools for measuring hydrogen peroxide. This assay has been used in various types of biological and biochemical studies, including LPMO research, but suffers from pitfalls that need to be accounted for. In this Chapter, we discuss this method and its use for assessing the often rate-limiting in situ formation of H2O2 in LPMO reactions. We show that, after accounting for multiple potential side reactions, quantitative data on H2O2 production obtained with the HRP/Amplex Red assay provide useful clues for understanding the catalytic activity of LPMOs, including the impact of reductants and transition metal ions.

On the impact of carbohydrate-binding modules (CBMs) in lytic polysaccharide monooxygenases (LPMOs)

Lytic polysaccharide monooxygenases (LPMOs) have revolutionized our understanding of how enzymes degrade insoluble polysaccharides. Compared with the substantial knowledge developed on the structure and mode of action of the catalytic LPMO domains, the (multi)modularity of LPMOs has received less attention. The presence of other domains, in particular carbohydrate-binding modules (CBMs), tethered to LPMOs has profound implications for the catalytic performance of the full-length enzymes. In the last few years, studies on LPMO modularity have led to advancements in elucidating how CBMs, other domains, and linker regions influence LPMO structure and function. This mini review summarizes recent literature, with particular focus on comparative truncation studies, to provide an overview of the diversity in LPMO modularity and the functional implications of this diversity.

Evaluation of Enzymatic Hydrolysis of Sugarcane Bagasse Using Combination of Enzymes or Co-Substrate to Boost Lytic Polysaccharide Monooxygenases Action

This study evaluated innovative approaches for the enzymatic hydrolysis of lignocellulosic biomass. More specifically, assays were performed to evaluate the supplementation of the commercial cellulolytic cocktail Cellic® CTec2 (CC2) with LPMO (GcLPMO9B), H2O2, or cello-oligosaccharide dehydrogenase (CelDH) FgCelDH7C in order to boost the LPMO action and improve the saccharification efficiency of biomass into monosaccharides. The enzymatic hydrolysis was carried out using sugarcane bagasse pretreated by hydrodynamic cavitation-assisted oxidative process, 10% (w/w) solid loading, and 30 FPU CC2/g dry biomass. The results were compared in terms of sugars release and revealed an important influence of the supplementations at the initial 6 h of hydrolysis. While the addition of CelDH led to a steady increase in glucose production to reach 101.1 mg of glucose/g DM, accounting for the highest value achieved after 72 h of hydrolysis, boosting the LPMOs activity by the supplementation of pure LPMOs or the LPMO co-substrate H2O2 were also effective to improve the cellulose conversion, increasing the initial reaction rate of the hydrolysis. These results revealed that LPMOs play an important role on enzymatic hydrolysis of cellulose and boosting their action can help to improve the reaction rate and increase the hydrolysis yield. LPMOs-CelDH oxidative pairs represent a novel potent combination for use in the enzymatic hydrolysis of lignocellulose biomass. Finally, the strategies presented in this study are promising approaches for application in lignocellulosic biorefineries, especially using sugarcane bagasse as a feedstock.

Structural perturbations of substrate binding and oxidation state changes in a lytic polysaccharide monooxygenase

LPMOs are enzymes which catalyse the oxidation of a C-H bond within polysaccharides, leading to their oxidative cleavage. To achieve this, LPMOs employ highly reactive oxidising intermediates, the generation of which is likely coupled to substrate binding to the enzyme. The nature of this coupling is unknown. Here we report a statistical comparison for four three-dimensional structures of an AA9 LPMO crystallised in the same space group but in different oxidation and substrate-binding states, to determine which significant structural perturbations occur at the enzyme upon either oxidation state change or the binding of substrate. In a novel step, we determine the global random error associated with the positional coordinates of atoms using the method of moments to ascertain the statistical estimators of Gaussian distributions of pairwise RMS differences between individual atoms in different structures. The results show that a change in the oxidation state of the copper leads to no significant structural changes, and that binding of the substrate leads to a single change in the conformation of a tryptophan residue. This tryptophan has previously been identified as part of a charge transfer pathway between the active site and the external surface of the protein, and the structural change identified herein may be part of the substrate-enzyme coupling mechanism.