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

Protonation State Insights into the Influence of Biocatalytic Function for Acetylcholinesterase Mediated by Neonicotinoids

The catalytic efficiency of acetylcholinesterase (AChE) is likely regulated by the protonation states and conformational adaptations of its catalytic residues. While neonicotinoid insecticides are recognized for impairing AChE function through neurotoxic mechanisms, the precise molecular mechanisms governing this inhibition remain poorly characterized. This investigation elucidates how structural variations among neonicotinoids modulate the protonation equilibria of Glu-202 and His-447 in AChE's catalytic triad. Comparative analysis reveals that nitro-substituted neonicotinoids (imidacloprid, clothianidin) induce more pronounced protonation state transitions compared to their cyano-containing counterparts (thiacloprid, acetamiprid). Specifically, the strong electron-withdrawing nitro groups facilitate the conversion of Glu-202 from the deprotonation (GLU) to protonation (GLH) state and His-447 from the δ- (HID) to ε-position protonation (HIE) state through enhanced electrostatic interactions. These electronic perturbations trigger structural reorganization within the active site, evidenced by nitro group-directed residue realignment and subsequent H-bond formation. Energy decomposition analysis identifies electrostatic contributions as the primary determinant of binding affinity differences, with nitro-neonicotinoids exhibiting stronger interactions than cyano-neonicotinoids. QM/MM metadynamics reveals that substantial protonation state alterations disrupt AChE's biocatalytic function, particularly its capacity for acetylcholine hydrolysis. Finally, SH-SY5Y-based cellular assays show that imidacloprid exhibits the strongest inhibitory effect on AChE intracellular activity, while thiacloprid and acetamiprid show weaker inhibitory effects, aligning with the computational predictions. This study provides insights into the protonation-state-induced biocatalytic function for acetylcholinesterase mediated by neonicotinoids, contributing to the assessment of exogenous ligand-induced potential ecological and human health risks.

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.

Functional characterization of two AA10 lytic polysaccharide monooxygenases from Cellulomonas gelida

Lytic polysaccharide monooxygenases (LPMOs) are redox enzymes targeting the crystalline region of recalcitrant polysaccharides such as cellulose and chitin. Functional characterization of two LPMOs from the cellulose-degrading soil bacterium Cellulomonas gelida, CgLPMO10A and CgLPMO10B, showed expected activities on cellulose but also revealed novel features of AA10 LPMOs. While clustering together with strictly C1-oxidizing and strictly cellulose-active AA10 LPMOs, CgLPMO10A exhibits activity on both cellulose and chitin, oxidizing the C1 carbon of both substrates. This combination of substrate and oxidative specificity has not been previously observed for family 10 LPMOs and may be due to a conspicuous divergence in two hydrophobic residues on the substrate-binding surface. CgLPMO10B oxidizes cellulose at both the C1 and C4 positions and is also active on chitin, in line with predictions based on phylogeny. Interestingly, while coming from the same organism and both acting on cellulose, the two enzymes have markedly different redox properties with CgLPMO10B displaying the lowest redox potential and the highest oxidase activity observed for an AA10 LPMO so far. These results provide insight into the LPMO machinery of C. gelida and expand the known catalytic repertoire of bacterial LPMOs.

Easing Intermediates Search by Combining Spectroscopy and Multivariate Curve Reconstruction: [CuI(6,6'-dimethyl-2,2'-bipyridyl)2]PF6 Oxidation as Case Study

Despite their prevalence in catalysis, complex reaction mixtures are not trivial to investigate and disentangle. Different approaches can be applied to characterize them, even featuring low-dimensionality data sets. The liquid-phase reaction of [CuI(6,6'-dimethyl-2,2'-bipyridyl)2]PF6 (CuI) with tert-butyl hydroperoxide is investigated: two CuII species are found upon oxidation of the pristine complex, characterized by different spectroscopic and kinetics fingerprints. Coupling EPR and UV-vis spectroscopies with chemometric methods (namely, multivariate curve reconstruction, MCR) allowed for easily retrieving pure spectral features and concentration profiles. Spectrokinetic analysis independently showed an optimal agreement with kinetic outcomes from MCR. Finally, hypotheses on the nature of the CuII species are drawn on the basis of EPR fitting and quantum chemistry computations on a series of candidate structures. Beyond the accurate characterization of a model system, this study demonstrates the potential of coupling multivariate statistical techniques, experiments, and computations toward a quantitative understanding of electronic and kinetic information on complex chemical systems.

Electrochemical kinetic fingerprinting of single-molecule coordinations in confined nanopores

Metal centers are essential for enzyme catalysis, stabilizing the active site, facilitating electron transfer, and maintaining the structure through coordination with amino acids. In this study, K238H-AeL nanopores with histidine sites were designed as single-molecule reactors for the measurement of single-molecule coordination reactions. The coordination mechanism of Au(III) with histidine and glutamate in biological nanopore confined space was explored. Specifically, Au(III) interacts with the nitrogen (N) atom in the histidine imidazole ring of the K238H-AeL nanopore and the oxygen (O) atom in glutamate to form a stable K238H-Au-Cl2 complex. The formation mechanism of this complex was further validated through single-molecule nanopore analysis, mass spectrometry, and molecular dynamics simulations. Introducing histidine and negative charge amino acids with carboxyl group into different positions within the nanopore revealed that the formation of the histidine-Au coordination bond in the confined space requires a suitable distance between the ligand and the central metal atom. By analyzing the association and dissociation rates of the single Au(III) ion under the applied voltages, it was found that a confined nanopore increased the bonding rate constant of Au(III)-histidine coordination reactions by around 10-100 times compared to that in the bulk solution and the optimal voltage for single-molecule. Therefore, nanopore techniques for tracking single-molecule reactions could offer valuable insights into designing metalloenzymes in metal-catalyzed organic reactions.

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.

Mutational study of a lytic polysaccharide monooxygenase from Myceliophthora thermophila (MtLPMO9F): Structural insights into substrate specificity and regioselectivity

Lytic polysaccharide monooxygenases (LPMOs) are key enzymes for the biotechnological exploitation of lignocellulosic biomass, yet their efficient application depends on the in-depth understanding of their mechanism of action. Here, we describe the structural and mutational characterization of a C4-active LPMO from Myceliophthora thermophila, MtLPMO9F, that belongs to auxiliary activity family 9 (AA9). MtLPMO9F is active on cellulose, cello-oligosaccharides and xyloglucan. The crystal structure of MtLPMO9F catalytic domain, determined at 2.3 Å resolution, revealed a double conformation for loop L3 with a potential implication in the formation of aglycon subsites. Product analysis of reactions with cello-oligosaccharides showed a prevalent -4 to +2 binding mode. Subsequent biochemical characterization of 4 MtLPMO9F point mutants further provided insights in LPMO structure-function relationships regarding both substrate binding and the role of second-coordination sphere residues.

LPMO-Catalyzed Oxidation of Cellulosic Fibers with Controlled Addition of a Reductant and H2O2

Cellulose-derived biomaterials offer a sustainable and versatile platform for various applications. Enzymatic engineering of these fibers, particularly using lytic polysaccharide monooxygenases (LPMOs), shows promise due to the ability to introduce functional groups onto cellulose surfaces, potentially enabling further functionalization. However, harnessing LPMOs for fiber engineering remains challenging, partly because controlling the enzymatic reaction is difficult and partly because limited information is available about how LPMOs modify the fibers. In this study, we explored controlling LPMO-mediated fiber oxidation by sequentially adding a reductant (gallic acid, GA) and H2O2, using three different carbohydrate-binding module (CBM)-containing LPMOs. An in-depth analysis of the soluble products and the M n, M w, and carbonyl content in the fiber fraction indicates that fiber oxidation can indeed be controlled by adjusting the amount of GA and H2O2 added to the reaction. In particular, at lower overall dosages of GA and H2O2, corresponding to low oxidation levels, fiber oxidation occurs rapidly with almost no release of soluble oxidized products. Conversely, at higher dosages, fiber oxidation levels off, while oxidized oligosaccharides continue to be released and the fibers are eroded. Importantly, next to demonstrating controlled fiber oxidation, this study shows that different cellulose-active LPMOs modify the fibers in different manners.

Secondary sphere interactions modulate peroxynitrite scavenging by the E2 domain of amyloid precursor protein

Peroxynitrite (ONOO-) is a highly reactive nitrogen species that can cause significant damage to proteins, lipids, and DNA. Various enzymes, including metalloenzymes, play crucial roles in reducing ONOO- concentrations to protect cellular components. While the interaction of ONOO- with heme proteins is well known, the reduction by Cu-containing proteins is less studied. Amyloid precursor protein (APP), implicated in Alzheimer's disease, has an E2 domain that binds copper ions with a dissociation constant of KD ∼ 10-12 M and is proposed to be involved in iron homeostasis, copper trafficking, and oxidative stress response. Our recent studies using EXAFS, UV-Vis, and EPR spectroscopy revealed a previously unidentified labile water ligand in the Cu(II) site of the E2 domain, suggesting reactivity with anionic substrates like ONOO-. Experimental data showed that Cu(I)-E2 reduces ONOO- at a significant rate (1.1 × 105 M-1 s-1), comparable to native peroxynitrite scavengers, while maintaining active site integrity through multiple redox cycles. This study further investigates the mechanism of ONOO- reduction by Cu(I)-E2 using the Griess assay, demonstrating that reduction occurs via single electron transfer, forming nitrite and nitrate. This process aligns with previous findings that Cu(I)-E2 is oxidized to Cu(II)-E2 upon ONOO- reduction. Mutations at Lys435, affecting secondary sphere interactions, revealed that factors beyond electrostatics are involved in substrate recruitment. MD simulations suggest that steric hindrance from a newly formed hydrogen bond also plays a role. Understanding ONOO- reduction by the E2 domain of APP expands our knowledge of copper proteins in mitigating oxidative stress and elucidates their physiological and pathological roles, particularly in Alzheimer's disease.

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.

Selective oxidation of active site aromatic residues in engineered Cu proteins

Recent studies have revealed critical roles for the local environments surrounding metallocofactors, such as the newly identified CuD site in particulate methane monooxygenases (pMMOs) and the second sphere aromatic residues in lytic polysaccharide monooxygenases (LPMOs), implicated in the protection against oxidative damage. However, these features are subjects of continued debate. Our work utilizes biotin-streptavidin (Sav) technology to develop artificial metalloproteins (ArMs) that mimic the active sites of natural copper metalloenzymes. By engineering ArMs with aromatic residues within their secondary coordination spheres, we systematically investigate the influence of these residues on Cu reactivity and oxidant activation. We demonstrate that the placement and orientation of tyrosine relative to the Cu cofactor critically affect the oxidation outcomes upon exposure to hydrogen peroxide. A key finding is the interplay between the coordination of an active site asparagine and the incorporation of aromatic residues proximal to the artificial Cu cofactor, which are the only variants where oxidation of an engineered residues is observed. These findings underscore the importance of the secondary coordination sphere in modulating Cu center reactivity, suggest a role for amide coordination in C-H bond activation by pMMOs, and potential inactivation pathways in natural copper enzymes like LPMOs.

Copper(II)-Oxyl Formation in a Biomimetic Complex Activated by Hydrogen Peroxide: The Key Role of Trans-Bis(Hydroxo) Species

Enzymes in nature, such as the copper-based lytic polysaccharide monooxygenases (LPMOs), have gained significant attention for their exceptional performance in C-H activation reactions. The use of H2O2 by LPMOs enzymes has also increased the interest in understanding the oxidation mechanism promoted by this oxidant. While some literature proposes Fenton-like chemistry involving the formation of Cu(II)-OH species and the hydroxyl radical, others contend that Cu(I) activation by H2O2 yields a Cu(II)-oxyl intermediate. In this study, we focused on a bioinspired Cu(I) complex to investigate the reaction mechanism of its oxidation by H2O2 using density functional theory and ab initio molecular dynamics simulations. The latter approach was found to be critical for finding the key Cu intermediates. Our results show that the highly flexible coordination environment of copper strongly influences the nature of the oxidized Cu(II) species. Furthermore, they suggest the favorable formation of trans-Cu(II)-(OH)2 moieties in low-coordinated Cu(II) species. This trans configuration hinders the formation of Cu(II)-oxyl species, facilitating intramolecular H-abstraction reactions in line with experimentally observed ligand oxidation processes.

Copper-oxygen adducts: new trends in characterization and properties towards C-H activation

This review summarizes the latest discoveries in the field of C-H activation by copper monoxygenases and more particularly by their bioinspired systems. This work first describes the recent background on copper-containing enzymes along with additional interpretations about the nature of the active copper-oxygen intermediates. It then focuses on relevant examples of bioinorganic synthetic copper-oxygen intermediates according to their nuclearity (mono to polynuclear). This includes a detailed description of the spectroscopic features of these adducts as well as their reactivity towards the oxidation of recalcitrant Csp3 -H bonds. The last part is devoted to the significant expansion of heterogeneous catalytic systems based on copper-oxygen cores (i.e. within zeolite frameworks).

Integrated Experimental and Theoretical Investigation of Copper Active Site Properties of a Lytic Polysaccharide Monooxygenase from Serratia marcescens

In this paper, we employed a multidisciplinary approach, combining experimental techniques and density functional theory (DFT) calculations to elucidate key features of the copper coordination environment of the bacterial lytic polysaccharide monooxygenase (LPMO) from Serratia marcescens (SmAA10). The structure of the holo-enzyme was successfully obtained by X-ray crystallography. We then determined the copper(II) binding affinity using competing ligands and observed that the affinity of the histidine brace ligands for copper is significantly higher than previously described. UV-vis, advanced electron paramagnetic resonance (EPR), and X-ray absorption spectroscopy (XAS) techniques, including high-energy resolution fluorescence detected (HERFD) XAS, were further used to gain insight into the copper environment in both the Cu(II) and Cu(I) redox states. The experimental data were successfully rationalized by DFT models, offering valuable information on the electronic structure and coordination geometry of the copper center. Finally, the Cu(II)/Cu(I) redox potential was determined using two different methods at ca. 350 mV vs NHE and rationalized by DFT calculations. This integrated approach not only advances our knowledge of the active site properties of SmAA10 but also establishes a robust framework for future studies of similar enzymatic systems.

Impact of the Copper Second Coordination Sphere on Catalytic Performance and Substrate Specificity of a Bacterial Lytic Polysaccharide Monooxygenase

Lytic polysaccharide monooxygenases (LPMOs) catalyze the oxidative cleavage of glycosidic bonds in recalcitrant polysaccharides, such as cellulose and chitin, using a single copper cofactor bound in a conserved histidine brace with a more variable second coordination sphere. Cellulose-active LPMOs in the fungal AA9 family and in a subset of bacterial AA10 enzymes contain a His-Gln-Tyr second sphere motif, whereas other cellulose-active AA10s have an Arg-Glu-Phe motif. To shine a light on the impact of this variation, we generated single, double, and triple mutations changing the His216-Gln219-Tyr221 motif in cellulose- and chitin-oxidizing MaAA10B toward Arg-Glu-Phe. These mutations generally reduced enzyme performance due to rapid inactivation under turnover conditions, showing that catalytic fine-tuning of the histidine brace is complex and that the roles of these second sphere residues are strongly interconnected. Studies of copper reactivity showed remarkable effects, such as an increase in oxidase activity following the Q219E mutation and a strong dependence of this effect on the presence of Tyr at position 221. In reductant-driven reactions, differences in oxidase activity, which lead to different levels of in situ generated H2O2, correlated with differences in polysaccharide-degrading ability. The single Q219E mutant displayed a marked increase in activity on chitin in both reductant-driven reactions and reactions fueled by exogenously added H2O2. Thus, it seems that the evolution of substrate specificity in LPMOs involves both the extended substrate-binding surface and the second coordination sphere.

Mimicking the Reactivity of LPMOs with a Mononuclear Cu Complex

Lytic polysaccharide monooxygenases (LPMOs) are Cu-dependent metalloenzymes that catalyze the hydroxylation of strong C-H bonds in polysaccharides using O2 or H2O2 as oxidants (monooxygenase/peroxygenase). In the absence of C-H substrate, LPMOs reduce O2 to H2O2 (oxidase) and H2O2 to H2O (peroxidase) using proton/electron donors. This rich oxidative reactivity is promoted by a mononuclear Cu center in which some of the amino acid residues surrounding the metal might can accept and donate protons and/or electrons during O2 and H2O2 reduction. Herein, we utilize a podal ligand containing H-bond/proton donors (LH2) to analyze the reactivity of mononuclear Cu species towards O2 and H2O2. [(LH2)CuI]1+ (1), [(LH2)CuII]2+ (2), [(LH-)CuII]1+ (3), [(LH2)CuII(OH)]1+ (4), and [(LH2)CuII(OOH)]1+ (5) were synthesized and characterized by structural and spectroscopic means. Complex 1 reacts with O2 to produce 5, which releases H2O2 to generate 3, suggesting that O2 is used by LPMOs to generate H2O2. The reaction of 1 with H2O2 produces 4 and hydroxyl radical, which reacts with C-H substrates in a Fenton-like fashion. Complex 3, which generate 1 via a reversible protonation/reduction, binds H2O and H2O2 to produce 4 and 5, respectively, a mechanism that could be used by LPMOs to control oxidative reactivity.

The rotamer of the second-sphere histidine in AA9 lytic polysaccharide monooxygenase is pH dependent

Lytic polysaccharide monooxygenases (LPMOs) catalyze a reaction that is crucial for the biological decomposition of various biopolymers and for the industrial conversion of plant biomass. Despite the importance of LPMOs, the exact molecular-level nature of the reaction mechanism is still debated today. Here, we investigated the pH-dependent conformation of a second-sphere histidine (His) that we call the stacking histidine, which is conserved in fungal AA9 LPMOs and is speculated to assist catalysis in several of the LPMO reaction pathways. Using constant-pH and accelerated molecular dynamics simulations, we monitored the dynamics of the stacking His in different protonation states for both the resting Cu(II) and active Cu(I) forms of two fungal LPMOs. Consistent with experimental crystallographic and neutron diffraction data, our calculations suggest that the side chain of the protonated and positively charged form is rotated out of the active site toward the solvent. Importantly, only one of the possible neutral states of histidine (HIE state) is observed in the stacking orientation at neutral pH or when bound to cellulose. Our data predict that, in solution, the stacking His may act as a stabilizer (via hydrogen bonding) of the Cu(II)-superoxo complex after the LPMO-Cu(I) has reacted with O2 in solution, which, in fine, leads to H2O2 formation. Also, our data indicate that the HIE-stacking His is a poor acid/base catalyst when bound to the substrate and, in agreement with the literature, may play an important stabilizing role (via hydrogen bonding) during the peroxygenase catalysis. Our study reveals the pH titration midpoint values of the pH-dependent orientation of the stacking His should be considered when modeling and interpreting LPMO reactions, whether it be for classical LPMO kinetics or in industry-oriented enzymatic cocktails, and for understanding LPMO behavior in slightly acidic natural processes such as fungal wood decay.

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.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Electrochemical Monitoring of Heterogeneous Peroxygenase Reactions Unravels LPMO Kinetics

Biological conversion of plant biomass depends on peroxygenases and peroxidases acting on insoluble polysaccharides and lignin. Among these are cellulose- and hemicellulose-degrading lytic polysaccharide monooxygenases (LPMOs), which have revolutionized our concept of biomass degradation. Major obstacles limiting mechanistic and functional understanding of these unique peroxygenases are their complex and insoluble substrates and the hard-to-measure H2O2 consumption, resulting in the lack of suitable kinetic assays. We report a versatile and robust electrochemical method for real-time monitoring and kinetic characterization of LPMOs and other H2O2-dependent interfacial enzymes based on a rotating disc electrode for the sensitive and selective quantitation of H2O2 at biologically relevant concentrations. The H2O2 sensor works in suspensions of insoluble substrates as well as in homogeneous solutions. Our characterization of multiple LPMOs provides unprecedented insights into the substrate specificity, kinetics, and stability of these enzymes. High turnover and total turnover numbers demonstrate that LPMOs are fast and durable biocatalysts.