Bastien Bissaro

Oxidative cleavage of polysaccharides by monocopper enzymes depends on H2O2

Enzymes currently known as lytic polysaccharide monooxygenases (LPMOs) play an important role in the conversion of recalcitrant polysaccharides, but their mode of action has remained largely enigmatic. It is generally believed that catalysis by LPMOs requires molecular oxygen and a reductant that delivers two electrons per catalytic cycle. Using enzyme assays, mass spectrometry and experiments with labeled oxygen atoms, we show here that H2O2, rather than O2, is the preferred co-substrate of LPMOs. By controlling H2O2 supply, stable reaction kinetics are achieved, the LPMOs work in the absence of O2, and the reductant is consumed in priming rather than in stoichiometric amounts. The use of H2O2 by a monocopper enzyme that is otherwise cofactor-free offers new perspectives regarding the mode of action of copper enzymes. Furthermore, these findings have implications for the enzymatic conversion of biomass in Nature and in industrial biorefining.

Structural diversity of lytic polysaccharide monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) catalyze the oxidative cleavage of glycosidic bonds and represent a promising resource for development of industrial enzyme cocktails for biomass processing. LPMOs show high sequence and modular diversity and are known, so far, to cleave insoluble substrates such as cellulose, chitin and starch, as well as hemicelluloses such as beta-glucan, xyloglucan and xylan. All LPMOs share a catalytic histidine brace motif to bind copper, but differ strongly when it comes to the nature and arrangement of residues on the substrate-binding surface. In recent years, the number of available LPMO structures has increased rapidly, including the first structure of an enzyme-substrate complex. The insights gained from these structures is reviewed below.

Fueling biomass-degrading oxidative enzymes by light-driven water oxidation

Photosynthesis may be described as light-driven oxidation of water and subsequent use of the generated reducing equivalents to fix CO2 and synthesize higher energy organic compounds, such as carbohydrates. The transposition of the sustainable and atom-efficient strategy of water oxidation to in vitro controlled biocatalytic reactions is poorly studied but is of high interest for the development of photobiocatalysis, and eco-friendly catalytic tools in a wider sense. Here we demonstrate that light-driven oxidation of water catalysed by vanadium-doped TiO2 (V-TiO2), a re-usable photocatalyst, can provide the electrons that lytic polysaccharide monooxygenases (LPMOs) need to oxidatively deconstruct biomass polysaccharides. The demonstration that electrons may be generated by water oxidation alleviates the need for an externally added electron donor, which so far has been a prerequisite for LPMO activity. Importantly, photocatalytic LPMO activation was achieved in the absence of redox mediators, which represents the first demonstration of mediator-free electron transfer from V-TiO2 particles to a redox enzyme, expanding the repertoire of known and conceivable photobiocatalytic reactions. Fundamentally, this photobiocatalytic system allows activation and tight control of LPMO activity, thus offering new tools for mechanistic studies of these industrially important and ubiquitous enzymes. The latter is illustrated by real-time studies of the redox state of an LPMO, using the controllable light-V-TiO2 technology for LPMO reduction and a novel fluorescence method for monitoring re-oxidation. We also show that the light-V-TiO2 technology may be used to study pre-activation of LPMOs.

Structural determinants of bacterial lytic polysaccharide monooxygenase functionality

Bacterial lytic polysaccharide monooxygenases (LPMO10s) use redox chemistry to cleave glycosidic bonds in the two foremost recalcitrant polysaccharides found in nature, namely cellulose and chitin. Analysis of correlated mutations revealed that the substrate-binding and copper-containing surface of LPMO10s composes a network of co-evolved residues and interactions, whose roles in LPMO functionality are unclear. Here, we mutated a subset of these correlated residues in a newly characterized C1/C4-oxidizing LPMO10 from Micromonospora aurantiaca (MaLPMO10B) to the corresponding residues in strictly C1-oxidizing LPMO10s. We found that surface properties near the catalytic copper, i.e. side chains likely to be involved in substrate positioning, are major determinants of the C1:C4 ratio. Several MaLPMO10B mutants almost completely lost C4-oxidizing activity while maintaining C1-oxidizing activity. These mutants also lost chitin-oxidizing activity, which is typically observed for C1/C4-oxidizing, but not for C1-oxidizing, cellulose-active LPMO10s. Selective loss in C1-oxidizing activity was not observed. Additional mutational experiments disclosed that neither truncation of the MaLPMO10B family 2 carbohydrate-binding module nor mutations altering access to the solvent-exposed axial copper coordination site significantly change the C1:C4 ratio. Importantly, several of the mutations that altered interactions with the substrate exhibited reduced stability. This effect could be explained by productive substrate binding that protects LPMOs from oxidative self-inactivation. We discuss these stability issues in view of recent findings on LPMO catalysis, such as the involvement of H2O2. Our results show that residues on the substrate-binding surface of LPMOs have co-evolved to optimize several of the interconnected properties: substrate binding and specificity, oxidative regioselectivity, catalytic efficiency, and stability.

Kinetics of H2O2-driven degradation of chitin by a bacterial lytic polysaccharide monooxygenase.

Silja Kuusk, Bastien Bissaro, Piret Kuusk, Zarah Forsberg, Vincent G. H. Eijsink, Morten Sørlie, and Priit Väljamäe There was an error in the molar extinction coefficient used to calculate the concentration of CBP21 from the absorbance at 280 nm. The correct value is 35,200 M 1 cm 1 rather than 29,450 M 1 cm 1 that was used in the article. For this reason, the reported concentration of CBP21 was 1.2-fold higher than the actual concentration with the exception of the data presented in Fig. 5, where the correct concentration was reported. This error affects only the reported kcat value, which should be 6.7 s 1 rather than 5.6 s , and does not affect the conclusions of this work. ADDITIONS AND CORRECTIONS

Methylation of the N-terminal histidine protects a lytic polysaccharide monooxygenase from auto-oxidative inactivation: Role of Histidine Methylation in Fungal LPMOs

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 pKa 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 H2O2 boost LPMO activity, whereas excess H2O2 leads to LPMO inactivation. The methylated variant of TaLPMO9A, produced in Aspergillus oryzae, was more resistant to excess H2O2 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.

Multipoint Precision Binding of Substrate Protects Lytic Polysaccharide Monooxygenases from Self-Destructive Off-Pathway Processes

Lytic polysaccharide monooxygenases (LPMOs) play a crucial role in the degradation of polysaccharides in biomass by catalyzing powerful oxidative chemistry using only a single copper ion as a cofactor. Despite the natural abundance and importance of these powerful monocopper enzymes, the structural determinants of their functionality have remained largely unknown. We have used site-directed mutagenesis to probe the roles of 13 conserved amino acids located on the flat substrate-binding surface of CBP21, a chitin-active family AA10 LPMO from Serratia marcescens, also known as SmLPMO10A. Single mutations of residues that do not interact with the catalytic copper site, but rather are involved in substrate binding had remarkably strong effects on overall enzyme performance. Analysis of product formation over time showed that these mutations primarily affected enzyme stability. Investigation of protein integrity using proteomics technologies showed that loss of activity was caused by oxidation of essential residues in the enzyme active site. For most enzyme variants, reduced enzyme stability correlated with a reduced level of binding to chitin, suggesting that adhesion to the substrate prevents oxidative off-pathway processes that lead to enzyme inactivation. Thus, the extended and highly evolvable surfaces of LPMOs are tailored for precise multipoint substrate binding, which provides the confinement that is needed to harness and control the remarkable oxidative power of these enzymes. These findings are important for the optimized industrial use of LPMOs as well as the design of LPMO-inspired catalysts.

How a Lytic Polysaccharide Monooxygenase Binds Crystalline Chitin

Lytic polysaccharide monooxygenases (LPMOs) are major players in biomass conversion, both in Nature and in the biorefining industry. How the monocopper LPMO active site is positioned relative to the crystalline substrate surface to catalyze powerful, but potentially self-destructive, oxidative chemistry is one of the major questions in the field. We have adopted a multidisciplinary approach, combining biochemical, spectroscopic, and molecular modeling methods to study chitin binding by the well-studied LPMO from Serratia marcescens SmAA10A (or CBP21). The orientation of the enzyme on a single-chain substrate was determined by analyzing enzyme cutting patterns. Building on this analysis, molecular dynamics (MD) simulations were performed to study interactions between the LPMO and three different surface topologies of crystalline chitin. The resulting atomistic models showed that most enzyme-substrate interactions involve the polysaccharide chain that is to be cleaved. The models also revealed a constrained active site geometry as well as a tunnel connecting the bulk solvent to the copper site, through which only small molecules such as H2O, O2, and H2O2 can diffuse. Furthermore, MD simulations, quantum mechanics/molecular mechanics calculations, and electron paramagnetic resonance spectroscopy demonstrate that rearrangement of Cu-coordinating water molecules is necessary when binding the substrate and also provide a rationale for the experimentally observed C1 oxidative regiospecificity of SmAA10A. This study provides a first, experimentally supported, atomistic view of the interactions between an LPMO and crystalline chitin. The confinement of the catalytic center is likely crucially important for controlling the oxidative chemistry performed by LPMOs and will help guide future mechanistic studies.