Structure-function characterization reveals new catalytic diversity in the galactose oxidase and glyoxal oxidase family

Expansion of Auxiliary Activity Family 5 sequence space via biochemical characterization of six new copper radical oxidases

Bacterial and fungal copper radical oxidases (CROs) from Auxiliary Activity Family 5 (AA5) are implicated in morphogenesis and pathogenesis. The unique catalytic properties of CROs also make these enzymes attractive biocatalysts for the transformation of small molecules and biopolymers. Despite a recent increase in the number of characterized AA5 members, especially from subfamily 2 (AA5_2), the catalytic diversity of the family as a whole remains underexplored. In the present study, phylogenetic analysis guided the selection of six AA5_2 members from diverse fungi for recombinant expression in Komagataella pfaffii (syn. Pichia pastoris) and biochemical characterization in vitro. Five of the targets displayed predominant galactose 6-oxidase activity (EC 1.1.3.9), and one was a broad-specificity aryl alcohol oxidase (EC 1.1.3.7) with maximum activity on the platform chemical 5-hydroxymethyl furfural (EC 1.1.3.47). Sequence alignment comparing previously characterized AA5_2 members to those from this study indicated various amino acid substitutions at active site positions implicated in the modulation of specificity.IMPORTANCEEnzyme discovery and characterization underpin advances in microbial biology and the application of biocatalysts in industrial processes. On one hand, oxidative processes are central to fungal saprotrophy and pathogenesis. On the other hand, controlled oxidation of small molecules and (bio)polymers valorizes these compounds and introduces versatile functional groups for further modification. The biochemical characterization of six new copper radical oxidases further illuminates the catalytic diversity of these enzymes, which will inform future biological studies and biotechnological applications.

Homo- and Heterogeneous Benzyl Alcohol Catalytic Oxidation Promoted by Mononuclear Copper(II) Complexes: The Influence of the Ligand upon Product Conversion

The catalytic properties of three copper complexes, [Cu(en)2](ClO4)2 (1), [Cu(amp)2](ClO4)2, (2) and [Cu(bpy)2](ClO4)2 (3) (where en = ethylenediamine, amp = 2-aminomethylpyridine and bpy = 2,2'-bipyridine), were explored upon the oxidation of benzyl alcohol (BnOH). Maximized conversions of the substrates to their respective products were obtained using a multivariate analysis approach, a powerful tool that allowed multiple variables to be optimized simultaneously, thus creating a more economical, fast and effective technique. Considering the studies in a fluid solution (homogeneous), all complexes strongly depended on the amount of the oxidizing agent (H2O2), followed by the catalyst load. In contrast, time seemed to be statistically less relevant for complexes 1 and 3 and not relevant for 2. All complexes showed high selectivity in their optimized conditions, and only benzaldehyde (BA) was obtained as a viable product. Quantitatively, the catalytic activity observed was 3 > 2 > 1, which is related to the π-acceptor character of the ligands employed in the study. Density functional theory (DFT) studies could corroborate this feature by correlating the geometric index for square pyramid Cu(II)-OOH species, which should be generated in the solution during the catalytic process. Complex 3 was successfully immobilized in silica-coated magnetic nanoparticles (Fe3O4@SiO2), and its oxidative activity was evaluated through heterogenous catalysis assays. Substrate conversion promoted by 3-Fe3O4@SiO2 generated only BA as a viable product, and the supported catalyst's recyclability was proven. Reduced catalytic conversions in the presence of the radical scavenger (2,2,6,6-tetrametil-piperidi-1-nil)oxil (TEMPO) indicate that radical and non-radical mechanisms are involved.

Genome evolution and transcriptome plasticity is associated with adaptation to monocot and dicot plants in Colletotrichum fungi

BACKGROUND

Colletotrichum fungi infect a wide diversity of monocot and dicot hosts, causing diseases on almost all economically important plants worldwide. Colletotrichum is also a suitable model for studying gene family evolution on a fine scale to uncover events in the genome associated with biological changes.

RESULTS

Here we present the genome sequences of 30 Colletotrichum species covering the diversity within the genus. Evolutionary analyses revealed that the Colletotrichum ancestor diverged in the late Cretaceous in parallel with the diversification of flowering plants. We provide evidence of independent host jumps from dicots to monocots during the evolution of Colletotrichum, coinciding with a progressive shrinking of the plant cell wall degradative arsenal and expansions in lineage-specific gene families. Comparative transcriptomics of 4 species adapted to different hosts revealed similarity in gene content but high diversity in the modulation of their transcription profiles on different plant substrates. Combining genomics and transcriptomics, we identified a set of core genes such as specific transcription factors, putatively involved in plant cell wall degradation.

CONCLUSIONS

These results indicate that the ancestral Colletotrichum were associated with dicot plants and certain branches progressively adapted to different monocot hosts, reshaping the gene content and its regulation.

Effective biosynthesis of 2,5-furandicarboxylic acid from 5-hydroxymethylfurfural via a bi-enzymatic cascade system using bacterial laccase and fungal alcohol oxidase

BACKGROUND

As a cost-effective and eco-friendly approach, biocatalysis has great potential for the transformation of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA). However, the compatibility of each enzyme in the cascade reaction limits the transformation efficiency of HMF to FDCA.

RESULTS

Coupled with an alcohol oxidase from Colletotrichum gloeosporioides (CglAlcOx), this study aims to study the potential of bacterial laccase from Bacillus pumilus (BpLac) in an enzymatic cascade for 2,5-furandicarboxylic acid (FDCA) biosynthesis from 5-hydroxymethylfurfural (HMF). BpLac showed 100% selectivity for HMF oxidation and generated 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). CglAlcOx was capable of oxidizing HMFCA to 2-formyl-5-furancarboxylic acid (FFCA). Both BpLac and CglAlcOx could oxidize FFCA to FDCA. At the 5 mM scale, a complete transformation of HMF with a 97.5% yield of FDCA was achieved by coupling BpLac with CglAlcOx in the cascade reaction. The FDCA productivity in the reaction was 5.3 mg/L/h. Notably, BpLac could alleviate the inhibitory effect of FFCA on CglAlcOx activity and boost the transformation efficiency of HMF to FDCA. Moreover, the reaction was scaled up to 40 times the volume, and FDCA titer reached 2.6 mM with a yield of 58.77% at 168 h.

CONCLUSIONS

This work provides a candidate and novel insight for better design of an enzymatic cascade in FDCA production.

Deciphering the effect of amine versus imine ligands of copper(II) complexes in 2-aminophenol oxidation

A series of amine (1-6) and imine (5',6') based copper(II) complexes with tridentate (NNO) ligand donors were synthesized and characterized using modern analytical techniques. All the complexes were subjected to 2-aminophenol (OAP) oxidation to form 2-aminophenoxazin-3-one, as a functional analogue of an enzyme, phenoxazinone synthase. In addition, a critical comparison of the reactivity using the amine-based complexes with their respective imine counterparts was achieved in both experimental as well as theoretical studies. For instance, the kinetic measurement revealed that the imine-based copper(II) complexes (kcat, 2.4 × 105-6.2 × 106 h-1) are better than amine-based (kcat, 6.3 × 104-3.9 × 105 h-1) complexes. The complex-substrate adducts [Cu(L3)(OAP)] (7) and [Cu(L3')(OAP)] (7') were characterized for both systems by mass spectrometry. Further, the DFT study was performed with amine- (3) and imine- (3') based copper(II) complexes, to compare their efficacy in the oxidation of OAP. The mechanistic investigations reveal that the key elementary step to determine the reactivity of 3 and 3' is the proton-coupled electron transfer (PCET) step occurring from the intermediates 7/7'. Further, the computed HOMO-LUMO energy gap of 7' was smaller than 7 by 0.8 eV, which indicates the facile PCET compared to that of 7. Moreover, the coupling of the OAP moiety using imine-complexes (ΔGR.E = -5.8 kcal/mol) was found to be thermodynamically more favorable than amine complexes (ΔGR.E = +3.3 kcal/mol). Overall, the theoretical findings are in good agreement with the experimental results.

Enzymatic reactions towards aldehydes: An overview

Many aldehydes are volatile compounds with distinct and characteristic olfactory properties. The aldehydic functional group is reactive and, as such, an invaluable chemical multi-tool to make all sorts of products. Owing to the reactivity, the selective synthesis of aldehydic is a challenging task. Nature has evolved a number of enzymatic reactions to produce aldehydes, and this review provides an overview of aldehyde-forming reactions in biological systems and beyond. Whereas some of these biotransformations are still in their infancy in terms of synthetic applicability, others are developed to an extent that allows their implementation as industrial biocatalysts.

Expanding the Physiological Role of Aryl-Alcohol Flavooxidases as Quinone Reductases

Aryl-alcohol oxidases (AAOs) are members of the glucose-methanol-choline oxidase/dehydrogenase (GMC) superfamily. These extracellular flavoproteins have been described as auxiliary enzymes in the degradation of lignin by several white-rot basidiomycetes. In this context, they oxidize fungal secondary metabolites and lignin-derived compounds using O₂ as an electron acceptor, and supply H₂O₂ to ligninolytic peroxidases. Their substrate specificity, including mechanistic aspects of the oxidation reaction, has been characterized in Pleurotus eryngii AAO, taken as a model enzyme of this GMC superfamily. AAOs show broad reducing-substrate specificity in agreement with their role in lignin degradation, being able to oxidize both nonphenolic and phenolic aryl alcohols (and hydrated aldehydes). In the present work, the AAOs from Pleurotus ostreatus and Bjerkandera adusta were heterologously expressed in Escherichia coli, and their physicochemical properties and oxidizing abilities were compared with those of the well-known recombinant AAO from P. eryngii. In addition, electron acceptors different from O₂, such as p-benzoquinone and the artificial redox dye 2,6-Dichlorophenolindophenol, were also studied. Differences in reducing-substrate specificity were found between the AAO enzymes from B. adusta and the two Pleurotus species. Moreover, the three AAOs oxidized aryl alcohols concomitantly with the reduction of p-benzoquinone, with similar or even higher efficiencies than when using their preferred oxidizing-substrate, O₂. IMPORTANCE In this work, quinone reductase activity is analyzed in three AAO flavooxidases, whose preferred oxidizing-substrate is O₂. The results presented, including reactions in the presence of both oxidizing substrates-benzoquinone and molecular oxygen-suggest that such aryl-alcohol dehydrogenase activity, although less important than its oxidase activity in terms of maximal turnover, may have a physiological role during fungal decay of lignocellulose by the reduction of quinones (and phenoxy radicals) from lignin degradation, preventing repolymerization. Moreover, the resulting hydroquinones would participate in redox-cycling reactions for the production of hydroxyl free radical involved in the oxidative attack of the plant cell-wall. Hydroquinones can also act as mediators for laccases and peroxidases in lignin degradation in the form of semiquinone radicals, as well as activators of lytic polysaccharide monooxygenases in the attack of crystalline cellulose. Moreover, reduction of these, and other phenoxy radicals produced by laccases and peroxidases, promotes lignin degradation by limiting repolymerization reactions. These findings expand the role of AAO in lignin biodegradation.

Tailoring Galactose Oxidase for Self-Powered Benzyl Alcohol Sensing

Benzyl alcohol (BnOH) is a widely-used preservative in a variety of cosmetics, but the excess addition (≥1.0 %) may cause strong symptoms such as nausea, gastrointestinal irritation, convulsion, even death, making it crucial to monitor and control the addition quantity. Herein, we have developed a test-strip-like BnOH detection method via tailoring a galactose oxidase (GOase) towards BnOH oxidation and preparing a self-powered electrochromic strip for BnOH concentration visualization. A double-substituted GOase variant (Y329S/R330F), on the basis of the reported GOase M₁ , has been obtained by semi-rational design with a 24.6-fold improved activity towards BnOH compared to GOase M₁ . The GOase Y329S/R330F electrode has a response to BnOH with a linear range of 0.04 to 3.25 mM (R² =0.9985), a sensitivity of 122.78 μA mM^-1  cm^-2 , and a detection limit of 0.03 mM (S/N=3). Coupling an electrochromic Prussian blue (PB) cathode helps the successful sensing visualization without any further power supply. The present sensing is more convenient and user-friendly than the generally used gas chromatography (GC) and high performance liquid chromatography (HPLC), and brings a more accessible solution to the field of quality controlling.

Copper radical oxidases: galactose oxidase, glyoxal oxidase, and beyond!

The copper radical oxidases (CROs) are an evolutionary and functionally diverse group of enzymes established by the historically significant galactose 6-oxidase and glyoxal oxidase from fungi. Inducted in 2013, CROs now constitute Auxiliary Activity Family 5 (AA5) in the Carbohydrate-Active Enzymes (CAZy) classification. CROs catalyse the two-electron oxidation of their substrates using oxygen as the final electron acceptor and are particularly distinguished by a cross-linked tyrosine-cysteine co-factor that is integral to radical stabilization. Recently, there has been a significant increase in the biochemically and structurally characterized CROs, which has revealed an expanded natural diversity of catalytic activities in the family. This review provides a brief historical introduction to CRO biochemistry and structural biology as a foundation for an update on current advances in CRO enzymology, biotechnology, and biology across kingdoms of life.

Tandem metalloenzymes gate plant cell entry by pathogenic fungi

Global food security is endangered by fungal phytopathogens causing devastating crop production losses. Many of these pathogens use specialized appressoria cells to puncture plant cuticles. Here, we unveil a pair of alcohol oxidase-peroxidase enzymes to be essential for pathogenicity. Using Colletotrichum orbiculare, we show that the enzyme pair is cosecreted by the fungus early during plant penetration and that single and double mutants have impaired penetration ability. Molecular modeling, biochemical, and biophysical approaches revealed a fine-tuned interplay between these metalloenzymes, which oxidize plant cuticular long-chain alcohols into aldehydes. We show that the enzyme pair is involved in transcriptional regulation of genes necessary for host penetration. The identification of these infection-specific metalloenzymes opens new avenues on the role of wax-derived compounds and the design of oxidase-specific inhibitors for crop protection.

Active-Site Engineering Switches Carbohydrate Regiospecificity in a Fungal Copper Radical Oxidase

Copper radical oxidases (CROs) from Auxiliary Activity Family 5, Subfamily 2 (AA5_2), are organic cofactor-free biocatalysts for the selective oxidation of alcohols to the corresponding aldehydes. AA5_2 CROs comprise canonical galactose-6-oxidases as well as the more recently discovered general alcohol oxidases and aryl alcohol oxidases. Guided by primary and tertiary protein structural analyses, we targeted a distinct extended loop in the active site of a Colletotrichum graminicola aryl alcohol oxidase (CgrAAO) to explore its effect on catalysis in the broader context of AA5_2. Deletion of this loop, which is bracketed by a conserved disulfide bridge, significantly reduced the inherent activity of the enzyme toward extended galacto-oligosaccharides, as anticipated from molecular modeling. Unexpectedly, kinetic and product analysis on a range of monosaccharides and disaccharides revealed that an altered carbohydrate specificity in CgrAAO-Δloop was accompanied by a complete change in regiospecificity from C-6 to C-1 oxidation, thereby generating aldonic acids. C-1 regiospecificity is unprecedented in AA5 enzymes and is classically associated with flavin-dependent carbohydrate oxidases of Auxiliary Activity Family 3. Thus, this work further highlights the catalytic adaptability of the unique mononuclear copper radical active site and provides a basis for the design of improved biocatalysts for diverse potential applications.

Basidiomycota Fungi and ROS: Genomic Perspective on Key Enzymes Involved in Generation and Mitigation of Reactive Oxygen Species

Our review includes a genomic survey of a multitude of reactive oxygen species (ROS) related intra- and extracellular enzymes and proteins among fungi of Basidiomycota, following their taxonomic classification within the systematic classes and orders, and focusing on different fungal lifestyles (saprobic, symbiotic, pathogenic). Intra- and extracellular ROS metabolism-involved enzymes (49 different protein families, summing 4170 protein models) were searched as protein encoding genes among 63 genomes selected according to current taxonomy. Extracellular and intracellular ROS metabolism and mechanisms in Basidiomycota are illustrated in detail. In brief, it may be concluded that differences between the set of extracellular enzymes activated by ROS, especially by H2O2, and involved in generation of H2O2, follow the differences in fungal lifestyles. The wood and plant biomass degrading white-rot fungi and the litter-decomposing species of Agaricomycetes contain the highest counts for genes encoding various extracellular peroxidases, mono- and peroxygenases, and oxidases. These findings further confirm the necessity of the multigene families of various extracellular oxidoreductases for efficient and complete degradation of wood lignocelluloses by fungi. High variations in the sizes of the extracellular ROS-involved gene families were found, however, among species with mycorrhizal symbiotic lifestyle. In addition, there are some differences among the sets of intracellular thiol-mediation involving proteins, and existence of enzyme mechanisms for quenching of intracellular H2O2 and ROS. In animal- and plant-pathogenic species, extracellular ROS enzymes are absent or rare. In these fungi, intracellular peroxidases are seemingly in minor role than in the independent saprobic, filamentous species of Basidiomycota. Noteworthy is that our genomic survey and review of the literature point to that there are differences both in generation of extracellular ROS as well as in mechanisms of response to oxidative stress and mitigation of ROS between fungi of Basidiomycota and Ascomycota.

α-Dioxygenases (α-DOXs): Promising biocatalysts for the environmentally friendly production of aroma compounds

Fatty aldehydes (FALs) can be derived from fatty acids (FAs) and related compounds and are frequently used as flavors and fragrances. Although chemical methods have been conventionally used, their selective biotechnological production aiming at more efficient and eco‐friendly synthetic routes is in demand. α‐Dioxygenases (α‐DOXs) are heme‐dependent oxidative enzymes biologically involved in the initial step of plant FA α‐oxidation during which molecular oxygen is incorporated into the C_α‐position of a FA (C_n) to generate the intermediate FA hydroperoxide, which is subsequently converted into the shortened corresponding FAL (C_n‐1). α‐DOXs are promising biocatalysts for the flavor and fragrance industries, they do not require NAD(P)H as cofactors or redox partner proteins, and they have a broad substrate scope. Here, we highlight recent advances in the biocatalytic utilization of α‐DOXs with emphasis on newly discovered cyanobacterial α‐DOXs as well as analytical methods to measure α‐DOX activity in vitro and in vivo.

Tunable Production of (R)- or (S)-Citronellal from Geraniol via a Bienzymatic Cascade Using a Copper Radical Alcohol Oxidase and Old Yellow Enzyme

Biocatalytic pathways for the synthesis of (-)-menthol, the most sold flavor worldwide, are highly sought-after. To access the key intermediate (R)-citronellal used in current major industrial production routes, we established a one-pot bienzymatic cascade from inexpensive geraniol, overcoming the problematic biocatalytic reduction of the mixture of (E/Z)-isomers in citral by harnessing a copper radical oxidase (CgrAlcOx) and an old yellow enzyme (OYE). The cascade using OYE2 delivered 95.1% conversion to (R)-citronellal with 95.9% ee, a 62 mg scale-up affording high yield and similar optical purity. An alternative OYE, GluER, gave (S)-citronellal from geraniol with 95.3% conversion and 99.2% ee.

2D and 3D maximum-quantum NMR and diffusion spectroscopy for the characterization of enzymatic reaction mixtures

Using MaxQ-NMR, we characterized enzymatic reaction mixtures containing several compounds (substrate, final product, and various intermediates). This approach enables, in a first analytical step, the counting of the molecules present in the samples.

A simple and direct ionic chromatography method to monitor galactose oxidase activity

Galactose oxidase (GalOx, EC.1.1.3.9) is one of the most extensively studied copper radical oxidases (CROs). The reaction catalyzed by GalOx leads to the oxidation of the C-6 hydroxyl group of galactose and galactosides (including galactosylated polysaccharides and glycoproteins) to the corresponding aldehydes, coupled to the reduction of dioxygen to hydrogen peroxide. Despite more than 60 years of research including mechanistic studies, enzyme engineering and application development, GalOx activity remains primarily monitored by indirect measurement of the co-product hydrogen peroxide. Here, we describe a simple direct method to measure GalOx activity through the identification of galactosylated oxidized products using high-performance anion-exchange chromatography coupled to pulsed amperometric detection (HPAEC-PAD). Using galactose and lactose as representative substrates, we were able to separate and detect the C-6 oxidized products, which were confirmed by LC-MS and NMR analyses to exist in their hydrated (geminal-diol) forms. We show that the HPAEC-PAD method is superior to other methods in terms of sensitivity as we could detect down to 0.08 μM of Lac^OX (eq. 30 μg L⁻¹). We believe the method will prove useful for qualitative detection of galactose oxidase activity in biological samples or for quantitative purposes to analyze enzyme kinetics or to compare enzyme variants in directed evolution programs.

Galactose oxidase (GalOx, EC.1.1.3.9) is one of the most extensively studied copper radical oxidases. Here, we show it can be monitored through the release of oxidized galactosylated products using a simple, direct and sensitive HPAEC-PAD method.

A survey of substrate specificity among Auxiliary Activity Family 5 copper radical oxidases

There is significant contemporary interest in the application of enzymes to replace or augment chemical reagents toward the development of more environmentally sound and sustainable processes. In particular, copper radical oxidases (CRO) from Auxiliary Activity Family 5 Subfamily 2 (AA5_2) are attractive, organic cofactor-free catalysts for the chemoselective oxidation of alcohols to the corresponding aldehydes. These enzymes were first defined by the archetypal galactose-6-oxidase (GalOx, EC 1.1.3.13) from the fungus Fusarium graminearum. The recent discovery of specific alcohol oxidases (EC 1.1.3.7) and aryl alcohol oxidases (EC 1.1.3.47) within AA5_2 has indicated a potentially broad substrate scope among fungal CROs. However, only relatively few AA5_2 members have been characterized to date. Guided by sequence similarity network and phylogenetic analysis, twelve AA5_2 homologs have been recombinantly produced and biochemically characterized in the present study. As defined by their predominant activities, these comprise four galactose 6-oxidases, two raffinose oxidases, four broad-specificity primary alcohol oxidases, and two non-carbohydrate alcohol oxidases. Of particular relevance to applications in biomass valorization, detailed product analysis revealed that two CROs produce the bioplastics monomer furan-2,5-dicarboxylic acid (FDCA) directly from 5-hydroxymethylfurfural (HMF). Furthermore, several CROs could desymmetrize glycerol (a by-product of the biodiesel industry) to D- or L-glyceraldehyde. This study furthers our understanding of CROs by doubling the number of characterized AA5_2 members, which may find future applications as biocatalysts in diverse processes.

Identification of Copper-Containing Oxidoreductases in the Secretomes of Three Colletotrichum Species with a Focus on Copper Radical Oxidases for the Biocatalytic Production of Fatty Aldehydes

Copper radical alcohol oxidases (CRO-AlcOx), which have been recently discovered among fungal phytopathogens, are attractive for the production of fragrant fatty aldehydes. With the initial objective to investigate the secretion of CRO-AlcOx by natural fungal strains, we undertook time course analyses of the secretomes of three Colletotrichum species (C. graminicola, C. tabacum, and C. destructivum) using proteomics. The addition of a copper-manganese-ethanol mixture in the absence of any plant-biomass mimicking compounds to Colletotrichum cultures unexpectedly induced the secretion of up to 400 proteins, 29 to 52% of which were carbohydrate-active enzymes (CAZymes), including a wide diversity of copper-containing oxidoreductases from the auxiliary activities (AA) class (AA1, AA3, AA5, AA7, AA9, AA11, AA12, AA13, and AA16). Under these specific conditions, while a CRO-glyoxal oxidase from the AA5_1 subfamily was among the most abundantly secreted proteins, the targeted AA5_2 CRO-AlcOx were secreted at lower levels, suggesting heterologous expression as a more promising strategy for CRO-AlcOx production and utilization. C. tabacum and C. destructivum CRO-AlcOx were thus expressed in Pichia pastoris, and their preference toward both aromatic and aliphatic primary alcohols was assessed. The CRO-AlcOx from C. destructivum was further investigated in applied settings, revealing a full conversion of C₆ and C₈ alcohols into their corresponding fragrant aldehydes. IMPORTANCE In the context of the industrial shift toward greener processes, the biocatalytic production of aldehydes is of utmost interest owing to their importance for their use as flavor and fragrance ingredients. Copper radical alcohol oxidases (CRO-AlcOx) have the potential to become platform enzymes for the oxidation of alcohols to aldehydes. However, the secretion of CRO-AlcOx by natural fungal strains has never been explored, while the use of crude fungal secretomes is an appealing approach for industrial applications to alleviate various costs pertaining to biocatalyst production. While investigating this primary objective, the secretomics studies revealed unexpected results showing that under the oxidative stress conditions we probed, Colletotrichum species can secrete a broad diversity of copper-containing enzymes (laccases, sugar oxidoreductases, and lytic polysaccharide monooxygenases [LPMOs]) usually assigned to "plant cell wall degradation," despite the absence of any plant-biomass mimicking compound. However, in these conditions, only small amounts of CRO-AlcOx were secreted, pointing out recombinant expression as the most promising path for their biocatalytic application.

A Lignocellulolytic Colletotrichum sp. OH with Broad-Spectrum Tolerance to Lignocellulosic Pretreatment Compounds and Derivatives and the Efficiency to Produce Hydrogen Peroxide and 5-Hydroxymethylfurfural Tolerant Cellulases

Fungal endophytes are an emerging source of novel traits and biomolecules suitable for lignocellulosic biomass treatment. This work documents the toxicity tolerance of Colletotrichum sp. OH toward various lignocellulosic pretreatment-derived inhibitors. The effects of aldehydes (vanillin, p-hydroxybenzaldehyde, furfural, 5-hydroxymethylfurfural; HMF), acids (gallic, formic, levulinic, and p-hydroxybenzoic acid), phenolics (hydroquinone, p-coumaric acid), and two pretreatment chemicals (hydrogen peroxide and ionic liquid), on the mycelium growth, biomass accumulation, and lignocellulolytic enzyme activities, were tested. The reported Colletotrichum sp. OH was naturally tolerant to high concentrations of single inhibitors like HMF (IC50; 17.5 mM), levulinic acid (IC50; 29.7 mM), hydroquinone (IC50; 10.76 mM), and H2O2 (IC50; 50 mM). The lignocellulolytic enzymes displayed a wide range of single and mixed inhibitor tolerance profiles. The enzymes β-glucosidase and endoglucanase showed H2O2- and HMF-dependent activity enhancements. The enzyme β-glucosidase activity was 34% higher in 75 mM and retained 20% activity in 125 mM H2O2. Further, β-glucosidase activity increased to 24 and 32% in the presence of 17.76 and 8.8 mM HMF. This research suggests that the Colletotrichum sp. OH, or its enzymes, can be used to pretreat plant biomass, hydrolyze it, and remove inhibitory by-products.

A Putative Lignin Copper Oxidase from Trichoderma reesei

The ability of Trichoderma reesei, a fungus widely used for the commercial production of hemicellulases and cellulases, to grow and modify technical soda lignin was investigated. By quantifying fungal genomic DNA, T. reesei showed growth and sporulation in solid and liquid cultures containing lignin alone. The analysis of released soluble lignin and residual insoluble lignin was indicative of enzymatic oxidative conversion of phenolic lignin side chains and the modification of lignin structure by cleaving the β-O-4 linkages. The results also showed that polymerization reactions were taking place. A proteomic analysis conducted to investigate secreted proteins at days 3, 7, and 14 of growth revealed the presence of five auxiliary activity (AA) enzymes in the secretome: AA6, AA9, two AA3 enzymes), and the only copper radical oxidase encoded in the genome of T. reesei. This enzyme was heterologously produced and characterized, and its activity on lignin-derived molecules was investigated. Phylogenetic characterization demonstrated that this enzyme belonged to the AA5_1 family, which includes characterized glyoxal oxidases. However, the enzyme displayed overlapping physicochemical and catalytic properties across the AA5 family. The enzyme was remarkably stable at high pH and oxidized both, alcohols and aldehydes with preference to the alcohol group. It was also active on lignin-derived phenolic molecules as well as simple carbohydrates. HPSEC and LC-MS analyses on the reactions of the produced protein on lignin dimers (SS ββ, SS βO4 and GG β5) uncovered the polymerizing activity of this enzyme, which was accordingly named lignin copper oxidase (TrLOx). Polymers of up 10 units were formed by hydroxy group oxidation and radical formation. The activations of lignin molecules by TrLOx along with the co-secretion of this enzyme with reductases and FAD flavoproteins oxidoreductases during growth on lignin suggest a synergistic mechanism for lignin breakdown.

Biocatalytic oxidation of fatty alcohols into aldehydes for the flavors and fragrances industry

From Egyptian mummies to the Chanel n°5 perfume, fatty aldehydes have long been used and keep impacting our senses in a wide range of foods, beverages and perfumes. Natural sources of fatty aldehydes are threatened by qualitative and quantitative variability while traditional chemical routes are insufficient to answer the society shift toward more sustainable and natural products. The production of fatty aldehydes using biotechnologies is therefore the most promising alternative for the flavors and fragrances industry. In this review, after drawing the portrait of the origin and characteristics of fragrant fatty aldehydes, we present the three main classes of enzymes that catalyze the reaction of fatty alcohols oxidation into aldehydes, namely alcohol dehydrogenases, flavin-dependent alcohol oxidases and copper radical alcohol oxidases. The constraints, challenges and opportunities to implement these oxidative enzymes in the flavors and fragrances industry are then discussed. By setting the scene on the biocatalytic production of fatty aldehydes, and providing a critical assessment of its potential, we expect this review to contribute to the development of biotechnology-based solutions in the flavors and fragrances industry.

Two Fusarium copper radical oxidases with high activity on aryl alcohols

BACKGROUND

Biomass valorization has been suggested as a sustainable alternative to petroleum-based energy and commodities. In this context, the copper radical oxidases (CROs) from Auxiliary Activity Family 5/Subfamily 2 (AA5_2) are attractive biocatalysts for the selective oxidation of primary alcohols to aldehydes. Originally defined by the archetypal galactose 6-oxidase from Fusarium graminearum, fungal AA5_2 members have recently been shown to comprise a wide range of specificities for aromatic, aliphatic and furan-based alcohols. This suggests a broader substrate scope of native CROs for applications. However, only 10% of the annotated AA5_2 members have been characterized to date.

RESULTS

Here, we define two homologues from the filamentous fungi Fusarium graminearum and F. oxysporum as predominant aryl alcohol oxidases (AAOs) through recombinant production in Pichia pastoris, detailed kinetic characterization, and enzyme product analysis. Despite possessing generally similar active-site architectures to the archetypal FgrGalOx, FgrAAO and FoxAAO have weak activity on carbohydrates, but instead efficiently oxidize specific aryl alcohols. Notably, both FgrAAO and FoxAAO oxidize hydroxymethyl furfural (HMF) directly to 5-formyl-2-furoic acid (FFCA), and desymmetrize the bioproduct glycerol to the uncommon L-isomer of glyceraldehyde.

CONCLUSIONS

This work expands understanding of the catalytic diversity of CRO from AA5_2 to include unique representatives from Fusarium species that depart from the well-known galactose 6-oxidase activity of this family. Detailed enzymological analysis highlights the potential biotechnological applications of these orthologs in the production of renewable plastic polymer precursors and other chemicals.

Spectroelectrochemical investigation of the glyoxal oxidase activation mechanism

Glyoxal oxidase (GLOX) is an extracellular source of H₂O₂ in white-rot secretomes, where it acts in concert with peroxidases to degrade lignin. It has been reported that GLOX requires activation prior to catalytic turnover and that a peroxidase system can fulfill this task. In this study, we verify that an oxidation product of horseradish peroxidase, the radical cation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), is an activator for GLOX. A spectroelectrochemical cell was used to generate the activating radical species, to continuously measure its concentration, and to simultaneously measure the catalytic activity of GLOX based on its O₂ consumption. The results show that GLOX can undergo multiple catalytic turnovers upon activation and that activity increases with the activator concentration. However, we also found that the ABTS cation radical can serve as an electron acceptor which becomes visible in the absence of O₂. Furthermore, GLOX activity is highly restrained by the naturally occurring, low O₂ concentration. We conclude that GLOX is indeed an auxiliary enzyme for H₂O₂ production in white-rot secretomes. Its turnover rate is strongly regulated by the availability of O₂ and the radical generating activity of peroxidases present in the secretome, which acts as a feedback loop for GLOX activity.

Fungal Treatment for the Valorization of Technical Soda Lignin

Technical lignins produced as a by-product in biorefinery processes represent a potential source of renewable carbon. In consideration of the possibilities of the industrial transformation of this substrate into various valuable bio-based molecules, the biological deconstruction of a technical soda lignin by filamentous fungi was investigated. The ability of three basidiomycetes (Polyporus brumalis, Pycnoporus sanguineus and Leiotrametes menziesii) to modify this material, the resultant structural and chemical changes, and the secreted proteins during growth on this substrate were investigated. The three fungi could grow on the technical lignin alone, and the growth rate increased when the media were supplemented with glucose or maltose. The proteomic analysis of the culture supernatants after three days of growth revealed the secretion of numerous Carbohydrate-Active Enzymes (CAZymes). The secretomic profiles varied widely between the strains and the presence of technical lignin alone triggered the early secretion of many lignin-acting oxidoreductases. The secretomes were notably rich in glycoside hydrolases and H₂O₂-producing auxiliary activity enzymes with copper radical oxidases being induced on lignin for all strains. The lignin treatment by fungi modified both the soluble and insoluble lignin fractions. A significant decrease in the amount of soluble higher molar mass compounds was observed in the case of P. sanguineus. This strain was also responsible for the modification of the lower molar mass compounds of the lignin insoluble fraction and a 40% decrease in the thioacidolysis yield. The similarity in the activities of P. sanguineus and P. brumalis in modifying the functional groups of the technical lignin were observed, the results suggest that the lignin has undergone structural changes, or at least changes in its composition, and pave the route for the utilization of filamentous fungi to functionalize technical lignins and produce the enzymes of interest for biorefinery applications.

Formation of Monofluorinated Radical Cofactor in Galactose Oxidase through Copper-Mediated C-F Bond Scission

Galactose oxidase (GAO) contains a Cu(II)-ligand radical cofactor. The cofactor, which is autocatalytically generated through the oxidation of the copper, consists of a cysteine-tyrosine radical (Cys-Tyr^•) as a copper ligand. The formation of the cross-linked thioether bond is accompanied by a C-H bond scission on Tyr272 with few details known thus far. Here, we report the genetic incorporation of 3,5-dichlorotyrosine (Cl₂-Tyr) and 3,5-difluorotyrosine (F₂-Tyr) to replace Tyr272 in the GAO^V previously optimized for expression through directed evolution. The proteins with an unnatural tyrosine residue are catalytically competent. We determined the high-resolution crystal structures of the GAO^V, Cl₂-Tyr272, and F₂-Tyr272 incorporated variants at 1.48, 1.23, and 1.80 Å resolution, respectively. The structural data showed only one halogen remained in the cofactor, indicating that an oxidative carbon-chlorine/fluorine bond scission has occurred during the autocatalytic process of cofactor biogenesis. Using hydroxyurea as a radical scavenger, the spin-coupled hidden Cu(II) was observed by EPR spectroscopy. Thus, the structurally defined catalytic center with genetic unnatural tyrosine substitution is in the radical containing form as in the wild-type, i.e., Cu(II)-(Cl-Tyr^•-Cys) or Cu(II)-(F-Tyr^•-Cys). These findings illustrate a previously unobserved C-F/C-Cl bond cleavage in biology mediated by a mononuclear copper center.

The vast repertoire of carbohydrate oxidases: An overview

Carbohydrates are widely abundant molecules present in a variety of forms. For their biosynthesis and modification, nature has evolved a plethora of carbohydrate-acting enzymes. Many of these enzymes are of particular interest for biotechnological applications, where they can be used as biocatalysts or biosensors. Among the enzymes catalysing conversions of carbohydrates are the carbohydrate oxidases. These oxidative enzymes belong to different structural families and use different cofactors to perform the oxidation reaction of CH-OH bonds in carbohydrates. The variety of carbohydrate oxidases available in nature reflects their specificity towards different sugars and selectivity of the oxidation site. Thanks to their properties, carbohydrate oxidases have received a lot of attention in basic and applied research, such that nowadays their role in biotechnological processes is of paramount importance. In this review we provide an overview of the available knowledge concerning the known carbohydrate oxidases. The oxidases are first classified according to their structural features. After a description on their mechanism of action, substrate acceptance and characterisation, we report on the engineering of the different carbohydrate oxidases to enhance their employment in biocatalysis and biotechnology. In the last part of the review we highlight some practical applications for which such enzymes have been exploited.

Glucose counteracts wood-dependent induction of lignocellulolytic enzyme secretion in monokaryon and dikaryon submerged cultures of the white-rot basidiomycete Pleurotus ostreatus

The secretome complexity and lignocellulose degrading capacity of Pleurotus ostreatus monokaryons mkPC9 and mkPC15 and mated dikaryon dkN001 were studied in submerged liquid cultures containing wood, glucose, and wood plus glucose as carbon sources. The study revealed that this white-rot basidiomycete attacks all the components of the plant cell wall. P. ostreatus secretes a variety of glycoside hydrolases, carbohydrate esterases, and polysaccharide lyases, especially when wood is the only carbon source. The presence of wood increased the secretome complexity, whereas glucose diminished the secretion of enzymes involved in cellulose, hemicellulose and pectin degradation. In contrast, the presence of glucose did not influence the secretion of redox enzymes or proteases, which shows the specificity of glucose on the secretion of cellulolytic enzymes. The comparison of the secretomes of monokaryons and dikaryons reveals that secretome complexity is unrelated to the nuclear composition of the strain.

Determination of biocatalytic parameters of a copper radical oxidase using real-time reaction progress monitoring

VTNA is applied to reaction progress curves to glean key kinetic and mechanistic details for a copper radical oxidase.

Design of an in vitro biocatalytic cascade for the manufacture of islatravir

Enzyme-catalyzed reactions have begun to transform pharmaceutical manufacturing, offering levels of selectivity and tunability that can dramatically improve chemical synthesis. Combining enzymatic reactions into multistep biocatalytic cascades brings additional benefits. Cascades avoid the waste generated by purification of intermediates. They also allow reactions to be linked together to overcome an unfavorable equilibrium or avoid the accumulation of unstable or inhibitory intermediates. We report an in vitro biocatalytic cascade synthesis of the investigational HIV treatment islatravir. Five enzymes were engineered through directed evolution to act on non-natural substrates. These were combined with four auxiliary enzymes to construct islatravir from simple building blocks in a three-step biocatalytic cascade. The overall synthesis requires fewer than half the number of steps of the previously reported routes.

The Cellulosome Paradigm in An Extreme Alkaline Environment

Rapid decomposition of plant biomass in soda lakes is associated with microbial activity of anaerobic cellulose-degrading communities. The alkaliphilic bacterium, Clostridium alkalicellulosi, is the single known isolate from a soda lake that demonstrates cellulolytic activity. This microorganism secretes cellulolytic enzymes that degrade cellulose under anaerobic and alkaliphilic conditions. A previous study indicated that the protein fraction of cellulose-grown cultures showed similarities in composition and size to known components of the archetypical cellulosome Clostridium thermocellum. Bioinformatic analysis of the C. alkalicellulosi draft genome sequence revealed 44 cohesins, organized into 22 different scaffoldins, and 142 dockerin-containing proteins. The modular organization of the scaffoldins shared similarities to those of C. thermocellum and Acetivibrio cellulolyticus, whereas some exhibited unconventional arrangements containing peptidases and oxidative enzymes. The binding interactions among cohesins and dockerins assessed by ELISA, revealed a complex network of cellulosome assemblies and suggested both cell-associated and cell-free systems. Based on these interactions, C. alkalicellulosi cellulosomal systems have the genetic potential to create elaborate complexes, which could integrate up to 105 enzymatic subunits. The alkalistable C. alkalicellulosi cellulosomal systems and their enzymes would be amenable to biotechnological processes, such as treatment of lignocellulosic biomass following prior alkaline pretreatment.

Effects on hyphal morphology and development by the putative copper radical oxidase glx1 in Trichoderma virens suggest a novel role as a cell wall associated enzyme

Trichoderma spp. have been characterized for their capacity to act as biological control agents against several pathogens through the activity of secondary metabolites and cell wall degrading enzymes. However, only T. reesei has been widely studied for the ability to assimilate lignocellulose substrates. Protein analysis by SDS-PAGE of culture filtrate of T. virens revealed the presence of an unknown ∼77 kDa band protein (GLX1) that showed sequence homology to glyoxal-like oxidase genes involved in lignin degradation. The analysis and biochemical characterization of the 1,119 amino acid coded protein showed the presence of five carbohydrate-binding modules (CBMs) with affinity for colloidal chitin, and a functional glyoxal oxidase catalytic domain that is involved in the production of hydrogen peroxide when methylglyoxal was used as a substrate. The silencing of the glx1 gene resulted in mutants with more than 90% expression reduction and the absence of glyoxal oxidase catalytic activity. These mutants showed delayed hyphal growth, reduced colony and conidial hydrophobicity, but showed no changes in their biocontrol ability. Most significantly, mutants exhibited a loss of growth directionality resulting in a curled phenotype that was eliminated in the presence of exogenous H₂O₂. Here we present evidence that in T. virens, glx1 is not involved in the breakdown of lignin but instead is responsible for normal hyphal growth and morphology and likely does this through free radical production within the fungal cell wall. This is the first time that a glyoxal oxidase protein has been isolated and characterized in ascomycete fungi.

Stable M(II)-Radicals and Nickel(III) Complexes of a Bis(phenol) N-Heterocyclic Carbene Chelated to Group 10 Metal Ions

The tetradentate ligand based on (1-imidazolium-3,5-di tert-butylphenol) units was prepared and chelated to group 10 metal ions (Ni(II), Pd(II), and Pt(II)), affording complexes 1, 2, and 3, respectively. The X-ray crystal structures of 1-3 show a square planar metal ion coordinated to two N-heterocyclic carbenes and two phenolate moieties. The cyclic voltammetry curves of complexes 1-3 show two reversible oxidation waves in the range 0.11-0.21 V ( E_1/2¹) and 0.55-0.65 V ( E_1/2²) vs Fc⁺/Fc, which are assigned to the successive oxidations of the phenolate moieties. One-electron oxidation affords mononuclear ( S = 1/2) systems. Complex 1⁺·SbF₆^- was remarkably stable, and its structure was characterized. The coordination sphere is slightly dissymmetric, while the typical patterns of phenoxyl radicals were observed within the ligand framework. Complex 1⁺ exhibits a rhombic signal at g = 2.087, 2.016, and 1.992, confirming its predominant phenoxyl radical character. The g-values are slightly smaller for 2⁺ (2.021, 2.008, and 1.983) and larger for 3⁺ (2.140, 1.999, and 1.885) yet consistent with phenoxyl radical species. The electronic spectra of 1⁺-3⁺ display an intervalence charge-transfer (IVCT) transition at 2396, 2600, and 2294 nm, respectively. Its intensity supports the description of cations 1⁺ and 3⁺ as mixed-valent (Class II/III) compounds according to the Robin Day classification. Complex 2⁺ behaves as a mixed-valent class II radical compound. In the presence of pyridine, radical species 1⁺ is successively converted into stable mono and bis(adducts), which are both Ni(III) complexes. Dications 1⁺²-3⁺² were prepared electrochemically. They are electron paramagnetic resonance (EPR)-silent and do not show IVCT transition in their NIR spectra, consistent with a bis(radical) formulation. The proposed electronic structures are fully supported by density functional theory calculations.

A family AA5_2 carbohydrate oxidase from Penicillium rubens displays functional overlap across the AA5 family

Copper radical alcohol oxidases belonging to auxiliary activity family 5, subfamily 2 (AA5_2) catalyze the oxidation of galactose and galactosides, as well as aliphatic alcohols. Despite their broad applied potential, so far very few AA5_2 members have been biochemically characterized. We report the recombinant production and biochemical characterization of an AA5_2 oxidase from Penicillium rubens Wisconsin 54–1255 (PruAA5_2A), which groups within an unmapped clade phylogenetically distant from those comprising AA5_2 members characterized to date. PruAA5_2 preferentially oxidized raffinose over galactose; however, its catalytic efficiency was 6.5 times higher on glycolaldehyde dimer compared to raffinose. Deep sequence analysis of characterized AA5_2 members highlighted amino acid pairs correlated to substrate range and conserved within the family. Moreover, PruAA5_2 activity spans substrate preferences previously reported for AA5 subfamily 1 and 2 members, identifying possible functional overlap across the AA5 family.

Mannich base Cu(II) complexes as biomimetic oxidative catalyst

Galactose Oxidase (GOase) and catechol oxidase (COase) are the metalloenzymes of copper having monomeric and dimeric sites of coordination, respectively. This paper summarizes the results of our studies on the structural, spectral and catalytic properties of new mononuclear copper (II) complexes [CuL(OAc)] (1), and [CuL₂] (2), (HL = 2,4‑dichloro‑6‑{[(2'‑dimethyl‑aminoethyl)methylamino]methyl}‑phenol) which can mimic the functionalities of the metalloenzymes GOase and COase. The structure of the compounds has been elucidated by X-ray crystallography and the mimicked Cu(II) catalysts were further characterized by EPR. These mimicked models were used for GOase and COase catalysis. The GOase catalytic results were identified by GC-MS and, analyzed by HPLC at room temperature. The conversion of benzyl alcohol to benzaldehyde were significant in presence of a strong base, Bu₄NOMe in comparison to the neutral medium. Apart from that, despite of being monomeric in nature, both the homogeneous catalysts are very prone to participate in COase mimicking oxidation reaction. Nevertheless, during COase catalysis, complex 1 was found to convert 3,5‑ditertarybutyl catechol (3,5-DTBC) to 3,5‑ditertarybutyl quinone (3,5-DTBQ) having greater rate constant, k_cat or turn over number (TON) value over complex 2. The generation of reactive intermediates during COase catalysis were accounted by electrospray ionization mass spectrometry (ESI-MS). Through mechanistic approach, we found that H₂O₂ is the byproduct for both the GOase and COase catalysis, thus, confirming the generation of reactive oxygen species during catalysis. Notably, complex 1 having mono-ligand coordinating atmosphere has superior catalytic activity for both cases in comparison to complex 2, that is having di-ligand environment.

RUBY, a Putative Galactose Oxidase, Influences Pectin Properties and Promotes Cell-To-Cell Adhesion in the Seed Coat Epidermis of Arabidopsis

Cell-to-cell adhesion is essential for establishment of multicellularity. In plants, such adhesion is mediated through a middle lamella composed primarily of pectic polysaccharides. The molecular interactions that influence cell-to-cell adhesion are not fully understood. We have used Arabidopsis (Arabidopsis thaliana) seed coat mucilage as a model system to investigate interactions between cell wall carbohydrates. Using a forward-genetic approach, we have discovered a gene, RUBY PARTICLES IN MUCILAGE (RUBY), encoding a protein that is annotated as a member of the Auxiliary Activity 5 (AA5) family of Carbohydrate-Active Enzymes (Gal/glyoxal oxidases) and is secreted to the apoplast late in the differentiation of seed coat epidermal cells. We show that RUBY is required for the Gal oxidase activity of intact seeds; the oxidation of Gal in side-chains of rhamnogalacturonan-I (RG-I) present in mucilage-modified2 (mum2) mucilage, but not in wild-type mucilage; the retention of branched RG-I in the seed following extrusion; and the enhancement of cell-to-cell adhesion in the seed coat epidermis. These data support the hypothesis that RUBY is a Gal oxidase that strengthens pectin cohesion within the middle lamella, and possibly the mucilage of wild-type seed coat epidermal cells, through oxidation of RG-I Gal side-chains.

Polysaccharide monooxygenase-catalyzed oxidation of cellulose to glucuronic acid-containing cello-oligosaccharides

BACKGROUND

Polysaccharide monooxygenases (PMOs) play an important role in the enzymatic degradation of cellulose. They have been demonstrated to able to C6-oxidize cellulose to produce C6-hexodialdoses. However, the biological function of C6 oxidation of PMOs remains unknown. In particular, it is unclear whether C6-hexodialdoses can be further oxidized to uronic acid (glucuronic acid-containing oligosaccharides).

RESULTS

A PMO gene, Hipmo1, was isolated from Humicola insolens and expressed in Pichia pastoris. This PMO (HiPMO1), belonging to the auxiliary activity 9 (AA9) family, was shown to able to cleave cellulose to yield non-oxidized and oxidized cello-oligosaccharides. The enzyme oxidizes C6 positions in cellulose to form glucuronic acid-containing cello-oligosaccharides, followed by hydrolysis with beta-glucosidase and beta-glucuronidase to yield glucose, glucuronic acid, and saccharic acid. This indicates that HiPMO1 can catalyze C6 oxidation of hydroxyl groups of cellulose to carboxylic groups.

CONCLUSIONS

HiPMO1 oxidizes C6 of cellulose to form glucuronic acid-containing cello-oligosaccharides followed by hydrolysis with beta-glucosidase and beta-glucuronidase to yield glucose, glucuronic acid, and saccharic acid, and even possibly by beta-eliminative cleavage to produce unsaturated cello-oligosaccharides. This study provides a new mechanism for cellulose cleavage by C6 oxidation of HiPMO1.

Dual oxidase/oxygenase reactivity and resonance Raman spectra of {Cu3O2} moiety with perfluoro-t-butoxide ligands

A mechanism for the formation of O-donor trinuclear {Cu₃O₂} moiety is reported.

An Engineered Alcohol Oxidase for the Oxidation of Primary Alcohols

Structure-guided directed evolution of choline oxidase has been carried out by using the oxidation of hexan-1-ol to hexanal as the target reaction. A six-amino-acid variant was identified with a 20-fold increased k_cat compared to that of the wild-type enzyme. This variant enabled the oxidation of 10 mm hexanol to hexanal in less than 24 h with 100 % conversion. Furthermore, this variant showed a marked increase in thermostability with a corresponding increase in T_m of 20 °C. Improved solvent tolerance was demonstrated with organic solvents including ethyl acetate, heptane and cyclohexane, thereby enabling improved conversions to the aldehyde by up to 30 % above conversion for the solvent-free system. Despite the evolution of choline oxidase towards hexan-1-ol, this new variant also showed increased specific activities (by up to 100-fold) for around 50 primary aliphatic, unsaturated, branched, cyclic, benzylic and halogenated alcohols.

Oxidoreductases and Reactive Oxygen Species in Conversion of Lignocellulosic Biomass

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

Characterization of a New Glyoxal Oxidase from the Thermophilic Fungus Myceliophthora thermophila M77: Hydrogen Peroxide Production Retained in 5-Hydroxymethylfurfural Oxidation

Myceliophthora thermophyla is a thermophilic industrially relevant fungus that secretes an assortment of hydrolytic and oxidative enzymes for lignocellulose degradation. Among them is glyoxal oxidase (MtGLOx), an extracellular oxidoreductase that oxidizes several aldehydes and α-hydroxy carbonyl substrates coupled to the reduction of O2 to H2O2. This copper metalloprotein belongs to a class of enzymes called radical copper oxidases (CRO) and to the “auxiliary activities” subfamily AA5_1 that is based on the Carbohydrate-Active enZYmes (CAZy) database. Only a few members of this family have been characterized to date. Here, we report the recombinant production, characterization, and structure-function analysis of MtGLOx. Electron Paramagnetic Resonance (EPR) spectroscopy confirmed MtGLOx to be a radical-coupled copper complex and small angle X-ray scattering (SAXS) revealed an extended spatial arrangement of the catalytic and four N-terminal WSC domains. Furthermore, we demonstrate that methylglyoxal and 5-hydroxymethylfurfural (HMF), a fermentation inhibitor, are substrates for the enzyme.

An Azoaromatic Ligand as Four Electron Four Proton Reservoir: Catalytic Dehydrogenation of Alcohols by Its Zinc(II) Complex

Electroprotic storage materials, though invaluable in energy-related research, are scanty among non-natural compounds. Herein, we report a zinc(II) complex of the ligand 2,6-bis(phenylazo)pyridine (L), which acts as a multiple electron and proton reservoir during catalytic dehydrogenation of alcohols to aldehydes/ketones. The redox-inactive metal ion Zn(II) serves as an oxophilic Lewis acid, while the ligand behaves as efficient storage of electron and proton. Synthesis, X-ray structure, and spectral characterizations of the catalyst, ZnLCl₂ (1a) along with the two hydrogenated complexes of 1a, ZnH₂LCl₂ (1b), and ZnH₄LCl₂ (1c) are reported. It has been argued that the reversible azo-hydrazo redox couple of 1a controls aerobic dehydrogenation of alcohols. Hydrogenated complexes are hyper-reactive and quantitatively reduce O₂ and para-benzoquinone to H₂O₂ and para-hydroquinone, respectively. Plausible mechanistic pathways for alcohol oxidation are discussed based on controlled experiments, isotope labeling, and spectral analysis of intermediates.