Spectroelectrochemical investigation of the glyoxal oxidase activation mechanism

Mapping microhabitats of lignocellulose decomposition by a microbial consortium

The leaf-cutter ant fungal garden ecosystem is a naturally evolved model system for efficient plant biomass degradation. Degradation processes mediated by the symbiotic fungus Leucoagaricus gongylophorus are difficult to characterize due to dynamic metabolisms and spatial complexity of the system. Herein, we performed microscale imaging across 12-µm-thick adjacent sections of Atta cephalotes fungal gardens and applied a metabolome-informed proteome imaging approach to map lignin degradation. This approach combines two spatial multiomics mass spectrometry modalities that enabled us to visualize colocalized metabolites and proteins across and through the fungal garden. Spatially profiled metabolites revealed an accumulation of lignin-related products, outlining morphologically unique lignin microhabitats. Metaproteomic analyses of these microhabitats revealed carbohydrate-degrading enzymes, indicating a prominent fungal role in lignocellulose decomposition. Integration of metabolome-informed proteome imaging data provides a comprehensive view of underlying biological pathways to inform our understanding of metabolic fungal pathways in plant matter degradation within the micrometer-scale environment.

Glyoxal oxidase-mediated detoxification of reactive carbonyl species contributes to virulence, stress tolerance, and development in a pathogenic fungus

Reactive carbonyl and oxygen species (RCS/ROS), often generated as metabolic byproducts, particularly under conditions of pathology, can cause direct damage to proteins, lipids, and nucleic acids. Glyoxal oxidases (Gloxs) oxidize aldehydes to carboxylic acids, generating hydrogen peroxide (H2O2). Although best characterized for their roles in lignin degradation, Glox in plant fungal pathogens are known to contribute to virulence, however, the mechanism underlying such effects are unclear. Here, we show that Glox in the insect pathogenic fungus, Metarhizium acridum, is highly expressed in mycelia and during formation of infection structures (appressoria), with the enzyme localizing to the cell membrane. MaGlox targeted gene disruption mutants showed RCS and ROS accumulation, resulting in cell toxicity, induction of apoptosis and increased autophagy, inhibiting normal fungal growth and development. The ability of the MaGlox mutant to scavenge RCS was significantly reduced, and the mutant exhibited increased susceptibility to aldehydes, oxidative and cell wall perturbing agents but not toward osmotic stress, with altered cell wall contents. The ΔMaGlox mutant was impaired in its ability to penetrate the host cuticle and evade host immune defense resulting in attenuated pathogenicity. Overexpression of MaGlox promoted fungal growth and conidial germination, increased tolerance to H2O2, but had little to other phenotypic effects. Transcriptomic analyses revealed downregulation of genes related to cell wall synthesis, conidiation, stress tolerance, and host cuticle penetration in the ΔMaGlox mutant. These findings demonstrate that MaGlox-mediated scavenging of RCS is required for virulence, and contributes to normal fungal growth and development, stress resistance.

Identification of redox activators for continuous reactivation of glyoxal oxidase from Trametes versicolor in a two-enzyme reaction cascade

Glyoxal oxidases, belonging to the group of copper radical oxidases (CROs), oxidize aldehydes to carboxylic acids, while reducing O2 to H2O2. Their activity on furan derivatives like 5-hydroxymethylfurfural (HMF) makes these enzymes promising biocatalysts for the environmentally friendly synthesis of the bioplastics precursor 2,5-furandicarboxylic acid (FDCA). However, glyoxal oxidases suffer from inactivation, which requires the identification of suitable redox activators for efficient substrate conversion. Furthermore, only a few glyoxal oxidases have been expressed and characterized so far. Here, we report on a new glyoxal oxidase from Trametes versicolor (TvGLOX) that was expressed at high levels in Pichia pastoris (reclassified as Komagataella phaffii). TvGLOX was found to catalyze the oxidation of aldehyde groups in glyoxylic acid, methyl glyoxal, HMF, 2,5-diformylfuran (DFF) and 5-formyl-2-furancarboxylic acid (FFCA), but barely accepted alcohol groups as in 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), preventing formation of FDCA from HMF. Various redox activators were tested for TvGLOX reactivation during catalyzed reactions. Among them, a combination of horseradish peroxidase and its substrate 2,2'-azino-di-(3-ethylbenzthiazoline sulfonic acid) (ABTS) most efficiently reactivated TvGLOX. Through continuous reactivation of TvGLOX in a two-enzyme system employing a recombinant Moesziomyces antarcticus aryl-alcohol oxidase (MaAAO) almost complete conversion of 8 mM HMF to FDCA was achieved within 24 h.

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.

Enzymatic and Microbial Electrochemistry: Approaches and Methods

The coupling of enzymes and/or intact bacteria with electrodes has been vastly investigated due to the wide range of existing applications. These span from biomedical and biosensing to energy production purposes and bioelectrosynthesis, whether for theoretical research or pure applied industrial processes. Both enzymes and bacteria offer a potential biotechnological alternative to noble/rare metal-dependent catalytic processes. However, when developing these biohybrid electrochemical systems, it is of the utmost importance to investigate how the approaches utilized to couple biocatalysts and electrodes influence the resulting bioelectrocatalytic response. Accordingly, this tutorial review starts by recalling some basic principles and applications of bioelectrochemistry, presenting the electrode and/or biocatalyst modifications that facilitate the interaction between the biotic and abiotic components of bioelectrochemical systems. Focus is then directed toward the methods used to evaluate the effectiveness of enzyme/bacteria-electrode interaction and the insights that they provide. The basic concepts of electrochemical methods widely employed in enzymatic and microbial electrochemistry, such as amperometry and voltammetry, are initially presented to later focus on various complementary methods such as spectroelectrochemistry, fluorescence spectroscopy and microscopy, and surface analytical/characterization techniques such as quartz crystal microbalance and atomic force microscopy. The tutorial review is thus aimed at students and graduate students approaching the field of enzymatic and microbial electrochemistry, while also providing a critical and up-to-date reference for senior researchers working in the field.

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.