Characterization of a new aryl-alcohol oxidase secreted by the phytopathogenic fungus Ustilago maydis

Agar plate assay for rapid screening of aryl-alcohol oxidase mutant libraries in Pichia pastoris

Directed evolution is a powerful tool for developing biocatalysts with tailor-made properties for biocatalytic applications. High-throughput activity screening of large mutant libraries generated by e.g. means of directed evolution is usually performed in 96-well microtiter plates requiring, however, time-consuming and laborious enzyme expression, cell harvesting and activity measurements. In addition, automated liquid handling systems are needed to cope with the high number of colonies to be screened. Herein, we developed an agar plate-based assay for simple and fast screening of H₂O₂-producing aryl-alcohol oxidase (AAO) mutant libraries in Pichia pastoris. Buffered minimal methanol agar plates were supplemented with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), horseradish peroxidase (HRP) and the target substrate. AAO activity is visualized by formation of green zones around AAO-secreting P. pastoris colonies due to ABTS oxidation by HRP which is fueled with H₂O₂ by AAO in course of substrate oxidation. Colonies were screened within 24h for AAO activity using different AAO substrates like veratryl alcohol, p-anisyl alcohol or trans,trans-2,4-hexadien-1-ol and even low AAO activity towards 5-hydroxymethylfurfural could be detected within 48h. The developed agar plate-based assay can be extended to other substrates and might also be applied for fast and substrate-specific screening of other H₂O₂-producing oxidases in P. pastoris.

Characterization of a thermotolerant aryl-alcohol oxidase from Moesziomyces antarcticus oxidizing 5-hydroxymethyl-2-furancarboxylic acid

The development of enzymatic processes for the environmentally friendly production of 2,5-furandicarboxylic acid (FDCA), a renewable precursor for bioplastics, from 5-hydroxymethylfurfural (HMF) has gained increasing attention over the last years. Aryl-alcohol oxidases (AAOs) catalyze the oxidation of HMF to 5-formyl-2-furancarboxylic acid (FFCA) through 2,5-diformylfuran (DFF) and have thus been applied in enzymatic reaction cascades for the production of FDCA. AAOs are flavoproteins that oxidize a broad range of benzylic and aliphatic allylic primary alcohols to the corresponding aldehydes, and in some cases further to acids, while reducing molecular oxygen to hydrogen peroxide. These promising biocatalysts can also be used for the synthesis of flavors, fragrances, and chemical building blocks, but their industrial applicability suffers from low production yield in natural and heterologous hosts. Here we report on heterologous expression of a new aryl-alcohol oxidase, MaAAO, from Moesziomyces antarcticus at high yields in the methylotrophic yeast Pichia pastoris (recently reclassified as Komagataella phaffii). Fed-batch fermentation of recombinant P. pastoris yielded around 750 mg of active enzyme per liter of culture. Purified MaAAO was highly stable at pH 2-9 and exhibited high thermal stability with almost 95% residual activity after 48 h at 57.5 °C. MaAAO accepts a broad range of benzylic primary alcohols, aliphatic allylic alcohols, and furan derivatives like HMF as substrates and some oxidation products thereof like piperonal or perillaldehyde serve as building blocks for pharmaceuticals or show health-promoting effects. Besides this, MaAAO oxidized 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) to FFCA, which has not been shown for any other AAO so far. Combining MaAAO with an unspecific peroxygenase oxidizing HMFCA to FFCA in one pot resulted in complete conversion of HMF to FDCA within 144 h. MaAAO is thus a promising biocatalyst for the production of precursors for bioplastics and bioactive compounds. KEY POINTS: • MaAAO from M. antarcticus was expressed in P. pastoris at 750 mg/l. • MaAAO oxidized 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). • Complete conversion of HMF to 2,5-furandicarboxylic acid by combining MaAAO and UPO.

Two adjacent C-terminal mutations enable expression of aryl-alcohol oxidase from Pleurotus eryngii in Pichia pastoris

Fungal aryl-alcohol oxidases (AAOs) are attractive biocatalysts because they selectively oxidize a broad range of aromatic and aliphatic allylic primary alcohols while yielding hydrogen peroxide as the only by-product. However, their use is hampered by challenging and often unsuccessful heterologous expression. Production of PeAAO1 from Pleurotus eryngii ATCC 90787 in Pichia pastoris failed, while PeAAO2 from P. eryngii P34 with an amino acid identity of 99% was expressed at high yields. By successively introducing mutations in PeAAO1 to mimic the sequence of PeAAO2, the double mutant PeAAO1 ER with mutations K583E and Q584R was constructed, that was successfully expressed in P. pastoris. Functional expression was enhanced up to 155 U/l via further replacements D361N (variant NER) or V367A (variant AER). Fed-batch cultivation of recombinant P. pastoris yielded up to 116 mg/l of active variants. Glycosylated PeAAO1 variants demonstrated high stability and catalytic efficiencies similar to PeAAO2. Interestingly, P. pastoris expressing PeAAO1 variant ER contained roughly 13 gene copies but showed similar volumetric activity as NER and AER with one to two gene copies and four times lower mRNA levels. Additional H-bonds and salt bridges introduced by mutations K583E and Q584R might facilitate heterologous expression by enhanced protein folding.Key points• PeAAO1 not expressed in P. pastoris and PeAAO2 well-expressed in Pichia differ at 7 positions.• Expression of PeAAO1 in P. pastoris achieved through mutagenesis based on PeAAO2 sequence.• Combination of K583E and Q584R is essential for expression of PeAAO1 in P. pastoris.

Sugar oxidoreductases and LPMOs - two sides of the same polysaccharide degradation story?

Lytic polysaccharide monooxygenases (LPMOs) catalyze the oxidative cleavage of glycosidic bonds in recalcitrant polysaccharides such as chitin and cellulose and their discovery has revolutionized our understanding of enzymatic biomass conversion. The discovery of LPMOs raises interesting new questions regarding the roles of other oxidoreductases and abiotic redox processes in biomass conversion. LPMOs need reducing power and an oxygen co-substrate and biomass degrading ecosystems contain a multitude of redox enzymes that affect the availability of both. For example, biomass degrading fungi produce multiple sugar oxidoreductases whose biological functions so far have remained somewhat enigmatic. It is now conceivable that these redox enzymes, in particular H₂O₂-producing sugar oxidases, could play a role in fueling and controlling LPMO reactions. Here, we shortly review contemporary issues in the LPMO field, paying particular attention to the possible roles of sugar oxidoreductases.

Pecularities and applications of aryl-alcohol oxidases from fungi

Abstract

Aryl-alcohol oxidases (AAOs) are FAD-containing enzymes that oxidize a broad range of aromatic as well as aliphatic allylic alcohols to aldehydes. Their broad substrate spectrum accompanied by the only need for molecular oxygen as cosubstrate and production of hydrogen peroxide as sole by-product makes these enzymes very promising biocatalysts. AAOs were used in the synthesis of flavors, fragrances, and other high-value-added compounds and building blocks as well as in dye decolorization and pulp biobleaching. Furthermore, AAOs offer a huge potential as efficient suppliers of hydrogen peroxide for peroxidase- and peroxygenase-catalyzed reactions. A prerequisite for application as biocatalysts at larger scale is the production of AAOs in sufficient amounts. Heterologous expression of these predominantly fungal enzymes is, however, quite challenging. This review summarizes different approaches aiming at enhancing heterologous expression of AAOs and gives an update on substrates accepted by these promising enzymes as well as potential fields of their application.

Key points

• Aryl-alcohol oxidases (AAOs) supply ligninolytic peroxidases with H₂O₂.

• AAOs accept a broad spectrum of aromatic and aliphatic allylic alcohols.

• AAOs are potential biocatalysts for the production of high-value-added bio-based chemicals.

Process optimization and scale-up production of fungal aryl alcohol oxidase from genetically modified Aspergillus nidulans in stirred-tank bioreactor

Microbial production of aryl alcohol oxidase (AAO) has attracted increasing attention due to the central role of AAO in enzymatic lignin depolymerization. However, large-scale production of AAO has not been reached because of the low yield and inefficient fermentation process. This study aims to optimize the process parameters and scale-up production of AAO using Aspergillus nidulans in a stirred-tank bioreactor. Effects of pH and dissolved oxygen on AAO production at bioreactor scale were particularly investigated. Results revealed that pH control significantly affected protein production and increasing dissolved oxygen level stimulated AAO production. The greatest AAO activity (1906 U/L) and protein concentration (1.19 g/L) were achieved in 48 h at 60% dissolved oxygen with pH controlled at 6.0. The yield and productivity (in 48 h) were 31.2 U/g maltose and 39.7 U/L/h, respectively. In addition, crude AAO was concentrated and partially purified by ultrafiltration and verified by protein identification.

High-level expression of aryl-alcohol oxidase 2 from Pleurotus eryngii in Pichia pastoris for production of fragrances and bioactive precursors

Abstract

The fungal secretome comprises various oxidative enzymes participating in the degradation of lignocellulosic biomass as a central step in carbon recycling. Among the secreted enzymes, aryl-alcohol oxidases (AAOs) are of interest for biotechnological applications including production of bio-based precursors for plastics, bioactive compounds, and flavors and fragrances. Aryl-alcohol oxidase 2 (PeAAO2) from the fungus Pleurotus eryngii was heterologously expressed and secreted at one of the highest yields reported so far of 315 mg/l using the methylotrophic yeast Pichia pastoris (recently reclassified as Komagataella phaffii). The glycosylated PeAAO2 exhibited a high stability in a broad pH range between pH 3.0 and 9.0 and high thermal stability up to 55 °C. Substrate screening with 41 compounds revealed that PeAAO2 oxidized typical AAO substrates like p-anisyl alcohol, veratryl alcohol, and trans,trans-2,4-hexadienol with up to 8-fold higher activity than benzyl alcohol. Several compounds not yet reported as substrates for AAOs were oxidized by PeAAO2 as well. Among them, cumic alcohol and piperonyl alcohol were oxidized to cuminaldehyde and piperonal with high catalytic efficiencies of 84.1 and 600.2 mM⁻¹ s⁻¹, respectively. While the fragrance and flavor compound piperonal also serves as starting material for agrochemical and pharmaceutical building blocks, various positive health effects have been attributed to cuminaldehyde including anticancer, antidiabetic, and neuroprotective effects. PeAAO2 is thus a promising biocatalyst for biotechnological applications.

Key points

• Aryl-alcohol oxidase PeAAO2 from P. eryngii was produced in P. pastoris at 315 mg/l.

• Purified enzyme exhibited stability over a broad pH and temperature range.

• Oxidation products cuminaldehyde and piperonal are of biotechnological interest.

Graphical abstract

Electronic supplementary material

The online version of this article (10.1007/s00253-020-10878-4) contains supplementary material, which is available to authorized users.

Reaction mechanisms and applications of aryl-alcohol oxidase

Aryl-alcohol oxidases (AAO) constitute a family of FAD-containing enzymes, included in the glucose-methanol-choline oxidase/dehydrogenase superfamily of proteins. They are commonly found in fungi, where their eco-physiological role is to produce hydrogen peroxide that activates ligninolytic peroxidases in white-rot (lignin-degrading) basidiomycetes or to trigger the Fenton reactions in brown-rot (carbohydrate-degrading) basidiomycetes. These enzymes catalyze the oxidation of a plethora of aromatic, and some aliphatic, polyunsaturated alcohols bearing conjugated primary hydroxyl group. Besides, the enzymes show activity on the hydrated forms of the corresponding aldehydes. Some AAO features, such as the broad range of substrates that it can oxidize (with the only need of molecular oxygen as co-substrate) and its stereoselective mechanism, confer good properties to these enzymes as industrial biocatalysts. In fact, AAO can be used for different biotechnological applications, such as flavor synthesis, secondary alcohol deracemization and oxidation of furfurals for the production of furandicarboxylic acid as a chemical building block. Also, AAO can participate in processes of interest in the wood biorefinery and textile industries as an auxiliary enzyme providing hydrogen peroxide to ligninolytic or dye-decolorizing peroxidases. Both rational design and directed molecular evolution have been employed to engineer AAO for some of the above biotechnological applications.

Enzymatic versatility and thermostability of a new aryl-alcohol oxidase from Thermothelomyces thermophilus M77

Background Fungal aryl-alcohol oxidases (AAOx) are extracellular flavoenzymes that belong to glucose-methanol-choline oxidoreductase family and are responsible for the selective conversion of primary aromatic alcohols into aldehydes and aromatic aldehydes to their corresponding acids, with concomitant production of hydrogen peroxide (H₂O₂) as by-product. The H₂O₂ can be provided to lignin degradation pathway, a biotechnological property explored in biofuel production. In the thermophilic fungus Thermothelomyces thermophilus (formerly Myceliophthora thermophila), just one AAOx was identified in the exo-proteome. Methods The glycosylated and non-refolded crystal structure of an AAOx from T. thermophilus at 2.6 Å resolution was elucidated by X-ray crystallography combined with small-angle X-ray scattering (SAXS) studies. Moreover, biochemical analyses were carried out to shed light on enzyme substrate specificity and thermostability. Results This flavoenzyme harbors a flavin adenine dinucleotide as a cofactor and is able to oxidize aromatic substrates and 5-HMF. Our results also show that the enzyme has similar oxidation rates for bulky or simple aromatic substrates such as cinnamyl and veratryl alcohols. Moreover, the crystal structure of MtAAOx reveals an open active site, which might explain observed specificity of the enzyme. Conclusions MtAAOx shows previously undescribed structural differences such as a fully accessible catalytic tunnel, heavy glycosylation and Ca²⁺ binding site providing evidences for thermostability and activity of the enzymes from AA3_2 subfamily. General significance Structural and biochemical analyses of MtAAOx could be important for comprehension of aryl-alcohol oxidases structure-function relationships and provide additional molecular tools to be used in future biotechnological applications.

Cosubstrate strategy for enhancing lignocellulose degradation during rumen fermentation in vitro: Characteristics and microorganism composition

To enhance the degradation of wheat straw (WS) and corn straw (CS) in rumen fermentation, characterization of degradation and ruminal microorganisms of monosubstrate (WS/CS) groups and a cosubstrate strategy with food waste (FW) group was performed. The cellulose, hemicellulose, and lignin degradation efficiency of WS and CS; soluble chemical oxygen demand; volatile fatty acid yields; and activity of ligninolytic, cellulolytic, and hemicellulolytic enzymes for the cosubstrate group were improved compared with those for the corresponding monosubstrate groups. An accurate and a good of fit of the Weibull kinetic model, decreased crystallinity index values, and characteristic absorbance bands in the Fourier transform-infrared spectra further confirmed that cosubstrate addition with FW decreased the resistance of cellulose and hemicellulose to biodegradation. High-throughput sequencing results suggested that the bacterial diversity in CS rumen fermentation and fungal diversity and richness in WS rumen fermentation were promoted with FW as a cosubstrate. The cosubstrate addition with FW significantly affected the composition of the ruminal bacteria and fungi in rumen fermentation. The relative abundances (RAs) of rumen bacteria were increased in the cosubstrate CS/WS and FW fermentation conditions, and the enhancement of CS degradation with FW supplementation was stronger than that of WS rumen fermentation with FW supplementation. The RAs of the ruminal fungal genera Ustilago and Fusarium were promoted in CS and WS fermentation with FW, respectively. Moreover, the fermentation properties and rumen flora in the FW rumen fermentation also provided some evidence to suggest an enhancement of the cosubstrate strategy compared with the monosubstrate strategy.

Directed evolution of the aryl-alcohol oxidase: Beyond the lab bench

Aryl-alcohol oxidase (AAO) is a fungal GMC flavoprotein secreted by white-rot fungi that supplies H2O2 to the ligninolytic consortium. This enzyme can oxidize a wide array of aromatic alcohols in a highly enantioselective manner, an important trait in organic synthesis. The best strategy to adapt AAO to industrial needs is to engineer its properties by directed evolution, aided by computational analysis. The aim of this review is to describe the strategies and challenges we faced when undertaking laboratory evolution of AAO. After a comprehensive introduction into the structure of AAO, its function and potential applications, the different directed evolution enterprises designed to express the enzyme in an active and soluble form in yeast are described, as well as those to unlock new activities involving the oxidation of secondary aromatic alcohols and the synthesis of furandicarboxylic acids.

Smut fungal strategies for the successful infection

The smut fungi include a large number of plant pathogens that establish obligate biotrophic relationships with their host. Throughout the whole life inside plant tissue, smut fungi keep plant cells alive and acquire nutrients via biotrophic interfaces. This mini-review mainly summarizes the interactions between smut fungi and their host plants during the infection process. Despite various strategies recruited by plants to defense invading pathogens, smut fungi successfully evolved an arsenal for colonization. Mating of two compatible haploids gives rise to parasitic mycelium, which can sense plant surface cues such as fatty acids and hydrophobic surface, and induce the formation of appressoria for surface penetration. Plants can recognize fungal invading and activate defense response, including callose and lignin deposition, programmed cell death, and SA signaling pathway. To suppress plant immunity and alter the metabolic pathway of host plants, a cocktail of effectors is secreted by smut fungi depending on the plant organ and cell type that is infected.

Pycnoporus cinnabarinus glyoxal oxidases display differential catalytic efficiencies on 5-hydroxymethylfurfural and its oxidized derivatives

Background

5-Hydroxymethylfurfural (HMF), a major residual component of a lignocellulosic bio-refinery process, can be transformed into fundamental building blocks for green chemistry via oxidation. While chemical methods are well established, interest is also being directed into the enzymatic oxidation of HMF into the bio-plastic precursor 2,5-furandicarboxylic acid (FDCA).

Results

We demonstrate that three glyoxal oxidases (PciGLOX) isoenzymes from the Basidiomycete fungus Pycnoporus cinnabarinus were able to oxidize HMF, with PciGLOX2 and PciGLOX3 being the most efficient. The major reaction product obtained with the three isoenzymes was 5-hydroxymethyl-2-furancarboxylic (HMFCA), a precursor in polyesters and pharmaceuticals production, and very little subsequent conversion of this compound was observed. However, small concentrations of FDCA, a substitute for terephthalic acid in the production of polyesters, were also obtained. The oxidation of HMF was significantly boosted in the presence of catalase for PciGLOX2, leading to 70% HMFCA yield. The highest conversion percentages were observed on 2,5-furandicarboxaldehyde (DFF), a minor product from the reaction of PciGLOX on HMF. To bypass HMFCA accumulation and exploit the efficiency of PciGLOX in oxidizing DFF and 5-formyl-2-furan carboxylic acid (FFCA) towards FDCA production, HMF was oxidized in a cascade reaction with an aryl alcohol oxidase (UmaAAO). After 2 h of reaction, UmaAAO completely oxidized HMF to DFF and further to FFCA, with FDCA only being detected when PciGLOX3 was added to the reaction. The maximum yield of 16% FDCA was obtained 24 h after the addition of PciGLOX3 in the presence of catalase.

Conclusions

At least two conversion pathways for HMF oxidation can be considered for PciGLOX; however, the highest selectivity was seen towards the production of the valuable polyester precursor HMFCA. The three isoenzymes showed differences in their catalytic efficiencies and substrate specificities when reacted with HMF derivatives.

Electronic supplementary material

The online version of this article (10.1186/s40694-019-0067-8) contains supplementary material, which is available to authorized users.

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 an aryl-alcohol oxidase from the plant saprophytic basidiomycete Coprinopsis cinerea with broad substrate specificity against aromatic alcohols

OBJECTIVES

The aim of the study was to obtain information about the enzymatic properties of aryl-alcohol oxidase from the plant saprophytic basidiomycete Coprinopsis cinerea (rCcAAO), which is classified into the auxiliary activities family 3 subfamily 2 (AA3_2).

RESULTS

The gene encoding AAO from the plant saprophytic basidiomycete Coprinopsis cinerea (CcAAO) was cloned, and the recombinant CcAAO (rCcAAO) was heterologously expressed in the methylotrophic yeast Pichia pastoris. The purified rCcAAO showed significant activity not only against trans,trans-2,4-hexadien-1-ol but also against a broad range of aromatic alcohols including aromatic compounds that were reported to be poor substrates for known AAOs. Moreover, site-directed mutagenesis analysis demonstrated that mutants with substitutions from leucine to phenylalanine and tryptophan at position 416 exhibited decreases of activity for aromatic alcohols but still maintained the activity for trans,trans-2,4-hexadien-1-ol.

CONCLUSIONS

Leucine 416 in CcAAO contributes to the broad substrate specificity against various aromatic alcohols, which is useful for the production of hydrogen peroxide using this enzyme.

Multiplicity of enzymatic functions in the CAZy AA3 family

The CAZy auxiliary activity family 3 (AA3) comprises enzymes from the glucose-methanol-choline (GMC) family of oxidoreductases, which assist the activity of other AA family enzymes via their reaction products or support the action of glycoside hydrolases in lignocellulose degradation. The AA3 family is further divided into four subfamilies, which include cellobiose dehydrogenase, glucose oxidoreductases, aryl-alcohol oxidase, alcohol (methanol) oxidase, and pyranose oxidoreductases. These different enzymes catalyze a wide variety of redox reactions with respect to substrates and co-substrates. The common feature of AA3 family members is the formation of key metabolites such as H2O2 or hydroquinones, which are required by other AA enzymes. The multiplicity of enzymatic functions in the AA3 family is reflected by the multigenicity of AA3 genes in fungi, which also depends on their lifestyle. We provide an overview of the phylogenetic, molecular, and catalytic properties of AA3 enzymes and discuss their interactions with other carbohydrate-active enzymes.

Recent advances in enzymatic oxidation of alcohols

Enzymatic alcohol oxidation plays an important role in chemical synthesis. In the past two years, new alcohol oxidation enzymes were developed through genome-mining and protein engineering, such as new copper radical oxidases with broad substrate scope, alcohol dehydrogenases with altered cofactor preference and a flavin-dependent alcohol oxidase with enhanced oxygen coupling. New cofactor recycling methods were reported for alcohol dehydrogenase-catalyzed oxidation with photocatalyst and coupled glutaredoxin-glutathione reductase as promising examples. Different alcohol oxidation systems were used for the oxidation of primary and secondary alcohols, especially in the cascade conversion of alcohols to lactones, lactams, chiral amines, chiral alcohols and hydroxyketones. Among them, biocatalyst with low enantioselectivity demonstrated an interesting feature for complete conversion of racemic secondary alcohols through non-enantioselective oxidation.

Inactivation of Cellobiose Dehydrogenases Modifies the Cellulose Degradation Mechanism of Podospora anserina

Conversion of biomass into high-value products, including biofuels, is of great interest to developing sustainable biorefineries. Fungi are an inexhaustible source of enzymes to degrade plant biomass. Cellobiose dehydrogenases (CDHs) play an important role in the breakdown through synergistic action with fungal lytic polysaccharide monooxygenases (LPMOs). The three CDH genes of the model fungus Podospora anserina were inactivated, resulting in single and multiple CDH mutants. We detected almost no difference in growth and fertility of the mutants on various lignocellulose sources, except on crystalline cellulose, on which a 2-fold decrease in fertility of the mutants lacking P. anserina CDH1 (PaCDH1) and PaCDH2 was observed. A striking difference between wild-type and mutant secretomes was observed. The secretome of the mutant lacking all CDHs contained five beta-glucosidases, whereas the wild type had only one. P. anserina seems to compensate for the lack of CDH with secretion of beta-glucosidases. The addition of P. anserina LPMO to either the wild-type or mutant secretome resulted in improvement of cellulose degradation in both cases, suggesting that other redox partners present in the mutant secretome provided electrons to LPMOs. Overall, the data showed that oxidative degradation of cellulosic biomass relies on different types of mechanisms in fungi.

IMPORTANCE

Plant biomass degradation by fungi is a complex process involving dozens of enzymes. The roles of each enzyme or enzyme class are not fully understood, and utilization of a model amenable to genetic analysis should increase the comprehension of how fungi cope with highly recalcitrant biomass. Here, we report that the cellobiose dehydrogenases of the model fungus Podospora anserina enable it to consume crystalline cellulose yet seem to play a minor role on actual substrates, such as wood shavings or miscanthus. Analysis of secreted proteins suggests that Podospora anserina compensates for the lack of cellobiose dehydrogenase by increasing beta-glucosidase expression and using an alternate electron donor for LPMO.

Profiling microbial lignocellulose degradation and utilization by emergent omics technologies

The use of plant materials to generate renewable biofuels and other high-value chemicals is the sustainable and preferable option, but will require considerable improvements to increase the rate and efficiency of lignocellulose depolymerization. This review highlights novel and emerging technologies that are being developed and deployed to characterize the process of lignocellulose degradation. The review will also illustrate how microbial communities deconstruct and metabolize lignocellulose by identifying the necessary genes and enzyme activities along with the reaction products. These technologies include multi-omic measurements, cell sorting and isolation, nuclear magnetic resonance spectroscopy (NMR), activity-based protein profiling, and direct measurement of enzyme activity. The recalcitrant nature of lignocellulose necessitates the need to characterize the methods microbes employ to deconstruct lignocellulose to inform new strategies on how to greatly improve biofuel conversion processes. New technologies are yielding important insights into microbial functions and strategies employed to degrade lignocellulose, providing a mechanistic blueprint in order to advance biofuel production.

Single-domain flavoenzymes trigger lytic polysaccharide monooxygenases for oxidative degradation of cellulose

The enzymatic conversion of plant biomass has been recently revolutionized by the discovery of lytic polysaccharide monooxygenases (LPMOs) that carry out oxidative cleavage of polysaccharides. These very powerful enzymes are abundant in fungal saprotrophs. LPMOs require activation by electrons that can be provided by cellobiose dehydrogenases (CDHs), but as some fungi lack CDH-encoding genes, other recycling enzymes must exist. We investigated the ability of AA3_2 flavoenzymes secreted under lignocellulolytic conditions to trigger oxidative cellulose degradation by AA9 LPMOs. Among the flavoenzymes tested, we show that glucose dehydrogenase and aryl-alcohol quinone oxidoreductases are catalytically efficient electron donors for LPMOs. These single-domain flavoenzymes display redox potentials compatible with electron transfer between partners. Our findings extend the array of enzymes which regulate the oxidative degradation of cellulose by lignocellulolytic fungi.

Activities of Secreted Aryl Alcohol Quinone Oxidoreductases from Pycnoporus cinnabarinus Provide Insights into Fungal Degradation of Plant Biomass

Auxiliary activities family 3 subfamily 2 (AA3_2) from the CAZy database comprises various functions related to ligninolytic enzymes, such as fungal aryl alcohol oxidases (AAO) and glucose oxidases, both of which are flavoenzymes. The recent study of the Pycnoporus cinnabarinus CIRM BRFM 137 genome combined with its secretome revealed that four AA3_2 enzymes are secreted during biomass degradation. One of these AA3_2 enzymes, scf184803.g17, has recently been produced heterologously in Aspergillus niger Based on the enzyme's activity and specificity, it was assigned to the glucose dehydrogenases (PcinnabarinusGDH [PcGDH]). Here, we analyze the distribution of the other three AA3_2 enzymes (scf185002.g8, scf184611.g7, and scf184746.g13) to assess their putative functions. These proteins showed the highest homology with aryl alcohol oxidase from Pleurotus eryngii Biochemical characterization demonstrated that they were also flavoenzymes harboring flavin adenine dinucleotide (FAD) as a cofactor and able to oxidize a wide variety of phenolic and nonphenolic aryl alcohols and one aliphatic polyunsaturated primary alcohol. Though presenting homology with fungal AAOs, these enzymes exhibited greater efficiency in reducing electron acceptors (quinones and one artificial acceptor) than molecular oxygen and so were defined as aryl-alcohol:quinone oxidoreductases (AAQOs) with two enzymes possessing residual oxidase activity (PcAAQO2 and PcAAQO3). Structural comparison of PcAAQO homology models with P. eryngii AAO demonstrated a wider substrate access channel connecting the active-site cavity to the solvent, explaining the absence of activity with molecular oxygen. Finally, the ability of PcAAQOs to reduce radical intermediates generated by laccase from P. cinnabarinus was demonstrated, shedding light on the ligninolytic system of this fungus.

Lytic polysaccharide monooxygenases and other oxidative enzymes are abundantly secreted by Aspergillus nidulans grown on different starches

BACKGROUND

Starch is the second most abundant plant-derived biomass and a major feedstock in non-food industrial applications and first generation biofuel production. In contrast to lignocellulose, detailed insight into fungal degradation of starch is currently lacking. This study explores the secretomes of Aspergillus nidulans grown on cereal starches from wheat and high-amylose (HA) maize, as well as legume starch from pea for 5 days.

RESULTS

Aspergillus nidulans grew efficiently on cereal starches, whereas growth on pea starch was poor. The secretomes at days 3-5 were starch-type dependent as also reflected by amylolytic activity measurements. Nearly half of the 312 proteins in the secretomes were carbohydrate-active enzymes (CAZymes), mostly glycoside hydrolases (GHs) and oxidative auxiliary activities (AAs). The abundance of the GH13 α-amylase (AmyB) decreased with time, as opposed to other starch-degrading enzymes, e.g., the GH13 AmyF, GH15 glucoamylases (GlaA and GlaB), and the GH31 α-glucosidase (AgdE). Two AA13 LPMOs displayed similar secretion patterns as amylolytic hydrolases and were among the most abundant CAZymes. The starch-active AnLPMO13A that possesses a CBM20 carbohydrate-binding module dominated the starch-binding secretome fraction. A striking observation is the co-secretion of several redox-active enzymes with the starch-active AA13 LPMOs and GHs, some at high abundance. Notably nine AA9 LPMOs, six AA3 sub-family 2 (AA_2) oxidoreductases, and ten AA7 glyco-oligosaccharide oxidases were identified in the secretomes in addition to other non-CAZyme oxidoreductases.

CONCLUSIONS

The co-secretion and high abundance of AA13 LPMOs are indicative of a key role in starch granule deconstruction. The increase in AA13 LPMO abundance with culture time may reflect accumulation of a more resistant starch fraction towards the later stages of the culture. The identification of AmyR sites upstream AA13 LPMOs unveils co-regulation of LPMOs featuring in starch utilization. Differential deployment of amylolytic hydrolases and LPMOs over time suggests additional regulatory mechanisms. The abundant co-secretion of distinct AA3 and AA7 oxidoreductases merits further studies into their roles and possible interplay with LPMOs and other enzymes in the deconstruction of starchy substrates. The study reports for the first time the biological significance of LPMOs in starch degradation and the temporal interplay between these and amylolytic hydrolases.