Phanerochaete chrysosporium produces a diverse array of extracellular enzymes when grown on sorghum

Inactivation of siderophore iron-chelating moieties by the fungal wheat root symbiont Pyrenophora biseptata

We investigated the ability of four plant and soil-associated fungi to modify or degrade siderophore structures leading to reduced siderophore iron-affinity in iron-limited and iron-replete cultures. Pyrenophora biseptata, a melanized fungus from wheat roots, was effective in inactivating siderophore iron-chelating moieties. In the supernatant solution, the tris-hydroxamate siderophore desferrioxamine B (DFOB) underwent a stepwise reduction of the three hydroxamate groups in DFOB to amides leading to a progressive loss in iron affinity. A mechanism is suggested based on the formation of transient ferrous iron followed by reduction of the siderophore hydroxamate groups during fungal high-affinity reductive iron uptake. P. biseptata also produced its own tris-hydroxamate siderophores (neocoprogen I and II, coprogen and dimerum acid) in iron-limited media and we observed loss of hydroxamate chelating groups during incubation in a manner analogous to DFOB. A redox-based reaction was also involved with the tris-catecholate siderophore protochelin in which oxidation of the catechol groups to quinones was observed. The new siderophore inactivating activity of the wheat symbiont P. biseptata is potentially widespread among fungi with implications for the availability of iron to plants and the surrounding microbiome in siderophore-rich environments.

Roles of various enzymes in the biotransformation of 6:2 fluorotelomer alcohol (6:2 FTOH) by a white-rot fungus

Six-carbon-chained polyfluoroalkyl substances, such as 6:2 fluorotelomer alcohol (6:2 FTOH), are being used to replace longer chained compounds in the manufacture of various commercial products. This study examined the effects of growth substrates and nutrients on specific intracellular and extracellular enzymes mediating 6:2 FTOH aerobic biotransformation by the white-rot fungus, Phanerochaete chrysosporium. Cellulolytic conditions with limited glucose were a suitable composition, resulting in high 5:3 FTCA yield (37 mol%), which is a key intermediate in 6:2 FTOH degradation without forming significant amounts of terminal perfluorocarboxylic acids (PFCAs). Sulfate and ethylenediaminetetraacetic acid (EDTA) were also essential for 5:3 FTCA production, but, at lower levels, resulted in the buildup of 5:2 sFTOH (52 mol%) and 6:2 FTUCA (20 mol%), respectively. In non-ligninolytic nutrient-rich medium, 45 mol% 6:2 FTOH was transformed but produced only 12.7 mol% 5:3 FTCA. Enzyme activity studies imply that cellulolytic conditions induce the intracellular cytochrome P450 system. In contrast, extracellular peroxidase synthesis is independent of 6:2 FTOH exposure. Gene expression studies further verified that peroxidases were relevant in catalyzing the downstream transformations from 5:3 FTCA. Collectively, the identification of nutrients and enzymatic systems will help elucidate underlying mechanisms and biogeochemical conditions favorable for fungal transformation of PFCA precursors in the environment.

Morphological growth pattern of Phanerochaete chrysosporium cultivated on different Miscanthus x giganteus biomass fractions

BACKGROUND

Solid-state fermentation is a fungal culture technique used to produce compounds and products of industrial interest. The growth behaviour of filamentous fungi on solid media is challenging to study due to the intermixity of the substrate and the growing organism. Several strategies are available to measure indirectly the fungal biomass during the fermentation such as following the biochemical production of mycelium-specific components or microscopic observation. The microscopic observation of the development of the mycelium, on lignocellulosic substrate, has not been reported. In this study, we set up an experimental protocol based on microscopy and image processing through which we investigated the growth pattern of Phanerochaete chrysosporium on different Miscanthus x giganteus biomass fractions.

RESULTS

Object coalescence, the occupied surface area, and radial expansion of the colony were measured in time. The substrate was sterilized by autoclaving, which could be considered a type of pre-treatment. The fastest growth rate was measured on the unfractionated biomass, followed by the soluble fraction of the biomass, then the residual solid fractions. The growth rate on the different fractions of the substrate was additive, suggesting that both the solid and soluble fractions were used by the fungus. Based on the FTIR analysis, there were differences in composition between the solid and soluble fractions of the substrate, but the main components for growth were always present. We propose using this novel method for measuring the very initial fungal growth by following the variation of the number of objects over time. Once growth is established, the growth can be followed by measurement of the occupied surface by the mycelium.

CONCLUSION

Our data showed that the growth was affected from the very beginning by the nature of the substrate. The most extensive colonization of the surface was observed with the unfractionated substrate containing both soluble and solid components. The methodology was practical and may be applied to investigate the growth of other fungi, including the influence of environmental parameters on the fungal growth.

A Seed Mucilage-Degrading Fungus From the Rhizosphere Strengthens the Plant-Soil-Microbe Continuum and Potentially Regulates Root Nutrients of a Cold Desert Shrub

Seed mucilage plays important roles in the adaptation of desert plants to the stressful environment. Artemisia sphaerocephala is an important pioneer plant in the Central Asian cold desert, and it produces a large quantity of seed mucilage. Seed mucilage of A. sphaerocephala can be degraded by soil microbes, but it is unknown which microorganisms can degrade mucilage or how the mucilage-degrading microorganisms affect rhizosphere microbial communities or root nutrients. Here, mucilage-degrading microorganisms were isolated from the rhizosphere of A. sphaerocephala, were screened by incubation with mucilage stained with Congo red, and were identified by sequencing and phylogenetic analyses. Fungal-bacterial networks based on high-throughput sequencing of rhizosphere microbes were constructed to explore the seasonal dynamic of interactions between a mucilage-degrading microorganism and its closely related microorganisms. The structural equation model was used to analyze effects of the mucilage-degrading microorganism, rhizosphere fungal-bacterial communities, and soil physicochemical properties on root C and N. The fungus Phanerochaete chrysosporium was identified as a mucilage-degrading microorganism. Relative abundance of the mucilage-degrading fungus (MDF) was highest in May. Subnetworks showed that the abundance of fungi and bacteria closely related to the MDF also were highest in May. Interactions between the MDF and related fungi and bacteria were positive, which might enhance mucilage degradation. In addition, the MDF might regulate root C and N by affecting rhizosphere microbial community structure. Our results suggest that MDF from the rhizosphere strengthens the plant-soil-microbe continuum, thereby potentially regulating microbial interactions and root nutrients of A. sphaerocephala.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Phanerochaete chrysosporium strain B-22, a nematophagous fungus parasitizing Meloidogyne incognita

The root-knot nematode Meloidogyne incognita has a wide host range and it is one of the most economically important crop parasites worldwide. Biological control has been a good approach for reducing M. incognita infection, for which many nematophagous fungi are reportedly applicable. However, the controlling effects of Phanerochaete chrysosporium strain B-22 are still unclear. In the present study we characterized the parasitism of this strain on M. incognita eggs, second-stage juveniles (J2), and adult females. The highest corrected mortality was 71.9% at 3 × 10⁸ colony forming units (CFU) mL^-1 and the estimated median lethal concentration of the fungus was 0.96 × 10⁸ CFU mL^-1. Two days after treatment with Phanerochaete chrysosporium strain B-22 eggshells were dissolved. A strong lethal effect was noted against J2, as the fungal spores developed in their body walls, germinated, and the resulting hyphae crossed the juvenile cuticle to dissolve it, thereby causing shrinkage and deformation of the juvenile body wall. The spores and hyphae also attacked adult females, causing the shrinkage and dissolution of their bodies and leakage of contents after five days. Greenhouse experiments revealed that different concentrations of the fungal spores effectively controlled M. incognita. In the roots, the highest inhibition rate for adult females, juveniles, egg mass, and gall index was 84.61%, 78.91%, 84.25%, and 79.48%, respectively. The highest juvenile inhibition rate was 89.18% in the soil. Phanerochaete chrysosporium strain B-22 also improved tomato plant growth, therefore being safe for tomato plants while effectively parasitizing M. incognita. This strain is thus a promising biocontrol agent against M. incognita.

An AA9-LPMO containing a CBM1 domain in Aspergillus nidulans is active on cellulose and cleaves cello-oligosaccharides

Lytic polysaccharide monooxygenases (LPMOs) are copper dependent enzymes that carry out oxidative cleavage of cellulose and other polysaccharides. Aspergillus nidulans, an ascomycete fungus that contains multiple AA9 LPMOs in the genome, offers an excellent model system to study their activity during the oxidative degradation of biomass. AN1602, a dual domain AA9-LPMO in A. nidulans appended with a carbohydrate-binding module, CBM1, was expressed in Pichia pastoris for analyzing oxidative cleavage on cellulosic substrates. The mass spectral and HPAEC analyses showed that the enzyme cleaves phosphoric acid swollen cellulose (PASC) in the presence of a reducing agent, yielding a range of cello-oligosaccharides. In addition to the polymeric substrate cellulose, AN1602 is also active on soluble cellohexaose, a property that is restricted to only a few characterized LPMOs. Product analysis of AN1602 cleaved cellohexaose revealed that C4 was the sole site of oxidation. The sequence and predicted structure of the catalytic domain of AN1602 matched very closely to known C4 cellohexaose active enzymes.

Xyloglucan breakdown by endo-xyloglucanase family 74 from Aspergillus fumigatus

Xyloglucan is the most abundant hemicellulose in primary walls of spermatophytes except for grasses. Xyloglucan-degrading enzymes are important in lignocellulosic biomass hydrolysis because they remove xyloglucan, which is abundant in monocot-derived biomass. Fungal genomes encode numerous xyloglucanase genes, belonging to at least six glycoside hydrolase (GH) families. GH74 endo-xyloglucanases cleave xyloglucan backbones with unsubstituted glucose at the -1 subsite or prefer xylosyl-substituted residues in the -1 subsite. In this work, 137 GH74-related genes were detected by examining 293 Eurotiomycete genomes and Ascomycete fungi contained one or no GH74 xyloglucanase gene per genome. Another interesting feature is that the triad of tryptophan residues along the catalytic cleft was found to be widely conserved among Ascomycetes. The GH74 from Aspergillus fumigatus (AfXEG74) was chosen as an example to conduct comprehensive biochemical studies to determine the catalytic mechanism. AfXEG74 has no CBM and cleaves the xyloglucan backbone between the unsubstituted glucose and xylose-substituted glucose at specific positions, along the XX motif when linked to regions deprived of galactosyl branches. It resembles an endo-processive activity, which after initial random hydrolysis releases xyloglucan-oligosaccharides as major reaction products. This work provides insights on phylogenetic diversity and catalytic mechanism of GH74 xyloglucanases from Ascomycete fungi.

Myceliophthora thermophila M77 utilizes hydrolytic and oxidative mechanisms to deconstruct biomass

Biomass is abundant, renewable and useful for biofuel production as well as chemical priming for plastics and composites. Deconstruction of biomass by enzymes is perceived as recalcitrant while an inclusive breakdown mechanism remains to be discovered. Fungi such as Myceliophthora thermophila M77 appear to decompose natural biomass sources quite well. This work reports on this fungus fermentation property while producing cellulolytic enzymes using natural biomass substrates. Little hydrolytic activity was detected, insufficient to explain the large amount of biomass depleted in the process. Furthermore, this work makes a comprehensive account of extracellular proteins and describes how secretomes redirect their qualitative protein content based on the nature and chemistry of the nutritional source. Fungus grown on purified cellulose or on natural biomass produced secretomes constituted by: cellobiohydrolases, cellobiose dehydrogenase, β-1,3 glucanase, β-glucosidases, aldose epimerase, glyoxal oxidase, GH74 xyloglucanase, galactosidase, aldolactonase and polysaccharide monooxygenases. Fungus grown on a mixture of purified hemicellulose fractions (xylans, arabinans and arabinoxylans) produced many enzymes, some of which are listed here: xylosidase, mixed β-1,3(4) glucanase, β-1,3 glucanases, β-glucosidases, β-mannosidase, β-glucosidases, galactosidase, chitinases, polysaccharide lyase, endo β-1,6 galactanase and aldose epimerase. Secretomes produced on natural biomass displayed a comprehensive set of enzymes involved in hydrolysis and oxidation of cellulose, hemicellulose-pectin and lignin. The participation of oxidation reactions coupled to lignin decomposition in the breakdown of natural biomass may explain the discrepancy observed for cellulose decomposition in relation to natural biomass fermentation experiments.

Enzymatic Strategies and Carbon Use Efficiency of a Litter-Decomposing Fungus Grown on Maize Leaves, Stems, and Roots

Soil microorganisms can control the soil cycles of carbon (C), and depending on their C-use efficiency (CUE), these microorganisms either contribute to C stabilization in soil or produce CO₂ when decomposing organic matter. However, little is known regarding the enzyme investment of microbial decomposers and the effects on their CUE. Our objective was to elucidate the strategies of litter-decomposing fungi as a function of litter quality. Fungal biosynthesis and respiration were accounted for by quantifying the investment in enzyme synthesis and enzyme efficiency. The basidiomycete Phanerochaete chrysosporium was grown on the leaves, stems, and roots of maize over 126 days in controlled conditions. We periodically measured the fungal biomass, enzyme activity, and chemical composition of the remaining litter and continuously measured the evolved C–CO₂. The CUE observed for the maize litter was highest in the leaves (0.63), intermediate in the roots (0.40), and lowest in the stems (0.38). However, the enzyme efficiency and investment in enzyme synthesis did not follow the same pattern. The amount of litter C decomposed per mole of C-acquiring hydrolase activity was 354 μg C in the leaves, 246 μg C in the roots, and 1541 μg C in the stems (enzyme efficiency: stems > leaves > roots). The fungus exhibited the highest investment in C-acquiring enzyme when grown on the roots and produced 40–80% less enzyme activity when grown on the stems and leaves (investment in enzymes: roots > leaves > stems). The CUE was dependent on the initial availability and replenishment of the soluble substrate fraction with the degradation products. The production of these compounds was either limited because of the low enzyme efficiency, which occurred in the roots, or because of the low investments in enzyme synthesis, which occurred in the stems. Fungal biosynthesis relied on the ability of the fungus to invest in enzyme synthesis and the efficient interactions between the enzymes and the substrate. The investment decreased when N was limited, whereas the efficiency of the C-acquiring enzymes was primarily explained by the hemicellulose content and its embedment in recalcitrant lignin linkages. Our results are crucial for modeling microbial allocation strategies.

Enzymatic cellulose oxidation is linked to lignin by long-range electron transfer

Enzymatic oxidation of cell wall polysaccharides by lytic polysaccharide monooxygenases (LPMOs) plays a pivotal role in the degradation of plant biomass. While experiments have shown that LPMOs are copper dependent enzymes requiring an electron donor, the mechanism and origin of the electron supply in biological systems are only partly understood. We show here that insoluble high molecular weight lignin functions as a reservoir of electrons facilitating LPMO activity. The electrons are donated to the enzyme by long-range electron transfer involving soluble low molecular weight lignins present in plant cell walls. Electron transfer was confirmed by electron paramagnetic resonance spectroscopy showing that LPMO activity on cellulose changes the level of unpaired electrons in the lignin. The discovery of a long-range electron transfer mechanism links the biodegradation of cellulose and lignin and sheds new light on how oxidative enzymes present in plant degraders may act in concert.

Genomics review of holocellulose deconstruction by aspergilli

SUMMARY

Biomass is constructed of dense recalcitrant polymeric materials: proteins, lignin, and holocellulose, a fraction constituting fibrous cellulose wrapped in hemicellulose-pectin. Bacteria and fungi are abundant in soil and forest floors, actively recycling biomass mainly by extracting sugars from holocellulose degradation. Here we review the genome-wide contents of seven Aspergillus species and unravel hundreds of gene models encoding holocellulose-degrading enzymes. Numerous apparent gene duplications followed functional evolution, grouping similar genes into smaller coherent functional families according to specialized structural features, domain organization, biochemical activity, and genus genome distribution. Aspergilli contain about 37 cellulase gene models, clustered in two mechanistic categories: 27 hydrolyze and 10 oxidize glycosidic bonds. Within the oxidative enzymes, we found two cellobiose dehydrogenases that produce oxygen radicals utilized by eight lytic polysaccharide monooxygenases that oxidize glycosidic linkages, breaking crystalline cellulose chains and making them accessible to hydrolytic enzymes. Among the hydrolases, six cellobiohydrolases with a tunnel-like structural fold embrace single crystalline cellulose chains and cooperate at nonreducing or reducing end termini, splitting off cellobiose. Five endoglucanases group into four structural families and interact randomly and internally with cellulose through an open cleft catalytic domain, and finally, seven extracellular β-glucosidases cleave cellobiose and related oligomers into glucose. Aspergilli contain, on average, 30 hemicellulase and 7 accessory gene models, distributed among 9 distinct functional categories: the backbone-attacking enzymes xylanase, mannosidase, arabinase, and xyloglucanase, the short-side-chain-removing enzymes xylan α-1,2-glucuronidase, arabinofuranosidase, and xylosidase, and the accessory enzymes acetyl xylan and feruloyl esterases.

Characterization of a novel manganese dependent endoglucanase belongs in GH family 5 from Phanerochaete chrysosporium

The cDNA encoding a putative glycoside hydrolase family 5, which has been predicted to be an endoglucanase (PcEg5A), was cloned from Phanerochaete chrysosporium and expressed in Pichia pastoris. PcEg5A contains a carbohydrate-binding domain and two important amino acids, E209 and E319, playing as proton donor and nucleophile in substrate catalytic domain. SDS-PAGE analysis indicated that the recombinant endoglucanase 5A (rPcEg5A) has a molecular size of 43 kDa which corresponds with the theoretical calculation. Optimum pH and temperature were found to be 4.5-6.0, and 50°C-60°C, respectively. Moreover, rPcEg5A exhibited maximal activity in the pH range of 3.0-8.0, whereas over 50% of activity still remained at 20°C and 80°C. rPcEg5A was stable at 60°C for 12 h incubation, indicating that rPcEg5A is a thermostable enzyme. Manganese ion enhanced the enzyme activity by 77%, indicating that rPcEg5A is a metal dependent enzyme. The addition of rPcEg5A to cellobiase (cellobiohydrolase and β-glucosidase) resulted in a 53% increasing saccharification of NaOH-pretreated barley straw, whereas the glucose release was 47% higher than that cellobiase treatment alone. Our study suggested that rPcEg5A is an enzyme with great potential for biomass saccharification.

Classification of fungal and bacterial lytic polysaccharide monooxygenases

Background

Lytic polysaccharide monooxygenases are important enzymes for the decomposition of recalcitrant biological macromolecules such as plant cell wall and chitin polymers. These enzymes were originally designated glycoside hydrolase family 61 and carbohydrate-binding module family 33 but are now classified as auxiliary activities 9, 10 and 11 in the CAZy database. To obtain a systematic analysis of the divergent families of lytic polysaccharide monooxygenases we used Peptide Pattern Recognition to divide 5396 protein sequences resembling enzymes from families AA9 (1828 proteins), AA10 (2799 proteins) and AA11 (769 proteins) into subfamilies.

Results

The results showed that the lytic polysaccharide monooxygenases have two conserved regions identified by conserved peptides specific for each AA family. The peptides were used for in silico PCR discovery of the lytic polysaccharide monooxygenases in 79 fungal and 95 bacterial genomes. The bacterial genomes encoded 0 – 7 AA10s (average 0.6). No AA9 or AA11 were found in the bacteria. The fungal genomes encoded 0 – 40 AA9s (average 7) and 0 – 15 AA11s (average 2) and two of the fungi possessed a gene encoding a putative AA10. The AA9s were mainly found in plant cell wall-degrading asco- and basidiomycetes in agreement with the described role of AA9 enzymes. In contrast, the AA11 proteins were found in 36 of the 39 ascomycetes and in only two of the 32 basidiomycetes and their abundance did not correlate to the degradation of cellulose and hemicellulose.

Conclusions

These results provides an overview of the sequence characteristics and occurrence of the divergent AA9, AA10 and AA11 families and pave the way for systematic investigations of the of lytic polysaccharide monooxygenases and for structure-function studies of these enzymes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1601-6) contains supplementary material, which is available to authorized users.

Removal of humic acid from composted hog waste by the white-rot fungus, Phanerochaete chrysosporium

The potential hazards of humic acid (HA) associated with hog waste effluent, coupled with increasing awareness of environmental problems, have prompted many countries to control disposal of effluents into water bodies and to maximize removal of HA. Here we employed the white-rot fungus, Phanerochaete chrysosporium, to degrade the HA in composted hog waste effluent, evaluated by the response surface method. Preliminary experiments demonstrate that pH, temperature and quantity of inoculum are significant variables determining success of the fungus. In total, 13 experiments were conducted with three variables designated as A (pH), B (temperature) and C (inoculum amount). The optimal conditions for reduction of HA by P. chrysosporium are pH 6, 31.5°C and an inoculum quantity of 5.86 g. Predicted and experimental results exhibit strong agreement, indicating efficiency in the model obtained by response surface method. Therefore, P. chrysosporium is an effective micro-organism for removal of HA from composted hog waste effluent.

Substrate specificity and regioselectivity of fungal AA9 lytic polysaccharide monooxygenases secreted by Podospora anserina

BACKGROUND

The understanding of enzymatic polysaccharide degradation has progressed intensely in the past few years with the identification of a new class of fungal-secreted enzymes, the lytic polysaccharide monooxygenases (LPMOs) that enhance cellulose conversion. In the fungal kingdom, saprotrophic fungi display a high number of genes encoding LPMOs from family AA9 but the functional relevance of this redundancy is not fully understood.

RESULTS

In this study, we investigated a set of AA9 LPMOs identified in the secretomes of the coprophilous ascomycete Podospora anserina, a biomass degrader of recalcitrant substrates. Their activity was assayed on cellulose in synergy with the cellobiose dehydrogenase from the same organism. We showed that the total release of oxidized oligosaccharides from cellulose was higher for PaLPMO9A, PaLPMO9E, and PaLPMO9H that harbored a carbohydrate-binding module from the family CBM1. Investigation of their regioselective mode of action revealed that PaLPMO9A and PaLPMO9H oxidatively cleaved at both C1 and C4 positions while PaLPMO9E released only C1-oxidized products. Rapid cleavage of cellulose was observed using PaLPMO9H that was the most versatile in terms of substrate specificity as it also displayed activity on cello-oligosaccharides and β-(1,4)-linked hemicellulose polysaccharides (e.g., xyloglucan, glucomannan).

CONCLUSIONS

This study provides insights into the mode of cleavage and substrate specificities of fungal AA9 LPMOs that will facilitate their application for the development of future biorefineries.

Transcriptome and exoproteome analysis of utilization of plant-derived biomass by Myceliophthora thermophila

Myceliophthora thermophila is a thermophilic fungus whose genome encodes a wide range of carbohydrate-active enzymes (CAZymes) involved in plant biomass degradation. Such enzymes have potential applications in turning different kinds of lignocellulosic feedstock into sugar precursors for biofuels and chemicals. The present study examined and compared the transcriptomes and exoproteomes of M. thermophila during cultivation on different types of complex biomass to gain insight into how its secreted enzymatic machinery varies with different sources of lignocellulose. In the transcriptome analysis three monocot (barley, oat, triticale) and three dicot (alfalfa, canola, flax) plants were used whereas in the proteome analysis additional substrates, i.e. wood and corn stover pulps, were included. A core set of 59 genes encoding CAZymes was up-regulated in response to both monocot and dicot straws, including nine polysaccharide monooxygenases and GH10, but not GH11, xylanases. Genes encoding additional xylanolytic enzymes were up-regulated during growth on monocot straws, while genes encoding additional pectinolytic enzymes were up-regulated in response to dicot biomass. Exoproteome analysis was generally consistent with the conclusions drawn from transcriptome analysis, but additional CAZymes that accumulated to high levels were identified. Despite the wide variety of biomass sources tested some CAZy family members were not expressed under any condition. The results of this study provide a comprehensive view from both transcriptome and exoproteome levels, of how M. thermophila responds to a wide range of biomass sources using its genomic resources.

Discovery of LPMO activity on hemicelluloses shows the importance of oxidative processes in plant cell wall degradation

The recently discovered lytic polysaccharide monooxygenases (LPMOs) are known to carry out oxidative cleavage of glycoside bonds in chitin and cellulose, thus boosting the activity of well-known hydrolytic depolymerizing enzymes. Because biomass-degrading microorganisms tend to produce a plethora of LPMOs, and considering the complexity and copolymeric nature of the plant cell wall, it has been speculated that some LPMOs may act on other substrates, in particular the hemicelluloses that tether to cellulose microfibrils. We demonstrate that an LPMO from Neurospora crassa, NcLPMO9C, indeed degrades various hemicelluloses, in particular xyloglucan. This activity was discovered using a glycan microarray-based screening method for detection of substrate specificities of carbohydrate-active enzymes, and further explored using defined oligomeric hemicelluloses, isolated polymeric hemicelluloses and cell walls. Products generated by NcLPMO9C were analyzed using high performance anion exchange chromatography and multidimensional mass spectrometry. We show that NcLPMO9C generates oxidized products from a variety of substrates and that its product profile differs from those of hydrolytic enzymes acting on the same substrates. The enzyme particularly acts on the glucose backbone of xyloglucan, accepting various substitutions (xylose, galactose) in almost all positions. Because the attachment of xyloglucan to cellulose hampers depolymerization of the latter, it is possible that the beneficial effect of the LPMOs that are present in current commercial cellulase mixtures in part is due to hitherto undetected LPMO activities on recalcitrant hemicellulose structures.

Fast solubilization of recalcitrant cellulosic biomass by the basidiomycete fungus Laetisaria arvalis involves successive secretion of oxidative and hydrolytic enzymes

BACKGROUND

Enzymatic breakdown of lignocellulosic biomass is a known bottleneck for the production of high-value molecules and biofuels from renewable sources. Filamentous fungi are the predominant natural source of enzymes acting on lignocellulose. We describe the extraordinary cellulose-deconstructing capacity of the basidiomycete Laetisaria arvalis, a soil-inhabiting fungus.

RESULTS

The L. arvalis strain displayed the capacity to grow on wheat straw as the sole carbon source and to fully digest cellulose filter paper. The cellulolytic activity exhibited in the secretomes of L. arvalis was up to 7.5 times higher than that of a reference Trichoderma reesei industrial strain, resulting in a significant improvement of the glucose release from steam-exploded wheat straw. Global transcriptome and secretome analyses revealed that L. arvalis produces a unique repertoire of carbohydrate-active enzymes in the fungal taxa, including a complete set of enzymes acting on cellulose. Temporal analyses of secretomes indicated that the unusual degradation efficiency of L. arvalis relies on its early response to the carbon source, and on the finely tuned sequential secretion of several lytic polysaccharide monooxygenases and hydrolytic enzymes targeting cellulose.

CONCLUSIONS

The present study illustrates the adaptation of a litter-rot fungus to the rapid breakdown of recalcitrant plant biomass. The cellulolytic capabilities of this basidiomycete fungus result from the rapid, selective and successive secretion of oxidative and hydrolytic enzymes. These enzymes expressed at critical times during biomass degradation may inspire the design of improved enzyme cocktails for the conversion of plant cell wall resources into fermentable sugars.

Recent insights into copper-containing lytic polysaccharide mono-oxygenases

Recently the role of oxidative enzymes in the degradation of polysaccharides by saprophytic bacteria and fungi was uncovered, challenging the classical model of polysaccharide degradation of being solely via a hydrolytic pathway. 3D structural analyses of lytic polysaccharide mono-oxygenases of both bacterial AA10 (formerly CBM33) and fungal AA9 (formerly GH61) enzymes revealed structures with β-sandwich folds containing an active site with a metal coordinated by an N-terminal histidine. Following some initial confusion about the identity of the metal ion it has now been shown that these enzymes are copper-dependent oxygenases. Here we assess recent developments in the academic literature, focussing on the structures of the copper active sites. We provide critical comparisons with known small-molecules studies of copper-oxygen complexes and with copper methane monoxygenase, another of nature's powerful copper oxygenases.

The copper active site of CBM33 polysaccharide oxygenases

The capacity of metal-dependent fungal and bacterial polysaccharide oxygenases, termed GH61 and CBM33, respectively, to potentiate the enzymatic degradation of cellulose opens new possibilities for the conversion of recalcitrant biomass to biofuels. GH61s have already been shown to be unique metalloenzymes containing an active site with a mononuclear copper ion coordinated by two histidines, one of which is an unusual τ-N-methylated N-terminal histidine. We now report the structural and spectroscopic characterization of the corresponding copper CBM33 enzymes. CBM33 binds copper with high affinity at a mononuclear site, significantly stabilizing the enzyme. X-band EPR spectroscopy of Cu(II)-CBM33 shows a mononuclear type 2 copper site with the copper ion in a distorted axial coordination sphere, into which azide will coordinate as evidenced by the concomitant formation of a new absorption band in the UV/vis spectrum at 390 nm. The enzyme's three-dimensional structure contains copper, which has been photoreduced to Cu(I) by the incident X-rays, confirmed by X-ray absorption/fluorescence studies of both aqueous solution and intact crystals of Cu-CBM33. The single copper(I) ion is ligated in a T-shaped configuration by three nitrogen atoms from two histidine side chains and the amino terminus, similar to the endogenous copper coordination geometry found in fungal GH61.