Efficient function and characterization of GH10 xylanase (Xyl10g) from Gloeophyllum trabeum in lignocellulose degradation

Thermophilic Degradation of Hemicellulose, a Critical Feedstock in the Production of Bioenergy and Other Value-Added Products

Renewable fuels have gained importance as the world moves toward diversifying its energy portfolio. A critical step in the biomass-to-bioenergy initiative is deconstruction of plant cell wall polysaccharides to their unit sugars for subsequent fermentation to fuels. To acquire carbon and energy for their metabolic processes, diverse microorganisms have evolved genes encoding enzymes that depolymerize polysaccharides to their carbon/energy-rich building blocks. The microbial enzymes mostly target the energy present in cellulose, hemicellulose, and pectin, three major forms of energy storage in plants. In the effort to develop bioenergy as an alternative to fossil fuel, a common strategy is to harness microbial enzymes to hydrolyze cellulose to glucose for fermentation to fuels. However, the conversion of plant biomass to renewable fuels will require both cellulose and hemicellulose, the two largest components of the plant cell wall, as feedstock to improve economic feasibility. Here, we explore the enzymes and strategies evolved by two well-studied bacteria to depolymerize the hemicelluloses xylan/arabinoxylan and mannan. The sets of enzymes, in addition to their applications in biofuels and value-added chemical production, have utility in animal feed enzymes, a rapidly developing industry with potential to minimize adverse impacts of animal agriculture on the environment.

Potential of esterase DmtH in transforming plastic additive dimethyl terephthalate to less toxic mono-methyl terephthalate

Dimethyl terephthalate (DMT) is a primary ingredient widely used in the manufacture of polyesters and industrial plastics; its environmental fate is of concern due to its global use. Microorganisms play key roles in the dissipation of DMT from the environment; however, the enzymes responsible for the initial transformation of DMT and the possible altered toxicity due to this biotransformation have not been extensively studied. To reduce DMT toxicity, we identified the esterase gene dmtH involved in the initial transformation of DMT from the AOPP herbicide-transforming strain Sphingobium sp. C3. DmtH shows 24-41% identity with α/β-hydrolases and belongs to subfamily V of bacterial esterases. The purified recombinant DmtH was capable of transforming DMT to mono-methyl terephthalate (MMT) and potentially transforming other p-phthalic acid esters, including diallyl terephthalate (DAT) and diethyl terephthalate (DET). Using C. elegans as an assay model, we observed the severe toxicity of DMT in inducing reactive oxygen species (ROS) production, decreasing locomotion behavior, reducing lifespan, altering molecular basis for oxidative stress, and inducing mitochondrial stress. In contrast, exposure to MMT did not cause obvious toxicity, induce oxidative stress, and activate mitochondrial stress in nematodes. Our study highlights the usefulness of Sphingobium sp. C3 and its esterase DmtH in transforming p-phthalic acid esters and reducing the toxicity of DMT to organisms.

Advanced strategy to produce insecticidal destruxins from lignocellulosic biomass Miscanthus

BACKGROUND

Biorefineries are widely recognized as the most feasible solution to the problem of achieving environmental sustainability along with economic growth. Furthermore, pine wilt disease has caused severe environmental and economic damage worldwide to date. Herein, a highly efficient, advanced process for producing destruxins (DTXs) from Miscanthus (MCT) is reported, along with an application strategy.

RESULTS

The acetic acid-sodium chlorite pretreatment of MCT (AASC-MCT) is found to improve the monosaccharide production. Through biocatalytic conversion processes (simultaneous saccharification and cultivation), Metarhizium anisopliae JEF-279 can efficiently produce DTXs from 1% (w/v) AASC-MCT, i.e., DTX E (334.8 mg/L), A (288.8 mg/L), and B (48.6 mg/L). Monochamus alternatus (MA, Japanese pine sawyer) is known to act as a mediator transferring Bursaphelenchus xylophilus to pinewood. As B. xylophilus is associated with the occurrence of pine wilt disease, biological control of MA is a major strategy or controlling this disease. In this study, upon the application of a mixture of DTXs and protease-containing culture filtrate (PCF), complete mortality of MA is observed after a 5-day incubation. The MA immune system response is believed to cause an overexpression of actin and tropomyosin as a defense mechanism against the flaccid paralysis induced by the DTXs and PCF treatment.

CONCLUSIONS

These results suggest that MCT can be used as a major feedstock in the biorefinery industry and that DTXs can be applied as an insecticide for biological control of pine wilt disease via MA termination.

Improvement of the catalytic efficiency of a hyperthermophilic xylanase from Bispora sp. MEY-1

Extremophilic xylanases have attracted great scientific and industrial interest. In this study, a GH10 xylanase-encoding gene, Xyl10E, was cloned from Bispora sp. MEY-1 and expressed in Pichia pastoris GS115. Deduced Xyl10E shares the highest identities of 62% and 57% with characterized family GH10 xylanases from Talaromyces leycettanus and Penicillium canescens (structure 4F8X), respectively. Xyl10E was most active at 93 to 95°C and pH 4.0, retained more than 75% or 48% of the initial activity when heated at 80°C or 90°C for 30 min, respectively, and hardly lost activity at pH 1.0 to 7.0, but was completely inhibited by SDS. Two residues, A160 and A161, located on loop 4, were identified to play roles in catalysis. Mutants A160D/E demonstrated higher affinity to substrate with lower K_m values, while mutants A161D/E mainly displayed elevated V_max values. All of these mutants had significantly improved catalytic efficiency. According to the molecular dynamics simulation, the mutation of A160E was able to affect the important substrate binding site Y204 and then improve the substrate affinity, and the mutation of A161D was capable of forming a hydrogen bond with the substrate to promote the substrate binding or accelerate the product release. This study introduces a highly thermophilic fungal xylanase and reveals the importance of loop 4 for catalytic efficiency.

Combined effect of enzyme inducers and nitrate on selective lignin degradation in wheat straw by Ganoderma lobatum

Lignin is one of the main barriers to obtaining added-value products from cellulosic fraction of lignocellulosic biomass due to its random aromatic structure and strong association with cellulose and hemicellulose. Inorganic and organic compounds have been used as enzyme inducers to increase the ligninolytic potential of white-rot fungi, without considering their effect on the selectivity of degradation. In this study, the selective lignin degradation in wheat straw by Ganoderma lobatum was optimized using a central composite design to evaluate the combined effect of Fe²⁺ and Mn²⁺ as inducers of ligninolytic enzymes and NO₃^- as an additional nitrogen source. Selective lignin degradation was promoted to maximize lignin degradation and minimize weight losses. The optimal conditions were 0.18 M NO₃^-, 0.73 mM Fe²⁺, and 1 mM Mn²⁺, which resulted in 50.0% lignin degradation and 18.5% weight loss after 40 days of fungal treatment. A decrease in absorbance at 1505 and 900 cm^-1 in fungal-treated samples was observed in the FTIR spectra, indicating lignin and cellulose degradation in fungal-treated wheat straw, respectively. The main ligninolytic enzymes detected during lignin degradation were manganese-dependent and manganese-independent peroxidases. Additionally, confocal laser scanning microscopy revealed that lignin degradation in wheat straw by G. lobatum resulted in higher cellulose accessibility. We concluded that the addition of enzyme inducers and NO₃^- promotes selective lignin degradation in wheat straw by G. lobatum.

Lignocellulose degrading extremozymes produced by Pichia pastoris: current status and future prospects

In this review article, extremophilic lignocellulosic enzymes with special interest on xylanases, β-mannanases, laccases and finally cellulases, namely, endoglucanases, exoglucanases and β-glucosidases produced by Pichia pastoris are reviewed for the first time. Recombinant lignocellulosic extremozymes are discussed from the perspectives of their potential application areas; characteristics of recombinant and native enzymes; the effects of P. pastoris expression system on recombinant extremozymes; and their expression levels and applied strategies to increase the enzyme expression yield. Further, effects of enzyme domains on activity and stability, protein engineering via molecular dynamics simulation and computational prediction, and site-directed mutagenesis and amino acid modifications done are also focused. Superior enzyme characteristics and improved stability due to the proper post-translational modifications and better protein folding performed by P. pastoris make this host favourable for extremozyme production. Especially, glycosylation contributes to the structure, function and stability of enzymes, as generally glycosylated enzymes produced by P. pastoris exhibit better thermostability than non-glycosylated enzymes. However, there has been limited study on enzyme engineering to improve catalytic efficiency and stability of lignocellulosic enzymes. Thus, in the future, studies should focus on protein engineering to improve stability and catalytic efficiency via computational modelling, mutations, domain replacements and fusion enzyme technology. Also metagenomic data need to be used more extensively to produce novel enzymes with extreme characteristics and stability.

Single step purification of concanavalin A (Con A) and bio-sugar production from jack bean using glucosylated magnetic nano matrix

Jack bean (JB, Canavalia ensiformis) is the source of bio-based products, such as proteins and bio-sugars that contribute to modern molecular biology and biomedical research. In this study, the use of jack bean was evaluated as a source for concanavalin A (Con A) and bio-sugar production. A novel method for purifying Con A from JBs was successfully developed using a glucosylated magnetic nano matrix (GMNM) as a physical support, which facilitated easy separation and purification of Con A. In addition, the enzymatic conversion rate of 2% (w/v) Con A extracted residue to bio-sugar was 98.4%. Therefore, this new approach for the production of Con A and bio-sugar is potentially useful for obtaining bio-based products from jack bean.

Thermostable microbial xylanases for pulp and paper industries: trends, applications and further perspectives

Xylanases are enzymes with biotechnological relevance in a number of fields, including food, feed, biofuel, and textile industries. Their most significant application is in the paper and pulp industry, where they are used as a biobleaching agent, showing clear economic and environmental advantages over chemical alternatives. Since this process requires high temperatures and alkali media, the identification of thermostable and alkali stable xylanases represents a major biotechnological goal in this field. Moreover, thermostability is a desirable property for many other applications of xylanases. The review makes an overview of xylanase producing microorganisms and their current implementation in paper biobleaching. Future perspectives are analyzed focusing in the efforts carried out to generate thermostable enzymes by means of modern biotechnological tools, including metagenomic analysis, enzyme molecular engineering and nanotechnology. Furthermore, structural and mutagenesis studies have revealed critical sites for stability of xylanases from glycoside hydrolase families GH10 and GH11, which constitute the main classes of these enzymes. The overall conclusions of these works are summarized here and provide relevant information about putative weak spots within xylanase structures to be targeted in future protein engineering approaches.

Molecular breeding of lignin-degrading brown-rot fungus Gloeophyllum trabeum by homologous expression of laccase gene

The basidiomycete Gloeophyllum trabeum KU-41 can degrade Japanese cedar wood efficiently. To construct a strain better suited for biofuel production from Japanese cedar wood, we developed a gene transformation system for G. trabeum KU-41 using the hygromycin phosphotransferase-encoding gene (hpt) as a marker. The endogenous laccase candidate gene (Gtlcc3) was fused with the promoter of the G. trabeum glyceraldehyde-3-phosphate dehydrogenase-encoding gene and co-transformed with the hpt-bearing pAH marker plasmid. We obtained 44 co-transformants, and identified co-transformant L#61, which showed the highest laccase activity among all the transformants. Moreover, strain L#61 was able to degrade lignin in Japanese cedar wood-containing medium, in contrast to wild-type G. trabeum KU-41 and to a typical white-rot fungus Phanerochaete chrysosporium. By using strain L#61, direct ethanol production from Japanese cedar wood was improved compared to wild type. To our knowledge, this study is the first report of the molecular breeding of lignin-degrading brown-rot fungus and direct ethanol production from softwoods by co-transformation with laccase overproduction constructs.

Efficient approach for bioethanol production from red seaweed Gelidium amansii

Gelidium amansii (GA), a red seaweed species, is a popular source of food and chemicals due to its high galactose and glucose content. In this study, we investigated the potential of bioethanol production from autoclave-treated GA (ATGA). The proposed method involved autoclaving GA for 60min for hydrolysis to glucose. Separate hydrolysis and fermentation processing (SHF) achieved a maximum ethanol concentration of 3.33mg/mL, with a conversion yield of 74.7% after 6h (2% substrate loading, w/v). In contrast, simultaneous saccharification and fermentation (SSF) produced an ethanol concentration of 3.78mg/mL, with an ethanol conversion yield of 84.9% after 12h. We also recorded an ethanol concentration of 25.7mg/mL from SSF processing of 15% (w/v) dry matter from ATGA after 24h. These results indicate that autoclaving can improve the glucose and ethanol conversion yield of GA, and that SSF is superior to SHF for ethanol production.