Characterization and biotechnological application of recombinant xylanases from Aspergillus nidulans

A novel endo-1,4-β-xylanase from Alicyclobacillus mali FL18: Biochemical characterization and its synergistic action with β-xylosidase in hemicellulose deconstruction

A novel endo-1,4-β-xylanase-encoding gene was identified in Alicyclobacillus mali FL18 and the recombinant protein, named AmXyn, was purified and biochemically characterized. The monomeric enzyme worked optimally at pH 6.6 and 80 °C on beechwood xylan with a specific activity of 440.00 ± 0.02 U/mg and a good catalytic efficiency (kcat/KM = 91.89 s-1mLmg-1). In addition, the enzyme did not display any activity on cellulose, suggesting a possible application in paper biobleaching processes. To develop an enzymatic mixture for xylan degradation, the association between AmXyn and the previously characterized β-xylosidase AmβXyl, deriving from the same microorganism, was assessed. The two enzymes had similar temperature and pH optima and showed the highest degree of synergy when AmXyn and AmβXyl were added sequentially to beechwood xylan, making this mixture cost-competitive and suitable for industrial use. Therefore, this enzymatic cocktail was also employed for the hydrolysis of wheat bran residue. TLC and HPAEC-PAD analyses revealed a high conversion rate to xylose (91.56 %), placing AmXyn and AmβXyl among the most promising biocatalysts for the saccharification of agricultural waste.

Safe production of Aspergillus terreus xylanase from Ricinus communis: gene identification, molecular docking, characterization, production of xylooligosaccharides, and its biological activities

BACKGROUND

The production of industrial enzymes such as xylanase using sufficient cost-effective substrates from potent microorganisms is considered economically feasible. Studies have reported castor cake (Ricinus communis) as the most potent and inexpensive alternative carbon source for production of xylanase C by using Aspergillus terreus (A. terreus).

RESULTS

A. terreus strain RGS Eg-NRC, a local isolate from agro-wastes, was first identified by sequencing the internal transcribed spacer region of a nuclear DNA encoding gene cluster deposited in GenBank (accession number MW282328). Before optimization of xylanase production, A. terreus produced 20.23 U/g of xylanase after 7 days using castor cake as a substrate in a solid-state fermentation (SSF) system that was employed to achieve ricin detoxification and stimulate xylanase production. Physicochemical parameters for the production of xylanase were optimized by using a one-variable-at-a-time approach and two statistical methods (two-level Plackett-Burman design and central composite design, CCD). The maximum xylanase yield after optimization was increased by 12.1-fold (245 U/g). A 60-70% saturation of ammonium sulfate resulted in partially purified xylanase with a specific activity of 3.9 IU/mg protein. At 60 °C and pH 6, the partially purified xylanase had the highest activity, and the activation energy (Ea) was 23.919 kJmol. Subsequently, antioxidant capacity and cytotoxicity tests in normal Ehrlich ascites carcinoma human cells demonstrated xylooligosaccharides produced by the xylanase degradation of xylan as a potent antioxidant and moderate antitumor agent. Further investigations with sodium dodecyl sulfate polyacrylamide gel electrophoresis then determined the molecular weight of partially purified xylanase C to be 36 kDa. Based on the conserved regions, observations revealed that xylanase C belonged to the glycosyl hydrolase family 10. Next, the xylanase-encoding gene (xynC), which has an open reading frame of 981 bp and encodes a protein with 326 amino acids, was isolated, sequenced, and submitted to the NCBI GenBank database (accession number LC595779.1). Molecular docking analysis finally revealed that Glu156, Glu262, and Lys75 residues were involved in the substrate-binding and protein-ligand interaction site of modeled xylanase, with a binding affinity of -8.7 kcal. mol^-1. CONCLUSION: The high production of safe and efficient xylanase could be achieved using economical materials such as Ricinus communis.

Structural and biochemical analysis reveals how ferulic acid improves catalytic efficiency of Humicola grisea xylanase

Humicolagrisea var. thermoidea is an aerobic and thermophilic fungus that secretes the GH11 xylanase HXYN2 in the presence of sugarcane bagasse. In this study, HXYN2 was expressed in Pichiapastoris and characterized biochemically and structurally in the presence of beechwood xylan substrate and ferulic acid (FA). HXYN2 is a thermally stable protein, as indicated by circular dichroism, with greater activity in the range of 40–50 °C and pH 5.0–9.0, with optimal temperature and pH of 50 °C and 6.0, respectively. FA resulted in a 75% increase in enzyme activity and a 2.5-fold increase in catalytic velocity, catalytic efficiency, and catalytic rate constant (k_cat), with no alteration in enzyme affinity for the substrate. Fluorescence quenching indicated that FA forms a complex with HXYN2 interacting with solvent-exposed tryptophan residues. The binding constants ranged from moderate (pH 7.0 and 9.0) to strong (pH 4.0) affinity. Isothermal titration calorimetry, structural models and molecular docking suggested that hydrogen bonds and hydrophobic interactions occur in the aglycone region inducing conformational changes in the active site driven by initial and final enthalpy- and entropy processes, respectively. These results indicate a potential for biotechnological application for HXYN2, such as in the bioconversion of plant residues rich in ferulic acid.

Loop engineering of a thermostable GH10 xylanase to improve low-temperature catalytic performance for better synergistic biomass-degrading abilities

Lignocellulosic biorefining for producing biofuels poses technical challenges. It is usually conducted over a long time using heat, making it energy intensive. In this study, we lowered the energy consumption of this process through an optimized enzyme and pretreatment strategy. First, the dominant mutant M137E/N269G of Bispora sp. MEY-1XYL10C_ΔN was obtained by directed evolution with highcatalytic efficiency (970 mL/s∙mg)and specific activity (2090 U/mg)at 37 °C, and thermostability was improved (T₅₀ increased by5 °C). After pretreatment with seawater immersionfollowing steam explosion,bagasse was co-treated with cellulase and M137E/N269G under mild conditions (37 °C), the resulting highest yield of fermentable sugars reached 219 µmol/g of bagasse,46% higher than that of the non-seawater treatment group, with the highest degree of synergy of 2.0. Pretreatment with seawater following steam explosion and synergistic hydrolysis through high activity xylanase and cellulase helped to achieve low energy degradation of lignocellulosic biomass.

Biochemical characterization of xylanase GH11 isolated from Aspergillus niger BCC14405 (XylB) and its application in xylooligosaccharide production

OBJECTIVE

To develop an endo-β-1,4-xylanase with high specificity for production of prebiotic xylooligosaccharides that optimally works at moderate temperature desirable to reduce the energy cost in the production process.

RESULTS

The xylB gene, encoding for a glycosyl hydrolase family 11 xylanase from a thermoresistant fungus, Aspergillus niger BCC14405 was expressed in a methylotrophic yeast P. pastoris KM71 in a secreted form. The recombinant XylB showed a high specific activity of 3852 and 169 U mg^-1 protein on beechwood xylan and arabinoxylan, respectively with no detectable side activities against different forms of cellulose (Avicel Ò PH101 microcrystalline cellulose, phosphoric acid swollen cellulose and carboxymethylcellulose). The enzyme worked optimally at 45 °C, pH 6.0. It showed a specific cleavage pattern by releasing xylobiose (X2) as the major product from xylooligosaccharides (X3 to X6) substrates. The highest XOS yield of 708 mg g^-1 substrate comprising X2, X3 and X6 was obtained from beechwood xylan hydrolysis.

CONCLUSION

The enzyme is potent for XOS production and for saccharification of lignocellulosic biomass.

Two-Step Saccharification of the Xylan Portion of Sugarcane Waste by Recombinant Xylanolytic Enzymes for Enhanced Xylose Production

Sugarcane bagasse (SB) and sugarcane trash (SCT) containing 30% hemicellulose content are the waste from the sugarcane industry. Hemicellulose being heterogeneous, more complex, and less abundant than cellulose remains less explored. The optimized conditions for the pretreatment of SB and SCT for maximizing the delignification are soaking in aqueous ammonia (SAA), 18.5 wt %, followed by heating at 70 °C for 14 h. The optimization of hydrolysis of SAA pretreated (ptd) SB and SCT by the Box-Behnken design in the first step of saccharification by xylanase (CtXyn11A) and α-l-arabinofuranosidase (PsGH43_12) resulted in the total reducing sugar (TRS) yield of xylooligosaccharides (TRS_(XOS)) of 93.2 mg/g ptd SB and 85.1 mg/g ptd SCT, respectively. The second step of saccharification by xylosidase (BoGH43) gave the TRS yield of 164.7 mg/g ptd SB and 147.2 mg/g ptd SCT. The high-performance liquid chromatography analysis of hydrolysate obtained after the second step of saccharification showed 69.6% xylan-to-xylose conversion for SB and 64.1% for SCT. This study demonstrated the optimization of the pretreatment method and of the enzymatic saccharification by recombinant xylanolytic enzymes, resulting in the efficient saccharification of ptd hemicellulose to TRS by giving 73.5% conversion for SB and 71.1% for SCT. These optimized conditions for the pretreatment and saccharification of sugarcane waste can also be used at a large scale.

Optimization of β-1,4-Endoxylanase Production by an Aspergillus niger Strain Growing on Wheat Straw and Application in Xylooligosaccharides Production

Plant biomass constitutes the main source of renewable carbon on the planet. Its valorization has traditionally been focused on the use of cellulose, although hemicellulose is the second most abundant group of polysaccharides on Earth. The main enzymes involved in plant biomass degradation are glycosyl hydrolases, and filamentous fungi are good producers of these enzymes. In this study, a new strain of Aspergillus niger was used for hemicellulase production under solid-state fermentation using wheat straw as single-carbon source. Physicochemical parameters for the production of an endoxylanase were optimized by using a One-Factor-at-a-Time (OFAT) approach and response surface methodology (RSM). Maximum xylanase yield after RSM optimization was increased 3-fold, and 1.41- fold purification was achieved after ultrafiltration and ion-exchange chromatography, with about 6.2% yield. The highest activity of the purified xylanase was observed at 50 °C and pH 6. The enzyme displayed high thermal and pH stability, with more than 90% residual activity between pH 3.0–9.0 and between 30–40 °C, after 24 h of incubation, with half-lives of 30 min at 50 and 60 °C. The enzyme was mostly active against wheat arabinoxylan, and its kinetic parameters were analyzed (K_m = 26.06 mg·mL⁻¹ and V_max = 5.647 U·mg⁻¹). Wheat straw xylan hydrolysis with the purified β-1,4 endoxylanase showed that it was able to release xylooligosaccharides, making it suitable for different applications in food technology.

Genomic and Postgenomic Diversity of Fungal Plant Biomass Degradation Approaches

Plant biomass degradation by fungi is a widely studied and applied field of science, due to its relevance for the global carbon cycle and many biotechnological applications. Before the genome era, many of the in-depth studies focused on a relatively small number of species, whereas now, many species can be addressed in detail, revealing the large variety in the approach used by fungi to degrade plant biomass. This variation is found at many levels and includes genomic adaptation to the preferred biomass component, but also different approaches to degrade this component by diverse sets of activities encoded in the genome. Even larger differences have been observed using transcriptome and proteome studies, even between closely related species, suggesting a high level of adaptation in individual species. A better understanding of the drivers of this diversity could be highly valuable in developing more efficient biotechnology approaches for the enzymatic conversion of plant biomass.

Redesigning the Aspergillus nidulans xylanase regulatory pathway to enhance cellulase production with xylose as the carbon and inducer source

Background

Biomass contains cellulose (C6-sugars), hemicellulose (C5-sugars) and lignin. Biomass ranks amongst the most abundant hydrocarbon resources on earth. However, biomass is recalcitrant to enzymatic digestion by cellulases. Physicochemical pretreatment methods make cellulose accessible but partially destroy hemicellulose, producing a C5-sugar-rich liquor. Typically, digestion of pretreated LCB is performed with commercial cellulase preparations, but C5-sugars could in principle be used for “on site” production of cellulases by genetically engineered microorganism, thereby reducing costs.

Results

Here we report a succession of genetic interventions in Aspergillus nidulans that redesign the natural regulatory circuitry of cellulase genes in such a way that recombinant strains use C5-sugar liquors (xylose) to grow a vegetative tissue and simultaneously accumulate large amounts of cellulases. Overexpression of XlnR showed that under xylose-induction conditions only xylanase C was produced. XlnR overexpression strains were constructed that use the xynCp promoter to drive the production of cellobiohydrolases, endoglucanases and β-glucosidase. All five cellulases accumulated at high levels when grown on xylose. Production of cellulases in the presence of pretreated-biomass C5-sugar liquors was investigated, and cellulases accumulated to much higher enzyme titers than those obtained for traditional fungal cell factories with cellulase-inducing substrates.

Conclusions

By replacing expensive substrates with a cheap by-product carbon source, the use of C5-sugar liquors directly derived from LCB pretreatment processes not only reduces enzyme production costs, but also lowers operational costs by eliminating the need for off-site enzyme production, purification, concentration, transport and dilution.

Structural and functional characterisation of xylanase purified from Penicillium chrysogenum produced in response to raw agricultural waste

Commercial interest in plant cell wall degrading enzymes (PCWDE) is motivated by their potential for energy or bioproduct generation that reduced dependency on non-renewable (fossil-derived) feedstock. Therefore, underlying work analysed the Penicillium chrysogenum isolate for PCWDE production by employing different biomass as a carbon source. Among the produced enzymes, three xylanase isoforms were observed in the culture filtrate containing sugarcane bagasse. Xylanase (PcX1) presenting 35 kDa molecular mass was purified by gel filtration and anion exchange chromatography. Unfolding was probed and analysed using fluorescence, circular dichroism and enzyme assay methods. Secondary structure contents were estimated by circular dichroism 45% α-helix and 10% β-sheet, consistent with the 3D structure predicted by homology. PcX1 optimally active at pH 5.0 and 30 °C, presenting t_1/2 19 h at 30 °C and 6 h at 40 °C. Thermodynamic parameters/melting temperature 51.4 °C confirmed the PcX1 stability at pH 5.0. PcX1 have a higher affinity for oat spelt xylan, K_M 1.2 mg·mL^-1, in comparison to birchwood xylan K_M 29.86 mg·mL^-1, activity was inhibited by Cu⁺² and activated by Zn⁺². PcX1 exhibited significant tolerance for vanillin, trans-ferulic acid, ρ-coumaric acid, syringaldehyde and 4-hydroxybenzoic acid, activity slightly inhibited (17%) by gallic and tannic acid.

Enzymes and Dairy Products

The use of enzymes in food production, including dairy products, is below expected due the high costs associated with their production and purification. Microbial enzymes have great potential for industrial applications since they can be produced via large-scale fermentation and they are easily expressed by cloning in well-known cultivated microorganisms. The combination of different procedures such as over-expression techniques and the use of low costs induction sources has resulted in the production of enzymes to be used in high added-value dairy products. The addition of glucose oxidase to probiotic yogurts has been indicated as an alternative to the maintenance of probiotic functionality. Bile salt hydrolase contributes to prevention of hypercholesterolemia which is interesting to produce new functional dairy products. This chapter discusses enzyme sources and their relevance in dairy products, the production of enzymes using cloning and super-expression techniques, as well as enzymes related to functional dairy products.

Adding tetrahydrofuran to dilute acid pretreatment provides new insights into substrate changes that greatly enhance biomass deconstruction by Clostridium thermocellum and fungal enzymes

BACKGROUND

Consolidated bioprocessing (CBP) by anaerobes, such as Clostridium thermocellum, which combine enzyme production, hydrolysis, and fermentation are promising alternatives to historical economic challenges of using fungal enzymes for biological conversion of lignocellulosic biomass. However, limited research has integrated CBP with real pretreated biomass, and understanding how pretreatment impacts subsequent deconstruction by CBP vs. fungal enzymes can provide valuable insights into CBP and suggest other novel biomass deconstruction strategies. This study focused on determining the effect of pretreatment by dilute sulfuric acid alone (DA) and with tetrahydrofuran (THF) addition via co-solvent-enhanced lignocellulosic fractionation (CELF) on deconstruction of corn stover and Populus with much different recalcitrance by C. thermocellum vs. fungal enzymes and changes in pretreated biomass related to these differences.

RESULTS

Coupling CELF fractionation of corn stover and Populus with subsequent CBP by the anaerobe C. thermocellum completely solubilized polysaccharides left in the pretreated solids within only 48 h without adding enzymes. These results were better than those from the conventional DA followed by either CBP or fungal enzymes or CELF followed by fungal enzyme hydrolysis, especially at viable enzyme loadings. Enzyme adsorption on CELF-pretreated corn stover and CELF-pretreated Populus solids were virtually equal, while DA improved the enzyme accessibility for corn stover more than Populus. Confocal scanning light microscopy (CSLM), transmission electron microscopy (TEM), and NMR characterization of solids from both pretreatments revealed differences in cell wall structure and lignin composition, location, coalescence, and migration-enhanced digestibility of CELF-pretreated solids.

CONCLUSIONS

Adding THF to DA pretreatment (CELF) greatly enhanced deconstruction of corn stover and Populus by fungal enzymes and C. thermocellum CBP, and the CELF-CBP tandem was agnostic to feedstock recalcitrance. Composition measurements, material balances, cellulase adsorption, and CSLM and TEM imaging revealed adding THF enhanced the enzyme accessibility, cell wall fractures, and cellular dislocation and cell wall delamination. Overall, enhanced deconstruction of CELF solids by enzymes and particularly by C. thermocellum could be related to lignin removal and alteration, thereby pointing to these factors being key contributors to biomass recalcitrance as a barrier to low-cost biological conversion to sustainable fuels.