Purification and biochemical properties of a thermostable xylose-tolerant β-D-xylosidase from Scytalidium thermophilum

Exploring the potential of a new thermotolerant xylanase from Rasamsonia composticola (XylRc): production using agro-residues, biochemical studies, and application to sugarcane bagasse saccharification

Xylanases from thermophilic fungi have a wide range of commercial applications in the bioconversion of lignocellulosic materials and biobleaching in the pulp and paper industry. In this study, an endoxylanase from the thermophilic fungus Rasamsonia composticola (XylRc) was produced using waste wheat bran and pretreated sugarcane bagasse (PSB) in solid-state fermentation. The enzyme was purified, biochemically characterized, and used for the saccharification of sugarcane bagasse. XylRc was purified 30.6-fold with a 22% yield. The analysis using sodium dodecyl sulphate-polyacrylamide gel electrophoresis revealed a molecular weight of 53 kDa, with optimal temperature and pH values of 80 °C and 5.5, respectively. Thin-layer chromatography suggests that the enzyme is an endoxylanase and belongs to the glycoside hydrolase 10 family. The enzyme was stimulated by the presence of K+, Ca2+, Mg2+, and Co2+ and remained stable in the presence of the surfactant Triton X-100. XylRc was also stimulated by organic solvents butanol (113%), ethanol (175%), isopropanol (176%), and acetone (185%). The Km and Vmax values for oat spelt and birchwood xylan were 6.7 ± 0.7 mg/mL, 2.3 ± 0.59 mg/mL, 446.7 ± 12.7 µmol/min/mg, and 173.7 ± 6.5 µmol/min/mg, respectively. XylRc was unaffected by different phenolic compounds: ferulic, tannic, cinnamic, benzoic, and coumaric acids at concentrations of 2.5-10 mg/mL. The results of saccharification of PSB showed that supplementation of a commercial enzymatic cocktail (Cellic® CTec2) with XylRc (1:1 w/v) led to an increase in the degree of synergism (DS) in total reducing sugar (1.28) and glucose released (1.05) compared to the control (Cellic® HTec2). In summary, XylRc demonstrated significant potential for applications in lignocellulosic biomass hydrolysis, making it an attractive alternative for producing xylooligosaccharides and xylose, which can serve as precursors for biofuel production.

High-level production of xylose from agricultural wastes using GH11 endo-xylanase and GH43 β-xylosidase from Bacillus sp

As a promising feedstock, alkali-extracted xylan from lignocellulosic biomass is desired for producing xylose, which can be used for renewable biofuels production. In this study, an efficient pathway has been established for low-cost and high-yield production of xylose by hydrolysis of alkali-extracted xylan from agricultural wastes using an endo-1,4-xylanase (XYLA) from Bacillus safensis TCCC 111022 and a β-xylosidase (XYLO) from B. pumilus TCCC 11573. The optimum activities of recombinant XYLA (rXYLA) and XYLO (rXYLO) were 60 ℃ and pH 8.0, and 30 ℃ and pH 7.0, respectively. They were stable over a broad pH range (pH 6.0-11.0 and 7.0-10.0). rXYLO showed a relatively high xylose tolerance up to 100 mM. Furthermore, the yield of xylose from wheat straw, rice straw, corn stover, corncob and sugarcane bagasse by rXYLA and rXYLO was 63.77%, 71.76%, 68.55%, 53.81%, and 58.58%, respectively. This study demonstrated a strategy to produce xylose from agricultural wastes by integrating alkali-extracted xylan and enzymatic hydrolysis.

Biochemical Basis of Xylooligosaccharide Utilisation by Gut Bacteria

Xylan is one of the major structural components of the plant cell wall. Xylan present in the human diet reaches the large intestine undigested and becomes a substrate to species of the gut microbiota. Here, we characterised the capacity of Limosilactobacillus reuteri and Blautia producta strains to utilise xylan derivatives. We showed that L. reuteri ATCC 53608 and B. producta ATCC 27340 produced β-D-xylosidases, enabling growth on xylooligosaccharide (XOS). The recombinant enzymes were highly active on artificial (p-nitrophenyl β-D-xylopyranoside) and natural (xylobiose, xylotriose, and xylotetraose) substrates, and showed transxylosylation activity and tolerance to xylose inhibition. The enzymes belong to glycoside hydrolase family 120 with Asp as nucleophile and Glu as proton donor, as shown by homology modelling and confirmed by site-directed mutagenesis. In silico analysis revealed that these enzymes were part of a gene cluster in L. reuteri but not in Blautia strains, and quantitative proteomics identified other enzymes and transporters involved in B. producta XOS utilisation. Based on these findings, we proposed a model for an XOS metabolism pathway in L. reuteri and B. producta strains. Together with phylogenetic analyses, the data also revealed the extended xylanolytic potential of the gut microbiota.

Definitive Screening Design and Artificial Neural Network for Modeling a Rapid Biodegradation of Date Palm Fronds by a New Trichoderma sp. PWN6 into Citric Acid

Generally, the bioconversion of lignocellulolytics into a new biomolecule is carried out through two or more steps. The current study used one-step bioprocessing of date palm fronds (DPF) into citric acid as a natural product, using a pioneer strain of Trichodermaharzianum (PWN6) that has been selected from six tested isolates based on the highest organic acid (OA) productivity (195.41 µmol/g), with the lowest amount of the released glucose. Trichoderma sp. PWN6 was morphologically and molecularly identified, and the GenBank accession number was MW78912.1. Both definitive screening design (DSD) and artificial neural network (ANN) were applied, for the first time, for modeling the bioconversion process of DPF. Although both models are capable of making accurate predictions, the ANN model outperforms the DSD model in terms of OA production, as ANN is characterized by a higher value of R² (0.963) and validation R² (0.967), and lower values of the RMSE (13.44), MDA (11.06), and SSE (9749.5). Citric acid was the only identified OA as was confirmed by GC-MS and UPLC, with a total of 1.5%. In conclusion, DPF together with T. harzianum PWN6 is considered an excellent new combination for citric acid biosynthesis, after modeling with artificial intelligence procedure.

Crude Citric Acid of Trichoderma asperellum: Tomato Growth Promotor and Suppressor of Fusarium oxysporum f. sp. lycopersici

Presently, the bioprocessing of agricultural residues to various bioactive compounds is of great concern, with the potential to be used as plant growth promoters and as a reductive of various diseases. Lycopersiconesculentum, one of the most consumed crops in the human diet, is attacked by Fusarium wilt disease, so the main aim is to biocontrol the pathogen. Several fungal species were isolated from decayed maize stover (MS). Trichodermaasperellum was chosen based on its organic acid productivity and was molecularly identified (GenBank accession number is MW195019). Citric acid (CA) was the major detected organic acid by HPLC. In vitro, CA of T.asperellum at 75% completely repressed the growth of Fusariumoxysporum f. sp. lycopersici (FOL). In vivo, soaking tomato seeds in CA enhanced the seed germination and vigor index. T. asperellum and/or its CA suppressed the wilt disease caused by FOL compared to control. There was a proportional increment of plant growth and yield, as well as improvements in the biochemical parameters (chlorophyll pigments, total phenolic contents and peroxidase, and polyphenol oxidase activities), suggesting targeting both the bioconversion of MS into CA and biological control of FOL.

β-Xylosidases: Structural Diversity, Catalytic Mechanism, and Inhibition by Monosaccharides

Xylan, a prominent component of cellulosic biomass, has a high potential for degradation into reducing sugars, and subsequent conversion into bioethanol. This process requires a range of xylanolytic enzymes. Among them, β-xylosidases are crucial, because they hydrolyze more glycosidic bonds than any of the other xylanolytic enzymes. They also enhance the efficiency of the process by degrading xylooligosaccharides, which are potent inhibitors of other hemicellulose-/xylan-converting enzymes. On the other hand, the β-xylosidase itself is also inhibited by monosaccharides that may be generated in high concentrations during the saccharification process. Structurally, β-xylosidases are diverse enzymes with different substrate specificities and enzyme mechanisms. Here, we review the structural diversity and catalytic mechanisms of β-xylosidases, and discuss their inhibition by monosaccharides.

Characterization of a novel salt-, xylose- and alkali-tolerant GH43 bifunctional β-xylosidase/α-l-arabinofuranosidase from the gut bacterial genome

A GH43 bifunctional β-xylosidase encoding gene (XylRBM26) was cloned from Massilia sp. RBM26 and successfully expressed in Escherichia coli. Recombinant XylRBM26 exhibited β-xylosidase and α-l-arabinofuranosidase activities. When 4-nitrophenyl-β-d-xylopyranoside was used as a substrate, the enzyme reached optimal activity at pH 6.5 and 50°C and remained stable at pH 5.0-10.0. Purified XylRBM26 presented good salt tolerance and retained 96.6% activity in 3.5 M NaCl and 77.9% initial activity even in 4.0 M NaCl. In addition, it exhibited high tolerance to xylose with Ki value of 500 mM. This study was the first to identify and characterize NaCl-tolerant β-xylosidase/α-l-arabinofuranosidase from the gut microbiota. The enzyme's salt, xylose, and alkali stability and resistance to various chemicals make it a potential biocatalyst for the saccharification of lignocellulose, the food industry, and industrial processes conducted in sea water.

Expression and characterisation of a pH and salt tolerant, thermostable and xylose tolerant recombinant GH43 β-xylosidase from Thermobifida halotolerans YIM 90462T for promoting hemicellulose degradation

A gene encoding a β-xylosidase (designated as Thxyl43A) was cloned from strain Thermobifida halotolerans YIM 90462^T. The open reading frame of this gene encodes 550 amino acid residues. The gene was over-expressed in Escherichia coli and the recombinant protein was purified. The monomeric Thxyl43A protein presented a molecular mass of 61.5 kDa. When p-nitrophenyl-β-d-xylopyranoside was used as the substrate, recombinant Thxyl43A exhibited optimal activity at 55 °C and pH 4.0 to 7.0, being thermostable by maintaining 47% of its activity after 30 h incubation at 55 °C. The recombinant enzyme retained more than 80% residual activity after incubation at pH range of 4.0 to 12.0 for 24 h, respectively, which indicated notable thermostability and pH stability of Thxyl43A. Moreover, Thxyl43A displayed high catalytic activity (> 60%) in presence of 5-35% NaCl (w/v) or 1-20% ionic liquid (w/v) or 1-50 mM xylose. These properties suggest that Thxyl43A has potential for promoting hemicellulose degradation and other industrial applications.

Characterization of a novel thermostable and xylose-tolerant GH 39 β-xylosidase from Dictyoglomus thermophilum

Background

β-D-xylosidase is a vital exoglycosidase with the ability to hydrolyze xylooligosaccharides to xylose and to biotransform some saponins by cleaving outer β-xylose. β-D-xylosidase is widely used as one of the xylanolytic enzymes in a diverse range of applications, such as fuel, food and the pharmaceutical industry; therefore, more and more studies have focused on the thermostable and xylose-tolerant β-D-xylosidases.

Results

A thermostable β-xylosidase gene (xln-DT) of 1509 bp was cloned from Dictyoglomus thermophilum and expressed in E.coli BL21. According to the amino acid and phylogeny analyses, the β-xylosidase Xln-DT is a novel β-xylosidase of the GH family 39. The recombinant β-xylosidase was purified, showing unique bands on SDS-PAGE, and had a protein molecular weight of 58.7 kDa. The β-xylosidase Xln-DT showed an optimal activity at pH 6.0 and 75 °C, with p-nitrophenyl-β-D-xylopyranoside (pNPX) as a substrate. Xln-DT displayed stability over a pH range of 4.0-7.5 for 24 h and displayed thermotolerance below 85 °C. The values of the kinetic parameters K_m and V_max for pNPX were 1.66 mM and 78.46 U/mg, respectively. In particular, Xln-DT displayed high tolerance to xylose, with 60% activity in the presence of 3 M xylose. Xln-DT showed significant effects on the hydrolyzation of xylobiose. After 3 h, all the xylobiose tested was degraded into xylose. Moreover, β-xylosidase Xln-DT had a high selectivity for cleaving the outer xylose moieties of natural saponins, such as notoginsenoside R1 and astragaloside IV, which produced the ginsenoside Rg1 with stronger anti-fatigue activity and produced cycloastragenol with stronger anti-aging activity, respectively.

Conclusion

This study provides a novel GH 39 β-xylosidase displaying extraordinary properties of highly catalytic activity at temperatures above 75 °C, remarkable hydrolyzing activity of xylooligosaccharides and rare saponins producing ability in the pharmaceutical and commercial industries.

Electronic supplementary material

The online version of this article (10.1186/s12896-018-0440-3) contains supplementary material, which is available to authorized users.

Characterization of a furan aldehyde-tolerant β-xylosidase/α-arabinosidase obtained through a synthetic metagenomics approach

AIMS

The aim of the study was to characterize 10 hemicellulolytic enzymes obtained from a wheat straw-degrading microbial consortium.

METHODS AND RESULTS

Based on previous metagenomics analyses, 10 glycosyl hydrolases were selected, codon-optimized, synthetized, cloned and expressed in Escherichia coli. Nine of the overexpressed recombinant proteins accumulated in cellular inclusion bodies, whereas one, a 37·5-kDa protein encoded by gene xylM1989, was found in the soluble fractions. The resulting protein, denoted XylM1989, showed β-xylosidase and α-arabinosidase activities. It fell in the GH43 family and resembled a Sphingobacterium sp. protein. The XylM1989 showed optimum activity at 20°C and pH 8·0. Interestingly, it kept approximately 80% of its β-xylosidase activity in the presence of 0·5% (w/v) furfural and 0·1% (w/v) 5-hydroxymethylfurfural. Additionally, the presence of Ca²⁺ , Mg²⁺ and Mn²⁺ ions increased the enzymatic activity and conferred complete tolerance to 500 mmol l^-1 of xylose. Protein XylM1989 is also able to release sugars from complex polysaccharides.

CONCLUSION

We report the characterization of a novel bifunctional hemicellulolytic enzyme obtained through a targeted synthetic metagenomics approach.

SIGNIFICANCE AND IMPACT OF THE STUDY

The properties of XylM1989 turn this protein into a promising enzyme that could be useful for the efficient saccharification of plant biomass.

Purification and characterization of an extracellular β-xylosidase from Pseudozyma hubeiensis NCIM 3574 (PhXyl), an unexplored yeast

This paper reports on the production of β-xylosidase from an unexplored yeast, Pseudozyma hubeinsis. The expression of this enzyme could be induced by beech wood xylan when the yeast was grown at 27 °C. The enzyme was purified to homogeneity as a glycoprotein with 23 % glycosylation. The purification protocol involved ammonium sulphate precipitation, QAE-Sephadex A50 ion exchange chromatography and sephacryl-200 column chromatography which resulted in 8.3-fold purification with 53.12 % final recovery. The purified enzyme showed prominent single band on SDS-PAGE. It is a monomeric protein of 110 kDa molecular weight confirmed by SDS-PAGE followed by MALDI-TOF mass spectrometry (112.3 kDa). The enzyme was optimally active at 60 °C and pH 4.5 and stable at pH range (4–9) and at 50 °C for 4 h. Chemical modification studies revealed that active site of the purified enzyme comprised of carboxyl, tyrosine and tryptophan residues. The carboxyl residue is involved in catalysis and tryptophan residue is solely involved in substrate binding. The best match from the search of the NCBInr database was with gi|808364558 glycoside hydrolase of Pseudozyma hubeiensis SY62 with 26 % sequence coverage confirming that it is a glycoside hydrolase/beta-glucosidase. From the search of customized SWISSPROT database, it was revealed that SWISSPROT does not contain any entries that are similar to the purified enzyme.

Novel pH-Stable Glycoside Hydrolase Family 3 β-Xylosidase from Talaromyces amestolkiae: an Enzyme Displaying Regioselective Transxylosylation

This paper reports on a novel β-xylosidase from the hemicellulolytic fungus Talaromyces amestolkiae. The expression of this enzyme, called BxTW1, could be induced by beechwood xylan and was purified as a glycoprotein from culture supernatants. We characterized the gene encoding this enzyme as an intronless gene belonging to the glycoside hydrolase gene family 3 (GH3). BxTW1 exhibited transxylosylation activity in a regioselective way. This feature would allow the synthesis of oligosaccharides or other compounds not available from natural sources, such as alkyl glycosides displaying antimicrobial or surfactant properties. Regioselective transxylosylation, an uncommon combination, makes the synthesis reproducible, which is desirable for its potential industrial application. BxTW1 showed high pH stability and Cu(2+) tolerance. The enzyme displayed a pI of 7.6, a molecular mass around 200 kDa in its active dimeric form, and Km and Vmax values of 0.17 mM and 52.0 U/mg, respectively, using commercial p-nitrophenyl-β-d-xylopyranoside as the substrate. The catalytic efficiencies for the hydrolysis of xylooligosaccharides were remarkably high, making it suitable for different applications in food and bioenergy industries.

Production and characterization of β-glucosidase obtained by the solid-state cultivation of the thermophilic fungus Thermomucor indicae-seudaticae N31

In this paper, several agro-industrial wastes (soybean meal and wheat straw, rice and peanut husks, corn cob and corn stover, and sugarcane bagasse) were tested for the production of β-glucosidase by the cultivation of thermophilic fungus Thermomucor indicae-seudaticae N31 in solid-state fermentation (SSF). Among the tested substrates, the highest yields were obtained in soybean meal. Other fermentation parameters were also evaluated, such as initial pH, merge substrates, and fermentation time, as well as the physicochemical characterization of the enzyme. The best results were obtained after 192 h of fermentation with the initial pH adjusted to 6.0. The substrate mixture did not improve the enzyme production by microorganism. The β-glucosidase showed best catalytic activity at pH 4.5 and at 75 °C and remained stable in the pH range from 4.5 to 9.5 and the temperature range 40-75 °C. The enzyme showed 80 % of its activity at a concentration of 15 mM glucose and remained stable up to 20 % ethanol.

Highly thermostable GH39 β-xylosidase from a Geobacillus sp. strain WSUCF1

Background

Complete enzymatic hydrolysis of xylan to xylose requires the action of endoxylanase and β-xylosidase. β-xylosidases play an important part in hydrolyzing xylo-oligosaccharides to xylose. Thermostable β-xylosidases have been a focus of attention as industrially important enzymes due to their long shelf life and role in the relief of end-product inhibition of xylanases caused by xylo-oligosaccharides. Therefore, a highly thermostable β-xylosidase with high specific activity has significant potential in lignocellulose bioconversion.

Results

A gene encoding a highly thermostable GH39 β-xylosidase was cloned from Geobacillus sp. strain WSUCF1 and expressed in Escherichia coli. Recombinant β-xylosidase was active over a wide range of temperatures and pH with optimum temperature of 70°C and pH 6.5. It exhibited very high thermostability, retaining 50% activity at 70°C after 9 days. WSUCF1 β-xylosidase is more thermostable than β-xylosidases reported from other thermophiles (growth temperature ≤ 70°C). Specific activity was 133 U/mg when incubated with p-nitrophenyl xylopyranoside, with K_m and V_max values of 2.38 mM and 147 U/mg, respectively. SDS-PAGE analysis indicated that the recombinant enzyme had a mass of 58 kDa, but omitting heating prior to electrophoresis increased the apparent mass to 230 kDa, suggesting the enzyme exists as a tetramer. Enzyme exhibited high tolerance to xylose, retained approximately 70% of relative activity at 210 mM xylose concentration. Thin layer chromatography showed that the enzyme had potential to convert xylo-oligomers (xylobiose, triose, tetraose, and pentaose) into fermentable xylose. WSUCF1 β-xylosidase along with WSUCF1 endo-xylanase synergistically converted the xylan into fermentable xylose with more than 90% conversion.

Conclusions

Properties of the WSUCF1 β-xylosidase i.e. high tolerance to elevated temperatures, high specific activity, conversion of xylo-oligomers to xylose, and resistance to inhibition from xylose, make this enzyme potentially suitable for various biotechnological applications.

Characterisation of a recombinant β-xylosidase (xylA) from Aspergillus oryzae expressed in Pichia pastoris

β-xylosidases catalyse the hydrolysis of short chain xylooligosaccharides from their non-reducing ends into xylose. In this study we report the heterologous expression of Aspergillus oryzae β-xylosidase (XylA) in Pichia pastoris under the control of the glyceraldehyde-3-phosphate dehydrogenase promoter. The recombinant enzyme was optimally active at 55°C and pH 4.5 with K_m and V_max values of 1.0 mM and 250 μmol min⁻¹ mg⁻¹ respectively against 4-nitrophenyl β-xylopyranoside. Xylose was a competitive inhibitor with a K_i of 2.72 mM, whereas fructose was an uncompetitive inhibitor reducing substrate binding affinity (K_m) and conversion efficiency (V_max). The enzyme was characterised to be an exo-cutting enzyme releasing xylose from the non-reducing ends of β-1,4 linked xylooligosaccharides (X₂, X₃ and X₄). Catalytic conversion of X₂, X₃ and X₄ decreased (V_max and k_cat) with increasing chain length.

High-level production of thermotolerant β-xylosidase of Aspergillus sp. BCC125 in Pichia pastoris: characterization and its application in ethanol production

A gene coding for thermotolerant β-xylosidase from Aspergillus sp. BCC125 was characterized. The recombinant enzyme was expressed in methylotrophic yeast Pichia pastoris KM71 and especially high yield of secreted enzyme was obtained. β-xylosidase possessed high enzyme efficiency (Kcat/Km=198.8mM(-1)s(-1)) toward pNP-β-D-xylopyranoside (pNPβX) with optimal temperature and pH for activity of 60°C and pH 4.0-5.0, respectively. The identified β-xylosidase showed clear synergism with previously identified xylanase for hydrolysis of xylan in vitro as well as simultaneous saccharification and fermentation process (SSF) in vivo with Pichia stipitis.

Biochemical properties of a novel thermostable and highly xylose-tolerant β-xylosidase/α-arabinosidase from Thermotoga thermarum

BACKGROUND

β-Xylosidase is an important constituent of the hemicellulase system and it plays an important role in hydrolyzing xylooligosaccharides to xylose. Xylose, a useful monose, has been utilized in a wide range of applications such as food, light, chemical as well as energy industry. Therefore, the xylose-tolerant β-xylosidase with high specific activity for bioconversion of xylooligosaccharides has a great potential in the fields as above.

RESULTS

A β-xylosidase gene (Tth xynB3) of 2,322 bp was cloned from the extremely thermophilic bacterium Thermotoga thermarum DSM 5069 that encodes a protein containing 774 amino acid residues, and was expressed in Escherichia coli BL21 (DE3). The phylogenetic trees of β-xylosidases were constructed using Neighbor-Joining (NJ) and Maximum-Parsimony (MP) methods. The phylogeny and amino acid analysis indicated that the Tth xynB3 β-xylosidase was a novel β-xylosidase of GH3. The optimal activity of the Tth xynB3 β-xylosidase was obtained at pH 6.0 and 95°C and was stable over a pH range of 5.0-7.5 and exhibited 2 h half-life at 85°C. The kinetic parameters Km and Vmax values for p-nitrophenyl-β-D-xylopyranoside and p-nitrophenyl-α-L-arabinofuranoside were 0.27 mM and 223.3 U/mg, 0.21 mM and 75 U/mg, respectively. The kcat/Km values for p-nitrophenyl-β-D-xylopyranoside and p-nitrophenyl-α-L-arabinofuranoside were 1,173.4 mM-1 s-1 and 505.9 mM-1 s-1, respectively. It displayed high tolerance to xylose, with Ki value approximately 1000 mM. It was stimulated by xylose at higher concentration up to 500 mM, above which the enzyme activity of Tth xynB3 β-xylosidase was gradually decreased. However, it still remained approximately 50% of its original activity even if the concentration of xylose was as high as 1000 mM. It was also discovered that the Tth xynB3 β-xylosidase exhibited a high hydrolytic activity on xylooligosaccharides. When 5% substrate was incubated with 0.3 U Tth xynB3 β-xylosidase in 200 μL reaction system for 3 h, almost all the substrate was biodegraded into xylose.

CONCLUSIONS

The article provides a useful and novel β-xylosidase displaying extraordinary and desirable properties: high xylose tolerance and catalytic activity at temperatures above 75°C, thermally stable and excellent hydrolytic activity on xylooligosaccharides.

Optimization of β-glucosidase, β-xylosidase and xylanase production by Colletotrichum graminicola under solid-state fermentation and application in raw sugarcane trash saccharification

Efficient, low-cost enzymatic hydrolysis of lignocellulosic residues is essential for cost-effective production of bioethanol. The production of β-glucosidase, β-xylosidase and xylanase by Colletotrichum graminicola was optimized using Response Surface Methodology (RSM). Maximal production occurred in wheat bran. Sugarcane trash, peanut hulls and corncob enhanced β-glucosidase, β-xylosidase and xylanase production, respectively. Maximal levels after optimization reached 159.3 ± 12.7 U g⁻¹, 128.1 ± 6.4 U g⁻¹ and 378.1 ± 23.3 U g⁻¹, respectively, but the enzymes were produced simultaneously at good levels under culture conditions optimized for each one of them. Optima of pH and temperature were 5.0 and 65 °C for the three enzymes, which maintained full activity for 72 h at 50 °C and for 120 min at 60 °C (β-glucosidase) or 65 °C (β-xylosidase and xylanase). Mixed with Trichoderma reesei cellulases, C. graminicola crude extract hydrolyzed raw sugarcane trash with glucose yield of 33.1% after 48 h, demonstrating good potential to compose efficient cocktails for lignocellulosic materials hydrolysis.

A novel xylan degrading β-D-xylosidase: purification and biochemical characterization

Aspergillus ochraceus, a thermotolerant fungus isolated in Brazil from decomposing materials, produced an extracellular β-xylosidase that was purified using DEAE-cellulose ion exchange chromatography, Sephadex G-100 and Biogel P-60 gel filtration. β-xylosidase is a glycoprotein (39 % carbohydrate content) and has a molecular mass of 137 kDa by SDS-PAGE, with optimal temperature and pH at 70 °C and 3.0-5.5, respectively. β-xylosidase was stable in acidic pH (3.0-6.0) and 70 °C for 1 h. The enzyme was activated by 5 mM MnCl₂ (28 %) and MgCl₂ (20 %) salts. The β-xylosidase produced by A. ochraceus preferentially hydrolyzed p-nitrophenyl-β-D-xylopyranoside, exhibiting apparent K(m) and V(max) values of 0.66 mM and 39 U (mg protein)⁻¹ respectively, and to a lesser extent p-nitrophenyl-β-D-glucopyranoside. The enzyme was able to hydrolyze xylan from different sources, suggesting a novel β-D-xylosidase that degrades xylan. HPLC analysis revealed xylans of different compositions which allowed explaining the differences in specificity observed by β-xylosidase. TLC confirmed the capacity of the enzyme in hydrolyzing xylan and larger xylo-oligosaccharides, as xylopentaose.

Plant cell walls to ethanol

Conversion of plant cell walls to ethanol constitutes second generation bioethanol production. The process consists of several steps: biomass selection/genetic modification, physiochemical pretreatment, enzymatic saccharification, fermentation and separation. Ultimately, it is desirable to combine as many of the biochemical steps as possible in a single organism to achieve CBP (consolidated bioprocessing). A commercially ready CBP organism is currently unreported. Production of second generation bioethanol is hindered by economics, particularly in the cost of pretreatment (including waste management and solvent recovery), the cost of saccharification enzymes (particularly exocellulases and endocellulases displaying kcat ~1 s−1 on crystalline cellulose), and the inefficiency of co-fermentation of 5- and 6-carbon monosaccharides (owing in part to redox cofactor imbalances in Saccharomyces cerevisiae).

High-level expression of extracellular secretion of a β-xylosidase gene from Paecilomyces thermophila in Escherichia coli

A novel β-xylosidase gene (designated as PtXyl43) from thermophilic fungus Paecilomycesthermophila was cloned and extracellularly expressed in Escherichia coli. PtXyl43 belonging to glycoside hydrolase (GH) family 43 has an open reading frame of 1017 bp, encoding 338 amino acids without a predicted signal peptide. No introns were found by comparison of the PtXyl43 genomic DNA and cDNA sequences. The recombinant β-xylosidase (PtXyl43) was secreted into the culture medium in E. coli with a yield of 98.0 U mL(-1) in shake-flask cultures. PtXyl43 was purified 1.2-fold to homogeneity with a recovery yield of 61.5% from the cell-free culture supernatant. It appeared as a single protein band on SDS-PAGE with a molecular mass of approx 52.3 kDa. The enzyme exhibited an optimal activity at 55 °C and pH 7.0, respectively. This is the first report on the cloning and expression of a GH family 43 β-xylosidase gene from thermophilic fungi.

Characterization of a novel beta-xylosidase, XylC, from Thermoanaerobacterium saccharolyticum JW/SL-YS485

The 1,914-bp open reading frame of xylC from Thermoanaerobacterium saccharolyticum JW/SL-YS485 encodes a calculated 73-kDa β-xylosidase, XylC, different from any glycosyl hydrolase in the database and representing a novel glycohydrolase family. Hydrolysis occurred under retention of the anomeric configuration, and transglycosylation occurred in the presence of alcohols as acceptors. With the use of vector pHsh, expression of XylC, the third β-xylosidase in this bacterium, increased approximately 4-fold when a loop within the translational initiation region in the mRNA was removed by site-directed mutagenesis. The increased expression of xylC(m) is due to removal of a stem-loop structure without a change of the amino acid sequence of the heterologously expressed enzyme (XylC(rec)). When gel filtration was applied, purified XylC had molecular masses of 210 kDa and 265 kDa using native gradient gel electrophoresis. The protein consisted of 78-kDa subunits based on SDS gel electrophoresis and contained 6% carbohydrates. XylC and XylC(rec) exhibited maximum activity at 65°C and pH(65°C) 6.0, a 1-h half-life at 67°C, a K(m) for p-nitrophenyl-β-D-xyloside of 28 mM, and a V(max) of 276 U/mg and retained 70% activity in the presence of 200 mM xylose, suggesting potential for industrial applications.

Thermostable enzymes as biocatalysts in the biofuel industry

Lignocellulose is the most abundant carbohydrate source in nature and represents an ideal renewable energy source. Thermostable enzymes that hydrolyze lignocellulose to its component sugars have significant advantages for improving the conversion rate of biomass over their mesophilic counterparts. We review here the recent literature on the development and use of thermostable enzymes for the depolymerization of lignocellulosic feedstocks for biofuel production. Furthermore, we discuss the protein structure, mechanisms of thermostability, and specific strategies that can be used to improve the thermal stability of lignocellulosic biocatalysts.

A xylose-tolerant beta-xylosidase from Paecilomyces thermophila: characterization and its co-action with the endogenous xylanase

An extracellular beta-xylosidase from the thermophilic fungus Paecilomyces thermophila J18 was purified 31.9-fold to homogeneity with a recovery yield of 2.27% from the cell-free culture supernatant. It appeared as a single protein band on SDS-PAGE with a molecular mass of approx 53.5 kDa. The molecular mass of beta-xylosidase was 51.8 kDa determined by Superdex 75 gel filtration. The enzyme was a glycoprotein with a carbohydrate content of 61.5%. It exhibited an optimal activity at 55 degrees C and pH 6.5, respectively. The enzyme was stable in the range of pH 6.0-9.0 and at 55 degrees C. The purified enzyme hydrolyzed xylobiose and higher xylooligosaccharides but was inactive against xylan substrates. It released xylose from xylooligosaccharides with a degree of polymerization ranging between 2 and 5. The rate of xylose released from xylooligosaccharides by the purified enzyme increased with increasing chain length. It had a K(m) of 4.3mM for p-nitrophenol-beta-d-xylopyranoside and was competitively inhibited by xylose with a K(i) value of 139 mM. Release of reducing sugars from xylans by a purified xylanase produced by the same organism increased markedly in the presence of beta-xylosidase. During 24-hour hydrolysis, the amounts of reducing sugar released in the presence of added beta-xylosidase were about 1.5-1.73 times that of the reaction employing the xylanase alone. This is the first report on the purification and characterization of a beta-xylosidase from Paecilomyces thermophila.

Xylanases from fungi: properties and industrial applications

Xylan is the principal type of hemicellulose. It is a linear polymer of beta-D-xylopyranosyl units linked by (1-4) glycosidic bonds. In nature, the polysaccharide backbone may be added to 4-O-methyl-alpha-D-glucuronopyranosyl units, acetyl groups, alpha-L-arabinofuranosyl, etc., in variable proportions. An enzymatic complex is responsible for the hydrolysis of xylan, but the main enzymes involved are endo-1,4-beta-xylanase and beta-xylosidase. These enzymes are produced by fungi, bacteria, yeast, marine algae, protozoans, snails, crustaceans, insect, seeds, etc., but the principal commercial source is filamentous fungi. Recently, there has been much industrial interest in xylan and its hydrolytic enzymatic complex, as a supplement in animal feed, for the manufacture of bread, food and drinks, textiles, bleaching of cellulose pulp, ethanol and xylitol production. This review describes some properties of xylan and its metabolism, as well as the biochemical properties of xylanases and their commercial applications.