A novel neutral xylanase with high SDS resistance from Volvariella volvacea : characterization and its synergistic hydrolysis of wheat bran with acetyl xylan esterase

Recombinant acetylxylan esterase of Halalkalibacterium halodurans NAH-Egypt: molecular and biochemical study

Acetylxylan esterase plays a crucial role in xylan hydrolysis as the acetyl side-groups restrict endoxylanase action by stearic hindrance. In this study, an acetylxylan esterase (AXE-HAS10: 960 bp & 319 a.a) putative ORF from Halalkalibacterium halodurans NAH-Egypt was extensively studied through heterologous overexpression in Escherichia coli, biochemical characterization, and structural modeling. The AXE-HAS10 tertiary structure was predicted by the Local Meta Threading Server. AXE-HAS10 belongs to the carbohydrate esterase Family 7. Purified to homogeneity AXE-HAS10 showed specific activity (36.99 U/mg), fold purification (11.42), and molecular mass (41.39 kDa). AXE-HAS10 showed optimal pH (8.5) and temperature (40 oC). After 15 h of incubation at pH 7.0–9.0, AXE-HAS10 maintained 100% activity. After 120 min at 35 and 40 oC, the retained activity was 80 and 50%, respectively. At 10 mM Mn2+, Fe3+, K+, and Ca2+ after 30 min, retained activity was 329 ± 15, 212 ± 5.2, 123 ± 1.4, and 120 ± 3.0%, respectively. After 30 min of preincubation with triton x-100, SDS, and CTAB at 0.1% (v/v), the retained activity was 150 ± 19, 88 ± 4, and 82 ± 7%, respectively. At 6.0 M NaCl after 30 min, retained activity was 58%. A 1.44-fold enhancement of beechwood xylan hydrolysis was achieved by AXE-HAS10 and Penicillium chrysogenum DSM105774 β-xylanase concurrently. Present data underpins AXE-HAS10 as a promising AXE for industrial exploitation.

Production of xylooligosaccharides, bioethanol, and lignin from structural components of barley straw pretreated with a steam explosion

Barley straw (BS) is a potential source to obtain bioethanol and value-added products such as xylooligosaccharides (XOS) and lignin for application in diverse industries. In this study, BS was submitted to steam explosion pretreatment to valorize the main components of this lignocellulose biomass. For hemicellulose fraction valorization, different combinations of endo-β-(1,4)-D-xylanase enzyme with accessory enzymes (α-L-arabinofuranosidase, feruloy -esterase and acetylxylan-esterase) have been studied to produce XOS with a low degree of polymerization. The application of accessory enzymes combined with endo-β-(1,4)-D-xylanase enzymes turned out to be the most effective strategy for the formation of XOS. The solid fraction obtained after the pretreatment was submitted to presacharification and simultaneous saccharification and fermentation process for bioethanol production. The resulting lignin-rich residue was characterized. In this integrated process, 13.0 g XOS (DP2-DP6), 12.6 g ethanol and 16.6 g lignin were obtained from 100 g of BS, achieving the goal of valorizing this agricultural residue.

Manganese promoted wheat straw decomposition by regulating microbial communities and enzyme activities

AIMS

This study investigated the dose-effect of manganese (Mn) addition on wheat straw (WS) decomposition, and explored the potential mechanisms of Mn involved in the acceleration of WS decomposition in regards to the soil microbial communities and enzyme activities.

METHODS AND RESULTS

A 180-day incubation experiment was performed to examine the decomposition of WS under four Mn levels, that is, 0, 0.25, 1 and 2 mg g^-1 . The effects of microbial communities and enzyme activities were evaluated using control (0 mg g^-1 ) and Mn (0.25 mg g^-1 ) treatments. Our results revealed that Mn (0.25 mg g^-1 ) addition significantly increased WS decomposition, and enhanced the release of carbon and nitrogen. Optimal Mn addition (0.25 mg g^-1 ) also caused significant increases in the activity of neutral xylanase (NEX), laccase (Lac), manganese peroxidase (MnP) and lignin peroxidase (LiP) within the incubation period. Mn (0.25 mg g^-1 ) addition also enriched some operational taxonomic units (OTUs) that, in turn, had the potential ability to decompose crop straw, such as secreting lignocellulolytic enzymes.

CONCLUSIONS

Mn (0.25 mg g^-1 ) could promote WS decomposition through enrichment of the microbial species involved in biomass decomposition, which enhanced the lignocellulose-degrading enzyme activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides evidence for Mn to promote WS biodegradation after Mn application, opening new windows to improve the utilization efficiency of crop residues.

Alkaline organosolv pretreatment of different sorghum stem parts for enhancing the total reducing sugar yields and p-coumaric acid release

BACKGROUND

The sorghum stem can be divided into the pith and rind parts with obvious differences in cell type and chemical composition, thus arising the different recalcitrance to enzyme hydrolysis and demand for different pretreatment conditions. The introduction of organic solvents in the pretreatment can reduce over-degradation of cellulose and hemicellulose, but significance of organic solvent addition in pretreatment of different parts of sorghum stem is still unclear. Valorization of each component is critical for economy of sorghum biorefinery. Therefore, in this study, NaOH-ethanol pretreatment condition for different parts of the sorghum stem was optimized to maximize p-coumaric acid release and total reducing sugar recovery.

RESULT

Ethanol addition improved p-coumaric acid release and delignification efficiency, but significantly reduced hemicellulose deconstruction in NaOH-ethanol pretreatment. Optimization using the response surface methodology revealed that the pith, rind and whole stem require different NaOH-ethanol pretreatment conditions for maximal p-coumaric acid release and xylan preservation. By respective optimal NaOH-ethanol pretreatment, the p-coumaric acid release yields reached 94.07%, 97.24% and 95.05% from pith, rind and whole stem, which increased by 8.16%, 8.38% and 8.39% compared to those of NaOH-pretreated samples. The xylan recoveries of pith, rind and whole stem reached 76.80%, 88.46% and 85.01%, respectively, which increased by 47.75%, 15.11% and 35.97% compared to NaOH pretreatment. Adding xylanase significantly enhanced the enzymatic saccharification of pretreated residues. The total reducing sugar yields after respective optimal NaOH-ethanol pretreatment and enzymatic hydrolysis reached 84.06%, 82.29% and 84.09% for pith, rind and whole stem, respectively, which increased by 29.56%, 23.67% and 25.56% compared to those of NaOH-pretreated samples. Considering the separation cost of the different stem parts, whole sorghum stem can be directly used as feedstock in industrial biorefinery.

CONCLUSION

These results indicated that NaOH-ethanol is effective for the efficient fractionation and pretreatment of sorghum biomass. This work will help to understand the differences of different parts of sorghum stem under NaOH-ethanol pretreatment, thereby improving the full-component utilization of sorghum stem.

Functional Expression and Characterization of Acetyl Xylan Esterases CE Family 7 from Lactobacillus antri and Bacillus halodurans

Acetyl xylan esterase (AXE; E.C. 3.1.1.72) is one of the accessory enzymes for xylan degradation, which can remove the terminal acetate residues from xylan polymers. In this study, two genes encoding putative AXEs (LaAXE and BhAXE) were cloned from Lactobacillus antri DSM 16041 and Bacillus halodurans C-125, and constitutively expressed in Escherichia coli. They possess considerable activities towards various substrates such as p-nitrophenyl acetate, 4-methylumbelliferyl acetate, glucose pentaacetate, and 7-amino cephalosporanic acid. LaAXE and BhAXE showed the highest activities at pH 7.0 and 8.0 at 50°C, respectively. These enzymes are AXE members of carbohydrate esterase (CE) family 7 with the cephalosporine-C deacetylase activity for the production of antibiotics precursors. The simultaneous treatment of LaAXE with Thermotoga neapolitana β-xylanase showed 1.44-fold higher synergistic degradation of beechwood xylan than the single treatment of xylanase, whereas BhAXE showed no significant synergism. It was suggested that LaAXE can deacetylate beechwood xylan and enhance the successive accessibility of xylanase towards the resulting substrates. The novel LaAXE originated from a lactic acid bacterium will be utilized for the enzymatic production of D-xylose and xylooligosaccharides.

A mini review of xylanolytic enzymes with regards to their synergistic interactions during hetero-xylan degradation

This review examines the recent models describing the mode of action of various xylanolytic enzymes and how these enzymes can be applied (sequentially or simultaneously) with their distinctive roles in mind to achieve efficient xylan degradation. With respect to homeosynergy, synergism appears to be as a result of β-xylanase and/or oligosaccharide reducing-end β-xylanase liberating xylo-oligomers (XOS) that are preferred substrates of the processive β-xylosidase. With regards to hetero-synergism, two cross relationships appear to exist and seem to be the reason for synergism between the enzymes during xylan degradation. These cross relations are the debranching enzymes such as α-glucuronidase or side-chain cleaving enzymes such as carbohydrate esterases (CE) removing decorations that would have hindered back-bone-cleaving enzymes, while backbone-cleaving-enzymes liberate XOS that are preferred substrates of the debranching and side-chain-cleaving enzymes. This interaction is demonstrated by high yields in co-production of xylan substituents such as arabinose, glucuronic acid and ferulic acid, and XOS. Finally, lytic polysaccharide monooxygenases (LPMO) have also been implicated in boosting whole lignocellulosic biomass or insoluble xylan degradation by glycoside hydrolases (GH) by possibly disrupting entangled xylan residues. Since it has been observed that the same enzyme (same Enzyme Commission, EC, classification) from different GH or CE and/or AA families can display different synergistic interactions with other enzymes due to different substrate specificities and properties, in this review, we propose an approach of enzyme selection (and mode of application thereof) during xylan degradation, as this can improve the economic viability of the degradation of xylan for producing precursors of value added products.

Action of different types of endoxylanases on eucalyptus xylan in situ

Most studies of the mode of action of industrially important endoxylanases have been done on alkali extracted-plant xylan. In just few cases, the native form of the polysaccharide, acetylated xylan, was used as a substrate. In this work action of xylanases belonging to three glycoside hydrolase families, GH10, GH11, and GH30 was investigated on acetylglucuronoxylan directly in hardwood cell walls. Powdered eucalyptus wood was used as xylanase substrate. Enzyme-generated fragments were characterized by TLC, MALDI ToF MS, and NMR spectroscopy. All three xylanases generated from eucalyptus wood powder acetylated xylooligosaccharides. Those released by GH10 enzyme were the shortest, and those released by GH30 xylanase were of the largest diversity. For GH30 xylanase the 4-O-methyl-D-glucuronic acid (MeGlcA) side residues function as substrate specificity determinants regardless the acetylation of the neighboring hydroxyl group. Much simpler xylooligosaccharide patterns were observed when xylanases were applied in combination with carbohydrate esterase family 6 acetylxylan esterase. In the presence of the esterase, all aldouronic acids remained 3-O-acetylated on the xylopyranosyl (Xylp) residue substituted with MeGlcA. The 3-O-acetyl group, in contrast to the acetyl groups of otherwise unsubstituted Xylp residues, does not affect the mode of action of endoxylanases, but contributes to recalcitrance of the acidic xylan fragments. The results confirm importance of acetylxylan esterases in microbial degradation of acetylated hardwood glucuronoxylan. They also point to still unresolved question of efficient enzymatic removal of the 3-O-acetyl group on MeGlcA-substituted Xylp residues negatively affecting the saccharification yields.

Characterization of a novel SGNH-type esterase from Lactobacillus plantarum

Lactic acid bacteria (LAB) are sources of a large variety of microbial ester hydrolases because they can produce a wide range of short-chain esters, phenolic alcohols, and fatty acids. Here, a novel SGNH-type esterase (LpSGNH1) from Lactobacillus plantarum WCFS1 was identified, functionally characterized, and immobilized for biotechnological applications. Homologs of LpSGNH1 are also found in many lactic acid bacteria (LAB) species. Biochemical features of LpSGNH1 were investigated using mass spectrometry, gel filtration chromatography, enzyme kinetics, fluorescence, and circular dichroism (CD) spectroscopy. LpSGNH1 were retained its activity under conditions that would be encountered during fermentations. Interestingly, LpSGNH1 exhibited the ability to act on a broad range of substrates including ketoprofen acetate, cefotaxime (CTX), and 7-aminocephalosporanic acid (7-ACA) as well as glucose pentaacetate, acetylxylan, and acetylalginate, which make LpSGNH1 a great candidate for extensive industrial applications. Furthermore, cross-linked enzyme aggregates of LpSGNH1 (CLEA-LpSGNH1) displayed recycling ability and thermal stability compared to free LpSGNH1, which could be useful for industrial applications. This work highlights the importance of LpSGNH1 in the preparation of commercial compounds, and LpSGNH1 can be used as a model system of SGNH esterases in lactic acid bacteria.

Replacement of carbohydrate binding modules improves acetyl xylan esterase activity and its synergistic hydrolysis of different substrates with xylanase

BACKGROUND

Acetylation of the xylan backbone was a major obstacle to enzymatic decomposition. Removal of acetyl groups by acetyl xylan esterases (AXEs) is essential for completely enzymatic hydrolysis of xylan. Appended carbohydrate binding modules (CBMs) can promote the enzymatic deconstruction of plant cell walls by targeting and proximity effects. Fungal acetyl xylan esterases are strictly appended to cellulose-specific CBM1. It is still unclear whether xylan-specific CBMs have a greater advantage than CBM1 in potentiating the activity of fungal deacetylating enzymes and its synergistic hydrolysis of different substrates with xylanase.

RESULTS

Three recombinant AXE1s fused with different xylan-specific CBMs, together with wild-type AXE1 with CBM1 and CBM1-deleted mutant AXE1dC, were constructed in this study. The optimal temperature and pH of recombinant AXE1s was 50 °C and 8.0 (except AXE1dC-CBM6), respectively. Cellulose-specific CBM1 in AXE1 obviously contributed to its catalytic action against substrates compared with AXE1dC. However, replacement of CBM1 with xylan-specific CBM4-2 significantly enhanced AXE1 thermostability and catalytic activity against soluble substrate 4-methylumbelliferyl acetate. Whereas replacements with xylan-specific CBM6 and CBM22-2 were more effective in enzymatic release of acetic acid from destarched wheat bran, NaClO₂-treated wheat straw, and water-insoluble wheat arabinoxylan compared to AXE1. Moreover, replacement with CBM6 and CBM22-2 also resulted in higher degree releases of reducing sugar and acetic acid from different substrates when simultaneous hydrolysis with xylanase. A good linear relationship exists between the acetic acid and reducing sugar release.

CONCLUSIONS

Our findings suggested that the replacement with CBM6 and CBM22-2 not only significantly improved the catalysis efficiency of AXE1, but also increased its synergistic hydrolysis of different substrates with xylanase, indicating the significance of targeting effect in AXE1 catalysis mediated by xylan-specific CBMs.

Towards enzymatic breakdown of complex plant xylan structures: State of the art

Significant progress over the past few years has been achieved in the enzymology of microbial degradation and saccharification of plant xylan, after cellulose being the most abundant natural renewable polysaccharide. Several new types of xylan depolymerizing and debranching enzymes have been described in microorganisms. Despite the increasing variety of known glycoside hydrolases and carbohydrate esterases, some xylan structures still appear quite recalcitrant. This review focuses on the mode of action of different types of depolymerizing endoxylanases and their cooperation with β-xylosidase and accessory enzymes in breakdown of complex highly branched xylan structures. Emphasis is placed on the enzymatic hydrolysis of alkali-extracted deesterified polysaccharide as well as acetylated xylan isolated from plant cell walls under non-alkaline conditions. It is also shown how the combination of selected endoxylanases and debranching enzymes can determine the nature of prebiotic xylooligosaccharides or lead to complete hydrolysis of the polysaccharide. The article also highlights the possibility for discovery of novel xylanolytic enzymes, construction of multifunctional chimeric enzymes and xylanosomes in parallel with increasing knowledge on the fine structure of the polysaccharide.

Combination of Xylanase and Debranching Enzymes Specific to Wheat Arabinoxylan Improve the Growth Performance and Gut Health of Broilers

Arabinoxylan (AX) is the major antinutritional factor of wheat. This study evaluated the synergistic effects of xylanase and debranching enzymes (arabinofuranosidase [ABF] and feruloyl esterase [FAE]) on AX. During in vitro tests, the addition of ABF or FAE accelerated the hydrolysis of water-soluble AX (WE-AX) and water-insoluble AX (WU-AX) and produced more xylan oligosaccharides (XOS) than xylanase alone. XOS obtained from WE-AX stimulated greater proliferation of Lactobacillus brevis and Bacillus subtilis than did fructo-oligosaccharides (FOS) and glucose. During in vivo trials, xylanase increased the average daily growth (ADG), decreased the feed-conversion ratio (FCR), and reduced the digesta viscosity of jejunum and intestinal lesions of broilers fed a wheat-based diet on day 36. ABF or FAE additions further improved these effects. Broilers fed a combination of xylanase, ABF, and FAE exhibited the best growth. In conclusion, the synergistic effects among xylanase, ABF, and FAE increased AX degradation, which improve the growth performance and gut health of broilers.

Hyperthermostable Thermotoga maritima xylanase XYN10B shows high activity at high temperatures in the presence of biomass-dissolving hydrophilic ionic liquids

The gene of Thermotoga maritima GH10 xylanase (TmXYN10B) was synthesised to study the extreme limits of this hyperthermostable enzyme at high temperatures in the presence of biomass-dissolving hydrophilic ionic liquids (ILs). TmXYN10B expressed from Pichia pastoris showed maximal activity at 100 °C and retained 92 % of maximal activity at 105 °C in a 30-min assay. Although the temperature optimum of activity was lowered by 1-ethyl-3-methylimidazolium acetate ([EMIM]OAc), TmXYN10B retained partial activity in 15-35 % hydrophilic ILs, even at 75-90 °C. TmXYN10B retained over 80 % of its activity at 90 °C in 15 % [EMIM]OAc and 15-25 % 1-ethyl-3-methylimidazolium dimethylphosphate ([EMIM]DMP) during 22-h reactions. [EMIM]OAc may rigidify the enzyme and lower V max. However, only minor changes in kinetic parameter K m showed that competitive inhibition by [EMIM]OAc of TmXYN10B is minimal. In conclusion, when extended enzymatic reactions under extreme conditions are required, TmXYN10B shows extraordinary potential.

Novel thermophilic hemicellulases for the conversion of lignocellulose for second generation biorefineries

The biotransformation of lignocellulose biomasses into fermentable sugars is a very complex procedure including, as one of the most critical steps, the (hemi) cellulose hydrolysis by specific enzymatic cocktails. We explored here, the potential of stable glycoside hydrolases from thermophilic organisms, so far not used in commercial enzymatic preparations, for the conversion of glucuronoxylan, the major hemicellulose of several energy crops. Searches in the genomes of thermophilic bacteria led to the identification, efficient production, and detailed characterization of novel xylanase and α-glucuronidase from Alicyclobacillus acidocaldarius (GH10-XA and GH67-GA, respectively) and a α-glucuronidase from Caldicellulosiruptor saccharolyticus (GH67-GC). Remarkably, GH10-XA, if compared to other thermophilic xylanases from this family, coupled good specificity on beechwood xylan and the best stability at 65 °C (3.5 days). In addition, GH67-GC was the most stable α-glucuronidases from this family and the first able to hydrolyse both aldouronic acid and aryl-α-glucuronic acid substrates. These enzymes, led to the very efficient hydrolysis of beechwood xylan by using 7- to 9-fold less protein (concentrations <0.3 μM) and in much less reaction time (2h vs 12h) if compared to other known biotransformations catalyzed by thermophilic enzymes. In addition, remarkably, together with a thermophilic β-xylosidase, they catalyzed the production of xylose from the smart cooking pre-treated biomass of one of the most promising energy crops for second generation biorefineries. We demonstrated that search by the CAZy Data Bank of currently available genomes and detailed enzymatic characterization of recombinant enzymes allow the identification of glycoside hydrolases with novel and interesting properties and applications.

Insight into glycoside hydrolases for debranched xylan degradation from extremely thermophilic bacterium Caldicellulosiruptor lactoaceticus

Caldicellulosiruptor lactoaceticus 6A, an anaerobic and extremely thermophilic bacterium, uses natural xylan as carbon source. The encoded genes of C. lactoaceticus 6A for glycoside hydrolase (GH) provide a platform for xylan degradation. The GH family 10 xylanase (Xyn10A) and GH67 α-glucuronidase (Agu67A) from C. lactoaceticus 6A were heterologously expressed, purified and characterized. Both Xyn10A and Agu67A are predicted as intracellular enzymes as no signal peptides identified. Xyn10A and Agu67A had molecular weight of 47.0 kDa and 80.0 kDa respectively as determined by SDS-PAGE, while both appeared as homodimer when analyzed by gel filtration. Xyn10A displayed the highest activity at 80 °C and pH 6.5, as 75 °C and pH 6.5 for Agu67A. Xyn10A had good stability at 75 °C, 80 °C, and pH 4.5-8.5, respectively, and was sensitive to various metal ions and reagents. Xyn10A possessed hydrolytic activity towards xylo-oligosaccharides (XOs) and beechwood xylan. At optimum conditions, the specific activity of Xyn10A was 44.6 IU/mg with beechwood xylan as substrate, and liberated branched XOs, xylobiose, and xylose. Agu67A was active on branched XOs with methyl-glucuronic acids (MeGlcA) sub-chains, and primarily generated XOs equivalents and MeGlcA. The specific activity of Agu67A was 1.3 IU/mg with aldobiouronic acid as substrate. The synergistic action of Xyn10A and Agu67A was observed with MeGlcA branched XOs and xylan as substrates, both backbone and branched chain of substrates were degraded, and liberated xylose, xylobiose, and MeGlcA. The synergism of Xyn10A and Agu67A provided not only a thermophilic method for natural xylan degradation, but also insight into the mechanisms for xylan utilization of C. lactoaceticus.

Biochemical characterization of two thermostable xylanolytic enzymes encoded by a gene cluster of Caldicellulosiruptor owensensis

The xylanolytic extremely thermophilic bacterium Caldicellulosiruptor owensensis provides a promising platform for xylan utilization. In the present study, two novel xylanolytic enzymes, GH10 endo-β-1,4-xylanase (Coxyn A) and GH39 β-1,4-xylosidase (Coxyl A) encoded in one gene cluster of C.owensensis were heterogeneously expressed and biochemically characterized. The optimum temperature of the two xylanlytic enzymes was 75°C, and the respective optimum pH for Coxyn A and Coxyl A was 7.0 and 5.0. The difference of Coxyn A and Coxyl A in solution was existing as monomer and homodimer respectively, it was also observed in predicted secondary structure. Under optimum condition, the catalytic efficiency (k_cat/K_m) of Coxyn A was 366 mg ml⁻¹ s⁻¹ on beechwood xylan, and the catalytic efficiency (k_cat/K_m) of Coxyl A was 2253 mM⁻¹ s⁻¹ on pNP-β-D-xylopyranoside. Coxyn A degraded xylan to oligosaccharides, which were converted to monomer by Coxyl A. The two intracellular enzymes might be responsible for xylooligosaccharides utilization in C.owensensis, also provide a potential way for xylan degradation in vitro.

Engineering the expression and characterization of two novel laccase isoenzymes from Coprinus comatus in Pichia pastoris by fusing an additional ten amino acids tag at N-terminus

The detail understanding of physiological/biochemical characteristics of individual laccase isoenzymes in fungi is necessary for fundamental and application purposes, but our knowledge is still limited for most of fungi due to difficult to express laccases heterologously. In this study, two novel laccase genes, named lac3 and lac4, encoding proteins of 547 and 532-amino acids preceded by 28 and 16-residue signal peptides, respectively, were cloned from the edible basidiomycete Coprinus comatus. They showed 70% identity but much lower homology with other fungal laccases at protein level (less than 58%). Two novel laccase isoenzymes were successfully expressed in Pichia pastoris by fusing an additional 10 amino acids (Thr-Pro-Phe-Pro-Pro-Phe-Asn-Thr-Asn-Ser) tag at N-terminus, and the volumetric activities could be dramatically enhanced from undetectable level to 689 and 1465 IU/l for Lac3 and Lac4, respectively. Both laccases possessed the lowest Km and highest kcat/Km value towards syringaldazine, followed by ABTS, guaiacol and 2,6-dimethylphenol similar as the low redox potential laccases from other microorganisms. Lac3 and Lac4 showed resistant to SDS, and retained 31.86% and 43.08% activity in the presence of 100 mM SDS, respectively. Lac3 exhibited higher decolorization efficiency than Lac4 for eleven out of thirteen different dyes, which may attribute to the relatively higher catalytic efficiency of Lac3 than Lac4 (in terms of kcat/Km) towards syringaldazine and ABTS. The mild synergistic decolorization by two laccases was observed for triphenylmethane dyes but not for anthraquinone and azo dyes.

Two promising alkaline β-glucosidases isolated by functional metagenomics from agricultural soil, including one showing high tolerance towards harsh detergents, oxidants and glucose

New β-glucosidase activities were identified by screening metagenomic libraries constructed with DNA isolated from the topsoil of a winter wheat field. Two of the corresponding proteins, displaying an unusual preference for alkaline conditions, were selected for purification by Ni-NTA chromatography. AS-Esc6, a 762-amino-acid enzyme belonging to glycoside hydrolase family 3, proved to be a mesophilic aryl-β-glucosidase with maximal activity around pH 8 and 40 °C. A similar pH optimum was found for AS-Esc10, a 475-amino-acid GH1-family enzyme, but this enzyme remained significantly active across a wider pH range and was also markedly more stable than AS-Esc6 at pH greater than 10. AS-Esc10 was found to degrade cellobiose and diverse aryl glycosides, with an optimal temperature of 60 °C and good stability up to 50 °C. Unlike AS-Esc6, which showed a classically low inhibitory constant for glucose (14 mM), AS-Esc10 showed enhanced activity in the presence of molar concentrations of glucose. AS-Esc10 was highly tolerant to hydrogen peroxide and also to sodium dodecyl sulfate, this being indicative of kinetic stability. This unique combination of properties makes AS-Esc10 a particularly promising candidate whose potential in biotechnological applications is worth exploring further.