Purification and Characterization of Aeromonas caviae ME-1 Xylanase V, Which Produces Exclusively Xylobiose from Xylan

A novel thermophilic recombinant obligate xylobiohydrolase (AcGH30A) from Acetivibrio clariflavus orchestrates the deconstruction of xylan polysaccharides

GH30 xylobiohydrolases, an expanding enzyme category, need deeper insights for optimal use. The primary aim of this study was to characterize a new xylobiohydrolase, AcGH30A of GH30 family from Acetivibrio clariflavus. The gene encoding AcGH30A was cloned using pET28a(+) vector and expressed in E. coli BL21(DE3) cells. AcGH30A was purified by immobilized metal-ion affinity chromatography. SDS-PAGE analysis of AcGH30A showed molecular mass of ~58 kDa. AcGH30A showed optimum temperature 80 °C and optimum pH 7.0. AcGH30A was stable (maintaining >80 % of control activity) in pH range, 4-7 and temperature range, 30 °C -70 °C when incubated for 90 min. AcGH30A displayed melting temperature, 72 °C and half-life, 21 days at 4 °C. The enzyme activity of AcGH30A was enhanced by 10 mM Ca2+ and Mg2+ ions by 25 % and 21 %, respectively, whereas 10 mM Co2+, Zn2+, Fe2+, and Cu2+ ions significantly reduced it. AcGH30A showed activity against various xylan polysaccharides displaying highest Vmax, 139 U.mg-1 and KM, 0.71 mg.ml-1 against 4-O-methyl glucuronoxylan under optimum conditions. TLC, HPLC and LC-MS analyses of AcGH30A hydrolyzed products from xylan substrates revealed the release of sole product, xylobiose, confirming it as an obligate xylobiohydrolase. AcGH30A being a highly thermostable enzyme can be potentially utlilized in various biotechnological applications.

Effect of N-terminal modification on the mode of action between the Xyn11A and Xylotetraose

The purpose of this study was to gain an insight into the effects of mutation-induced binding pocket tilting of the Xyn11A xylanase from Bacillus firmus K-1 in producing a unique hydrolysis characteristic. In this study, the wildtype Xyn11A and its K40L mutant were compared for their hydrolysis patterns on beechwood xylan and xylooligosaccharides of sizes 2 to 6. According to our thin-layer chromatography experiment, the K40L mutant produced a larger amount of xylotetraose leftover than the wildtype. Kinetic determination of the WT and K40L mutant suggested that the higher X4 leftover on TLC was reflected in the decreasing catalytic efficiency (k_cat/K_m) between enzyme and X4. The mechanisms underlying this efficiency loss were examined through atomistic molecular dynamics (MD) simulations. The MD trajectory analysis showed that the mutation-induced binding pocket tilting resulted in an additional hydrophobic contact between the reducing end of X4 and Trp128. Meanwhile, the interactions between the non-reducing end and the Arg112 residue near the active site became lost, which could decrease the catalytic efficiency. This work suggested that the protein engineering to fine-tune the hydrolysis pattern for some desired xylooligosaccharide products was possible.

A novel GH30 xylobiohydrolase from Acremonium alcalophilum releasing xylobiose from the non-reducing end

Xylanases of the GH30 family are grouped to subfamilies GH30-7 and GH30-8. The GH30-8 members are of bacterial origin and well characterized, while the GH30-7 members are from fungal sources and their properties are quite diverse. Here, a heterologous expression and characterization of the GH30-7 xylanase AaXyn30A from a cellulolytic fungus Acremonium alcalophilum is reported. From various polymeric and oligomeric substrates AaXyn30A generates xylobiose as the main product. It was proven that xylobiose is released from the non-reducing end of all tested substrates, thus the enzyme behaves as a typical non-reducing-end acting xylobiohydrolase. AaXyn30A is active on different types of xylan, exhibiting the highest activity on rhodymenan (linear β-1,3-β-1,4-xylan) from which also an isomeric xylotriose Xyl-β-1,3-Xyl-β-1,4-Xyl is formed. Production of xylobiose from glucuronoxylan is at later stage accompanied by a release of aldouronic acids differing from those liberated by the bacterial GH30-8 glucuronoxylanases.

Structure-guided design combined with evolutionary diversity led to the discovery of the xylose-releasing exo-xylanase activity in the glycoside hydrolase family 43

Rational design is an important tool for sculpting functional and stability properties of proteins and its potential can be much magnified when combined with in vitro and natural evolutionary diversity. Herein, we report the structure-guided design of a xylose-releasing exo-β-1,4-xylanase from an inactive member of glycoside hydrolase family 43 (GH43). Structural analysis revealed a nonconserved substitution (Lys²⁴⁷ ) that results in the disruption of the hydrogen bond network that supports catalysis. The mutation of this residue to a conserved serine restored the catalytic activity and crystal structure elucidation of the mutant confirmed the recovery of the proper orientation of the catalytically relevant histidine. Interestingly, the tailored enzyme can cleave both xylooligosaccharides and xylan, releasing xylose as the main product, being the first xylose-releasing exo-β-1,4-xylanase reported in the GH43 family. This enzyme presents a unique active-site topology when compared with closely related β-xylosidases, which is the absence of a hydrophobic barrier at the positive-subsite region, allowing the accommodation of long substrates. Therefore, the combination of rational design for catalytic activation along with naturally occurring differences in the substrate binding interface led to the discovery of a novel activity within the GH43 family. In addition, these results demonstrate the importance of solvation of the β-propeller hollow for GH43 catalytic function and expand our mechanistic understanding about the diverse modes of action of GH43 members, a key and polyspecific carbohydrate-active enzyme family abundant in most plant cell-wall-degrading microorganisms.

Characterization of a purified thermostable xylanase from Caldicoprobacter algeriensis sp. nov. strain TH7C1(T)

The present study investigates the purification and biochemical characterization of an extracellular thermostable xylanase (called XYN35) from Caldicoprobacter algeriensis sp. nov., strain TH7C1(T), a thermophilic, anaerobic strain isolated from the hydrothermal hot spring of Guelma (Algeria). The maximum xylanase activity recorded after 24 h of incubation at 70 °C and in an optimized medium containing 10 g/L mix birchwood- and oats spelt-xylan was 250 U/mL. The pure protein was obtained after heat treatment (1 h at 70 °C), followed by sequential column chromatographies on Sephacryl S-200 gel filtration and Mono-S Sepharose anion-exchange. Matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF/MS) analysis indicated that the purified enzyme is a monomer with a molecular mass of 35,075.10 Da. The results from amino-acid sequence analysis revealed high homology between the 21 NH2-terminal residues of XYN35 and those of bacterial xylanases. The enzyme showed optimum activity at pH 11 and 70 °C. While XYN35 was activated by Ca(2+), Mn(2+), and Mg(2+), it was completely inhibited by Hg(2+) and Cd(2+). The xylanase showed higher specific activity on soluble oat-spelt xylan, followed by beechwood xylan. This enzyme was also noted to obey the Michaelis-Menten kinetics, with Km and kcat values on oat-spelt xylan being 1.33 mg/mL and 400 min(-1), respectively. Thin-layer chromatography soluble oat-spelt xylan (TLC) analysis showed that the final hydrolyzed products of the enzyme from birchwood xylan were xylose, xylobiose, and xylotriose. Taken together, the results indicated that the XYN35 enzyme has a number of attractive biochemical properties that make it a potential promising candidate for future application in the pulp bleaching industry.

Expression of Aeromonas punctata ME-1 exo-xylanase X in E. coli for efficient hydrolysis of xylan to xylose

exo-Xylanase X from Aeromonas punctata ME-1 was functionally expressed in Escherichia coli with a carboxy terminal His tag (6×) and a molecular mass of 39.42 kDa, which is in agreement with the prediction from its amino acid composition. The recombinant exo-xylanase reached 186 mg l(-1) after induction by isopropyl β-D-1-thiogalactopyranoside. Its optimal temperature and pH were 50 °C and 6, respectively. The enzyme showed not only an exo-xylanase activity with K m of 3.90 mg ml(-1) and V max of 12.9 U μg(-1) for hydrolysis of Remazol Brilliant Blue-xylan but also a considerable exo-glucanase activity (27.9 U mg(-1)) on P-nitrophenyl β-D-cellobioside. It hydrolyzed xylan predominantly to xylobiose, xylotriose, xylotetraose, and xylose. An enzyme mixture of exo-xylanase and endo-xylanase (50 μg ml(-1) each) yielded a larger amount (330 mg l(-1)) of xylose from beechwood xylan than the controls (270 and 150 mg l(-1)) using them alone at 100 μg ml(-1), indicating a synergistic action between the two xylanases favoring the hydrolysis of beechwood xylan to release more xylose.

Expression of Aspergillus niger IA-001 Endo-β-1,4-xylanase in Pichia pastoris and analysis of the enzymic characterization

The xylanaseB (XynB) (JX560731.1) gene of Aspergillus niger IA-001 was optimized according to the codon usage of Pichia pastoris and expressed in P. pastoris GS115. The optimized XynB expression level was increased 2.8 times relative to that of the wild-type XynB, and the dual-copy XynB (optimized) expression level was increased 1.9 times relative to that of the single-copy XynB (optimized). The activity of the dual-copy XynB ((XynB-opt)2) was maximized at 15,158.23 ± 45.11 U/mL after 120 h of shaking. The optimal temperature and pH of (XynB-opt)2 were 50 °C and 5.0, respectively. (XynB-opt)2 showed a high specific activity of 6,853.00 ± 20.08 U/mg. IC analysis of the standard xylooligosaccharides showed that (XynB-opt)2 was an endo-xylanase with X2 as the main degradation product. (XynB-opt)2 was highly specific towards different natural xylans. After 24 h of hydrolysis, more than 90 % of the total hydrolysis products of xylan were X2 and X1, almost no X4 ~ X6. In addition, the enzyme exhibited resistance to many metal ions and low pH values. The superior catalytic properties of (XynB-opt)2 suggested its great potential as an effective additive in animal feed industry.

Methods for isolation and cultivation of filamentous fungi

Filamentous fungi are important organisms for basic discovery, industry, and human health. Their natural growth environments are extremely variable, a fact reflected by the numerous methods developed for their isolation and cultivation. Fungal culture in the laboratory is usually carried out on agar plates, shake flasks, and bench top fermenters starting with an inoculum that typically features fungal spores. Here we discuss the most popular methods for the isolation and cultivation of filamentous fungi for various purposes with the emphasis on enzyme production and molecular microbiology.

Characterization of a new Providencia sp. strain X1 producing multiple xylanases on wheat bran

Providencia sp. strain X1 showing the highest xylanase activity among six bacterial isolates was isolated from saw-dust decomposing site. Strain X1 produced cellulase-free extracellular xylanase, which was higher in wheat bran medium than in xylan medium, when cultivated at pH 8.0 and 35°C. Zymogram analysis of crude preparation of enzymes obtained while growing on wheat bran and birchwood xylan revealed the presence of seven and two distinct xylanases with estimated molecular weight of 33; 35; 40; 48; 60; 75; and 95 kDa and 33 and 44 kDa, respectively. The crude xylanases were produced on wheat bran medium and showed optimum activity at pH 9.0 and 60°C. The thermotolerance studies showed activity retention of 100% and 85% at 40°C and 60°C after 30 min preincubation at pH 9.0. It was tolerant to lignin, ferulic acid, syringic acid, and guaiacol and retained 90% activity after ethanol treatment. The enzyme preparation was also tolerant to methanol and acetone and showed good activity retention in the presence of metal ions such as Fe²⁺, Mg²⁺, Zn²⁺, and Ca²⁺. The crude enzyme preparation was classified as endoxylanase based on the product pattern of xylan hydrolysis. Pretreatment of kraft pulp with crude xylanases for 3 h at 60°C led to a decrease in kappa number by 28.5%. The properties of present xylanases make them potentially useful for industrial applications.

Xylanase XYN IV from Trichoderma reesei showing exo- and endo-xylanase activity

A minor xylanase, named XYN IV, was purified from the cellulolytic system of the fungus Trichoderma reesei Rut C30. The enzyme was discovered on the basis of its ability to attack aldotetraohexenuronic acid (HexA-2Xyl-4Xyl-4Xyl, HexA(3)Xyl(3)), releasing the reducing-end xylose residue. XYN IV exhibited catalytic properties incompatible with previously described endo-β-1,4-xylanases of this fungus, XYN I, XYN II and XYN III, and the xylan-hydrolyzing endo-β-1,4-glucanase EG I. XYN IV was able to degrade several different β-1,4-xylans, but was inactive on β-1,4-mannans and β-1,4-glucans. It showed both exo-and endo-xylanase activity. Rhodymenan, a linear soluble β-1,3-β-1,4-xylan, was as the best substrate. Linear xylooligosaccharides were attacked exclusively at the first glycosidic linkage from the reducing end. The gene xyn4, encoding XYN IV, was also isolated. It showed clear homology with xylanases classified in glycoside hydrolase family 30, which also includes glucanases and mannanases. The xyn4 gene was expressed slightly when grown on xylose and xylitol, clearly on arabinose, arabitol, sophorose, xylobiose, xylan and cellulose, but not on glucose or sorbitol, resembling induction of other xylanolytic enzymes from T. reesei. A recombinant enzyme prepared in a Pichia pastoris expression system exhibited identical catalytic properties to the enzyme isolated from the T. reesei culture medium. The physiological role of this unique enzyme remains unknown, but it may involve liberation of xylose from the reducing end of branched oligosaccharides that are resistant toward β-xylosidase and other types of endoxylanases. In terms of its catalytic properties, XYN IV differs from bacterial GH family 30 glucuronoxylanases that recognize 4-O-methyl-D-glucuronic acid (MeGlcA) substituents as substrate specificity determinants.

Isolation, purification, and characterization of xylanase produced by a new species of bacillus in solid state fermentation

A thermoalkalophilic new species of Bacillus, similar to Bacillus arseniciselenatis DSM 15340, produced extracellular xylanase under solid state fermentation when wheat bran is used as carbon source. The extracellular xylanase was isolated by ammonium sulfate (80%) precipitation and purified using ion exchange chromatography. The molecular weight of xylanase was ~29.8 kDa. The optimum temperature and pH for the enzyme activity were 50°C and pH 8.0. The enzyme was active on birchwood xylan and little active on p-nitrophenyl xylopyranoside but not on Avicel, CMC, cellobiose, and starch, showing its absolute substrate specificity. For birchwood xylan, the enzyme gave a Km 5.26 mg/mL and Vmax 277.7 μmol/min/mg, respectively. In addition, the xylanase was also capable of producing high-quality xylo-oligosaccharides, which indicated its application potential not only in pulp biobleaching processes but also in the nutraceutical industry.

Purification and biochemical characterization of a highly thermostable xylanase from Actinomadura sp. strain Cpt20 isolated from poultry compost

An extracellular thermostable xylanase from a newly isolated thermophilic Actinomadura sp. strain Cpt20 was purified and characterized. Based on matrix-assisted laser desorption-ionization time-of-flight mass spectrometry analysis, the purified enzyme is a monomer with a molecular mass of 20,110.13 Da. The 19 residue N-terminal sequence of the enzyme showed 84% homology with those of actinomycete endoxylanases. The optimum pH and temperature values for xylanase activity were pH 10 and 80 °C, respectively. This xylanase was stable within a pH range of 5-10 and up to a temperature of 90 °C. It showed high thermostability at 60 °C for 5 days and half-life times at 90 °C and 100 °C were 2 and 1 h, respectively. The xylanase was specific for xylans, showing higher specific activity on soluble oat-spelt xylan followed by beechwood xylan. This enzyme obeyed the Michaelis-Menten kinetics, with the K (m) and k (cat) values being 1.55 mg soluble oat-spelt xylan/ml and 388 min(-1), respectively. While the xylanase from Actinomadura sp. Cpt20 was activated by Mn(2+), Ca(2+), and Cu(2+), it was, strongly inhibited by Hg(2+), Zn(2+), and Ba(2+). These properties make this enzyme a potential candidate for future use in biotechnological applications particularly in the pulp and paper industry.

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.

Rheofermentometer fermentation and breadmaking characteristics of dough containing xylo-oligosaccharide hydrolyzate from wheat bran

The sugar composition of a xylo-oligosaccharide enzymolysis solution (XES) hydrolyzed from wheat bran and bread containing XES were studied. The effects of the XES on fermentation properties of dough and on bread crumb and crust color characteristics are reported. The composition of sugars was determined by high performance liquid chromatography (HPLC), and the fermentation properties of dough and bread color were determined by Rheofermentometer F3 and Chroma Meter CR-400, respectively. xylo-Oligosaccharides (XOS) (51.3%) and xylose (39.2%) were the major sugars in the XES. XOS remained in the XES-containing bread after breadmaking processes (xylotriose, xylotetraose, and xylopentaose, 21.1%), with xylopentaose accounting for 14.4%. Maximum dough height (Hm) was significantly increased by XES, while the maximum gaseous release height (Hm') was not affected. XES increased the brownness index of the crumb and crust of bread when compared to the that of control bread. Consumer acceptability scores of control and XES containing breads were similar.

Purification and biochemical characterization of two xylanases from Aspergillus sydowii SBS 45

Two xylanases were isolated and purified from crude culture filtrate of Aspergillus sydowii SBS 45 after 9 days of growth on wheat bran containing 0.5% (w/v) birch wood xylan as the carbon source under solid-state fermentation. After a three-step purification scheme involving ammonium sulfate precipitation, gel filtration chromatography (Sephadex G-200), and anion exchange chromatography (DEAE-Sephadex A-50), xylanase I was purified 93.41 times, and xylanase II was purified 77.40 times with yields of 4.49 and 10.46, respectively. Molecular weights of xylanase I and II were 20.1 and 43 kDa, respectively, in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Optimum temperature was 50 degrees C, and optimum pH was 10.0 for both xylanase I and II. The Km value of xylanase I for birch wood xylan was 3.18 mg ml(-1) and for oat spelt xylan 6.45 mg ml(-1), while the Km value of xylanase II for birch wood xylan was 6.51 mg ml(-1) and for oat spelt xylan 7.69 mg ml(-1). Metal ions like Al3+, Ba2+, Ca2+, Na+, and Zn2+ enhanced the activity of xylanase I and II at 10 mM concentration. Among the additives, L-tryptophan enhanced the activity of xylanase I and II at 10-, 20-, and 30-mM concentrations. Both xylanases appeared to be glycoproteins.

Purification and characterization of thermoalkalophilic xylanase isolated from the Enterobacter sp. MTCC 5112

Thermoalkalophilic Enterobacter sp. MTCC 5112 was isolated from a sediment sample collected from the Mandovi estuary on the west coast of India. This culture produced extracellular xylanase. The xylanase enzyme was isolated by ammonium sulfate (80%) fractionation and purified to homogeneity using size exclusion and ion exchange chromatography. The molecular mass of the xylanase was approximately 43 kDa. The optimal pH of the xylanase activity was 9, and at room temperature it showed 100% stability at pH 7, 8 and 9 for 3 h. The optimal temperature for the enzyme activity was 100 degrees C at pH 9.0. At 80 degrees C and pH 9, 90% of the enzyme activity was retained after 40 min. At 70 and 60 degrees C, the enzyme retained 64% and 85% of its activity after 18 h, respectively, while at 50 degrees C and pH 9 the enzyme remained stable for days. For xylan, the enzyme gave a K(m) value of 3.3 mg ml(-1) and a V(max) value of 5,000 micromol min(-1) mg(-1) when the reaction was carried out at 100 degrees C and pH 9. In the presence of metal ions such as Co(2+), Zn(2+), Fe(2+), Cu(2+), Mg(2+) and Ca(2+) the activity of the enzyme increased, whereas strong inhibition of enzyme activity was observed in the presence of Hg(2+) and EDTA. To the best of our knowledge, this is the first report on the production of xylanase by this bacterium.

Expression of xyrA gene encoding for D-Xylose reductase of Candida tropicalis and production of xylitol in Escherichia coli

The D-Xylose reductase (XR) gene (xyrA) of Candida tropicalis IFO 0618 was expressed in Escherichia coli JM109. The enzymatic properties of each recombinant XR such as the Km value for D-xylose and NADPH, the substrate specificity for other sugars and the optimal pH were essentially the same as those of the corresponding enzyme of C. tropicalis. The recombinant XR was more heat-stable than C. tropicalis XR at 60 degrees C. E. coli, expressing the xyrA gene, successfully converted D-xylose to xylitol. When D-xylose (50 g/l) and D-glucose (5 g/l) were added to IPTG-induced cells, 13.3 g/l of xylitol was produced during 20 h of cultivation.

Crystallization and preliminary X-ray crystallographic studies of XynX, a family 10 xylanase from Aeromonas punctata ME-1

Xylanases catalyze the hydrolysis of beta-1,4-glycosidic linkages within the xylan backbone. XynX is a xylanase from Aeromonas punctata ME-1 and belongs to glycoside hydrolase family 10. While most xylanases show endo-type catalytic activities, XynX shows exo-like catalytic activities, selectively producing xylobiose from birchwood xylan. In this study, XynX was crystallized by the hanging-drop vapour-diffusion method. The crystals belonged to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 79.0, b = 88.6, c = 93.2 A, and diffracted to beyond 1.8 A resolution.

Characterization of a Paenibacillus cell-associated xylanase with high activity on aryl-xylosides: a new subclass of family 10 xylanases

The sequence of gene xynB encoding xylanase B from Paenibacillus sp. BP-23 was determined. It revealed an open reading frame of 999 nucleotides encoding a protein of 38,561 Da. The deduced amino acid sequence of xylanase B shows that the N-terminal region of the enzyme lacks the features of a signal peptide. When the xylan-degrading system of Paenibacillus sp. BP-23 was analysed in zymograms, it revealed that xylanase B was not secreted to the extracellular medium but instead remained cell-associated, even in late stationary-phase cultures. When xynB was expressed in a Bacillus subtilis secreting host, it also remained associated with the cells. Sequence homology analysis showed that xylanase B from Paenibacillus sp. BP-23 belongs to family 10 glycosyl hydrolases, exhibiting a distinctive high homology to six xylanases of this family. The homologous enzymes were also found to be devoid of a signal peptide and seem to constitute, together with xylanase B, a separate group of enzymes. They all have two conserved amino acid regions not found in the other family 10 xylanases, and cluster in a separate group after dendrogram analysis. We propose that these enzymes constitute a new subclass of family 10 xylanases, that are cell-associated, and that hydrolyse the xylooligosaccharides resulting from extracellular xylan hydrolysis. Xylanase B shows similar specific activity on aryl-xylosides and xylans. This can be correlated to some, not yet identified, trait of catalytic activity of the enzyme on plant xylan.

A cytoplasmic xylanase (XynX) of Aeromonas caviae ME-1 is released from the cytoplasm to the periplasm by osmotic downshock

Aeromonas caviae ME-1 is a multiple xylanase-producing gram-negative bacterium which was isolated from the gut contents of a wild silkworm, Samia cynthia pryeri. One of the xylanases produced by A. caviae ME-1, XynX (38 kDa, family 10 xylanase), hydrolyzes xylan to xylobiose and xylotetraose as final degradation products. Generally, xylanases are extracellular or cell surface enzymes. However, XynX is not exported to the extracellular fluid by A. caviae ME-1 and an Escherichia coli transformant harboring the xynX gene. In this study, we investigated the intracellular localization of XynX in A. caviae ME-1 and an E. coli transformant. XynX was found in the cytoplasm when the cells were grown under normal culture conditions. However, XynX was released from the cytoplasm to the periplasm during osmotic downshock. This release of XynX in the E. coli transformant was blocked in the presence of gadolinium chloride, which has been reported to be an inhibitor of bacterial mechanosensitive channels.

A role of xylanase, alpha-L-arabinofuranosidase, and xylosidase in xylan degradation

Renewable natural resources such as xylans are abundant in many agricultural wastes. Penicillium sp. AHT-1 is a strong producer of xylanolytic enzymes. The sequential activities of its xylanase, alpha-L-arabinofuranosidase, and beta-xylosidase on model hemicellulose oat-spelt xylan was investigated. Optimum production of the enzymes was found in culture containing oat-spelt xylan at 30 degrees C and initial pH 7.0 after 6 days. The enzymes were partially purified by ammonium sulphate fractionation and anion-exchange chromatography on DEAE-Toyopearl 650 S. The apparent molecular mass was 21 kDa, and the protein displayed an "endo" mode of action. The xylanase exhibited glycotansferase activity. It synthesized higher oligosaccharides from the initial substrates, and xylotriose was the shortest unit of substrate transglycosylated. Xylanolytic enzymes (enzyme mixture) produced by this Penicillium sp. interacted cooperatively and sequentially in the hydrolysis of oat-spelt xylan in the following order: alpha-L-arabinofuranosidase --> xylanase --> beta-xylosidase. All three enzymes exhibited optimal activity under the same conditions (temperature, pH, cultivation), indicating that they alone are sufficient to completely depolymerize the test xylan. Results indicate that the xylanolytic enzyme mixture of Penicillium sp. AHT-1 could be useful for bioconversion of xylan-rich plant wastes to value-added products.

The processing of high-molecular-weight xylanase (XynE, 110 kDa) fromAeromonas caviae ME-1 to 60-kDa xylanase (XynE60) inEscherichia coli and purification and characterization of XynE60

A xylanase gene (xynE) encoding XynE (110 kDa) was cloned from a lambda phage genomic library of Aeromonas caviae ME-1 which is a multiple-xylanase-producing bacterium. Upon nucleotide sequence analysis, we found that xynE comprises 2823 by and encodes a protein of 941 amino acid residues (104,153 Da), which was similar to endo-beta-1,4-xylanases which are categorized to glycosyl hydrolase family 10. An Escherichia coli transformant that harbored pXED30 carrying xynE produced 110-, 84-, 72-, and 66-kDa xylanases in the cell-free extract, and 72- and 66-kDa xylanases in the culture supernatant. We purified the 66-kDa xylanase to electrophoretic homogeneity from a culture supernatant by a series of column chromatographies. The calculated molecular mass of the purified xylanase determined by matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was 60,154.50 Da, and the xylanase was designated XynE60. Analysis of the N-terminal 10 amino acid residues and the determined molecular mass of XynE60 revealed that XynE60 is a product processed at the Gly26-Gly27, and Thr565-Ala566 sites of XynE by proteolytic cleavage. XynE60 showed optimal activity at 55 degrees C and pH 8.0, and was stable below 45 degrees C and at pH 7.0-8.5. The K(m) and V(max) of XynE60 were calculated to be 8.1 mg/ml and 6897 nkat/mg, respectively.

Processing of XynE (110-kDa) ofAeromonas caviae ME-1 to 72-kDa xylanase inEscherichia coli transformant

An extracellular 72-kDa xylanase from an Escherichia coli transformant that harbored pXED30 carrying xynE of Aeromonas caviae ME-1 was purified by extraction following SDS-PAGE to electrophoretic homogeneity. The analysis of N-terminal 10 amino acid residues (Gly-Ala-Arg-Ala-Gln-Ala-Ala-Ala-Asp-Val) revealed a 72-kDa product derived by the cleavage of Gly26-Gly27 at the N-terminal region of 110 kDa XynE. The 72-kDa xylanase from A. caviae ME-1 was found to have the same sequence as that of the N-terminal 10 amino acid residues. When OmpT-deficient E. coli mutants were used as hosts, the 72-kDa xylanase was detected in cell-free extracts, but not in the culture supernatant, suggesting that OmpT is not involved in the cleavage at the C-terminal region, but is involved in the secretion of 72-kDa xylanase to the culture medium.

XynX, a possible exo-xylanase of Aeromonas caviae ME-1 that produces exclusively xylobiose and xylotetraose from xylan

A gene, xynX, encoding a novel xylanase, was cloned from Aeromonas caviae ME-1. This gene encoded an enzyme that was constituted of 334 amino acid residues (38,580 Da) and was similar in sequence to Family 10 (Family F) beta-1,4 endo-xylanases. XynX produced only xylobiose and xylotetraose from birch wood xylan, and xylotriose, xylopentaose, and higher oligosaccharides were not detected in the TLC analysis. We designated it as X2/X4-forming xylanase. This enzyme does not have transglycosylation activity. These data suggested that this enzyme is a possible exo-xylanase. According to homology modeling, the enzyme has a ring-shaped (alpha/beta)8 barrel (TIM barrel) structure, typical of Family 10 endo-xylanases, with the extraordinary feature of a longer bottom-loop structure.

Xylanolytic enzymes from fungi and bacteria

The development of new analytical techniques and the commercial availability of new substrates have led to the purification and characterization of a large number of xylan-degrading enzymes. Furthermore, the introduction of recombinant DNA technology has resulted in the selection of xylanolytic enzymes that are more suitable for industrial applications. For a successful integration of xylanases in industrial processes, a detailed understanding of the mechanism of enzyme action is, however, required. This review gives an overview of various xylanolytic enzyme systems from bacteria and fungi that have been described recently in more detail.

Purification and characterization of xylanase from alkali-tolerant Aspergillus fischeri Fxn1

Alkali-tolerant Aspergillus fischeri Fxn1 produced two extracellular xylanases. The major xylanase (M(r) 31,000) was purified to electrophoretic homogeneity by ammonium sulfate precipitation, anion exchange chromatography and preparatory PAGE. Xylose was the major hydrolysis product from oat spelt and birch wood xylans. It was completely free of cellulolytic activities. The optimum pH and temperature were 6.0 and 60 degrees C, respectively. pH stability ranged from 5 to 9.5 and the t1/2 at 50 degrees C was 490 min. It had a Km of 4.88 mg ml-1 and a Vmax of 588 mumol min-1 mg-1. The activity was inhibited (95%) by AlCl3 (10 mM). This enzyme appears to be novel and will be useful for studies on the mechanism of hydrolysis of xylan by xylanolytic enzymes.

Biochemistry and genetics of microbial xylanases

Xylanases are classified into two major families (10 or F and 11 or G) of glycosyl hydrolases. Both use ion pair catalytic mechanisms and both retain anomeric configuration following hydrolysis. Family 10 xylanases are larger, more complex and produce smaller oligosaccharides; Family 11 xylanases are more specific for xylan. Alkaline-active and extreme-thermophilic enzymes are of particular interest. Such xylanases are being commercialized for bleaching pulps and other applications.

Xylanase IV, an Exoxylanase of Aeromonas caviae ME-1 Which Produces Xylotetraose as the Only Low-Molecular-Weight Oligosaccharide from Xylan

A novel xylanase (xylanase IV) which produces xylotetraose as the only low-molecular-weight oligosaccharide from oat spelt xylan was isolated from the culture medium of Aeromonas caviae ME-1. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the xylanase IV molecular weight was 41,000. Xylanase IV catalyzed the hydrolysis of oat spelt xylan, producing exclusively xylotetraose. The acid hydrolysate of the product gave d-xylose. The enzyme did not hydrolyze either p-nitrophenyl-(beta)-d-xyloside, small oligosaccharides (xylobiose and xylotetraose), or polysaccharides, such as starch, cellulose, carboxymethyl cellulose, laminarin, and (beta)-1,3-xylan.

Characterization and substrate specificity of an endo-beta-1,4-D-glucanase I (Avicelase I) from an extracellular multienzyme complex of Bacillus circulans

An endo-1,4-beta-D-glucanase I (Avicelase I; EC 3.2.1.4) was purified to homogeneity from an extracellular celluloxylanosome of Bacillus circulans F-2. The purification in the presence of 6 M urea yielded homogeneous enzyme. The enzyme had a monomeric structure, its relative molecular mass being 75 kDa as determined by gel filtration and 82 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The pI was 5.4, and the N-terminal amino acid sequence was ASNIGGWVGGNESGFEFG. The optimal pH was 4.5, and the enzyme was stable at pH 4 to 10. The enzyme has a temperature optimum of 50 degrees C, it was stable at 55 degrees C for 46 h, and it retains approximately 20% of its activity after 30 min at 80 degrees C. It showed high-level activity towards carboxymethyl cellulose (CMC) as well as p-nitrophenyl-beta-D-cellobioside, 4-methylumbelliferyl cellobioside, xylan, Avicel, filter paper, and some cello-oligosaccharides. Km values for birch xylan, CMC, and Avicel were 4.8, 7.2, and 87.0 mg/ml, respectively, while Vmax values were 256, 210, and 8.6 mumol x min-1 x mg-1, respectively. Cellotetraose was preferentially cleaved into cellobiose (G2) plus G2, and cellopentaose was cleaved into G2 plus cellotriose (G3), while cellohexaose was cleaved into cellotetraose plus G2 and to a lesser extent G3 plus G3. G3 was not cleaved at all. G2 was the main product of Avicel hydrolysis. Xylotetraose (X4) and xylobiose (X2) were mainly produced by the enzyme hydrolysis of xylan. G2 inhibited the activity of carboxymethyl cellulase and Avicelase, whereas Mg2+ stimulated it. The enzyme was completely inactivated by Hg2+, and it was inhibited by a thiol-blocking reagent. Hydrolysis of CMC took place, with a rapid decrease in viscosity but a slow liberation of reducing sugars. On the basis of these results, it appeared that the cellulase should be regarded as endo-type cellulase, although it hydrolyzed Avicel.