Identification, heterologous expression and characterization of a novel glycoside hydrolase family 30 xylanase from the fungus Penicillium purpurogenum

Crystal structure of reducing-end xylose-releasing exoxylanase in subfamily 7 of glycoside hydrolase family 30

TcXyn30A from Talaromyces cellulolyticus, which belongs to subfamily 7 of the glycoside hydrolase family 30 (GH30-7), releases xylose from the reducing end of xylan and xylooligosaccharides (XOSs), the so-called reducing-end xylose-releasing exoxylanase (ReX). In this study, the crystal structures of TcXyn30A with and without xylose at subsite +1 (the binding site of the xylose residue at the reducing end) were determined. This is the first report on the structure of ReX in the family GH30-7. TcXyn30A forms a dimer. The complex structure of TcXyn30A with xylose revealed that subsite +1 is located at the dimer interface. TcXyn30A recognizes xylose at subsite +1 composed of amino acid residues from each monomer and blocks substrate binding to subsite +2 by dimer formation. Thus, the dimeric conformation is responsible for ReX activity. The structural comparison between TcXyn30A and the homologous enzyme indicated that subsite -2 is composed of assembled three stacked Trp residues, Trp49, Trp333, and Trp334, allowing TcXyn30A to accommodate xylan and any branched XOSs decorated with a substitution such as α-1,2-linked 4-O-methyl-d-glucuronic acid or α-1,2- and/or -1,3-linked L-arabinofuranose. These findings provide an insight into the structural determinants for ReX activity of TcXyn30A.

Endo-1,4-β-xylanase-containing glycoside hydrolase families: Characteristics, singularities and similarities

Endo-1,4-β-xylanases (EC 3.2.1.8) are O-glycoside hydrolases that cleave the internal β-1,4-D-xylosidic linkages of the complex plant polysaccharide xylan. They are produced by a vast array of organisms where they play critical roles in xylan saccharification and plant cell wall hydrolysis. They are also important industrial biocatalysts with widespread application. A large and ever growing number of xylanases with wildly different properties and functionalites are known and a better understanding of these would enable a more effective use in various applications. The Carbohydrate-Active enZYmes database (CAZy), which classifies evolutionarily related proteins into a glycoside hydrolase family-subfamily organisational scheme has proven powerful in understanding these enzymes. Nevertheless, ambiguity currently exists as to the number of glycoside hydrolase families and subfamilies harbouring catalytic domains with true endoxylanase activity and as to the specific characteristics of each of these families/subfamilies. This review seeks to clarify this, identifying 9 glycoside hydrolase families containing enzymes with endo-1,4-β-xylanase activity and discussing their properties, similarities, differences and biotechnological perspectives. In particular, substrate specificities and hydrolysis patterns and the structural determinants of these are detailed, with taxonomic aspects of source organisms being also presented. Shortcomings in current knowledge and research areas that require further clarification are highlighted and suggestions for future directions provided. This review seeks to motivate further research on these enzymes and especially of the lesser known endo-1,4-β-xylanase containing families. A better understanding of these enzymes will serve as a foundation for the knowledge-based development of process-fitted endo-1,4-β-xylanases and will accelerate their development for use with even the most recalcitrant of substrates in the biobased industries of the future.

The first crystal structure of a xylobiose-bound xylobiohydrolase with high functional specificity from the bacterial glycoside hydrolase 30 subfamily 10

Xylobiose is a prebiotic sugar that has applications in functional foods. This report describes the first X-ray crystallographic structure models of apo and xylobiose bound forms of a xylobiohydrolase (XBH) from Acetivibrio clariflavus. This xylan active enzyme, a member of the recently described glycoside hydrolase family 30 (GH30) subfamily 10 phylogenetic clade has been shown to strictly release xylobiose as its primary hydrolysis product. Inspection of the apo-structure reveals a glycone region X₂ binding slot. When X₂ binds, the nonreducing xylose in the -2 subsite is highly coordinated with numerous hydrogen bond contacts while contacts in the -1 subsite mostly reflect interactions typical for GH30 and enzymes in clan A of the carbohydrate-active enzymes database (CAZy). This structure provides an explanation for the high functional specificity of this new bacterial GH30 XBH subfamily.

Glycoside Hydrolase family 30 harbors fungal subfamilies with distinct polysaccharide specificities

Efficient bioconversion of agro-industrial side streams requires a wide range of enzyme activities. Glycoside Hydrolase family 30 (GH30) is a diverse family that contains various catalytic functions and has so far been divided into ten subfamilies (GH30_1-10). In this study, a GH30 phylogenetic tree using over 150 amino acid sequences was contructed. The members of GH30 cluster into four subfamilies and eleven candidates from these subfamilies were selected for biochemical characterization. Novel enzyme activities were identified in GH30. GH30_3 enzymes possess β-(1→6)-glucanase activity. GH30_5 targets β-(1→6)-galactan with mainly β-(1→6)-galactobiohydrolase catalytic behavior. β-(1→4)-Xylanolytic enzymes belong to GH30_7 targeting β-(1→4)-xylan with several activities (e.g. xylobiohydrolase, endoxylanase). Additionally, a new fungal subfamily in GH30 was proposed, i.e. GH30_11, which displays β-(1→6)-galactobiohydrolase. This study confirmed that GH30 fungal subfamilies harbor distinct polysaccharide specificity and have high potential for the production of short (non-digestible) di- and oligosaccharides.

Unique features of the bifunctional GH30 from Thermothelomyces thermophila revealed by structural and mutational studies

Fungal xylanases belonging to family GH30_7, initially categorized as endo-glucuronoxylanases, are now known to differ both in terms of substrate specificity, as well as mode of action. Recently, TtXyn30A, a GH30_7 xylanase from Thermothelomyces thermophila, was shown to possess dual activity, acting on the xylan backbone in both an endo- and an exo- manner. Here, in an effort to identify the structural characteristics that append these functional properties to the enzyme, we present the biochemical characterization of various TtXyn30A mutants as well as its crystal structure, alone, and in complex with the reaction product. An auxiliary catalytic amino acid has been identified, while it is also shown that glucuronic acid recognition is not mediated by a conserved arginine residue, as shown by previously determined GH30 structures.

Catalytic Diversity of GH30 Xylanases

Catalytic properties of GH30 xylanases belonging to subfamilies 7 and 8 were compared on glucuronoxylan, modified glucuronoxylans, arabinoxylan, rhodymenan, and xylotetraose. Most of the tested bacterial GH30-8 enzymes are specific glucuronoxylanases (EC 3.2.1.136) requiring for action the presence of free carboxyl group of MeGlcA side residues. These enzymes were not active on arabinoxylan, rhodymenan and xylotetraose, and conversion of MeGlcA to its methyl ester or its reduction to MeGlc led to a remarkable drop in their specific activity. However, some GH30-8 members are nonspecific xylanases effectively hydrolyzing all tested substrates. In terms of catalytic activities, the GH30-7 subfamily is much more diverse. In addition to specific glucuronoxylanases, the GH30-7 subfamily contains nonspecific endoxylanases and predominantly exo-acting enzymes. The activity of GH30-7 specific glucuronoxylanases also depend on the presence of the MeGlcA carboxyl, but not so strictly as in bacterial enzymes. The modification of the carboxyl group of glucuronoxylan had only weak effect on the action of predominantly exo-acting enzymes, as well as nonspecific xylanases. Rhodymenan and xylotetraose were the best substrates for exo-acting enzymes, while arabinoxylan represented hardly degradable substrate for almost all tested GH30-7 enzymes. The results expand current knowledge on the catalytic properties of this relatively novel group of xylanases.

Xylanases of glycoside hydrolase family 30 - An overview

Xylan is the most abundant hemicellulose in nature and as such it is a huge source of renewable carbon. Its bioconversion requires a battery of xylanolytic enzymes. Of them the most important are the endo-β-1,4-xylanases which depolymerize the polysaccharide into smaller fragments. Most of the xylanases are members of glycoside hydrolase (GH) families 10 and 11, although they are classified in some other GH families. The relatively new xylanases of GH30 are of special interest. Initially, they appeared to be specific glucuronoxylanases, however, other specificities were found later among prokaryotic and in particular eukaryotic enzymes. This review gives an overview of the substrate and product specificities observed for the GH30 xylanases characterized to date. An emphasis is given to the structure-activity relationship in order to explain how minor differences in catalytic centre and its vicinity can alter catalytic properties from the endoxylanase into the reducing end xylose releasing exoxylanase or into the non-reducing end xylobiohydrolase. Biotechnological potential of the GH30 xylanases is also considered.

A novel bacterial GH30 xylobiohydrolase from Hungateiclostridium clariflavum

Typical bacterial GH30 xylanases are glucuronoxylanases requiring 4-O-methylglucuronic acid (MeGlcA) substitution of a xylan main chain for their action. They do not exhibit a significant activity on neutral xylooligosaccharides, arabinoxylan (AraX), or rhodymenan (Rho). In this work, the biochemical characterization of the bacterial Clocl_1795 xylanase from Hungateiclostridium (Clostridium) clariflavum DSM 19732 (HcXyn30A) is presented. Amino acid sequence analysis of HcXyn30A revealed that the enzyme does not contain amino acids known to be responsible for MeGlcA coordination in the -2b subsite of glucuronoxylanases. This suggested that the catalytic properties of HcXyn30A may differ from those of glucuronoxylanases. HcXyn30A shows similar specific activity on glucuronoxylan (GX) and Rho, while the specific activity on AraX is about 1000 times lower. HcXyn30A releases Xyl₂ as the main product from the non-reducing end of different polymeric and oligomeric substrates. Catalytic properties of HcXyn30A resemble the properties of the fungal GH30 xylobiohydrolase from Acremonium alcalophilum, AaXyn30A. HcXyn30A is the first representative of a prokaryotic xylobiohydrolase. Its unique specificity broadens the catalytic diversity of bacterial GH30 xylanases. KEY POINTS: • Bacterial GH30 xylobiohydrolase from H. clariflavum (HcXyn30A) has been characterized. • HcXyn30A releases xylobiose from the non-reducing end of different substrates. • HcXyn30A is the first representative of bacterial xylobiohydrolase.

Crystal structure of GH30-7 endoxylanase C from the filamentous fungus Talaromyces cellulolyticus

GH30-7 endoxylanase C from the cellulolytic fungus Talaromyces cellulolyticus (TcXyn30C) belongs to glycoside hydrolase family 30 subfamily 7, and specifically releases 22-(4-O-methyl-α-D-glucuronosyl)-xylobiose from glucuronoxylan, as well as various arabino-xylooligosaccharides from arabinoxylan. TcXyn30C has a modular structure consisting of a catalytic domain and a C-terminal cellulose-binding module 1 (CBM1). In this study, the crystal structure of a TcXyn30C mutant which lacks the CBM1 domain was determined at 1.65 Å resolution. The structure of the active site of TcXyn30C was compared with that of the bifunctional GH30-7 xylanase B from T. cellulolyticus (TcXyn30B), which exhibits glucuronoxylanase and xylobiohydrolase activities. The results revealed that TcXyn30C has a conserved structural feature for recognizing the 4-O-methyl-α-D-glucuronic acid (MeGlcA) substituent in subsite -2b. Additionally, the results demonstrated that Phe47 contributes significantly to catalysis by TcXyn30C. Phe47 is located in subsite -2b and also near the C-3 hydroxyl group of a xylose residue in subsite -2a. Substitution of Phe47 with an arginine residue caused a remarkable decrease in the catalytic efficiency towards arabinoxylan, suggesting the importance of Phe47 in arabinoxylan hydrolysis. These findings indicate that subsite -2b of TcXyn30C has unique structural features that interact with arabinofuranose and MeGlcA substituents.

Substrate recognition by a bifunctional GH30-7 xylanase B from Talaromyces cellulolyticus

Xylanase B, a member of subfamily 7 of the GH30 (glycoside hydrolase family 30) from Talaromyces cellulolyticus (TcXyn30B), is a bifunctional enzyme with glucuronoxylanase and xylobiohydrolase activities. In the present study, crystal structures of the native enzyme and the enzyme–product complex of TcXyn30B expressed in Pichia pastoris were determined at resolutions of 1.60 and 1.65 Å, respectively. The enzyme complexed with 2²‐(4‐O‐methyl‐α‐d‐glucuronyl)‐xylobiose (U^4m2X) revealed that TcXyn30B strictly recognizes both the C‐6 carboxyl group and the 4‐O‐methyl group of the 4‐O‐methyl‐α‐d‐glucuronyl side chain by the conserved residues in GH30‐7 endoxylanases. The crystal structure and site‐directed mutagenesis indicated that Asn‐93 on the β2‐α2‐loop interacts with the non‐reducing end of the xylose residue at subsite‐2 and is likely to be involved in xylobiohydrolase activity. These findings provide structural insight into the mechanisms of substrate recognition of GH30‐7 glucuronoxylanase and xylobiohydrolase.

Highly alkali-stable and cellulase-free xylanases from Fusarium sp. 21 and their application in clarification of orange juice

Xylanase is a versatile tool in the food, fiber biobleaching and biofuel industries. Here, to discover new enzyme with special properties, we cloned three xylanases (Xyn11A, Xyn11B, and Xyn11C) by mining the genome of the xylanase producing fungus strain Fusarium sp. 21, biochemically characterized these enzyme and explored their potential application in juice processing. Both Xyn11A and Xyn11B had an optimal pH of 6.0 and optimal temperature of 45 °C, and retained >90% of the residual activity at pH range of 5-10.5 for 24 h. Xyn11C displayed the maximum activity at pH 5.0 and 45 °C and outstanding pH stability with a minimal loss of activity in the pH range of 2.0-10.5. These three xylanases displayed a strong specificity towards beechwood and corncob xylan, with no activity for other substrates. Xyn11A showed much a higher activity against corncob xylan, while Xyn11B and Xyn11C presented higher activities against beechwood xylan. Xyn11A catalyzed the hydrolysis of beechwood xylan with a K_m of 4.25 ± 0.29 mg·mL^-1 and k_cat/K_m of 30.34 ± 0.65 mL·s^-1·mg^-1, while the hydrolysis of corncob xylan had Km and k_cat/K_m values of 14.73 ± 1.43 mg·mL^-1and 26.48 ± 0.11 mL·s^-1·mg^-1, respectively. Xyn11B and Xyn11C hydrolyzed beechwood xylan with Km of 9.8 ± 0.69 mg·mL^-1, and 4.89 ± 0.38 mg·mL^-1and k_cat/K_m values of 45.07 ± 1.66 mL^-1·mg^-1, and 26.95 ± 0.67 mL·s^-1·mg^-1, respectively. Beechwood xylan hydrolysates catalyzed by these three xylanases contained xylobiose, xylotriose and xylooligosaccharides (XOS). The clarification of orange juice was improved when treated with these three xylanases. Conclusively, the desirable pH stability and substrate specificity make Xyn11A, Xyn11B and Xyn11C have high potential for application in fiber biobleaching, wine and fruit juice clarification, as well as probiotic XOS production.

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.

Purification and characterization of an endo-xylanase from Trichoderma sp., with xylobiose as the main product from xylan hydrolysis

Fungal endo-β-1,4-xylanases (endo-xylanases) can hydrolyze xylan into xylooligosaccharides (XOS), and have potential biotechnological applications for the exploitation of natural renewable polysaccharides. In the current study, we aimed to screen and characterize an efficient fungal endo-xylanase from 100 natural humus-rich soil samples collected in Guizhou Province, China, using extracted sugarcane bagasse xylan (SBX) as the sole carbon source. Initially, 182 fungal isolates producing xylanases were selected, among which Trichoderma sp. strain TP3-36 was identified as showing the highest xylanase activity of 295 U/mL with xylobiose (X2) as the main product when beechwood xylan was used as substrate. Subsequently, a glycoside hydrolase family 11 endo-xylanase, TXyn11A, was purified from strain TP3-36, and its optimal pH and temperature for activity against beechwood xylan were identified to be 5.0 and 55 °C, respectively. TXyn11A was stable across a broad pH range (3.0-10.0), and exhibited strict substrate specificity, including xylan from beechwood, wheat, rye, and sugarcane bagasse, with K_m and V_max values of 5 mg/mL and 1250 μmol/mg min, respectively, toward beechwood xylan. Intriguingly, the main product obtained from hydrolysis of beechwood xylan by TXyn11A was xylobiose, whereas SBX hydrolysis resulted in both X2 and xylotriose. Overall, these characteristics of the endo-xylanase TXyn11A indicate several potential industrial applications.

GH30-7 Endoxylanase C from the Filamentous Fungus Talaromyces cellulolyticus

Glycoside hydrolase family 30 subfamily 7 (GH30-7) enzymes include various types of xylanases, such as glucuronoxylanase, endoxylanase, xylobiohydrolase, and reducing-end xylose-releasing exoxylanase. Here, we characterized the mode of action and gene expression of the GH30-7 endoxylanase from the cellulolytic fungus Talaromyces cellulolyticus (TcXyn30C). TcXyn30C has a modular structure consisting of a GH30-7 catalytic domain and a C-terminal cellulose binding module 1, whose cellulose-binding ability has been confirmed. Sequence alignment of GH30-7 xylanases exhibited that TcXyn30C has a conserved Phe residue at the position corresponding to a conserved Arg residue in GH30-7 glucuronoxylanases, which is required for the recognition of the 4-O-methyl-α-d-glucuronic acid (MeGlcA) substituent. TcXyn30C degraded both glucuronoxylan and arabinoxylan with similar kinetic constants and mainly produced linear xylooligosaccharides (XOSs) with 2 to 3 degrees of polymerization, in an endo manner. Notably, the hydrolysis of glucuronoxylan caused an accumulation of 2²-(MeGlcA)-xylobiose (U^4m2X). The production of this acidic XOS is likely to proceed via multistep reactions by putative glucuronoxylanase activity that produces 2²-(MeGlcA)-XOSs (X _n U^4m2X, n ≥ 0) in the initial stages of the hydrolysis and by specific release of U^4m2X from a mixture containing X _n U^4m2X. Our results suggest that the unique endoxylanase activity of TcXyn30C may be applicable to the production of linear and acidic XOSs. The gene xyn30C was located adjacent to the putative GH62 arabinofuranosidase gene (abf62C) in the T. cellulolyticus genome. The expression of both genes was induced by cellulose. The results suggest that TcXyn30C may be involved in xylan removal in the hydrolysis of lignocellulose by the T. cellulolyticus cellulolytic system.IMPORTANCE Xylooligosaccharides (XOSs), which are composed of xylose units with a β-1,4 linkage, have recently gained interest as prebiotics in the food and feed industry. Apart from linear XOSs, branched XOSs decorated with a substituent such as methyl glucuronic acid and arabinose also have potential applications. Endoxylanase is a promising tool in producing XOSs from xylan. The structural variety of XOSs generated depends on the substrate specificity of the enzyme as well as the distribution of the substituents in xylan. Thus, the exploration of endoxylanases with novel specificities is expected to be useful in the provision of a series of XOSs. In this study, the endoxylanase TcXyn30C from Talaromyces cellulolyticus was characterized as a unique glycoside hydrolase belonging to the family GH30-7, which specifically releases 2²-(4-O-methyl-α-d-glucuronosyl)-xylobiose from hardwood xylan. This study provides new insights into the production of linear and branched XOSs by GH30-7 endoxylanase.

Penicillium purpurogenum produces a novel, acidic, GH3 beta-xylosidase: Heterologous expression and characterization of the enzyme

Xylan, a component of plant cell walls, is composed of a backbone of β-1,4-linked xylopyranosyl units with a number of substituents. The complete degradation of xylan requires the action of several enzymes, among them β-xylosidase. The fungus Penicillium purpurogenum secretes a number of enzymes participating in the degradation of xylan. In this study, a β-xylosidase from this fungus was expressed in Pichia pastoris, and characterized. This enzyme (Xyl2) is a member of glycoside hydrolase family 3; it consists of a sequence of 792 residues including a signal peptide of 20 residues, with a theoretical molecular mass for the mature protein of 84.2 KDa and an isoelectric point of 5.07. The highest identity with a characterized fungal enzyme, is with a β-xylosidase from Aspergillus oryzae (70%). The optimal activity of Xyl2 is found at pH 2.0 and 28 °C. The enzyme is most stable at pH 2.0 and conserves 40% of activity at 42 °C (after 1h incubation). The kinetic parameters for p-nitrophenyl-β-d-xylopyranoside are: K_M 0.53 mM, kcat 1*10⁷ s^-1 and kcat/K_M 1.9*10¹⁰ M^-1 s^-1. The enzyme is about 10% active on p-nitrophenyl-α-l-arabinofuranoside. Xyl2 exhibits a high hydrolytic activity on xylooligosaccharides; it liberates xylose from beechwood and birchwood glucuronoxylan and it acts synergistically with endoxylanases in the degradation of xylan. Its low pH optimum make this enzyme particularly useful in potential applications requiring a low pH such as increasing the flavor of wine.

A novel fungal GH30 xylanase with xylobiohydrolase auxiliary activity

BACKGROUND

The main representatives of hemicellulose are xylans, usually decorated β-1,4-linked d-xylose polymers, which are hydrolyzed by xylanases. The efficient utilization and complete hydrolysis of xylans necessitate the understanding of the mode of action of xylan degrading enzymes. The glycoside hydrolase family 30 (GH30) xylanases comprise a less studied group of such enzymes, and differences regarding the substrate recognition have been reported between fungal and bacterial GH30 xylanases. Besides their role in the utilization of lignocellulosic biomass for bioenergy, such enzymes could be used for the tailored production of prebiotic xylooligosaccharides (XOS) due to their substrate specificity.

RESULTS

The expression of a putative GH30_7 xylanase from the fungus Thermothelomyces thermophila (synonyms Myceliophthora thermophila, Sporotrichum thermophile) in Pichia pastoris resulted in the production and isolation of a novel xylanase with unique catalytic properties. The novel enzyme designated TtXyn30A, exhibited an endo- mode of action similar to that of bacterial GH30 xylanases that require 4-O-methyl-d-glucuronic acid (MeGlcA) decorations, in contrast to most characterized fungal ones. However, TtXyn30A also exhibited an exo-acting catalytic behavior by releasing the disaccharide xylobiose from the non-reducing end of XOS. The hydrolysis products from beechwood glucuronoxylan were MeGlcA substituted XOS, and xylobiose. The major uronic XOS (UXOS) were the aldotriuronic and aldotetrauronic acid after longer incubation indicating the ability of TtXyn30A to cleave linear parts of xylan and UXOS as well.

CONCLUSIONS

Hereby, we reported the heterologous production and biochemical characterization of a novel fungal GH30 xylanase exhibiting endo- and exo-xylanase activity. To date, considering its novel catalytic properties, TtXyn30A shows differences with most characterized fungal and bacterial GH30 xylanases. The discovered xylobiohydrolase mode of action offers new insights into fungal enzymatic systems that are employed for the utilization of lignocellulosic biomass. The recombinant xylanase could be used for the production of X2 and UXOS from glucuronoxylan, which in turn would be utilized as prebiotics carrying manifold health benefits.