Contribution of a family 1 carbohydrate-binding module in thermostable glycoside hydrolase 10 xylanase from Talaromyces cellulolyticus toward synergistic enzymatic hydrolysis of lignocellulose

A novel cellulolytic/xylanolytic SbAA14 from Sordaria brevicollis with a branched chain preference and its synergistic effects with glycoside hydrolases on lignocellulose

In this study, the novel auxiliary activity (AA) family 14 lytic polysaccharide monooxygenase (LPMO) SbAA14 from Sordaria brevicollis was successfully characterized. It was active against heteroxylan, xyloglucan and cellulose in β-cellulose and released native oligosaccharides and corresponding C1- and/or C4-oxidized products. SbAA14 showed a branched chain preference, because partial removal of arabinosyl substituents from heteroxylan led to a decrease in activity. SbAA14 had synergistic effects with the debranching enzyme EpABF62C in an enzyme- and ascorbic acid-dependent manner. SbAA14 had synergistic effects with the GH10 endoxylanase EpXYN1, and the degree of synergy was greater with step-by-step addition than with simultaneous addition. SbAA14 could also synergize with Celluclast® 1.5 L on NaOH-pretreated wheat straw and on NaOH-pretreated and hydrogen peroxide-acetic acid (HPAC)-H2SO4-pretreated bamboo substrates. The greatest synergistic effect between SbAA14 and Celluclast® 1.5 L was observed for HPAC-H2SO4-200 mM pretreated bamboo, in which the degree of synergy reached approximately 1.61. The distinctive substrate preference of SbAA14 indicated that it is a novel AA14 LPMO that may act mainly on heteroxylan with numerous arabinosyl substituents between cellulose fibers rather than on recalcitrant xylan tightly associated with cellulose. These findings broaden the understanding of enigmatic AA14 LPMOs and provide new insights into the substrate specificities and biological functionalities of AA14 LPMOs in fungi.

Functional identification of two novel carbohydrate-binding modules of glucuronoxylanase CrXyl30 and their contribution to the lignocellulose saccharification

BACKGROUND

Glycoside hydrolase (GH) family 30 xylanases are a distinct group of xylanases, most of which have a highly specific catalytic activity for glucuronoxylan. Since GH30 xylanases do not normally carry carbohydrate-binding modules (CBMs), our knowledge of the function of their CBMs is lacking.

RESULTS

In this work, the CBM functions of CrXyl30 were investigated. CrXyl30 was a GH30 glucuronoxylanase containing tandem CBM13 (CrCBM13) and CBM2 (CrCBM2) at its C terminus, which was identified in a lignocellulolytic bacterial consortium previously. Both CBMs could bind insoluble and soluble xylan, with CrCBM13 having binding specificity for the xylan with L-arabinosyl substitutions, whereas CrCBM2 targeted L-arabinosyl side chains themselves. Such binding abilities of these two CBMs were completely different from other CBMs in their respective families. Phylogenetic analysis also suggested that both CrCBM13 and CrCBM2 belong to novel branches. Inspection of the simulated structure of CrCBM13 identified a pocket that just accommodates the side chain of 3(2)-alpha-L-arabinofuranosyl-xylotriose, which forms hydrogen bonds with three of the five amino acid residues involved in ligand interaction. The truncation of either CrCBM13 or CrCBM2 did not alter the substrate specificity and optimal reaction conditions of CrXyl30, whereas truncation of CrCBM2 decreased the k_cat/K_m value by 83% (± 0%). Moreover, the absence of CrCBM2 and CrCBM13 resulted in a 5% (± 1%) and a 7% (± 0%) decrease, respectively, in the amount of reducing sugar released by the synergistic hydrolysis of delignified corncob whose hemicellulose is arabinoglucuronoxylan, respectively. In addition, fusion of CrCBM2 with a GH10 xylanase enhanced its catalytic activity against the branched xylan and improved the synergistic hydrolysis efficiency by more than fivefold when delignified corncob was used as substrate. Such a strong stimulation of hydrolysis resulted from the enhancement of hemicellulose hydrolysis on the one hand, and the cellulose hydrolysis is also improved according to the lignocellulose conversion rate measured by HPLC.

CONCLUSIONS

This study identifies the functions of two novel CBMs in CrXyl30 and shows the good potential of such CBMs specific for branched ligands in the development of efficient enzyme preparations.

A thermostable and CBM2-linked GH10 xylanase from Thermobifida fusca for paper bleaching

Xylanases have the potential to be used as bio-deinking and bio-bleaching materials and their application will decrease the consumption of the chlorine-based chemicals currently used for this purpose. However, xylanases with specific properties could act effectively, such as having significant thermostability and alkali resistance, etc. In this study, we found that TfXyl10A, a xylanase from Thermobifida fusca, was greatly induced to transcript by microcrystalline cellulose (MCC) substrate. Biochemical characterization showed that TfXyl10A is optimally effective at temperature of 80 °C and pH of 9.0. After removing the carbohydrate-binding module (CBM) and linker regions, the optimum temperature of TfXyl10A-CD was reduced by 10°C (to 70°C), at which the enzyme’s temperature tolerance was also weakened. While truncating only the CBM domain (TfXyl10AdC) had no significant effect on its thermostability. Importantly, polysaccharide-binding experiment showed that the auxiliary domain CBM2 could specifically bind to cellulose substrates, which endowed xylanase TfXyl10A with the ability to degrade xylan surrounding cellulose. These results indicated that TfXyl10A might be an excellent candidate in bio-bleaching processes of paper industry. In addition, the features of active-site architecture of TfXyl10A in GH10 family were further analyzed. By mutating each residue at the -2 and -1 subsites to alanine, the binding force and enzyme activity of mutants were observably decreased. Interestingly, the mutant E51A, locating at the distal -3 subsite, exhibited 90% increase in relative activity compared with wild-type (WT) enzyme TfXyl10A-CD (the catalytic domain of TfXyl110A). This study explored the function of a GH10 xylanase containing a CBM2 domain and the contribution of amino acids in active-site architecture to catalytic activity. The results obtained provide guidance for the rational design of xylanases for industrial applications under high heat and alkali-based operating conditions, such as paper bleaching.

Comparison of the Biochemical Properties and Roles in the Xyloglucan-Rich Biomass Degradation of a GH74 Xyloglucanase and Its CBM-Deleted Variant from Thielavia terrestris

Xyloglucan is closely associated with cellulose and still retained with some modification in pretreated lignocellulose; however, its influence on lignocellulose biodegradation is less understood. TtGH74 from Thielavia terrestris displayed much higher catalytic activity than previously characterized fungal GH74 xyloglucanases. The carbohydrate-binding module 1 (CBM1) deleted variant (TtGH74ΔCBM) had the same optimum temperature and pH but an elevated thermostability. TtGH74 displayed a high binding affinity on xyloglucan and cellulose, while TtGH74ΔCBM completely lost the adsorption capability on cellulose. Their hydrolysis action alone or in combination with other glycoside hydrolases on the free xyloglucan, xyloglucan-coated phosphoric acid-swollen cellulose or pretreated corn bran and apple pomace was compared. CBM1 might not be essential for the hydrolysis of free xyloglucan but still effective for the associated xyloglucan to an extent. TtGH74 alone or synergistically acting with the CBH1/EG1 mixture was more effective in the hydrolysis of xyloglucan in corn bran, while TtGH74ΔCBM showed relatively higher catalytic activity on apple pomace, indicating that the role and significance of CBM1 are substrate-specific. The degrees of synergy for TtGH74 or TtGH74ΔCBM with the CBH1/EG1 mixture reached 1.22–2.02. The addition of GH10 xylanase in TtGH74 or the TtGH74ΔCBM/CBH1/EG1 mixture further improved the overall hydrolysis efficiency, and the degrees of synergy were up to 1.50–2.16.

A Fungal Versatile GH10 Endoxylanase and Its Glycosynthase Variant: Synthesis of Xylooligosaccharides and Glycosides of Bioactive Phenolic Compounds

The study of endoxylanases as catalysts to valorize hemicellulosic residues and to obtain glycosides with improved properties is a topic of great industrial interest. In this work, a GH10 β-1,4-endoxylanase (XynSOS), from the ascomycetous fungus Talaromyces amestolkiae, has been heterologously produced in Pichia pastoris, purified, and characterized. rXynSOS is a highly glycosylated monomeric enzyme of 53 kDa that contains a functional CBM1 domain and shows its optimal activity on azurine cross-linked (AZCL)-beechwood xylan at 70 °C and pH 5. Substrate specificity and kinetic studies confirmed its versatility and high affinity for beechwood xylan and wheat arabinoxylan. Moreover, rXynSOS was capable of transglycosylating phenolic compounds, although with low efficiencies. For expanding its synthetic capacity, a glycosynthase variant of rXynSOS was developed by directed mutagenesis, replacing its nucleophile catalytic residue E236 by a glycine (rXynSOS-E236G). This novel glycosynthase was able to synthesize β-1,4-xylooligosaccharides (XOS) of different lengths (four, six, eight, and ten xylose units), which are known to be emerging prebiotics. rXynSOS-E236G was also much more active than the native enzyme in the glycosylation of a broad range of phenolic compounds with antioxidant properties. The interesting capabilities of rXynSOS and its glycosynthase variant make them promising tools for biotechnological applications.

Pretreatment Affects Profits From Xylanase During Enzymatic Saccharification of Corn Stover Through Changing the Interaction Between Lignin and Xylanase Protein

Effective pretreatment is vital to improve the biomass conversion efficiency, which often requires the addition of xylanase as an accessory enzyme to enhance enzymatic saccharification of corn stover. In this study, we investigated the effect of two sophisticated pretreatment methods including ammonium sulfite (AS) and steam explosion (SE) on the xylanase profits involved in enzymatic hydrolysis of corn stover. We further explored the interactions between lignin and xylanase Xyn10A protein. Our results showed that the conversion rates of glucan and xylan in corn stover by AS pretreatment were higher by Xyn10A supplementation than that by SE pretreatment. Compared with the lignin from SE pretreated corn stover, the lignin from AS pretreated corn stover had a lower Xyn10A initial adsorption velocity (13.56 vs. 10.89 mg g⁻¹ min⁻¹) and adsorption capacity (49.46 vs. 27.42 mg g⁻¹ of lignin) and weakened binding strength (310.6 vs. 215.9 L g⁻¹). Our study demonstrated the low absolute zeta potential and strong hydrophilicity of the lignin may partly account for relative weak interaction between xylanase protein and lignin from AS pretreated corn stover. In conclusion, our results suggested that AS pretreatment weakened the inhibition of lignin to enzyme, promoted the enzymatic hydrolysis of corn stover, and decreased the cost of enzyme in bioconversion.

Lignocellulose biomass bioconversion during composting: Mechanism of action of lignocellulase, pretreatment methods and future perspectives

Composting is a biodegradation and transformation process that converts lignocellulosic biomass into value-added products, such as humic substances (HSs). However, the recalcitrant nature of lignocellulose hinders the utilization of cellulose and hemicellulose, decreasing the bioconversion efficiency of lignocellulose. Pretreatment is an essential step to disrupt the structure of lignocellulosic biomass. Many pretreatment methods for composting may cause microbial inactivation and death. Thus, the pretreatment methods suitable for composting can promote the degradation and transformation of lignocellulosic biomass. Therefore, this review summarizes the pretreatment methods suitable for composting. Microbial consortium pretreatment, Fenton pretreatment and surfactant-assisted pretreatment for composting may improve the bioconversion process. Microbial consortium pretreatment is a cost-effective pretreatment method to enhance HSs yields during composting. On the other hand, the efficiency of enzyme production during composting is very important for the degradation of lignocellulose, whose action mechanism is unknown. Therefore, this review describes the mechanism of action of lignocellulase, the predominant microbes producing lignocellulase and their related genes. Finally, optimizing pretreatment conditions and increasing enzymatic hydrolysis to improve the quality of composts by controlling suitable microenvironmental factors and core target microbial activities as a research focus in the bioconversion of lignocellulose during composting in the future.

Identification and characterization of a thermostable GH11 xylanase from Paenibacillus campinasensis NTU-11 and the distinct roles of its carbohydrate-binding domain and linker sequence

An extracellular thermostable xylanase (XynNTU) from Paenibacillus campinasensis NTU-11, consisted of a glycoside hydrolase (GH) family 11 catalytic domain, a Gly/Pro-rich linker sequence (LS) and a family 6 carbohydrate-binding module (CBM6), was identified and expressed in E. coli BL21. The purified XynNTU had a specific activity of 2750 U/mg and an optimal activity at 60 °C and pH 7.0, and retained a residual activity of 58.4% after incubation (60 °C, 48 h). Two truncated mutants, CBM6-truncated form XynNTU-CDLS, CBM6 and linker-truncated form XynNTU-CD, possessed similar values of optimum pH and temperature as the native XynNTU. XynNTU-CD displayed a lower thermostability than XynNTU, whereas for XynNTU-CDLS, more than 90% of residual activity was remained (60 °C, 48 h), indicating that this enzyme presented a higher thermostability than that of the majority of reported GH11 xylanases. Furthermore, XynNTU and two mutants maintained more than 70% of residual activity at pH values of 5-9. Kinetic measurements suggested that CBM6 had a crucial function in the ability of the enzyme to bind and hydrolyze xylan substrates, while LS had a relatively mild influence. Collectively, a noticeable thermostability and a high specific activity of XynNTU and its truncated form XynNTU-CDLS highlights their potentials for diverse 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.

Cooperation of hydrolysis modes among xylanases reveals the mechanism of hemicellulose hydrolysis by Penicillium chrysogenum P33

Background

Xylanases randomly cleave the internal β-1,4-glycosidic bonds in the xylan backbone and are grouped into different families in the carbohydrate-active enzyme (CAZy) database. Although multiple xylanases are detected in single strains of many filamentous fungi, no study has been reported on the composition, synergistic effect, and mode of action in a complete set of xylanases secreted by the same microorganism.

Results

All three xylanases secreted by Penicillium chrysogenum P33 were expressed and characterized. The enzymes Xyl1 and Xyl3 belong to the GH10 family and Xyl3 contains a CBM1 domain at its C-terminal, whereas Xyl2 belongs to the GH11 family. The optimal temperature/pH values were 35 °C/6.0, 50 °C/5.0 and 55 °C/6.0 for Xyl1, Xyl2, and Xyl3, respectively. The three xylanases exhibited synergistic effects, with the maximum synergy observed between Xyl3 and Xyl2, which are from different families. The synergy between xylanases could also improve the hydrolysis of cellulase (C), with the maximum amount of reducing sugars (5.68 mg/mL) observed using the combination of C + Xyl2 + Xyl3. Although the enzymatic activity of Xyl1 toward xylan was low, it was shown to be capable of hydrolyzing xylooligosaccharides into xylose. Xyl2 was shown to hydrolyze xylan to long-chain xylooligosaccharides, whereas Xyl3 hydrolyzed xylan to xylooligosaccharides with a lower degree of polymerization.

Conclusions

Synergistic effect exists among different xylanases, and it was higher between xylanases from different families. The cooperation of hydrolysis modes comprised the primary mechanism for the observed synergy between different xylanases. This study demonstrated, for the first time, that the hydrolysates of GH11 xylanases can be further hydrolyzed by GH10 xylanases, but not vice versa.

Mode of Action of GH30-7 Reducing-End Xylose-Releasing Exoxylanase A (Xyn30A) from the Filamentous Fungus Talaromyces cellulolyticus

In this study, we characterized the mode of action of reducing-end xylose-releasing exoxylanase (Rex), which belongs to the glycoside hydrolase family 30-7 (GH30-7). GH30-7 Rex, isolated from the cellulolytic fungus Talaromyces cellulolyticus (Xyn30A), exists as a dimer. The purified Xyn30A released xylose from linear xylooligosaccharides (XOSs) 3 to 6 xylose units in length with similar kinetic constants. Hydrolysis of branched, borohydride-reduced, and p-nitrophenyl XOSs clarified that Xyn30A possesses a Rex activity. ¹H nuclear magnetic resonance (¹H NMR) analysis of xylotriose hydrolysate indicated that Xyn30A degraded XOSs via a retaining mechanism and without recognizing an anomeric structure at the reducing end. Hydrolysis of xylan by Xyn30A revealed that the enzyme continuously liberated both xylose and two types of acidic XOSs: 2²-(4-O-methyl-α-d-glucuronyl)-xylotriose (MeGlcA²Xyl₃) and 2²-(MeGlcA)-xylobiose (MeGlcA²Xyl₂). These acidic products were also detected during hydrolysis using a mixture of MeGlcA²Xyl _n (n = 2 to 14) as the substrate. This indicates that Xyn30A can release MeGlcA²Xyl _n (n = 2 and 3) in an exo manner. Comparison of subsites in Xyn30A and GH30-7 glucuronoxylanase using homology modeling suggested that the binding of the reducing-end residue at subsite +2 was partially prevented by a Gln residue conserved in GH30-7 Rex; additionally, the Arg residue at subsite -2b, which is conserved in glucuronoxylanase, was not found in Xyn30A. Our results lead us to propose that GH30-7 Rex plays a complementary role in hydrolysis of xylan by fungal cellulolytic systems.IMPORTANCE Endo- and exo-type xylanases depolymerize xylan and play crucial roles in the assimilation of xylan in bacteria and fungi. Exoxylanases release xylose from the reducing or nonreducing ends of xylooligosaccharides; this is generated by the activity of endoxylanases. β-Xylosidase, which hydrolyzes xylose residues on the nonreducing end of a substrate, is well studied. However, the function of reducing-end xylose-releasing exoxylanases (Rex), especially in fungal cellulolytic systems, remains unclear. This study revealed the mode of xylan hydrolysis by Rex from the cellulolytic fungus Talaromyces cellulolyticus (Xyn30A), which belongs to the glycoside hydrolase family 30-7 (GH30-7). A conserved residue related to Rex activity is found in the substrate-binding site of Xyn30A. These findings will enhance our understanding of the function of GH30-7 Rex in the cooperative hydrolysis of xylan by fungal enzymes.

The composition of accessory enzymes of Penicillium chrysogenum P33 revealed by secretome and synergistic effects with commercial cellulase on lignocellulose hydrolysis

Herein, we report the secretome of Penicillium chrysogenum P33 under induction of lignocellulose for the first time. A total of 356 proteins were identified, including complete cellulases and numerous hemicellulases. Supplementing a commercial cellulase with increasing dosage of P33 enzyme cocktail from 1 to 5 mg/g substrate increased the release of reducing sugars from delignified corn stover by 21.4% to 106.8%. When 50% cellulase was replaced by P33 enzyme cocktail, release of reducing sugars was 78.6% higher than with cellulase alone. Meanwhile, glucan and xylan conversion was increased by 37% and 106%, respectively. P33 enzyme cocktail also enhanced commercial cellulase hydrolysis against four different delignified lignocellulosic biomass. These findings demonstrate that mixing appropriate amount of P33 cocktail with cellulase improves polysaccharide hydrolysis, suggesting P33 enzymes have great potential for industrial applications.

Effect of CBM1 and linker region on enzymatic properties of a novel thermostable dimeric GH10 xylanase (Xyn10A) from filamentous fungus Aspergillus fumigatus Z5

Xylanase with a high thermostability will satisfy the needs of raising the temperature of hydrolysis to improve the rheology of the broth in industry of biomass conversion. In this study, a xylanase gene (xyn10A), predicted to encode a hydrolase domain of GH10, a linker region and a CBM1 domain, was cloned from a superior lignocellulose degrading strain Aspergillus fumigatus Z5 and successfully expressed in Pichia pastoris X33. Xyn10A has a specific xylanase activity of 34.4 U mg⁻¹, and is optimally active at 90 °C and pH 6.0. Xyn10A shows quite stable at pHs ranging from 3.0 to 11.0, and keeps over 40% of xylanase activity after incubation at 70 °C for 1 h. Removal of CBM1 domain has a slight negative effect on its thermostability, but the further cleavage of linker region significantly decreased its stability at high temperature. The transfer of CBM1 and linker region to another GH10 xylanase can help to increase the thermostability. In addition, hydrolase domains between the two Xyn10A proteins naturally formed a dimer structure, which became more thermostable after removing the CBM1 or/and linker region. This thermostable Xyn10A is a suitable candidate for the highly efficient fungal enzyme cocktails for biomass conversion.

Simultaneous Silencing of Xylanase Genes in Botrytis cinerea

The endo-β-1,4-xylanase BcXyn11A is one of several plant cell-wall degrading enzymes that the phytopathogenic fungus Botrytis cinerea secretes during interaction with its hosts. In addition to its enzymatic activity, this protein also acts as an elicitor of the defense response in plants and has been identified as a virulence factor. In the present work, other four endoxylanase coding genes (Bcxyn11B, Bcxyn11C, Bcxyn10A, and Bcxyn10B) were identified in the B. cinerea genome and the expression of all five genes was analyzed by Q-RT- PCR in vitro and in planta. A cross-regulation between xylanase genes was identified analyzing their expression pattern in the ΔBcxyn11A mutant strain and a putative BcXyn11A-dependt induction of Bcxyn10B gene was found. In addition, multiple knockdown strains were obtained for the five endoxylanase genes by transformation of B. cinerea with a chimeric DNA construct composed of 50-nt sequences from the target genes. The silencing of each xylanase gene was analyzed in axenic cultures and during infection and the results showed that the efficiency of the multiple silencing depends on the growth conditions and on the cross-regulation between them. Although the simultaneous silencing of the five genes was observed by Q-RT-PCR when the silenced strains were grown on medium supplemented with tomato extract, the endoxylanase activity measured in the supernatants was reduced only by 40%. Unexpectedly, the silenced strains overexpressed the Bcxyn11A and Bcxyn11C genes during the infection of tomato leaves, making difficult the analysis of the role of the endo-β-1,4-xylanases in the virulence of the fungus.

A novel GH10 xylanase from Penicillium sp. accelerates saccharification of alkaline-pretreated bagasse by an enzyme from recombinant Trichoderma reesei expressing Aspergillus β-glucosidase

BACKGROUND

Trichoderma reesei is considered a candidate fungal enzyme producer for the economic saccharification of cellulosic biomass. However, performance of the saccharifying enzymes produced by T. reesei is insufficient. Therefore, many attempts have been made to improve its performance by heterologous protein expression. In this study, to increase the conversion efficiency of alkaline-pretreated bagasse to sugars, we conducted screening of biomass-degrading enzymes that showed synergistic effects with enzyme preparations produced by recombinant T. reesei.

RESULTS

Penicillium sp. strain KSM-F532 produced the most effective enzyme to promote the saccharification of alkaline-pretreated bagasse. Biomass-degrading enzymes from strain KSM-F532 were fractionated and analyzed, and a xylanase, named PspXyn10, was identified. The amino acid sequence of PspXyn10 was determined by cDNA analysis: the enzyme shows a modular structure consisting of glycoside hydrolase family 10 (GH10) and carbohydrate-binding module family 1 (CBM1) domains. Purified PspXyn10 was prepared from the supernatant of a recombinant T. reesei strain. The molecular weight of PspXyn10 was estimated to be 55 kDa, and its optimal temperature and pH for xylanase activity were 75 °C and pH 4.5, respectively. More than 80% of the xylanase activity was maintained at 65 °C for 10 min. With beechwood xylan as the substrate, the enzyme had a K_m of 2.2 mg/mL and a V_max of 332 μmol/min/mg. PspXyn10ΔCBM, which lacked the CBM1 domain, was prepared by limited proteolysis. PspXyn10ΔCBM showed increased activity against soluble xylan, but decreased saccharification efficiency of alkaline-pretreated bagasse. This result indicated that the CBM1 domain of PspXyn10 contributes to the enhancement of the saccharification efficiency of alkaline-pretreated bagasse. A recombinant T. reesei strain, named X2PX10, was constructed from strain X3AB1. X3AB1 is an Aspergillus aculeatus β-glucosidase-expressing T. reesei PC-3-7. X2PX10 also expressed PspXyn10 under the control of the xyn2 promoter. An enzyme preparation from X2PX10 showed almost the same saccharification efficiency of alkaline-pretreated bagasse at half the enzyme dosage as that used for an enzyme preparation from X3AB1.

CONCLUSIONS

Our results suggest that PspXyn10 promotes the saccharification of alkaline-pretreated bagasse more efficiently than TrXyn3, a GH10 family xylanase from T. reesei, and that the PspXyn10-expressing strain is suitable for enzyme production for biomass saccharification.

Characterisation of three fungal glucuronoyl esterases on glucuronic acid ester model compounds

The glucuronoyl esterases (GEs) that have been identified so far belong to family 15 of the carbohydrate esterases in the CAZy classification system and are presumed to target ester bonds between lignin alcohols and (4-O-methyl-)d-glucuronic acid residues of xylan. Few GEs have been cloned, expressed and characterised to date. Characterisation has been done on a variety of synthetic substrates; however, the number of commercially available substrates is very limited. We identified novel putative GEs from a wide taxonomic range of fungi and expressed the enzymes originating from Acremonium alcalophilum and Wolfiporia cocos as well as the previously described PcGE1 from Phanerochaete chrysosporium. All three fungal GEs were active on the commercially available compounds benzyl glucuronic acid (BnGlcA), allyl glucuronic acid (allylGlcA) and to a lower degree on methyl glucuronic acid (MeGlcA). The enzymes showed pH stability over a wide pH range and tolerated 6-h incubations of up to 50 °C. Kinetic parameters were determined for BnGlcA. This study shows the suitability of the commercially available model compounds BnGlcA, MeGlcA and allylGlcA in GE activity screening and characterisation experiments. We enriched the spectrum of characterised GEs with two new members of a relatively young enzyme family. Due to its biotechnological significance, this family deserves to be more extensively studied. The presented enzymes are promising candidates as auxiliary enzymes to improve saccharification of plant biomass.

Two degradation strategies for overcoming the recalcitrance of natural lignocellulosic xylan by polysaccharides-binding GH10 and GH11 xylanases of filamentous fungi

The recalcitrance of lignocellulose forms a strong barrier for the bioconversion of lignocellulosic biomass in chemical or biofuel industries. Filamentous fungi are major plant biomass decomposer, and capable of forming all the required enzymes. Here, they characterized the GH10 and GH11 endo-xylanases and a CE1 acetyl-xylan esterase (Axe1) from a superior biomass-degrading strain, Aspergillus fumigatus Z5, and examined how they interact in xylan degradation. Cellulose-binding (CBM1) domain inhibited GH10 xylanase activities for pure xylan, but afforded them an ability to hydrolyze washed corncob particles (WCCP). CBM1-containing GH10 xylanases also showed synergism with CBM1-containing Axe1 in WCCP hydrolysis, and this synergy was strictly dependent on the presence of their CBM1 domains. In contrast, GH11 xylanases had no CBM1, but still could bind xylan and hydrolyzed WCCP; however, no synergism displayed with Axe1. GH10 xylanases and GH11 xylanases showed a pronounced synergism in WCCP hydrolysis, which was dependent on the presence of the CBM1 in GH10 xylanases and absence from GH11 xylanases. They exhibit different mechanisms to bind to cellulose and xylan, and act in synergy when these two structures are intact. These findings will be helpful for the further development of highly efficient enzyme mixtures for lignocellulosic biomass conversion.

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.

Insights into the mechanism of enzymatic hydrolysis of xylan

Hemicelluloses are a vast group of complex, non-cellulosic heteropolysaccharides that are classified according to the principal monosaccharides present in its structure. Xylan is the most abundant hemicellulose found in lignocellulosic biomass. In the current trend of a more effective utilization of lignocellulosic biomass and developments of environmentally friendly industrial processes, increasing research activities have been directed to a practical application of the xylan component of plants and plant residues as biopolymer resources. A variety of enzymes, including main- and side-chain acting enzymes, are responsible for xylan breakdown. Xylanase is a main-chain enzyme that randomly cleaves the β-1,4 linkages between the xylopyranosyl residues in xylan backbone. This enzyme presents varying folds, mechanisms of action, substrate specificities, hydrolytic activities, and physicochemical characteristics. This review pays particular attention to different aspects of the mechanisms of action of xylan-degrading enzymes and their contribution to improve the production of bioproducts from plant biomass. Furthermore, the influence of phenolic compounds on xylanase activity is also discussed.

Sexual crossing of thermophilic fungus Myceliophthora heterothallica improved enzymatic degradation of sugar beet pulp

BACKGROUND

Enzymatic degradation of plant biomass requires a complex mixture of many different enzymes. Like most fungi, thermophilic Myceliophthora species therefore have a large set of enzymes targeting different linkages in plant polysaccharides. The majority of these enzymes have not been functionally characterized, and their role in plant biomass degradation is unknown. The biotechnological challenge is to select the right set of enzymes to efficiently degrade a particular biomass. This study describes a strategy using sexual crossing and screening with the thermophilic fungus Myceliophthora heterothallica to identify specific enzymes associated with improved sugar beet pulp saccharification.

RESULTS

Two genetically diverse M. heterothallica strains CBS 203.75 and CBS 663.74 were used to generate progenies with improved growth on sugar beet pulp. One progeny, named SBP.F1.2.11, had a different genetic pattern from the parental strains and had improved saccharification activity after the growth on 3 % sugar beet pulp. The improved SBP saccharification was not explained by altered activities of the major (hemi-)cellulases. Exo-proteome analysis of progeny and parental strains after 7-day growth on sugar beet pulp showed that only 17 of the 133 secreted CAZy enzymes were more abundant in progeny SBP.F1.2.11. Particularly one enzyme belonging to the carbohydrate esterase family 5 (CE5) was more abundant in SBP.F1.2.11. This CE5-CBM1 enzyme, named as Axe1, was phylogenetically related to acetyl xylan esterases. Biochemical characterization of Axe1 confirmed de-acetylation activity with optimal activities at 75-85 °C and pH 5.5-6.0. Supplementing Axe1 to CBS 203.75 enzyme set improved release of xylose and glucose from sugar beet pulp.

CONCLUSIONS

This study identified beneficial enzymes for sugar beet pulp saccharification by selecting progeny with improved growth on this particular substrate. Saccharification of sugar beet pulp was improved by supplementing enzyme mixtures with a previously uncharacterized CE5-CBM1 acetyl xylan esterase. This shows that sexual crossing and selection of M. heterothallica are the successful strategy to improve the composition of enzyme mixtures for efficient plant biomass degradation.