Effect of endoxylanase and α-L-arabinofuranosidase supplementation on the enzymatic hydrolysis of steam exploded wheat straw

Synergistic effects of distinct arabinofuranosidase specificities in lignocellulose degradation by different hemicellulases

Arabinoxylan is a prevalent hemicellulose type, notably heterogeneous and resistant to biodegradation. Arabinofuranosidases (Abfs) remove arabinofuranosyl decorations of arabinoxylan, thus enabling hydrolysis by xylanases. However, a variety of Abf and xylanase specificities have emerged in recent years, necessitating a deeper understanding of their role in biomass degradation. This work investigates the biochemical features of TtAbf43 from Thermothelomyces thermophila, which specifically removes the O-3-linked arabinofuranose moieties from di-substituted xylopyranoses. The enzyme also exhibited secondary hydrolytic activity on o-nitrophenyl-β-d-xylopyranoside and arabinan. The hydrolysis of pretreated wheat and corn bran substrates was assessed using TtAbf43 and AnAbf51, two enzymes with distinct catalytic specificities. The Abfs enhanced the performance of endo-xylanases TmXyn10 and AnXyn11, promoting the release of xylo-oligomers, while the xylanases, in turn, stimulated arabinose release by the Abfs. Additionally, the Abfs facilitated the endo- and exo-activities of the bifunctional xylobiohydrolase/glucuronoxylanase TtXyn30A for the release of xylobiose and short aldouronic acids from biomass. AnAbf51 also acted in synergy with the acetyl xylan esterase OCE6 and the exo-deacetylase TtCE16B in debranching enzymatically derived oligomers from lignocellulose, whereas TtAbf43 remained unaffected by the esterases. These diverse synergistic relationships among different hemicellulases could assist the development of new enzymatic approaches for efficient biomass valorization.

Bioprospecting of endophytic fungi from semidesert candelilla (Euphorbia antisyphilitica Zucc): Potential for extracellular enzyme production

Endophytic microbial communities colonize plants growing under various abiotic stress conditions. Candelilla (Euphorbia antisyphilitica Zucc.) is a shrub that develops functionally in arid and semi-arid zones of Mexico; these conditions generate an association between the plant and the microorganisms, contributing to the production of enzymes as a defense mechanism for resistance to abiotic stress. The objective of this research was to isolate and identify endophyte fungi of candelilla and bioprospection of these endophytic fungi for enzyme production using candelilla by-products. Fungi were isolated and identified using ITS1/ITS4 sequencing. Their potency index (PI) was evaluated in producing endoglucanase, xylanase, amylase, and laccase. Fermentation was carried out at 30°C for 8 days at 200 rpm, with measurements every 2 days, using candelilla by-products as substrate. All fungi exhibited higher cellulase, amylase, and laccase activities on the 2nd, 6th, and 8th day of fermentation, respectively, of fermentation. The fungus Aspergillus niger ITD-IN4.1 showed the highest amylase activity (246.84 U/mg), the genus Neurospora showed the highest cellulase activity, reaching up to 13.45 FPU/mg, and the strain Neurospora sp. ITD-IN5.2 showed the highest laccase activity (3.46 U/mg). This work provides the first report on the endophytic diversity of E. antisyphilitica and its potential role in enzyme production.

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

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

Continuous-Flow Microreactor Accelerates Molecular Collisions for Lignin Depolymerization to Phenolic Monomers and Oligomers

Effective depolymerization of lignin is the most important step for its comprehensive utilization. So far, most of the studies on depolymerization of lignin focused on batch processing, whereas only a few studies relied on the microreactor. In this study, we developed a continuous-flow microreactor for depolymerization of lignin into monomeric and oligomeric compounds. The yields of monomers and oligomers can be adjusted by varying the temperature, pressure, residence time, NaOH dosage, and solvent. Under optimized conditions, the lignin conversion rate was 77.73 wt %, and the monomer yield was 13.26 wt %, with 77.81% being phenolic compounds. In addition, comparative characterizations on the raw lignin and products demonstrated that the oil products were mainly composed of phenolic tetramers and trimers, and the effective cleavage of the β-O-4 linkage of S-type lignin was responsible for the high yield of 2,6-dimethoxyphenol. It indicated that raw lignin could be effectively depolymerized continuously using the continuous-flow microreactor, and it will be a new strategy for comprehensive utilization of lignin to produce fine-chemical intermediates.

Highly efficient synergistic activity of an α-L-arabinofuranosidase for degradation of arabinoxylan in barley/wheat

Here, an α-L-arabinofuranosidase (termed TtAbf62) from Thermothelomyces thermophilus is described, which efficiently removes arabinofuranosyl side chains and facilitates arabinoxylan digestion. The specific activity of TtAbf62 (179.07 U/mg) toward wheat arabinoxylan was the highest among all characterized glycoside hydrolase family 62 enzymes. TtAbf62 in combination with endoxylanase and β-xylosidase strongly promoted hydrolysis of barley and wheat. The release of reducing sugars was significantly higher for the three-enzyme combination relative to the sum of single-enzyme treatments: 85.71% for barley hydrolysis and 33.33% for wheat hydrolysis. HPLC analysis showed that TtAbf62 acted selectively on monosubstituted (C-2 or C-3) xylopyranosyl residues rather than double-substituted residues. Site-directed mutagenesis and interactional analyses of enzyme-substrate binding structures revealed the catalytic sites of TtAbf62 formed different polysaccharide-catalytic binding modes with arabinoxylo-oligosaccharides. Our findings demonstrate a "multienzyme cocktail" formed by TtAbf62 with other hydrolases strongly improves the efficiency of hemicellulose conversion and increases biomass hydrolysis through synergistic interaction.

Expression and Characterization of Two α-l-Arabinofuranosidases from Talaromyces amestolkiae: Role of These Enzymes in Biomass Valorization

α-l-arabinofuranosidases are glycosyl hydrolases that catalyze the break between α-l-arabinofuranosyl substituents or between α-l-arabinofuranosides and xylose from xylan or xylooligosaccharide backbones. While they belong to several glycosyl hydrolase (GH) families, there are only 24 characterized GH62 arabinofuranosidases, making them a small and underrepresented group, with many of their features remaining unknown. Aside from their applications in the food industry, arabinofuranosidases can also aid in the processing of complex lignocellulosic materials, where cellulose, hemicelluloses, and lignin are closely linked. These materials can be fully converted into sugar monomers to produce secondary products like second-generation bioethanol. Alternatively, they can be partially hydrolyzed to release xylooligosaccharides, which have prebiotic properties. While endoxylanases and β-xylosidases are also necessary to fully break down the xylose backbone from xylan, these enzymes are limited when it comes to branched polysaccharides. In this article, two new GH62 α-l-arabinofuranosidases from Talaromyces amestolkiae (named ARA1 and ARA-2) have been heterologously expressed and characterized. ARA-1 is more sensitive to changes in pH and temperature, whereas ARA-2 is a robust enzyme with wide pH and temperature tolerance. Both enzymes preferentially act on arabinoxylan over arabinan, although ARA-1 has twice the catalytic efficiency of ARA-2 on this substrate. The production of xylooligosaccharides from arabinoxylan catalyzed by a T. amestolkiae endoxylanase was significantly increased upon pretreatment of the polysaccharide with ARA-1 or ARA-2, with the highest synergism values reported to date. Finally, both enzymes (ARA-1 or ARA-2 and endoxylanase) were successfully applied to enhance saccharification by combining them with a β-xylosidase already characterized from the same fungus.

Endophytes in Lignin Valorization: A Novel Approach

Lignin, one of the essential components of lignocellulosic biomass, comprises an abundant renewable aromatic resource on the planet earth. Although 15%––40% of lignocellulose pertains to lignin, its annual valorization rate is less than 2% which raises the concern to harness and/or develop effective technologies for its valorization. The basic hindrance lies in the structural heterogeneity, complexity, and stability of lignin that collectively makes it difficult to depolymerize and yield common products. Recently, microbial delignification, an eco-friendly and cheaper technique, has attracted the attention due to the diverse metabolisms of microbes that can channelize multiple lignin-based products into specific target compounds. Also, endophytes, a fascinating group of microbes residing asymptomatically within the plant tissues, exhibit marvellous lignin deconstruction potential. Apart from novel sources for potent and stable ligninases, endophytes share immense ability of depolymerizing lignin into desired valuable products. Despite their efficacy, ligninolytic studies on endophytes are meagre with incomplete understanding of the pathways involved at the molecular level. In the recent years, improvement of thermochemical methods has received much attention, however, we lagged in exploring the novel microbial groups for their delignification efficiency and optimization of this ability. This review summarizes the currently available knowledge about endophytic delignification potential with special emphasis on underlying mechanism of biological funnelling for the production of valuable products. It also highlights the recent advancements in developing the most intriguing methods to depolymerize lignin. Comparative account of thermochemical and biological techniques is accentuated with special emphasis on biological/microbial degradation. Exploring potent biological agents for delignification and focussing on the basic challenges in enhancing lignin valorization and overcoming them could make this renewable resource a promising tool to accomplish Sustainable Development Goals (SDG’s) which are supposed to be achieved by 2030.

Development of different pretreatments and related technologies for efficient biomass conversion of lignocellulose

Lignocellulose, a kind of biological resource widely existing in nature, which can be transformed into value-added biochemical products through saccharification, fermentation or chemical catalysis. Pretreatments are the necessary step to increase the accessibility and digestibility of lignocellulose. This paper comprehensively reviewed different pretreatment progress of lignocellulose in recent year, including mechanical/thermal, biological, inorganic solvent, organic solvent and unconventional physical-chemical pretreatments, focusing on quantifying the influence of pretreatments on subsequent biomass conversion. In addition, related pretreatment techniques such as genetic engineering, reactor configurations, downstream process and visualization technology of pretreatment were discussed. Finally, this review presented the challenge of lignocellulose pretreatment in the future.

Coordinately express hemicellulolytic enzymes in Kluyveromyces marxianus to improve the saccharification and ethanol production from corncobs

BACKGROUND

Hemicellulose acts as one factor contributing to the recalcitrance of lignocellulose that prevents cellulases to degrade the cellulose efficiently even in low quantities. Supplement of hemicellulases can enhance the performance of commercial cellulases in the enzymatic hydrolyses of lignocellulose. Kluyveromyce marxianus is an attractive yeast for cellulosic ethanol fermentation, as well as a promising host for heterologous protein production, since it has remarkable thermotolerance, high growth rate, and broad substrate spectrum etc. In this study, we attempted to coordinately express multiple hemicellulases in K. marxianus through a 2A-mediated ribosome skipping to self-cleave polyproteins, and investigated their capabilities for saccharification and ethanol production from corncobs.

RESULTS

Two polycistronic genes IMPX and IMPαX were constructed to test the self-cleavage of P2A sequence from the Foot-and-Mouth Disease virus (FMDV) in K. marxianus. The IMPX gene consisted of a β-mannanase gene M330 (without the stop codon), a P2A sequence and a β-xylanase gene Xyn-CDBFV in turn. In the IMPαX gene, there was an additional α-factor signal sequence in frame with the N-terminus of Xyn-CDBFV. The extracellular β-mannanase activities of the IMPX and IMPαX strains were 21.34 and 15.50 U/mL, respectively, but the extracellular β-xylanase activity of IMPαX strain was much higher than that of the IMPX strain, which was 136.17 and 42.07 U/mL, respectively. Subsequently, two recombinant strains, the IXPαR and IMPαXPαR, were constructed to coordinately and secretorily express two xylantic enzymes, Xyn-CDBFV and β-D-xylosidase RuXyn1, or three hemicellulolytic enzymes including M330, Xyn-CDBFV and RuXyn1. In fed-batch fermentation, extracellular activities of β-xylanase and β-xylosidase in the IXPαR strain were 1664.2 and 0.90 U/mL. Similarly, the IMPαXPαR strain secreted the three enzymes, β-mannanase, β-xylanase, and β-xylosidase, with the activities of 159.8, 2210.5, and 1.25 U/mL, respectively. Hemicellulolases of both strains enhanced the yields of glucose and xylose from diluted acid pretreated (DAP) corncobs when acted synergistically with commercial cellulases. In hybrid saccharification and fermentation (HSF) of DAP corncobs, hemicellulases of the IMPαXPαR strain increased the ethanol yield by 8.7% at 144 h compared with the control. However, both ethanol and xylose yields were increased by 12.7 and 18.2%, respectively, at 120 h in HSF of aqueous ammonia pretreated (AAP) corncobs with this strain. Our results indicated that coordinate expression of hemicellulolytic enzymes in K. marxianus promoted the saccharification and ethanol production from corncobs.

CONCLUSIONS

The FMDV P2A sequence showed high efficiency in self-cleavage of polyproteins in K. marxianus and could be used for secretory expression of multiple enzymes in the presence of their signal sequences. The IMPαXPαR strain coexpressed three hemicellulolytic enzymes improved the saccharification and ethanol production from corncobs, and could be used as a promising strain for ethanol production from lignocelluloses.

Arabinofuranosidases: Characteristics, microbial production, and potential in waste valorization and industrial applications

Alpha-L-arabinofuranoside arabinofuranohydrolase (ARA), more commonly known as alpha-L-arabinofuranosidase (E.C. number 3.2.1.55), is a hydrolytic enzyme, catalyzing the cleavage of alpha-L-arabinose by acting on the non-reducing ends of alpha-L-arabinofuranosides, alpha-L-arabinans containing (1,3)- and/or (1,5)-linked arabinoxylans and arabinogalactans. ARA functions as debranching enzyme removing arabinose substituents from arabinoxylan and arabinoxylooligomers, thereby, boosting the hydrolysis of arabinoxylan fraction of hemicellulose and improving bioconversion of lignocellulosic biomass. Previously, comprehensive information on this enzyme has not been reviewed thoroughly. Therefore, the main aim of this review is to highlight the important properties of this interesting enzyme, microorganisms used for its production, and enhanced production using genetic engineering approach. An account on synergism with other biomass hydrolyzing enzymes and various industrial applications of this enzyme has also been provided along with an outlook on further research and development.

Improved ethanol production from the slurry of pretreated brewers' spent grain through different co-fermentation strategies

The aim of this work was to bioconvert all sugars in BSG into ethanol using a process scheme that includes the enzymatic hydrolysis of the whole slurry resulting from the pretreatment of BSG with phosphoric and sulfuric acid using previously optimised conditions, followed by the co-fermentation of the mixed sugars. More than 90% of the sugars in raw BSG were recovered in the pretreatment and the subsequent enzymatic hydrolysis of the whole slurry. The co-fermentation of the enzymatic hydrolysates with Escherichia coli was then compared with that the co-culture of Scheffersomyces stipitis and Saccharomyces cerevisiae, which resulted in lower ethanol production. The co-fermentation strategy with a single microorganism (E. coli) when BSG was pretreated with phosphoric acid resulted into the highest ethanol concentration, 39 g/L, which means that 222 L of ethanol can be obtained from a ton of BSG without detoxification requirements.

Synergism of cellulase, pectinase and xylanase on hydrolyzing differently pretreated sweet potato residues

The synergism of cellulase (C), pectinase (P), and xylanase (X) for the saccharification of sweet potato residues (SPR) was investigated. The removal of starch from SPR was easily achieved by using amylase, but the cellulose conversion of de-starched SPR was relatively low, thus dilute H₂SO₄, NaOH, and H₂O₂ pretreatment was conducted to improve the enzymatic digestibility. The lignin content of NaOH pretreated SPR was the lowest, whereas H₂SO₄ pretreatment resulted in the lowest contents of hemicellulose and pectin. The combination of C, P, and X exhibited different sugar production patterns, C-P displayed synergistic action on glucose and galactose production from each type of SPR, C-X also exhibited synergistic effect on glucose production except when H₂SO₄ pretreated SPR was used, whereas no synergism between P-X on monosaccharide production was observed. The presence of synergism between cellulase and mixed accessory enzymes [C-(PX)] on glucose formation was determined by C-X, and the degree of synergism between C-P and C-(PX) on glucose production had a positive relationship with pectin content. The highest cellulose conversion of 96.2% was obtained from NaOH pretreated SPR using mixed enzymes comprising C, P, and X with the ratio of 8:1:1.

The boosting effect of recombinant hemicellulases on the enzymatic hydrolysis of steam-treated sugarcane bagasse

To increase the efficiency of enzyme cocktails in deconstructing cellulose and hemicelluloses present in the plant cell wall, a combination of enzymes with complementary activities is required. Xylan is the main hemicellulose component of energy crops and for its complete hydrolysis a system consisting of several enzymes acting cooperatively, including endoxylanases (XYN), β-xylosidases (XYL) and α-l-arabinofuranosidases (ABF) is necessary. The current work aimed at evaluating the effect of recombinant hemicellulolytic enzymes on the enzymatic hydrolysis of steam-exploded sugarcane bagasse (SEB). One recombinant endoxylanase (HXYN2) and one recombinant β-xylosidase (HXYLA) from Humicola grisea var thermoidea, together with an α-l-arabinofuranosidase (AFB3) from Penicillium pupurogenum, all produced in Pichia pastoris, were used to formulate an efficient enzyme mixture for SEB hydrolysis using a 2³ Central Composite Rotatable Design (CCRD). The most potent enzyme for SEB hydrolysis was ABF3. Subsequently, the optimal enzyme mixture was used in combination with commercial cellulases (Accellerase 1500), either simultaneously or in sequential experiments. The supplementation of Accellerase 1500 with hemicellulases enhanced the glucose yield from SEB hydrolysis by 14.6%, but this effect could be raised to 50% when hemicellulases were added prior to hydrolysis with commercial cellulases. These results were supported by scanning electron microscopy, which revealed the effect of enzymatic hydrolysis on SEB fibers. Our results show the potential of complementary enzyme activities to improve enzymatic hydrolysis of SEB, thus improving the efficiency of the hydrolytic process.

Impacts of cellulase deactivation at the moving air-liquid interface on cellulose conversions at low enzyme loadings

BACKGROUND

We recently confirmed that the deactivation of T. reesei cellulases at the air-liquid interface reduces microcrystalline cellulose conversion at low enzyme loadings in shaken flasks. It is one of the main causes for lowering of cellulose conversions at low enzyme loadings. However, supplementing cellulases with small quantities of surface-active additives in shaken flasks can increase cellulose conversions at low enzyme loadings. It was also shown that cellulose conversions at low enzyme loadings can be increased in unshaken flasks if the reactions are carried for a longer time. This study further explores these recent findings to better understand the impact of air-liquid interfacial phenomena on enzymatic hydrolysis of cellulose contained in Avicel, Sigmacell, α-cellulose, cotton linters, and filter paper. The impacts of solids and enzyme loadings, supplementation with nonionic surfactant Tween 20 and xylanases, and application of different types of mixing and reactor designs on cellulose hydrolysis were also evaluated.

RESULTS

Avicel cellulose conversions at high solid loading were more than doubled by minimizing loss of cellulases to the air-liquid interface. Maximum cellulose conversions were high for surface-active supplemented shaken flasks or unshaken flasks because of low cellulase deactivation at the air-liquid interface. The nonionic surfactant Tween 20 was unable to completely prevent cellulase deactivation in shaken flasks and only reduced cellulose conversions at unreasonably high concentrations.

CONCLUSIONS

High dynamic interfacial areas created through baffles in reactor vessels, low volumes in high-capacity vessels, or high shaking speeds severely limited cellulose conversions at low enzyme loadings. Precipitation of cellulases due to aggregation at the air-liquid interface caused their continuous deactivation in shaken flasks and severely limited solubilization of cellulose.

Insight into the role of α-arabinofuranosidase in biomass hydrolysis: cellulose digestibility and inhibition by xylooligomers

BACKGROUND

α-l-Arabinofuranosidase (ARA), a debranching enzyme that can remove arabinose substituents from arabinoxylan and arabinoxylooligomers (AXOS), promotes the hydrolysis of the arabinoxylan fraction of biomass; however, the impact of ARA on the overall digestibility of cellulose is controversial. In this study, we investigated the effects of the addition of ARA on cellulase hydrolytic action.

RESULTS

We found that approximately 15% of the xylan was converted into AXOS during the hydrolysis of aqueous ammonia-pretreated corn stover and that this AXOS fraction was approximately 12% substituted with arabinose. The addition of ARA removes a portion of the arabinose decoration, but the resulting less-substituted AXOS inhibited cellulase action much more effectively; showing an increase of 45.7%. Kinetic experiments revealed that AXOS with a lower degree of arabinose substitution showed stronger affinity for the active site of cellobiohydrolase, which could be the mechanism of increased inhibition.

CONCLUSIONS

Our findings strongly suggest that the ratio of ARA and other xylanases should be carefully selected to avoid the strong inhibition caused by the less-substituted AXOS during the hydrolysis of arabinoxylan-containing biomass. This study advances our understanding of the inhibitory mechanism of xylooligomers and provides critical new insights into the relationship of ARA addition and cellulose digestibility.

Molecular and physiological basis of Saccharomyces cerevisiae tolerance to adverse lignocellulose-based process conditions

Lignocellulose-based biorefineries have been gaining increasing attention to substitute current petroleum-based refineries. Biomass processing requires a pretreatment step to break lignocellulosic biomass recalcitrant structure, which results in the release of a broad range of microbial inhibitors, mainly weak acids, furans, and phenolic compounds. Saccharomyces cerevisiae is the most commonly used organism for ethanol production; however, it can be severely distressed by these lignocellulose-derived inhibitors, in addition to other challenging conditions, such as pentose sugar utilization and the high temperatures required for an efficient simultaneous saccharification and fermentation step. Therefore, a better understanding of the yeast response and adaptation towards the presence of these multiple stresses is of crucial importance to design strategies to improve yeast robustness and bioconversion capacity from lignocellulosic biomass. This review includes an overview of the main inhibitors derived from diverse raw material resultants from different biomass pretreatments, and describes the main mechanisms of yeast response to their presence, as well as to the presence of stresses imposed by xylose utilization and high-temperature conditions, with a special emphasis on the synergistic effect of multiple inhibitors/stressors. Furthermore, successful cases of tolerance improvement of S. cerevisiae are highlighted, in particular those associated with other process-related physiologically relevant conditions. Decoding the overall yeast response mechanisms will pave the way for the integrated development of sustainable yeast cell-based biorefineries.

The mechanism by which a distinguishing arabinofuranosidase can cope with internal di-substitutions in arabinoxylans

BACKGROUND

Arabinoxylan is an abundant polysaccharide in industrially relevant biomasses such as sugarcane, corn stover and grasses. However, the arabinofuranosyl di-substitutions that decorate the xylan backbone are recalcitrant to most known arabinofuranosidases (Abfs).

RESULTS

In this work, we identified a novel GH51 Abf (XacAbf51) that forms trimers in solution and can cope efficiently with both mono- and di-substitutions at terminal or internal xylopyranosyl units of arabinoxylan. Using mass spectrometry, the kinetic parameters of the hydrolysis of 33-α-l-arabinofuranosyl-xylotetraose and 23,33-di-α-l-arabinofuranosyl-xylotetraose by XacAbf51 were determined, demonstrating the capacity of this enzyme to cleave arabinofuranosyl linkages of internal mono- and di-substituted xylopyranosyl units. Complementation studies of fungal enzyme cocktails with XacAbf51 revealed an increase of up to 20% in the release of reducing sugars from pretreated sugarcane bagasse, showing the biotechnological potential of a generalist GH51 in biomass saccharification. To elucidate the structural basis for the recognition of internal di-substitutions, the crystal structure of XacAbf51 was determined unveiling the existence of a pocket strategically arranged near to the - 1 subsite that can accommodate a second arabinofuranosyl decoration, a feature not described for any other GH51 Abf structurally characterized so far.

CONCLUSIONS

In summary, this study reports the first kinetic characterization of internal di-substitution release by a GH51 Abf, provides the structural basis for this activity and reveals a promising candidate for industrial processes involving plant cell wall depolymerization.

Integral process assessment of sugarcane agricultural crop residues conversion to ethanol

This work focuses a whole process assessment on post-harvesting sugarcane residues for 2G ethanol production by different saccharification-fermentation conditions at high solids loading, performed after steam explosion, alkaline and acidic pretreatments. Carbohydrate recoveries and enzymatic digestibility results showed that alkali and steam explosion pretreatments were effective for the biomass assayed. Due to a significant improvement (60%) of the glucose released by combining hemicellulases and cellulases only after the NaOH pretreatment, the most favorable process settled comprised an alkali-based pretreatment followed by a pre-saccharification and simultaneous saccharification and fermentation (PSSF). The produced ethanol reached 4.8% (w/w) as a result of an 80% conversion of the glucose from the pretreated biomass. Finally, an ethanol concentration of 3.2% (w/w) was obtained by means of a steam explosion followed by PSSF, representing a suitable start point to further develop a low environmental impact alternative for ethanol production.

Review of Second Generation Bioethanol Production from Residual Biomass

In the context of climate change and the depletion of fossil fuels, there is a great need for alternatives to petroleum in the transport sector. This review provides an overview of the production of second generation bioethanol, which is distinguished from the first generation and subsequent generations of biofuels by its use of lignocellulosic biomass as raw material. The structural components of the lignocellulosic biomass such as cellulose, hemicellulose and lignin, are presented along with technological unit steps including pretreatment, enzymatic hydrolysis, fermentation, distillation and dehydration. The purpose of the pretreatment step is to increase the surface area of carbohydrate available for enzymatic saccharification, while minimizing the content of inhibitors. Performing the enzymatic hydrolysis releases fermentable sugars, which are converted by microbial catalysts into ethanol. The hydrolysates obtained after the pretreatment and enzymatic hydrolysis contain a wide spectrum of sugars, predominantly glucose and xylose. Genetically engineered microorganisms are therefore needed to carry out co-fermentation. The excess of harmful inhibitors in the hydrolysate, such as weak organic acids, furan derivatives and phenol components, can be removed by detoxification before fermentation. Effective saccharification further requires using exogenous hemicellulases and cellulolytic enzymes. Conventional species of distiller's yeast are unable to ferment pentoses into ethanol, and only a very few natural microorganisms, including yeast species like Candida shehatae, Pichia (Scheffersomyces) stipitis, and Pachysolen tannophilus, metabolize xylose to ethanol. Enzymatic hydrolysis and fermentation can be performed in a number of ways: by separate saccharification and fermentation, simultaneous saccharification and fermentation or consolidated bioprocessing. Pentose-fermenting microorganisms can be obtained through genetic engineering, by introducing xylose-encoding genes into metabolism of a selected microorganism to optimize its use of xylose accumulated in the hydrolysate.

Microwave-Assisted Oxalic Acid Pretreatment for the Enhancing of Enzyme Hydrolysis in the Production of Xylose and Arabinose from Bagasse

In this study, highly-efficient hydrolysis of bagasse into xylose and arabinose sugars (C5 sugars) was developed by microwave-assisted oxalic acid pretreatment under mild reaction conditions. The effects of acid and hydrolysis conditions on the C5 sugar yields were discussed. The results showed that oxalic acid performed better than hydrochloric acid and maleic acid, and was a promising alternative to sulfuric acid for xylose production at the same acid concentration. The maximum yields of xylose (95.7%) and arabinose (91.5%) were achieved via the microwave-assisted oxalic acid pretreatment (120 °C, 10 min, 0.4 mol/L, solid–liquid ratio of 1:50 g/mL), indicating that almost all xylan-type hemicelluloses were released from the cell wall and hydrolyzed into C5 sugars. After pretreatment, more than 90% of the cellulose in the residual bagasse was converted to glucose (92.2%) by enzymatic hydrolysis. This approach could realize the highly-efficient hydrolysis of xylan from bagasse into C5 sugars, which would enhance the enzyme hydrolysis of treated bagasse into glucose.

Deactivation of Cellulase at the Air-Liquid Interface Is the Main Cause of Incomplete Cellulose Conversion at Low Enzyme Loadings

Amphiphilic additives such as bovine serum albumin (BSA) and Tween have been used to improve cellulose hydrolysis by cellulases. However, there has been a lack of clarity to explain their mechanism of action in enzymatic hydrolysis of pure or low-lignin cellulosic substrates. In this work, a commercial Trichoderma reesei enzyme preparation and the amphiphilic additives BSA and Tween 20 were applied for hydrolysis of pure Avicel cellulose. The results showed that these additives only had large effects on cellulose conversion at low enzyme to substrate ratios when the reaction flasks were shaken. Furthermore, changes in the air-liquid interfacial area profoundly affected cellulose conversion, but surfactants reduced or prevented cellulase deactivation at the air-liquid interface. Not shaking the flasks or adding low amounts of surfactant resulted in near theoretical cellulose conversion at low enzyme loadings given enough reaction time. At low enzyme loadings, hydrolysis of cellulose in lignocellulosic biomass with low lignin content suffered from enhanced enzyme deactivation at the air-liquid interface.

Characterization of cellulolytic microbial consortium enriched on Napier grass using metagenomic approaches

Energy grass is a promising substrate for production of biogas by anaerobic digestion. However, the conversion efficiency is limited by the enzymatically recalcitrant nature of cellulosic wastes. In this study, an active, structurally stable mesophilic lignocellulolytic degrading microbial consortium (Np-LMC) was constructed from forest compost soil microbiota by successive subcultivation on Napier grass under facultative anoxic conditions. According to tagged 16S rRNA gene amplicon sequencing, increasing abundance of facultative Proteobacteria was found in the middle of batch cycle which was then subsequently replaced by the cellulose degraders Firmicutes and Bacteroidetes along with decreasing CMCase, xylanase, and β-glucanase activity profiles in the supernatant after 5 days of incubation. Anaerobic/facultative bacteria Dysgonomonas and Sedimentibacter and aerobic bacteria Comamonas were the major genera found in Np-LMC. The consortium was active on degradation of the native and delignified grass. Direct shotgun sequencing of the consortium metagenome revealed relatively high abundance of genes encoding for various lignocellulose degrading enzymes in 23 glycosyl hydrolase (GH) families compared to previously reported cellulolytic microbial communities in mammalian digestive tracts. Enzymes attacking cellulose and hemicellulose were dominated by GH2, 3, 5, 9, 10, 26, 28 and 43 in addition to a variety of carbohydrate esterases (CE) and auxiliary activities (AA), reflecting adaptation of the enzyme systems to the native herbaceous substrate. The consortium identified here represents the microcosm specifically bred on energy grass, with potential for enhancing degradation of fibrous substrates in bioenergy industry.

Contrasted enzymatic cocktails reveal the importance of cellulases and hemicellulases activity ratios for the hydrolysis of cellulose in presence of xylans

Various enzymatic cocktails were produced from two Trichoderma reesei strains, a cellulase hyperproducer strain and a strain with β-glucosidase activity overexpression. By using various carbon sources (lactose, glucose, xylose, hemicellulosic hydrolysate) for strains growth, contrasted enzymatic activities were obtained. The enzymatic cocktails presented various levels of efficiency for the hydrolysis of cellulose Avicel into glucose, in presence of xylans, or not. These latter were also hydrolyzed with different extents according to cocktails. The most efficient cocktails (TR1 and TR3) on Avicel were richer in filter paper activity (FPU) and presented a low ratio FPU/β-glucosidase activity. Cocktails TR2 and TR5 which were produced on the higher amount of hemicellulosic hydrolysate, possess both high xylanase and β-xylosidase activities, and were the most efficient for xylans hydrolysis. When hydrolysis of Avicel was conducted in presence of xylans, a decrease of glucose release occurred for all cocktails compared to hydrolysis of Avicel alone. Mixing TR1 and TR5 cocktails with two different ratios of proteins (1/1 and 1/4) resulted in a gain of efficiency for glucose release during hydrolysis of Avicel in presence of xylans compared to TR5 alone. Our results demonstrate the importance of combining hemicellulase and cellulase activities to improve the yields of glucose release from Avicel in presence of xylans. In this context, strategies involving enzymes production with carbon sources comprising mixed C5 and C6 sugars or combining different cocktails produced on C5 or on C6 sugars are of interest for processes developed in the context of lignocellulosic biorefinery.

Biological pretreatment of lignocellulosic biomass--An overview

Pretreatment is an important step involved in the production of bioethanol from lignocelluosic biomass. Though several pretreatment regimes are available, biological pretreatment seems to be promising being an eco-friendly process and there is no inhibitor generation during the process. In the current scenario there are few limitations in using this strategy for pilot scale process. The first and foremost one is the long incubation time for effective delignification. This can be minimized to an extent by using suitable microbial consortium. There is an urgent need for research and development activities and fine tuning of the process for the development of an economically viable process. This review presents an overview of various aspects of biological pretreatment, enzymes involved in the process, parameters affecting biological pretreatment as well as future perspectives.

Facilitating the enzymatic saccharification of pulped bamboo residues by degrading the remained xylan and lignin-carbohydrates complexes

Kraft pulping was performed on bamboo residues and its impact on the chemical compositions and the enzymatic digestibility of the samples were investigated. To improve the digestibility of sample by degrading the xylan and lignin-carbohydrates complexes (LCCs), xylanase and α-L-arabinofuranosidase (AF) were supplemented with cellulase. The results showed more carbohydrates were remained in the samples pulped with low effective alkali (EA) charge, compared to conventional kraft pulping. When 120 IU/g xylanase and 15 IU/g AF were supplemented with 20 FPU/g cellulase, the xylan degradation yield of the sample pulped with 12% EA charge increased from 68.20% to 88.35%, resulting in an increased enzymatic saccharification efficiency from 58.98% to 83.23%. The amount of LCCs in this sample decreased from 8.63/100C9 to 2.99/100C9 after saccharification with these enzymes. The results indicated that degrading the remained xylan and LCCs in the pulp could improve its enzymatic digestibility.

Characterization and identification of the xylanolytic enzymes from Aspergillus fumigatus Z5

BACKGROUND

Plant biomass, the most abundant natural material on earth, represents a vast source of food and energy in nature. As the main component of plant biomass, xylan is a complex polysaccharide comprising a linear β(1,4)-linked backbone of xylosyl residues substituted by acetyl, arabinosyl, glucuronysyl and 4-O-methylglucuronycyl residues.

RESULTS

Aspergillus fumigatus Z5 is an efficient plant biomass depolymerization fungus. In this study, its crude xylanolytic enzymes were characterized and identified by two-dimensional gel electrophoresis (2-DE). The optimal temperature for the crude xylanases was close to 60 °C, the highest xylanase activity was achieved at pH ranged from 3 to 6, and the crude xylanases also showed a very broad region of pH (3-11) stability. The maximal xylanase activity of 21.45 U · ml(-1) was observed in the fourth day of cultivation at 50 °C and 150 rpm with 2 % xylan as the sole carbon source. Zymogram analysis indicated that there were more than seven secreted proteins with xylanase activity. In the crude enzyme, two major endoxylanases, five cellulases and several associated enzymes were identified to be involved in the hydrolysis of polysaccharides. Of the total 13 xylanase genes in the Z5 genome, 11 were observed using q-PCR to be induced by xylan, one of which, An endo-1,4-β-xylanase with a low secretion level, was also expressed and characterized. The final hydrolysis products of xylan by crude enzyme mainly consisted of xylobiose.

CONCLUSIONS

This study provides a comprehensive understanding of the depolymerization of xylan by Z5 and will help to design enzymatic strategies for plant biomass utilization.

Unraveling the effects of laccase treatment on enzymatic hydrolysis of steam-exploded wheat straw

Laccase enzymes are promising detoxifying agents during lignocellulosic bioethanol production from wheat straw. However, they affect the enzymatic hydrolysis of this material by lowering the glucose recovery yields. This work aimed at explaining the negative effects of laccase on enzymatic hydrolysis. Relative glucose recovery in presence of laccase (10IU/g substrate) with model cellulosic substrate (Sigmacell) at 10% (w/v) was almost 10% points lower (P<0.01) than in the absence of laccase. This fact could be due to an increase in the competition of cellulose binding sites between the enzymes and a slight inhibition of β-glucosidase activity. However, enzymatic hydrolysis and infrared spectra of laccase-treated and untreated wheat straw filtered pretreated residue (WS-FPR), revealed that a grafting process of phenoxy radicals onto the lignin fiber could be the cause of diminished accessibility of cellulases to cellulose in pretreated wheat straw.

Competitive inhibition of cellobiohydrolase I by manno-oligosaccharides

In the hydrolysis of softwood, significant amounts of manno-oligosaccharides (MOS) are released from mannan, the major hemicelluloses in softwood. However, the impact of MOS on the performance of cellulases is not yet clear. In this work, the effect of mannan and MOS in cellulose hydrolysis by cellulases, especially cellobiohydrolase I (CBHI) from Thermoascus aurantiacus (Ta Cel7A), was studied. The glucose yield of Avicel decreased with an increasing amount of added mannan. Commercial cellulases contained mannan hydrolysing enzymes, and β-glucosidase played an important role in mannan hydrolysis. Addition of 10mg/ml mannan reduced the glucose yield of Avicel (at 20g/l) from 40.1 to 24.3%. No inhibition of β-glucosidase by mannan was observed. The negative effects of mannan and MOS on the hydrolytic action of cellulases indicated that the inhibitory effect was at least partly attributed to the inhibition of Ta Cel7A (CBHI), but not on β-glucosidase. Kinetic experiments showed that MOS were competitive inhibitors of the CBHI from T. aurantiacus, and mannobiose had a stronger inhibitory effect on CBHI than mannotriose or mannotetraose. For efficient hydrolysis of softwood, it was necessary to add supplementary enzymes to hydrolyze both mannan and MOS to less inhibitory product, mannose.

Synergism of cellulase, xylanase, and pectinase on hydrolyzing sugarcane bagasse resulting from different pretreatment technologies

Sugarcane bagasse (SCB) resulting from different pretreatments was hydrolyzed by enzyme cocktails based on replacement of cellulase (Celluclast 1.5 L:Novozym 188=1FPU:4pNPGU) by xylanase or pectinase at different proportions. Lignin content of NaOH pretreated SCB and hemicellulose content of H2SO4 pretreated SCB were the lowest. NaOH pretreatment showed the best for monosaccharide production among the four pretreatments. Synergism was apparently observed between cellulase and xylanase for monosaccharide production from steam exploded SCB (SESB), NaOH, and H2O2 pretreated SCB. No synergism was observed between cellulase and pectinase for producing glucose. Additionally, no synergism was present when H2SO4 pretreated SCB was used. Replacement of 20% of the cellulase by xylanase enhanced the glucose yield by 6.6%, 8.8%, and 9.5% from SESB, NaOH, and H2O2 pretreated SCB, respectively. Degree of synergism between cellulase and xylanase had positive relationship with xylan content and was affected by hydrolysis time.

Fed-batch SSCF using steam-exploded wheat straw at high dry matter consistencies and a xylose-fermenting Saccharomyces cerevisiae strain: effect of laccase supplementation

BACKGROUND

Lignocellulosic bioethanol is expected to play an important role in fossil fuel replacement in the short term. Process integration, improvements in water economy, and increased ethanol titers are key considerations for cost-effective large-scale production. The use of whole steam-pretreated slurries under high dry matter (DM) conditions and conversion of all fermentable sugars offer promising alternatives to achieve these goals.

RESULTS

Wheat straw slurry obtained from steam explosion showed high concentrations of degradation compounds, hindering the fermentation performance of the evolved xylose-recombinant Saccharomyces cerevisiae KE6-12 strain. Fermentability tests using the liquid fraction showed a higher number of colony-forming units (CFU) and higher xylose consumption rates when treating the medium with laccase. During batch simultaneous saccharification and co-fermentation (SSCF) processes, cell growth was totally inhibited at 12% DM (w/v) in untreated slurries. However, under these conditions laccase treatment prior to addition of yeast reduced the total phenolic content of the slurry and enabled the fermentation. During this process, an ethanol concentration of 19 g/L was obtained, corresponding to an ethanol yield of 39% of the theoretical yield. By changing the operation from batch mode to fed-batch mode, the concentration of inhibitors at the start of the process was reduced and 8 g/L of ethanol were obtained in untreated slurries with a final consistency of 16% DM (w/v). When fed-batch SSCF medium was supplemented with laccase 33 hours after yeast inoculation, no effect on ethanol yield or cell viability was found compared to untreated fermentations. However, if the laccase supplementation (21 hours after yeast inoculation) took place before the first addition of substrate (at 25 hours), improved cell viability and an increased ethanol titer of up to 32 g/L (51% of the theoretical) were found.

CONCLUSIONS

Laccase treatment in SSCF processes reduces the inhibitory effect that degradation compounds have on the fermenting microorganism. Furthermore, in combination with fed-batch operational mode, laccase supplementation allows the fermentation of wheat straw slurry at high DM consistencies, improving final ethanol concentrations and yields.

The capability of endophytic fungi for production of hemicellulases and related enzymes

Background

There is an imperative necessity for alternative sources of energy able to reduce the world dependence of fossil oil. One of the most successful options is ethanol obtained mainly from sugarcane and corn fermentation. The foremost residue from sugarcane industry is the bagasse, a rich lignocellulosic raw material uses for the production of ethanol second generation (2G). New cellulolytic and hemicellulytic enzymes are needed, in order to optimize the degradation of bagasse and production of ethanol 2G.

Results

The ability to produce hemicellulases and related enzymes, suitable for lignocellulosic biomass deconstruction, was explored using 110 endophytic fungi and 9 fungi isolated from spoiled books in Brazil. Two initial selections were performed, one employing the esculin gel diffusion assay, and the other by culturing on agar plate media with beechwood xylan and liquor from the hydrothermal pretreatment of sugar cane bagasse. A total of 56 isolates were then grown at 29°C on steam-exploded delignified sugar cane bagasse (DEB) plus soybean bran (SB) (3:1), with measurement of the xylanase, pectinase, β-glucosidase, CMCase, and FPase activities. Twelve strains were selected, and their enzyme extracts were assessed using different substrates. Finally, the best six strains were grown under xylan and pectin, and several glycohydrolases activities were also assessed. These strains were identified morphologically and by sequencing the internal transcribed spacer (ITS) regions and the partial β-tubulin gene (BT2). The best six strains were identified as Aspergillus niger DR02, Trichoderma atroviride DR17 and DR19, Alternaria sp. DR45, Annulohypoxylon stigyum DR47 and Talaromyces wortmannii DR49. These strains produced glycohydrolases with different profiles, and production was highly influenced by the carbon sources in the media.

Conclusions

The selected endophytic fungi Aspergillus niger DR02, Trichoderma atroviride DR17 and DR19, Alternaria sp. DR45, Annulohypoxylon stigyum DR47 and Talaromyces wortmannii DR49 are excellent producers of hydrolytic enzymes to be used as part of blends to decompose sugarcane biomass at industrial level.

In situ laccase treatment enhances the fermentability of steam-exploded wheat straw in SSCF processes at high dry matter consistencies

This work evaluates the in situ detoxification of inhibitory lignocellulosic broths by laccases to facilitate their fermentation by the xylose-consuming Saccharomyces cerevisiae F12. Treatment of wheat straw slurries with laccases prior to SSCF processes decreased the total phenolic content by 50-80%, reducing the lag phase and increasing the cell viability. After laccase treatment, a negative impact on enzymatic hydrolysis was observed. This effect, together with the low enzymatic hydrolysis yields when increasing consistency, resulted in a decrease in final ethanol yields. Furthermore, when using high substrate loading (20% DM (w/v)), high concentration of inhibitors prevailed in broths and the absence of an extra nitrogen source led to a total cell growth inhibition within the first 24h in non-treated samples. This inhibition of growth at 20% DM (w/v) was overcome by laccase treatment with no addition of nitrogen, allowing S. cerevisiae F12 to produce more than 22 g/L of ethanol.

Polysaccharide hydrolysis with engineered Escherichia coli for the production of biocommodities

Escherichia coli can ferment a broad range of sugars, including pentoses, hexoses, uronic acids, and polyols. These features make E. coli a suitable microorganism for the development of biocatalysts to be used in the production of biocommodities and biofuels by metabolic engineering. E. coli cannot directly ferment polysaccharides because it does not produce and secrete the necessary saccharolytic enzymes; however, there are many genetic tools that can be used to confer this ability on this prokaryote. The construction of saccharolytic E. coli strains will reduce costs and simplify the production process because the saccharification and fermentation can be conducted in a single reactor with a reduced concentration or absence of additional external saccharolytic enzymes. Recent advances in metabolic engineering, surface display, and excretion of hydrolytic enzymes provide a framework for developing E. coli strains for the so-called consolidated bioprocessing. This review presents the different strategies toward the development of E. coli strains that have the ability to display and secrete saccharolytic enzymes to hydrolyze different sugar-polymeric substrates and reduce the loading of saccharolytic enzymes.

In situ rheometry of concentrated cellulose fibre suspensions and relationships with enzymatic hydrolysis

This work combines physical and biochemical analyses to scrutinize liquefaction and saccharification of complex lignocellulose materials. A multilevel analysis (macroscopic: rheology, microscopic: particle size and morphology and molecular: sugar product) was conducted at the lab-scale with three matrices: microcrystalline cellulose (MCC), Whatman paper (WP) and extruded paper-pulp (PP). A methodology to determine on-line viscosity is proposed and validated using the concept of Metzner and Otto (1957) and Rieger and Novak's (1973). The substrate suspensions exhibited a shear-thinning behaviour with respect to the power law. A structured rheological model was established to account for the suspension viscosity as a function of shear rate and substrate concentration. The critical volume fractions indicate the transition between diluted, semi-diluted and concentrated regimes. The enzymatic hydrolysis was performed with various solid contents: MCC 273.6 gdm/L, WP 56.0 gdm/L, PP 35.1 gdm/L. During hydrolysis, the suspension viscosity decreased rapidly. The fibre diameter decreased two fold within 2 h of starting hydrolysis whereas limited bioconversion was obtained (10-15%).

Understanding the cellulolytic system of Trichoderma harzianum P49P11 and enhancing saccharification of pretreated sugarcane bagasse by supplementation with pectinase and α-L-arabinofuranosidase

Supplementation of cellulase cocktails with accessory enzymes can contribute to a higher hydrolytic capacity in releasing fermentable sugars from plant biomass. This study investigated which enzymes were complementary to the enzyme set of Trichoderma harzianum in the degradation of sugarcane bagasse. Specific activities of T. harzianum extract on different substrates were compared with the extracts of Penicillium echinulatum and Trichoderma reesei, and two commercial cellulase preparations. Complementary analysis of the secretome of T. harzianum was also used to identify which enzymes were produced during growth on pretreated sugarcane bagasse. These analyses enabled the selection of the enzymes pectinase and α-L-arabinofuranosidase (AF) to be further investigated as supplements to the T. harzianum extract. The effect of enzyme supplementation on the efficiency of sugarcane bagasse saccharification was evaluated using response surface methodology. The supplementation of T. harzianum enzymatic extract with pectinase and AF increased the efficiency of hydrolysis by up to 116%.