Synergistic Cellulose Hydrolysis Dominated by a Multi-Modular Processive Endoglucanase from Clostridium cellulosi

Metagenome and metabolome insights into the energy compensation and exogenous toxin degradation of gut microbiota in high-altitude rhesus macaques (Macaca mulatta)

There have been many reports on the genetic mechanism in rhesus macaques (RMs) for environmental adaptation to high altitudes, but the synergistic involvement of gut microbiota in this adaptation remains unclear. Here we performed fecal metagenomic and metabolomic studies on samples from high- and low-altitude populations to assess the synergistic role of gut microbiota in the adaptation of RMs to high-altitude environments. Microbiota taxonomic annotation yielded 7471 microbiota species. There were 37 bacterial species whose abundance was significantly enriched in the high-altitude populations, 16 of which were previously reported to be related to the host's dietary digestion and energy metabolism. Further functional gene enrichment found a stronger potential for gut microbiota to synthesize energy substrate acetyl-CoA using CO₂ and energy substrate pyruvate using oxaloacetate, as well as a stronger potential to transform acetyl-CoA to energy substrate acetate in high-altitude populations. Interestingly, there were no apparent differences between low-altitude and high-altitude populations in terms of genes enriched in the main pathways by which the microbiota consumed the three energy substrates, and none of the three energy substrates were detected in the fecal metabolites. These results strongly suggest that gut microbiota plays an important energy compensatory role that helps RMs to adapt to high-altitude environments. Further functional enrichment after metabolite source analysis indicated the abundance of metabolites related to the degradation of exogenous toxins was also significantly higher in high-altitude populations, which suggested a contributory role of gut microbiota to the degradation of exogenous toxins in wild RMs adapted to high-altitude environments.

Optimization and biochemical characterization of a thermotolerant processive cellulase, PtCel1, of Parageobacillus thermoglucosidasius NBCB1

Vermicomposting involves enrichment of microorganisms that are able to resist higher temperatures and perform simultaneous degradation of lignocellulose, and therefore, such microbial communities are a potential source of cellulolytic enzymes. This study aimed to optimize the production of a processive cellulase by Parageobacillus thermoglucosidasius NBCB1 isolated from vermicompost, under submerged fermentation of rice straw and to characterize the purified enzyme for industrial suitability. Cellulase production in basal medium (7.27 IU/mg) was enhanced to 61 IU/mg by One Factor At a Time approach, which was further improved to 78.46 IU/mg by genetic algorithm based artificial neural networking. The cellulase PtCel1 purified from bacterial culture showed a molecular weight of ≈33 kD, had activity on both crystalline (305 IU/mg) and amorphous (184 IU/mg) cellulose as substrates. It had pH and temperature optima of 5.5°C and 60°C, respectively, and retained 100% activity upon preincubation at 60°C for 1 h indicating thermostability. PtCel1 was tolerant to sodium dodecyl sulfate, glucose and mannose; and the various metal chlorides, such as sodium, magnesium, calcium and zinc, acted as inducers giving 77.54%, 45.15%, 61.10%, and 169.14% augmentation of activity, respectively. Its efficiency on cellulosic substrates and robustness against aforementioned chemical and thermal environment makes it suitable for industrial applications.

Positive role of non-catalytic proteins on mitigating inhibitory effects of lignin and enhancing cellulase activity in enzymatic hydrolysis: Application, mechanism, and prospective

Fermentable sugar production from lignocellulosic biomass has received considerable attention and has been dramatic progress recently. However, due to low enzymatic hydrolysis (EH) yields and rates, a high dosage of the costly enzyme is required, which is a bottleneck for commercial applications. Over the last decades, various strategies have been developed to reduce cellulase enzyme costs. The progress of the non-catalytic additive proteins in mitigating inhibition in EH is discussed in detail in this review. The low efficiency of EH is mostly due to soluble lignin compounds, insoluble lignin, and harsh thermal and mechanical conditions of the EH process. Adding non-catalytic proteins into the EH is considered a simple and efficient approach to boost hydrolysis yield. This review discussed the multiple mechanical steps involved in the EH process. The effect of physicochemical properties of modified lignin on EH and its interaction with cellulase and cellulose are identified and discussed, which include hydrogen bonding, hydrophobic, electrostatic, and cation-π interactions, as well as physical barriers. Moreover, the effects of different conditions of EH that lead to cellulase deactivation by thermal and mechanical mechanisms are also explained. Finally, recent advances in the development, potential mechanisms, and economic feasibility of non-catalytic proteins on EH are evaluated and perspectives are presented.

Engineering Caldicellulosiruptor bescii with Surface Layer Homology Domain-Linked Glycoside Hydrolases Improves Plant Biomass Solubilization

Extremely thermophilic Caldicellulosiruptor species solubilize carbohydrates from lignocellulose through glycoside hydrolases (GHs) that can be extracellular, intracellular, or cell surface layer (S-layer) associated. Caldicellulosiruptor genomes sequenced so far encode at least one surface layer homology domain glycoside hydrolase (SLH-GH), representing six different classes of these enzymes; these can have multiple binding and catalytic domains. Biochemical characterization of a representative from each class was done to determine their biocatalytic features: four SLH-GHs from Caldicellulosiruptor kronotskyensis (Calkro_0111, Calkro_0402, Calkro_0072, and Calkro_2036) and two from Caldicellulosiruptor hydrothermalis (Calhy_1629 and Calhy_2383). Calkro_0111, Calkro_0072, and Calhy_2383 exhibited β-1,3-glucanase activity, Calkro_0402 was active on both β-1,3/1,4-glucan and β-1,4-xylan, Calkro_2036 exhibited activity on both β-1,3/1,4-glucan and β-1,4-glucan, and Calhy_1629 was active only on arabinan. Caldicellulosiruptor bescii, the only species with molecular genetic tools as well as already a strong cellulose degrader, contains only one SLH-GH, Athe_0594, a glucanase that is a homolog of Calkro_2036; the other 5 classes of SLH-GHs are absent in C. bescii. The C. bescii secretome, supplemented with individual enzymes or cocktails of SLH-GHs, increased in vitro sugar release from sugar cane bagasse and poplar. Expression of non-native SLH-GHs in vivo, either associated with the S-layer or as freely secreted enzymes, improved total carbohydrate solubilization of sugar cane bagasse and poplar by up to 45% and 23%, respectively. Most notably, expression of Calkro_0402, a xylanase/glucanase, improved xylose solubilization from poplar and bagasse by over 70% by C. bescii. While Caldicellulosiruptor species are already prolific lignocellulose degraders, they can be further improved by the strategy described here. IMPORTANCE Caldicellulosiruptor species hold promise as microorganisms that can solubilize the carbohydrate portion of lignocellulose and subsequently convert fermentable sugars into bio-based chemicals and fuels. Members of the genus have surface layer (S-layer) homology domain-associated glycoside hydrolases (SLH-GHs) that mediate attachment to biomass as well as hydrolysis of carbohydrates. Caldicellulosiruptor bescii, the most studied member of the genus, has only one SLH-GH. Expression of SLH-GHs from other Caldicellulosiruptor species in C. bescii significantly improved degradation of sugar cane bagasse and poplar. This suggests that this extremely thermophilic bacterium can be engineered to further improve its ability to degrade specific plant biomasses by inserting genes encoding SLH-GHs recruited from other Caldicellulosiruptor species.

Enzymatic generation of short chain cello-oligosaccharides from Miscanthus using different pretreatments

Enzyme combinations producing short-chain cello-oligosaccharides (COS) as major bio-products from cellulose of Miscanthus Mx2779 accessed through different pretreatment methods were compared. Over short hydrolysis times, processive endoglucanase TfCel9a produced a high percentage of cellotetraose and cellopentaose and is synergistic with endoglucanase CcCel9m for producing short oligomers from amorphous cellulose but had low activity on untreated Miscanthus. Hydrolysis of the latter improved when these were combined with a mutant cellobio/triohydrolase OsCelC7(-105) and a lytic polysaccharide monooxygenase TrCel61a, a combination which also produced the highest COS yields from phosphoric acid swollen cellulose. Steam explosion pretreatment of Miscanthus increased COS yields, with/without phosphoric acid swelling, while increased swelling time (from 20 to 45 min) also increased yields but decreased the need for TrCel61a. The highest COS yields (933 mg/g glucan) and most stable product profile were obtained using ionic liquid [C₂mim][OAc] pretreatment and the three enzyme mixture TfCel9a, Cel9m and OsCel7a(-105).

Degradation of switchgrass by Bacillus subtilis 1AJ3 and expression of a beta-glycoside hydrolase

Increasing demand for carbon neutrality has led to the development of new techniques and modes of low carbon production. The utilization of microbiology to convert low-cost renewable resources into more valuable chemicals is particularly important. Here, we investigated the ability of a cellulolytic bacterium, Bacillus subtilis 1AJ3, in switchgrass lignocellulose degradation. After 5 days of culture with the strain under 37°C, cellulose, xylan, and acid-insoluble lignin degradation rates were 16.13, 14.24, and 13.91%, respectively. Gas chromatography-mass spectrometry (GC-MS) analysis and field emission scanning electron microscopy (FE-SEM) indicated that the lignin and surface of switchgrass were degraded after incubation with the bacterial strain. Strain 1AJ3 can grow well below 60°C, which satisfies the optimum temperature (50°C) condition of most cellulases; subsequent results emphasize that acid-heat incubation conditions increase the reducing sugar content in a wide range of cellulosic biomass degraded by B. subtilis 1AJ3. To obtain more reducing sugars, we focused on β-glycoside hydrolase, which plays an important role in last steps of cellulose degradation to oligosaccharides. A β-glycoside hydrolase (Bgl-16A) was characterized by cloning and expression in Escherichia coli BL21 and further determined to belong to glycoside hydrolase (GH) 16 family. The Bgl-16A had an enzymatic activity of 365.29 ± 10.43 U/mg, and the enzyme's mode of action was explained by molecular docking. Moreover, the critical influence on temperature (50°C) of Bgl-16A also explained the high-efficiency degradation of biomass by strain under acid-heat conditions. In terms of potential applications, both the strain and the recombinant enzyme showed that coffee grounds would be a suitable and valuable substrate. This study provides a new understanding of cellulose degradation by B. subtilis 1AJ3 that both the enzyme action mode and optimum temperature limitation by cellulases could impact the degradation. It also gave new sight to unique advantage utilization in the industrial production of green manufacturing.

Biochemical and Molecular Dynamics Study of a Novel GH 43 α-l-Arabinofuranosidase/β-Xylosidase From Caldicellulosiruptor saccharolyticus DSM8903

The complete hydrolysis of xylan can be facilitated by the coordinated action of xylanase and other de-branching enzymes. Here, a GH43 α-l-arabinofuranosidase/β-xylosidase (CAX43) from Caldicellulosiruptor saccharolyticus was cloned, sequenced, and biochemically investigated. The interaction of the enzyme with various substrates was also studied. With a half-life of 120 h at 70°C, the produced protein performed maximum activity at pH 6.0 and 70°C. The enzyme demonstrated a higher activity (271.062 ± 4.83 U/mg) against para nitrophenol (pNP) α-L-arabinofuranosides. With xylanase (XynA), the enzyme had a higher degree of synergy (2.30) in a molar ratio of 10:10 (nM). The interaction of the enzyme with three substrates, pNP α-L-arabinofuranosides, pNP β-D-xylopyranosides, and sugar beet arabinan, was investigated using protein modeling, molecular docking, and molecular dynamics (MD) simulation. During the simulation time, the root mean square deviation (RMSD) of the enzyme was below 2.5 Å, demonstrating structural stability. Six, five, and seven binding-interacting residues were confirmed against pNP α-L-arabinofuranosides, pNP β-D-xylopyranosides, and arabinan, respectively, in molecular docking experiments. This biochemical and in silico study gives a new window for understanding the GH43 family’s structural stability and substrate recognition, potentially leading to biological insights and rational enzyme engineering for a new generation of enzymes that perform better and have greater biorefinery utilization.

Rice β-glucosidase Os12BGlu38 is required for synthesis of intine cell wall and pollen fertility

Glycoside hydrolase family1 β-glucosidases play a variety of roles in plants, but their in planta functions are largely unknown in rice (Oryza sativa). In this study, the biological function of Os12BGlu38, a rice β-glucosidase, expressed in bicellular to mature pollen, was examined. Genotype analysis of progeny of the self-fertilized heterozygous Os12BGlu38 T-DNA mutant, os12bglu38-1, found no homozygotes and a 1:1 ratio of wild type to heterozygotes. Reciprocal cross analysis demonstrated that Os12BGlu38 deficiency cannot be inherited through the male gamete. In cytological analysis, the mature mutant pollen appeared shrunken and empty. Histochemical staining and TEM showed that mutant pollen lacked intine cell wall, which was rescued by introduction of wild-type Os12BGlu38 genomic DNA. Metabolite profiling analysis revealed that cutin monomers and waxes, the components of the pollen exine layer, were increased in anthers carrying pollen of os12bglu38-1 compared with wild type and complemented lines. Os12BGlu38 fused with green fluorescent protein was localized to the plasma membrane in rice and tobacco. Recombinant Os12BGlu38 exhibited β-glucosidase activity on the universal substrate p-nitrophenyl β-d-glucoside and some oligosaccharides and glycosides. These findings provide evidence that function of a plasma membrane-associated β-glucosidase is necessary for proper intine development.

Defining the Frontiers of Synergism between Cellulolytic Enzymes for Improved Hydrolysis of Lignocellulosic Feedstocks

Lignocellulose has economic potential as a bio-resource for the production of value-added products (VAPs) and biofuels. The commercialization of biofuels and VAPs requires efficient enzyme cocktail activities that can lower their costs. However, the basis of the synergism between enzymes that compose cellulolytic enzyme cocktails for depolymerizing lignocellulose is not understood. This review aims to address the degree of synergism (DS) thresholds between the cellulolytic enzymes and how this can be used in the formulation of effective cellulolytic enzyme cocktails. DS is a powerful tool that distinguishes between enzymes’ synergism and anti-synergism during the hydrolysis of biomass. It has been established that cellulases, or cellulases and lytic polysaccharide monooxygenases (LPMOs), always synergize during cellulose hydrolysis. However, recent evidence suggests that this is not always the case, as synergism depends on the specific mechanism of action of each enzyme in the combination. Additionally, expansins, nonenzymatic proteins responsible for loosening cell wall fibers, seem to also synergize with cellulases during biomass depolymerization. This review highlighted the following four key factors linked to DS: (1) a DS threshold at which the enzymes synergize and produce a higher product yield than their theoretical sum, (2) a DS threshold at which the enzymes display synergism, but not a higher product yield, (3) a DS threshold at which enzymes do not synergize, and (4) a DS threshold that displays anti-synergy. This review deconvolutes the DS concept for cellulolytic enzymes, to postulate an experimental design approach for achieving higher synergism and cellulose conversion yields.

Characterization and evaluation of a natural derived bacterial consortium for efficient lignocellulosic biomass valorization

A consortium (HPP) with improved ability in biomass conversion was achieved by adjusting the proportion of Pseudoxanthomonas taiwanensis in a natural consortium (HP), but the mechanism behind was unknown. Herein, the diversities of microbial community structure and gene functions of the consortia were analyzed first, and found that HPP had a more balanced microbial structure with enriched gene pathways related to cellular processes, environmental information processing and metabolism. Then, key genes responsible for biomass conversion were further analyzed, finding that their abundance and distribution contributed to HPP's efficient biomass conversion. Finally, consolidated bioprocessing of agricultural wastes by HPP was carried out to verify its enhanced ability, and ethanol with the highest yield that was ever reported was achieved at 0.28 g/g. This is the first study which reported the underlying mechanisms for synergistic effects of microbial consortia, and will guide the artificial construction of complex microbial consortium for specific purpose.

Revisiting the Phenomenon of Cellulase Action: Not All Endo- and Exo-Cellulase Interactions Are Synergistic

The conventional endo–exo synergism model has extensively been supported in literature, which is based on the perception that endoglucanases (EGs) expose or create accessible sites on the cellulose chain to facilitate the action of processive cellobiohydrolases (CBHs). However, there is a lack of information on why some bacterial and fungal CBHs and EGs do not exhibit synergism. Therefore, the present study evaluated and compared the synergistic relationships between cellulases from different microbial sources and provided insights into how different GH families govern synergism. The results showed that CmixA2 (a mixture of TlCel7A and CtCel5A) displayed the highest effect with BaCel5A (degree of synergy for reducing sugars and glucose of 1.47 and 1.41, respectively) in a protein mass ratio of 75–25%. No synergism was detected between CmixB1/B2 (as well as CmixC1/C2) and any of the EGs, and the combinations did not improve the overall cellulose hydrolysis. These findings further support the hypothesis that “not all endo-to exo-cellulase interactions are synergistic”, and that the extent of synergism is dependent on the composition of cellulase systems from various sources and their compatibility in the cellulase cocktail. This method of screening for maximal compatibility between exo- and endo-cellulases constitutes a critical step towards the design of improved synergistic cellulose-degrading cocktails for industrial-scale biomass degradation.

New thermostable endoglucanase from Spirochaeta thermophila and its mutants with altered substrate preferences

Endoglucanases are key elements in several industrial applications, such as cellulosic biomass hydrolysis, cellulose fiber modification for the production paper and composite materials, and in nanocellulose production. In all of these applications, the desired function of the endoglucanase is to create nicks in the amorphous regions of the cellulose. However, endoglucanase can be diverted from its activity on the fibers by other substrates-soluble oligosaccharides. This issue was addressed in the current study using enzyme engineering and an enzyme evolution approach. To this end, a hypothetical endoglucanase from a thermostable bacterium Spirochaeta thermophila was for the first time cloned and characterized. The wild-type enzyme was used as a starting point for mutagenesis and molecular evolution toward a preference for the higher molecular weight substrates. The best of the evolved enzymes was more active than the wild-type enzyme toward high molecular weight substrate at temperatures below 45 °C (3-fold more active at 30 °C) and showed little or no activity with low molecular weight substrates. These findings can be instrumental in bioeconomy sectors, such as second-generation biofuels and biomaterials from lignocellulosic biomass. KEY POINTS: • A new thermostable endoglucanase was characterized. • The substrate specificity of this endoglucanase was changed by means of genetic engineering. • A mutant with a preference for long molecular weight substrate was obtained and proposed to be beneficial for cellulose fiber modification.

Artificially selecting bacterial communities using propagule strategies

Artificial selection is a promising approach to manipulate microbial communities. Here, we report the outcome of two artificial selection experiments at the microbial community level. Both used "propagule" selection strategies, whereby the best-performing communities are used as the inocula to form a new generation of communities. Both experiments were contrasted to a random selection control. The first experiment used a defined set of strains as the starting inoculum, and the function under selection was the amylolytic activity of the consortia. The second experiment used multiple soil communities as the starting inocula, and the function under selection was the communities' cross-feeding potential. In both experiments, the selected communities reached a higher mean function than the control. In the first experiment, this was caused by a decline in function of the control, rather than an improvement of the selected line. In the second experiment, this response was fueled by the large initial variance in function across communities, and stopped when the top-performing community "fixed" in the metacommunity. Our results are in agreement with basic expectations from breeding theory, pointing to some of the limitations of community-level selection experiments that can inform the design of future studies.

Three glycoside hydrolase family 12 enzymes display diversity in substrate specificities and synergistic action between each other

PoCel12A, PoCel12B, and PoCel12C are genes that encode glycoside hydrolase family 12 (GH12) enzymes in Penicillium oxalicum. PoCel12A and PoCel12B are typical GH12 enzymes that belong to fungal subfamilies 12-1 and 12-2, respectively. PoCel12C contains a low-complexity region (LCR) domain, which is not found in PoCel12A or PoCel12B and independent of fungal subfamily 12-1 or 12-2. Recombinant enzymes (named rCel12A, rCel12B and rCel12C) demonstrate existing diversity in the substrate specificities. Although most members in GH family 12 are typical endoglucanases and preferentially hydrolyze β-1,4-glucan (e.g., carboxymethylcellulose), recombinant PoCel12A is a non-typical endo-(1-4)-β-glucanase; it preferentially hydrolyzes mix-linked β-glucan (barley β-glucan, β-1,3-1,4-glucan) and slightly hydrolyzes β-1,4-glucan (carboxymethylcellulose). Recombinant PoCel12B possesses a significantly high activity against xyloglucan. A specific activity of rCel12B toward xyloglucan (239 µmol/min/mg) is the second-highest value known. Recombinant PoCel12C shows low activity toward β-glucan, carboxymethylcellulose, or xyloglucan. All three enzymes can degrade phosphoric acid-swollen cellulose (PASC). However, the hydrolysis products toward PASC by enzymes are different: the main hydrolysis products are cellotriose, cellotetraose, and cellobiose for rCel12A, rCel12B, and rCel12C, correspondingly. A synergistic action toward PASC among rCel12A and rCel12B is observed, thereby suggesting a potential application for preparing enzyme cocktails used in lignocellulose hydrolysis.

Endoglucanase improve the growth of homofermentative Lactobacillus spp. in ensilages

Endoglucanase, an important component of cellulases, is used as additives in ensiling of forage crops. However, its detailed role is unclear in ensilages. In the present study, two endoglucanases Cel5 and Cel9 produced by strain Paenibacillus panacisoli SDMCC050309, previously isolated from ensiled corn stover, were identified in the cultures by microcrystalline cellulose absorption coupled with zymogram analysis. After heterologously expressed in Escherichia coli DE3 and purified, these two proteins were biochemically characterized. Cel5 was 61 kDa and showed maximal activity at pH 7.0 and 45 °C, while the maximum activity was at pH 8.0 and 65 °C for Cel9 with 97 kDa in size. Both of them could degrade carboxymethyl cellulose into cellooligosaccharides, in which cellobiose and cellotriose could be used as substrates for the growth of homofermentative strains Lactobacillus plantarum CGMCC6888 and L. farciminis CCTCC AB2016237, but not for the heterofermentative strains L. brevis SDMCC050297 and L. parafarraginis SDMCC050300. Therefore, we concluded that the added endoglucanase contributed to enhance the growth of homofermentative lactic acid bacteria for high level of lactic acid production in ensilages.

Biochemical characterization and low-resolution SAXS structure of two-domain endoglucanase BlCel9 from Bacillus licheniformis

Lignocellulose feedstock constitutes the most abundant carbon source in the biosphere; however, its recalcitrance remains a challenge for microbial conversion into biofuel and bioproducts. Bacillus licheniformis is a microbial mesophilic bacterium capable of secreting a large number of glycoside hydrolase (GH) enzymes, including a glycoside hydrolase from GH family 9 (BlCel9). Here, we conducted biochemical and biophysical studies of recombinant BlCel9, and its low-resolution molecular shape was retrieved from small angle X-ray scattering (SAXS) data. BlCel9 is an endoglucanase exhibiting maximum catalytic efficiency at pH 7.0 and 60 °C. Furthermore, it retains 80% of catalytic activity within a broad range of pH values (5.5-8.5) and temperatures (up to 50 °C) for extended periods of time (over 48 h). It exhibits the highest hydrolytic activity against phosphoric acid swollen cellulose (PASC), followed by bacterial cellulose (BC), filter paper (FP), and to a lesser extent carboxymethylcellulose (CMC). The HPAEC-PAD analysis of the hydrolytic products demonstrated that the end product of the enzymatic hydrolysis is primarily cellobiose, and also small amounts of glucose, cellotriose, and cellotetraose are produced. SAXS data analysis revealed that the enzyme adopts a monomeric state in solution and has a molecular mass of 65.8 kDa as estimated from SAXS data. The BlCel9 has an elongated shape composed of an N-terminal family 3 carbohydrate-binding module (CBM3c) and a C-terminal GH9 catalytic domain joined together by 20 amino acid residue long linker peptides. The domains are closely juxtaposed in an extended conformation and form a relatively rigid structure in solution, indicating that the interactions between the CBM3c and GH9 catalytic domains might play a key role in cooperative cellulose biomass recognition and hydrolysis.

Novel Ethanol- and 5-Hydroxymethyl Furfural-Stimulated β-Glucosidase Retrieved From a Brazilian Secondary Atlantic Forest Soil Metagenome

Beta-glucosidases are key enzymes involved in lignocellulosic biomass degradation for bioethanol production, which complete the final step during cellulose hydrolysis by converting cellobiose into glucose. Currently, industry requires enzymes with improved catalytic performance or tolerance to process-specific parameters. In this sense, metagenomics has become a powerful tool for accessing and exploring the biochemical biodiversity present in different natural environments. Here, we report the identification of a novel β-glucosidase from metagenomic DNA isolated from soil samples enriched with decaying plant matter from a Secondary Atlantic Forest region. For this, we employed a functional screening approach using an optimized and synthetic broad host-range vector for library production. The novel β-glucosidase – named Lfa2 – displays three GH3-family conserved domains and conserved catalytic amino acids D283 and E487. The purified enzyme was most active in pH 5.5 and at 50°C, and showed hydrolytic activity toward several pNP synthetic substrates containing β-glucose, β-galactose, β-xylose, β-fucose, and α-arabinopyranose, as well as toward cellobiose. Lfa2 showed considerable glucose tolerance, exhibiting an IC₅₀ of 300 mM glucose and 30% of remaining activity in 600 mM glucose. In addition, Lfa2 retained full or slightly enhanced activity in the presence of several metal ions. Further, β-glucosidase activity was increased by 1.7-fold in the presence of 10% (v/v) ethanol, a concentration that can be reached in conventional fermentation processes. Similarly, Lfa2 showed 1.7-fold enhanced activity at high concentrations of 5-hydroxymethyl furfural, one of the most important cellulase inhibitors in pretreated sugarcane bagasse hydrolysates. Moreover, the synergistic effect of Lfa2 on Bacillus subtilis GH5-CBM3 endoglucanase activity was demonstrated by the increased production of glucose (1.6-fold). Together, these results indicate that β-glucosidase Lfa2 is a promissory enzyme candidate for utilization in diverse industrial applications, such as cellulosic biomass degradation or flavor enhancement in winemaking and grape processing.

Two Distinct α-l-Arabinofuranosidases in Caldicellulosiruptor Species Drive Degradation of Arabinose-Based Polysaccharides

Species in the extremely thermophilic genus Caldicellulosiruptor can degrade unpretreated plant biomass through the action of multimodular glycoside hydrolases. To date, most focus with these bacteria has been on hydrolysis of glucans and xylans, while the biodegradation mechanism for arabinose-based polysaccharides remains unclear. Here, putative α-l-arabinofuranosidases (AbFs) were identified in Caldicellulosiruptor species by homology to less-thermophilic versions of these enzymes. From this screen, an extracellular XynF was determined to be a key factor in hydrolyzing α-1,2-, α-1,3-, and α-1,5-l-arabinofuranosyl residues of arabinose-based polysaccharides. Combined with a GH11 xylanase (XynA), XynF increased arabinoxylan hydrolysis more than 6-fold compared to the level seen with XynA alone, likely the result of XynF removing arabinofuranosyl side chains to generate linear xylans that were readily degraded. A second AbF, the intracellular AbF51, preferentially cleaved the α-1,5-l-arabinofuranosyl glycoside bonds within sugar beet arabinan. β-Xylosidases, such as GH39 Xyl39B, facilitated the hydrolysis of arabinofuranosyl residues at the nonreducing terminus of the arabinose-branched xylo-oligosaccharides by AbF51. These results demonstrate the separate but complementary contributions of extracellular XynF and cytosolic AbF51 in processing the bioconversion of arabinose-containing oligosaccharides to fermentable monosaccharides.IMPORTANCE Degradation of hemicellulose, due to its complex chemical structure, presents a major challenge during bioconversion of lignocellulosic biomass to biobased fuels and chemicals. Degradation of arabinose-containing polysaccharides, in particular, can be a key bottleneck in this process. Among Caldicellulosiruptor species, the multimodular arabinofuranosidase XynF is present in only selected members of this genus. This enzyme exhibited high hydrolysis activity, broad specificity, and strong synergism with other hemicellulases acting on arabino-polysaccharides. An intracellular arabinofuranosidase, AbF51, occurs in all Caldicellulosiruptor species and, in conjunction with xylosidases, processes the bioconversion of arabinose-branched oligosaccharides to fermentable monosaccharides. Taken together, the data suggest that plant biomass degradation in Caldicellulosiruptor species involves extracellular XynF that acts synergistically with other hemicellulases to digest arabino-polysaccharides that are subsequently transported and degraded further by intracellular AbF51 to produce short-chain arabino sugars.

Biochemical characterization and synergism of cellulolytic enzyme system from Chaetomium globosum on rice straw saccharification

Background

Efficient hydrolysis of lignocellulosic materials to sugars for conversion to biofuels and chemicals is a key step in biorefinery. Designing an active saccharifying enzyme system with synergy among their components is considered a promising approach.

Results

In this study, a lignocellulose-degrading enzyme system of Chaetomium globosum BCC5776 (CG-Cel) was characterized for its activity and proteomic profiles, and synergism with accessory enzymes. The highest cellulase productivity of 0.40 FPU/mL was found for CG-Cel under the optimized submerged fermentation conditions on 1% (w/v) EPFB (empty palm fruit bunch), 2% microcrystalline cellulose (Avicel®) and 1% soybean meal (SBM) at 30 °C, pH 5.8 for 6 d. CG-Cel worked optimally at 50–60 °C in an acidic pH range. Proteomics analysis by LC/MS/MS revealed a complex enzyme system composed of core cellulases and accessory hydrolytic/non-hydrolytic enzymes attacking plant biopolymers. A synergistic enzyme system comprising the CG-Cel, a β-glucosidase (Novozyme® 188) and a hemicellulase Accellerase® XY was optimized on saccharification of alkaline-pretreated rice straw by a mixture design approach. Applying a full cubic model, the optimal ratio of ternary enzyme mixture containing CG-Cel: Novozyme® 188: Accellerase® XY of 44.4:20.6:35.0 showed synergistic enhancement on reducing sugar yield with a glucose releasing efficiency of 256.4 mg/FPU, equivalent to a 2.9 times compared with that from CG-Cel alone.

Conclusions

The work showed an approach for developing an active synergistic enzyme system based on the newly characterized C. globosum for lignocellulose saccharification and modification in bio-industries.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-016-0312-7) contains supplementary material, which is available to authorized users.