Current perspectives in enzymatic saccharification of lignocellulosic biomass

Plant cell wall enzymatic deconstruction: Bridging the gap between micro and nano scales

Understanding lignocellulosic biomass resistance to enzymatic deconstruction is crucial for its sustainable conversion into bioproducts. Despite scientific advances, quantitative morphological analysis of plant deconstruction at cell and tissue scales remains under-explored. In this study, an original pipeline is devised, involving four-dimensional (space  + time) fluorescence confocal imaging, and a novel computational tool, to track and quantify deconstruction at cell and tissue scales. By applying this pipeline to poplar wood, dynamics of cellular parameters was computed and cellulose conversion during enzymatic deconstruction was measured. Results showed that enzymatic deconstruction predominantly impacts cell wall volume rather than surface area. Additionally, a negative correlation was observed between pre-hydrolysis compactness measures and volumetric cell wall deconstruction rate, whose strength was modulated by enzymatic activity. Results also revealed a strong positive correlation between average volumetric cell wall deconstruction rate and cellulose conversion rate. These findings link key deconstruction parameters across nano and micro scales.

Comparative genomics reveals cellobiose hydrolysis mechanism of Ruminiclostridium thermocellum M3, a cellulosic saccharification bacterium

The cellulosome of Ruminiclostridium thermocellum was one of the most efficient cellulase systems in nature. However, the product of cellulose degradation by R. thermocellum is cellobiose, which leads to the feedback inhibition of cellulosome, and it limits the R. thermocellum application in the field of cellulosic biomass consolidated bioprocessing (CBP) industry. In a previous study, R. thermocellum M3, which can hydrolyze cellulosic feedstocks into monosaccharides, was isolated from horse manure. In this study, the complete genome of R. thermocellum M3 was sequenced and assembled. The genome of R. thermocellum M3 was compared with the other R. thermocellum to reveal the mechanism of cellulosic saccharification by R. thermocellum M3. In addition, we predicted the key genes for the elimination of feedback inhibition of cellobiose in R. thermocellum. The results indicated that the whole genome sequence of R. thermocellum M3 consisted of 3.6 Mb of chromosomes with a 38.9% of GC%. To be specific, eight gene islands and 271 carbohydrate-active enzyme-encoded proteins were detected. Moreover, the results of gene function annotation showed that 2,071, 2,120, and 1,246 genes were annotated into the Clusters of Orthologous Groups (COG), Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases, respectively, and most of the genes were involved in carbohydrate metabolism and enzymatic catalysis. Different from other R. thermocellum, strain M3 has three proteins related to β-glucosidase, and the cellobiose hydrolysis was enhanced by the synergy of gene BglA and BglX. Meanwhile, the GH42 family, CBM36 family, and AA8 family might participate in cellobiose degradation.

Heterotrophic Cultivation of the Cyanobacterium Pseudanabaena sp. on Forest Biomass Hydrolysates toward Sustainable Biodiesel Production

Environmental pollution, greenhouse gas emissions, depletion of fossil fuels, and a growing population have sparked a search for new and renewable energy sources such as biodiesel. The use of waste or residues as substrates for microbial growth can favor the implementation of a biorefinery concept with reduced environmental footprint. Cyanobacteria constitute microorganisms with enhanced ability to use industrial effluents, wastewaters, forest residues for growth, and concomitant production of added-value compounds. In this study, a recently isolated cyanobacterium strain of Pseudanabaena sp. was cultivated on hydrolysates from pretreated forest biomass (silver birch and Norway spruce), and the production of biodiesel-grade lipids was assessed. Optimizing carbon source concentration and the (C/N) carbon-to-nitrogen ratio resulted in 66.45% w/w lipid content when microalgae were grown on glucose, compared to 62.95% and 63.79% w/w when grown on spruce and birch hydrolysate, respectively. Importantly, the lipid profile was suitable for the production of high-quality biodiesel. The present study demonstrates how this new cyanobacterial strain could be used as a biofactory, converting residual resources into green biofuel.

Decomposition of Saccharides and Alcohols in Solution Plasma for Hydrogen Production

Solution plasma or in-liquid plasma, which is generated by gas-phase discharge within bubbles in a solution, is an exciting reaction field for biomass conversion. However, it is not fully elucidated how the solution plasma works to degrade biomass or how biomass is degraded in it. In this study, various saccharides and alcohols, mainly sucrose, were treated in solution plasma using a high-voltage pulse power supply to study the degradation mechanisms. Hydrolysis and gasification were observed in the solution-plasma treatment of sucrose. The former was mainly influenced by the water temperature, and the latter was mainly influenced by the discharge power. Therefore, it was inferred that hydrolysis occurred in the hot-compressed water region around the plasma, and gasification occurred at the interface between the plasma and water. Gasification of saccharides and alcohols produced H2-rich gases, but gasification was faster for high-volatility alcohols and slower for non-volatile saccharides. The formation of H2-rich gas can be attributed to H2 formation by the water–gas shift reaction of CO and direct H2 formation from water, in addition to H2 from the sample.

Prospects on bio-based 2,3-butanediol and acetoin production: Recent progress and advances

The bio-based platform chemicals 2,3-butanediol (BDO) and acetoin have various applications in chemical, cosmetics, food, agriculture, and pharmaceutical industries, whereas the derivatives of BDO could be used as fuel additives, polymer and synthetic rubber production. This review summarizes the novel technological developments in adapting genetic and metabolic engineering strategies for selection and construction of chassis strains for BDO and acetoin production. The valorization of renewable feedstocks and bioprocess development for the upstream and downstream stages of bio-based BDO and acetoin production are discussed. The techno-economic aspects evaluating the viability and industrial potential of bio-based BDO production are presented. The commercialization of bio-based BDO and acetoin production requires the utilization of crude renewable resources, the chassis strains with high fermentation production efficiencies and development of sustainable purification or conversion technologies.

Metabolic engineering of 1,2-propanediol production from cellobiose using beta-glucosidase-expressing E. coli

Microbial 1,2-propanediol production using renewable feedstock is a promising method for the sustainable production of value-added fuels and chemicals. We demonstrated the metabolically engineered Escherichia coli for improvement of 1,2-propanediol production using glucose and cellobiose. The deletion of competing pathways improved 1,2-propanediol production. To reduce carbon flux toward downstream glycolysis, the phosphotransferase system (PTS) was inactivated by ptsG gene deletion. The resultant strain, GL3/PD, produced 1.48 ± 0.01 g/L of 1,2-propanediol from 20 g/L of glucose. A sugar supply was engineered by coexpression of β-glucosidase (BGL). The strain expressing BGL produced 1,2-propanediol from cellobiose at a concentration of 0.90 ± 0.11 g/L with a yield of 0.15 ± 0.01 g/g glucose (cellobiose 1 g is equal to glucose 1.1 g). As cellobiose or cellooligosaccharides a carbon source, the feasibility of producing 1,2-propanediol using an E. coli strain engineered for β-glucosidase expression are demonstrated.

One-pot production of lactic acid from rice straw pretreated with ionic liquid

Production of platform chemicals has been advocated as a sustainable option to tackle the problems associated with agro-waste management. In this report, for the first time, efforts were made to effectively produce second-generation lactic acid from rice straw pretreated with imidazolium ionic liquid [EMIM][OAc] and subsequently fermented with a promising Lactobacillus plantarum SKL-22 strain saccharified with a commercial cellulase enzyme. Medium optimization was carried out to enhance the lactic acid (LA) yield by response surface methodology. In a 5 L bioreactor, the process was further upscale, and a yield increment of 1.11% was observed. The process using rice straw as substrate led to a LA yield of 36.75 g/L from L. plantarum SKL-22 in a single pot bioprocess. Overall, the above finding has shown the ability of L. plantarum SKL-22 to produce LA from the hydrolysate of rice straw. This study presented a novel environmental-friendly method for LA production.

High-resolution crystal structure and biochemical characterization of a GH11 endoxylanase from Nectria haematococca

Enzymatic degradation of vegetal biomass offers versatile procedures to improve the production of alternative fuels and other biomass-based products. Here we present the three-dimensional structure of a xylanase from Nectria haematococca (NhGH11) at 1.0 Å resolution and its functional properties. The atomic resolution structure provides details and insights about the complex hydrogen bonding network of the active site region and allowed a detailed comparison with homologous structures. Complementary biochemical studies showed that the xylanase can catalyze the hydrolysis of complex xylan into simple xylose aldopentose subunits of different lengths. NhGH11 can catalyze the efficient breakdown of beechwood xylan, xylan polysaccharide, and wheat arabinoxylan with turnover numbers of 1730.6 ± 318.1 min^-1, 1648.2 ± 249.3 min^-1 and 2410.8 ± 517.5 min^-1 respectively. NhGH11 showed maximum catalytic activity at pH 6.0 and 45 °C. The mesophilic character of NhGH11 can be explained by distinct structural features in comparison to thermophilic GH11 enzymes, including the number of hydrogen bonds, side chain interactions and number of buried water molecules. The enzymatic activity of NhGH11 is not very sensitive to metal ions and chemical reagents that are typically present in associated industrial production processes. The data we present highlights the potential of NhGH11 to be applied in industrial biomass degradation processes.

Metabolic Engineering of Shikimic Acid-Producing Corynebacterium glutamicum From Glucose and Cellobiose Retaining Its Phosphotransferase System Function and Pyruvate Kinase Activities

The production of aromatic compounds by microbial production is a promising and sustainable approach for producing biomolecules for various applications. We describe the metabolic engineering of Corynebacterium glutamicum to increase its production of shikimic acid. Shikimic acid and its precursor-consuming pathways were blocked by the deletion of the shikimate kinase, 3-dehydroshikimate dehydratase, shikimate dehydratase, and dihydroxyacetone phosphate phosphatase genes. Plasmid-based expression of shikimate pathway genes revealed that 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase, encoded by aroG, and DHQ synthase, encoded by aroB, are key enzymes for shikimic acid production in C. glutamicum. We constructed a C. glutamicum strain with aroG, aroB and aroE3 integrated. This strain produced 13.1 g/L of shikimic acid from 50 g/L of glucose, a yield of 0.26 g-shikimic acid/g-glucose, and retained both its phosphotransferase system and its pyruvate kinase activity. We also endowed β-glucosidase secreting ability to this strain. When cellobiose was used as a carbon source, the strain produced shikimic acid at 13.8 g/L with the yield of 0.25 g-shikimic acid/g-glucose (1 g of cellobiose corresponds to 1.1 g of glucose). These results demonstrate the feasibility of producing shikimic acid and its derivatives using an engineered C. glutamicum strain from cellobiose as well as glucose.

Accelerated biomethane production from lignocellulosic biomass: Pretreated by mixed enzymes secreted by Trichoderma viride and Aspergillus sp

Biological pretreatment is a promising technology to increase biogas yield. The methane yield and microbial community resulting from anaerobic digestion of maize straw after pretreatment of enzymes [extracted from Trichoderma viride (ETv) and Aspergillus sp. (EAs)] at different mixing ratios [5/0, 4/1, 3/2, 2/3, 1/4, 0/5] were evaluated. The methane yields from mixed enzymes pretreatment were higher than single enzymes pretreatments of ETv and EAs. The optimal enzymes mixing ratio of ETv and EAs was found to be 2/3, with the cumulative methane yield 512.64 mL/g TS_added, which was 31.74% higher than the control. Enzymatic pretreatment promoted an increase in the abundance of bacteria and archaea associated with cellulose decomposition. The majority of bacteria and archaea were assigned to Bacteroidetes, Firmicutes and Methanosaeta.

Camelina sativa meal hydrolysate as sustainable biomass for the production of carotenoids by Rhodosporidium toruloides

BACKGROUND

As the circular economy advocates a near total waste reduction, the industry has shown an increased interest toward the exploitation of various residual biomasses. The origin and availability of biomass used as feedstock strongly affect the sustainability of biorefineries, where it is converted in energy and chemicals. Here, we explored the valorization of Camelina meal, the leftover residue from Camelina sativa oil extraction. In fact, in addition to Camelina meal use as animal feed, there is an increasing interest in further valorizing its macromolecular content or its nutritional value.

RESULTS

Camelina meal hydrolysates were used as nutrient and energy sources for the fermentation of the carotenoid-producing yeast Rhodosporidium toruloides in shake flasks. Total acid hydrolysis revealed that carbohydrates accounted for a maximum of 31 ± 1.0% of Camelina meal. However, because acid hydrolysis is not optimal for subsequent microbial fermentation, an enzymatic hydrolysis protocol was assessed, yielding a maximum sugar recovery of 53.3%. Separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), and SSF preceded by presaccharification of Camelina meal hydrolysate produced 5 ± 0.7, 16 ± 1.9, and 13 ± 2.6 mg/L of carotenoids, respectively. Importantly, the presence of water-insoluble solids, which normally inhibit microbial growth, correlated with a higher titer of carotenoids, suggesting that the latter could act as scavengers.

CONCLUSIONS

This study paves the way for the exploitation of Camelina meal as feedstock in biorefinery processes. The process under development provides an example of how different final products can be obtained from this side stream, such as pure carotenoids and carotenoid-enriched Camelina meal, can potentially increase the initial value of the source material. The obtained data will help assess the feasibility of using Camelina meal to generate high value-added products.

Intensification of dilute acid hydrolysis of spent tea powder using ultrasound for enhanced production of reducing sugars

Spent tea (ST) powder is one of the potential sustainable sources available abundantly and can be utilized to produce reducing sugars required for production of platform chemicals. The current study aims at intensifying the reducing sugars production based on ultrasound assisted dilute acid hydrolysis (UADAH). The effects of reaction time, solid liquid ratio, acid concentration and temperature on the yield of reducing sugars were investigated initially for UADAH process based on ultrasonic (US) horn. The highest yield of 24.75 g/L for the reducing sugars was obtained at solid liquid ratio of 1:8, acid concentration of 1% w/v and temperature of 60 °C within 120 min. Use of oxidants like hydrogen peroxide (H₂O₂) and Fenton's reagent to further intensify the production has also been studied. Use of H₂O₂ at optimum loading of 0.75 g/L resulted in reducing sugars yield of 26.2 g/L within 75 min while using same H₂O₂ loading with FeSO₄ at loading of 0.75 g/L along with UADAH reduced the reaction time to 60 min for almost similar yield. Large scale studies performed using US flow cell revealed that yield of reducing sugars as 22.4 g/L is obtained in 120 min in the case of only UADAH, while in the case of UADAH along with H₂O₂ and Fenton's reagent, similar yield of reducing sugars was obtained in only 90 and 60 min respectively. UADAH in combination with oxidants has been demonstrated as an effective and intensified approach to produce reducing sugars from spent tea powder available as sustainable source.

Feasibility study of on-site solid-state enzyme production by Aspergillus oryzae

BACKGROUND

The development of biorefinery systems that use lignocellulosic biomass as a renewable carbon source to produce fuels and chemicals is attracting increasing attention. The process cost of enzymatic saccharification of biomass is a major challenge for commercialization. To decrease this cost, researchers have proposed on-site solid-state fermentation (SSF). This study investigated the feasibility of using Aspergillus oryzae as a host microorganism for SSF recombinant enzyme production with ammonia-treated rice straw as model biomass. Eight A. oryzae strains were tested, all of which are used in the food industry. We evaluated the effects of acetic acid, a fermentation inhibitor. We also developed a platform strain for targeted recombinant enzyme production by gene engineering technologies.

RESULTS

The SSF validation test showed variation in the visibility of mycelium growth and secreted protein in all eight A. oryzae strains. The strains used to produce shoyu and miso grew better under test conditions. The ammonia-treated rice straw contained noticeable amounts of acetic acid. This acetic acid enhanced the protein production by A. oryzae in a liquid-state fermentation test. The newly developed platform strain successfully secreted three foreign saccharifying enzymes.

CONCLUSIONS

A. oryzae is a promising candidate as a host microorganism for on-site SSF recombinant enzyme production, which bodes well for the future development of a more cost-efficient saccharifying enzyme production system.

Recognition of the neutral sugars conversion induced by bacterial community during lignocellulose wastes composting

The aim of this study was to explore the conversion characteristics of neutral sugars during different lignocellulose wastes composting from rice straw (RS), leaf (L) and mushroom dreg (MD). The results showed that the changes of neutral sugars were different during different wastes composting, but the changes of various hexose or pentose were similar during composting of the same material. The diversity of bacterial community led to different conversion characteristics of neutral sugars. During RS composting, each neutral sugar was transformed by a specific group of bacteria. However, a group of bacteria could transform multiple neutral sugars during MD and L composting. Furthermore, GM/AX value was first applied to composting, which could be used to characterize the conversion of neutral sugars during composting. This will help to provide more efficient recommendations for lignocellulose wastes treatment and accelerating humic substances synthesis during composting.

Genetically Engineered Proteins to Improve Biomass Conversion: New Advances and Challenges for Tailoring Biocatalysts

Protein engineering emerged as a powerful approach to generate more robust and efficient biocatalysts for bio-based economy applications, an alternative to ecologically toxic chemistries that rely on petroleum. On the quest for environmentally friendly technologies, sustainable and low-cost resources such as lignocellulosic plant-derived biomass are being used for the production of biofuels and fine chemicals. Since most of the enzymes used in the biorefinery industry act in suboptimal conditions, modification of their catalytic properties through protein rational design and in vitro evolution techniques allows the improvement of enzymatic parameters such as specificity, activity, efficiency, secretability, and stability, leading to better yields in the production lines. This review focuses on the current application of protein engineering techniques for improving the catalytic performance of enzymes used to break down lignocellulosic polymers. We discuss the use of both classical and modern methods reported in the literature in the last five years that allowed the boosting of biocatalysts for biomass degradation.

Optimization of conditions for the production of lignocellulolytic enzymes by Sphingobacterium sp. ksn-11 utilizing agro-wastes under submerged condition

The present work was aimed at studying the production of lignocellulolytic enzymes, namely cellulase, xylanase, pectinase, mannanase, and laccase by a newly isolated bacterium Sphingobacterium sp. ksn-11, utilizing various agro-residues as a substrate under submerged conditions. The production of lignocellulolytic enzymes was found to be maximum at the loading of 10%(w/v) agro-residues. The enzyme secretion was enhanced by two-fold at 2 mM CaCO_3, optimum pH 7, and temperature 40°. The Field Emission Gun-Scanning Electron Microscope (FEG-SEM) results have shown the degradative effect of lignocellulases; cellulase, xylanase, mannanase, pectinase, and laccase on corn husk with 3.55 U/ml, 79.22 U/ml, 12.43 U/ml, 64.66 U/ml, and 21.12 U/ml of activity, respectively. The hydrolyzed corn husk found to be good adsorbent for polyphenols released during hydrolysis of corn husk providing suitable conditions for stability of lignocellulases. Sphingobacterium sp. ksn is proved to be a promising candidate for lignocellulolytic enzymes in view of demand for enzymes in the biofuel industry.

Proteomic profiling of Sporotrichum thermophile under the effect of ionic liquids: manifestation of an oxidative stress response

Sporotrichum thermophile, a known producer of industrial enzymes exhibited stability in the presence of ionic liquids (ILs).The study reports, for the first time, the stress response of S. thermophile upon exposure to ILs. In vitro assay showed increased anti-oxidative enzyme levels indicating ROS-mediated oxidative stress by ILs. The proteomic profile and identification of differential proteins confirmed the fungal adaptations by (i) increased expression of glycolytic enzymes and ATP synthases (ii) downregulation of TCA cycle and protein synthesis machinery components (iii) expression of HSP70 and catalase/peroxidase. These changes are indicative of metabolic regulation of many important pathways and how ILs can be used to manipulate protein behavior.

Heterologous expression of Neurospora crassa cbh1 gene in Pichia pastoris resulted in production of a neutral cellobiohydrolase I

The high production cost of cellulase is one of the limitations that hinder the commercialization of lignocellulose-based biorefineries. As one of the important cellulases, Neurospora crassa cellulase is not so intensively investigated as T. reesei cellulase. In this study, the cbh1gene (NCU07340) cloned from N. crassa was expressed in Pichia pastoris under the control of alcohol oxidase 1 (AOX1) promotor. Six transformants with the highest resistance to G418 were selected by two rounds of transformant screening, among which the most robust producer of the recombinant cellobiohydrolase I (CBHI) has an Avicelase activity of 0.22 U/mL. After fermentation optimization, it was improved to 0.30 U/mL. Interestingly, the optimal temperature and pH of the recombinant CBHI were 60°C and 7.2, respectively, and it was quite stable within the wide ranges of temperature and pH. This work is a good example for the future improvement and optimization of N. crassa cellulase.

Simultaneous enhancement of the beta-exo synergism and exo-exo synergism in Trichoderma reesei cellulase to increase the cellulose degrading capability

Background

Cellulase is the one of the largest contributors to the high production costs of the lignocellulose-based biorefineries. As the most widely used cellulase producer, Trichoderma reesei has two weaknesses, deficiencies in β-glucosidase and cellobiohydrolase II. This work aimed at solving this problem by simultaneous enhancement of the beta–exo synergism and exo–exo synergism in T. reesei cellulase to increase the cellulose degrading capability, i.e. enhanced co-expression of the β-glucosidase gene the cellobiohydrolase II gene of T. reesei.

Results

Enhanced co-expression of the β-glucosidase gene and the cellobiohydrolase II gene in T. reesei using the strong promoter Pcbh1 was found successful in overcoming the two weaknesses. Filter paper activities of T. reesei cellulase were greatly elevated, which were 7.21 ± 0.45 (E7, Aabgl1 and Trcbh2) and 7.69 ± 0.42 (F6, Anbgl1 and Trcbh2) FPIU/mL. They were much higher than that of the parental strain Rut-C30, 2.45 ± 0.36 FPIU/mL. Enzymatic hydrolysis yields were also improved, from 67.22 ± 1.61% by Rut-C30 cellulase to 87.98 ± 0.65% by E7 cellulase and 86.50 ± 1.01% by F6 cellulase. The substrate loading for 1 g glucose release from SECS were decreased, from 2.9637 g SECS using Rut-C30 cellulase to 2.0291 g SECS using E7 cellulase and 2.0573 g SECS using F6 cellulase. As a result, the efficiency of the process from SECS to glucose was substantially improved.

Conclusions

Enhanced co-expression of the β-glucosidase gene and the cellobiohydrolase II gene in T. reesei using the strong promoter Pcbh1 in T. reesei was proven triumphal in the simultaneous enhancement of the beta–exo synergism and exo–exo synergism in T. reesei cellulase. This strategy also improved the cellulase production, enzymatic hydrolysis yield and the efficiency of the process from SECS to glucose in the context of on-site cellulase production. This work is a commendable attempt in the cellulase composition optimization at the transcriptional level.

Effective one-step saccharification of lignocellulosic biomass using magnetite-biocatalysts containing saccharifying enzymes

Lignocellulosic biomass, packed with sugars, is one of the most available renewable resources for biofuels and bioproducts production. To release the sugars for the production, enzymatic hydrolysis (saccharification) of pretreated lignocellulosic biomass are required. However, the saccharification process is costly, inefficient, and requires multi-step operations. This is in part due to the high cost and the limited selection of commercial enzymes which commonly have different optimal pH and temperatures. Here we reported a one-step saccharification of pretreated lignocellulosic biomass using immobilized biocatalysts containing five different saccharifying enzymes (SEs) with a similar optimum pH and temperature. The five SEs - endo-1,4-β-d-glucanase (an endoglucanase, eglS), cellobiohydrolase (an exoglucanase, cbhA), and β-glucosidase (bglH), endo-1,4-β-xylanase (an endoxylanase, xynC) and β-xylosidase (bxlB) - were successfully expressed and produced by E. coli BL21. Better saccharification of pretreated corn husks was observed when using the five crude SE enzymes than those using two commonly used SEs, endo-1,4-β-d-glucanase and β-glucosidase. The five SEs were cross-linked in the absence or the presence of magnetic nanoparticles (hereafter referred as SE-CLEAs and M-SE-CLEAs, respectively). By using SE-CLEAs, the highest amount of reduced sugar (250 mg/g biomass) was measured. The activity of immobilized SEs is better than free crude SEs. The M-SE-CLEAs can be reused at least 3 times for effective saccharification of pretreated lignocellulosic biomass.

Addition of β-galactosidase boosts the xyloglucan degradation capability of endoglucanase Cel9D from Clostridium thermocellum

BACKGROUND

Increasing the efficiency of enzymatic biomass degradation is crucial for a more economically feasible conversion of abundantly available plant feedstock. Synergistic effects between the enzymes deployed in the hydrolysis of various hemicelluloses have been demonstrated, which can reduce process costs by lowering the amount of enzyme required for the reaction. Xyloglucan is the only major hemicellulose for which no such effects have been described yet.

RESULTS

We report the beneficial combination of two enzymes for the degradation of the hemicellulose xyloglucan. The addition of β-galactosidase Bga2B from Clostridium stercorarium to an in vitro hydrolysis reaction of a model xyloglucan substrate increased the enzymatic efficiency of endoglucanase Cel9D from Clostridium thermocellum to up to 22-fold. Furthermore, the total amount of enzyme required for high hydrolysis yields was lowered by nearly 80%. Increased yields were also observed when using a natural complex substrate-tamarind kernel powder.

CONCLUSION

The findings of this study may improve the valorization of feedstocks containing high-xyloglucan amounts. The combination of the endoglucanase Cel9D and the β-galactosidase Bga2B can be used to efficiently produce the heptasaccharide XXXG. The exploitation of one specific oligosaccharide may open up possibilities for the use as a prebiotic or platform chemical in additional reactions.

Effectiveness of cross-linked enzyme aggregates of cellulolytic enzymes in hydrolyzing wheat straw

Development of industrially potent cellulolytic enzymes is one of the greatest challenges faced in lignocellulosic feed-stock based bio-refining. In the current work cross-linked enzyme aggregates (CLEAs) of commercial cellulase mix were successfully prepared and their performance to be used as potential industrial enzymes in terms of stability and wheat straw hydrolysis was evaluated. The CLEAs were more stable compared to native enzymes with half-lives being 2.30-, 1.56-, 3.07- and 1.67-fold higher at 70°C for filter paper activity (FPA), endoglucanase, β-glucosidase and xylanase, respectively. CLEAs retained 77.4% of endoglucanase and 85.9% of xylanase activity after five cycles of hydrolysis of soluble substrates such as carboxymethyl cellulose and xylan, respectively. A maximum saccharification yield of 31.8% by soluble enzymes and 32.9% by CLEAs were obtained when alkali-pretreated wheat straw was subjected to hydrolysis. On repeated batch hydrolysis for five consecutive cycles of 24 h each, the CLEAs showed an overall higher saccharification yield of 43.3% compared to 31.8% with soluble enzymes.

One-pot bioprocess for lactic acid production from lignocellulosic agro-wastes by using ionic liquid stable Lactobacillus brevis

The lignocellulosic agro-wastes are an attractive renewable resource in biorefinery for production of value-added platform chemicals and biofuels. The study describes use of different agro-wastes as substrate for production of lactic acid, a C3-platform chemical and high demand industrial product by Lactobacillus brevis in a one-pot bioprocess. The simultaneous saccharification and co-fermentation (SSCF) process was achieved by L. brevis governed fermentation of sugars, derived from saccharification of ionic liquid pretreated feedstocks by nanoimmobilized cellulase, which was further recovered and used for consecutive cycle. The lactic acid yields of 0.22, 0.49, 0.52 g/g were obtained from cottonseed cake, wheat straw and sugarcane bagasse, respectively. The ionic liquid-tolerant L. brevis, cellulolytic reusable nanoimmobilized enzyme coupled with valorization of renewable feedstocks points towards a holistic approach for future biorefineries with sustainable production of bioproducts.

Assessment of hazelnut husk as a lignocellulosic feedstock for the production of fermentable sugars and lignocellulolytic enzymes

The present work focuses firstly on the evaluation of the effect of laccase on enzymatic hydrolysis of hazelnut husk which is one of the most abundant lignocellulosic agricultural residues generated in Turkey. In this respect, the co-enzymatic treatment of hazelnut husk by cellulase and laccase, without a conventional pretreatment step is evaluated. Using 2.75 FPU/g substrate (40 g/L substrate) and a ratio of 131 laccase U/FPU achieved the highest reducing sugars concentration. Gas chromatography mass spectrometry confirmed that the hydrolysate was composed of glucose, xylose, mannose, arabinose and galactose. The inclusion of laccase in the enzyme mixture [carboxymethyl cellulase (CMCase) and β-glucosidase] increased the final glucose content of the reducing sugars from 20 to 50%. Therefore, a very significant increase in glucose content of the final reducing sugars concentration was obtained by laccase addition. Furthermore, the production of cellulases and laccase by Pycnoporus sanguineus DSM 3024 using hazelnut husk as substrate was also investigated. Among the hazelnut husk concentrations tested (1.5, 6, 12, 18 g/L), the highest CMCase concentration was obtained using 12 g/L husk concentration on the 10th day of fermentation. Besides CMCase, P. sanguineus DSM 3024 produced β-glucosidase and laccase using hazelnut husk as carbon source. In addition to CMCase and β-glucosidase, the highest laccase activity measured was 2240 ± 98 U/L (8.89 ± 0.39 U/mg). To the best of our knowledge, this is the first study to report hazelnut husk hydrolysis in the absence of pretreatment procedures.

A new method to evaluate temperature vs. pH activity profiles for biotechnological relevant enzymes

BACKGROUND

Glycoside hydrolases are important for various industrial and scientific applications. Determination of their temperature as well as pH optima and range is crucial to evaluate whether an enzyme is suitable for application in a biotechnological process. These basic characteristics of enzymes are generally determined by two separate measurements. However, these lead to a two-dimensional assessment of the pH range at one temperature (and vice versa) and do not allow prediction of the relative enzymatic performance at any pH/temperature combination of interest. In this work, we demonstrate a new method that is based on experimental data and visualizes the relationship among pH, temperature, and activity at a glance in a three-dimensional contour plot.

RESULTS

In this study, we present a method to determine the relative activity of an enzyme at 96 different combinations of pH and temperature in parallel. For this purpose, we used a gradient PCR cycler and a citrate-phosphate-based buffer system in microtiter plates. The approach was successfully tested with various substrates and diverse assays for glycoside hydrolases. Furthermore, its applicability was demonstrated for single enzymes using the endoglucanase Cel8A from Clostridium thermocellum as well as the commercially available complex enzyme mixture Celluclast^®. Thereby, we developed a fast and adaptable method to determine simultaneously both pH and temperature ranges of enzymes over a wide range of conditions, an easy transformation of the experimental data into a contour plot for visualization, and the necessary controls. With our method, the suitability of an enzyme or enzyme mixture for any chosen combination of temperature and pH can easily be assessed at a glance.

CONCLUSIONS

We propose a method that offers significant advantages over commonly used methods to determine the pH and temperature ranges of enzymes. The overall relationship among pH, temperature, and activity is visualized. Our method could be applied to evaluate exactly what conditions have to be met for optimal utilization of an enzyme or enzyme mixture for both lab-scale and industrial processes. Adaptation to other enzymes, including proteases, should be possible and the method may also lead to a platform for additional applications, such as inactivation kinetics analysis.

Development of cellulase-nanoconjugates with enhanced ionic liquid and thermal stability for in situ lignocellulose saccharification

The present work aimed to improve catalytic efficiency of Trichoderma reesei cellulase for enhanced saccharification. The cellulase was immobilized on two nanomatrices i.e. magnetic and silica nanoparticles with immobilization efficiency of 85% and 76% respectively. The nanobioconjugates exhibited increase in V_max, temperature optimum, pH and thermal stability as compared with free enzyme. These could be efficiently reused for five repeated cycles and were stable in 1-ethyl-3-methylimidazoliumacetate [EMIM][Ac], an ionic liquid. Ionic liquids (IL) are used as green solvents to dissolve lignocellulosic biomass and facilitate better saccharification. The cellulase immobilized on magnetic nanoparticles was used for in situ saccharification of [EMIM][Ac] pretreated sugarcane bagasse and wheat straw for two cycles. The structural deconstruction and decrease in biomass crystallinity was confirmed by SEM, XRD and FTIR. The high hydrolysis yields (∼89%) obtained in this one-pot process coupled with IL stability and recycled use of immobilized cellulase, potentiates its usefulness in biorefineries.

Fermentation Process and Metabolic Flux of Ethanol Production from the Detoxified Hydrolyzate of Cassava Residue

With the growth of the world population, energy problems are becoming increasingly severe; therefore, sustainable energy sources have gained enormous importance. With respect to ethanol fuel production, biomass is gradually replacing grain as the main raw material. In this study, we explored the fermentation of five- and six-carbon sugars, the main biomass degradation products, into alcohol. We conducted mutagenic screening specifically for Candida tropicalis CICC1779 to obtain a strain that effectively used xylose (Candida tropicalis CICC1779-Dyd). By subsequently studying fermentation conditions under different initial liquid volume oxygen transfer coefficients (k_Lα), and coupling control of the aeration rate and agitation speed under optimal conditions, the optimal dissolved oxygen change curve was obtained. In addition, we constructed metabolic flow charts and equations to obtain a better understanding of the fermentation mechanism and to improve the ethanol yield. In our experiment, the ethanol production of the wild type stain was 17.58 g·L⁻¹ at a k_Lα of 120. The highest ethanol yield of the mutagenic strains was 24.85 g·L⁻¹. The ethanol yield increased to 26.56 g·L⁻¹ when the dissolved oxygen content was optimized, and the conversion of sugar into alcohol reached 0.447 g·g⁻¹ glucose (the theoretical titer of yeast-metabolized xylose was 0.46 g ethanol/g xylose and the glucose ethanol fermentation titer was 0.51 g ethanol/g glucose). Finally, the detected activity of xylose reductase and xylose dehydrogenase was higher in the mutant strain than in the original, which indicated that the mutant strain (CICC1779-Dyd) could effectively utilize xylose for metabolism.

A Chrysoporthe cubensis enzyme cocktail produced from a low-cost carbon source with high biomass hydrolysis efficiency

Low cost and high efficiency cellulolytic cocktails can consolidate lignocellulosic ethanol technologies. Sugarcane bagasse (SCB) is a low cost agro-industrial residue, and its use as a carbon source can reduce the costs of fungi cultivation for enzyme production. Chrysoporthe cubensis grown under solid state fermentation (SSF) with wheat bran has potential to produce efficient enzymatic extracts for SCB saccharification. This fungus was grown under submersed fermentation (SmF) and SSF with in natura SCB, pretreated with acid or alkali and with others carbon sources. In natura SCB induced the highest carboxymethylcellulase (CMCase), xylanase, β-xylosidase, α-galactosidase and mannanase activities by C. cubensis under SSF. In natura and washed SCB, inducers of enzyme production under SSF, did not induce high cellulases and hemicellulases production by C. cubensis in SmF. The C. cubensis enzymatic extract produced under SSF with in natura SCB as a carbon source was more efficient for lignocelulolic biomass hydrolysis than extracts produced under SSF with wheat bran and commercial cellulolytic extract. Chrysoporthe cubensis showed high potential for cellulases and hemicellulases production, especially when grown under SSF with in natura SCB as carbon source.

Xylanase production by Aureobasidium pullulans on globe artichoke stem: Bioprocess optimization, enzyme characterization and application in saccharification of lignocellulosic biomass

Statistical optimization of the factors affecting xylanase production by Aureobasidium pullulans NRRL Y-2311-1 on globe artichoke stem was performed for the first time. The optimization strategies used resulted in almost six-fold enhancement of xylanase production (66.48 U/ml). Biochemical and thermal characterization of the crude xylanase preparation was performed to elucidate its feasibility for different industrial applications. The optimum conditions for xylanase activity were pH 4.0 and 30-50°C. The enzyme was very stable over a wide pH range of 3.0-8.0. The thermal stability studies revealed an inactivation energy of 183 kJ/mol. Thermodynamic parameters (enthalpy, entropy, and Gibbs free energy) for thermal inactivation were also determined. Primary application of the crude xylanase preparation in saccharification of corn cob subjected to different pretreatment techniques has been evaluated. The crude xylanase preparation was very promising for saccharification of corn cob pretreated with aqueous ammonia. The maximum yield of reducing sugar was 357 mg/g dry substrate, which revealed that the crude xylanase from A. pullulans could be a very good alternative in saccharification of lignocellulosic biomass for biological fuel generation. This study also provides a basis for further exploitation of globe artichoke by-products in microbial production of several other industrially significant metabolites.

Bioprocessing of wheat bran for the production of lignocellulolytic enzyme cocktail by Cotylidia pannosa under submerged conditions

Characterization and production of efficient lignocellulytic enzyme cocktails for biomass conversion is the need for biofuel industry. The present investigation reports the modeling and optimization studies of lignocellulolytic enzyme cocktail production by Cotylidia pannosa under submerged conditions. The predominant enzyme activities of cellulase, xylanase and laccase were produced in the cocktail through submerged conditions using wheat bran as a substrate. A central composite design approach was utilized to model the production process using temperature, pH, incubation time and agitation as input variables with the goal of optimizing the output variables namely cellulase, xylanase and laccase activities. The effect of individual, square and interaction terms on cellulase, xylanase and laccase activities were depicted through the non-linear regression equations with significant R(2) and P-values. An optimized value of 20 U/ml, 17 U/ml and 13 U/ml of cellulase, xylanase and laccase activities, respectively, were obtained with a media pH of 5.0 in 77 h at 31C, 140 rpm using wheatbran as a substrate. Overall, the present study introduces a fungal strain, capable of producing lignocellulolytic enzyme cocktail for subsequent applications in biofuel industry.