Effect of enzyme supplementation at moderate cellulase loadings on initial glucose and xylose release from corn stover solids pretreated by leading technologies

Are cellulases slow? Kinetic and thermodynamic limitations for enzymatic breakdown of cellulose

Cellulases are of paramount interest for upcoming biorefineries that utilize residue from agriculture and forestry to produce sustainable fuels and chemicals. Specifically, cellulases are used for the conversion of recalcitrant plant biomass to fermentable sugars in a so-called saccharification process. The vast literature on enzymatic saccharification frequently refers to low catalytic rates of cellulases as a main bottleneck for industrial implementation, but such statements are rarely supported by kinetic or thermodynamic considerations. In this perspective, we first discuss activation barriers and equilibrium conditions for the hydrolysis of cellulose and how these parameters influence enzymatic turnover. Next, we propose a simple framework for kinetic description of cellulolytic enzyme reactions and show how this can pave the way for comparative biochemical analyses of cellulases acting on their native, insoluble substrate. This latter analysis emphasizes that cellulases are characterized by extraordinarily low off-rate constants, while other kinetic parameters including specificity constants and rate constants for association and bond cleavage are quite like parameters reported for related enzymes acting on soluble substrates.

Valorization of Lignocellulose with One-Step Acidified Monophasic Phenoxyethanol Fractionation

Effective fractionation of lignocelluosic biomass and subsequent valorization of all three major components under mild conditions were achieved. Pretreatment with acidified monophasic phenoxyethanol (EPH) efficiently removed 92.6 % lignin and 80 % xylan from poplar at 110 °C in 60 min, yielding high-value EPH-xyloside, EPH-modified lignin (EPHL), and a solid residue nearly purely composed of carbohydrates. After removing the grafted acetyl groups using 1 % NaOH at 50 °C, the highest enzymatic digestibility reached 92.3 %. EPHL could be recovered in high yield and purity with an uncondensed structure, while xylose was converted to EPH-xyloside, a potential precursor in biomedical industries. Additionally, the acidified monophasic EPH solvent could effectively fractionate biomass from species other than hardwood, achieving over 70 % delignification from recalcitrant pinewood under the same mild conditions, demonstrating the high potential of monophasic EPH pretreatment.

Application of an aqueous enzymatic-ultrasound cavitation method for the separation of Sapium sebiferum seed kernel oil

An aqueous enzymatic-ultrasound cavitation extraction (AEUCE) method was developed to separate Sapium sebiferum seed kernel oil. In this process, neutral proteinase was screened as the propriate enzyme. The Plackett-Burman and Box-Behnken designs were employed to optimize AEUCE. We determined the optimal extraction conditions, producing an oil yield of 84.22 ± 3.17 %. Gas chromatography-mass spectrometry (GC-MS) analysis indicated that the S. sebiferum seed kernel oil was abundant in unsaturated fatty acids (>92 %) and that the compositions of the fatty acid profiles extracted by AEUCE were similar to those obtained from Soxhlet extraction, but their contents were slightly different. The physicochemical properties analysis showed that the oil extracted by AEUCE was comparable to that obtained from Soxhlet extraction. The results showed that the developed AEUCE is an efficient technique that can separate high-quality plant oils. The S. sebiferum seed kernel oil obtained from this extraction method is a promising substitute for vegetable oils used in biodiesel production.

Glycosyl hydrolase 11 (xynA) gene with xylanase activity from thermophilic bacteria isolated from thermal springs

Background

Hemicellulose is one of the copious polymer in lignocellulosic biomass (LCB). It is primarily composed of xylan linked by β-1,4 glycosidic bonds. Xylanase preferentially cleaves the β-1,4-glycosidic bonds in the xylan backbone resulting in complete hydrolysis of the biomass. Thermostable variants of glycoside hydrolases act as robust catalysts, not only in degradation but also during processing, to obtain specific carbohydrate-containing chemicals and materials (Ramasamy et al. in Madras Agric J 107(special):1. 10.29321/MAJ.2020.000382, 2020).

Results

The xylanase production by two thermophilic bacteria isolated from thermal springs was evaluated. In addition, the gene encoding this industrially vital enzyme was isolated and characterized, and its protein structure was analyzed. The thermophilic bacteria producing xylanases were isolated from augmented sawdust and banana fiber biomass from hot springs of Himachal Pradesh and identified as Bacillus subtilis VSDB5 and Bacillus licheniformis KBFB4 using 16S rRNA gene sequencing. The persistent xylanase activity revealed that the enzyme is secreted extracellularly with the maximum activity of 0.76 IU mL−1 and 1.0 IU mL−1 at 6 h and 12 h of growth by KBFB4 and VSDB5, respectively, under submerged fermentation. Both the strains exhibited the maximum activity at pH 6 and a temperature of 50 °C. The xylanases of KBFB4 and VSDB5 were thermostable and retained 40% of their activity at 60 °C after incubation for 30 min. Xylanase of VSDB5 had wide thermotolerance and retained 20% of its activity from 60 to 80 °C, whereas xylanase of KBFB4 showed wide alkali tolerance and retained 80% of its activity until pH 10. The xylanase (xynA)-encoding gene (650 bp) cloned from both the strains using specific primers showed 98 to 99% homology to β-1,4-endoxylanase gene. Further in silico analysis predicted that the xylanase protein, with a molecular weight of 23 kDa, had a high pI (9.44–9.65), which explained the alkaline nature of the enzyme and greater aliphatic index (56.29). This finding suggested that the protein is thermostable. Multiple sequence alignment and homology modeling of the protein sequence revealed that the gene product belonged to the GH11 family, indicating its possible application in bioconversion.

Conclusion

The strains B. subtilis VSDB5 and B. licheniformis KBFB4 obtained from hot springs of Himachal Pradesh produced potent and alkali-tolerant thermostable xylanases, which belong to the GH11 family. The enzyme can be supplemented in industrial applications for biomass conversion at high temperatures and pH (or in processes involving alkali treatment).

Graphical Abstract

Analysis of the galactomannan binding ability of β-mannosidases, BtMan2A and CmMan5A, regarding their activity and synergism with a β-mannanase

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BtMan2A preferred short manno-oligomers, while CmMan5A preferred longer ones; DP >2.

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BtMan2A displayed stronger irreversible binding to galactomannan than CmMan5A.

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BtMan2A binding to galactomannan did not affect its activity, while CmMan5A lost activity.

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BtMan2A binding was pH-dependent, with increased binding ability at lower pH.

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CmMan5A synergised with CcManA, while BtMan2A did not – even though the enzyme was active.

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High loadings of BtMan2A abolished CcManA activity; at protein ratios ≥ 5:1.

Both β-mannanases and β-mannosidases are required for mannan-backbone degradation into mannose. In this study, two β-mannosidases of glycoside hydrolase (GH) families 2 (BtMan2A) and 5 (CmMan5A) were evaluated for their substrate specificities and galactomannan binding ability. BtMan2A preferred short manno-oligomers, while CmMan5A preferred longer ones; DP >2, and galactomannans. BtMan2A displayed irreversible galactomannan binding, which was pH-dependent, with higher binding observed at low pH, while CmMan5A had limited binding. Docking and molecular dynamics (MD) simulations showed that BtMan2A galactomannan binding was stronger under acidic conditions (-8.4 kcal/mol) than in a neutral environment (-7.6 kcal/mol), and the galactomannan ligand was more unstable under neutral conditions than acidic conditions. Qualitative surface plasmon resonance (SPR) experimentally confirmed the reduced binding capacity of BtMan2A at pH 7. Finally, synergistic β-mannanase to β-mannosidase (BtMan2A or CmMan5A) ratios required for maximal galactomannan hydrolysis were determined. All CcManA to CmMan5A combinations were synergistic (≈1.2-fold), while combinations of CcManA with BtMan2A (≈1.0-fold) yielded no hydrolysis improvement. In conclusion, the low specific activity of BtMan2A towards long and galactose-containing oligomers and its non-catalytic galactomannan binding ability led to no synergy with the mannanase, making GH2 mannosidases ineffective for use in cocktails for mannan degradation.

Analytical Characterization and Inhibitor Detection in Liquid Phases Obtained After Steam Refining of Corn Stover and Maize Silage

The utilization of agricultural products and residues for the production of value-added and biobased products is a highly relevant topic in present research. Due to the natural recalcitrance of lignocellulosic biomass against enzymatic degradation, pretreatments are important requirement for further processes. For the raw material in this study, corn stover (CS) as highly available agricultural residue and maize silage (MS) as model substrate for an ensiled agricultural product were pretreated by steam refining. However, after processing a liquid fraction and fibers are present. Subsequent to steaming the fiber fraction is well characterized. Nonetheless, in depth characterizations of the filtrates are also important for their subsequent utilization. Decreasing molar masses from 7,900 g/mol to 1,100 g/mol for CS filtrates and 100.000–12.900 g/mol for MS filtrates were determined with increasing severity. Due to their proven inhibitory effect on microorganisms weak acids, furans and phenolic compounds within the liquid phased were analyzed. Especially formic acid increases with increasing severity from 0.27 to 1.20% based on raw material for CS and from 0.07 to 0.23% based on raw material for MS. Further GC/MS measurements indicate, that up to 8.25% (CS filtrate) and 5.23% (MS filtrates) of the total peak area is related to inhibitory phenols. Considering the data, detoxification strategies are of non-negligible importance for filtrates after steam refining and should be considered for further research and process or parameter optimizations. An alternative may be the application of milder process conditions in order to prevent the formation of inhibitory degradation products or the dilution of the gained filtrates.

A novel mechanism of β-glucosidase stimulation through a monosaccharide binding-induced conformational change

It is urgent the transition from a fossil fuel-based economy to a sustainable bioeconomy based on bioconversion technologies using renewable plant biomass feedstocks to produce high chemicals, bioplastics, and biofuels. β-Glucosidases are key enzymes responsible for degrading the plant cell wall polymers, as they cleave glucan-based oligo- and polysaccharides to generate glucose. Monosaccharide-tolerant or -stimulated β-glucosidases have been reported in the past decade. Here, we describe a novel mechanism of β-glucosidase stimulation by glucose and xylose. The glycoside hydrolase 1 family β-glucosidase from Thermotoga petrophila (TpBgl1) displays a typical glucose stimulation mechanism based on an increased V_max and decreased K_m in response to glucose. Through molecular docking and dynamics analyses, we mapped putative monosaccharide binding regions (BRs) on the surface of TpBgl1. Our results indicate that after interaction with glucose or xylose at BR1 site, an adjacent loop region assumes an extended conformation, which increases the entrance to the TpBgl1 active site, improving product formation. Biochemical assays with TpBgl1 BR1 mutants, TpBgl1^D49A/Y410A and TpBgl1^D49K/Y410H, resulted in decreasing and abolishing monosaccharide stimulation, respectively. These mutations also impaired the BR1 looping extension responsible for monosaccharide stimulation. This study provides a molecular basis for the rational design of β-glucosidases for biotechnological applications.

Understanding the structural characteristics of water-soluble phenolic compounds from four pretreatments of corn stover and their inhibitory effects on enzymatic hydrolysis and fermentation

BACKGROUND

For bioethanol production from lignocellulosic biomass, phenolics derived from pretreatment have been generally considered as highly inhibitory towards enzymatic hydrolysis and fermentation. As phenolics are produced from lignin degradation during pretreatment, it is likely that the pretreatment will exert a strong impact on the structure of phenolics, resulting in varied levels of inhibition of the bioconversion process. Despite the extensive studies on pretreatment, it remains unclear how pretreatment process affects the properties of generated phenolics and how the inhibitory effect of phenolics from different pretreatment varies on enzymatic hydrolysis and fermentation.

RESULTS

In this study, the structural properties of phenolic compounds derived from four typical pretreatment [dilute acid (DA), liquid hot water pretreatment (LHW), ammonia fiber expansion (AFEX) and alkaline pretreatment (AL)] were characterized, and their effect on both enzymatic hydrolysis and fermentation were evaluated. The inhibitory effect of phenolics on enzymatic hydrolysis followed the order: AFEX > LHW > DA > AL, while the inhibitory effect of phenolics on Zymomonas mobilis 8b strain fermentation followed the order: AL > LHW > DA > AFEX. Interestingly, this study revealed that phenolics derived from AFEX showed more severe inhibitory effect on enzymatic hydrolysis than those from the other pretreatments at the same phenolics concentrations (note: AFEX produced much less amount of phenolics compared to AL and DA), while they exhibited the lowest inhibitory effect on fermentation. The composition of phenolics from different pretreatments was analyzed and model phenolics were applied to explore the reason for this difference. The results suggested that the amide group in phenolics might account for this difference.

CONCLUSIONS

Pretreatment process greatly affects the properties of generated phenolics and the inhibitory effects of phenolics on enzymatic hydrolysis and fermentation. This study provides new insight for further pretreatment modification and hydrolysate detoxification to minimize phenolics-caused inhibition and enhance the efficiency of enzymatic hydrolysis and fermentation.

Constraints and advances in high-solids enzymatic hydrolysis of lignocellulosic biomass: a critical review

The industrial production of sugar syrups from lignocellulosic materials requires the conduction of the enzymatic hydrolysis step at high-solids loadings (i.e., with over 15% solids [w/w] in the reaction mixture). Such conditions result in sugar syrups with increased concentrations and in improvements in both capital and operational costs, making the process more economically feasible. However, this approach still poses several technical hindrances that impact the process efficiency, known as the "high-solids effect" (i.e., the decrease in glucan conversion yields as solids load increases). The purpose of this review was to present the findings on the main limitations and advances in high-solids enzymatic hydrolysis in an updated and comprehensive manner. The causes for the rheological limitations at the onset of the high-solids operation as well as those influencing the "high-solids effect" will be discussed. The subject of water constraint, which results in a highly viscous system and impairs mixing, and by extension, mass and heat transfer, will be analyzed under the perspective of the limitations imposed to the action of the cellulolytic enzymes. The "high-solids effect" will be further discussed vis-à-vis enzymes end-product inhibition and the inhibitory effect of compounds formed during the biomass pretreatment as well as the enzymes' unproductive adsorption to lignin. This review also presents the scientific and technological advances being introduced to lessen high-solids hydrolysis hindrances, such as the development of more efficient enzyme formulations, biomass and enzyme feeding strategies, reactor and impeller designs as well as process strategies to alleviate the end-product inhibition. We surveyed the academic literature in the form of scientific papers as well as patents to showcase the efforts on technological development and industrial implementation of the use of lignocellulosic materials as renewable feedstocks. Using a critical approach, we expect that this review will aid in the identification of areas with higher demand for scientific and technological efforts.

Lignocellulosic Biomass: Understanding Recalcitrance and Predicting Hydrolysis

Lignocellulosic biomass (LB) is an abundant and renewable resource from plants mainly composed of polysaccharides (cellulose and hemicelluloses) and an aromatic polymer (lignin). LB has a high potential as an alternative to fossil resources to produce second-generation biofuels and biosourced chemicals and materials without compromising global food security. One of the major limitations to LB valorisation is its recalcitrance to enzymatic hydrolysis caused by the heterogeneous multi-scale structure of plant cell walls. Factors affecting LB recalcitrance are strongly interconnected and difficult to dissociate. They can be divided into structural factors (cellulose specific surface area, cellulose crystallinity, degree of polymerization, pore size and volume) and chemical factors (composition and content in lignin, hemicelluloses, acetyl groups). Goal of this review is to propose an up-to-date survey of the relative impact of chemical and structural factors on biomass recalcitrance and of the most advanced techniques to evaluate these factors. Also, recent spectral and water-related measurements accurately predicting hydrolysis are presented. Overall, combination of relevant factors and specific measurements gathering simultaneously structural and chemical information should help to develop robust and efficient LB conversion processes into bioproducts.

A review on commercial-scale high-value products that can be produced alongside cellulosic ethanol

The demand for fossil derivate fuels and chemicals has increased, augmenting concerns on climate change, global economic stability, and sustainability on fossil resources. Therefore, the production of fuels and chemicals from alternative and renewable resources has attracted considerable and growing attention. Ethanol is a promising biofuel that can reduce the consumption of gasoline in the transportation sector and related greenhouse gas (GHG) emissions. Lignocellulosic biomass is a promising feedstock to produce bioethanol (cellulosic ethanol) because of its abundance and low cost. Since the conversion of lignocellulose to ethanol is complex and expensive, the cellulosic ethanol price cannot compete with those of the fossil derivate fuels. A promising strategy to lower the production cost of cellulosic ethanol is developing a biorefinery which produces ethanol and other high-value chemicals from lignocellulose. The selection of such chemicals is difficult because there are hundreds of products that can be produced from lignocellulose. Multiple reviews and reports have described a small group of lignocellulose derivate compounds that have the potential to be commercialized. Some of these products are in the bench scale and require extensive research and time before they can be industrially produced. This review examines chemicals and materials with a Technology Readiness Level (TRL) of at least 8, which have reached a commercial scale and could be shortly or immediately integrated into a cellulosic ethanol process.

Hemicellulose from Plant Biomass in Medical and Pharmaceutical Application: A Critical Review

BACKGROUND

Due to the non-toxicity, abundance and biodegradability, recently more and more attention has been focused on the exploration of hemicellulose as the potential substrate for the production of liquid fuels and other value-added chemicals and materials in different fields. This review aims to summarize the current knowledge on the promising application of nature hemicellulose and its derivative products including its degradation products, its new derivatives and hemicellulosebased medical biodegradable materials in the medical and pharmaceutical field, especially for inmmune regulation, bacteria inhibition, drug release, anti-caries, scaffold materials and anti-tumor.

METHODS

We searched the related papers about the medical and pharmaceutical application of hemicellulose and its derivative products, and summarized their preparation methods, properties and use effects.

RESULTS

Two hundred and twenty-seven papers were included in this review. Forty-seven papers introduced the extraction and application in immune regulation of nature hemicellulose, such as xylan, mannan, xyloglucan (XG) and β-glucan. Seventy-seven papers mentioned the preparation and application of degradation products of hemicellulose for adjusting intestinal function, maintaining blood glucose levels, enhancing the immunity and alleviating human fatigue fields such as xylooligosaccharides, xylitol, xylose, arabinose, etc. The preparation of hemicellulose derivatives were described in thirty-two papers such as hemicellulose esters, hemicellulose ethers and their effects on anticoagulants, adsorption of creatinine, the addition of immune cells and the inhibition of harmful bacteria. Finally, the preparations of hemicellulose-based materials such as hydrogels and membrane for the field of drug release, cell immobilization, cancer therapy and wound dressings were presented using fifty-five papers.

CONCLUSION

The structure of hemicellulose-based products has the significant impact on properties and the use effect for the immunity, and treating various diseases of human. However, some efforts should be made to explore and improve the properties of hemicellulose-based products and design the new materials to broaden hemicellulose applications.

Enhanced depolymerization and utilization of raw lignocellulosic material by co-cultures of Ruminiclostridium thermocellum with hemicellulose-utilizing partners

Ruminiclostridium thermocellum is one of the most promising candidates for consolidated bioprocessing (CBP) of low-cost lignocellulosic materials to biofuels but it still shows poor performance in its ability to deconstruct untreated lignocellulosic substrates. One promising approach to increase R. thermocellum's rate of hydrolysis is to co-culture this cellulose-specialist with partners that possess synergistic hydrolysis enzymes and metabolic capabilities. We have created co-cultures of R. thermocellum with two hemicellulose utilizers, Ruminiclostridium stercorarium and Thermoanaerobacter thermohydrosulfuricus, both of which secrete xylanolytic enzymes and utilize the pentose oligo- and monosaccharides that inhibit R. thermocellum's hydrolysis and metabolism. When grown on milled wheat straw, the co-cultures were able to solubilize up to 58% more of the total polysaccharides than the R. thermocellum mono-culture control. Repeated passaging of the co-cultures on wheat straw yielded stable populations with reduced R. thermocellum cell numbers, indicating competition for cellodextrins released from cellulose hydrolysis, although these stabilized co-cultures were still able to outperform the mono-culture controls. Repeated passaging on Avicel cellulose also yielded stable populations. Overall, the observed synergism suggests that co-culturing R. thermocellum with other members is a viable option for increasing the rate and extent of untreated lignocellulose deconstruction by R. thermocellum for CBP purposes.

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.

Improving enzymatic digestibility of wheat straw pretreated by a cellulase-free xylanase-secreting Pseudomonas boreopolis G22 with simultaneous production of bioflocculants

BACKGROUND

Xylan removal by bacterial pretreatments has been confirmed to increase the digestibility of biomass. Here, an effective xylan removal technique has been developed to enhance the digestibility of wheat straw and simultaneously produce bioflocculants by a cellulase-free xylanase-secreting strain, Pseudomonas boreopolis G22.

RESULTS

The results indicated that P. boreopolis G22 is an alkaliphilic strain which can secrete abundant amounts of xylanase. This xylanase had activity levels of 2.67-1.75 U mL^-1 after an incubation period of 5-25 days. The xylanase showed peak activity levels at pH 8.6, and retained more than 85% relative activity in the pH range of 7.2-9.8. After 15 days of cultivation, the hemicellulose contents of the wheat straw were significantly decreased by 32.5%, while its cellulose contents were increased by 27.3%, compared to that of the control. The maximum reducing sugars released from the 15-day-pretreated wheat straw were 1.8-fold higher than that of the untreated wheat straw, under optimal enzymatic hydrolysis conditions. In addition, a maximum bioflocculant yield of 2.08 g L^-1 was extracted from the fermentation broth after 15 days of incubation. The aforementioned bioflocculants could be used to efficiently decolorize a dye solution.

CONCLUSIONS

The results indicate that the cellulase-free xylanase-secreting P. boreopolis G22 may be a potential strain for wheat straw pretreatments. The strain G22 does not only enhance the enzymatic digestibility of wheat straw, but also simultaneously produces a number of bioflocculants that can be used for various industrial applications.

Biomass augmentation through thermochemical pretreatments greatly enhances digestion of switchgrass by Clostridium thermocellum

BACKGROUND

The thermophilic anaerobic bacterium Clostridium thermocellum is a multifunctional ethanol producer, capable of both saccharification and fermentation, that is central to the consolidated bioprocessing (CBP) approach of converting lignocellulosic biomass to ethanol without external enzyme supplementation. Although CBP organisms have evolved efficient machinery for biomass deconstruction, achieving complete solubilization requires targeted approaches, such as pretreatment, to prepare recalcitrant biomass feedstocks for further biological digestion. Here, differences between how C. thermocellum and fungal cellulases respond to senescent switchgrass prepared by four different pretreatment techniques revealed relationships between biomass substrate composition and its digestion by the two biological approaches.

RESULTS

Alamo switchgrass was pretreated using hydrothermal, dilute acid, dilute alkali, and co-solvent-enhanced lignocellulosic fractionation (CELF) pretreatments to produce solids with varying glucan, xylan, and lignin compositions. C. thermocellum achieved highest sugar release and metabolite production from de-lignified switchgrass prepared by CELF and dilute alkali pretreatments demonstrating greater resilience to the presence of hemicellulose sugars than fungal enzymes. 100% glucan solubilization and glucan plus xylan release from switchgrass were achieved using the CELF-CBP combination. Lower glucan solubilization and metabolite production by C. thermocellum was observed on solids prepared by dilute acid and hydrothermal pretreatments with higher xylan removal from switchgrass than lignin removal. Further, C. thermocellum (2% by volume inoculum) showed ~ 48% glucan solubilization compared to < 10% through fungal enzymatic hydrolysis (15 and 65 mg protein/g glucan loadings) of unpretreated switchgrass indicating the effectiveness of C. thermocellum's cellulosome. Overall, C. thermocellum performed equivalent to 65 and better than 15 mg protein/g glucan fungal enzymatic hydrolysis on all substrates except CELF-pretreated substrates. CELF pretreatments of switchgrass produced solids that were highly digestible regardless of whether C. thermocellum or fungal enzymes were chosen.

CONCLUSIONS

The unparalleled comprehensive nature of this work with a comparison of four pretreatment and two biological digestion techniques provides a strong platform for future integration of pretreatment with CBP. Lignin removal had a more positive impact on biological digestion of switchgrass than xylan removal from the biomass. However, the impact of switchgrass structural properties, including cellulose, hemicellulose, and lignin characterization, would provide a better understanding of lignocellulose deconstruction.

Physico-Chemical Conversion of Lignocellulose: Inhibitor Effects and Detoxification Strategies: A Mini Review

A pretreatment of lignocellulosic biomass to produce biofuels, polymers, and other chemicals plays a vital role in the biochemical conversion process toward disrupting the closely associated structures of the cellulose-hemicellulose-lignin molecules. Various pretreatment steps alter the chemical/physical structure of lignocellulosic materials by solubilizing hemicellulose and/or lignin, decreasing the particle sizes of substrate and the crystalline portions of cellulose, and increasing the surface area of biomass. These modifications enhance the hydrolysis of cellulose by increasing accessibilities of acids or enzymes onto the surface of cellulose. However, lignocellulose-derived byproducts, which can inhibit and/or deactivate enzyme and microbial biocatalysts, are formed, including furan derivatives, lignin-derived phenolics, and carboxylic acids. These generation of compounds during pretreatment with inhibitory effects can lead to negative effects on subsequent steps in sugar flat-form processes. A number of physico-chemical pretreatment methods such as steam explosion, ammonia fiber explosion (AFEX), and liquid hot water (LHW) have been suggested and developed for minimizing formation of inhibitory compounds and alleviating their effects on ethanol production processes. This work reviews the physico-chemical pretreatment methods used for various biomass sources, formation of lignocellulose-derived inhibitors, and their contributions to enzymatic hydrolysis and microbial activities. Furthermore, we provide an overview of the current strategies to alleviate inhibitory compounds present in the hydrolysates or slurries.

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.

Adding tetrahydrofuran to dilute acid pretreatment provides new insights into substrate changes that greatly enhance biomass deconstruction by Clostridium thermocellum and fungal enzymes

BACKGROUND

Consolidated bioprocessing (CBP) by anaerobes, such as Clostridium thermocellum, which combine enzyme production, hydrolysis, and fermentation are promising alternatives to historical economic challenges of using fungal enzymes for biological conversion of lignocellulosic biomass. However, limited research has integrated CBP with real pretreated biomass, and understanding how pretreatment impacts subsequent deconstruction by CBP vs. fungal enzymes can provide valuable insights into CBP and suggest other novel biomass deconstruction strategies. This study focused on determining the effect of pretreatment by dilute sulfuric acid alone (DA) and with tetrahydrofuran (THF) addition via co-solvent-enhanced lignocellulosic fractionation (CELF) on deconstruction of corn stover and Populus with much different recalcitrance by C. thermocellum vs. fungal enzymes and changes in pretreated biomass related to these differences.

RESULTS

Coupling CELF fractionation of corn stover and Populus with subsequent CBP by the anaerobe C. thermocellum completely solubilized polysaccharides left in the pretreated solids within only 48 h without adding enzymes. These results were better than those from the conventional DA followed by either CBP or fungal enzymes or CELF followed by fungal enzyme hydrolysis, especially at viable enzyme loadings. Enzyme adsorption on CELF-pretreated corn stover and CELF-pretreated Populus solids were virtually equal, while DA improved the enzyme accessibility for corn stover more than Populus. Confocal scanning light microscopy (CSLM), transmission electron microscopy (TEM), and NMR characterization of solids from both pretreatments revealed differences in cell wall structure and lignin composition, location, coalescence, and migration-enhanced digestibility of CELF-pretreated solids.

CONCLUSIONS

Adding THF to DA pretreatment (CELF) greatly enhanced deconstruction of corn stover and Populus by fungal enzymes and C. thermocellum CBP, and the CELF-CBP tandem was agnostic to feedstock recalcitrance. Composition measurements, material balances, cellulase adsorption, and CSLM and TEM imaging revealed adding THF enhanced the enzyme accessibility, cell wall fractures, and cellular dislocation and cell wall delamination. Overall, enhanced deconstruction of CELF solids by enzymes and particularly by C. thermocellum could be related to lignin removal and alteration, thereby pointing to these factors being key contributors to biomass recalcitrance as a barrier to low-cost biological conversion to sustainable fuels.

Additives enhancing enzymatic hydrolysis of lignocellulosic biomass

Linked to the development of cellulolytic enzyme cocktails from Myceliophthora thermophila, we studied the effect of different additives on the enzymatic hydrolysis yield. The hydrolysis of pretreated corn stover (PCS), sugar cane straw (PSCS) and microcrystalline cellulose (Avicel) was performed under industrial conditions using high solid loadings, limited mixing, and low enzyme dosages. The addition of polyethylene glycol (PEG4000) allowed to increase the glucose yields by 10%, 7.5%, and 32%, respectively in the three materials. PEG4000 did not have significant effect on the stability of the main individual enzymes but increased beta-glucosidase and endoglucanase activity by 20% and 60% respectively. Moreover, the presence of PEG4000 accelerated cellulase-catalyzed hydrolysis reducing up to 25% the liquefaction time. However, a preliminary economical assessment concludes that even with these improvements, a lower contribution of PEG4000 to the 2G bioethanol production costs would be needed to reach commercial feasibility.

OrganoCat pretreatment of perennial plants: Synergies between a biogenic fractionation and valuable feedstocks

A successful biorefinery needs to align suitable pretreatment with sustainable production of biomasses. Herein, four perennial plants, (Sida, Silphium, Miscanthus and Szarvasi) regarded as promising feedstocks for biorefineries were subjected to the OrganoCat pretreatment. The technology was successfully applied to the different perennial plants revealing that pretreatment of grasses was more efficient than of non-grasses. Thorough analyses of the lignocellulose - before and after fractionation - enabled a detailed description of the fate of cellulosic, non-cellulosic polysaccharides and lignin during the pretreatment. Especially Szarvasi pulp displayed outstanding results in terms of fractionation efficiency and enzymatic digestibility, though in all cases successful lignocellulose fractionation was observed. These insights into the structural composition of different perennial plant species and the impact of the OrganoCat pretreatment on the plant material leads to useful information to strategically adapt such processes to the individual lignocellulosic material aiming for a full valorisation.

Using an Inducible Promoter of a Gene Encoding Penicillium verruculosum Glucoamylase for Production of Enzyme Preparations with Enhanced Cellulase Performance

Background

Penicillium verruculosum is an efficient producer of highly active cellulase multienzyme system. One of the approaches for enhancing cellulase performance in hydrolysis of cellulosic substrates is to enrich the reaction system with β -glucosidase and/or accessory enzymes, such as lytic polysaccharide monooxygenases (LPMO) displaying a synergism with cellulases.

Results

Genes bglI, encoding β-glucosidase from Aspergillus niger (AnBGL), and eglIV, encoding LPMO (formerly endoglucanase IV) from Trichoderma reesei (TrLPMO), were cloned and expressed by P. verruculosum B1-537 strain under the control of the inducible gla1 gene promoter. Content of the heterologous AnBGL in the secreted multienzyme cocktails (hBGL1, hBGL2 and hBGL3) varied from 4 to 10% of the total protein, while the content of TrLPMO in the hLPMO sample was ~3%. The glucose yields in 48-h hydrolysis of Avicel and milled aspen wood by the hBGL1, hBGL2 and hBGL3 preparations increased by up to 99 and 80%, respectively, relative to control enzyme preparations without the heterologous AnBGL (at protein loading 5 mg/g substrate for all enzyme samples). The heterologous TrLPMO in the hLPMO preparation boosted the conversion of the lignocellulosic substrate by 10–43%; however, in hydrolysis of Avicel the hLPMO sample was less effective than the control preparations. The highest product yield in hydrolysis of aspen wood was obtained when the hBGL2 and hLPMO preparations were used at the ratio 1:1.

Conclusions

The enzyme preparations produced by recombinant P. verruculosum strains, expressing the heterologous AnBGL or TrLPMO under the control of the gla1 gene promoter in a starch-containing medium, proved to be more effective in hydrolysis of a lignocellulosic substrate than control enzyme preparations without the heterologous enzymes. The enzyme composition containing both AnBGL and TrLPMO demonstrated the highest performance in lignocellulose hydrolysis, providing a background for developing a fungal strain capable to express both heterologous enzymes simultaneously.

Nutritional and metabolic implications of replacing cornstarch with D-xylose in broiler chickens fed corn and soybean meal-based diet

Effects of substituting cornstarch with D-xylose on growth performance, nutrients digestibility, serum metabolites, and expression of select hepatic genes involved in glucose and lipid metabolism were investigated in broiler chickens. A total of 360 one-day-old male Ross chicks were fed 3 diets (n = 24; 5 chicks/cage) for 21 days. A control corn-soybean meal-based diet with 25% cornstarch was formulated to meet specifications. Two additional diets were formulated by substituting cornstarch with 5 or 15% D-xylose w/w. Growth performance and digestibility by index method were determined in 12 replicate cages. Birds in these replicates had free access to feed and water, the BW and feed intake (FI) were monitored weekly and the excreta samples were collected on d 18 to 20. The other 12 replicates were used for blood and liver sampling by serial slaughter. On d 18, baseline (t0) birds were sampled following a 12 h overnight fasting and birds allowed 30 min access to the feed; samples were subsequently taken at 60, 120, 180, 240, and 300 min post feeding. Serum metabolites (glucose, xylose, and insulin) were assayed at all time points, whereas expression of hepatic transcripts was evaluated at zero, 180 and 300 min. Xylose linearly reduced (P < 0.05) FI, BWG, gross energy digestibility, and feed conversion ratio (FCR) but increased (P < 0.05) serum xylose level. Serum glucose and insulin levels were higher (P < 0.05) in the post-fed state compared with baseline, irrespective of treatments. There was an interaction (P < 0.05) between diet and sampling time on the expression of hepatic genes. At t0, xylose linearly increased (P < 0.05) the expression of pyruvate carboxylase, Acetyl Co-A acethyltransferase 2 (ACAT2), and glucose transporter 2. Xylose linearly reduced (P < 0.05) the expression of ACAT2 at 300 min post feeding. In conclusion, 5% or more xylose reduced growth performance and utilization of nutrients linked to hepatic enzymes and transcription factors involved in glucose and lipid metabolism.

Delignification of miscanthus using ethylenediamine (EDA) with or without ammonia and subsequent enzymatic hydrolysis to sugars

Pretreatment of miscanthus is essential for efficient enzymatic production of cellulosic ethanol. This study reports a possible pretreatment method for miscanthus using aqueous ethylenediamine (EDA) for 30 min at 180 °C with or without ammonia. The mass ratio of miscanthus to EDA was varied from 1:3, 1:1, and 1:0.5, keeping the mass ratio of miscanthus to liquid (EDA + Water) constant at 1:8. The ammonia-to-miscanthus ratio was 1:0.25. After pretreatment with a ratio of 1:3 miscanthus to EDA, about 75 % of the lignin was removed from the raw miscanthus with 90 % retention of cellulose and 50 % of hemicellulose in the recovered solid. Enzymatic hydrolysis of the recovered solid miscanthus gave 63 % glucose and 62 % xylose conversion after 72 h. EDA provides an effective pretreatment for miscanthus, achieving good delignification and enhanced sugar yield by enzyme hydrolysis. Results using aqueous EDA with or without ammonia are much better than those using hot water and compare favorably with those using aqueous ammonia. The delignification efficiency of EDA pretreatment is high compared to that for hot-water pretreatment and is nearly as efficient as that obtained for aqueous-ammonia pretreatment.

The access of Trichoderma reesei 6A to cellulose is blocked by isolated hemicelluloses and their derivatives in biomass hydrolysis

Mannan inhibited action of CBHII from Trichoderma reesei by retarding the adsorption of CBHII to cellulose.

New perspective on glycoside hydrolase binding to lignin from pretreated corn stover

BACKGROUND

Non-specific binding of cellulases to lignin has been implicated as a major factor in the loss of cellulase activity during biomass conversion to sugars. It is believed that this binding may strongly impact process economics through loss of enzyme activities during hydrolysis and enzyme recycling scenarios. The current model suggests glycoside hydrolase activities are lost though non-specific/non-productive binding of carbohydrate-binding domains to lignin, limiting catalytic site access to the carbohydrate components of the cell wall.

RESULTS

In this study, we have compared component enzyme affinities of a commercial Trichoderma reesei cellulase formulation, Cellic CTec2, towards extracted corn stover lignin using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and p-nitrophenyl substrate activities to monitor component binding, activity loss, and total protein binding. Protein binding was strongly affected by pH and ionic strength. β-d-glucosidases and xylanases, which do not have carbohydrate-binding modules (CBMs) and are basic proteins, demonstrated the strongest binding at low ionic strength, suggesting that CBMs are not the dominant factor in enzyme adsorption to lignin. Despite strong adsorption to insoluble lignin, β-d-glucosidase and xylanase activities remained high, with process yields decreasing only 4-15 % depending on lignin concentration.

CONCLUSION

We propose that specific enzyme adsorption to lignin from a mixture of biomass-hydrolyzing enzymes is a competitive affinity where β-d-glucosidases and xylanases can displace CBM interactions with lignin. Process parameters, such as temperature, pH, and salt concentration influence the individual enzymes' affinity for lignin, and both hydrophobic and electrostatic interactions are responsible for this binding phenomenon. Moreover, our results suggest that concern regarding loss of critical cell wall degrading enzymes to lignin adsorption may be unwarranted when complex enzyme mixtures are used to digest biomass.

Probing the Functions of Carbohydrate Binding Modules in the CBEL Protein from the Oomycete Phytophthora parasitica

Oomycetes are microorganisms that are distantly related to true fungi and many members of this phylum are major plant pathogens. Oomycetes express proteins that are able to interact with plant cell wall polysaccharides, such as cellulose. This interaction is thought to be mediated by carbohydrate-binding modules that are classified into CBM family 1 in the CAZy database. In this study, the two CBMs (1–1 and 1–2) that form part of the cell wall glycoprotein, CBEL, from Phytophthora parasitica have been submitted to detailed characterization, first to better quantify their interaction with cellulose and second to determine whether these CBMs can be useful for biotechnological applications, such as biomass hydrolysis. A variety of biophysical techniques were used to study the interaction of the CBMs with various substrates and the data obtained indicate that CBEL’s CBM1-1 exhibits much greater cellulose binding ability than CBM1-2. Engineering of the family 11 xylanase from Talaromyces versatilis (TvXynB), an enzyme that naturally bears a fungal family 1 CBM, has produced two variants. The first one lacks its native CBM, whereas the second contains the CBEL CBM1-1. The study of these enzymes has revealed that wild type TvXynB binds to cellulose, via its CBM1, and that the substitution of its CBM by oomycetal CBM1-1 does not affect its activity on wheat straw. However, intriguingly the addition of CBEL during the hydrolysis of wheat straw actually potentiates the action of TvXynB variant lacking a CBM1. This suggests that the potentiating effect of CBM1-1 might not require the formation of a covalent linkage to TvXynB.

Hydrothermal Pretreatment of Date Palm (Phoenix dactylifera L.) Leaflets and Rachis to Enhance Enzymatic Digestibility and Bioethanol Potential

Date palm residues are one of the most promising lignocellulosic biomass for bioethanol production in the Middle East. In this study, leaflets and rachis were subjected to hydrothermal pretreatment to overcome the recalcitrance of the biomass for enzymatic conversion. Evident morphological, structural, and chemical changes were observed by scanning electron microscopy, X-ray diffraction, and infrared spectroscopy after pretreatment. High glucan (>90% for both leaflets and rachis) and xylan (>75% for leaflets and >79% for rachis) recovery were achieved. Under the optimal condition of hydrothermal pretreatment (210°C/10 min) highly digestible (glucan convertibility, 100% to leaflets, 78% to rachis) and fermentable (ethanol yield, 96% to leaflets, 80% to rachis) solid fractions were obtained. Fermentability test of the liquid fractions proved that no considerable inhibitors to Saccharomyces cerevisiae were produced in hydrothermal pretreatment. Given the high sugar recovery, enzymatic digestibility, and ethanol yield, production of bioethanol by hydrothermal pretreatment could be a promising way of valorization of date palm residues in this region.

Mimicking the Fenton reaction-induced wood decay by fungi for pretreatment of lignocellulose

In this study, the Fenton reaction, which is naturally used by fungi for wood decay, was employed to pretreat rice straw and increase the enzymatic digestibility for the saccharification of lignocellulosic biomass. Using an optimized Fenton's reagent (FeCl3 and H2O2) for pretreatment, an enzymatic digestibility that was 93.2% of the theoretical glucose yield was obtained. This is the first report of the application of the Fenton reaction to lignocellulose pretreatment at a moderate temperature (i.e., 25°C) and with a relatively high loading of biomass (i.e., 10% (w/v)). Substantial improvement in the process economics of cellulosic fuel and chemical production can be achieved by replacing the conventional pretreatment with this Fenton-mimicking process.

Comparison and evaluation of concurrent saccharification and anaerobic digestion of Napier grass after pretreatment by three microbial consortia

Napier grass is potentially a viable feedstock for biofuel production. The present study investigated biological pretreatment of Napier grass by three microbial consortia followed by saccharification and anaerobic digestion. The pretreatment efficiencies of three microbial consortia were compared in terms of degradation ability, saccharide and biogas yield. The lignocellulose loss rates of Napier grass varied largely. The biomass pretreated by the consortium WSD-5 gave 43.4% and 66.2% total sugar yield under low and moderate loadings of commercial enzyme mixtures, while the highest yield was 83.2% pretreated by the consortium MC1 under a high enzyme loading. The maximum methane yield of pretreated samples by the consortia MC1, WSD-5 and XDC-2 were 259, 279, 247ml/g VS, respectively, which were 1.39, 1.49 and 1.32times greater than the values of the untreated controls. This study showed that pretreatments by MC1, WSD-5 and XDC-2 were capable of significantly enhancing both the saccharide and methane yields from Napier grass.

Sugar loss and enzyme inhibition due to oligosaccharide accumulation during high solids-loading enzymatic hydrolysis

BACKGROUND

Accumulation of recalcitrant oligosaccharides during high-solids loading enzymatic hydrolysis of cellulosic biomass reduces biofuel yields and increases processing costs for a cellulosic biorefinery. Recalcitrant oligosaccharides in AFEX-pretreated corn stover hydrolysate accumulate to the extent of about 18-25 % of the total soluble sugars in the hydrolysate and 12-18 % of the total polysaccharides in the inlet biomass (untreated), equivalent to a yield loss of about 7-9 kg of monomeric sugars per 100 kg of inlet dry biomass (untreated). These oligosaccharides represent a yield loss and also inhibit commercial hydrolytic enzymes, with both being serious bottlenecks for economical biofuel production from cellulosic biomass. Very little is understood about the nature of these oligomers and why they are recalcitrant to commercial enzymes. This work presents a robust method for separating recalcitrant oligosaccharides from high solid loading hydrolysate in gramme quantities. Composition analysis, recalcitrance study and enzyme inhibition study were performed to understand their chemical nature.

RESULTS

Oligosaccharide accumulation occurs during high solid loading enzymatic hydrolysis of corn stover (CS) irrespective of using different pretreated corn stover (dilute acid: DA, ionic liquids: IL, and ammonia fibre expansion: AFEX). The methodology for large-scale separation of recalcitrant oligosaccharides from 25 % solids-loading AFEX-corn stover hydrolysate using charcoal fractionation and size exclusion chromatography is reported for the first time. Oligosaccharides with higher degree of polymerization (DP) were recalcitrant towards commercial enzyme mixtures [Ctec2, Htec2 and Multifect pectinase (MP)] compared to lower DP oligosaccharides. Enzyme inhibition studies using processed substrates (Avicel and xylan) showed that low DP oligosaccharides also inhibit commercial enzymes. Addition of monomeric sugars to oligosaccharides increases the inhibitory effects of oligosaccharides on commercial enzymes.

CONCLUSION

The carbohydrate composition of the recalcitrant oligosaccharides, ratios of different DP oligomers and their distribution profiles were determined. Recalcitrance and enzyme inhibition studies help determine whether the commercial enzyme mixtures lack the enzyme activities required to completely de-polymerize the plant cell wall. Such studies clarify the reasons for oligosaccharide accumulation and contribute to strategies by which oligosaccharides can be converted into fermentable sugars and provide higher biofuel yields with less enzyme.

Pilot scale study on steam explosion and mass balance for higher sugar recovery from rice straw

Pretreatment of rice straw on pilot scale steam explosion has been attempted to achieve maximum sugar recovery. Three different reaction media viz. water, sulfuric acid and phosphoric acid (0.5%, w/w) were explored for pretreatment by varying operating temperature (160, 180 and 200°C) and reaction time (5 and 10min). Using water and 0.5% SA showed almost similar sugar recovery (∼87%) at 200 and 180°C respectively. However, detailed studies showed that the former caused higher production of oligomeric sugars (13.56g/L) than the later (3.34g/L). Monomeric sugar, followed the reverse trend (7.83 and 11.62g/L respectively). Higher oligomers have a pronounced effect in reducing enzymatic sugar yield as observed in case of water. Mass balance studies for water and SA assisted SE gave total saccharification yield as 81.8% and 77.1% respectively. However, techno-economical viability will have a trade-off between these advantages and disadvantages offered by the pretreatment medium.