Lignin-enzyme interaction: Mechanism, mitigation approach, modeling, and research prospects

Pretreatment of biomass with ethanol/deep eutectic solvent towards higher component recovery and obtaining lignin with high β-O-4 content

Deep eutectic solvent (DES) is an ideal solvent for extracting lignin in biomass pretreatment process. However, excessive breakage of the β-O-4 bonds of lignin remained a challenge for DES-pretreated biomass. In this study, a novel pretreatment system of choline chloride-citrate acid DES combined with ethanol for the pretreatment of bamboo was developed. The chemical characteristics of extracted lignin of bamboo before and after pretreatment were analyzed by gel permeation chromatography (GPC) and nuclear magnetic resonance spectroscopy (NMR). The results showed that the lignin extracted by ethanol/DES had moderate and uniform molecular weight (Mn: 3081-4314 Da, Mw: 3130-5399 Da), and was structurally intact (maintaining 40.29 % β-O-4 content), which was about five times higher than DES-extracted lignin, and contained a high number of S units (up to 80 %). Ethanol/DES system resulted in high removal of lignin up to 78.81 % and the highest enzymatic digestibility of glucose (72.68 %) and xylan (92.95 %), respectively. In addition, recovered DES provided similar glucose digestibility yields and delignification performance. The Ethanol/DES pretreatment developed herein provided a viable method for maintaining the structural integrity of lignin and preparing lignin with high β-O-4 content whilst with a relatively high components recovery.

Modifying lignin composition and xylan O-acetylation induces changes in cell wall composition, extractability, and digestibility

BACKGROUND

Lignin and xylan are important determinants of cell wall structure and lignocellulosic biomass digestibility. Genetic manipulations that individually modify either lignin or xylan structure improve polysaccharide digestibility. However, the effects of their simultaneous modifications have not been explored in a similar context. Here, both individual and combinatorial modification in xylan and lignin was studied by analysing the effect on plant cell wall properties, biotic stress responses and integrity sensing.

RESULTS

Arabidopsis plant co-harbouring mutation in FERULATE 5-HYDROXYLASE (F5H) and overexpressing Aspergillus niger acetyl xylan esterase (35S:AnAXE1) were generated and displayed normal growth attributes with intact xylem architecture. This fah1-2/35S:AnAXE1 cross was named as hyper G lignin and hypoacetylated (HrGHypAc) line. The HrGHypAc plants showed increased crystalline cellulose content with enhanced digestibility after chemical and enzymatic pre-treatment. Moreover, both parents and HrGHypAc without and after pre-treating with glucuronyl esterase and alpha glucuronidase exhibited an increase in xylose release after xylanase digestion as compared to wild type. The de-pectinated fraction in HrGHypAc displayed elevated levels of xylan and cellulose. Furthermore, the transcriptomic analysis revealed differential expression in cell wall biosynthetic, transcription factors and wall-associated kinases genes implying the role of lignin and xylan modification on cellular regulatory processes.

CONCLUSIONS

Simultaneous modification in xylan and lignin enhances cellulose content with improved saccharification efficiency. These modifications loosen cell wall complexity and hence resulted in enhanced xylose and xylobiose release with or without pretreatment after xylanase digestion in both parent and HrGHypAc. This study also revealed that the disruption of xylan and lignin structure is possible without compromising either growth and development or defense responses against Pseudomonas syringae infection.

The role of glycosylation in non-productive adsorption of cellulase to lignin isolated from pretreated corn stover

Glycosylation, a general post-translational modification for fungal cellulase, has been shown to affect cellulase binding to its substrate. However, the exact impact of glycosylation on cellulase-lignin interaction remain unclear. Here, we demonstrated that the lignin isolated from tetrahydrofuran-pretreated corn stover exhibits strong adsorption capability to cellulase due to its negatively charged and porous structure. For the cellulases with varying glycosylation levels, the less-glycosylated protein showed high adsorption capability to lignin, and that trend was observed for the main cellulase components secreted by Penicillium oxilicum, including endoglucanase PoCel5B, cellobiohydrolase PoCel7A-2, and β-glucosidase PoBgl1. Additionally, N-glycan sites and motifs were examined using mass spectrometry, and protein structures with N-glycans were constructed, where PoBgl1 and PoCel7A-2 contained 13 and 1 glycosylated sites respectively. The results of molecular dynamics simulations indicated that the N-glycans impacted on the solvent-accessible surface area and secondary structure of protein, and the binding conformation of lignin fragment on cellulase, resulting in a decrease in binding energy (14 kcal/mol for PoBgl1 and 13 kcal/mol for PoCel7A-2), particularly for van der Waals and electrostatic interaction. Those findings suggested that glycosylation negatively impacted the lignin-cellulase interaction, providing a theoretical basis for the rational engineering of enzymes to reduce lignin-enzyme interaction.

The Future of Graphene: Preparation from Biomass Waste and Sports Applications

At present, the main raw material for producing graphene is graphite ore. However, researchers actively seek alternative resources due to their high cost and environmental problems. Biomass waste has attracted much attention due to its carbon-rich structure and renewability, emerging as a potential raw material for graphene production to be used in sports equipment. However, further progress is required on the quality of graphene produced from waste biomass. This paper, therefore, summarizes the properties, structures, and production processes of graphene and its derivatives, as well as the inherent advantages of biomass waste-derived graphene. Finally, this paper reviews graphene's importance and application prospects in sports since this wonder material has made sports equipment available with high-strength and lightweight quality. Moreover, its outstanding thermal and electrical conductivity is exploited to prepare wearable sensors to collect more accurate sports data, thus helping to improve athletes' training levels and competitive performance. Although the large-scale production of biomass waste-derived graphene has yet to be realized, it is expected that its application will expand to various other fields due to the associated low cost and environmental friendliness of the preparation technique.

Evaluating the mechanism of soybean meal protein for boosting the laccase-catalyzed of thymol onto lignosulfonate via restraining non-specific adsorption

The control of laccase-catalyzed efficiency often relies on the utilization of modifying enzyme molecules and shielding agents. However, their elevated costs or carcinogenicity led to the inability for large-scale application. To address this concern, we found that a low-cost protein from soybean meal can reduce lignin's ineffective adsorption onto enzymes for improving the efficiency of thymol grafting to lignosulfonate. The results demonstrated that by adding 0.5 mg/mL of additional soybean meal protein, the thymol reaction ratio of the modified lignosulfonate (L-0.5 S) significantly boosted from 18.1 % to 35.0 %, with the minimal inhibitory concentrations of the L-0.5 S against Aspergillus niger dramatically improved from 12.5 mg/mL to 3.1 mg/mL. Multiple characterization methods were employed to better understand the benefit of the modification under the addition of the soybean meal protein. The CO and R1-O group content increased from 20.5 % to 37.8 % and from 65.1 % to 75.5 %, respectively. The proposed potential reaction mechanism was further substantiated by the physicochemical properties. The incorporation of soybean meal effectively mitigated the non-specific adsorption of lignosulfonate, resulting in a reduction of the surface area of lignin from 235.0 to 139.2 m2/g. The utilization of soybean meal as a cost-effective and efficient shielding agent significantly enhanced the efficiency of subsequent enzyme catalysis. Consequently, the application of soybean meal in commercial enzyme catalysis holds considerable appeal and amplifies the relevance of this study in preservative industries.

Molecular insights into inhibiting effects of lignin on cellulase investigated by molecular dynamics simulation

The hydrolysis of lignocellulose into fermentable monosaccharides using cellulases represents a critical stage in lignocellulosic bioconversion. However, the inactivation of cellulase in the presence of lignin is attributed to the high cost of biofinery. To address this challenge, a comprehensive investigation into the structure-function relationship underlying lignin-driven cellulase inactivation is essential. In this study, molecular docking and molecular dynamics (MD) simulations were employed to explore the impacts of lignin fragments on the catalytic efficiency of cellulase at the atomic level. The findings revealed that soluble lignin fragments and cellulose could spontaneously form stable complexes with cellulase, indicating a competitive binding scenario. The enzyme's structure remained unchanged upon binding to lignin. Furthermore, specific amino acid residues have been identified as involved in interactions with lignin and cellulose. Hydrophobic interactions were found to dominate the binding of lignin to cellulase. Based on the mechanisms underlying the interactions between lignin fragments and cellulase, decreased hydrophobicity and change in the charge of lignin may mitigate the inhibition of cellulase. Furthermore, site mutations and chemical modification are also feasible to improve the efficiency of cellulase. This study may contribute valuable insights into the design of more lignin-resistant enzymes and the optimization of lignocellulosic pretreatment technologies.Communicated by Ramaswamy H. Sarma.

Characterization of sugarcane mutants developed through gamma irradiations for their lignin content and caffeic acid-O-methyl transferase (COMT) gene mutations

PURPOSE

Bagasse, the residue left after extracting juice from sugarcane stalks, is rich in lignocellulosic biomass. The lignin present in this plant biomass is the key factor that hinders the efficient extraction of ethanol from the bagasse. In the current study, γ-irradiated sugarcane mutants were evaluated for variation in lignin content and its corresponding caffeic acid-O-methyl transferase (COMT) gene.

MATERIALS AND METHODS

The acetyl bromide method was used to estimate lignin content in sugarcane mutants. PCR-based cloning of the COMT gene was performed in low lignin mutants as well as control plants in E. coli (strain DH5α) to understand the mechanism of variation at the molecular level. The Sanger sequencing for cloned gene was performed to check variation in gene sequence.

RESULTS

In comparison to the control (21.5%), the mutant plants' lignin content ranged from 13 to 28%. The Sanger sequencing revealed approximately the same length of the gene from mutants as well as a control plant. In comparison to the reference gene, the mutated gene showed SNPs and indels in different regions, which may have an impact on lignin content.

CONCLUSIONS

Therefore, γ-irradiated mutagenesis is an acceptable approach to develop novel mutants of sugarcane with low lignin content to enhance bioethanol production from waste material using bioprocess technology.

Insight into key obstacles and technological strategy for enzymatic digestion of full cellulose fraction from poplar sawdust

Lignocellulosic biomass mainly consists of hemicellulose, lignin, and cellulose, which differently affect the enzymatic digestibility of cellulose. As for the typical representative for inert woody biomass, three components of cellulose were proposed conceptually for poplar sawdust, i.e., active cellulose, inert cellulose, and resistant cellulose. Dilute sulfuric acid pretreatment, hydrogen peroxide-sulfuric acid delignification, and sulfuric acid-assisted glycerol swelling were, respectively, proven to break the three obstacle mechanisms that affect the cellulase of poplar. The removal of key obstacles improved the cellulase digestibility of poplar enzyme-hydrolyzed residues by 188.7 %, and glucose yield increased from 34.6 % to 99.9 %. Therefore, a total of 39.5 g glucose was obtained from 100 g poplar sawdust by integrating the above three technologies. This work presented insight into and removed the key obstacles to enzymatic digestibility of poplar cellulose and developed an integrated technology to effectively convert full cellulose fraction to glucose from woody biomass.

Improving enzymatic efficiency of β-glucosidases in cellulase system by altering its binding behavior to the insoluble substrate during bioconversion of lignocellulose

The enzymatic efficiency of β-glucosidases is influenced by their binding behavior onto insoluble substrates (cellulose and lignin) during bioconversion of lignocellulose. This study suggested that the Bgl3 protein (Aspergillus fumigatus) showed strong adsorption affinity to lignin and the Bgl1 protein (Penicillium oxalicum) tended to adsorb to cellulose. It indicated that the various surface properties of the fibronectin type Ш-like domain (FnIII) led to different binding properties of β-glucosidases by investigating their binding mechanism. By engineering β-glucosidases' FnIII domain, Bgl3-1 and Bgl1-3 were constructed, which both showed lower binding capacities to insoluble substrates. As well as for Bgl1-3, its sensitivity to the phenolic component was also eased. Based on that, the reconstructed protein showed high catalytic efficiency during the enzymatic hydrolysis of corn stover by effectively transforming cellobiose to glucose. Thus, this study provided a new strategy to engineer β-glucosidases to enhance their performance in the cellulase system.

Enzyme-Based Biostimulants Influence Physiological and Biochemical Responses of Lactuca sativa L

Biostimulants (BSs) are natural materials (i.e., organic or inorganic compounds, and/or microorganisms) having beneficial effects on plant growth and productivity, and able to improve resilience/tolerance to biotic and abiotic stresses. Therefore, they represent an innovative alternative to the phyto- and agrochemicals, being environmentally friendly and a valuable tool to cope with extreme climate conditions. The objective of this study was to investigate the effects of several biomolecules (i.e., Xylanase, β-Glucosidase, Chitinase, and Tramesan), alone or in combinations, on lettuce plant growth and quality. With this aim, the influence of these biomolecules on biomass, pigment content, and antioxidant properties in treated plants were investigated. Our results showed that Xylanase and, to a lesser extent, β-Glucosidase, have potentially biostimulant activity for lettuce cultivation, positively influencing carotenoids, total polyphenols, and ascorbic acid contents; similar effects were found with respect to antioxidative properties. Furthermore, the effect of the more promising molecules (Xylanase and β-Glucosidase) was also evaluated in kiwifruit cultured cells to test their putative role as sustainable input for plant cell biofactories. The absence of phytotoxic effects of both molecules at low doses (0.1 and 0.01 µM), and the significantly enhanced cell biomass growth, indicates a positive impact on kiwifruit cells.

Greater Importance of Structural Changes Over Lignin Removal in Impacting the Enzymatic Hydrolysis of Crop Straws

The importance of structural changes and lignin removal in influencing the enzymatic hydrolysis of crop straws was investigated. Alkaline pretreatment of corn stover (CS) and rice straw (RS) was conducted to remove lignin. It was found that alkaline pretreatment caused more lignin removal from CS than RS under the same condition, and CS and RS treated at 40 °C (CS40 and RS40) attained less lignin removal than those treated at 80 °C (CS80 and RS80). However, CS40 achieved glucan conversion efficiency of 70.77% which was 11.85% less than that of RS40, and similar glucan conversion efficiencies were obtained for RS 40 and RS80. Structural observation showed that the connection among single fiber bundle of CS40 was tighter than that of RS40, while those of RS40 and RS80 were loose. It meant that structural changes might be more important than lignin removal to influence enzymatic hydrolysis of crop straws.

Exploring how lignin structure influences the interaction between carbohydrate-binding module and lignin using AFM

Nonproductive adsorption of cellulase onto the residual lignin in substrate seriously hinders the enzymatic hydrolysis. To understand how lignin structure affects lignin-cellulase interaction, the carbohydrate-binding module (CBM) functionalized atomic force microscope tip was used to measure CBM-lignin interaction by single-molecule dynamic force spectroscopy in this work. The results showed that sulfonated lignin (SL) has the greatest adhesion force to CBM (4.74 nN), while those of masson pine milled wood lignin (MWL), poplar MWL and herbaceous MWLs were 2.85, 1.03 and 0.27-0.61 nN, respectively. It provides direct quantitative evidence for the significance of lignin structure on lignin-cellulase interaction. The CBM-MWLs interaction decreased sharply to 0.054-0.083 nN while SL was added, indicating the primary mechanism of SL promoting lignocellulose hydrolysis was significantly reducing the nonproductive adsorption of substrate lignin on cellulase. Finally, the "competitive adsorption" mechanism was proposed to interpret why SL effectively promotes the enzymatic hydrolysis of lignin-containing substrates.

The Biomodified Lignin Platform: A Review

A reliance on fossil fuel has led to the increased emission of greenhouse gases (GHGs). The excessive consumption of raw materials today makes the search for sustainable resources more pressing than ever. Technical lignins are mainly used in low-value applications such as heat and electricity generation. Green enzyme-based modifications of technical lignin have generated a number of functional lignin-based polymers, fillers, coatings, and many other applications and materials. These bio-modified technical lignins often display similar properties in terms of their durability and elasticity as fossil-based materials while also being biodegradable. Therefore, it is possible to replace a wide range of environmentally damaging materials with lignin-based ones. By researching publications from the last 20 years focusing on the latest findings utilizing databases, a comprehensive collection on this topic was crafted. This review summarizes the recent progress made in enzymatically modifying technical lignins utilizing laccases, peroxidases, and lipases. The underlying enzymatic reaction mechanisms and processes are being elucidated and the application possibilities discussed. In addition, the environmental assessment of novel technical lignin-based products as well as the developments, opportunities, and challenges are highlighted.

Strategies of pretreatment of feedstocks for optimized bioethanol production: distinct and integrated approaches

Bioethanol is recognized as a valuable substitute for renewable energy sources to meet the fuel and energy demand of the nation, considered an environmentally friendly resource obtained from agricultural residues such as sugarcane bagasse, rice straw, husk, wheat straw and corn stover. The energy demand is sustained using lignocellulosic biomass to produce bioethanol. Lignocellulosic biomass (LCBs) is the point of attention in replacing the dependence on fossil fuels. The recalcitrant structure of the lignocellulosic biomass is disrupted using effective pretreatment techniques that separate complex interlinked structures among cellulose, hemicellulose, and lignin. Pretreatment of biomass involves various physical, chemical, biological, and physiochemical protocols which are of importance, dependent upon their individual or combined dissolution effect. Physical pretreatment involves a reduction in the size of the biomass using mechanical, extrusion, irradiation, and sonification methods while chemical pretreatment involves the breaking of various bonds present in the LCB structure. This can be obtained by using an acidic, alkaline, ionic liquid, and organosolvent methods. Biological pretreatment is considered an environment-friendly and safe process involving various bacterial and fungal microorganisms. Distinct pretreatment methods, when combined and utilized in synchronization lead to more effective disruption of LCB, making biomass more accessible for further processing. These could be utilized in terms of their effectiveness for a particular type of cellulosic fiber and are namely steam explosion, liquid hot water, ammonia fibre explosion, CO₂ explosion, and wet air oxidation methods. The present review encircles various distinct and integrated pretreatment processes developed till now and their advancement according to the current trend and future aspects to make lignocellulosic biomass available for further hydrolysis and fermentation.

Enzymatic Hydrolysis Strategies for Cellulosic Sugars Production to Obtain Bioethanol from Eucalyptus globulus Bark

Cellulosic sugars production for the valorization of lignocellulosic biomass residues in an industrial site has economic benefits and is promising if integrated into a biorefinery. Enzymatic hydrolysis (EH) of pretreated Eucalyptus globulus bark, an industrial residue of low-economic value widely available in Portuguese pulp and paper mills, could be an excellent approach to attain resource circularity and pulp mill profitability. This work evaluated the potential for improving cellulosic sugars concentrations by operating with high solids loading and introducing the additives Triton X-100, PEG 4000 and Tween 80 using a commercial enzymatic consortium with a dosage of 25 FPU gcarbohydrates−1. Additives did not improve enzymatic hydrolysis performance, but the effect of increasing solids loading to 14% (w/v) in batch operation was accomplished. The fed-batch operation strategy was investigated and, when starting with 11% (w/v) solids loading, allowed the feeding of 3% (w/v) fresh feedstock sequentially at 2, 4 and 6 h, attaining 20% (w/v) total solids loading. After 24 h of operation, the concentration of cellulosic sugars reached 161 g L−1, corresponding to an EH conversion efficiency of 76%. Finally, the fermentability of the fed-batch hydrolysate using the Ethanol Red® strain was evaluated in a 5 L bioreactor scale. The present results demonstrate that Eucalyptus globulus bark, previously pretreated by kraft pulping, is a promising feedstock for cellulosic sugars production, allowing it to become the raw material for feeding a wide range of bioprocesses.

Journey of lignin from a roadblock to bridge for lignocellulose biorefineries: A comprehensive review

The grave concerns arisen as a result of environmental pollution and diminishing fossil fuel reserves in the 21st century have shifted the focus on the use of sustainable and environment friendly alternative resources. Lignocellulosic biomass constituted by cellulose, hemicellulose and lignin is an abundantly available natural bioresource. Lignin, a natural biopolymer has over the years gained much importance as a high value material with commercial importance. The present review provides an in-depth knowledge on the journey of lignin from being considered a roadblock to a bridge connecting diverse industries with widescale applications. The successful valorization of lignin for the production of bio-based platform chemicals and fuels has been the subject of intensive investigation. A deeper understanding of lignin characteristics and factors governing the biomass conversion into valuable products can support improved biomass consumption. The components of lignocellulosic biomass might be totally transformed into a variety of value-added products with the improvements in bioprocess techniques that valorize lignin. In this review, the recent advances in the lignin extraction and depolymerization methods that may help in achieving the cost-economics of the bioprocess are summarized and compared. The industrial potential of lignin-derived products such as aromatics, biopolymers, biofuels and agrochemicals are also outlined. Additionally, assessment of the recent research trends in lignin valorization into value-added chemicals has been done and present scenario of technological-industrial applications of lignin with economic perspectives is highlighted.

Bacterial Community Drives the Carbon Source Degradation during the Composting of Cinnamomum camphora Leaf Industrial Extracted Residues

The increasing production of industrial aromatic plant residues (IAPRs) are potentially environmental risky, and composting is a promising solution to resolve the coming IAPR problems. Carbon source degradation is a basic but important field in compost research; however, we still lack a clear understanding of carbon source degradation and the corresponding relationship to microbial community variation during IAPR composting, which hampers the improvement of IAPR composting efficiency and the promotion of this technology. In this study, samples were chosen on the first day, the 10th day, the 20th day, and the last day during the composting of Cinnamomum camphora leaf IAPRs, and the microbial community composition, main carbon source composition, and several enzyme activities were measured accordingly. The results showed that during composting, the hemicellulose had the highest reduction (200 g kg−1), followed by cellulose (143 g kg−1), lignin (15.5 g kg−1), starch (5.48 g kg−1), and soluble sugar (0.56 g kg−1), which supported that hemicellulose and cellulose were the main carbon source to microbes during composting. The relative abundance of the main bacterial phylum Firmicute decreased from 85.1% to 40.3% while Actinobactreia increased from 14.4% to 36.7%, and the relative abundance of main fungal class Eurotiomycetes decreased from 60.9% to 19.6% while Sordariomycetes increased from 16.9% to 69.7%. Though principal coordinates analysis found that both bacterial and fungal community composition significantly varied during composting (p < 0.05), structure equation modeling (SEM) supported that bacterial composition rather than fungal counterpart was more responsible for the change in carbon source composition, as the standard total effects offered by bacterial composition (−0.768) was about five times the fungal composition (−0.144). Enzyme2 (comprised of xylanase, laccase, cellulase and manganese peroxidase) provided −0.801 standard total effects to carbon source composition, while Enzyme1 (comprised of lignin peroxidase and polyphenol oxidase) had only 0.172. Furthermore, xylanase and laccase were the only two enzymes appeared in co-occurrence network, clustered with nearly all the carbon sources concerned (except starch) in module-II. Xylanase, hemicellulose, and cellulose were linked to higher numbers of OTUs, more than laccase and other carbon sources. In addition, there were 11 BOTUs but only 1 FOTUs directly interacted to xylanase, hemicellulose, and cellulose simultaneously, three of them were Limnochordaceae and two were Savagea, which highlighted the potential core function in lignocellulose degradation provided by bacterial members, especially Limnochordaceae and Savagea. Thus, the results supported that during composting of Cinnamomum camphora leaf IAPRs, the degradation of dominate carbon sources, hemicellulose and cellulose, was mainly driven by bacterial community rather than fungal community. In addition, the bacterial originated xylanase and laccase played potentially core roles in the functional modules. This research clearly investigated the microbial dynamics of carbon source degradation during the composting of Cinnamomum camphora leaf IAPRs, and offers valuable information about and new insight into future IAPRs waste treatment.

Bioremediation potential of laccase for catalysis of glyphosate, isoproturon, lignin, and parathion: Molecular docking, dynamics, and simulation

The present study aimed to investigate the catalytic degradation produced by laccase in the detoxification of glyphosate, isoproturon, lignin polymer, and parathion. We explored laccase-glyphosate, laccase-lignin polymer, laccase-isoproturon, and laccase-parathion using molecular docking (MD) and molecular dynamics simulation (MDS) approaches. The results suggest that laccase interacts well with glyphosate, lignin polymer, isoproturon, and parathion during biodegradation. We calculated the root mean square deviations (RMSD) of laccase-glyphosate, laccase-lignin polymer, laccase-isoproturon, and laccase-parathion as 0.24 ± 0.02, 0.59 ± 0.32, 0.43 ± 0.07, and 0.43 ± 0.06 nm, respectively. In an aqueous solution, the stability of laccase with glyphosate, lignin polymer, isoproturon, and parathion is mediated through the formation of hydrophobic interactions, hydrogen bonds, and van der Waals interactions. The presence of xenobiotic toxic compounds in the active site changed the conformation of laccase. MDS of the laccase-substrate complexes confirmed their stability during catalytic degradation. Laccase assay results confirmed that the degradation of syringol, dihydroconiferyl alcohol, guaiacol, parathion, isoproturon, and glyphosate were 100%, 99.31%, 95.69%, 60.96%, 54.51%, and 48.34% within 2 h, respectively. Taken together, we describe a novel method to understand the molecular-level biodegradation of xenobiotic compounds through laccase and its potential application in contaminant removal.

In-situ lignin sulfonation for enhancing enzymatic hydrolysis of poplar using mild organic solvent pretreatment

Biomass pretreatment is an essential strategy to overcome biomass recalcitrance and promote lignocellulosic bioconversion. Here, a reusable organic solvent system (formic acid-methanesulfonic acid) was explored to pretreat poplar under a mild temperature (below 100 °C). The results showed that the co-solvent system could extract basically complete hemicelluloses and part of lignin with original cellulose retained in the pretreated substrates. Meanwhile, sulfonic acid groups were introduced into lignin structure remained in the substrates. The glucose conversion yield of the substrates with a higher concentration of sulfonic acid groups (13.2 mmol/kg) reached 45.9 % by reducing the hydrophobic interaction between lignin and cellulase, showing 89.3 % improvement compared with that of the substrates treated with single formic acid. This progressive study aimed to develop a new strategy to realize sulfonation and promote enzymatic hydrolysis of substrates by using mild organic solvent pretreatment.

Synthesis of temperature and pH responsive lignin-grafted sulfobetaine for efficiently recycling cellulase

Recycling cellulase can reduce the cost of lignocellulosic enzymatic hydrolysis. Here, a lignin-grafted sulfobetaine (LSB) was first synthesized by grafting sulfobetaine (SB) on enzymatic hydrolysis lignin (EHL). LSB had a sensitive response of pH and temperature. LSB dissolved under the conditions of lignocellulosic enzymatic hydrolysis (pH 5.0, 50 °C). After hydrolysis, LSB co-precipitated with cellulase when lowering pH of the hydrolysate to 4.0 and cooling to 25 °C. When 3.0 g/L LSB-100 was added to the hydrolysis system of corncob residue (CCR), 70 % of amount of cellulase was saved. LSB had a remarkable response and stronger cellulase recovery capacity. This was attributed that carboxylate radical in LSB was protonated, and positive and negative ions of SB associated to form salt at 25 °C. This work provides a new idea for reducing the cost for preparing fermentable sugars from lignocellulose, and increasing the added value of EHL.

Effective fractionation of lignocellulose components and lignin valorization by combination of deep eutectic solvent with ethanol

Introduction: A combination of deep eutectic solvent with ethanol was developed for pretreatment of Broussonetia papyrifera to effectively extract lignin and promote the subsequent enzymatic hydrolysis. Methods: In order to further explore the optimal conditions for enzymatic hydrolysis, a central composite design method was applied. Results and Discussion: The correlation between each factor and glucose yield was obtained, and the optimal conditions was 160°C, 60 min, the ratio of DES to E was 1/1 (mol/mol). The results showed that compared with control, the glucose yield increased by 130.67% under the optimal pretreatment conditions. Furthermore, the specific surface area of biomass was increased by 66.95%, and the content of xylan and lignin was decreased by 86.71% and 85.83%. The correlation between xylan/lignin removal and enzymatic hydrolysis showed that the removal of lignin facilitated the glucose yield more significantly than that of xylan. To further explore the lignin valorization, the structural and antioxidant analysis of recovered lignin revealed that high temperature was favorable for lignin with good antioxidant performance. This pretreatment is a promising method for separating lignin with high antioxidant activity and improving cellulose digestibility.

Enzymes and enzymatic mechanisms in enzymatic degradation of lignocellulosic biomass: A mini-review

Enzymatic hydrolysis is the key step limiting the efficiency of the biorefinery of lignocellulosic biomass. Enzymes involved in enzymatic hydrolysis and their interactions with biomass should be comprehended to form the basis for looking for strategies to improve process efficiency. This article updates the contemporary research on the properties of key enzymes in the lignocellulose biorefinery and their interactions with biomass, adsorption, and hydrolysis. The advanced analytical techniques to track the interactions for exploiting mechanisms are discussed. The challenges and prospects for future research are outlined.

Advances and perspectives on mass transfer and enzymatic hydrolysis in the enzyme-mediated lignocellulosic biorefinery: A review

Enzymatic hydrolysis is a critical process for the cellulase-mediated lignocellulosic biorefinery to produce sugar syrups that can be converted into a whole range of biofuels and biochemicals. Such a process operating at high-solid loadings (i.e., scarcely any free water or roughly ≥ 15% solids, w/w) is considered more economically feasible, as it can generate a high sugar concentration at low operation and capital costs. However, this approach remains restricted and incurs "high-solid effects", ultimately causing the lower hydrolysis yields with increasing solid loadings. The lack of available water leads to a highly viscous system with impaired mixing that exhibits strong transfer resistance and reaction limitation imposed on enzyme action. Evidently, high-solid enzymatic hydrolysis involves multi-scale mass transfer and multi-phase enzyme reaction, and thus requires a synergistic perspective of transfer and biotransformation to assess the interactions among water, biomass components, and cellulase enzymes. Porous particle characteristics of biomass and its interface properties determine the water form and distribution state surrounding the particles, which are summarized in this review aiming to identify the water-driven multi-scale/multi-phase bioprocesses. Further aided by the cognition of rheological behavior of biomass slurry, solute transfer theories, and enzyme kinetics, the coupling effects of flow-transfer-reaction are revealed under high-solid conditions. Based on the above basic features, this review lucidly explains the causes of high-solid hydrolysis hindrances, highlights the mismatched issues between transfer and reaction, and more importantly, presents the advanced strategies for transfer and reaction enhancements from the viewpoint of process optimization, reactor design, as well as enzyme/auxiliary additive customization.

Molecular docking and molecular dynamics simulation approaches for evaluation of laccase-mediated biodegradation of various industrial dyes

Dyes are being increasingly utilized across the globe, but there is no appropriate method of bioremediation for their full mineralization from the environment. Laccases are key enzymes that help microbes to degrade dyes as well as their intermediate metabolites. Various dyes have been reported to be degraded by bacteria, but it is still unclear how these enzymes function during dye degradation. To effectively eradicate toxic dyes from the system, it is essential to understand the molecular function of enzymes. As a result, the interaction of laccase with different toxic dyes was investigated using molecular docking. Based on the highest binding energy we have screened ten dyes with positive interaction with laccase. Evaluating the MD simulation results, three out of ten dyes were more stable as potential targets for degradation by laccase of Bacillus subtilis. As a result, subsequent research focused solely on the results of three substrates: pigment red, fuchsin base, and Sudan IV. Analysis of MD simulation revealed that pigments red 23, fuchsin base, and Sudan IV form hydrogen and hydrophobic bond as well as Vander Waals interactions with the active site of laccase to keep it stable in aqueous solution. The conformation of laccase is greatly altered by the inclusion of all three substrates in the active site. The MD simulation findings show that laccase complexes remain stable throughout the catalytic reaction. Therefore, this research provides a molecular understanding of laccase expression and its role in the bioremediation of the pigments red 23, fuchsin base, and Sudan IV.Communicated by Ramaswamy H. Sarma.

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

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

Improve Enzymatic Hydrolysis of Lignocellulosic Biomass by Modifying Lignin Structure via Sulfite Pretreatment and Using Lignin Blockers

Even traditional pretreatments can partially remove or degrade lignin and hemicellulose from lignocellulosic biomass for enhancing its enzymatic digestibility, the remaining lignin in pretreated biomass still restricts its enzymatic hydrolysis by limiting cellulose accessibility and lignin-enzyme nonproductive interaction. Therefore, many pretreatments that can modify lignin structure in a unique way and approaches to block the lignin’s adverse impact have been proposed to directly improve the enzymatic digestibility of pretreated biomass. In this review, recent development in sulfite pretreatment that can transform the native lignin into lignosulfonate and subsequently enhance saccharification of pretreated biomass under certain conditions was summarized. In addition, we also reviewed the approaches of the addition of reactive agents to block the lignin’s reactive sites and limit the cellulase-enzyme adsorption during hydrolysis. It is our hope that this summary can provide a guideline for workers engaged in biorefining for the goal of reaching high enzymatic digestibility of lignocellulose.

Understanding of promoting enzymatic hydrolysis of combined hydrothermal and deep eutectic solvent pretreated poplars by Tween 80

In this study, lignin blockers including non-catalytic protein and surfactants were employed to promote enzymatic digestibility of pretreated poplars. Among them, Tween 80 exhibited the most pronounced facilitation, improving the glucose yield from 26.6% to 99.6% at a low enzyme loading (10 FPU/g glucan), and readily reduced the required cellulase loading by 75%. The underlying mechanism for this remarkable improvement on glucose yields by Tween 80 was elucidated. The impacts of Tween 80 on the enzyme-lignin interaction were explored by quartz crystal microbalance analysis, revealing that the binding rate of Tween 80 on lignin surfaces was 3-fold higher than that of enzyme. More importantly, Tween 80 remarkably decreased the binding capacity and binding rate of enzyme on lignins. Furthermore, the substrate properties dominating the increase in glucose yields with Tween 80 were explored. The results facilitate to understand the underlying mechanism of the promotion of surfactants on enzymatic hydrolysis.

Tailoring Lignin-Based Spherical Particles as a Support for Lipase Immobilization

Lignin-based spherical particles have recently gained popularity due to their characteristic and the usage of biopolymeric material. In this study, lignin-based spherical particles were prepared using choline chloride at different pH values, ranging from 2 to 10. Their dispersive, microstructural, and physicochemical properties were studied by a variety of techniques, including scanning electron microscopy, Fourier transform infrared spectroscopy, and zeta potential analysis. The best results were obtained for the particles prepared at pH 5 and 7, which had a spherical shape without a tendency to form aggregates and agglomerates. The lignin-based spherical particles were used for the immobilization of lipase, a model enzyme capable of catalyzing a wide range of transformations. It was shown that the highest relative activity of immobilized lipase was obtained after 24 h of immobilization at 30 °C and pH 7, using 100 mg of the support. Moreover, the immobilized lipase exhibited enhanced stability under harsh process conditions, and demonstrated high reusability, up to 87% after 10 cycles, depending on the support used. In the future, the described approach to enzyme immobilization based on lignin spheres may play a significant role in the catalytic synthesis of organic and fine chemicals, with high utility value.

Revealing the mechanism of surfactant-promoted enzymatic hydrolysis of dilute acid pretreated bamboo

To improve the enzymatic digestibility of dilute acid pretreated bamboo residue (DABR), surfactants including PEG 4000 and Tween 80 were added to prevent the non-productive adsorption between residual lignin and enzyme. At the optimal loadings (e.g., 0.2 and 0.3 g surfactant/g lignin), the enzymatic digestibility of DABR improved from 29.4% to 64.6% and 61.6% for PEG 4000 and Tween 80, respectively. Furthermore, the promoting mechanism of these surfactants on enzymatic hydrolysis was investigated by real-time surface plasmon resonance (SPR) and fluorescence spectroscopy. Results from SPR analysis showed that Tween 80 outperformed PEG 4000 in terms of dissociating the irreversible cellulase adsorption onto lignin. Fluorescence quenching mechanism revealed that PEG 4000 and Tween 80 intervened the interaction between lignin and cellulase by hydrogen bonds/Van der Waals and hydrophobic action, respectively. This work provided an in-depth understanding of the mechanisms of PEG 4000 and Tween 80 on enhancing the enzymatic hydrolysis efficiency.

Research Progress on Lignocellulosic Biomass Degradation Catalyzed by Enzymatic Nanomaterials

Lignocellulose biomass (LCB) has extensive applications in many fields such as bioenergy, food, medicines, and raw materials for producing value-added products. One of the keys to efficient utilization of LCB is to obtain directly available oligo- and monomers (e.g., glucose). With the characteristics of easy recovery and separation, high efficiency, economy, and environmental protection, immobilized enzymes have been developed as heterogeneous catalysts to degrade LCB effectively. In this review, applications and mechanisms of LCB-degrading enzymes are discussed, and the nanomaterials and methods used to immobilize enzymes are also discussed. Finally, the research progress of lignocellulose biodegradation catalyzed by nano-enzymes was discussed.

Lignin fractionation to realize the comprehensive elucidation of structure-inhibition relationship of lignins in enzymatic hydrolysis

A better understanding of the relationship between lignin structures and their inhibitory effects in enzymatic saccharification would facilitate the development of lignocellulose biorefinery process. However, the heterogeneity of lignins challenges the elucidation of lignin structure-inhibition correlation. In this study, two types of lignin fractions including ethanol soluble lignins and ethanol insoluble lignins were respectively isolated from the poplars pretreated with various severities. The impacts of pretreatment severities on the structural changes of lignin fractions were studied from the perspective of inter-units linkages, condensed aromatic substructure, and hydroxyl groups. Furthermore, it was observed that lignin addition strongly inhibited the enzymatic saccharification of pure cellulose by 13.3 ∼ 56.3%. Lignin inhibition extents were increased with the elevated pretreatment severity. The relationships between the lignin structural features and lignin inhibition were analyzed, which revealed that the contents of condensed aromatic units and phenolic hydroxyl were crucial factors determining the lignin inhibition.

Current status, challenges and prospects for lignin valorization by using Rhodococcus sp

Lignin represents the most abundant renewable aromatics in nature, which has complicated and heterogeneous structure. The rapid development of biotransformation technology has brought new opportunities to achieve the complete lignin valorization. Especially, Rhodococcus sp. possesses excellent capabilities to metabolize aromatic hydrocarbons degraded from lignin. Furthermore, it can convert these toxic compounds into high value added bioproducts, such as microbial lipids, polyhydroxyalkanoate and carotenoid et al. Accordingly, this review will discuss the potentials of Rhodococcus sp. as a cell factory for lignin biotransformation, including phenol tolerance, lignin depolymerization and lignin-derived aromatic hydrocarbon metabolism. The detailed metabolic mechanism for lignin biotransformation and bioproducts spectrum of Rhodococcus sp. will be comprehensively discussed. The available molecular tools for the conversion of lignin by Rhodococcus sp. will be reviewed, and the possible direction for lignin biotransformation in the future will also be proposed.

Fluorescent Imaging of Extracellular Fungal Enzymes Bound onto Plant Cell Walls

Lignocelluloytic enzymes are industrially applied as biocatalysts for the deconstruction of recalcitrant plant biomass. To study their biocatalytic and physiological function, the assessment of their binding behavior and spatial distribution on lignocellulosic material is a crucial prerequisite. In this study, selected hydrolases and oxidoreductases from the white rot fungus Phanerochaete chrysosporium were localized on model substrates as well as poplar wood by confocal laser scanning microscopy. Two different detection approaches were investigated: direct tagging of the enzymes and tagging specific antibodies generated against the enzymes. Site-directed mutagenesis was employed to introduce a single surface-exposed cysteine residue for the maleimide site-specific conjugation. Specific polyclonal antibodies were produced against the enzymes and were labeled using N-hydroxysuccinimide (NHS) ester as a cross-linker. Both methods allowed the visualization of cell wall-bound enzymes but showed slightly different fluorescent yields. Using native poplar thin sections, we identified the innermost secondary cell wall layer as the preferential attack point for cellulose-degrading enzymes. Alkali pretreatment resulted in a partial delignification and promoted substrate accessibility and enzyme binding. The methods presented in this study are suitable for the visualization of enzymes during catalytic biomass degradation and can be further exploited for interaction studies of lignocellulolytic enzymes in biorefineries.

A structure-activity understanding of the interaction between lignin and various cellulase domains

To elucidate the structure-activity relationship between lignin and various cellulase domains, four lignin fractions with specific structures and molecular weight were prepared from bamboo kraft lignin (BKL) and used to investigate the adsorption mechanism between different cellulase domains by fluorescence spectroscopy and SDS-PAGE. Endo-cellulase 6B exhibited a higher affinity to BKL fractions than the carbohydrate-binding module (CBM4A) of cellulase, which is positively correlated to molecular weight. The thermodynamic mechanism showed that the adsorption between BKL fractions and endo-cellulase 6B was dominated by van der Waals and electrostatic forces, while hydrophobic force is the driver for BKL fractions to adsorb CBM4A. Structure-activity relationship between lignin fractions and cellulase domain revealed that thermodynamics and interaction forces were more easily affected by the structure of BKL, including S/G ratio, molecular weight and hydrophobicity. The aforementioned results demonstrated that lignin's structure plays a critical role in its adsorption with various cellulase domains.

Integrated hydrothermal and deep eutectic solvent-mediated fractionation of lignocellulosic biocomponents for enhanced accessibility and efficient conversion in anaerobic digestion

Effective fractionation of lignocellulosic biocomponents of lignocellulosic biomass can increase its utilization in anaerobic digestion for high yield biomethane production. A hydrothermal process was optimized and integrated with a deep eutectic solvent (DES) pretreatment to preferentially fractionate hemicellulose, cellulose, and lignin in rice straw. The optimized hydrothermal process resulted in 96% hemicellulose solubilization at moderately low combined pretreatment severity (log S = 2.26), allowing increased hemicellulosic sugar recovery with minimal formation of inhibitory byproducts. Subsequent DES pretreatment resulted in highly bioaccessible cellulosic pulp, removing 81.3% of lignin that can be recovered and converted into value-added products. Anaerobic digestion of hemicellulosic fraction and cellulosic pulp using a microbial methanogenic consortium seed acclimatized to the lignocellulosic inhibitors resulted in a 33.4% higher yield of methane (467.84 mL g^-1 VS_initial) than with anaerobic digester sludge seed. This integrated approach can facilitate and maximize the targeted utilization of different biocomponents through sustainable biorefining.

Structural Changes of Alkali Lignin under Ozone Treatment and Effect of Ozone-Oxidized Alkali Lignin on Cellulose Digestibility

In this study, the structural changes of alkali lignin induced by ozonation were investigated, and the effect of ozone-treated alkali lignin and its mechanism on Avicel enzymatic hydrolysis was examined. The physicochemical properties of alkali lignin were analyzed by FTIR, 1H-13C HSQC NMR, and GPC. It was revealed that ozone pretreatment increased the content of carboxyl and/or aldehyde groups and the negative zeta potential of alkali lignin, which enhanced the electrostatic repulsion between alkali lignin and cellulase; The S/G ratio was reduced, indicating the hydrophobic interaction was diminished. The Langmuir adsorption isotherm showed that the cellulase binding strength of ozone pretreated alkali lignin (OL-pH3, OL-pH7, and OL-pH12 were 16.67, 13.87, and 44.05 mL/g, respectively) was significantly lower than that of alkali lignin (161.29 mL/g). The 72 h hydrolysis yields of Avicel added with OL-pH3, OL-pH7, and OL-pH12 were 55.4%, 58.6%, and 54.9% respectively, which were 2.6–6.3% higher than that of Avicel added with AL (52.3%). This research aimed to reduce the non-productive adsorption between cellulase and lignin by investigating the structural changes of lignin caused by ozone treatment. For the first time, we discovered that ozone-treated alkali lignin has a further promotion effect on the enzymatic digestion of cellulose, providing a green and feasible pretreatment process for the enzymatic hydrolysis of lignocellulose and aiding in the more efficient utilization of biomass.

The use of steam pretreatment to enhance pellet durability and the enzyme-mediated hydrolysis of pellets to fermentable sugars

Although densified wood pellets are an attractive biomass feedstock for bioenergy and biofuels production, partly due to their ease of transport, their friability and hygroscopic nature (attraction of moisture) have proven problematic in terms of storage and handling. Pre-steaming the biomass was shown to reduce the need for size reduction, significantly increasing pellet durability by relocating the plant cell wall lignin to the fibre surface and consequently enhancing binding between particles. Although steam pretreatment has been shown to facilitate enzyme-mediated hydrolysis of biomass, by increasing cellulose accessibility, drying and pelletization partially impeded enzymatic hydrolysis. However, the incorporation of alkaline deacetylation or neutral sulfonation step prior to pre-steaming was shown to mitigate many of the negative effects of drying. Although drying and pelletization did not significantly impact the redistribution of lignin, a mild mechanical refining step was shown to further enhance the hydrolysis of the cellulose component of the pelletized biomass.

Enhancing enzymatic digestibility of bamboo residues using a three-constituent deep eutectic solvent pretreatment

A three-constituent deep eutectic solvent (3c-DES) pretreatment with choline chloride-oxalic acid-ethylene glycol was applied to examine its effectiveness on bamboo residues. The 3c-DES pretreatment can remove 91.09% xylan and significantly improved the 72 h hydrolysis yield of D. sinicus by 6.3 and 1.7 times as compared with the liquid hot water and two-constituent deep eutectic solvent (2c-DES) pretreatment. The introduction of ethylene glycol (EG) into choline chloride (ChCl)/ oxalic acid (OA) decreased the content of surface lignin and the condensation of lignin, which contributed to the increase of hydrophilic nature and cellulose accessibility in substrates. Moreover, higher glucose (85.72%) and xylose (91.05%) yields of 3c-DES pretreated bamboo were achieved with the addition of Tween 80. The 3c-DES system provides an alternative approach for the development of efficient bamboo pretreatment, and had broad space for bamboo biorefinery in southern China.

The inhibition of p-hydroxyphenyl hydroxyl group in residual lignin on enzymatic hydrolysis of cellulose and its underlying mechanism

The controlling factors of the inhibition on enzymatic hydrolysis caused by residual lignin were identified with molecular level understanding of the mechanism. Residual lignin samples with different properties were isolated, characterized and added into the enzymatic hydrolysis of Avicel. It was found that the phenolic hydroxyl group (OH) was the main inhibitor in residual lignin, and the p-hydroxyphenyl OH was the crucial sub-structure that exhibited the highest inhibition and non-productive adsorption, ascribing to its higher electrophilicity and lower steric hindrance. The H-bond interaction and π-π stacking between phenolic OH of lignin and phenolic OH of tyrosine on the planar face of carbohydrate binding module of cellulase were probably responsible for the non-productive adsorption. The binding sites of H-bonds may be the H in phenolic OH of lignin and the O in phenolic OH of tyrosine, respectively, and that of the π-π stacking may be the benzene rings of them.

Surfactants in biorefineries: Role, challenges & perspectives

The use of lignocellulosic biomass (LCB) as feedstock has received increasing attention as an alternative to fossil-based refineries. Initial steps such as pretreatment and enzymatic hydrolysis are essential to breakdown the complex structure of LCB to make the sugar molecules available to obtain bioproducts by fermentation. However, these steps increase the cost of the bioproduct and often reduces its competitiveness against synthetic products. Currently, the use of surfactants has shown considerable potential to enhance lignocellulosic biomass processing. This review addresses the main mechanisms and role of surfactants as key molecules in various steps of biorefinery processes, viz., increasing the removal of lignin and hemicellulose during the pretreatments, increasing enzymatic stability and enhancing the accessibility of enzymes to the polymeric fractions, and improving the downstream process during fermentation. Further, technical advances, challenges in application of surfactants, and future perspectives to augment the production of several high value-added bioproducts have been discussed.

A review on physico-chemical delignification as a pretreatment of lignocellulosic biomass for enhanced bioconversion

Effective pretreatment of lignocellulosic biomass (LCB) is one of the most important steps in biorefinery, ensuring the quality and commercial viability of the overall bioprocess. Lignin recalcitrance in LCB is a major bottleneck in biological conversion as the polymerization of lignin with hemicellulose hinders enzyme accessibility and further bioconversion to fuels and chemicals. Therefore, there is a need to delignify LCB to ease further bioprocessing. The efficiency of delignification, quality and quantity of the desired products, and generation of inhibitors depend upon the type of pretreatment employed. This review summarizes different single and integrated physicochemical pretreatments for delignification. Additionally, conditions required for effective delignification and the advantages and drawbacks of each method were evaluated. Advances in overcoming the recalcitrance of residual lignin to saccharification and the methods to recover lignin after delignification are also discussed. Efficient lignin recovery and valorization strategies provide an avenue for the sustainable lignocellulose biorefinery.

Current understanding and optimization strategies for efficient lignin-enzyme interaction: A review

From energy perspective, with abundant polysaccharides (45-85%), the renewable lignocellulosic is recognized as the 2nd generation feedstock for bioethanol and bio-based products production. Enzymatic hydrolysis is a critical pathway to yield fermentable monosaccharides from pretreated substrates of lignocellulose. Nevertheless, the lignin presence in lignocellulosic substrates leads to the low substrate enzymatic digestibility ascribed to the nonproductive adsorption. It has been reported that the water-soluble lignin (low molecular weight, sulfonated/sulfomethylated and graft polymer) enhance the rate of enzymatic digestibility, however, the catalytic mechanism of lignin-enzyme interaction remains elusive. In this review, optimization strategies for enzymatic hydrolysis based on the lignin structural modification, enzyme engineering, and different additives are critically reviewed. Lignin-enzyme interaction mechanism is also discussed (lignin and various cellulases). In addition, the mathematical models and simulation of lignin, cellulose and enzyme aims for promoting an integrated biomass-conversion process for sustainable production of value-added biofuels.

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

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

Comparison of sulfomethylated lignin from poplar and masson pine on cellulase adsorption and the enzymatic hydrolysis of wheat straw

In this work, effects of sulfomethylated lignins (SLs) prepared from masson pine (SL_M) and poplar (SL_P) on enzymatic hydrolysis and cellulase-lignin interaction were comparatively investigated. The results showed that both SL_M and SL_P significantly promoted the substrate enzymatic digestibility. The total sugar yield increased from 38.6% to 74.4% and ∼ 100%, respectively at 10 FPU/g-cellulose of cellulase dosage. The protein content in hydrolysate linearly increased with the addition of SL (0 - 1.6 g/g-substrate lignin), which suggested the competitive adsorption of cellulase may occur to substrate lignin and SLs. Further structural analysis of lignin revealed the high S/(V + H) ratio was directly related to the high enzymatic saccharification efficiency. The strong interaction between SL and cellulase decreased the nonproductive adsorption of cellulase onto substrate lignin and increased the accessibility of cellulase to carbohydrate, which was considered to be the key factor for the improvement of substrate enzymatic digestibility.

Comparative Evaluation of Adsorption of Major Enzymes in a Cellulase Cocktail Obtained from Trichoderma reesei onto Different Types of Lignin

Cellulase adsorption onto lignin decreases the productivity of enzymatic hydrolysis of lignocellulosic biomass. Here, adsorption of enzymes onto different types of lignin was investigated, and the five major enzymes—cellobiohydrolases (CBHs), endoglucanase (Cel7B), β-glucosidase (Cel3A), xylanase (XYNIV), and mannanase (Man5A)—in a cellulase cocktail obtained from Trichoderma reesei were individually analyzed through SDS-PAGE and zymogram assay. Lignin was isolated from woody (oak and pine lignin) and herbaceous (rice straw and kenaf lignin) plants. The relative adsorption of CBHs compared to the control was in the range of 14.15–18.61%. The carbohydrate binding motif (CBM) of the CBHs contributed to higher adsorption levels in oak and kenaf lignin, compared to those in pine and rice lignin. The adsorption of endoglucanase (Cel7B) by herbaceous plant lignin was two times higher than that of woody lignin, whereas XYNIV showed the opposite pattern. β-glucosidase (Cel3A) displayed the highest and lowest adsorption ratios on rice straw and kenaf lignin, respectively. Mannanase (Man5A) was found to have the lowest adsorption ratio on pine lignin. Our results showed that the hydrophobic properties of CBM and the enzyme structures are key factors in adsorption onto lignin, whereas the properties of specific lignin types indirectly affect adsorption.

Saccharification of acid-alkali pretreated sugarcane bagasse using immobilized enzymes from Phomopsis stipata

In this study, a mild-temperature two-step dilute acid and alkaline pretreatment (DA-AL) process was developed to generate highly digestible cellulose pulp from sugarcane bagasse for producing fermentable sugars by novel thermophilic cellulases derived from Phomopsis stipata SC 04. First, DA pretreatment of sugarcane bagasse at 2% (w/v) H2SO4 and 121 °C for 71 min, followed by AL pretreatment at 2.2% (w/v) NaOH and 110 °C for 100 min led to the pulp containing 86% cellulose. The cellulose pulp was hydrolyzed by the immobilized P. stipata cellulase on Ca-alginate beads, following optimization of immobilization conditions. The results showed that mixing the cellulase extract and sodium alginate solutions at a volume ratio of 1:4 led to the highest immobilization efficiencies of 99.83% for β-glucosidase and 97.52% for endoglucanase while the enzyme leakage was the lowest. The use of the immobilized cellulases led to a cellulose digestibility of 30% in the initial batch and recycling of the immobilized cellulases reduced cellulose digestibility to 18% after s recycling for two times (a total of third rounds). Overall, this study provides useful information in the use of a mild pretreatment process to produce highly digestible cellulose pulp and in the immobilization of thermophilic cellulases to produce fermentable sugars from pretreated biomass.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-03101-2.

Current breakthroughs in the hardwood biorefineries: Hydrothermal processing for the co-production of xylooligosaccharides and bioethanol

The development of lignocellulosic biorefineries requires a first stage of pretreatment which enables the efficient valorization of all fractions present in this renewable material. In this sense, this review aims to show the main advantages of hydrothermal treatment as a first step of a biorefinery infrastructure using hardwood as raw material, as well as, main drawback to overcome. Hydrothermal treatment of hardwood highlights for its high selectivity for hemicelluloses solubilization as xylooligosaccharides (XOS). Nevertheless, the suitable conditions for XOS production are inadequate to achieve an elevate cellulose to glucose conversion. Hence, several strategies namely the combination of hydrothermal treatment with delignification process, in situ modification of lignin and the mixture with another renewable resources (concretely, seaweeds, and by-products generated in the food industry with high sugar content) were pinpointed as promising alternative to increase the final ethanol concentration coupled with XOS recovery in the hydrolysate.

In-situ lignin modification with polyethylene glycol-epoxides to boost enzymatic hydrolysis of combined-pretreated masson pine

Acid pretreatment was insufficient to disrupt the recalcitrance derived from lignins in softwood, thus a lignin-targeting post-treatment was required. In this study, a combined acid and alkali pretreatment with polyethylene glycol-epoxides (PEG-epoxides) was developed on masson pine. Results showed although the combined pretreatment achieved a limited delignification, but a remarkably increment of 15.9-34.9% on hydrolysis yields was achieved. This was ascribed to the successful incorporation of hydrophilic PEG chains to residual lignins. Moreover, the improvement on enzymatic digestibility varied with the PEG chain lengths in modifiers. The underlying reasons for this improvement were primarily investigated by monitoring the lignin properties as well as water retention values variation after in-situ lignin modification by PEG-epoxides with varied molecular weights. It indicated that the enzymatic hydrolysis improvement was mainly due to both reduced enzyme nonspecific adsorption and increased fiber swelling. Results will give new insights to resolve the challenge on softwood biorefinery.