Understanding the Nonproductive Enzyme Adsorption and Physicochemical Properties of Residual Lignins in Moso Bamboo Pretreated with Sulfuric Acid and Kraft Pulping

Lignin detaching from the oxidative delignified softwood during enzymatic hydrolysis and its effect on carbohydrate saccharification

Most studies about the influence of lignin on enzymatic digestibility of lignocellulose have focused on the content and properties, but less on the detaching behavior of lignin. The samples were prepared from Pinus massoniana wood chips by kraft cooking followed by delignification using oxygen/alkali (KP-O) and chlorine dioxide (KP-D), respectively. Two oxidative delignified samples with a similar lignin content were subject to enzymatic hydrolysis at both pH of 5.0 and 5.5 to investigate the effects of lignin detached rate (LDR) on substrate enzymatic digestibility (SED). The LDRs and the SEDs from both samples increased with the enzymatic hydrolysis time, and the situations of KP-D were much higher than those of KP-O under the same enzymatic hydrolysis time. The results of enzymatic hydrolysis at an elevated pH of 5.5 and the changes in concentration of free cellulase of the two samples indicated that the lignin detaching increased the free cellulase concentration, and thus promoted the enzymatic digestibility. Moreover, lignin distribution analysis by X-ray photoelectric spectroscopy and confocal laser scanning microscopy indicated surface lignin being preferentially detached. This work provided a reference for rationally designing pretreatment strategies, which can improve the efficiency of enzyme hydrolysis of lignocellulosic biomass.

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.

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.

Hydrothermal pretreatment contributes to accelerate maturity during the composting of lignocellulosic solid wastes

The aim of this work was to study the optimal conditions and mechanism of lignocellulose degradation in the hydrothermal pretreatment coupled with aerobic fermentation (HTPAF). The optimized process parameters in the hydrothermal pretreatment (HTP) were discussed. The response relationship between enzyme activity and microbial community in HTPAF were explored. The results showed that with the moisture content of 50%-90%, the lignin content decreased by 150 mg/g after treatment at 120 °C for 6 h, and a loose pore structure was formed on the surface of the chestnut shells after HTP. The compost maturity time was shortened to 12 days. The dominant microbial genera in HTPAF were Gallicola, Moheibacter and Atopostipes, which were significant different with that of the traditional composting. HTPAF is beneficial to increase the maximum temperature of aerobic fermentation and quickly degrade lignin to shorten the maturity time.

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.

Recent advances in understanding the effects of lignin structural characteristics on enzymatic hydrolysis

Enzymatic hydrolysis of lignocellulose for bioethanol production shows a great potential to remit the rapid consumption of fossil fuels, given the fact that lignocellulose feedstocks are abundant, cost-efficient, and renewable. Lignin results in low enzymatic saccharification by forming the steric hindrance, non-productive adsorption of cellulase onto lignin, and deactivating the cellulase. In general, the non-productive binding of cellulase on lignin is widely known as the major cause for inhibiting the enzymatic hydrolysis. Pretreatment is an effective way to remove lignin and improve the enzymatic digestibility of lignocellulose. Along with removing lignin, the pretreatment can modify the lignin structure, which significantly affects the non-productive adsorption of cellulase onto lignin. To relieve the inhibitory effect of lignin on enzymatic hydrolysis, enormous efforts have been made to elucidate the correlation of lignin structure with lignin-enzyme interactions but with different views. In addition, contrary to the traditional belief that lignin inhibits enzymatic hydrolysis, in recent years, the addition of water-soluble lignin such as lignosulfonate or low molecular-weight lignin exerts a positive effect on enzymatic hydrolysis, which gives a new insight into the lignin-enzyme interactions. For throwing light on their structure-interaction relationship during enzymatic hydrolysis, the effect of residual lignin in substrate and introduced lignin in hydrolysate on enzymatic hydrolysis are critically reviewed, aiming at realizing the targeted regulation of lignin structure for improving the saccharification of lignocellulose. The review is also focused on exploring the lignin-enzyme interactions to mitigate the negative impact of lignin and reducing the cost of enzymatic hydrolysis of lignocellulose.

Unlocking the secret of lignin-enzyme interactions: Recent advances in developing state-of-the-art analytical techniques

Bioconversion of renewable lignocellulosics to produce liquid fuels and chemicals is one of the most effective ways to solve the problem of fossil resource shortage, energy security, and environmental challenges. Among the many biorefinery pathways, hydrolysis of lignocellulosics to fermentable monosaccharides by cellulase is arguably the most critical step of lignocellulose bioconversion. In the process of enzymatic hydrolysis, the direct physical contact between enzymes and cellulose is an essential prerequisite for the hydrolysis to occur. However, lignin is considered one of the most recalcitrant factors hindering the accessibility of cellulose by binding to cellulase unproductively, which reduces the saccharification rate and yield of sugars. This results in high costs for the saccharification of carbohydrates. The various interactions between enzymes and lignin have been explored from different perspectives in literature, and a basic lignin inhibition mechanism has been proposed. However, the exact interaction between lignin and enzyme as well as the recently reported promotion of some types of lignin on enzymatic hydrolysis is still unclear at the molecular level. Multiple analytical techniques have been developed, and fully unlocking the secret of lignin-enzyme interactions would require a continuous improvement of the currently available analytical techniques. This review summarizes the current commonly used advanced research analytical techniques for investigating the interaction between lignin and enzyme, including quartz crystal microbalance with dissipation (QCM-D), surface plasmon resonance (SPR), attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, atomic force microscopy (AFM), nuclear magnetic resonance (NMR) spectroscopy, fluorescence spectroscopy (FLS), and molecular dynamics (MD) simulations. Interdisciplinary integration of these analytical methods is pursued to provide new insight into the interactions between lignin and enzymes. This review will serve as a resource for future research seeking to develop new methodologies for a better understanding of the basic mechanism of lignin-enzyme binding during the critical hydrolysis process.

Production of xylo-oligosaccharides from poplar by acetic acid pretreatment and its impact on inhibitory effect of poplar lignin

Recently, efficient production of xylo-oligosaccharides (XOS) from poplar by acetic acid (AA) pretreatment was developed; but the effect of residual lignin on subsequent cellulase hydrolysis was unclear. Herein, XOS was produced from poplar by AA pretreatment and the effect of AA pretreatment on lignin inhibition to cellulase hydrolysis was investigated. The results indicated that a high XOS yield of 55.8% was obtained, and the inhibition degree of lignin in poplar increased from 1.0% to 6.8% after AA pretreatment. Lignin was acetylated and its molecular weight decreased from 12,211 to 2871 g/mol after AA pretreatment. The increase of S/G ratio, phenolic hydroxyl, and condensed units of lignin after AA pretreatment might be reasons for this intensified inhibition. The results advanced our understanding of the structural and inhibitory properties of lignin after production of XOS from poplar with AA pretreatment, and provided references for efficient cellulase hydrolysis of poplar after AA pretreatment.

Evaluating the potential of a novel hardwood biomass using a superbase ionic liquid

Lignocellulosic biomass, being ubiquitous and easily accessible, bears a huge potential for sustainable energy and other products.

NMR elucidation of nonproductive binding sites of lignin models with carbohydrate-binding module of cellobiohydrolase I

BACKGROUND

Highly efficient enzymatic saccharification of pretreated lignocellulose is a key step in achieving lignocellulosic biorefinery. Cellobiohydrolase I (Cel7A) secreted by Trichoderma reesei is an industrially used cellulase that possesses carbohydrate-binding module 1 (TrCBM1) at the C-terminal domain. The nonproductive binding of TrCBM1 to lignin significantly decreases the enzymatic saccharification efficiency and increases the cost of biomass conversion because of the additionally required enzymes. Understanding the interaction mechanism between lignin and TrCBM1 is essential for realizing a cost-effective biofuel production; however, the binding sites in lignin have not been clearly elucidated.

RESULTS

Three types of ¹³C-labeled β-O-4 lignin oligomer models were synthesized and characterized. The 2D ¹H-¹³C heteronuclear single-quantum correlation (HSQC) spectra of the ¹³C-labeled lignin models confirmed that the three types of the ¹³C labels were correctly incorporated in the (1) aromatic rings and β positions, (2) α positions, and (3) methoxy groups, respectively. The TrCBM1-binding sites in lignin were analyzed by observing NMR chemical shift perturbations (CSPs) using the synthetic ¹³C-labeled β-O-4 lignin oligomer models. Obvious CSPs were observed in signals from the aromatic regions in oligomers bound to TrCBM1, whereas perturbations in the signals from aliphatic regions and methoxy groups were insignificant. These findings indicated that hydrophobic interactions and π-π stacking were dominating factors in nonproductive binding. The synthetic lignin models have two configurations whose terminal units were differently aligned and donated C^(I) and C^(II). The C^(I) ring showed remarkable perturbation compared with the C^(II), which indicated that the binding of TrCBM1 was markedly affected by the configuration of the lignin models. The long-chain lignin models (degree of polymerization (DP) 4.16-4.70) clearly bound to TrCBM1. The interactions of TrCBM1 with the short-chain lignin models (DP 2.64-3.12) were insignificant, indicating that a DP greater than 4 was necessary for TrCBM1 binding.

CONCLUSION

The CSP analysis using ¹³C-labeled β-O-4 lignin oligomer models enabled the identification of the TrCBM1 binding sites in lignins at the atomic level. This specific interaction analysis will provide insights for new molecular designs of cellulase having a controlled affinity to cellulose and lignin for a cost-effective biorefinery process.

Non-productive celluase binding onto deep eutectic solvent (DES) extracted lignin from willow and corn stover with inhibitory effects on enzymatic hydrolysis of cellulose

In this work, deep eutectic solvent (DES) was prepared by mixing choline chloride (ChCl) with lactic acid (LA), and effects of cellulase non-productive binding onto DES-extracted lignin from willow and corn stover on enzymatic hydrolysis of cellulose was investigated. The correlation between hydrolysis yield of cellulose and chemical features of lignin was evaluated, and a potential inhibitory mechanism was proposed. Condensation of lignin was observed during DES treatment, and these condensed aromatic structures had an increased tendency to adsorb enzymes through hydrophobic interactions. As well as hydrophobic interactions mediated by lignin condensation, an increase in phenolic hydroxyl groups resulted in a greater amount of hydrogen bonds between cellulases and lignin that appeared to inhibit enzymatic hydrolysis yields of cellulose (39.96-42.86 % to 31.96-32.68 %). Although large amounts of COOHs were generated, the elevated electrostatic repulsion as a result of ionic groups was insufficient to decrease non-productive adsorption.

Real-Time QCM-D Monitoring of the Adsorption-Desorption of Expansin on Lignin

Expansin has nonhydrolytic disruptive activity and synergistically acts with cellulases to enhance the hydrolysis of cellulose. The adsorption-desorption of expansin on noncellulosic lignin can greatly affect the action of expansin on lignocellulose. In this study, three lignins with different sources (kraft lignin (KL), sodium lignin sulfonate (SLS), and enzymatic hydrolysis lignin (EHL)) were selected as the substrates. The real-time adsorption-desorption of Bacillus subtilis expansin (BsEXLX1) on lignins was monitored using quartz crystal microgravimetry with dissipation (QCM-D). The effects of temperature and Tween 80 on the adsorption-desorption behaviors were also investigated. The results show that BsEXLX1 exhibited high binding ability on lignin and achieved maximum adsorption of 283.2, 273.8, and 266.9 ng cm^-2 at 25 °C on KL, SLS, and EHL, respectively. The maximum adsorption decreased to 148.2-192.8 ng cm^-2 when the temperature increased from 25 to 45 °C. Moreover, Tween 80 competitively bound to lignin and significantly prevented expansin adsorption. After irreversible adsorption of Tween 80, the maximum adsorption of BsEXLX1 greatly decreased to 33.3, 37.2, and 10.3 ng cm^-2 at 25 °C on KL, SLS, and EHL, respectively. Finally, a kinetic model was developed to analyze the adsorption-desorption process of BsEXLX1. BsEXLX1 has a higher adsorption rate constant (k_A) and a lower desorption rate constant (k_D) on KL than on SLS and EHL. The findings of this study provide useful insights into the adsorption-desorption of expansin on lignin.

Ethanol Production from Hydrolyzed Kraft Pulp by Mono- and Co-Cultures of Yeasts: The Challenge of C6 and C5 Sugars Consumption

Second-generation bioethanol production’s main bottleneck is the need for a costly and technically difficult pretreatment due to the recalcitrance of lignocellulosic biomass (LCB). Chemical pulping can be considered as a LCB pretreatment since it removes lignin and targets hemicelluloses to some extent. Chemical pulps could be used to produce ethanol. The present study aimed to investigate the batch ethanol production from unbleached Kraft pulp of Eucalyptus globulus by separate hydrolysis and fermentation (SHF). Enzymatic hydrolysis of the pulp resulted in a glucose yield of 96.1 ± 3.6% and a xylose yield of 94.0 ± 7.1%. In an Erlenmeyer flask, fermentation of the hydrolysate using Saccharomyces cerevisiae showed better results than Scheffersomyces stipitis. At both the Erlenmeyer flask and bioreactor scale, co-cultures of S. cerevisiae and S. stipitis did not show significant improvements in the fermentation performance. The best result was provided by S. cerevisiae alone in a bioreactor, which fermented the Kraft pulp hydrolysate with an ethanol yield of 0.433 g·g−1 and a volumetric ethanol productivity of 0.733 g·L−1·h−1, and a maximum ethanol concentration of 19.24 g·L−1 was attained. Bioethanol production using the SHF of unbleached Kraft pulp of E. globulus provides a high yield and productivity.

Exploring why sodium lignosulfonate influenced enzymatic hydrolysis efficiency of cellulose from the perspective of substrate-enzyme adsorption

BACKGROUND

Cellulase adsorbed on cellulose is productive and helpful to produce reducing sugars in enzymatic hydrolysis of lignocellulose; however, cellulase adsorbed on lignin is non-productive. Increasing productive adsorption of cellulase on cellulose would be beneficial in improving enzymatic hydrolysis. Adding lignin that was more hydrophilic in hydrolysis system could increase productive adsorption and promote hydrolysis. However, the effect mechanism is still worth exploring further. In this study, lignosulfonate (LS), a type of hydrophilic lignin, was used to study its effect on cellulosic hydrolysis.

RESULTS

The effect of LS on the enzymatic hydrolysis of pure cellulose (Avicel) and lignocellulose [dilute acid (DA) treated sugarcane bagasse (SCB)] was investigated by analyzing enzymatic hydrolysis efficiency, productive and non-productive cellulase adsorptions, zeta potential and particle size distribution of substrates. The result showed that after adding LS, the productive cellulase adsorption on Avicel reduced. Adding LS to Avicel suspension could form the Avicel-LS complexes. The particles were charged more negatively and the average particle size was smaller than Avicel before adding LS. In addition, adding LS to cellulase solution formed the LS-cellulase complexes. For DA-SCB, adding LS decreased the non-productive cellulase adsorption on DA-SCB from 3.92 to 2.99 mg/g lignin and increased the productive adsorption of cellulase on DA-SCB from 2.00 to 3.44 mg/g cellulose. Besides, the addition of LS promoted the formation of LS-lignin complexes and LS-cellulase complexes, and the complexes had more negative charges and smaller average sizes than DA-SCB lignin and cellulase particles before adding LS.

CONCLUSIONS

In this study, LS inhibited Avicel's hydrolysis, but enhanced DA-SCB's hydrolysis. This stemmed from the fact that LS could bind cellulase and Avicel, and occupied the binding sites of cellulase and Avicel. Thus, a decreased productive adsorption of cellulase on Avicel arose. Regarding DA-SCB, adding LS, which enhanced hydrolysis efficiency of DA-SCB, increased the electrostatic repulsion between DA-SCB lignin and cellulase, and therefore, decreased non-productive adsorption of cellulase on DA-SCB lignin and enhanced productive adsorption of cellulase on DA-SCB cellulose.

Improving enzymatic hydrolysis of acid-pretreated bamboo residues using amphiphilic surfactant derived from dehydroabietic acid

In this work, amphiphilic surfactant was obtained using dehydroabietic acid from pine rosin and then pre-adsorbed with acid-pretreated bamboo residues (AP-BR) to block the residual lignin adsorption site, which is expected to improve its enzymatic digestibility. Results from cryogenic-transmission electron microscopy (Cryo-TEM) indicated amphiphilic surfactant with PEG with polymerization degree of 34 (D-34) aggregated to form worm-like micelles, which improved enzymatic hydrolysis yield of AP-BR from 24.3% to 71.9% by pre-adsorbing with 0.8 g/L. Amphiphilic surfactants pre-adsorbed on AP-BR could reduce hydrophobicity of AP-BR, adsorption affinity and adsorption capacity of lignin for cellulase from 0.51 L/g to 0.48-0.32 L/g, from 2.9 mL/mg to 1.8-1.4 mL/mg, and from 122.3 mg/g to 101.9-21.4 mg/g, respectively. These changed properties showed compelling positive contributions (R² > 0.9) for free enzymes in the supernatants and sequently for final enzymatic hydrolysis yield, which was caused by blocking non-productively hydrophobic adsorption between lignin and cellulase.

Engineering grass biomass for sustainable and enhanced bioethanol production

Bioethanol from lignocellulosic biomass is a promising step for the future energy requirements. Grass is a potential lignocellulosic biomass which can be utilised for biorefinery-based bioethanol production. Grass biomass is a suitable feedstock for bioethanol production due to its all the year around production, requirement of less fertile land and noninterference with food system. However, the processes involved, i.e. pretreatment, enzymatic hydrolysis and fermentation for bioethanol production from grass biomass, are both time consuming and costly. Developing the grass biomass in planta for enhanced bioethanol production is a promising step for maximum utilisation of this valuable feedstock and, thus, is the focus of the present review. Modern breeding techniques and transgenic processes are attractive methods which can be utilised for development of the feedstock. However, the outcomes are not always predictable and the time period required for obtaining a robust variety is generation dependent. Sophisticated genome editing technologies such as synthetic genetic circuits (SGC) or clustered regularly interspaced short palindromic repeats (CRISPR) systems are advantageous for induction of desired traits/heritable mutations in a foreseeable genome location in the 1st mutant generation. Although, its application in grass biomass for bioethanol is limited, these sophisticated techniques are anticipated to exhibit more flexibility in engineering the expression pattern for qualitative and qualitative traits. Nevertheless, the fundamentals rendered by the genetics of the transgenic crops will remain the basis of such developments for obtaining biorefinery-based bioethanol concepts from grass biomass. Grasses which are abundant and widespread in nature epitomise attractive lignocellulosic feedstocks for bioethanol production. The complexity offered by the grass cell wall in terms of lignin recalcitrance and its binding to polysaccharides forms a barricade for its commercialization as a biofuel feedstock. Inspired by the possibilities for rewiring the genetic makeup of grass biomass for reduced lignin and lignin-polysaccharide linkages along with increase in carbohydrates, innovative approaches for in planta modifications are forging ahead. In this review, we highlight the progress made in the field of transgenic grasses for bioethanol production and focus our understanding on improvements of simple breeding techniques and post-harvest techniques for development in shortening of lignin-carbohydrate and carbohydrate-carbohydrate linkages. Further, we discuss about the designer lignins which are aimed for qualitable lignins and also emphasise on remodelling of polysaccharides and mixed-linkage glucans for enhancing carbohydrate content and in planta saccharification efficiency. As a final point, we discuss the role of synthetic genetic circuits and CRISPR systems in targeted improvement of cell wall components without compromising the plant growth and health. It is anticipated that this review can provide a rational approach towards a better understanding of application of in planta genetic engineering aspects for designing synthetic genetic circuits which can promote grass feedstocks for biorefinery-based bioethanol concepts.

Substrate-Related Factors Affecting Cellulosome-Induced Hydrolysis for Lignocellulose Valorization

Cellulosomes are an extracellular supramolecular multienzyme complex that can efficiently degrade cellulose and hemicelluloses in plant cell walls. The structural and unique subunit arrangement of cellulosomes can promote its adhesion to the insoluble substrates, thus providing individual microbial cells with a direct competence in the utilization of cellulosic biomass. Significant progress has been achieved in revealing the structures and functions of cellulosomes, but a knowledge gap still exists in understanding the interaction between cellulosome and lignocellulosic substrate for those derived from biorefinery pretreatment of agricultural crops. The cellulosomic saccharification of lignocellulose is affected by various substrate-related physical and chemical factors, including native (untreated) wood lignin content, the extent of lignin and xylan removal by pretreatment, lignin structure, substrate size, and of course substrate pore surface area or substrate accessibility to cellulose. Herein, we summarize the cellulosome structure, substrate-related factors, and regulatory mechanisms in the host cells. We discuss the latest advances in specific strategies of cellulosome-induced hydrolysis, which can function in the reaction kinetics and the overall progress of biorefineries based on lignocellulosic feedstocks.

NMR Analysis on Molecular Interaction of Lignin with Amino Acid Residues of Carbohydrate-Binding Module from Trichoderma reesei Cel7A

Lignocellulosic biomass is anticipated to serve as a platform for green chemicals and fuels. Nonproductive binding of lignin to cellulolytic enzymes should be avoided for conversion of lignocellulose through enzymatic saccharification. Although carbohydrate-binding modules (CBMs) of cellulolytic enzymes strongly bind to lignin, the adsorption mechanism at molecular level is still unclear. Here, we report NMR-based analyses of binding sites on CBM1 of cellobiohydrolase I (Cel7A) from a hyper-cellulase-producing fungus, Trichoderma reesei, with cellohexaose and lignins from Japanese cedar (C-MWL) and Eucalyptus globulus (E-MWL). A method was established to obtain properly folded TrCBM1. Only TrCBM1 that was expressed in freshly transformed E. coli had intact conformation. Chemical shift perturbation analyses revealed that TrCBM1 adsorbed cellohexaose in highly specific manner via two subsites, flat plane surface and cleft, which were located on the opposite side of the protein surface. Importantly, MWLs were adsorbed at multiple binding sites, including the subsites, having higher affinity than cellohexaose. G6 and Q7 were involved in lignin binding on the flat plane surface of TrCBM1, while cellohexaose preferentially interacted with N29 and Q34. TrCBM1 used much larger surface area to bind with C-MWL than E-MWL, indicating the mechanisms of adsorption toward hardwood and softwood lignins are different.

Overcoming biomass recalcitrance by synergistic pretreatment of mechanical activation and metal salt for enhancing enzymatic conversion of lignocellulose

BACKGROUND

Due to biomass recalcitrance, including complexity of lignocellulosic matrix, crystallinity of cellulose, and inhibition of lignin, the bioconversion of lignocellulosic biomass is difficult and inefficient. The aim of this study is to investigate an effective and green pretreatment method for overcoming biomass recalcitrance of lignocellulose.

RESULTS

An effective mechanical activation (MA) + metal salt (MAMS) technology was applied to pretreat sugarcane bagasse (SCB), a typical kind of lignocellulosic biomass, in a stirring ball mill. Chlorides and nitrates of Al and Fe showed better synergistic effect with MA, especially AlCl₃, ascribing to the interaction between metal salt and oxygen-containing groups induced by MA. Comparative studies showed that MAMS pretreatment effectively changed the recalcitrant structural characteristics of lignocellulosic matrix and reduced the inhibitory action of lignin on enzymatic conversion of SCB. The increase in hydroxyl and carboxyl groups of lignin induced by MAMS pretreatment led to the increase of its hydrophilicity, which could weaken the binding force between cellulase and lignin and reduce the nonproductive binding of cellulase enzymes to lignin.

CONCLUSIONS

MAMS pretreatment significantly enhanced the enzymatic digestibility of polysaccharides substrate by overcoming biomass recalcitrance without the removal of lignin from enzymatic hydrolysis system.

Enzymatic sugar production from elephant grass and reed straw through pretreatments and hydrolysis with addition of thioredoxin-His-S

BACKGROUND

The bioconversion of lignocellulose to fermentable C5/C6-saccharides is composed of pretreatment and enzymatic hydrolysis. Lignin, as one of the main components, resists lignocellulose to be bio-digested. Alkali and organosolv treatments were reported to be able to delignify feedstocks and loose lignocellulose structure. In addition, the use of additives was an alternative way to block lignin and reduce the binding of cellulases to lignin during hydrolysis. However, the relatively high cost of these additives limits their commercial application.

RESULTS

This study explored the feasibility of using elephant grass (Pennisetum purpureum) and reed straw (Phragmites australis), both of which are important fibrous plants with high biomass, no-occupation of cultivated land, and soil phytoremediation, as feedstocks for bio-saccharification. Compared with typical agricultural residues, elephant grass and reed straw contained high contents of cellulose and hemicellulose. However, lignin droplets on the surface of elephant grass and the high lignin content in reed straw limited their hydrolysis performances. High hydrolysis yield was obtained for reed straw after organosolv and alkali pretreatments via increasing cellulose content and removing lignin. However, the hydrolysis of elephant grass was only enhanced by organosolv pretreatment. Further study showed that the addition of bovine serum albumin (BSA) or thioredoxin with His- and S-Tags (Trx-His-S) improved the hydrolysis of alkali-pretreated elephant grass. In particular, Trx-His-S was first used as an additive in lignocellulose saccharification. Its structural and catalytic properties were supposed to be beneficial for enzymatic hydrolysis.

CONCLUSIONS

Elephant grass and reed straw could be used as feedstocks for bioconversion. Organosolv and alkali pretreatments improved their enzymatic sugar production; however, the increase in hydrolysis yield of pretreated elephant grass was not as effective as that of reed straw. During the hydrolysis of alkali-pretreated elephant grass, Trx-His-S performed well as additive, and its structural and catalytic capability was beneficial for enzymatic hydrolysis.

Second Generation Bioethanol Production: On the Use of Pulp and Paper Industry Wastes as Feedstock

Due to the health and environment impacts of fossil fuels utilization, biofuels have been investigated as a potential alternative renewable source of energy. Bioethanol is currently the most produced biofuel, mainly of first generation, resulting in food-fuel competition. Second generation bioethanol is produced from lignocellulosic biomass, but a costly and difficult pretreatment is required. The pulp and paper industry has the biggest income of biomass for non-food-chain production, and, simultaneously generates a high amount of residues. According to the circular economy model, these residues, rich in monosaccharides, or even in polysaccharides besides lignin, can be utilized as a proper feedstock for second generation bioethanol production. Biorefineries can be integrated in the existing pulp and paper industrial plants by exploiting the high level of technology and also the infrastructures and logistics that are required to fractionate and handle woody biomass. This would contribute to the diversification of products and the increase of profitability of pulp and paper industry with additional environmental benefits. This work reviews the literature supporting the feasibility of producing ethanol from Kraft pulp, spent sulfite liquor, and pulp and paper sludge, presenting and discussing the practical attempt of biorefineries implementation in pulp and paper mills for bioethanol production.

Enzyme adsorption properties on dilute acid pretreated biomass by low vacuum-scanning electron microscopy and structural analysis of lignin

In this study, enzyme adsorption properties were investigated as a function of the structural change of lignin in dilute acid pretreated biomass using oxalic and sulfuric acid catalysts under the same reaction conditions. Although the contents of glucan and lignin in the dilute acid pretreated biomass were similar regardless of the catalysts used, the enzymatic hydrolysis efficiency and degree of enzyme adsorption differed considerably. The highest efficiencies were 87.79% and 96.49% for the oxalic acid and sulfuric acid catalysts, respectively. The reasons for this observation were investigated by low vacuum-scanning electron microscopy and the structural analysis of lignin. In the oxalic acid pretreated biomass, the enzyme was irreversibly adsorbed onto the lignin. The oxalic acid pretreated biomass possessed a higher content of G-type lignin than did the sulfuric acid pretreated biomass. This type of lignin has a high affinity for the enzyme, inducing irreversible enzyme adsorption onto the biomass.

Enhancing enzymatic hydrolysis of sugarcane bagasse by ferric chloride catalyzed organosolv pretreatment and Tween 80

In this work, a FeCl₃-catalyzed organosolv pretreatment was employed at 160 °C to remove hemicellulose and lignin in sugarcane bagasse leaving the cellulose-enriched residue for enzymatic hydrolysis to sugars. The solubilized hemicellulose fractions consisted more monomer xylose than oligomer xylose. The FeCl₃-catalyzed organosolv pretreatment significantly improved the enzymatic hydrolysis, nearly 100% of cellulose components were converted to glucose after pretreatment with 0.05 M FeCl₃. Structural analysis was employed to reveal how pretreatment affected the enzymatic hydrolysis. With the addition of Tween 80, the same level of glucose was obtained with 50% reduction of enzyme dosage after 24 h. Furthermore, the influence of Tween 80 on different pretreatment systems was investigated, indicating that the improvement was increased as the lignin content increased, decreased with high enzyme loading and extending hydrolysis time. This work suggested that the addition of Tween 80 could improve the enzymatic hydrolysis, reduce the hydrolysis time and enzyme dosage.

Current Understanding of the Correlation of Lignin Structure with Biomass Recalcitrance

Lignin, a complex aromatic polymer in terrestrial plants, contributes significantly to biomass recalcitrance to microbial and/or enzymatic deconstruction. To reduce biomass recalcitrance, substantial endeavors have been exerted on pretreatment and lignin engineering in the past few decades. Lignin removal and/or alteration of lignin structure have been shown to result in reduced biomass recalcitrance with improved cell wall digestibility. While high lignin content is usually a barrier to a cost-efficient application of bioresources to biofuels, the direct correlation of lignin structure and its concomitant properties with biomass remains unclear due to the complexity of cell wall and lignin structure. Advancement in application of biorefinery to production of biofuels, chemicals, and bio-derived materials necessitates a fundamental understanding of the relationship of lignin structure and biomass recalcitrance. In this mini-review, we focus on recent investigations on the influence of lignin chemical properties on bioprocessability-pretreatment and enzymatic hydrolysis of biomass. Specifically, lignin-enzyme interactions and the effects of lignin compositional units, hydroxycinnamates, and lignin functional groups on biomass recalcitrance have been highlighted, which will be useful not only in addressing biomass recalcitrance but also in deploying renewable lignocelluloses efficiently.

Enhancement of In Situ Enzymatic Saccharification of Corn Stover by a Stepwise Sodium Hydroxide and Organic Acid Pretreatment

A stepwise pretreatment method that combines sodium hydroxide and organic acid pretreatments was proposed and investigated to maximize the recovery of main constituents of lignocellulose. The sodium hydroxide pretreatment was firstly optimized by a designed orthogonal experiment with the optimum pretreatment conditions determined as 1 wt% NaOH at 70 °C for 1 h, and 60.42 % of lignin was successfully removed during this stage. In the second stage, 0.5 % acetic acid was selected to pretreat the first-stage solid residue at 80 °C for 40 min in order to decompose hemicelluloses to soluble oligomers or monomers. Then, the whole slurry was subjected to in situ enzymatic saccharification by cellullase with a supplementation of xylanase to further degrade the xylooligosaccharides generated during the acetic acid pretreatment. The maximum reducing sugar and glucose yields achieved were 20.74 and 12.03 g/L, respectively. Furthermore, rapid ethanol fermentation and a yield of 80.3 % also testified this pretreatment method, and the in situ saccharification did not bring any negative impact on ethanol fermentation and has a broad application prospect.