Structural insights into the affinity of Cel7A carbohydrate-binding module for lignin

Investigating Cellulose Binding of Peptides Derived from Carbohydrate Binding Module 1

Carbohydrate-binding modules (CBM) have emerged as useful tools for a wide range of tasks, including the use as purification tags or for cellulose fiber modification. For this purpose, the CBM needs to be attached to a target protein leading to large constructs. We investigated if short peptides from the carbohydrate binding site of CBMs can bind in a similar way as native, full-length CBMs to nanocrystalline cellulose (NCC) or cotton linter paper. We designed our cellulose-binding peptides to be less hydrophobic and shorter than those previously reported. Starting from the binding site of Cel7A-CBM1, we incorporated the essential amino acids involved in cellulose binding into our peptides. These peptides, as well as control peptides with scrambled sequences or a lack of essential amino acids, bound to cellulose with similar affinity as CBM regardless of their secondary structure, sequence, or hydrophobicity. This unspecific mode of cellulose binding displayed by the presented peptides may be exploited to functionalize cellulose-based biomaterials by means of peptide-conjugates.

Carbohydrate binding modules: Compact yet potent accessories in the specific substrate binding and performance evolution of carbohydrate-active enzymes

Carbohydrate binding modules (CBMs) are independent non-catalytic domains widely found in carbohydrate-active enzymes (CAZymes), and they play an essential role in the substrate binding process of CAZymes by guiding the appended catalytic modules to the target substrates. Owing to their precise recognition and selective affinity for different substrates, CBMs have received increasing research attention over the past few decades. To date, CBMs from different origins have formed a large number of families that show a variety of substrate types, structural features, and ligand recognition mechanisms. Moreover, through the modification of specific sites of CBMs and the fusion of heterologous CBMs with catalytic domains, improved enzymatic properties and catalytic patterns of numerous CAZymes have been achieved. Based on cutting-edge technologies in computational biology, gene editing, and protein engineering, CBMs as auxiliary components have become portable and efficient tools for the evolution and application of CAZymes. With the aim to provide a theoretical reference for the functional research, rational design, and targeted utilization of novel CBMs in the future, we systematically reviewed the function-related characteristics and potentials of CAZyme-derived CBMs in this review, including substrate recognition and binding mechanisms, non-catalytic contributions to enzyme performances, module modifications, and innovative applications in various fields.

Lignin impairs Cel7A degradation of in vitro lignified cellulose by impeding enzyme movement and not by acting as a sink

BACKGROUND

Cellulose degradation by cellulases has been studied for decades due to the potential of using lignocellulosic biomass as a sustainable source of bioethanol. In plant cell walls, cellulose is bonded together and strengthened by the polyphenolic polymer, lignin. Because lignin is tightly linked to cellulose and is not digestible by cellulases, is thought to play a dominant role in limiting the efficient enzymatic degradation of plant biomass. Removal of lignin via pretreatments currently limits the cost-efficient production of ethanol from cellulose, motivating the need for a better understanding of how lignin inhibits cellulase-catalyzed degradation of lignocellulose. Work to date using bulk assays has suggested three possible inhibition mechanisms: lignin blocks access of the enzyme to cellulose, lignin impedes progress of the enzyme along cellulose, or lignin binds cellulases directly and acts as a sink.

RESULTS

We used single-molecule fluorescence microscopy to investigate the nanoscale dynamics of Cel7A from Trichoderma reesei, as it binds to and moves along purified bacterial cellulose in vitro. Lignified cellulose was generated by polymerizing coniferyl alcohol onto purified bacterial cellulose, and the degree of lignin incorporation into the cellulose meshwork was analyzed by optical and electron microscopy. We found that Cel7A preferentially bound to regions of cellulose where lignin was absent, and that in regions of high lignin density, Cel7A binding was inhibited. With increasing degrees of lignification, there was a decrease in the fraction of Cel7A that moved along cellulose rather than statically binding. Furthermore, with increasing lignification, the velocity of processive Cel7A movement decreased, as did the distance that individual Cel7A molecules moved during processive runs.

CONCLUSIONS

In an in vitro system that mimics lignified cellulose in plant cell walls, lignin did not act as a sink to sequester Cel7A and prevent it from interacting with cellulose. Instead, lignin both blocked access of Cel7A to cellulose and impeded the processive movement of Cel7A along cellulose. This work implies that strategies for improving biofuel production efficiency should target weakening interactions between lignin and cellulose surface, and further suggest that nonspecific adsorption of Cel7A to lignin is likely not a dominant mechanism of inhibition.

Carbohydrate-binding modules enhance H2O2 tolerance by promoting lytic polysaccharide monooxygenase active site H2O2 consumption

Lytic polysaccharide monooxygenases (LPMOs) oxidatively depolymerize recalcitrant polysaccharides, which is important for biomass conversion. The catalytic domains of many LPMOs are linked to carbohydrate-binding modules (CBMs) through flexible linkers, but the function of these CBMs in LPMO catalysis is not well understood. In this study, we utilized MtLPMO9L and MtLPMO9G derived from Myceliophthora thermophila to investigate the impact of CBMs on LPMO activity, with particular emphasis on their influence on H2O2 tolerance. Using truncated forms of MtLPMO9G generated by removing the CBM, we found reduced substrate binding affinity and enzymatic activity. Conversely, when the CBM was fused to the C terminus of the single-domain MtLPMO9L to create MtLPMO9L-CBM, we observed a substantial improvement in substrate binding affinity, enzymatic activity, and notably, H2O2 tolerance. Furthermore, molecular dynamics simulations confirmed that the CBM fusion enhances the proximity of the active site to the substrate, thereby promoting multilocal cleavage and impacting the exposure of the copper active site to H2O2. Importantly, the fusion of CBM resulted in more efficient consumption of H2O2 by LPMO, leading to improved enzymatic activity and reduced auto-oxidative damage of the copper active center.

An Oxidative Enzyme Boosting Mechanical and Optical Performance of Densified Wood Films

Nature has evolved elegant ways to alter the wood cell wall structure through carbohydrate-active enzymes, offering environmentally friendly solutions to tailor the microstructure of wood for high-performance materials. In this work, the cell wall structure of delignified wood is modified under mild reaction conditions using an oxidative enzyme, lytic polysaccharide monooxygenase (LPMO). LPMO oxidation results in nanofibrillation of cellulose microfibril bundles inside the wood cell wall, allowing densification of delignified wood under ambient conditions and low pressure into transparent anisotropic films. The enzymatic nanofibrillation facilitates microfibril fusion and enhances the adhesion between the adjacent wood fiber cells during densification process, thereby significantly improving the mechanical performance of the films in both longitudinal and transverse directions. These results improve the understanding of LPMO-induced microstructural changes in wood and offer an environmentally friendly alternative for harsh chemical treatments and energy-intensive densification processes thus representing a significant advance in sustainable production of high-performance wood-derived materials.

Understanding the in silico aspects of bacterial catabolic cascade for styrene degradation

Styrene is a nonpolar organic compound used in very high volume for the industrial scale production of commercially important polymers such as polystyrene resins as well as copolymers like acrylonitrile butadiene styrene, latex, and rubber. These resins are widely used in the manufacturing of various products including single-use plastics such as disposable cups and containers, protective packaging, heat insulation, and so forth. The large-scale utilization leads to the over-accumulation of styrene waste in the environment causing deleterious health risks including cancer, neurological impairment, dysbiosis of central nervous system, and respiratory problems. To eliminate the accumulating waste. Microbial enzyme-based system represents the most environmental friendly and sustainable approach for elimination of styrene waste. However, comprehensive understanding of the enzyme-substrate interaction and associated pathways would be crucial for developing large-scale disposal systems. This study aims to understand the molecular interaction between the protein-ligand complexes of the styrene catabolic reactions by bacterial enzymes of sty operon. Molecular docking analysis for catalytic enzymes namely, styrene monooxygenase (SMO), styrene oxide isomerase (SOI), and phenylacetaldehyde dehydrogenase (PAD) of the bacterial sty operon was carried out with their individual substrates, that is, styrene, styrene oxide, and phenylacetic acid, respectively. The binding energy, amino acids forming binding cavity, and binding interactions between the protein-ligand binding sites were calculated for each case. The obtained binding energies showed a stable association of these complexes indicating the future scope of their utilization for large-scale bioremediation of styrene, and its commercially used polymers and copolymers.

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.

CelS-Catalyzed Processive Cellulose Degradation and Cellobiose Extraction for the Production of Bioethanol

Bacterial cellulase enzymes are potent candidates for the efficient production of bioethanol, a promising alternative to fossil fuels, from cellulosic biomass. These enzymes catalyze the breakdown of cellulose in plant biomass into simple sugars and then to bioethanol. In the absence of the enzyme, the cellulosic biomass is recalcitrant to decomposition due to fermentation-resistant lignin and pectin coatings on the cellulose surface, which make them inaccessible for hydrolysis. Cellobiohydrolase CelS is a microbial enzyme that binds to cellulose fiber and efficiently cleaves it into a simple sugar (cellobiose) by a repeated processive chopping mechanism. The two contributing factors to the catalytic reaction rate and the yield of cellobiose are the efficient product expulsion from the product binding site of CelS and the movement of the substrate or cellulose chain into the active site. Despite progress in understanding product expulsion in other cellulases, much remains to be understood about the molecular mechanism of processive action of these enzymes. Here, nonequilibrium molecular dynamics simulations using suitable reaction coordinates are carried out to investigate the energetics and mechanism of the substrate dynamics and product expulsion in CelS. The calculated free energy barrier for the product expulsion is three times lower than that for the processive action indicating that product removal is relatively easier and faster than the sliding of the substrate to the catalytic active site. The water traffic near the active site in response to the product expulsion and the processive action is also explored.

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.

Alleviating Nonproductive Adsorption of Lignin on CBM through the Addition of Cationic Additives for Lignocellulosic Hydrolysis

The nonproductive adsorption of cellulase onto lignin significantly inhibited the enzymatic hydrolysis of lignocellulosic biomass. In this study, we constructed a rapid fluorescence detection (RFD) system, and using this system, we demonstrated that the addition of cationic additives DTAB or polyDADMAC greatly increased the partition coefficients of cellulose/lignin, reduced nonproductive adsorption, and enhanced the hydrolysis efficiency of lignocellulose compared to those of Tweens or PEGs. Moreover, the addition of polyDADMAC and DTAB increased the glucose yield released from the mixture of Avicel and AICS-lignin (MCL) by 16.9 and 20.6%, respectively, and reduced the inhibition rate of lignin by 16.9 and 20.7%, respectively. Interestingly, polyDADMAC or DTAB treatment performed more effectively for the enzymatic hydrolysis of pretreated lignocellulosic biomass, compared with MCL. We confirmed that the reduced hydrophobicity and increased zeta potential of lignin cocontribute to the dampening nonproductive adsorption of lignin. In particular, the zeta potential values of lignin and the partition coefficients of Avicel/lignin with the addition of additives showed a good correlation, suggesting that electrostatic force also plays a crucial role in the adsorbing of cellulase on lignin. This work will be conducive to decreasing the nonproductive binding of cellulase onto lignin and enhancing cellulose conversion.

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.

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.

Identifying the negative cooperation between major inhibitors of cellulase activity and minimizing their inhibitory potential during hydrolysis of acid-pretreated corn stover

Soluble compounds produced during the enzymatic hydrolysis of lignocelluloses hampers cellulose conversion. Cellobiose and vanillin most severely inhibited the effect of cellobiohydrolase I. A concentration-dependent negative cooperative effect was found between cellobiose and vanillin. The combined inhibitory effect was about 83.5% of the cellobiose and 88.1% of the vanillin when their concentration was 20 mg/ml. However, the negative synergy could be eliminated by excessive enzyme loading. Differences in their binding sites on the catalytic domain of cellobiohydrolase I lead to negative synergistic inhibition, which should be considered in devising strategies to alleviate this effect. Combined β-glucosidase and PEG addition at an appropriate dose was feasible to balance cost and hydrolytic efficiency. To achieve efficient hydrolysis, especially at high solid concentrations, it is important to understand the synergistic inhibition between these inhibitors.

Fungal cellulases: protein engineering and post-translational modifications

Enzymatic degradation of lignocelluloses into fermentable sugars to produce biofuels and other biomaterials is critical for environmentally sustainable development and energy resource supply. However, there are problems in enzymatic cellulose hydrolysis, such as the complex cellulase composition, low degradation efficiency, high production cost, and post-translational modifications (PTMs), all of which are closely related to specific characteristics of cellulases that remain unclear. These problems hinder the practical application of cellulases. Due to the rapid development of computer technology in recent years, computer-aided protein engineering is being widely used, which also brings new opportunities for the development of cellulases. Especially in recent years, a large number of studies have reported on the application of computer-aided protein engineering in the development of cellulases; however, these articles have not been systematically reviewed. This article focused on the aspect of protein engineering and PTMs of fungal cellulases. In this manuscript, the latest literatures and the distribution of potential sites of cellulases for engineering have been systematically summarized, which provide reference for further improvement of cellulase properties. KEY POINTS: •Rational design based on virtual mutagenesis can improve cellulase properties. •Modifying protein side chains and glycans helps obtain superior cellulases. •N-terminal glutamine-pyroglutamate conversion stabilizes fungal cellulases.

Effect of carbohydrate binding modules alterations on catalytic activity of glycoside hydrolase family 6 exoglucanase from Chaetomium thermophilum to cellulose

Exoglucanase (CBH) is the rate limiting enzyme in the process of cellulose degradation. The carbohydrate binding module (CBM) can improve the accessibility of cellulase to substrate, thereby promoting the enzymatic hydrolysis of cellulase. In this study, the influence of CBM on the properties of GH6 exoglucanase from Chaetomium thermophilum (CtCBH) is systematically explored from three perspectives: the fusion of exogenous CBM, the exogenous CBM replacement of its own CBM, and the deletion of its own CBM. The parental and reconstructed CtCBH presented the same optimum pH (6.0) and temperature (60 °C) for maximum activity. Fusion of exogenous CBM increased the binding capacity of CtCBH to Avicel by 8% and 9%, respectively, but it had no significant effect on its catalytic activity. The exogenous CBM replacement of its own CBM resulted in a 12% reduction in the binding ability of CtCBH to Avicel, and a 26% reduction in the catalytic activity of Avicel. The deletion of its own CBM significantly reduced the binding ability of CtCBH to Avicel by approximately 53%, but its catalytic activity was not obviously reduced. These observations suggest that binding ability of CBM is not necessary for the catalysis of CtCBH.

Engineered tyrosinases with broadened bio-catalysis scope: immobilization using nanocarriers and applications

Enzyme immobilization is a widely used technology for creating more stable, active, and reusable biocatalysts. The immobilization process also improves the enzyme's operating efficiency in industrial applications. Various support matrices have been designed and developed to enhance the biocatalytic efficiency of immobilized enzymes. Given their unique physicochemical attributes, including substantial surface area, rigidity, semi-conductivity, high enzyme loading, hyper catalytic activity, and size-assisted optical properties, nanomaterials have emerged as fascinating matrices for enzyme immobilization. Tyrosinase is a copper-containing monooxygenase that catalyzes the o-hydroxylation of monophenols to catechols and o-quinones. This enzyme possesses a wide range of uses in the medical, biotechnological, and food sectors. This article summarizes an array of nanostructured materials as carrier matrices for tyrosinase immobilization. Following a detailed background overview, various nanomaterials, as immobilization support matrices, including carbon nanotubes (CNTs), carbon dots (CDs), carbon black (CB), nanofibers, Graphene nanocomposite, platinum nanoparticles, nano-sized magnetic particles, lignin nanoparticles, layered double hydroxide (LDH) nanomaterials, gold nanoparticles (AuNPs), and zinc oxide nanoparticles have been discussed. Next, applied perspectives have been spotlights with particular reference to environmental pollutant sensing, phenolic compounds detection, pharmaceutical, and food industry (e.g., cereal processing, dairy processing, and meat processing), along with other miscellaneous applications.

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.

Carbohydrate-binding module O-mannosylation alters binding selectivity to cellulose and lignin

Improved understanding of the effect of protein glycosylation is expected to provide the foundation for the design of protein glycoengineering strategies. In this study, we examine the impact of O-glycosylation on the binding selectivity of a model Family 1 carbohydrate-binding module (CBM), which has been shown to be one of the primary sub-domains responsible for non-productive lignin binding in multi-modular cellulases. Specifically, we examine the relationship between glycan structure and the binding specificity of the CBM to cellulose and lignin substrates. We find that the glycosylation pattern of the CBM exhibits a strong influence on the binding affinity and the selectivity between both cellulose and lignin. In addition, the large set of binding data collected allows us to examine the relationship between binding affinity and the correlation in motion between pairs of glycosylation sites. Our results suggest that glycoforms displaying highly correlated motion in their glycosylation sites tend to bind cellulose with high affinity and lignin with low affinity. Taken together, this work helps lay the groundwork for future exploitation of glycoengineering as a tool to improve the performance of industrial enzymes.

Improved understanding of the effect of protein glycosylation is expected to provide the foundation for the design of protein glycoengineering strategies.

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

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

Insights into the mechanism(s) of digestion of crystalline cellulose by plant class C GH9 endoglucanases

Biofuels such as γ-valerolactone, bioethanol, and biodiesel are derived from potentially fermentable cellulose and vegetable oils. Plant class C GH9 endoglucanases are CBM49-encompassing hydrolases that cleave the β (1 → 4) glycosidic linkage of contiguous D-glucopyranose residues of crystalline cellulose. Here, I analyse 3D-homology models of characterised and putative class C enzymes to glean insights into the contribution of the GH9, linker, and CBM49 to the mechanism(s) of crystalline cellulose digestion. Crystalline cellulose may be accommodated in a surface groove which is imperfectly bounded by the GH9_CBM49, GH9_linker, and linker_CBM49 surfaces and thence digested in a solvent accessible subsurface cavity. The physical dimensions and distortions thereof, of the groove, are mediated in part by the bulky side chains of aromatic amino acids that comprise it and may also result in a strained geometry of the bound cellulose polymer. These data along with an almost complete absence of measurable cavities, along with poorly conserved, hydrophobic, and heterogeneous amino acid composition, increased atomic motion of the CBM49_linker junction, and docking experiements with ligands of lower degrees of polymerization suggests a modulatory rather than direct role for CBM49 in catalysis. Crystalline cellulose is the de facto substrate for CBM-containing plant and non-plant GH9 enzymes, a finding supported by exceptional sequence- and structural-homology. However, despite the implied similarity in general acid-base catalysis of crystalline cellulose, this study also highlights qualitative differences in substrate binding and glycosidic bond cleavage amongst class C members. Results presented may aid the development of novel plant-based GH9 endoglucanases that could extract and utilise potential fermentable carbohydrates from biomass.

Graphical Abstract

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.

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.

Altering the linker in processive GH5 endoglucanase 1 modulates lignin binding and catalytic properties

BACKGROUND

The non-productive adsorption of cellulases onto lignin in biomass is a key issue for the biofuel process economy. It would be helpful to reduce the inhibitory effect of lignin on enzymatic hydrolysis by engineering weak lignin-binding cellulases. Cellulase linkers are highly divergent in their lengths, compositions, and glycosylations. Numerous studies have revealed that linkers can facilitate optimal interactions between structured domains. Recently, efforts have focused on the contributions and mechanisms of carbohydrate-binding modules and catalytic domains that affect lignin affinity and processivity of cellulases, but our understanding of the effects of the linker regions on lignin adsorption and processivity of GH5 processive endoglucanases is still limited.

RESULTS

Eight GH5 endoglucanase 1 variants of varying length, flexibility, and sequence in the linker region were constructed. Their characteristics were then compared to the wild-type enzyme (EG1). Remarkably, significant differences in the lignin adsorption profiles and processivities were observed for EG1 and other variants. Our studies suggest that either the length or the specific amino acid composition of the linker has a prominent influence on the lignin-binding affinity of the enzymes. Comparatively, the processivity may depend primarily on the length of the linker and less so on the specific amino acid composition. EG1-ApCel5A, a variant with better performance in enzymatic hydrolysis in the presence of lignin, was obtained by replacing a longer, flexible linker. In total, up to between 28.2 and 30.1% more reducing sugars were generated from filter paper by EG1-ApCel5A in the presence of lignin compared to EG1.

CONCLUSIONS

Our results highlight the relevance of the linker region in the lignin adsorption and processivity of a processive endoglucanase. Our findings suggest that the linker region may be used as a target for the design of more active and weaker lignin-binding cellulases.

The adsorption properties of endoglucanase to lignin and their impact on hydrolysis

Nonproductive adsorption of cellulase to lignin dramatically influenced the hydrolysis efficiency of lignocellulose. By comparing the adsorption behaviors of CBH and EG, we found that the adsorption of EG to lignin showed lower adsorption velocity and capacity versus CBH. During the adsorption of EG to lignin, carbohydrate binding domain (CBM) and catalytic domain (CD) both played an important role by a two-step adsorption process, in which CD slowly bond on lignin and developed stronger interaction with lignin. The optimal binding position of EG on lignin was consistent with that on polysaccharide located in the open catalytic tunnel. So, the adsorption of EG to lignin not only limited the movement of enzyme, but also restricted the catalytic ability of enzyme, which dramatically influenced enzymatic hydrolysis. Increasing the proportion of EG in cellulase cocktails or engineering "weak lignin adsorbed" EG was necessary to relieve the influence of lignin adsorption on hydrolysis.

Binding and hydrolysis properties of engineered cellobiohydrolases and endoglucanases

Because cellulase was the main enzyme used in bioconversion of lignocellulose, it was a valid way to reduce the hydrolysis cost by increasing the adsorption and hydrolysis efficiency of cellulase. In this study, modified cellobiohydrolases (CBHs) and endoglucanases (EGs) were constructed. Two engineered cellulases CBH-TrCBM^{V27E,P30D,Link1} and EG-TrCBM^{V27E,P30D,Link1} well-performed during hydrolysis. Compared to wild-type enzymes, EG-TrCBM^{V27E,P30D,Link1} had relatively less adsorption ability to lignin and greater affinity to cellulose, especially Avicel. However, for CBH-TrCBM^{V27E,P30D,Link1}, the hydrolysis manner was changed and in favor to hydrolysis process, although the adsorption properties were unexpected. It suggested that various binding conformations of polysaccharide on CBMs hypothetically resulted in different functions of CBMs, including binding ability, processive and digestive properties on fiber surface. Fusion of T. r-CBM^{V27E,P30D,Link1} to cellulase, both CBH and EG, gave the destruction ability of enzyme and increased the accessible surface of substrate to cellulase, enhanced the adsorption and hydrolysis efficiency of cellulase.

Different pathways for 4-n-nonylphenol biodegradation by two Aspergillus strains derived from estuary sediment: Evidence from metabolites determination and key-gene identification

Nonylphenols (NPs) are known as Endocrine Disputing Chemicals (ECDs) and Persistent Organic Pollutants (POPs) and have attracted continuous attention. Biodegradation is one of the effective ways for pollutant removal in aquatic, sedimentary and soil environments. In this study, two estuarine derived fungi strains, NPF2 and NPF3, were screened from Moshui river estuarine sediment and identified as genus Aspergillus. The growth curves of the two strains as well as the removal and degradation rates for 4-n-NP in Potato Dextrose(PD)medium were used to evaluate their degradation ability. Both strains showed high efficiency for 4-n-NP degradation with 86.03% and 98.76% removal rates in 3 days for NPF2 and NPF3, respectively. Determination of degradation intermediates by LC-MS suggested that the mechanisms for 4-n-NP biodegradation by NPF2 and NPF3 are quite different. Some key functional genes for the two strains also provided supplementary evidences for the different biodegradation mechanism. On strain NPF2, with participation of Cox1, 2 and 3, 4-n-NP degradation starts from reaction at the terminal of the long alkyl chain. The chain reduces one carbon atom once within a cycle of hydroxylation, subsequent oxidation at α-C position and decarboxylation. However, on NPF3, with involvement of sMO, Cel7A, Cel7B and ATEG-00639, 4-n-NP degradation starts from benzene ring, converting into fatty acids. The latter bio-pathway was the first time reported for NPs degradation on fungi.

Functionalized Tyrosinase-Lignin Nanoparticles as Sustainable Catalysts for the Oxidation of Phenols

Sustainable catalysts for the oxidation of phenol derivatives under environmentally friendly conditions were prepared by the functionalization of lignin nanoparticles with tyrosinase. Lignin, the most abundant polyphenol in nature, is the main byproduct in the pulp and paper manufacturing industry and biorefinery. Tyrosinase has been immobilized by direct adsorption, encapsulation, and layer-by-layer deposition, with or without glutaraldehyde reticulation. Lignin nanoparticles were found to be stable to the tyrosinase activity. After the enzyme immobilization, they showed a moderate to high catalytic effect in the synthesis of catechol derivatives, with the efficacy of the catalyst being dependent on the specific immobilization procedures.

Origin, evolution, and divergence of plant class C GH9 endoglucanases

BACKGROUND

Glycoside hydrolases of the GH9 family encode cellulases that predominantly function as endoglucanases and have wide applications in the food, paper, pharmaceutical, and biofuel industries. The partitioning of plant GH9 endoglucanases, into classes A, B, and C, is based on the differential presence of transmembrane, signal peptide, and the carbohydrate binding module (CBM49). There is considerable debate on the distribution and the functions of these enzymes which may vary in different organisms. In light of these findings we examined the origin, emergence, and subsequent divergence of plant GH9 endoglucanases, with an emphasis on elucidating the role of CBM49 in the digestion of crystalline cellulose by class C members.

RESULTS

Since, the digestion of crystalline cellulose mandates the presence of a well-defined set of aromatic and polar amino acids and/or an attributable domain that can mediate this conversion, we hypothesize a vertical mode of transfer of genes that could favour the emergence of class C like GH9 endoglucanase activity in land plants from potentially ancestral non plant taxa. We demonstrated the concomitant occurrence of a GH9 domain with CBM49 and other homologous carbohydrate binding modules, in putative endoglucanase sequences from several non-plant taxa. In the absence of comparable full length CBMs, we have characterized several low strength patterns that could approximate the CBM49, thereby, extending support for digestion of crystalline cellulose to other segments of the protein. We also provide data suggestive of the ancestral role of putative class C GH9 endoglucanases in land plants, which includes detailed phylogenetics and the presence and subsequent loss of CBM49, transmembrane, and signal peptide regions in certain populations of early land plants. These findings suggest that classes A and B of modern vascular land plants may have emerged by diverging directly from CBM49 encompassing putative class C enzymes.

CONCLUSION

Our detailed phylogenetic and bioinformatics analysis of putative GH9 endoglucanase sequences across major taxa suggests that plant class C enzymes, despite their recent discovery, could function as the last common ancestor of classes A and B. Additionally, research into their ability to digest or inter-convert crystalline and amorphous forms of cellulose could make them lucrative candidates for engineering biofuel feedstock.

Relating Nanoscale Accessibility within Plant Cell Walls to Improved Enzyme Hydrolysis Yields in Corn Stover Subjected to Diverse Pretreatments

Simultaneous chemical modification and physical reorganization of plant cell walls via alkaline hydrogen peroxide or liquid hot water pretreatment can alter cell wall structural properties impacting nanoscale porosity. Nanoscale porosity was characterized using solute exclusion to assess accessible pore volumes, water retention value as a proxy for accessible water-cell walls surface area, and solute-induced cell wall swelling to measure cell wall rigidity. Key findings concluded that delignification by alkaline hydrogen peroxide pretreatment decreased cell wall rigidity and that the subsequent cell wall swelling resulted increased nanoscale porosity and improved enzyme binding and hydrolysis compared to limited swelling and increased accessible surface areas observed in liquid hot water pretreated biomass. The volume accessible to a 90 Å dextran probe within the cell wall was found to be correlated to both enzyme binding and glucose hydrolysis yields, indicating cell wall porosity is a key contributor to effective hydrolysis yields.

The Multi Domain Caldicellulosiruptor bescii CelA Cellulase Excels at the Hydrolysis of Crystalline Cellulose

The crystalline nature of cellulose microfibrils is one of the key factors influencing biomass recalcitrance which is a key technical and economic barrier to overcome to make cellulosic biofuels a commercial reality. To date, all known fungal enzymes tested have great difficulty degrading highly crystalline cellulosic substrates. We have demonstrated that the CelA cellulase from Caldicellulosiruptor bescii degrades highly crystalline cellulose as well as low crystallinity substrates making it the only known cellulase to function well on highly crystalline cellulose. Unlike the secretomes of cellulolytic fungi, which typically comprise multiple, single catalytic domain enzymes for biomass degradation, some bacterial systems employ an alternative strategy that utilizes multi-catalytic domain cellulases. Additionally, CelA is extremely thermostable and highly active at elevated temperatures, unlike commercial fungal cellulases. Furthermore we have determined that the factors negatively affecting digestion of lignocellulosic materials by C. bescii enzyme cocktails containing CelA appear to be significantly different from the performance barriers affecting fungal cellulases. Here, we explore the activity and degradation mechanism of CelA on a variety of pretreated substrates to better understand how the different bulk components of biomass, such as xylan and lignin, impact its performance.

Adsorption of Proteins on Colloidal Lignin Particles for Advanced Biomaterials

Coating of colloidal lignin particles (CLPs), or lignin nanoparticles (LNPs), with proteins was evaluated in order to establish a safe, self-assembly mediated modification technique to tune their surface chemistry. Gelatin and poly- l-lysine formed the most pronounced protein corona on the CLP surface, as determined by dynamic light scattering (DLS) and zeta potential measurements. Spherical morphology of individual protein coated CLPs was confirmed by transmission electron (TEM) and atomic force (AFM) microscopy. A mechanistic adsorption study with several random coiled and globular model proteins was carried out using quartz crystal microbalance with dissipation monitoring (QCM-D). The three-dimensional (3D) protein fold structure and certain amino acid interactions were decisive for the protein adsorption on the lignin surface. The main driving forces for protein adsorption were electrostatic, hydrophobic, and van der Waals interactions, and hydrogen bonding. The relative contributions of these interactions were highly dependent on the ionic strength of the surrounding medium. Capillary electrophoresis (CE) and Fourier transform infrared spectroscopy (FTIR) provided further evidence of the adsorption-enhancing role of specific amino acid residues such as serine and proline. These results have high impact on the utilization of lignin as colloidal particles in biomedicine and biodegradable materials, as the protein corona enables tailoring of the CLP surface chemistry for intended applications.

Lipopeptide produced from Bacillus sp. W112 improves the hydrolysis of lignocellulose by specifically reducing non-productive binding of cellulases with and without CBMs

BACKGROUND

Surfactants have attracted increasing interest for their capability to improve the enzymatic hydrolysis of lignocellulosic biomass. Compared to chemical surfactants, biosurfactants have a broader prospect for industrial applications because they are more environmentally friendly and more effective in some researches. Commercial cellulase preparations are mainly composed of endoglucanases (EGs) and cellobiohydrolases (CBHs) that possess carbohydrate-binding modules (CBMs). However, the effects of lipopeptide-type biosurfactants on enzymatic saccharification of lignocellulose and adsorption behaviors of cellulases with CBMs remain unclear.

RESULTS

In this study, we found that Bacillus sp. W112 could produce a lipopeptide-type biosurfactant from untreated biomass, such as wheat bran and Jerusalem artichoke tuber. The lipopeptide could enhance the enzymatic hydrolysis of dilute acid pretreated Giant Juncao grass (DA-GJG) by fungal and bacterial enzymes. The enhancement increased over a range of temperatures from 30 to 50 °C. Lipopeptide was shown to be more effective in promoting DA-GJG saccharification than chemical surfactants at low dosages, with a best stimulatory degree of 20.8% at 2% loading of the substrates (w/w). Lipopeptide increased the thermostability of EG and CBH in commercial cellulase cocktails. Moreover, the dual effects of lipopeptide on the adsorption behaviors of cellulases were found. It specifically lowered the non-productive binding of cellulases to lignin and increased the binding of cellulases to cellulose. In addition, we investigated the influence of lipopeptide on the adsorption behaviors of CBHs with CBMs for the first time. Our results showed that lipopeptide reduced the adsorption of CBM-deleted CBH to DA-GJG to a greater extent than that of intact CBH while the non-productive binding of intact CBH to lignin was reduced more, indicating that lipopeptide decreased the binding of CBMs onto lignin but not their combination with cellulose.

CONCLUSIONS

In this study, we found that lipopeptide from Bacillus sp. W112 promoted the enzymatic hydrolysis of DA-GJG at relative low loadings. The stimulatory effect could be attributed to increasing the cellulase thermostability, reducing non-productive adsorption of cellulases with CBMs caused by lignin and enhancing the binding of cellulases to cellulose.

Isolation and Characterization of a Glycosyl Hydrolase Family 16 β-Agarase from a Mangrove Soil Metagenomic Library

A mangrove soil metagenomic library was constructed and a β-agarase gene designated as AgaML was isolated by functional screening. The gene encoded for a 659-amino-acids polypeptide with an estimated molecular mass of 71.6 kDa. The deduced polypeptide sequences of AgaML showed the highest identity of 73% with the glycoside hydrolase family 16 β-agarase from Microbulbifer agarilyticus in the GenBank database. AgaML was cloned and highly expressed in Escherichia coli BL21(DE3). The purified recombinant protein, AgaML, showed optimal activity at 50 °C and pH 7.0. The kinetic parameters of Km and Vmax values toward agarose were 4.6 mg·mL(-1) and 967.5 μM·min(-1)·mg(-1), respectively. AgaML hydrolyzed the β-1,4-glycosidic linkages of agar to generate neoagarotetraose (NA4) and neoagarohexaose (NA6) as the main products. These characteristics suggest that AgaML has potential application in cosmetic, pharmaceuticals and food industries.

Using Carbohydrate Interaction Assays to Reveal Novel Binding Sites in Carbohydrate Active Enzymes

Carbohydrate active enzymes often contain auxiliary binding sites located either on independent domains termed carbohydrate binding modules (CBMs) or as so-called surface binding sites (SBSs) on the catalytic module at a certain distance from the active site. The SBSs are usually critical for the activity of their cognate enzyme, though they are not readily detected in the sequence of a protein, but normally require a crystal structure of a complex for their identification. A variety of methods, including affinity electrophoresis (AE), insoluble polysaccharide pulldown (IPP) and surface plasmon resonance (SPR) have been used to study auxiliary binding sites. These techniques are complementary as AE allows monitoring of binding to soluble polysaccharides, IPP to insoluble polysaccharides and SPR to oligosaccharides. Here we show that these methods are useful not only for analyzing known binding sites, but also for identifying new ones, even without structural data available. We further verify the chosen assays discriminate between known SBS/CBM containing enzymes and negative controls. Altogether 35 enzymes are screened for the presence of SBSs or CBMs and several novel binding sites are identified, including the first SBS ever reported in a cellulase. This work demonstrates that combinations of these methods can be used as a part of routine enzyme characterization to identify new binding sites and advance the study of SBSs and CBMs, allowing them to be detected in the absence of structural data.

The Effects of Noncellulosic Compounds on the Nanoscale Interaction Forces Measured between Carbohydrate-Binding Module and Lignocellulosic Biomass

The lack of fundamental understanding of the types of forces that govern how cellulose-degrading enzymes interact with cellulosic and noncellulosic components of lignocellulosic surfaces limits the design of new strategies for efficient conversion of biomass to bioethanol. In a step to improve our fundamental understanding of such interactions, nanoscale forces acting between a model cellulase-a carbohydrate-binding module (CBM) of cellobiohydrolase I (CBH I)-and a set of lignocellulosic substrates with controlled composition were measured using atomic force microscopy (AFM). The three model substrates investigated were kraft (KP), sulfite (SP), and organosolv (OPP) pulped substrates. These substrates varied in their surface lignin coverage, lignin type, and xylan and acetone extractives' content. Our results indicated that the overall adhesion forces of biomass to CBM increased linearly with surface lignin coverage with kraft lignin showing the highest forces among lignin types investigated. When the overall adhesion forces were decoupled into specific and nonspecific component forces via the Poisson statistical model, hydrophobic and Lifshitz-van der Waals (LW) forces dominated the binding forces of CBM to kraft lignin, whereas permanent dipole-dipole interactions and electrostatic forces facilitated the interactions of lignosulfonates to CBM. Xylan and acetone extractives' content increased the attractive forces between CBM and lignin-free substrates, most likely through hydrogen bonding forces. When the substrates treated differently were compared, it was found that both the differences in specific and nonspecific forces between lignin-containing and lignin-free substrates were the least for OPP. Therefore, cellulase enzymes represented by CBM would weakly bind to organosolv lignin. This will facilitate an easy enzyme recovery compared to other substrates treated with kraft or sulfite pulping. Our results also suggest that altering the surface hydrophobicity and the surface energy of lignin that facilitates the LW forces should be a priori to avoid nonproductive binding of cellulase to kraft lignin.

Effects of lignin and surfactant on adsorption and hydrolysis of cellulases on cellulose

BACKGROUND

Considerable works have been reported concerning the obstruction of enzymatic hydrolysis efficiency by lignin. However, there is a lack of information about the influence of lignin on the adsorption of cellulases on cellulose, along with the hydrolytic activity of the cellulases adsorbed on lignin. In addition, limited discovery has been reported about the influence of additives on cellulase desorption from lignin and lignocellulosic materials. In this work, the effects of lignin on cellulase adsorption and hydrolysis of Avicel were investigated and the effects of Tween 80 on cellulases adsorption and desorption on/from lignin and corn stover were explored.

RESULTS

The results showed that the maximum adsorption capacity of Avicel reduced from 276.9 to 179.7 and 112.1 mg/g cellulose with the addition of 1 and 10 mg lignin per gram Avicel, which indicated that lignin adsorbed on Avicel reduced surface area of cellulose and lignin available for cellulases. Cellulases adsorbed on lignin could be released by reaching new adsorption equilibrium between lignin and supernatants. In addition, cellulases desorbed from lignin still possess hydrolytic capacity. Tween 80 could adsorb onto both lignin and corn stover, and reduce the cellulase adsorption on them. Furthermore, Tween 80 could enhance desorption of cellulases from both lignin and corn stover, which might be due to the competitive adsorption between cellulases and Tween 80 on them.

CONCLUSIONS

The presence of lignin decreased the maximum adsorption capacity of cellulases on cellulose and the cellulases adsorbed on lignin could be released to supernatant, exhibiting hydrolytic activity. Tween 80 could alleviate the adsorption of cellulases and enhanced desorption of cellulases on/from lignin and corn stover. The conclusions of this work help us further understanding the role of lignin in the reduction of adsorption of cellulases on substrates, and the function of additives in cellulases adsorption and desorption on/from lignin and substrates.

Adsorption and mechanism of cellulase enzymes onto lignin isolated from corn stover pretreated with liquid hot water

BACKGROUND

In the bioconversion of lignocellulosic substrates, the adsorption behavior of cellulase onto lignin has a negative effect on enzymatic hydrolysis of cellulose, decreasing glucose production during enzymatic hydrolysis, thus decreasing the yield of fermentation and the production of useful products. Understanding the interaction between lignin and cellulase is necessary to optimize the components of cellulase mixture, genetically engineer high-efficiency cellulase, and reduce cost of bioconversion. Most lignin is not removed during liquid hot water (LHW) pretreatment, and the characteristics of lignin in solid substrate are also changed. To understand the interactions between cellulase and lignin, this study investigated the change in the characteristics of lignin obtained from corn stover, as well as the behavior of cellulase adsorption onto lignin, under various severities of LHW pretreatment.

RESULTS

LHW pretreatment removed most hemicellulose and some lignin in corn stover, as well as improved enzymatic digestibility of corn stover. After LHW pretreatment, the molecular weight of lignin obviously increased, whereas its polydispersity decreased and became more negative. The hydrophobicity and functional groups in lignin also changed. Adsorption of cellulase from Penicillium oxalicum onto lignin isolated from corn stover was enhanced after LHW pretreatment, and increased under increasing pretreatment severity. Different adsorption behaviors were observed in different lignin samples and components of cellulase mixtures, even in different cellobiohydrolases (CBHs), endo-beta-1, 4-glucanases (EGs). The greatest reduction in enzyme activity caused by lignin was observed in CBH, followed by that in xylanase and then in EG and β-Glucosidase (BGL). The adsorption behavior exerted different effects on subsequent enzymatic hydrolysis of various biomass substrates. Hydrophobic and electrostatic interactions may be important factors affecting different adsorption behaviors between lignin and cellulase.

CONCLUSIONS

LHW pretreatment changed the characteristics of the remaining lignin in corn stover, thus affected the adsorption behavior of lignin toward cellulase. For different protein components in cellulase solution from P. oxalicum, electrostatic action was a main factor influencing the adsorption of EG and xylanase onto lignin in corn stover, while hydrophobicity affected the adsorption of CBH and BGL onto lignin.

Mechanism of lignin inhibition of enzymatic biomass deconstruction

BACKGROUND

The conversion of plant biomass to ethanol via enzymatic cellulose hydrolysis offers a potentially sustainable route to biofuel production. However, the inhibition of enzymatic activity in pretreated biomass by lignin severely limits the efficiency of this process.

RESULTS

By performing atomic-detail molecular dynamics simulation of a biomass model containing cellulose, lignin, and cellulases (TrCel7A), we elucidate detailed lignin inhibition mechanisms. We find that lignin binds preferentially both to the elements of cellulose to which the cellulases also preferentially bind (the hydrophobic faces) and also to the specific residues on the cellulose-binding module of the cellulase that are critical for cellulose binding of TrCel7A (Y466, Y492, and Y493).

CONCLUSIONS

Lignin thus binds exactly where for industrial purposes it is least desired, providing a simple explanation of why hydrolysis yields increase with lignin removal.