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

Role of in situ surfactant modification of lignin structure and surface properties during glycerol pretreatment in modulating cellulase-lignin binding affinities

Surfactants are effective agents for enhancing lignocellulosic pretreatment, synergistically modifying lignin with polyols to improve substrate hydrolyzability while achieving comparable delignification. However, the mechanisms underlying the multiple modifications from dual in situ surfactant/polyols grafting that passivate lignin-cellulase interactions and their core affecting factors remain unclear. Following the previously developed polyethylene glycol (PEG) and Triton-assisted pretreatment, the intrinsic correlation among lignin structures, physical barriers, and cellulase interactions was analyzed in this study. The surfactant grafting onto lignin can significantly decrease non-productive cellulase adsorption by 5-59 % compared to the initial glycerol-modified lignin. Structurally, the changes in lignin aliphatic -OH (r > 0.93), H-OH (r > 0.98), and G-OH (r > 0.74) showed strong correlations with cellulase adsorption; the -COOH and C=O were not well-valiadted for assessing the non-productive interaction. Physically, surfactant modification also induced changes in lignin surface structure, with variations in specific surface area, pore size, and pore volume showing positive correlations (r > 0.71). The structurally modified lignin had a relatively strong affinity for exo-glucanase and β-glucosidase in enzyme cocktails, while it reduced the irreversible adsorption of lignin onto cellulases (up to 97 % of total adsorbed protein). The secondary structure of desorbed cellulases underwent obvious changes independent of lignin structural modifications, with lowering β-sheet content and increasing random coil content. Based on molecular forces, surfactant modification lowered the binding free energy of cellulases by 60.3-86.5 %, and the reduction in H-bonding interaction was predominant. This study provides mechanistic insights for constructing lignin-modified pretreatments to enhance the substrate hydrolyzability.

Elucidation of the relationship between lignin structure and its inhibitory effect on enzymatic hydrolysis

In this work, milled wood lignin (MWL) was isolated from three distinct raw materials, including reed, poplar and pine. The research focused on investigating the adsorption of lignin onto cellulase and its subsequent impact on enzymatic hydrolysis. Analysis revealed that reed lignin consisted of three structural units: syringal (S), guaiacyl (G), and p-hydroxyphenyl (H), with a G/(G + S + H) molar ratio of 0.26; Poplar lignin contained S and G, with a G/(G + S + H) molar ratio of 0.63, while pine lignin only contained G. Quartz crystal microbalance with dissipation (QCM-D) results demonstrated that lignin with different types of structural units had varying adsorption capacity for cellulase. Specifically, lignin from coniferous wood (pine) displayed stronger adsorption with cellulase compared to lignin derived from grasses (reed) and broad-leaved wood (poplar). Atomic force microscope (AFM) analysis further indicated that G units promoted the adsorption between cellulase and lignin. These findings underscored a strong correlation between the structure of lignin and its interaction with cellulase, with the proportion of G units playing a pivotal role. Notably, a lower proportion of G units in lignin was beneficial in reducing lignin adsorption onto cellulase, thereby enhancing the saccharification rate.

Elucidating the synergistic action between sulfonated lignin and lytic polysaccharide monooxygenases (LPMOs) in enhancing cellulose hydrolysis

Modern enzyme cocktails often include lytic polysaccharide monooxygenase (LPMO) as an accessory enzyme that enhances cellulose accessibility during hydrolysis. Although lignin is known to generally impede cellulose hydrolysis, previous research has demonstrated lignin's potential to act as a co-factor in boosting LPMO activity and that the negative impact of lignin limiting enzyme accessibility can be mitigated by sulfonated. When sulphonated lignin was added to microcrystalline cellulose (Avicel) the activity of the lytic polysaccharide monooxygenase (LPMO) was boosted, as determined when using a quartz crystal microbalance and dissipation monitoring (QCM-D). Further assessment via scanning electron microscopy, Simon's staining and nitrogen adsorption indicated that the addition of sulphonated lignin with the LPMO also increased cellulose accessibility.

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.

Achieving high enzymatic hydrolysis sugar yield of sodium hydroxide-pretreated wheat straw with a low cellulase dosage by adding sulfomethylated tannic acid

Sulfonated lignin can significantly enhance the enzymatic hydrolysis of lignocellulose substrates. Lignin is a type of polyphenol, therefore, sulfonated polyphenol, such as tannic acid, is likely to have similar effects. In order to obtain a low-cost and high-efficiency additive to improve enzymatic hydrolysis, sulfomethylated tannic acids (STAs) with different sulfonation degrees were prepared and their impact on enzymatic saccharification of sodium hydroxide-pretreated wheat straw were investigated. Tannic acid strongly inhibited, while STAs strongly promoted the substrate enzymatic digestibility. While adding 0.04 g/g-substrate STA containing 2.4 mmol/g sulfonate group, the glucose yield increased from 60.6% to 97.9% at a low cellulase dosage (5 FPU/g-glucan). The concentration of protein in enzymatic hydrolysate significantly increased with the added STAs, indicating that cellulase preferentially adsorbed with STAs, thereby reducing the amount of cellulase nonproductively anchored on substrate lignin. This result provides a reliable approach for establishing an efficient lignocellulosic enzyme hydrolysis system.