Novel catalytic potential of a hyperthermostable mono‑copper oxidase (LPMO-AOAA17) for the oxidation of lignin monomers and depolymerisation of lignin dimer in aqueous media

Catalytic Cleavage of the C-O Bonds in Lignin and Lignin Model Compounds by Metal Triflate Catalysts

The effective cleavage of C-O bonds in linkages of lignin was one of the significant strategies promoting lignin valorization. Herein, the strategy of C-O bonds cleavage of lignin using metal triflate as the catalyst was developed. The carboxylic acid or alcohol could be used as the nucleophile to stabilize the reactive intermediates formed during the depolymerization of lignin, and the corresponding ester/ether compounds could be obtained. This catalytic system was suitable for the C-O bond cleavage in α-O-4 and β-O-4 linkages with excellent efficiency. Additionally, reaction conditions were optimized. The reaction mixture was detected by 1 H NMR, and no other byproducts were found. As for treated lignin samples, the cleavage of C-O bonds in linkages was determined by 2D HSQC NMR, the increased content of the phenol hydroxyl group was proved by FT-IR, and the reduced molecular weight was investigated by GPC. Furthermore, multiple phenolic compounds were detected by GC-MS in the reaction mixtures.

Bacterial transformation of lignin: key enzymes and high-value products

Lignin, a natural organic polymer that is recyclable and inexpensive, serves as one of the most abundant green resources in nature. With the increasing consumption of fossil fuels and the deterioration of the environment, the development and utilization of renewable resources have attracted considerable attention. Therefore, the effective and comprehensive utilization of lignin has become an important global research topic, with the goal of environmental protection and economic development. This review focused on the bacteria and enzymes that can bio-transform lignin, focusing on the main ways that lignin can be utilized to produce high-value chemical products. Bacillus has demonstrated the most prominent effect on lignin degradation, with 89% lignin degradation by Bacillus cereus. Furthermore, several bacterial enzymes were discussed that can act on lignin, with the main enzymes consisting of dye-decolorizing peroxidases and laccase. Finally, low-molecular-weight lignin compounds were converted into value-added products through specific reaction pathways. These bacteria and enzymes may become potential candidates for efficient lignin degradation in the future, providing a method for lignin high-value conversion. In addition, the bacterial metabolic pathways convert lignin-derived aromatics into intermediates through the "biological funnel", achieving the biosynthesis of value-added products. The utilization of this "biological funnel" of aromatic compounds may address the heterogeneous issue of the aromatic products obtained via lignin depolymerization. This may also simplify the separation of downstream target products and provide avenues for the commercial application of lignin conversion into high-value products.

Current insights of factors interfering the stability of lytic polysaccharide monooxygenases

Cellulose and chitin are two of the most abundant biopolymers in nature, but they cannot be effectively utilized in industry due to their recalcitrance. This limitation was overcome by the advent of lytic polysaccharide monooxygenases (LPMOs), which promote the disruption of biopolymers through oxidative mechanism and provide a breakthrough in the action of hydrolytic enzymes. In the application of LPMOs to biomass degradation, the key to consistent and effective functioning lies in their stability. The efficient transformation of biomass resources using LPMOs depends on factors that interfere with their stability. This review discussed three aspects that affect LPMO stability: general external factors, structural factors, and factors in the enzyme-substrate reaction. It explains how these factors impact LPMO stability, discusses the resulting effects, and finally presents relevant measures and considerations, including potential resolutions. The review also provides suggestions for the application of LPMOs in polysaccharide degradation.

Recent advances in the efficient degradation of lignocellulosic metabolic networks by lytic polysaccharide monooxygenase

Along with long-term evolution, the plant cell wall generates lignocellulose and other anti-degradation barriers to confront hydrolysis by fungi. Lytic polysaccharide monooxygenase (LPMO) is a newly defined oxidase in lignocellulosic degradation systems that significantly fuels hydrolysis. LPMO accepts electrons from wide sources, such as cellobiose dehydrogenase (CDH), glucose-methanol-choline (GMC) oxidoreductases, and small phenols. In addition, the extracellular cometabolic network formed by cosubstrates improves the degradation efficiency, forming a stable and efficient lignocellulose degradation system. In recent years, using structural proteomics to explore the internal structure and the complex redox system of LPMOs has become a research hotspot. In this review, the diversity of LPMOs, catalytic domains, carbohydrate binding modules, direct electron transfer with CDH, cosubstrates, and degradation networks of LPMOs are explored, which can provide a systematic reference for the application of lignocellulosic degradation systems in industrial approaches.

BsLPMO10A from Bacillus subtilis boosts the depolymerization of diverse polysaccharides linked via β-1,4-glycosidic bonds

Lytic polysaccharide monooxygenase (LPMO) is known as an oxidatively cleaving enzyme in recalcitrant polysaccharide deconstruction. Herein, we report a novel AA10 LPMO derived from Bacillus subtilis (BsLPMO10A). A substrate specificity study revealed that the enzyme exhibited an extensive active-substrate spectrum, particularly for polysaccharides linked via β-1,4 glycosidic bonds, such as β-(Man1 → 4Man), β-(Glc1 → 4Glc) and β-(Xyl1 → 4Xyl). HPAEC-PAD and MALDI-TOF-MS analyses indicated that BsLPMO10A dominantly liberated native oligosaccharides with a degree of polymerization (DP) of 3-6 and C1-oxidized oligosaccharides ranging from DP3ox to DP6ox from mixed linkage glucans and beechwood xylan. Due to its synergistic action with a variety of glycoside hydrolases, including glucanase IDSGLUC5-38, xylanase TfXYN11-1, cellulase IDSGLUC5-11 and chitinase BtCHI18-1, BsLPMO10A dramatically accelerated glucan, xylan, cellulose and chitin saccharification. After co-reaction for 72 h, the reducing sugars in Icelandic moss lichenan, beechwood xylan, phosphoric acid swollen cellulose and chitin yielded 3176 ± 97, 7436 ± 165, 649 ± 44, and 2604 ± 130 μmol/L, which were 1.47-, 1.56-, 1.44- and 1.25-fold higher than those in the GHs alone groups, respectively (P < 0.001). In addition, the synergy of BsLPMO10A and GHs was further validated by the degradation of natural feedstuffs, the co-operation of BsLPMO10A and GHs released 3266 ± 182 and 1725 ± 107 μmol/L of reducing sugars from Oryza sativa L. and Arachis hypogaea L. straws, respectively, which were significantly higher than those produced by GHs alone (P < 0.001). Furthermore, BsLPMO10A also accelerated the liberation of reducing sugars from Celluclast® 1.5 L, a commercial cellulase cocktail, on filter paper, A. hypogaea L. and O. sativa L. straws by 49.58 % (P < 0.05), 72.19 % (P < 0.001) and 54.36 % (P < 0.05), respectively. This work has characterized BsLPMO10A with a broad active-substrate scope, providing a promising candidate for lignocellulosic biomass biorefinery.

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.

Microbial lignin valorization through depolymerization to aromatics conversion

Lignin is the most abundant source of renewable aromatic biopolymers and its valorization presents significant value for biorefinery sustainability, which promotes the utilization of renewable resources. However, it is challenging to fully convert the structurally complex, heterogeneous, and recalcitrant lignin into high-value products. The in-depth research on the lignin degradation mechanism, microbial metabolic pathways, and rational design of new systems using synthetic biology have significantly accelerated the development of lignin valorization. This review summarizes the key enzymes involved in lignin depolymerization, the mechanisms of microbial lignin conversion, and the lignin valorization application with integrated systems and synthetic biology. Current challenges and future strategies to further study lignin biodegradation and the trends of lignin valorization are also discussed.

Lytic polysaccharide monooxygenases (LPMOs) producing microbes: A novel approach for rapid recycling of agricultural wastes

Out of the huge quantity of agricultural wastes produced globally, rice straw is one of the most abundant ligno-cellulosic waste. For efficient utilization of these wastes, several cost-effective biological processes are available. The practice of field level in-situ or ex-situ decomposition of rice straw is having less degree of adoption due to its poor decomposition ability within a short time span between rice harvest and sowing of the next crop. Agricultural wastes including rice straw are in general utilized by using lignocellulose degrading microbes for industrial metabolite or compost production. However, bioconversion of crystalline cellulose and lignin present in the waste, into simple molecules is a challenging task. To resolve this issue, researchers have identified a novel new generation microbial enzyme i.e., lytic polysaccharide monooxygenases (LPMOs) and reported that the combination of LPMOs with other glycolytic enzymes are found efficient. This review explains the progress made in LPMOs and their role in lignocellulose bioconversion and the possibility of exploring LPMOs producers for rapid decomposition of agricultural wastes. Also, it provides insights to identify the knowledge gaps in improving the potential of the existing ligno-cellulolytic microbial consortium for efficient utilization of agricultural wastes at industrial and field levels.