Enzymatic Specific Production and Chemical Functionalization of Phenylpropanone Platform Monomers from Lignin

Dimensionally stable and durable wood by lignin impregnation

Cracking, warping, and decaying stemming from wood's poor dimensional stability and durability are the most annoying issues of natural wood. There is an urgent need to address these issues, of which, sustainable and green chemical treatments are favorably welcomed. Herein, we developed a facile method through the incorporation of environmentally friendly biopolymer lignin into wood cells for wood dimensional stability and durability enhancement. Enzymatic hydrolysis lignin (EHL) was dissolved into various solvents followed by impregnation and drying to incorporate lignin into wood cells. Impregnation treatment was developed to incorporate into wood to improve its dimensional stability, durability, and micromechanics. The anti-swelling efficiency reached up to 99.4 %, the moisture absorption decreased down to 0.55 %, the mass loss after brown rot decay decreased to 7.22 %, and the cell wall elasticity as well as hardness increased 8.7 % and 10.3 %, respectively. Analyses acquired from scanning electron microscopy, fluorescent microscopy, and Raman imaging revealed that the EHL was successfully colonized in cell lumen as well as in cell walls, thus improved wood dimensional stability and durability. Moreover, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy confirmed EHL interaction with the cell wall components, thus the wood mechanical property was not impaired significantly, whereas nanoindentation data indicated even slight mechanical enhancement on the cell walls. This facile approach can improve the wood properties in multiple aspects and remarkably enhance the outdoor performance of modified wood products. In addition, using lignin as a natural modifying agent to improve wood performance will have a great positive impact on the environment.

The etherase system of Novosphingobium sp. MBES04 functions as a sensor of lignin fragments through phenylpropanone production to induce specific transcriptional responses

The MBES04 strain of Novosphingobium accumulates phenylpropanone monomers as end-products of the etherase system, which specifically and reductively cleaves the β-O-4 ether bond (a major bond in lignin molecules). However, it does not utilise phenylpropanone monomers as an energy source. Here, we studied the response to the lignin-related perturbation to clarify the physiological significance of its etherase system. Transcriptome analysis revealed two gene clusters, each consisting of four tandemly linked genes, specifically induced by a lignin preparation extracted from hardwood (Eucalyptus globulus) and a β-O-4-type lignin model biaryl compound, but not by vanillin. The most strongly induced gene was a 2,4'-dihydroxyacetophenone dioxygenase-like protein, which leads to energy production through oxidative degradation. The other cluster was related to multidrug resistance. The former cluster was transcriptionally regulated by a common promoter, where a phenylpropanone monomer acted as one of the effectors responsible for gene induction. These results indicate that the physiological significance of the etherase system of the strain lies in its function as a sensor for lignin fragments. This may be a survival strategy to detect nutrients and gain tolerance to recalcitrant toxic compounds, while the strain preferentially utilises easily degradable aromatic compounds with lower energy demands for catabolism.

Catalytic and molecular properties of alkaliphilic and thermotolerant β-etherase from Altererythrobacter sp. B11

Phenylpropanone monomers, including guaiacyl hydroxypropanone, are important precursors for the synthesis of various chemicals. The monomers are obtained in a three-step cascade reaction catalyzed by a group of enzymes in the β-etherase system that cleaves the β-O-4 bond, the major bond in lignin. In this study, one of the β-etherase of the glutathione-S-transferase superfamily, AbLigF2, was discovered in genus Altererythrobacter, and the recombinant etherase was characterized. The enzyme showed maximal activity at 45 °C, maintained 30% of its activity after 2 h at 50 °C, and was the most thermostable among the previously reported enzymes. Moreover, N13, S14, and S115, located near the thiol group of glutathione, had a significant effect on the maximum reaction rate of enzyme activity. This study suggests that AbLigF2 has the potential to serve as a thermostable enzyme for lignin utilization and provides insights into its catalytic mechanism.

Enzymatic bioconversion process of lignin: mechanisms, reactions and kinetics

Lignin is a wasted renewable source of biomass-derived value-added chemicals. However, due to its material resistance to degradation, it remains highly underutilized. In order to develop new, catalysed and more environment friendly reaction processes for lignin valorization, science has turned a selective concentrated attention to microbial enzymes. This present work looks at the enzymes involved with the main reference focus on the different elementary mechanisms of action/conversion rate kinetics. Pathways, like with laccases/peroxidases, employ radicals, which more readily result in polymerization than de-polymerization. The β-etherase system interaction of proteins targets β-O-4 ether covalent bond, which targets lower molecular weight product species. Enzymatic activity is influenced by a wide variety of different factors which need to be considered in order to obtain the best functionality and synthesis yields.

Degradation of ester linkages in rice straw components by Sphingobium species recovered from the sea bottom using a non-secretory tannase-family α/β hydrolase

Microbial decomposition of allochthonous plant components imported into the aquatic environment is one of the vital steps of the carbon cycle on earth. To expand the knowledge of the biodegradation of complex plant materials in aquatic environments, we recovered a sunken wood from the bottom of Otsuchi Bay, situated in northeastern Japan in 2012. We isolated Sphingobium with high ferulic acid esterase activity. The strain, designated as OW59, grew on various aromatic compounds and sugars, occurring naturally in terrestrial plants. A genomic study of the strain suggested its role in degrading hemicelluloses. We identified a gene encoding a non-secretory tannase-family α/β hydrolase, which exhibited ferulic acid esterase activity. This enzyme shares the consensus catalytic triad (Ser-His-Asp) within the tannase family block X in the ESTHER database. The molecules, which had the same calculated elemental compositions, were produced consistently in both the enzymatic and microbial degradation of rice straw crude extracts. The non-secretory tannase-family α/β hydrolase activity may confer an important phenotypic feature on the strain to accelerate plant biomass degradation. Our study provides insights into the underlying biodegradation process of terrestrial plant polymers in aquatic environments.

Enzymatic Preparation and Characterization of Spherical Microparticles Composed of Artificial Lignin and TEMPO-Oxidized Cellulose Nanofiber

A one-pot and one-step enzymatic synthesis of submicron-order spherical microparticles composed of dehydrogenative polymers (DHPs) of coniferyl alcohol as a typical lignin precursor and TEMPO-oxidized cellulose nanofibers (TOCNFs) was investigated. Horseradish peroxidase enzymatically catalyzed the radical coupling of coniferyl alcohol in an aqueous suspension of TOCNFs, resulting in the formation of spherical microparticles with a diameter and sphericity index of approximately 0.8 μm and 0.95, respectively. The ζ-potential of TOCNF-functionalized DHP microspheres was about -40 mV, indicating that the colloidal systems had good stability. Nanofibrous components were clearly observed on the microparticle surface by scanning electron microscopy, while some TOCNFs were confirmed to be inside the microparticles by confocal laser scanning microscopy with Calcofluor white staining. As both cellulose and lignin are natural polymers known to biodegrade, even in the sea, these woody TOCNF-DHP microparticle nanocomposites were expected to be promising alternatives to fossil resource-derived microbeads in cosmetic applications.

Roles of two glutathione S-transferases in the final step of the β-aryl ether cleavage pathway in Sphingobium sp. strain SYK-6

Sphingobium sp. strain SYK-6 is an alphaproteobacterial degrader of lignin-derived aromatic compounds, which can degrade all the stereoisomers of β-aryl ether-type compounds. SYK-6 cells convert four stereoisomers of guaiacylglycerol-β-guaiacyl ether (GGE) into two enantiomers of α-(2-methoxyphenoxy)-β-hydroxypropiovanillone (MPHPV) through GGE α-carbon atom oxidation by stereoselective Cα-dehydrogenases encoded by ligD, ligL, and ligN. The ether linkages of the resulting MPHPV enantiomers are cleaved by stereoselective glutathione (GSH) S-transferases (GSTs) encoded by ligF, ligE, and ligP, generating (βR/βS)-α-glutathionyl-β-hydroxypropiovanillone (GS-HPV) and guaiacol. To date, it has been shown that the gene products of ligG and SLG_04120 (ligQ), both encoding GST, catalyze GSH removal from (βR/βS)-GS-HPV, forming achiral β-hydroxypropiovanillone. In this study, we verified the enzyme properties of LigG and LigQ and elucidated their roles in β-aryl ether catabolism. Purified LigG showed an approximately 300-fold higher specific activity for (βR)-GS-HPV than that for (βS)-GS-HPV, whereas purified LigQ showed an approximately six-fold higher specific activity for (βS)-GS-HPV than that for (βR)-GS-HPV. Analyses of mutants of ligG, ligQ, and both genes revealed that SYK-6 converted (βR)-GS-HPV using both LigG and LigQ, whereas only LigQ was involved in converting (βS)-GS-HPV. Furthermore, the disruption of both ligG and ligQ was observed to lead to the loss of the capability of SYK-6 to convert MPHPV. This suggests that GSH removal from GS-HPV catalyzed by LigG and LigQ, is essential for cellular GSH recycling during β-aryl ether catabolism.

A method for high-throughput screening hydrolase of lignin β-aryl ether linkage from directed evolution by glutathione (GSH) assay

Background

Lignin’s valorization plays a critical role in refining the bioresource. Considering that the β-aryl ether linkage (β-O-4 bond) accounts for 50–70% of lignin chemical linkage between aromatic rings, the hydrolase of lignin β-aryl ether linkage, especially the β-etherase, provided a promising way for the lignin depolymerization and valorization. As a result, it is essential to develop the effective high-throughput methods for screening the mutant library of β-etherase from directed evolution.

Results

Based on the enzymatic mechanism of β-O-4 bond’s cleavage by β-etherase, the LigF was selected as the model to study high-throughput method by GSH assay for screening the mutant library of β-etherase from directed evolution. After the primary study with purified LigF and cell lysate, the GSH assay was used to screen mutant library of β-etherase. The study on screening the mutant library with about 600 colonies indicated that the selected transformants all have one or two mutated sites in the gene sequence of LigF, and the activities from GSH assay of most selected transformants were the same as their activities from HPLC assay.

Conclusions

The results from the high-throughput screening of mutant library demonstrated that GSH assay could be applied to screen β-etherase mutant from directed evolution.

Database Mining for Novel Bacterial β-Etherases, Glutathione-Dependent Lignin-Degrading Enzymes

Lignin is the most abundant aromatic polymer in nature and a promising renewable source for the provision of aromatic platform chemicals and biofuels. β-Etherases are enzymes with a promising potential for application in lignin depolymerization due to their selectivity in the cleavage of β-O-4 aryl ether bonds. However, only a very limited number of these enzymes have been described and characterized so far. Using peptide pattern recognition (PPR) as well as phylogenetic analyses, 96 putatively novel β-etherases have been identified, some even originating from bacteria outside the order Sphingomonadales A set of 13 diverse enzymes was selected for biochemical characterization, and β-etherase activity was confirmed for all of them. Some enzymes displayed up to 3-fold higher activity than previously known β-etherases. Moreover, conserved sequence motifs specific for either LigE- or LigF-type enzymes were deduced from multiple-sequence alignments and the PPR-derived peptides. In combination with structural information available for the β-etherases LigE and LigF, insight into the potential structural and/or functional role of conserved residues within these sequence motifs is provided. Phylogenetic analyses further suggest the presence of additional bacterial enzymes with potential β-etherase activity outside the classical LigE- and LigF-type enzymes as well as the recently described heterodimeric β-etherases.IMPORTANCE The use of biomass as a renewable source and replacement for crude oil for the provision of chemicals and fuels is of major importance for current and future societies. Lignin, the most abundant aromatic polymer in nature, holds promise as a renewable starting material for the generation of required aromatic structures. However, a controlled and selective lignin depolymerization to yield desired aromatic structures is a very challenging task. In this regard, bacterial β-etherases are especially interesting, as they are able to cleave the most abundant bond type in lignin with high selectivity. With this study, we significantly expanded the toolbox of available β-etherases for application in lignin depolymerization and discovered more active as well as diverse enzymes than previously known. Moreover, the identification of further β-etherases by sequence database mining in the future will be facilitated considerably through our deduced etherase-specific sequence motifs.

Whole-cell cascade for the preparation of enantiopure β-O-4 aryl ether compounds with glutathione recycling

Bacterial β-etherases and glutathione lyases are glutathione-dependent enzymes that catalyze the selective cleavage of β-O-4 aryl ether bonds found in lignin. Their glutathione (GSH) dependence is regarded as major limitation for their application in the production of aromatics from lignin polymer and oligomers, as stoichiometric glutathione amounts are required. Thus, recycling of the GSH cofactor by a NAD(P)H-dependent glutathione reductase was proposed previously. Herein, the use of a whole-cell catalyst was studied for efficient β-O-4 aryl ether bond cleavage with intracellular GSH supply and recycling. After optimization of the whole-cell catalyst as well as reaction conditions, up to 5 mM lignin model substrate 2,6-methoxyphenoxy-α-veratrylglycerone (2,6-MP-VG) were efficiently converted into 2,6-methoxyphenol (2,6-MP) and veratryl glycerone (VG) without addition of external GSH. Unexpectedly, no glucose supply was required for glutathione recycling within the cells up to this substrate concentration. To demonstrate the applicability of this whole-cell approach, a whole-cell cascade combining a stereoselective β-etherase (either LigE from Sphingobium sp. SYK-6 or LigF-NA from Novosphingobium aromaticivorans) and a glutathione lyase (LigG-TD from Thiobacillus denitrificans) was employed in the kinetic resolution of racemic 2,6-MP-VG. This way, enantiopure (S)- and (R)-2,6-MP-VG were obtained on semi-preparative scale without the need for external GSH supply.

A heterodimeric glutathione S-transferase that stereospecifically breaks lignin's β(R)-aryl ether bond reveals the diversity of bacterial β-etherases

Lignin is a heterogeneous polymer of aromatic subunits that is a major component of lignocellulosic plant biomass. Understanding how microorganisms deconstruct lignin is important for understanding the global carbon cycle and could aid in developing systems for processing plant biomass into valuable commodities. Sphingomonad bacteria use stereospecific glutathione S-transferases (GSTs) called β-etherases to cleave the β-aryl ether (β-O-4) bond, the most common bond between aromatic subunits in lignin. Previously characterized bacterial β-etherases are homodimers that fall into two distinct GST subclasses: LigE homologues, which cleave the β(R) stereoisomer of the bond, and LigF homologues, which cleave the β(S) stereoisomer. Here, we report on a heterodimeric β-etherase (BaeAB) from the sphingomonad Novosphingobium aromaticivorans that stereospecifically cleaves the β(R)-aryl ether bond of the di-aromatic compound β-(2-methoxyphenoxy)-γ-hydroxypropiovanillone (MPHPV). BaeAB's subunits are phylogenetically distinct from each other and from other β-etherases, although they are evolutionarily related to LigF, despite the fact that BaeAB and LigF cleave different β-aryl ether bond stereoisomers. We identify amino acid residues in BaeAB's BaeA subunit important for substrate binding and catalysis, including an asparagine that is proposed to activate the GSH cofactor. We also show that BaeAB homologues from other sphingomonads can cleave β(R)-MPHPV and that they may be as common in bacteria as LigE homologues. Our results suggest that the ability to cleave the β-aryl ether bond arose independently at least twice in GSTs and that BaeAB homologues may be important for cleaving the β(R)-aryl ether bonds of lignin-derived oligomers in nature.

Structure-dependent antiviral activity of catechol derivatives in pyroligneous acid against the encephalomycarditis virus

The pyrolysis product, wood vinegar (WV), from Japanese larch exhibited strong antiviral activity against the encephalomycarditis virus (EMCV). Catechol, 3-methyl-, 4-methyl-, 4-ethyl-, and 3-methoxycatechol, and 2-methyl-1,4-benzenediol were identified as the major antiviral compounds. The viral inhibition ability of these compounds was affected by the structure and position of the substituent group attached to the aromatic skeleton. The IC50 of catechol was 0.67 mg mL-1 and those of its derivatives were <0.40 mg mL-1. Methyl and ethyl substitution in the para position relative to a hydroxyl group obviously increased the antiviral activities. The mode of antiviral action was investigated by adding catechol derivatives at different times of the viral life cycle. It was found that direct inactivations of EMCV by these compounds were the major pathway for the antiviral activity. The effect of catechol derivatives on the host immune system was studied by quantification of Il6 and Ifnb1 expression levels. Increased Il6 expression levels indicate NF-κB activation by reactive oxygen species from auto-oxidations of catechol derivatives, which is also a possible antiviral route. The present research provides indices for production of potent antiviral agents form lignocellulose biomass.

Robust Surface Plasmon Resonance Chips for Repetitive and Accurate Analysis of Lignin-Peptide Interactions

We have developed novel surface plasmon resonance (SPR) sensor chips whose surfaces bear newly synthesized functional self-assembled monolayer (SAM) anchoring lignin through covalent chemical bonds. The SPR sensor chips are remarkably robust and suitable for repetitive and accurate measurement of noncovalent lignin-peptide interactions, which is of significant interest in the chemical or biochemical conversion of renewable woody biomass to valuable chemical feedstocks. The lignin-anchored SAMs were prepared for the first time by click chemistry based on an azide-alkyne Huisgen cycloaddition: mixed SAMs are fabricated on gold thin film using a mixture of alkynyl and methyl thioalkyloligo(ethylene oxide) disulfides and then reacted with azidated milled wood lignins to furnish the functional SAMs anchoring lignins covalently. The resulting SAMs were characterized using infrared reflection-absorption, Raman, and X-ray photoelectron spectroscopies to confirm covalent immobilization of the lignins to the SAMs via triazole linkages and also to reveal that the SAM formation induces a helical conformation of the ethylene oxide chains. Further, SPR measurements of the noncovalent lignin-peptide interactions using lignin-binding peptides have demonstrated high reproducibility and durability of the prepared lignin-anchored sensor chips.

Alternatives for Chemical and Biochemical Lignin Valorization: Hot Topics from a Bibliometric Analysis of the Research Published During the 2000–2016 Period

A complete bibliometric analysis of the Scopus database was performed to identify the research trends related to lignin valorization from 2000 to 2016. The results from this analysis revealed an exponentially increasing number of publications and a high relevance of interdisciplinary collaboration. The simultaneous valorization of the three main components of lignocellulosic biomass (cellulose, hemicellulose, and lignin) has been revealed as a key aspect and optimal pretreatment is required for the subsequent lignin valorization. Research covers the determination of the lignin structure, isolation, and characterization; depolymerization by thermal and thermochemical methods; chemical, biochemical and biological conversion of depolymerized lignin; and lignin applications. Most methods for lignin depolymerization are focused on the selective cleavage of the β-O-4 linkage. Although many depolymerization methods have been developed, depolymerization with sodium hydroxide is the dominant process at industrial scale. Oxidative conversion of lignin is the most used method for the chemical lignin upgrading. Lignin uses can be classified according to its structure into lignin-derived aromatic compounds, lignin-derived carbon materials and lignin-derived polymeric materials. There are many advances in all approaches, but lignin-derived polymeric materials appear as a promising option.

Bioproduction of Benzylamine from Renewable Feedstocks via a Nine-Step Artificial Enzyme Cascade and Engineered Metabolic Pathways

Production of chemicals from renewable feedstocks has been an important task for sustainable chemical industry. Although microbial fermentation has been widely employed to produce many biochemicals, it is still very challenging to access non-natural chemicals. Two methods (biotransformation and fermentation) have been developed for the first bio-derived synthesis of benzylamine, a commodity non-natural amine with broad applications. Firstly, a nine-step artificial enzyme cascade was designed by biocatalytic retrosynthetic analysis and engineered in recombinant E. coli LZ243. Biotransformation of l-phenylalanine (60 mm) with the E. coli cells produced benzylamine (42 mm) in 70 % conversion. Importantly, the cascade biotransformation was scaled up to 100 mL and benzylamine was successfully isolated in 57 % yield. Secondly, an artificial biosynthesis pathway to benzylamine from glucose was developed by combining the nine-step cascade with an enhanced l-phenylalanine synthesis pathway in cells. Fermentation with E. coli LZ249 gave benzylamine in 4.3 mm concentration from glucose. In addition, one-pot syntheses of several useful benzylamines from the easily available styrenes were achieved, representing a new type of alkene transformation by formal oxidative cleavage and reductive amination.

Microbial β-etherases and glutathione lyases for lignin valorisation in biorefineries: current state and future perspectives

Lignin is the major aromatic biopolymer in nature, and it is considered a valuable feedstock for the future supply of aromatics. Hence, its valorisation in biorefineries is of high importance, and various chemical and enzymatic approaches for lignin depolymerisation have been reported. Among the enzymes known to act on lignin, β-etherases offer the possibility for a selective cleavage of the β-O-4 aryl ether bonds present in lignin. These enzymes, together with glutathione lyases, catalyse a reductive, glutathione-dependent ether bond cleavage displaying high stereospecificity. β-Etherases and glutathione lyases both belong to the superfamily of glutathione transferases, and several structures have been solved recently. Additionally, different approaches for their application in lignin valorisation have been reported in the last years. This review gives an overview on the current knowledge on β-etherases and glutathione lyases, their biochemical and structural features, and critically discusses their potential for application in biorefineries.

Bacterial Catabolism of β-Hydroxypropiovanillone and β-Hydroxypropiosyringone Produced in the Reductive Cleavage of Arylglycerol-β-Aryl Ether in Lignin

Sphingobium sp. strain SYK-6 converts four stereoisomers of arylglycerol-β-guaiacyl ether into achiral β-hydroxypropiovanillone (HPV) via three stereospecific reaction steps. Here, we determined the HPV catabolic pathway and characterized the HPV catabolic genes involved in the first two steps of the pathway. In SYK-6 cells, HPV was oxidized to vanilloyl acetic acid (VAA) via vanilloyl acetaldehyde (VAL). The resulting VAA was further converted into vanillate through the activation of VAA by coenzyme A. A syringyl-type HPV analog, β-hydroxypropiosyringone (HPS), was also catabolized via the same pathway. SLG_12830 (hpvZ), which belongs to the glucose-methanol-choline oxidoreductase family, was isolated as the HPV-converting enzyme gene. An hpvZ mutant completely lost the ability to convert HPV and HPS, indicating that hpvZ is essential for the conversion of both the substrates. HpvZ produced in Escherichia coli oxidized both HPV and HPS and other 3-phenyl-1-propanol derivatives. HpvZ localized to both the cytoplasm and membrane of SYK-6 and used ubiquinone derivatives as electron acceptors. Thirteen gene products of the 23 aldehyde dehydrogenase (ALDH) genes in SYK-6 were able to oxidize VAL into VAA. Mutant analyses suggested that multiple ALDH genes, including SLG_20400, contribute to the conversion of VAL. We examined whether the genes encoding feruloyl-CoA synthetase (ferA) and feruloyl-CoA hydratase/lyase (ferB and ferB2) are involved in the conversion of VAA. Only FerA exhibited activity toward VAA; however, disruption of ferA did not affect VAA conversion. These results indicate that another enzyme system is involved in VAA conversion.IMPORTANCE Cleavage of the β-aryl ether linkage is the most essential process in lignin biodegradation. Although the bacterial β-aryl ether cleavage pathway and catabolic genes have been well documented, there have been no reports regarding the catabolism of HPV or HPS, the products of cleavage of β-aryl ether compounds. HPV and HPS have also been found to be obtained from lignin by chemoselective catalytic oxidation by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone/tert-butyl nitrite/O₂, followed by cleavage of the β-aryl ether with zinc. Therefore, value-added chemicals are expected to be produced from these compounds. In this study, we determined the SYK-6 catabolic pathways for HPV and HPS and identified the catabolic genes involved in the first two steps of the pathways. Since SYK-6 catabolizes HPV through 2-pyrone-4,6-dicarboxylate, which is a building block for functional polymers, characterization of HPV catabolism is important not only for understanding the bacterial lignin catabolic system but also for lignin utilization.

In Vitro Enzymatic Depolymerization of Lignin with Release of Syringyl, Guaiacyl, and Tricin Units

New environmentally sound technologies are needed to derive valuable compounds from renewable resources. Lignin, an abundant polymer in terrestrial plants comprised predominantly of guaiacyl and syringyl monoaromatic phenylpropanoid units, is a potential natural source of aromatic compounds. In addition, the plant secondary metabolite tricin is a recently discovered and moderately abundant flavonoid in grasses. The most prevalent interunit linkage between guaiacyl, syringyl, and tricin units is the β-ether linkage. Previous studies have shown that bacterial β-etherase pathway enzymes catalyze glutathione-dependent cleavage of β-ether bonds in dimeric β-ether lignin model compounds. To date, however, it remains unclear whether the known β-etherase enzymes are active on lignin polymers. Here we report on enzymes that catalyze β-ether cleavage from bona fide lignin, under conditions that recycle the cosubstrates NAD+ and glutathione. Guaiacyl, syringyl, and tricin derivatives were identified as reaction products when different model compounds or lignin fractions were used as substrates. These results demonstrate an in vitro enzymatic system that can recycle cosubstrates while releasing aromatic monomers from model compounds as well as natural and engineered lignin oligomers. These findings can improve the ability to produce valuable aromatic compounds from a renewable resource like lignin.IMPORTANCE Many bacteria are predicted to contain enzymes that could convert renewable carbon sources into substitutes for compounds that are derived from petroleum. The β-etherase pathway present in sphingomonad bacteria could cleave the abundant β-O-4-aryl ether bonds in plant lignin, releasing a biobased source of aromatic compounds for the chemical industry. However, the activity of these enzymes on the complex aromatic oligomers found in plant lignin is unknown. Here we demonstrate biodegradation of lignin polymers using a minimal set of β-etherase pathway enzymes, the ability to recycle needed cofactors (glutathione and NAD+) in vitro, and the release of guaiacyl, syringyl, and tricin as depolymerized products from lignin. These observations provide critical evidence for the use and future optimization of these bacterial β-etherase pathway enzymes for industrial-level biotechnological applications designed to derive high-value monomeric aromatic compounds from lignin.

Bacterial catabolism of lignin-derived aromatics: New findings in a recent decade: Update on bacterial lignin catabolism

Lignin is the most abundant phenolic polymer; thus, its decomposition by microorganisms is fundamental to carbon cycling on earth. Lignin breakdown is initiated by depolymerization catalysed by extracellular oxidoreductases secreted by white-rot basidiomycetous fungi. On the other hand, bacteria play a predominant role in the mineralization of lignin-derived heterogeneous low-molecular-weight aromatic compounds. The outline of bacterial catabolic pathways for lignin-derived bi- and monoaryls are typically composed of the following sequential steps: (i) funnelling of a wide variety of lignin-derived aromatics into vanillate and syringate, (ii) O demethylation of vanillate and syringate to form catecholic derivatives and (iii) aromatic ring-cleavage of the catecholic derivatives to produce tricarboxylic acid cycle intermediates. Knowledge regarding bacterial catabolic systems for lignin-derived aromatic compounds is not only important for understanding the terrestrial carbon cycle but also valuable for promoting the shift to a low-carbon economy via biological lignin valorisation. This review summarizes recent progress in bacterial catabolic systems for lignin-derived aromatic compounds, including newly identified catabolic pathways and genes for decomposition of lignin-derived biaryls, transcriptional regulation and substrate uptake systems. Recent omics approaches on catabolism of lignin-derived aromatic compounds are also described.

Expanding the biomass derived chemical space

The derivatization and covalent modification of biomass derived platform chemicals expand the biomass derived chemical spaces allowing for the preparation of new bioactive molecules and materials.

Biorefinery aims at the conversion of biomass and renewable feedstocks into fuels and platform chemicals, in analogy to conventional oil refinery. In the past years, the scientific community has defined a number of primary building blocks that can be obtained by direct biomass decomposition. However, the large potential of this “renewable chemical space” to contribute to the generation of value added bio-active compounds and materials still remains unexplored. In general, biomass derived building blocks feature a diverse range of chemical functionalities. In order to be integrated into value-added compounds, they require additional functionalization and/or covalent modification thereby generating secondary building blocks. The latter can be thus regarded as functional components of bio-active molecules or materials and represent an expansion of the renewable chemical space. This perspective highlights the most recent developments and opportunities for the synthesis of secondary biomass derived building blocks and their application to the preparation of value added products.

Synthesis of nitrogenated lignin-derived compounds and reactivity with laccases. Study of their application in mild chemoenzymatic oxidative processes

The chemical synthesis of a series of nitrogenated lignin-derived compounds, their reactivity with laccases and further application in mild oxidative processes are here disclosed.