Pseudomonas sp. NGC7 as a microbial chassis for glucose-free muconate production from a variety of lignin-derived aromatics and its application to the production from sugar cane bagasse alkaline extract

Engineering a vanillate-producing strain of Pseudomonas sp. NGC7 corresponding to aromatic compounds derived from the continuous catalytic alkaline oxidation of sulfite lignin

INTRODUCTION

Lignin is a promising resource for obtaining aromatic materials, however, its heterogeneous structure poses a challenge for effective utilization. One approach to produce homogeneous aromatic materials from lignin involves the application of microbial catabolism, which is gaining attention. This current study focused on constructing a catabolic pathway in Pseudomonas sp. NGC7 to produce vanillate (VA) from aromatic compounds derived from the chemical depolymerization of sulfite lignin.

RESULTS

Alkaline oxidation of sulfite lignin was performed using a hydroxide nanorod copper foam [Cu(OH)2/CF]-equipped flow reactor. The flow reactor operated continuously for 50 h without clogging and it yielded a sulfite lignin stream containing acetovanillone (AV), vanillin (VN), and VA as the major aromatic monomers. The catabolic pathway of Pseudomonas sp. NGC7 was optimized to maximize VA production from aromatic monomers in the sulfite lignin stream derived from this oxidation process. Pseudomonas sp. NGC7 possesses four gene sets for vanillate O-demethylase, comprising the oxygenase component (vanA) and its oxidoreductase component (vanB). Among these, the vanA4B4 gene set was identified as the key contributor to VA catabolism. To facilitate the conversion of AV to VA, AV-converting enzyme genes from Sphingobium lignivorans SYK-6 were introduced. The ΔvanA4B4 strain, harboring these AV-converting genes, produced VA from the sulfite lignin stream with 91 mol%. Further disruption of vanA1B1, vanA2B2, vanA3B3, and a vanillin reductase gene, in addition to vanA4B4, and introduction of a 5-carboxyvanillate decarboxylase gene from S. lignivorans SYK-6 to utilize 5-carboxyvanillin and 5-carboxyvanillate from the sulfite lignin stream for VA production achieved a VA yield of 103 mol%.

CONCLUSION

Developing methods to overcome lignin heterogeneity is essential for its use as a raw material. Consolidating continuous alkaline oxidation of lignin in a Cu(OH)2/CF-packed flow reactor and biological funneling using an engineered catabolic pathway of Pseudomonas sp. NGC7 is a promising approach to produce VA for aromatic materials synthesis. NGC7 possesses a higher adaptability to various aromatic compounds generated from the alkaline oxidation of lignin and its natural ability to grow on p-hydroxyphenyl, guaiacyl, and syringyl compounds underscores its potential as a bacterial chassis for VA production from a wide range of lignin-derived aromatic compounds.

Direct catalytic oxidation of rice husk lignin with hydroxide nanorod-modified copper foam and muconate production by engineered Pseudomonas sp. NGC7

For the direct alkaline oxidation of rice husk lignin, we developed a copper foam-based heterogeneous catalyst that offers advantages in its recovery after the reaction mixture. The depolymerized products were utilized for muconate production by an engineered Pseudomonas sp. NGC7-based strain. A hydroxide nanorod-modified copper foam was prepared by the surface oxidation of copper foam, followed by alkaline oxidation of rice husk lignin over the catalyst. The catalyst was easily separated from the cellulosic residues in the reaction mixture, and the residues were then recovered by filtration. The resulting lignin stream was composed of a variety of aromatic monomers containing p-hydroxyphenyl, guaiacyl, and syringyl compounds. The catabolic activity of Pseudomonas sp. NGC7 was demonstrated to be more suitable for muconate production from a mixture comprising 4-hydroxybenzoate (a typical p-hydroxyphenyl compound), vanillate (a guaiacyl compound), and syringate (a syringyl compound), owing to its natural ability to grow on syringate. Thus, it was applied to produce muconate from a rice husk lignin stream prepared through hydroxide nanorod-modified copper foam-catalyzed alkaline oxidation by conferring the genes responsible for converting the acetophenone derivatives to their corresponding aromatic acids and protocatechuate decarboxylase to an NGC7-based strain deficient in protocatechuate 3,4-dioxygenase and muconate cycloisomerase. As a result, the engineered NGC7-based muconate-producing strain produced muconate selectively from the rice husk lignin stream at 93.7 mol% yield.

Bacterial valorization of lignin for the sustainable production of value-added bioproducts

As the most abundant aromatic biopolymer in the biosphere, lignin represents a promising alternative feedstock for the industrial production of various value-added bioproducts with enhanced economical value. However, the large-scale implementation of lignin valorization remains challenging because of the heterogeneity and irregular structure of lignin. General fragmentation and depolymerization processes often yield various products, but these approaches necessitate tedious purification steps to isolate target products. Moreover, microbial biocatalytic processes, especially bacterial-based systems with high metabolic activity, can depolymerize and further utilize lignin in an eco-friendly way. Considering that wild bacterial strains have evolved several metabolic pathways and enzymatic systems for lignin degradation, substantial efforts have been made to exploit their potential for lignin valorization. This review summarizes recent advances in lignin valorization for the production of value-added bioproducts based on bacterial systems. Additionally, the remaining challenges and available strategies for lignin biodegradation processes and future trends of bacterial lignin valorization are discussed.

Microbial production of cis,cis-muconic acid from aromatic compounds in engineered Pseudomonas

Industrial expansion has led to environmental pollution by xenobiotic compounds like polycyclic aromatic hydrocarbons and monoaromatic hydrocarbons. Pseudomonas spp. have broad metabolic potential for degrading aromatic compounds. The objective of this study was to develop a "biological funneling" strategy based on genetic modification to convert complex aromatic compounds into cis,cis-muconate (ccMA) using Pseudomonas putida B6-2 and P. brassicacearum MPDS as biocatalysts. The engineered strains B6-2 (B6-2ΔcatBΔsalC) and MPDS (MPDSΔsalC(pUCP18k-catA)) thrived with biphenyl or naphthalene as the sole carbon source and produced ccMA, attaining molar conversions of 95.3% (ccMA/biphenyl) and 100% (ccMA/naphthalene). Under mixed substrates, B6-2ΔcatBΔsalC grew on biphenyl as a carbon source and transformed ccMA from non-growth substrates benzoate or salicylate to obtain higher product concentration. Inserting exogenous clusters like bedDC1C2AB and xylCMAB allowed B6-2 recombinant strains to convert benzene and toluene to ccMA. In mixed substrates, constructed consortia of engineered strains B6-2 and MPDS specialized in catabolism of biphenyl and naphthalene; the highest molar conversion rate of ccMA from mixed substrates was 85.2% when B6-2ΔcatBΔsalC was added after 24 h of MPDSΔsalC(pUCP18k-catA) incubation with biphenyl and naphthalene. This study provides worthwhile insights into efficient production of ccMA from aromatic hydrocarbons by reusing complex pollutants.

Successful selective production of vanillic acid from depolymerized sulfite lignin and its application to poly(ethylene vanillate) synthesis

Towards lignin upgrading, vanillic acid (VA), a lignin-derived guaiacyl compound, was produced from sulfite lignin for successfully synthesizing poly(ethylene vanillate), an aromatic polymer. The engineered Sphingobium sp. SYK-6-based strain in which the genes responsible for VA/3-O-methyl gallic acid O-demethylase and syringic acid O-demethylase were disrupted was able to produce vanillic acid (VA) from the mixture consisting of acetovanillone, vanillin, VA, and other low-molecular-weight aromatics obtained by Cu(OH)₂-catalyzed alkaline depolymerization of sulfite lignin and membrane fractionation. From the bio-based VA, methyl-4-(2-hydroxyethoxy)-3-methoxybenzoate was synthesized via methylesterification, hydroxyethylation, and distillation, and then it was subjected to polymerization catalyzed by titanium tetraisopropoxide. The molecular weight of the obtained poly(ethylene vanillate) was evaluated to be M_w = 13,000 (M_w/M_n = 1.99) and its melting point was 261°C. The present work proved that poly(ethylene vanillate) is able to be synthesized using VA produced from lignin for the first time.

Lignin Valorization: Production of High Value-Added Compounds by Engineered Microorganisms

Lignin is the second most abundant polymer in nature, which is also widely generated during biomass fractionation in lignocellulose biorefineries. At present, most of technical lignin is simply burnt for energy supply although it represents the richest natural source of aromatics, and thus it is a promising feedstock for generation of value-added compounds. Lignin is heterogeneous in composition and recalcitrant to degradation, with this substantially hampering its use. Notably, microbes have evolved particular enzymes and specialized metabolic pathways to degrade this polymer and metabolize its various aromatic components. In recent years, novel pathways have been designed allowing to establish engineered microbial cell factories able to efficiently funnel the lignin degradation products into few metabolic intermediates, representing suitable starting points for the synthesis of a variety of valuable molecules. This review focuses on recent success cases (at the laboratory/pilot scale) based on systems metabolic engineering studies aimed at generating value-added and specialty chemicals, with much emphasis on the production of cis,cis-muconic acid, a building block of recognized industrial value for the synthesis of plastic materials. The upgrade of this global waste stream promises a sustainable product portfolio, which will become an industrial reality when economic issues related to process scale up will be tackled.