Biotransformation of ferulic acid to protocatechuic acid by Corynebacterium glutamicum ATCC 21420 engineered to express vanillate O-demethylase

Biomanufacturing of value-added chemicals from lignin

Lignin valorization faces persistent biomanufacturing challenges due to the heterogeneous and toxic carbon substrates derived from lignin depolymerization. To address the heterogeneous nature of aromatic feedstocks, plant cell wall engineering and 'lignin first' pretreatment methods have recently emerged. Next, to convert the resulting aromatic substrates into value-added chemicals, diverse microbial host systems also continue to be developed. This includes microbes that (1) lack aromatic metabolism, (2) metabolize aromatics but not sugars, and (3) co-metabolize both aromatics and sugars, each system presenting unique pros and cons. Considering the intrinsic complexity of lignin-derived substrate mixtures, emerging and non-model microbes with native metabolism for aromatics appear poised to provide the greatest impacts on lignin valorization via biomanufacturing.

Biological Valorization of Lignin-Derived Aromatics in Hydrolysate to Protocatechuic Acid by Engineered Pseudomonas putida KT2440

Alongside fermentable sugars, weak acids, and furan derivatives, lignocellulosic hydrolysates contain non-negligible amounts of lignin-derived aromatic compounds. The biological funnel of lignin offers a new strategy for the "natural" production of protocatechuic acid (PCA). Herein, Pseudomonas putida KT2440 was engineered to produce PCA from lignin-derived monomers in hydrolysates by knocking out protocatechuate 3,4-dioxygenase and overexpressing vanillate-O-demethylase endogenously, while acetic acid was used for cell growth. The sugar catabolism was further blocked to prevent the loss of fermentable sugar. Using the engineered strain, a total of 253.88 mg/L of PCA was obtained with a yield of 70.85% from corncob hydrolysate 1. The highest titer of 433.72 mg/L of PCA was achieved using corncob hydrolysate 2 without any additional nutrients. This study highlights the potential ability of engineered strains to address the challenges of PCA production from lignocellulosic hydrolysate, providing novel insights into the utilization of hydrolysates.

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.

Protocatechuic acid, ferulic acid and relevant defense enzymes correlate closely with walnut resistance to Xanthomonas arboricola pv. juglandis

BACKGROUND

Juglans regia L. is an important nut tree that has a wide range of distribution in temperate regions of the world. In some walnut orchards, walnut blight can become a problematic disease that affects the growth of walnut trees. To explore the correlation between biochemical response and walnut resistance, we inoculated four walnut cultivars with Xanthomonas arboricola pv. juglandis (Xaj). The walnut cultivars were, namely, 'Xiangling', 'Xiluo 2', 'Yuanfeng' and 'Xifu 2'. Total phenol content (TPC) and total flavonoid content (TFC) were measured, whereby nine major phenolic compounds and several relevant enzymes were identified.

RESULTS

The results showed that the most resistant and susceptible walnut varieties were 'Xiluo 2' and 'Xifu 2' respectively. The reaction of walnut to Xaj was characterized by the early accumulation of phenolic compounds in the infected site. After inoculation with Xaj, we found that the resistant variety 'Xiluo 2' show the significant differences with other varieties at different time points through the determination of related antioxidant enzymes such as catalase (CAT) and peroxidase (POD). Meanwhile, the phenylalanine ammonia lyase (PAL) of 'Xiluo 2' increased significantly at 8 day post infection (dpi) and made differences from the control samples, while other varieties changed little. And the polyphenol oxidase (PPO) was significantly higher than in the control at 16 dpi, maintaining the highest and the lowest activity in 'Xiluo 2' and 'Xifu 2' respectively. It was also found that the content of protocatechuic acid in all cultivars increased significantly at 4 dpi, and 'Xiluo 2' was significantly higher than that of the control. In the early stage of the disease, ferulic acid content increased significantly in 'Xiluo 2'.

CONCLUSION

Our findings confirmed that the metabolism of phenolic compounds and related defense enzymes are of great significance in the response of walnut to Xaj.

Creative biological lignin conversion routes toward lignin valorization

Lignin, the largest renewable aromatic resource, is a promising alternative feedstock for the sustainable production of various chemicals, fuels, and materials. Despite this potential, lignin is characterized by heterogeneous and macromolecular structures that must be addressed. In this review, we present biological lignin conversion routes (BLCRs) that offer opportunities for overcoming these challenges, making lignin valorization feasible. Funneling heterogeneous aromatics via a 'biological funnel' offers a high-specificity bioconversion route for aromatic platform chemicals. The inherent aromaticity of lignin drives atom-economic functionalization routes toward aromatic natural product generation. By harnessing the ligninolytic capacities of specific microbial systems, powerful aromatic ring-opening routes can be developed to generate various value-added products. Thus, BLCRs hold the promise to make lignin valorization feasible and enable a lignocellulose-based bioeconomy.

Biowastes for biodegradable bioplastics production and end-of-life scenarios in circular bioeconomy and biorefinery concept

Due to global urbanization, industrialization, and economic development, biowastes generation represents negative consequences on the environment and human health. The use of generated biowastes as a feedstock for biodegradable bioplastic production has opened a new avenue for environmental sustainability from the circular (bio)economy standpoint. Biodegradable bioplastic production can contribute to the sustainability pillars (environmental, economic, and social). Furthermore, bioenergy, biomass, and biopolymers production after recycling of biodegradable bioplastic can help to maintain the energy-environment balance. Several types of biodegradable bioplastic, such as starch-based, polyhydroxyalkanoates, polylactic acid, and polybutylene adipate terephthalate, can achieve this aim. In this review, an overview of the main biowastes valorization routes and the main biodegradable bioplastic types of production, application, and biodegradability are discussed to achieve the transition to the circular economy. Additionally, end-of-life scenarios (up-cycle and down-cycle) are reviewed to attain the maximum environmental, social, and economic benefit from biodegradable bioplastic products under biorefinery concept.

Recent advances in lignocellulosic biomass white biotechnology for bioplastics

Lignocellulosic biomass has great potential as an inedible feedstock for bioplastic synthesis, although its use is still limited compared to current edible feedstocks of glucose and starch. This review focuses on recent advances in the production of biopolymers and biomonomers from lignocellulosic feedstocks with downstream processing and chemical polymer syntheses. In microbial production, four routes composed of existing poly (lactic acid) and polyhydroxyalkanoates (PHAs) and the emerging biomonomers of itaconic acid and aromatic compounds were presented to review present challenges and future perspectives, focusing on the use of lignocellulosic feedstocks. Recently, advances in purification technologies decreased the number of processes and their environmental burden. Additionally, the unique structures and high-performance of emerging lignocellulose-based bioplastics have expanded the possibilities for the use of bioplastics. The sequence of processes provides insight into the emerging technologies that are needed for the practical use of bioplastics made from lignocellulosic biomass.

Construction of a p-coumaric and ferulic acid auto-regulatory system in Pseudomonas putida KT2440 for protocatechuate production from lignin-derived aromatics

Lignin is a robust and underutilized aromatic heteropolymer on earth. The effective valorization of lignin into value-added products is an attractive research topic in lignocellulosic biorefineries. However, a low bioconversion rate, high process cost, low yield, and high toxicity of substrates hinder its further applications. In this study, an auto-regulatory system was developed and identified as an effective solution to diminish these challenging bottlenecks. First, a lignin-derived standard model aromatic p-coumaric acid (p-CA)-responsive biosensor was successfully developed through a series of rational engineering approaches in Pseudomonas putida KT2440. Furthermore, an auto-regulatory system was established, which rapidly coexpressed key rate-limiting enzymes, 4-hydroxybenzoate hydroxylase, vanillate-O-demethylase, and transporter HcnK under the biosensor element, to convert the mixture of p-CA and ferulic acid into a value-added platform chemical protocatechuate with a titer of 12.7 g/L. This study demonstrated that the constructed auto-regulatory platform is effective and economical for lignin valorization.

Metabolic and process engineering for microbial production of protocatechuate with Corynebacterium glutamicum

3,4-Dihydroxybenzoate (protocatechuate, PCA) is a phenolic compound naturally found in edible vegetables and medicinal herbs. PCA is of high interest in the chemical industry and has wide potential for pharmaceutical applications. We designed and constructed a novel Corynebacterium glutamicum strain to enable the efficient utilization of d-xylose for microbial production of PCA. Shake flask cultivation of the engineered strain showed a maximum PCA titer of 62.1 ± 12.1 mM (9.6 ± 1.9 g L^-1 ) from d-xylose as the primary carbon and energy source. The corresponding yield was 0.33 C-mol PCA per C-mol d-xylose, which corresponds to 38% of the maximum theoretical yield. Under growth-decoupled bioreactor conditions, a comparable PCA titer and a total amount of 16.5 ± 1.1 g PCA could be achieved when d-glucose and d-xylose were combined as orthogonal carbon substrates for biocatalyst provision and product synthesis, respectively. Downstream processing of PCA was realized via electrochemically induced crystallization by taking advantage of the pH-dependent properties of PCA. This resulted in a maximum final purity of 95.4%. The established PCA production process represents a highly sustainable approach, which will serve as a blueprint for the bio-based production of other hydroxybenzoic acids from alternative sugar feedstocks.

A Timed Off-Switch for Dynamic Control of Gene Expression in Corynebacterium Glutamicum

Dynamic control of gene expression mainly relies on inducible systems, which require supplementation of (costly) inducer molecules. In contrast, synthetic regulatory circuits, which allow the timed shutdown of gene expression, are rarely available and therefore represent highly attractive tools for metabolic engineering. To achieve this, we utilized the VanR/P vanABK * regulatory system of Corynebacterium glutamicum, which consists of the transcriptional repressor VanR and a modified promoter of the vanABK operon (P vanABK *). VanR activity is modulated by one of the phenolic compounds ferulic acid, vanillin or vanillic acid, which are co-metabolized with d-glucose. Thus, gene expression in the presence of d-glucose is turned off if one of the effector molecules is depleted from the medium. To dynamically control the expression of the aceE gene, encoding the E1 subunit of the pyruvate dehydrogenase complex that is essential for growth on d-glucose, we replaced the native promoter by vanR/P vanABK * yielding C. glutamicum ΔP aceE ::vanR-P vanABK *. The biomass yield of this strain increased linearly with the supplemented amount of effector. After consumption of the phenolic compounds growth ceased, however, C. glutamicumΔP aceE ::vanR-P vanABK * continued to utilize the residual d-glucose to produce significant amounts of pyruvate, l-alanine, and l-valine. Interestingly, equimolar concentrations of the three phenolic compounds resulted in different biomass yields; and with increasing effector concentration, the product spectrum shifted from pyruvate over l-alanine to l-valine. To further test the suitability of the VanR/P vanABK * system, we overexpressed the l-valine biosynthesis genes ilvBNCE in C. glutamicum ΔP aceE ::vanR-P vanABK *, which resulted in efficient l-valine production with a yield of about 0.36 mol l-valine per mol d-glucose. These results demonstrate that the VanR/P vanABK * system is a valuable tool to control gene expression in C. glutamicum in a timed manner by the cheap and abundant phenolic compounds ferulic acid, vanillin, and vanillic acid.

In Situ Product Recovery of Bio-Based Industrial Platform Chemicals: A Guideline to Solvent Selection

In situ product recovery (ISPR), in the form of an extractive fermentation process, can increase productivity and product titers in the sustainable production of platform chemicals. To establish a guideline for the development of industrially relevant production processes for such bio-based compounds, a wide screening was performed, mapping the potential of an extensive range of solvents and solvent mixtures. Besides solvent biocompatibility with Saccharomyces cerevisiae, distribution coefficients of three organic acids (protocatechuic acid, adipic acid and para-aminobenzoic acid) and four fragrance compounds (2-phenylethanol, geraniol, trans-cinnamaldehyde and β-ionone) were determined. While for highly hydrophobic fragrance compounds, multiple pure solvents were identified that were able to extract more than 98%, reactive extraction mixtures were proven effective for more challenging compounds including organic acids and hydrophilic alcohols. For example, a reactive mixture consisting of 12.5% of the extractant CYTOP 503 in canola oil was found to be biocompatible and showed superior extraction efficiency for the challenging compounds as compared to any biocompatible single solvent. This mapping of biocompatible solvents and solvent mixtures for the extraction of various classes of industrial platform chemicals can be a tremendous step forward in the development of extractive fermentations.

Protocatechuic acid production from lignin-associated phenolics

Biobased chemicals are gaining popularity and market in attempts to mitigate the deteriorating environmental and sustainability issues. Components of renewable agricultural and forest biomass residues are projected to serve as abundant precursors to synthesis of expanding range of products. Agroindustrial wastes comprises of several phenolic compounds associated with lignin via ether linkages such as ferulic acid, p-coumaric, syringic acid and vanillin. These aromatic chemicals have myriad industrial applications. In this study, p-coumaric acid and ferulic acid were found to be two major components in corn bran derived lignin hydrolysate. Engineered Pseudomonas putida KT2440 was constructed and found to convert p-coumaric acid and vanillic acid to protocatechuic acid in >90% and >50% yields, respectively. Engineering the strain included deletion of the gene encoding protocatechuate 3,4-dioxygenase, and overexpression of vanillate-O-demethylase gene from Acinetobacter sp. ADP1.

Investigating the Efficiency of Hydroxycinnamic Acids to Inhibit the Production of Enniatins by Fusarium avenaceum and Modulate the Expression of Enniatins Biosynthetic Genes

Enniatins (ENNs) that belong to the group of emerging mycotoxins are widespread contaminants of agricultural commodities. There is currently insufficient evidence to rule out health concerns associated with long-term exposure to ENNs and efforts must be strengthened to define a control strategy. While the potential of plant compounds to counteract the contamination with legislated mycotoxins has been reported, little remains known regarding ENNs. The present study evidenced for the first time the efficiency of hydroxycinnamic acids to inhibit the fungal growth and ENNs yield by Fusarium avenaceum. Notably, 0.5 mM of exogenous ferulic, caffeic, and p-coumaric acids led to a drastic reduction of ENNs synthesis in pH4 broths, with ferulic acid being the most potent. The ENNs production inhibitory activity of ferulic acid was shown to be associated with a significant down-regulation of the expression of ENNs biosynthetic genes. To further investigate the bioactivity of ferulic acid, its metabolic fate was characterized in fungal broths and the capacity of F. avenaceum to metabolize it through a C2-cleavage type degradation was demonstrated. Overall, our data support the promising use of ferulic acid in ENNs control strategies, either as part of an environmentally friendly plant-care product or as a biomarker of plant resistance.

Protocatechuate overproduction by Corynebacterium glutamicum via simultaneous engineering of native and heterologous biosynthetic pathways

Protocatechuic acid (3, 4-dihydroxybenzoic acid, PCA) is a natural bioactive phenolic acid potentially valuable as a pharmaceutical raw material owing to its diverse pharmacological activities. Corynebacterium glutamicum forms PCA as a key intermediate in a native pathway to assimilate shikimate/quinate through direct conversion of the shikimate pathway intermediate 3-dehydroshikimate (DHS), which is catalyzed by qsuB-encoded DHS dehydratase (the DHS pathway). PCA can also be formed via an alternate pathway extending from chorismate by introducing heterologous chorismate pyruvate lyase that converts chorismate into 4-hydroxybenzoate (4-HBA), which is then converted into PCA catalyzed by endogenous 4-HBA 3-hydroxylase (the 4-HBA pathway). In this study, we generated three plasmid-free C. glutamicum strains overproducing PCA based on the markerless chromosomal recombination by engineering each or both of the above mentioned two PCA-biosynthetic pathways combined with engineering of the host metabolism to enhance the shikimate pathway flux and to block PCA consumption. Aerobic growth-arrested cell reactions were performed using the resulting engineered strains, which revealed that strains dependent on either the DHS or 4-HBA pathway as the sole PCA-biosynthetic route produced 43.8 and 26.2 g/L of PCA from glucose with a yield of 35.3% and 10.0% (mol/mol), respectively, indicating that PCA production through the DHS pathway is significantly efficient compared to that produced through the 4-HBA pathway. Remarkably, a strain simultaneously using both DHS and 4-HBA pathways achieved the highest reported PCA productivity of 82.7 g/L with a yield of 32.8% (mol/mol) from glucose in growth-arrested cell reaction. These results indicated that simultaneous engineering of both DHS and 4-HBA pathways is an efficient method for PCA production. The generated PCA-overproducing strain is plasmid-free and does not require supplementation of aromatic amino acids and vitamins due to the intact shikimate pathway, thereby representing a promising platform for the industrial bioproduction of PCA and derived chemicals from renewable sugars.

Advances and Prospects of Phenolic Acids Production, Biorefinery and Analysis

Biotechnological production of phenolic acids is attracting increased interest due to their superior antioxidant activity, as well as other antimicrobial, dietary, and health benefits. As secondary metabolites, primarily found in plants and fungi, they are effective free radical scavengers due to the phenolic group available in their structure. Therefore, phenolic acids are widely utilised by pharmaceutical, food, cosmetic, and chemical industries. A demand for phenolic acids is mostly satisfied by utilising chemically synthesised compounds, with only a low quantity obtained from natural sources. As an alternative to chemical synthesis, environmentally friendly bio-based technologies are necessary for development in large-scale production. One of the most promising sustainable technologies is the utilisation of microbial cell factories for biosynthesis of phenolic acids. In this paper, we perform a systematic comparison of the best known natural sources of phenolic acids. The advances and prospects in the development of microbial cell factories for biosynthesis of these bioactive compounds are discussed in more detail. A special consideration is given to the modern production methods and analytics of phenolic acids.