Integrated process development for grass biomass utilization through enzymatic saccharification and upgrading hydroxycinnamic acids via microbial funneling

Efficient biological funneling of lignin into 2-pyrone-4,6-dicarboxylic acid via electrocatalytic depolymerization and genetically engineered Pseudomonas putida KT2440

Lignin has been an abundant biomass resource with remarkable potential to produce value-added chemicals. The comprehensive process from lignin degradation to the biological conversion of its monomers remains a challenge for demonstrating the industrial applicability of lignin refinery. Herein, Pseudomonas putida KT-PDCV overexpressing homologous vanillate-O-methylase (VanAB) could efficiently produce 2-pyrone-4,6-dicarboxylic acid (PDC) from lignin-derived compounds (LDC), including S-unit monomers (e.g., syringate and syringaldehyde). The engineered strain efficiently consumed syringate with other types of LDCs, such as p-coumarate and ferulate, and produced PDC up to 67.2 mM from mixed model lignin with a molar yield of 98 %. The efficient electrolyzer degraded practical lignin into the S-unit-dominant mixture of LDCs with remarkable performance. In addition, P. putida KT-PDCV directly utilized the mixture of LDCs without significant susceptibility to impurities, yielding a PDC of 0.91 mM with a molar yield of 62.3 %.

Metabolic engineering of Escherichia coli for high-level production of the biodegradable polyester monomer 2-pyrone-4,6-dicarboxylic acid

2-Pyrone-4,6-dicarboxylic acid (PDC), a chemically stable pseudo-aromatic dicarboxylic acid, is a promising building block compound for manufacturing biodegradable polyesters. This study aimed to construct high-performance cell factories enabling the efficient production of PDC from glucose. Firstly, the effective enzymes of the PDC biosynthetic pathway were overexpressed on the chromosome of the 3-dehydroshikimate overproducing strain. Consequently, the one-step biosynthesis of PDC from glucose was achieved. Further, the PDC production was enhanced by multi-copy integration of the key gene PsligC encoding 4-carboxy-2-hydroxymuconate-6-semialdehyde dehydrogenase and co-expression of Vitreoscilla hemoglobin. Subsequently, the PDC production was substantially improved by redistributing the metabolic flux for cell growth and PDC biosynthesis based on dynamically downregulating the expression of pyruvate kinase. The resultant strain PDC50 produced 129.37 g/L PDC from glucose within 78 h under fed-batch fermentation conditions, with a yield of 0.528 mol/mol and an average productivity of 1.65 g/L/h. The findings of this study lay the foundation for the potential industrial production of PDC.

Lignin conversion to β-ketoadipic acid by Pseudomonas putida via metabolic engineering and bioprocess development

Bioconversion of a heterogeneous mixture of lignin-related aromatic compounds (LRCs) to a single product via microbial biocatalysts is a promising approach to valorize lignin. Here, Pseudomonas putida KT2440 was engineered to convert mixed p-coumaroyl- and coniferyl-type LRCs to β-ketoadipic acid, a precursor for performance-advantaged polymers. Expression of enzymes mediating aromatic O-demethylation, hydroxylation, and ring-opening steps was tuned, and a global regulator was deleted. β-ketoadipate titers of 44.5 and 25 grams per liter and productivities of 1.15 and 0.66 grams per liter per hour were achieved from model LRCs and corn stover-derived LRCs, respectively, the latter representing an overall yield of 0.10 grams per gram corn stover-derived lignin. Technoeconomic analysis of the bioprocess and downstream processing predicted a β-ketoadipate minimum selling price of $2.01 per kilogram, which is cost competitive with fossil carbon-derived adipic acid ($1.10 to 1.80 per kilogram). Overall, this work achieved bioproduction metrics with economic relevance for conversion of lignin-derived streams into a performance-advantaged bioproduct.

Advances in microbial pretreatment for biorefining of perennial grasses

Perennial grasses are potentially abundant sources of biomass for biorefineries, which can produce high yields with low input requirements, and many added environmental benefits. However, perennial grasses are highly recalcitrant to biodegradation and may require pretreatment before undergoing many biorefining pathways. Microbial pretreatment uses the ability of microorganisms or their enzymes to deconstruct plant biomass and enhance its biodegradability. This process can enhance the enzymatic digestibility of perennial grasses, enabling saccharification with cellulolytic enzymes to produce fermentable sugars and derived fermentation products. Similarly, microbial pretreatment can increase the methanation rate when the grasses are used to produce biogas through anaerobic digestion. Microorganisms can also increase the digestibility of the grasses to improve their quality as animal feed, enhance the properties of grass pellets, and improve biomass thermochemical conversion. Metabolites produced by fungi or bacteria during microbial pretreatment, such as ligninolytic and cellulolytic enzymes, can be further recovered as added-value products. Additionally, the action of the microorganisms can release chemicals with commercialization potential, such as hydroxycinnamic acids and oligosaccharides, from the grasses. This review explores the recent advances and remaining challenges in using microbial pretreatment for perennial grasses with the goal of obtaining added-value products through biorefining. It emphasizes recent trends in microbial pretreatment such as the use of microorganisms as part of microbial consortia or in unsterilized systems, the use and development of microorganisms and consortia capable of performing more than one biorefining step, and the use of cell-free systems based on microbial enzymes. KEY POINTS: • Microorganisms or enzymes can reduce the recalcitrance of grasses for biorefining • Microbial pretreatment effectiveness depends on the grass-microbe interaction • Microbial pretreatment can generate value added co-products to enhance feasibility.

High-level production of 2-pyrone-4,6-dicarboxylic acid from vanillic acid as a lignin-related aromatic compound by metabolically engineered fermentation to realize industrial valorization processes of lignin

2-Pyrone-4,6-dicarboxylic acid (PDC) is a valuable building block molecule produced from lignin-derived aromatic compounds by biological funneling. This study aimed to design a fermentation process for producing PDC from vanillic acid, which could be applied at an industrial production. Metabolomic analysis revealed that a high primary metabolic activity within cells was required to improve the production efficiency. Moreover, a medium with ammonium salts and no alkali metals was advantageous because it suppressed the formation of PDC-metal complexes. Resulting optimized process yielded the highest PDC titer and productivity ever reported: 99.9 g/L and 1.69 g/L/h, respectively. Per batch, 190 g of PDC was produced per liter of initial culture media, and the final liquid volume was 1.9 L. This study demonstrates the design of fermentation processes for the advanced industrial utilization of lignin by biological funneling.

Multi-step biosynthesis of the biodegradable polyester monomer 2-pyrone-4,6-dicarboxylic acid from glucose

BACKGROUND

2-Pyrone-4,6-dicarboxylic acid (PDC), a chemically stable pseudoaromatic dicarboxylic acid, represents a promising building block for the manufacture of biodegradable polyesters. Microbial production of PDC has been extensively investigated, but low titers and yields have limited industrial applications.

RESULTS

In this study, a multi-step biosynthesis strategy for the microbial production of PDC was demonstrated using engineered Escherichia coli whole-cell biocatalysts. The PDC biosynthetic pathway was first divided into three synthetic modules, namely the 3-dehydroshikimic acid (DHS) module, the protocatechuic acid (PCA) module and the PDC module. Several effective enzymes, including 3-dehydroshikimate dehydratase for the PCA module as well as protocatechuate 4,5-dioxygenase and 4-carboxy-2-hydroxymuconate-6-semialdehyde dehydrogenase for the PDC module were isolated and characterized. Then, the highly efficient whole-cell bioconversion systems for producing PCA and PDC were constructed and optimized, respectively. Finally, the efficient multi-step biosynthesis of PDC from glucose was achieved by smoothly integrating the above three biosynthetic modules, resulting in a final titer of 49.18 g/L with an overall 27.2% molar yield, which represented the highest titer for PDC production from glucose reported to date.

CONCLUSIONS

This study lays the foundation for the microbial production of PDC, including one-step de novo biosynthesis from glucose as well as the microbial transformation of monoaromatics.

Customized valorization of waste streams by Pseudomonas putida: State-of-the-art, challenges, and future trends

Preventing catastrophic climate events warrants prompt action to delay global warming, which threatens health and food security. In this context, waste management using engineered microbes has emerged as a long-term eco-friendly solution for addressing the global climate crisis and transitioning to clean energy. Notably, Pseudomonas putida can valorize industry-derived synthetic wastes including plastics, oils, food, and agricultural waste into products of interest, and it has been extensively explored for establishing a fully circular bioeconomy through the conversion of waste into bio-based products, including platform chemicals (e.g., cis,cis-muconic and adipic acid) and biopolymers (e.g., medium-chain length polyhydroxyalkanoate). However, the efficiency of waste pretreatment technologies, capability of microbial cell factories, and practicability of synthetic biology tools remain low, posing a challenge to the industrial application of P. putida. The present review discusses the state-of-the-art, challenges, and future prospects for divergent biosynthesis of versatile products from waste-derived feedstocks using P. putida.

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.