Toward biotechnological production of adipic acid and precursors from biorenewables

Computation-guided transcription factor biosensor specificity engineering for adipic acid detection

Transcription factor (TF)-based biosensors that connect small-molecule sensing with readouts such as fluorescence have proven to be useful synthetic biology tools for applications in biotechnology. However, the development of specific TF-based biosensors is hindered by the limited repertoire of TFs specific for molecules of interest since current construction methods rely on a limited set of characterized TFs. In this study, we present an approach for engineering the specificity of TFs through a computation-based workflow using molecular docking that enables targeted alteration of TF ligand specificity. Using this method, we engineer the LysR family BenM TF to alter its specificity from its cognate ligand cis,cis-muconic acid to adipic acid through a single amino acid substitution identified by our computational workflow. When implemented in a cell-free system, the engineered biosensor shows higher ligand sensitivity, expanding the potential applications of this circuit. We further investigate ligand binding through molecular dynamics to analyze the substitution, elucidating the impact of modulating a single amino acid position on the mechanism of BenM ligand binding. This study represents the first application of biomolecular modeling methods for altering BenM specificity and for gaining insights into how mutations influence the structural dynamics of BenM. Such methods can potentially be applied to other TFs to alter specificity and analyze the dynamics responsible for these changes, highlighting the applicability of computational tools for informing experiments. In addition, our developed adipic acid biosensor can be applied for the identification and engineering of enzymes to produce adipic acid.

Medium-chain dicarboxylic acids: chemistry, pharmacological properties, and applications in modern pharmaceutical and cosmetics industries

Succinic (SUA), glutaric (GLA), pimelic (PA), suberic (SUBA), adipic (ADA), azelaic (AZA), and sebacic acids (SA) make up the majority of medium-chain dicarboxylic acids (MCDAs) with chain lengths of C4-C10, and are widely utilised in the chemical, food, textile, pesticide, pharmaceutical, and liquid crystal sectors. The MCDAs' two carboxyl groups provide them with an incredibly broad variety of applications. The focus of significant scientific research now is on the increasingly varied pharmacological effects of MCDAs. However, only a few studies have compared the biological characteristics of MCDAs in the contemporary pharmaceutical and cosmetic sectors and thoroughly examined the most recent research and marketing initiatives for MCDAs. This review's objective is to offer a thorough analysis of academic works on MCDAs, to assess the usefulness of these substances' chemical-pharmacological properties for use in the contemporary pharmaceutical and cosmetic industries, and to investigate the direction of their possible applications in these two disciplines. In addition, this review investigates how these compounds are metabolised in the human body.

Functional production and biochemical investigation of an integral membrane enzyme for olefin biosynthesis

Integral membrane enzymes play essential roles in a plethora of biochemical processes. The fatty acid desaturases (FADS)-like superfamily is an important group of integral membrane enzymes that catalyze a wide array of reactions, including hydroxylation, desaturation, and cyclization; however, due to the membrane-bound nature, the majority of these enzymes have remained poorly understood. UndB is a member of the FADS-like superfamily, which catalyzes fatty acid decarboxylation, a chemically challenging reaction at the membrane interface. UndB reaction produces terminal olefins that are prominent biofuel candidates and building blocks of polymers with widespread industrial applications. Despite the great importance of UndB for several biotechnological applications, the enzyme has eluded comprehensive investigation. Here, we report details of the expression, solubilization, and purification of several constructs of UndB to achieve the optimally functional enzyme. We gained important insights into the biochemical, biophysical, and catalytic properties of UndB, including the thermal stability and factors influencing the enzyme activity. Additionally, we established the ability and kinetics of UndB to produce dienes by performing di-decarboxylation of diacids. We found that the reaction proceeds by forming a mono-carboxylic acid intermediate. Our findings shed light on the unexplored biochemical properties of the UndB and extend opportunities for its rigorous mechanistic and structural characterization.

Engineering of Yarrowia lipolytica as a platform strain for producing adipic acid from renewable resource

There is an increasing demand for bio-based dicarboxylic acids (DCA) as an eco-friendly alternatives to chemically synthesized DCA. Adipic acid, which is not naturally produced by microorganisms, is an essential DCA with significant industrial importance. This study aimed to develop a platform strain using Yarrowia lipolytica for efficient bioconversion of renewable resources into adipic acid. To prevent the complete oxidation of adipic acid, peroxisomal β-oxidation was engineered by selectively disrupting acyl-CoA oxidases. Furthermore, ω-oxidation activity was improved via introducing an additional copy of cytochrome P450 monooxygenase (ALK5) and reductase (CPR1) with fatty alcohol oxidase (FAO1). The production phase used SP92D medium in a two-stage bioconversion process, during which the engineered strain exhibited the highest production level, achieving a remarkable 9.7-fold increase compared to that of the parental strain. To our knowledge, this is the first report demonstrating that engineered Y. lipolytica can produce adipic acid from fatty acid methyl esters.

The Relationships between the Structure and Properties of PA56 and PA66 and Their Fibers

Bio-based polymers can reduce dependence on nonrenewable petrochemical resources and will be beneficial for future sustainable developments due to their low carbon footprint. In this work, the feasibility of bio-based polyamide 56 (PA56) substituting petroleum-based PA66 is systematically investigated. The crystallization, melting, and decomposition temperature of PA56 were all lower than that of PA66. PA56 formed a γ crystal type with larger grain size and took a longer amount of time to complete the crystallization process since its crystallization rate was lower than that of PA66. Compared with PA66, PA56 exhibited a higher tensile strength of 71.3 ± 1.9 MPa and specific strength of 64.8 ± 2.0 MPa but lower notched impact strength. More importantly, the limited oxygen index and vertical combustion measurement results indicated that the flame retardancy of PA56 was better than PA66, and the LOI values and the UL94 result of PA56 were 27.6% ± 0.9% and V-2. It is worth noting that the PA56 fiber had superior biodegradability compared to the PA66 fiber. PA56 showed significant biodegradation from the eighth week, whereas PA66 remained clean until the sixteenth week (without obvious biodegradation taking place). Eventually, PA56 did not show significant differences compared to PA66 in terms of thermal and mechanical properties. However, PA56 had great advantages in flame retardancy and biodegradability, indicating that the bio-based PA56 could potentially replace petroleum-based PA66 in many fields.

Improved production of adipic acid from a high loading of corn stover via an efficient and mild combination pretreatment

Adipic acid is one kind of important organic dibasic acid, which has crucial role in manufacturing plastics, lubricants, resins, fibers, etc. Using lignocellulose as feedstock for producing adipic acid can reduce production cost and improve bioresource utilization. After pretreated in the mixture of 7 wt% NaOH and 8 wt% ChCl-PEG10000 at 25 ^oC for 10 min, the surface of corn stover became loose and rough. The specific surface area was increased after the removal of lignin. A high loading of pretreated corn stover was enzymatically hydrolyzed by cellulase (20 FPU/g substrate) and xylanase (15 U/g substrate), and the yield of reducing sugars was as high as 75%. Biomass-hydrolysates obtained by enzymatic hydrolysis were efficiently fermented to produce adipic acid, and the yield was 0.45 g adipic acid per g reducing sugar. A sustainable approach for manufacturing adipic acid from lignocellulose via a room temperature pretreatment has great potential in future.

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.

Polymers without Petrochemicals: Sustainable Routes to Conventional Monomers

Access to a wide range of plastic materials has been rationalized by the increased demand from growing populations and the development of high-throughput production systems. Plastic materials at low costs with reliable properties have been utilized in many everyday products. Multibillion-dollar companies are established around these plastic materials, and each polymer takes years to optimize, secure intellectual property, comply with the regulatory bodies such as the Registration, Evaluation, Authorisation and Restriction of Chemicals and the Environmental Protection Agency and develop consumer confidence. Therefore, developing a fully sustainable new plastic material with even a slightly different chemical structure is a costly and long process. Hence, the production of the common plastic materials with exactly the same chemical structures that does not require any new registration processes better reflects the reality of how to address the critical future of sustainable plastics. In this review, we have highlighted the very recent examples on the synthesis of common monomers using chemicals from sustainable feedstocks that can be used as a like-for-like substitute to prepare conventional petrochemical-free thermoplastics.

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.

Antimicrobial Resistance and Recent Alternatives to Antibiotics for the Control of Bacterial Pathogens with an Emphasis on Foodborne Pathogens

Antimicrobial resistance (AMR) is one of the most important global public health problems. The imprudent use of antibiotics in humans and animals has resulted in the emergence of antibiotic-resistant bacteria. The dissemination of these strains and their resistant determinants could endanger antibiotic efficacy. Therefore, there is an urgent need to identify and develop novel strategies to combat antibiotic resistance. This review provides insights into the evolution and the mechanisms of AMR. Additionally, it discusses alternative approaches that might be used to control AMR, including probiotics, prebiotics, antimicrobial peptides, small molecules, organic acids, essential oils, bacteriophage, fecal transplants, and nanoparticles.

Recent advances in microbial production of diamines, aminocarboxylic acids, and diacids as potential platform chemicals and bio-based polyamides monomers

Recently, bio-based manufacturing processes of value-added platform chemicals and polymers in biorefineries using renewable resources have extensively been developed for sustainable and carbon dioxide (CO₂) neutral-based industry. Among them, bio-based diamines, aminocarboxylic acids, and diacids have been used as monomers for the synthesis of polyamides having different carbon numbers and ubiquitous and versatile industrial polymers and also as precursors for further chemical and biological processes to afford valuable chemicals. Until now, these platform bio-chemicals have successfully been produced by biorefinery processes employing enzymes and/or microbial host strains as main catalysts. In this review, we discuss recent advances in bio-based production of diamines, aminocarboxylic acids, and diacids, which has been developed and improved by systems metabolic engineering strategies of microbial consortia and optimization of microbial conversion processes including whole cell bioconversion and direct fermentative production.

Recent advances and perspectives on production of value-added organic acids through metabolic engineering

Organic acids are important consumable materials with a wide range of applications in the food, biopolymer and chemical industries. The global consumer organic acids market is estimated to increase to $36.86 billion by 2026. Conventionally, organic acids are produced from the chemical catalysis process with petrochemicals as raw materials, which posts severe environmental concerns and conflicts with our sustainable development goals. Most of the commonly used organic acids can be produced from various organisms. As a state-of-the-art technology, large-scale fermentative production of important organic acids with genetically-modified microbes has become an alternative to the chemical route to meet the market demand. Despite the fact that bio-based organic acid production from renewable cheap feedstock provides a viable solution, low productivity has impeded their industrial-scale application. With our deeper understanding of strain genetics, physiology and the availability of strain engineering tools, new technologies including synthetic biology, various metabolic engineering strategies, omics-based system biology tools, and high throughput screening methods are gradually established to bridge our knowledge gap. And they were further applied to modify the cellular reaction networks of potential microbial hosts and improve the strain performance, which facilitated the commercialization of consumable organic acids. Here we present the recent advances of metabolic engineering strategies to improve the production of important organic acids including fumaric acid, citric acid, itaconic acid, adipic acid, muconic acid, and we also discuss the current challenges and future perspectives on how we can develop a cost-efficient, green and sustainable process to produce these important chemicals from low-cost feedstocks.

A facile process for adipic acid production in high yield by oxidation of 1,6-hexanediol using the resting cells of Gluconobacter oxydans

Background

Adipic acid (AA) is one of the most important industrial chemicals used mainly for the production of Nylon 6,6 but also for making polyurethanes, plasticizers, and unsaturated polyester resins, and more recently as a component in the biodegradable polyester poly(butylene adipate terephthalate) (PBAT). The main route for AA production utilizes benzene as feedstock and generates copious amounts of the greenhouse gas NO₂. Hence, alternative clean production routes for AA from renewable bio-based feedstock are drawing increasing attention. We have earlier reported the potential of Gluconobacter oxydans cells to oxidize 1,6-hexanediol, a potentially biobased diol to AA.

Results

The present report involves a study on the effect of different parameters on the microbial transformation of 1,6-hexanediol to adipic acid, and subsequently testing the process on a larger lab scale for achieving maximal conversion and yield. Comparison of three wild-type strains of G. oxydans DSM50049, DSM2003, and DSM2343 for the whole-cell biotransformation of 10 g/L 1,6-hexanediol to adipic acid in batch mode at pH 7 and 30 °C led to the selection of G. oxydans DSM50049, which showed 100% conversion of the substrate with over 99% yield of adipic acid in 30 h. An increase in the concentrations of the substrate decreased the degree of conversion, while the product up to 25 g/L in batch and 40 g/L in fed-batch showed no inhibition on the conversion. Moreover, controlling the pH of the reaction at 5–5.5 was required for the cascade oxidation reactions to work. Cell recycling for the biotransformation resulted in a significant decrease in activity during the third cycle. Meanwhile, the fed-batch mode of transformation by intermittent addition of 1,6-hexanediol (30 g in total) in 1 L scale resulted in complete conversion with over 99% yield of adipic acid (approximately 37 g/L). The product was recovered in a pure form using downstream steps without the use of any solvent.

Conclusion

A facile, efficient microbial process for oxidation of 1,6-hexanediol to adipic acid, having potential for scale up was demonstrated. The entire process is performed in aqueous medium at ambient temperatures with minimal greenhouse gas emissions. The enzymes involved in catalyzing the oxidation steps are currently being identified.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01947-6.

Extraction, Isolation, and Purification of Value-Added Chemicals from Lignocellulosic Biomass

This review covers the operating conditions for extracting top value-added chemicals, such as levulinic acid, lactic acid, succinic acid, vanillic acid, 3-hydroxypropionic acid, xylitol, 2,5-furandicarboxylic acid, 5-hydroxymethyl furfural, chitosan, 2,3-butanediol, and xylo-oligosaccharides, from common lignocellulosic biomass. Operating principles of novel extraction methods, beyond pretreatments, such as Soxhlet extraction, ultrasound-assisted extraction, and enzymatic extraction, are also presented and reviewed. Post extraction, high-value biochemicals need to be isolated, which is achieved through a combination of one or more isolation and purification steps. The operating principles, as well as a review of isolation methods, such as membrane filtration and liquid–liquid extraction and purification using preparative chromatography, are also discussed.

Research Progress on the Construction of Artificial Pathways for the Biosynthesis of Adipic Acid by Engineered Microbes

Adipic acid is an important bulk chemical used in the nylon industry, as well as in food, plasticizers and pharmaceutical fields. It is thus considered one of the most important 12 platform chemicals. The current production of adipic acid relies on non-renewable petrochemical resources and emits large amounts of greenhouse gases. The bio-production of adipic acid from renewable resources via engineered microorganisms is regarded as a green and potential method to replace chemical conversion, and has attracted attention all over the world. Herein we review the current status of research on several artificial pathways for the biosynthesis of adipic acid, especially the reverse degradation pathway, which is a full biosynthetic method and has achieved the highest titer of adipic acid so far. Other artificial pathways including the fatty acid degradation pathway, the muconic acid conversion pathway, the polyketide pathway, the α-ketopimelate pathway and the lysine degradation pathway are also discussed. In addition, the challenges in the bio-production of adipic acid via these artificial pathways are analyzed and the prospects are presented with the intention of providing some significant points for the promotion of adipic acid biosynthesis.

Directed Evolution of (R)-2-Hydroxyglutarate Dehydrogenase Improves 2-Oxoadipate Reduction by 2 Orders of Magnitude

Pathway engineering is commonly employed to improve the production of various metabolites but may incur in bottlenecks due to the low catalytic activity of a particular reaction step. The reduction of 2-oxoadipate to (R)-2-hydroxyadipate is a key reaction in metabolic pathways that exploit 2-oxoadipate conversion via α-reduction to produce adipic acid, an industrially important platform chemical. Here, we engineered (R)-2-hydroxyglutarate dehydrogenase from Acidaminococcus fermentans (Hgdh) with the aim of improving 2-oxoadipate reduction. Using a combination of computational analysis, saturation mutagenesis, and random mutagenesis, three mutant variants with a 100-fold higher catalytic efficiency were obtained. As revealed by rational analysis of the mutations found in the variants, this improvement could be ascribed to a general synergistic effect where mutation A206V played a key role since it boosted the enzyme’s activity by 4.8-fold. The Hgdh variants with increased activity toward 2-oxoadipate generated within this study pave the way for the bio-based production of adipic acid.

The metabolic potential of plastics as biotechnological carbon sources - Review and targets for the future

The plastic crisis requires drastic measures, especially for the plastics' end-of-life. Mixed plastic fractions are currently difficult to recycle, but microbial metabolism might open new pathways. With new technologies for degradation of plastics to oligo- and monomers, these carbon sources can be used in biotechnology for the upcycling of plastic waste to valuable products, such as bioplastics and biosurfactants. We briefly summarize well-known monomer degradation pathways and computed their theoretical yields for industrially interesting products. With this information in hand, we calculated replacement scenarios of existing fossil-based synthesis routes for the same products. Thereby, we highlight fossil-based products for which plastic monomers might be attractive alternative carbon sources. Notably, not the highest yield of product on substrate of the biochemical route, but rather the (in-)efficiency of the petrochemical routes (i.e., carbon, energy use) determines the potential of biochemical plastic upcycling. Our results might serve as a guide for future metabolic engineering efforts towards a sustainable plastic economy.

Guiding stars to the field of dreams: Metabolically engineered pathways and microbial platforms for a sustainable lignin-based industry

Lignin is an important structural component of terrestrial plants and is readily generated during biomass fractionation in lignocellulose processing facilities. Due to lacking alternatives the majority of technical lignins is industrially simply burned into heat and energy. However, regarding its vast abundance and a chemically interesting richness in aromatics, lignin is presently regarded as the most under-utilized and promising feedstock for value-added applications. Notably, microbes have evolved powerful enzymes and pathways that break down lignin and metabolize its various aromatic components. This natural pathway atlas meanwhile serves as a guiding star for metabolic engineers to breed designed cell factories and efficiently upgrade this global waste stream. The metabolism of aromatic compounds, in combination with success stories from systems metabolic engineering, as reviewed here, promises a sustainable product portfolio from lignin, comprising bulk and specialty chemicals, biomaterials, and fuels.

Synthesis, Properties, and Enzymatic Hydrolysis of Poly(lactic acid)-co-Poly(propylene adipate) Block Copolymers Prepared by Reactive Extrusion

Poly(lactic acid) (PLA) is a biobased polyester with ever-growing applications in the fields of packaging and medicine. Despite its popularity, it suffers from inherent brittleness, a very slow degradation rate and a high production cost. To tune the properties of PLA, block copolymers with poly(propylene adipate) (PPAd) prepolymer were prepared by polymerizing L-lactide and PPAd oligomers via reactive extrusion (REX) in a torque rheometer. The effect of reaction temperature and composition on the molecular weight, chemical structure, and physicochemical properties of the copolymers was studied. The introduction of PPAd successfully increased the elongation and the biodegradation rate of PLA. REX is an efficient and economical alternative method for the fast and continuous synthesis of PLA-based copolymers with tunable properties.

Combinatorial pathway balancing provides biosynthetic access to 2-fluoro-cis,cis-muconate in engineered Pseudomonas putida

The wealth of bio-based building blocks produced by engineered microorganisms seldom include halogen atoms. Muconate is a platform chemical with a number of industrial applications that could be broadened by introducing fluorine atoms to tune its physicochemical properties. The soil bacterium Pseudomonas putida naturally assimilates benzoate via the ortho-cleavage pathway with cis,cis-muconate as intermediate. Here, we harnessed the native enzymatic machinery (encoded within the ben and cat gene clusters) to provide catalytic access to 2-fluoro-cis,cis-muconate (2-FMA) from fluorinated benzoates. The reactions in this pathway are highly imbalanced, leading to accumulation of toxic intermediates and limited substrate conversion. By disentangling regulatory patterns of ben and cat in response to fluorinated effectors, metabolic activities were adjusted to favor 2-FMA biosynthesis. After implementing this combinatorial approach, engineered P. putida converted 3-fluorobenzoate to 2-FMA at the maximum theoretical yield. Hence, this study illustrates how synthetic biology can expand the diversity of nature's biochemical catalysis.

Current Progress in Production of Building-Block Organic Acids by Consolidated Bioprocessing of Lignocellulose

Several organic acids have been indicated among the top value chemicals from biomass. Lignocellulose is among the most attractive feedstocks for biorefining processes owing to its high abundance and low cost. However, its highly complex nature and recalcitrance to biodegradation hinder development of cost-competitive fermentation processes. Here, current progress in development of single-pot fermentation (i.e., consolidated bioprocessing, CBP) of lignocellulosic biomass to high value organic acids will be examined, based on the potential of this approach to dramatically reduce process costs. Different strategies for CBP development will be considered such as: (i) design of microbial consortia consisting of (hemi)cellulolytic and valuable-compound producing strains; (ii) engineering of microorganisms that combine biomass-degrading and high-value compound-producing properties in a single strain. The present review will mainly focus on production of organic acids with application as building block chemicals (e.g., adipic, cis,cis-muconic, fumaric, itaconic, lactic, malic, and succinic acid) since polymer synthesis constitutes the largest sector in the chemical industry. Current research advances will be illustrated together with challenges and perspectives for future investigations. In addition, attention will be dedicated to development of acid tolerant microorganisms, an essential feature for improving titer and productivity of fermentative production of acids.

Exploring functionality of the reverse β-oxidation pathway in Corynebacterium glutamicum for production of adipic acid

BACKGROUND

Adipic acid, a six-carbon platform chemical mainly used in nylon production, can be produced via reverse β-oxidation in microbial systems. The advantages posed by Corynebacterium glutamicum as a model cell factory for implementing the pathway include: (1) availability of genetic tools, (2) excretion of succinate and acetate when the TCA cycle becomes overflown, (3) initiation of biosynthesis with succinyl-CoA and acetyl-CoA, and (4) established succinic acid production. Here, we implemented the reverse β-oxidation pathway in C. glutamicum and assessed its functionality for adipic acid biosynthesis.

RESULTS

To obtain a non-decarboxylative condensation product of acetyl-CoA and succinyl-CoA, and to subsequently remove CoA from the condensation product, we introduced heterologous 3-oxoadipyl-CoA thiolase and acyl-CoA thioesterase into C. glutamicum. No 3-oxoadipic acid could be detected in the cultivation broth, possibly due to its endogenous catabolism. To successfully biosynthesize and secrete 3-hydroxyadipic acid, 3-hydroxyadipyl-CoA dehydrogenase was introduced. Addition of 2,3-dehydroadipyl-CoA hydratase led to biosynthesis and excretion of trans-2-hexenedioic acid. Finally, trans-2-enoyl-CoA reductase was inserted to yield 37 µg/L of adipic acid.

CONCLUSIONS

In the present study, we engineered the reverse β-oxidation pathway in C. glutamicum and assessed its potential for producing adipic acid from glucose as starting material. The presence of adipic acid, albeit small amount, in the cultivation broth indicated that the synthetic genes were expressed and functional. Moreover, 2,3-dehydroadipyl-CoA hydratase and β-ketoadipyl-CoA thiolase were determined as potential target for further improvement of the pathway.

Binary Green Blends of Poly(lactic acid) with Poly(butylene adipate-co-butylene terephthalate) and Poly(butylene succinate-co-butylene adipate) and Their Nanocomposites

Poly(lactic acid) (PLA) is the most widely produced biobased, biodegradable and biocompatible polyester. Despite many of its properties are similar to those of common petroleum-based polymers, some drawbacks limit its utilization, especially high brittleness and low toughness. To overcome these problems and improve the ductility and the impact resistance, PLA is often blended with other biobased and biodegradable polymers. For this purpose, poly(butylene adipate-co-butylene terephthalate) (PBAT) and poly(butylene succinate-co-butylene adipate) (PBSA) are very advantageous copolymers, because their toughness and elongation at break are complementary to those of PLA. Similar to PLA, both these copolymers are biodegradable and can be produced from annual renewable resources. This literature review aims to collect results on the mechanical, thermal and morphological properties of PLA/PBAT and PLA/PBSA blends, as binary blends with and without addition of coupling agents. The effect of different compatibilizers on the PLA/PBAT and PLA/PBSA blends properties is here elucidated, to highlight how the PLA toughness and ductility can be improved and tuned by using appropriate additives. In addition, the incorporation of solid nanoparticles to the PLA/PBAT and PLA/PBSA blends is discussed in detail, to demonstrate how the nanofillers can act as morphology stabilizers, and so improve the properties of these PLA-based formulations, especially mechanical performance, thermal stability and gas/vapor barrier properties. Key points about the biodegradation of the blends and the nanocomposites are presented, together with current applications of these novel green materials.

Poly(Alkylene 2,5-Thiophenedicarboxylate) Polyesters: A New Class of Bio-Based High-Performance Polymers for Sustainable Packaging

In the present study, 100% bio-based polyesters of 2,5-thiophenedicarboxylic acid were synthesized via two-stage melt polycondensation using glycols containing 3 to 6 methylene groups. The so-prepared samples were characterised from the molecular point of view and processed into free-standing thin films. Afterward, both the purified powders and the films were subjected to structural and thermal characterisation. In the case of thin films, mechanical response and barrier properties to O₂ and CO₂ were also evaluated. From the results obtained, it emerged that the length of glycolic sub-units is an effective tool to modulate the chain mobility and, in turn, the kind and amount of ordered phases developed in the samples. In addition to the usual amorphous and 3D crystalline phases, in all the samples investigated it was possible to evidence a further phase characterised by a lower degree of order (mesophase) than the crystalline one, whose amount is strictly related to the glycol sub-unit length. The relative fraction of all these phases is responsible for the different mechanical and barrier performances. Last, but not least, a comparison between thiophene-based homopolymers and their furan-based homologues was carried out.

New Solvent and Coagulating Agent for Development of Chitosan Fibers by Wet Spinning

Adipic acid was evaluated as a novel solvent for wet spinning of chitosan fibers. A solvent with two carboxyl groups could act as a physical crosslinker between the chitosan chains, resulting in improved properties of the fibers. The performance of adipic acid was compared with conventional solvents, i.e., lactic, citric, and acetic acids. Chitosan solutions were injected into a coagulation bath to form monofilaments. Sodium hydroxide (NaOH) and its mixture with ethanol (EtOH) were used as coagulation agents. Scanning electron microscopy confirmed the formation of uniform chitosan monofilaments with an even surface when using adipic acid as solvent. These monofilaments generally showed higher mechanical strength compared to that of monofilaments produced using conventional solvents. The highest Young's modulus, 4.45 GPa, was recorded for adipic acid monofilaments coagulated in NaOH-EtOH. This monofilament also had a high tensile strength of 147.9 MPa. Furthermore, taking advantage of chitosan insolubility in sulfuric acid (H₂SO₄) at room temperature, chitosan fibers were successfully formed upon coagulation in H₂SO₄-EtOH. The dewatering of fibers using EtOH before drying resulted in a larger fiber diameter and lower mechanical strength. Adipic acid fibers made without dehydration illustrated 18% (for NaOH), 46% (for NaOH-EtOH), and 91% (for H₂SO₄-EtOH) higher tensile strength compared to those made with dehydration.

Biosynthesis of Commodity Chemicals From Oil Palm Empty Fruit Bunch Lignin

Lignin is one of the most abundant natural resources that can be exploited for the bioproduction of value-added commodity chemicals. Oil palm empty fruit bunches (OPEFBs), byproducts of palm oil production, are abundant lignocellulosic biomass but largely used for energy and regarded as waste. Pretreatment of OPEFB lignin can yield a mixture of aromatic compounds that can potentially serve as substrates to produce commercially important chemicals. However, separation of the mixture into desired individual substrates is required, which involves expensive steps that undermine the utility of OPEFB lignin. Here, we report successful engineering of microbial hosts that can directly utilize heterogeneous mixtures derived from OPEFB lignin to produce commodity chemicals, adipic acid and levulinic acid. Furthermore, the corresponding bioconversion pathway was placed under a genetic controller to autonomously activate the conversion process as the cells are fed with a depolymerized OPEFB lignin mixture. This study demonstrates a simple, one-pot biosynthesis approach that directly utilizes derivatives of agricultural waste to produce commodity chemicals.

Engineering the Reductive TCA Pathway to Dynamically Regulate the Biosynthesis of Adipic Acid in Escherichia coli

Adipic acid is a versatile aliphatic dicarboxylic acid. It is applied mainly in the polymerization of nylon-6,6, which accounts for 50.8% of the global consumption market of adipic acid. The microbial production of adipic acid avoids the usage of petroleum resources and the emission of harmful nitrogen oxides that are generated by traditional chemical synthetic approaches. However, in the fermentation process, the low theoretical yield and the usage of expensive inducers hinders the large-scale industrial production of adipic acid. To overcome these challenges, we established an oxygen-dependent dynamic regulation (ODDR) system to control the expression of key genes (sucD, pyc, mdh, and frdABCD) that could be induced to enhance the metabolic flux of the reductive TCA pathway under anaerobic conditions. Coupling of the constitutively expressed adipic acid synthetic pathway not only avoids the use of inducers but also increases the theoretical yield by nearly 50%. After the gene combination and operon structure were optimized, the reaction catalyzed by frdABCD was found to be the rate-limiting step. Further optimizing the relative expression levels of sucD, pyc, and frdABCD improved the titer of adipic acid 41.62-fold compared to the control strain Mad1415, demonstrating the superior performance of our ODDR system.

A yeast chemogenomic screen identifies pathways that modulate adipic acid toxicity

Adipic acid production by yeast fermentation is gaining attention as a renewable source of platform chemicals for making nylon products. However, adipic acid toxicity inhibits yeast growth and fermentation. Here, we performed a chemogenomic screen in Saccharomyces cerevisiae to understand the cellular basis of adipic acid toxicity. Our screen revealed that KGD1 (a key gene in the tricarboxylic acid cycle) deletion improved tolerance to adipic acid and its toxic precursor, catechol. Conversely, disrupting ergosterol biosynthesis as well as protein trafficking and vacuolar transport resulted in adipic acid hypersensitivity. Notably, we show that adipic acid disrupts the Membrane Compartment of Can1 (MCC) on the plasma membrane and impacts endocytosis. This was evidenced by the rapid internalization of Can1 for vacuolar degradation. As ergosterol is an essential component of the MCC and protein trafficking mechanisms are required for endocytosis, we highlight the importance of these cellular processes in modulating adipic acid toxicity.

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Deletion of the TCA cycle gene KGD1 improves tolerance to adipic acid and catechol

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Ergosterol and Pdr12 play non-overlapping roles protecting cell from adipic acid

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Adipic acid-induced plasma membrane localization of Pdr12 is independent of ergosterol

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Adipic acid disrupts the Membrane Compartment of Can1 (MCC) and induces endocytosis

Cell-based and cell-free biocatalysis for the production of D-glucaric acid

D-Glucaric acid (GA) is a value-added chemical produced from biomass, and has potential applications as a versatile platform chemical, food additive, metal sequestering agent, and therapeutic agent. Marketed GA is currently produced chemically, but increasing demand is driving the search for eco-friendlier and more efficient production approaches. Cell-based production of GA represents an alternative strategy for GA production. A series of synthetic pathways for GA have been ported into Escherichia coli, Saccharomyces cerevisiae and Pichia pastoris, respectively, and these engineered cells show the ability to synthesize GA de novo. Optimization of the GA metabolic pathways in host cells has leapt forward, and the titer and yield have increased rapidly. Meanwhile, cell-free multi-enzyme catalysis, in which the desired pathway is constructed in vitro from enzymes and cofactors involved in GA biosynthesis, has also realized efficient GA bioconversion. This review presents an overview of studies of the development of cell-based GA production, followed by a brief discussion of potential applications of biosensors that respond to GA in these biosynthesis routes.

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.

Fully biological production of adipic acid analogs from branched catechols

Microbial production of adipic acid from lignin-derived monomers, such as catechol, is a greener alternative to the petrochemical-based process. Here, we produced adipic acid from catechol using catechol 1,2-dioxygenase (CatA) and a muconic acid reductase (MAR) in Escherichia coli. As the reaction progressed, the pH of the media dropped from 7 to 4-5 and the muconic acid isomerized from the cis,cis (ccMA) to the cis,trans (ctMA) isomer. Feeding experiments suggested that cells preferentially uptook ctMA and that MAR efficiently reduced all muconic isomers to adipic acid. Intrigued by the substrate promiscuity of MAR, we probed its utility to produce branched chiral diacids. Using branched catechols likely found in pretreated lignin, we found that while MAR fully reduced 2-methyl-muconic acid to 2-methyl-adipic acid, MAR reduced only one double bond in 3-substituted muconic acids. In the future, MAR's substrate promiscuity could be leveraged to produce chiral-branched adipic acid analogs to generate branched, nylon-like polymers with reduced crystallinity.

Hydrogenation of Trans,Trans-Muconic Acid to Bio-Adipic Acid: Mechanism Identification and Kinetic Modelling

The hydrogenation of trans,trans-muconic acid was investigated on a Pt/C 5% (wt) catalyst in a batch slurry reactor at constant hydrogen pressure (4 bar) and temperature (323, 333 and 343 K), with the purpose of developing a kinetic model able to predict conversions and product distributions. A dual-site Langmuir–Hinshelwood–Hougen–Watson (LHHW) model with hydrogen dissociation provided good fitting of the experimental data. The model parameters were regressed by robust numerical methods to overcome the computational challenges of the model parameters’ collinearity. Different reaction mechanisms were tested; the best model involved two subsequent hydrogenation steps. The first step yielded from trans,trans-muconic acid a monounsaturated intermediate (trans-2-hexenedioic acid), which was further hydrogenated to adipic acid in the second step. The intermediate was subjected to an equilibrium isomerization with cis-2-hexenedioic acid. The activation energy values and the rate constants were calculated for the reactions, providing the first reference for trans,trans-muconic acid hydrogenation.

Structure-function investigation of 3-methylaspartate ammonia lyase reveals substrate molecular determinants for the deamination reaction

The enzymatic reactions leading to the deamination of β-lysine, lysine, or 2-aminoadipic acid are of great interest for the metabolic conversion of lysine to adipic acid. Enzymes able to carry out these reactions are not known, however ammonia lyases (EC 4.3.1.-) perform deamination on a wide range of substrates. We have studied 3-methylaspartate ammonia lyase (MAL, EC 4.3.1.2) as a potential candidate for protein engineering to enable deamination towards β-lysine, that we have shown to be a competitive inhibitor of MAL. We have characterized MAL activity, binding and inhibition properties on six different compounds that would allow to define the molecular determinants necessary for MAL to deaminate our substrate of interest. Docking calculations showed that β-lysine as well as the other compounds investigated could fit spatially into MAL catalytic pocket, although they probably are weak or very transient binders and we identified molecular determinants involved in the binding of the substrate. The hydrophobic interactions formed by the methyl group of 3-methylaspartic acid, together with the presence of the amino group on carbon 2, play an essential role in the appropriate binding of the substrate. The results showed that β-lysine is able to fit and bind in MAL catalytic pocket and can be potentially converted from inhibitor to substrate of MAL upon enzyme engineering. The characterization of the binding and inhibition properties of the substrates tested here provide the foundation for future and more extensive studies on engineering MAL that could lead to a MAL variant able to catalyse this challenging deamination reaction.

Recent Progress in Adipic Acid Synthesis Over Heterogeneous Catalysts

Adipic acid is one of the most important feedstocks for producing resins, nylons, lubricants, plasticizers. Current industrial petrochemical process, producing adipic acid from KA oil, catalyzed by nitric acid, has a serious pollution to the environment, due to the formation of waste nitrous oxide. Hence, developing cleaner methods to produce adipic acid has attracted much attention of both industry and academia. This mini-review article discussed advances on adipic acid synthesis from bio-renewable feedstocks, as well as most recent progress on cleaner technology from fossil fuels over novel catalytic materials. This work on recent advances in green adipic acid production will provide insights and guidance to further study of various other industrial processes for producing nylon precursors.

Genetic Manipulation of a Lipolytic Yeast Candida aaseri SH14 Using CRISPR-Cas9 System

A lipolytic yeast Candida aaseri SH14 that can utilise long-chain fatty acids as the sole carbon source was isolated from oil palm compost. To develop this strain as a platform yeast for the production of bio-based chemicals from renewable plant oils, a genetic manipulation system using CRISPR-Cas9 was developed. Episomal vectors for expression of Cas9 and sgRNA were constructed using an autonomously replicating sequence isolated from C. aaseri SH14. This system guaranteed temporal expression of Cas9 for genetic manipulation and rapid curing of the vector from transformed strains. A β-oxidation mutant was directly constructed by simultaneous disruption of six copies of acyl-CoA oxidases genes (AOX2, AOX4 and AOX5) in diploid cells using a single sgRNA with 70% efficiency and the Cas9 vector was efficiently removed. Blocking of β-oxidation in the triple AOX mutant was confirmed by the accumulation of dodecanedioic acid from dodecane. Targeted integration of the expression cassette for C. aaseri lipase2 was demonstrated with 60% efficiency using this CRISPR-Cas9 system. This genome engineering tool could accelerate industrial application of C. aaseri SH14 for production of bio-based chemicals from renewable oils.

Effect of Carbon Support, Capping Agent Amount, and Pd NPs Size for Bio-Adipic Acid Production from Muconic Acid and Sodium Muconate

The effect of support, stabilizing agent, and Pd nanoparticles (NPs) size was studied for sodium muconate and t,t-muconic acid hydrogenation to bio-adipic acid. Three different activated carbons (AC) were used (Norit, KB, and G60) and carbon morphology did not affect the substrate conversion, but it greatly influenced the adipic acid yield. 1% Pd/KB Darco catalyst, which has the highest surface area and Pd surface exposure, and the smallest NPs size displayed the highest activity. Furthermore, the effect of the amount of the protective agent was studied varying metal/protective agent weight ratios in the range of 1/0.00–1/1.20, using KB as the chosen support. For sodium muconate reduction 1% Pd/KB_1.2 catalyst gave the best results in terms of activity (0.73 s⁻¹), conversion, and adipic acid yield (94.8%), while for t,t-muconic acid hydrogenation the best activity result (0.85 s⁻¹) was obtained with 1% Pd/KB_0.0 catalyst. Correlating the results obtained from XPS and TEM analyses with catalytic results, we found that the amount of PVA (polyvinyl alcohol) influences mean Pd NPs size, Pd(0)/Pd(II) ratio, and Pd surface exposure. Pd(0)/Pd(II) ratio and Pd NPs size affected adipic acid yield and activity during sodium muconate hydrogenation, respectively, while adipic acid yield was related by exposed Pd amount during t,t-muconic acid hydrogenation. The synthesized catalysts showed higher activity than commercial 5% Pd/AC.

One-Pot Biocatalytic Transformation of Adipic Acid to 6-Aminocaproic Acid and 1,6-Hexamethylenediamine Using Carboxylic Acid Reductases and Transaminases

Production of platform chemicals from renewable feedstocks is becoming increasingly important due to concerns on environmental contamination, climate change, and depletion of fossil fuels. Adipic acid (AA), 6-aminocaproic acid (6-ACA) and 1,6-hexamethylenediamine (HMD) are key precursors for nylon synthesis, which are currently produced primarily from petroleum-based feedstocks. In recent years, the biosynthesis of adipic acid from renewable feedstocks has been demonstrated using both bacterial and yeast cells. Here we report the biocatalytic conversion/transformation of AA to 6-ACA and HMD by carboxylic acid reductases (CARs) and transaminases (TAs), which involves two rounds (cascades) of reduction/amination reactions (AA → 6-ACA → HMD). Using purified wild type CARs and TAs supplemented with cofactor regenerating systems for ATP, NADPH, and amine donor, we established a one-pot enzyme cascade catalyzing up to 95% conversion of AA to 6-ACA. To increase the cascade activity for the transformation of 6-ACA to HMD, we determined the crystal structure of the CAR substrate-binding domain in complex with AMP and succinate and engineered three mutant CARs with enhanced activity against 6-ACA. In combination with TAs, the CAR L342E protein showed 50-75% conversion of 6-ACA to HMD. For the transformation of AA to HMD (via 6-ACA), the wild type CAR was combined with the L342E variant and two different TAs resulting in up to 30% conversion to HMD and 70% to 6-ACA. Our results highlight the suitability of CARs and TAs for several rounds of reduction/amination reactions in one-pot cascade systems and their potential for the biobased synthesis of terminal amines.

Advances in microbial production of medium-chain dicarboxylic acids for nylon materials

Medium-chain dicarboxylic acids (MDCAs) are widely used in the production of nylon materials, and among which, succinic, glutaric, adipic, pimelic, suberic, azelaic and sebacic acids are particularly important for that purpose.

Organic acids as antimicrobial food agents: applications and microbial productions

Food safety is a global health and socioeconomic concern since many people still suffer from various acute and life-long diseases, which are caused by consumption of unsafe food. Therefore, ensuring safety of the food is one of the most essential issues in the food industry, which needs to be considered during not only food composition formulation but also handling and storage. For safety purpose, various chemical preservatives have been used so far in the foods. Recently, there has been renewed interest in replacing chemically originated food safety compounds with natural ones in the industry, which can also serve as antimicrobial agents. Among these natural compounds, organic acids possess the major portion. Therefore, in this paper, it is aimed to review and compile the applications, effectiveness, and microbial productions of various widely used organic acids as antimicrobial agents in the food industry.

Phosphorus-Containing Flame Retardants from Biobased Chemicals and Their Application in Polyesters and Epoxy Resins

Phosphorus-containing flame retardants synthesized from renewable resources have had a lot of impact in recent years. This article outlines the synthesis, characterization and evaluation of these compounds in polyesters and epoxy resins. The different approaches used in producing biobased flame retardant polyesters and epoxy resins are reported. While for the polyesters biomass derived compounds usually are phosphorylated and melt blended with the polymer, biobased flame retardants for epoxy resins are directly incorporated into the polymer structure by a using a phosphorylated biobased monomer or curing agent. Evaluating the efficiency of the flame retardant composites is done by discussing results obtained from UL94 vertical burning, limiting oxygen index (LOI) and cone calorimetry tests. The review ends with an outlook on future development trends of biobased flame retardant systems for polyesters and epoxy resins.