Production of C2-C4 diols from renewable bioresources: new metabolic pathways and metabolic engineering strategies

A metabolomics footprinting approach using GC-MS to study inhibitory effects of the fungal metabolite diplopyrone C against nosocomial pathogen biofilms

Seen initially as wonder drugs, the widespread and often inappropriate use of antibiotics led to the development of microbial resistances. As a result, a true emergency has arisen, and a significant need has emerged to discover and develop new safe and valuable antibiotics. The captivating chemical structure of the fungal metabolite diplopyrone C has caught our attention as an excellent candidate for a circumstantial study aimed at revealing its antimicrobial and antibiofilm activities. In this work, we describe the full analytical strategy from the isolation/identification to the evaluation of the metabolomics effect on target microorganisms of this fungal metabolite. Our results show interesting antimicrobial and antibiofilm activities of diplopyrone C against two frequently isolated nosocomial pathogens (i.e., the fungus Candida albicans and the gram-negative bacterium Klebsiella pneumoniae). Moreover, a GC-MS based metabolomics footprinting approach gave an insight into the uptake and excretion of metabolites from and into the culture medium as a response to the presence of this active substance. The workflow employed in this study is suitable to exploit natural resources for the search of lead compounds for drug development.

From waste management to circular economy: Leveraging thermophiles for sustainable growth and global resource optimization

Waste of any origin is one of the most serious global and man-made concerns of our day. It causes climate change, environmental degradation, and human health problems. Proper waste management practices, including waste reduction, safe handling, and appropriate treatment, are essential to mitigate these consequences. It is thus essential to implement effective waste management strategies that reduce waste at the source, promote recycling and reuse, and safely dispose of waste. Transitioning to a circular economy with policies involving governments, industries, and individuals is essential for sustainable growth and waste management. The review focuses on diverse kinds of environmental waste sources around the world, such as residential, industrial, commercial, municipal services, electronic wastes, wastewater sewerage, and agricultural wastes, and their challenges in efficiently valorizing them into useful products. It highlights the need for rational waste management, circularity, and sustainable growth, and the potential of a circular economy to address these challenges. The article has explored the role of thermophilic microbes in the bioremediation of waste. Thermophiles known for their thermostability and thermostable enzymes, have emerged to have diverse applications in biotechnology and various industrial processes. Several approaches have been explored to unlock the potential of thermophiles in achieving the objective of establishing a zero-carbon sustainable bio-economy and minimizing waste generation. Various thermophiles have demonstrated substantial potential in addressing different waste challenges. The review findings affirm that thermophilic microbes have emerged as pivotal and indispensable candidates for harnessing and valorizing a range of environmental wastes into valuable products, thereby fostering the bio-circular economy.

Metabolic Engineering of Escherichia coli for De Novo Production of 1,2-Butanediol

1,2-Butanediol (1,2-BDO) is an important platform chemical widely utilized in the synthesis of polyester polyols, plasticizers, cosmetics, and pharmaceuticals. However, no natural metabolic pathway for its biosynthesis has been identified, and biological production of 1,2-BDO from renewable bioresources has not been reported so far. In this study, we designed and experimentally verified a feasible non-natural synthesis pathway for the de novo production of 1,2-BDO from renewable carbohydrates for the first time. This pathway extends the l-threonine synthesis pathway by introducing two artificial metabolic modules to sequentially convert l-threonine into 2-hydroxybutyric acid and 1,2-BDO. Following key enzyme screening and enhancement of l-threonine synthesis module in the chassis microorganism, the best engineered Escherichia coli strain was able to produce 0.15 g/L 1,2-BDO using glucose as the sole carbon source. This work lays the foundation for the bioproduction of 1,2-BDO from renewable resources.

Engineering propionyl-CoA pools for de novo biosynthesis of odd-chain fatty acids in microbial cell factories

Odd-chain fatty acids (OcFAs) and their derivatives have attracted great interest due to their wide applications in the food, pharmaceutical and petrochemical industries. Microorganisms can naturally de novo produce fatty acids (FAs), where mainly, even-chain with acetyl-CoA instead of odd-chain with propionyl-CoA is used as the primer. Usually, the absence of the precursor propionyl-CoA is considered the main reason that limits the efficient production of OcFAs. It is thus crucial to explore/evaluate/identify promising propionyl-CoA biosynthetic pathways to achieve large-scale biosynthesis of OcFAs. This review discusses the latest advances in microbial metabolism engineering toward producing propionyl-CoA and considers future research directions and challenges toward optimized production of OcFAs.

Design, Evaluation, and Implementation of Synthetic Isopentyldiol Pathways in Escherichia coli

Isopentyldiol (IPDO) is an important raw material in the cosmetic industry. So far, IPDO is exclusively produced through chemical synthesis. Growing interest in natural personal care products has inspired the quest to develop a biobased process. We previously reported a biosynthetic route that produces IPDO via extending the leucine catabolism (route A), the efficiency of which, however, is not satisfactory. To address this issue, we computationally designed a novel non-natural IPDO synthesis pathway (route B) using RetroPath RL, the state-of-the-art tool for bioretrosynthesis based on artificial intelligence methods. We compared this new pathway with route A and two other intuitively designed routes for IPDO biosynthesis from various perspectives. Route B, which exhibits the highest thermodynamic driving force, least non-native reaction steps, and lowest energy requirements, appeared to hold the greatest potential for IPDO production. All three newly designed routes were then implemented in the Escherichia coli BL21(DE3) strain. Results show that the computationally designed route B can produce 2.2 mg/L IPDO from glucose but no IPDO production from routes C and D. These results highlight the importance and usefulness of in silico design and comprehensive evaluation of the potential efficiencies of candidate pathways in constructing novel non-natural pathways for the production of biochemicals.

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.

Effect of Short-Chain Fatty Acids on the Yield of 2,3-Butanediol by Saccharomyces cerevisiae W141: The Synergistic Effect of Acetic Acid and Dissolved Oxygen

As a platform chemical, 2,3-butanediol (2,3-BDO) has been widely used in various industrial fields. To improve the yield of 2,3-BDO produced by Saccharomyces cerevisiae W141, this paper explored the effects of exogenous short-chain fatty acids (SCFAs) as well as the synergistic effects of acetic acid and dissolved oxygen content on the yield of 2,3-BDO from the perspective of physiological metabolism. The results indicated that different SCFAs had different effects on the production of 2,3-BDO, and higher or lower concentrations of SCFAs were not conducive to the generation of 2,3-BDO. However, exogenically adding 1.0 g/L acetic acid significantly increased the yield of 2,3-BDO and the expression level of bdh1, a key gene in the synthesis of 2,3-BDO (p < 0.05). In addition, a dissolved oxygen concentration of 4.52 mg/L was proven to be the optimal condition for 2,3-BDO production. When the dissolved oxygen content and acetic acid concentration were 4.52 mg/L and 1.0 g/L, respectively, the maximum yield of 2,3-BDO was 3.25 ± 0.03 g/L, which was 66.59% higher than that produced by S. cerevisiae W141 alone. These results provide methodological guidance for the industrial production of 2,3-BDO by S. cerevisiae.

Unraveling propylene glycol-induced lipolysis of the biosynthesis pathway in ultra-high temperature milk using high resolution mass spectrometry untargeted lipidomics and proteomics

In July 2022, the food safety accident that excessive propylene glycol was detected in milk processing factory raised widespread concerns about quality and nutrition of milk with illegal additive. To the best of our knowledge, the influences of propylene glycol to lipids in milk had not been systematically explored. Therefore, spatiotemporal distributions of lipids related to propylene glycol reaction and changes of sensory quality were investigated by food exogenous. Briefly, 10 subclasses (Cer, DG, HexCer, LPC, LPE, PC, PE, PI, SPH and TG) included 147 lipids and 38 pivotal enzymes were annotated. Propylene glycol altered lysophospholipidase and phospholipase A₂ through altering structural order in lipids domains surrounding proteins to inhibit glycerophospholipid metabolism and initiated obvious changes in PC (10.45-27.91 mg kg^-1) and PE (12.92-49.02 mg kg^-1). This study offered insights into influences of propylene glycol doses and storage time on milk metabolism at molecular level to assess the quality of milk.

Genome-Scale Modeling and Systems Metabolic Engineering of Vibrio natriegens for the Production of 1,3-Propanediol

The fastest-growing bacterium Vibrio natriegens is a highly promising next-generation workhorse for molecular biology and industrial biotechnology. In this work, we described the workflows for developing genome-scale metabolic models and genome-editing protocols for engineering Vibrio natriegens. A case study for metabolic engineering of Vibrio natriegens for the production of 1,3-propanediol was also presented.

Critical Assessment of the Sustainability of Deep Eutectic Solvents: A Case Study on Six Choline Chloride-Based Mixtures

An outline of the advantages, in terms of sustainability, of Deep Eutectic Solvents (DESs) is provided, by analyzing some of the most popular DESs, obtained by the combination of choline chloride, as a hydrogen bond acceptor, and six hydrogen bond donors. The analysis is articulated into four main issues related to sustainability, which are recurrently mentioned in the literature, but are often taken for granted without any further critical elaboration, as the prominent green features of DESs: their low toxicity, good biodegradability, renewable sourcing, and low cost. This contribution is intended to provide a more tangible, evidence-based evaluation of the actual green credentials of the considered DESs, to reinforce or question their supposed sustainability, also in mutual comparison with one another.

New pathways and metabolic engineering strategies for microbial synthesis of diols

Diols are important bulk chemicals that are widely used in polymer, cosmetics, fuel, food, and pharmaceutical industries. The development of bioprocess to produce diols from renewable feedstocks has gained much interest in recent years and is contributing to reducing the carbon footprint of the chemical industry. Although bioproduction of some natural diols such as 1,3-propanediol and 2,3-butanediol has been commercialized, microbial production of most other diols is still challenging due to the lack of natural biosynthetic pathways. This review describes the recent efforts in the development of novel synthetic pathways and metabolic engineering strategies for the biological production of C2∼C5 diols. We also discussed the main challenges and future perspectives for the microbial processes toward industrial application.

If It Is Hard, It Is Worth Doing: Engineering Radical Enzymes from Anaerobes

With a pressing need for sustainable chemistries, radical enzymes from anaerobes offer a shortcut for many chemical transformations and deliver highly sought-after functionalizations such as late-stage C-H functionalization, C-C bond formation, and carbon-skeleton rearrangements, among others. The challenges in handling these oxygen-sensitive enzymes are reflected in their limited industrial exploitation, despite what they may deliver. With an influx of structures and mechanistic understanding, the scope for designed radical enzymes to deliver wanted processes becomes ever closer. Combined with new advances in computational methods and workflows for these complex systems, the outlook for an increased use of radical enzymes in future processes is exciting.

Conformational Flexibility as a Tool for Enabling Site-Selective Functionalization of Unactivated sp3 C-O Bonds in Cyclic Acetals

A dual catalytic manifold that enables site-selective functionalization of unactivated sp³ C–O bonds in cyclic acetals with aryl and alkyl halides is reported. The reaction is triggered by an appropriate σ*–p orbital overlap prior to sp³ C–O cleavage, thus highlighting the importance of conformational flexibility in both reactivity and site selectivity. The protocol is characterized by its excellent chemoselectivity profile, thus offering new vistas for activating strong σ sp³ C–O linkages.

Biosynthesizing structurally diverse diols via a general route combining oxidative and reductive formations of OH-groups

Diols encompass important bulk and fine chemicals for the chemical, pharmaceutical and cosmetic industries. During the past decades, biological production of C3-C5 diols from renewable feedstocks has received great interest. Here, we elaborate a general principle for effectively synthesizing structurally diverse diols by expanding amino acid metabolism. Specifically, we propose to combine oxidative and reductive formations of hydroxyl groups from amino acids in a thermodynamically favorable order of four reactions catalyzed by amino acid hydroxylase, L-amino acid deaminase, α-keto acid decarboxylase and aldehyde reductase consecutively. The oxidative formation of hydroxyl group from an alkyl group is energetically more attractive than the reductive pathway, which is exclusively used in the synthetic pathways of diols reported so far. We demonstrate this general route for microbial production of branched-chain diols in E. coli. Ten C3-C5 diols are synthesized. Six of them, namely isopentyldiol (IPDO), 2-methyl-1,3-butanediol (2-M-1,3-BDO), 2-methyl-1,4-butanediol (2-M-1,4-BDO), 2-methyl-1,3-propanediol (MPO), 2-ethyl-1,3-propanediol (2-E-1,3-PDO), 1,4-pentanediol (1,4-PTD), have not been biologically synthesized before. This work opens up opportunities for synthesizing structurally diverse diols and triols, especially by genome mining, rational design or directed evolution of proper enzymes.

Diols are important bulk and fine chemicals, but bioproduciton of branch-chain diols is hampered by the unknown biological route. Here, the authors report the expanding of amino acid metabolism for biosynthesis of branch-chain diols via a general route of combined oxidative and reductive formations of hydroxyl groups.

Synthesis of C2-C4 diols from bioresources: Pathways and metabolic intervention strategies

Diols are important platform chemicals with extensive industrial applications in biopolymer synthesis, cosmetics, and fuels. The increased dependence on non-renewable sources to meet the energy requirement of the population raised issues regarding fossil fuel depletion and environmental impacts. The utilization of biological methods for the synthesis of diols by utilizing renewable resources such as glycerol and agro-residual wastes gained attention worldwide because of its advantages. Among these, biotransformation of 1,3-propanediol (1,3-PDO) and 2,3-butanediol (2,3-BDO) were extensively studied and at present, these diols are produced commercially in large scale with high yield. Many important isomers of C2-C4 diols lack natural synthetic pathways and development of chassis strains for the synthesis can be accomplished by adopting synthetic biology approaches. This current review depicts an overall idea about the pathways involved in C2-C4 diol production, metabolic intervention strategies and technologies in recent years.

Systems metabolic engineering of Corynebacterium glutamicum for high-level production of 1,3-propanediol from glucose and xylose

Corynebacterium glutamicum is a versatile chassis which has been widely used to produce various amino acids and organic acids. In this study, we report the development of an efficient C. glutamicum strain to produce 1,3-propanediol (1,3-PDO) from glucose and xylose by systems metabolic engineering approaches, including (1) construction and optimization of two different glycerol synthesis modules; (2) combining glycerol and 1,3-PDO synthesis modules; (3) reducing 3-hydroxypropionate accumulation by clarifying a mechanism involving 1,3-PDO re-consumption; (4) reducing the accumulation of toxic 3-hydroxypropionaldehyde by pathway engineering; (5) engineering NADPH generation pathway and anaplerotic pathway. The final engineered strain can efficiently produce 1,3-PDO from glucose with a titer of 110.4 g/L, a yield of 0.42 g/g glucose, and a productivity of 2.30 g/L/h in fed-batch fermentation. By further introducing an optimized xylose metabolism module, the engineered strain can simultaneously utilize glucose and xylose to produce 1,3-PDO with a titer of 98.2 g/L and a yield of 0.38 g/g sugars. This result demonstrates that C. glutamicum is a potential chassis for the industrial production of 1,3-PDO from abundant lignocellulosic feedstocks.

Development of a plasmid stabilization system in Vibrio natriegens for the high production of 1,3-propanediol and 3-hydroxypropionate

Vibrio natriegens is a promising industrial chassis with a super-fast growth rate and high substrate uptake rates. V. natriegens was previously engineered to produce 1,3-propanediol (1,3-PDO) from glycerol by overexpressing the corresponding genes in a plasmid. However, antibiotic selection pressure for plasmid stability was not satisfactory and plasmid loss resulted in reduced productivity of the bioprocess. In this study, we developed an antibiotic-free plasmid stabilization system for V. natriegens. The system was achieved by shifting the glpD gene, one of the essential genes for glycerol degradation, from the chromosome to plasmid. With this system, engineered V. natriegens can stably maintain a large expression plasmid during the whole fed-batch fermentation and accumulated 69.5 g/L 1,3-PDO in 24 h, which was 23% higher than that based on antibiotic selection system. This system was also applied to engineering V. natriegens for the production of 3-hydroxypropionate (3-HP), enabling the engineered strain to accumulate 64.5 g/L 3-HP in 24 h, which was 30% higher than that based on antibiotic system. Overall, the developed strategy could be useful for engineering V. natriegens as a platform for the production of value-added chemicals from glycerol.

C4 Bacterial Volatiles Improve Plant Health

Plant growth-promoting rhizobacteria (PGPR) associated with plant roots can trigger plant growth promotion and induced systemic resistance. Several bacterial determinants including cell-wall components and secreted compounds have been identified to date. Here, we review a group of low-molecular-weight volatile compounds released by PGPR, which improve plant health, mostly by protecting plants against pathogen attack under greenhouse and field conditions. We particularly focus on C4 bacterial volatile compounds (BVCs), such as 2,3-butanediol and acetoin, which have been shown to activate the plant immune response and to promote plant growth at the molecular level as well as in large-scale field applications. We also disc/ uss the potential applications, metabolic engineering, and large-scale fermentation of C4 BVCs. The C4 bacterial volatiles act as airborne signals and therefore represent a new type of biocontrol agent. Further advances in the encapsulation procedure, together with the development of standards and guidelines, will promote the application of C4 volatiles in the field.

Recent advances in the microbial production of C4 alcohols by metabolically engineered microorganisms

BACKGROUND

The heavy global dependence on petroleum-based industries has led to serious environmental problems, including climate change and global warming. As a result, there have been calls for a paradigm shift towards the use of biorefineries, which employ natural and engineered microorganisms that can utilize various carbon sources from renewable resources as host strains for the carbon-neutral production of target products.

PURPOSE AND SCOPE

C4 alcohols are versatile chemicals that can be used directly as biofuels and bulk chemicals and in the production of value-added materials such as plastics, cosmetics, and pharmaceuticals. C4 alcohols can be effectively produced by microorganisms using DCEO biotechnology (tools to design, construct, evaluate, and optimize) and metabolic engineering strategies.

SUMMARY OF NEW SYNTHESIS AND CONCLUSIONS

In this review, we summarize the production strategies and various synthetic tools available for the production of C4 alcohols and discuss the potential development of microbial cell factories, including the optimization of fermentation processes, that offer cost competitiveness and potential industrial commercialization.

Efficient Production of 1,3-Propanediol from Diverse Carbohydrates via a Non-natural Pathway Using 3-Hydroxypropionic Acid as an Intermediate

1,3-Propanediol (1,3-PDO) is a promising platform chemical used to manufacture various polyesters, polyethers, and polyurethanes. Microbial production of 1,3-PDO using non-natural producers often requires adding expensive cofactors such as vitamin B₁₂, which increases the whole production cost. In this study, we proposed and engineered a non-natural 1,3-PDO synthetic pathway derived from acetyl-CoA, enabling efficient accumulation of 1,3-PDO in Escherichia coli without adding expensive cofactors. This functional pathway was established by introducing the malonyl-CoA-dependent 3-hydroxypropionic acid (3-HP) module and screening the key enzymes to convert 3-HP to 1,3-PDO. The best engineered strain can produce 2.93 g/L 1,3-PDO with a yield of 0.35 mol/mol glucose in shake flask cultivation (and 7.98 g/L in fed-batch fermentation), which is significantly higher than previous reports based on homoserine- or malate-derived non-natural pathways. We also demonstrated for the first time the feasibility of producing 1,3-PDO from diverse carbohydrates including xylose, glycerol, and acetate based on the same pathway. Thus, this study provides an alternative route for 1,3-PDO production.

Systems metabolic engineering of Vibrio natriegens for the production of 1,3-propanediol

The economic viability of current bio-production systems is often limited by its low productivity due to slow cell growth and low substrate uptake rate. The fastest-growing bacterium Vibrio natriegens is a highly promising next-generation workhorse of the biotechnology industry which can utilize various industrially relevant carbon sources with high substrate uptake rates. Here, we demonstrate the first systematic engineering example of V. natriegens for the heterologous production of 1,3-propanediol (1,3-PDO) from glycerol. Systems metabolic engineering strategies have been applied in this study to develop a superior 1,3-PDO producer, including: (1) heterologous pathway construction and optimization; (2) engineering cellular transcriptional regulators and global transcriptomic analysis; (3) enhancing intracellular reducing power by cofactor engineering; (4) reducing the accumulation of toxic intermediate by pathway engineering; (5) systematic engineering of glycerol oxidation pathway to eliminate byproduct formation. A final engineered strain can efficiently produce 1,3-PDO with a titer of 56.2 g/L, a yield of 0.61 mol/mol, and an average productivity of 2.36 g/L/h. The strategies described in this study would be useful for engineering V. natriegens as a potential chassis for the production of other useful chemicals and biofuels.

Recent advances in microbial biosynthesis of C3 - C5 diols: Genetics and process engineering approaches

Diols derived from renewable feedstocks have significant commercial interest in polymer, pharmaceutical, cosmetics, flavors and fragrances, food and feed industries. In C3-C5 diols biological processes of 1,3-propanediol, 1,2-propanediol and 2,3-butanediol have been commercialized as other isomers are non-natural metabolites and lack natural biosynthetic pathways. However, the developments in the field of systems and synthetic biology paved a new path to learn, build, construct, and test for efficient chassis strains. The current review addresses the recent advancements in metabolic engineering, construction of novel pathways, process developments aimed at enhancing in production of C3-C5 diols. The requisites on developing an efficient and sustainable commercial bioprocess for C3-C5 diols were also discussed.

Metabolic Engineering of Escherichia coli for De Novo Production of 1,5-Pentanediol from Glucose

1,5-Pentanediol (1,5-PDO) is an important C5 building block for the synthesis of different value-added polyurethanes and polyesters. However, no natural metabolic pathway exists for the biosynthesis of 1,5-PDO. Herein we designed and constructed a promising nonnatural pathway for de novo production of 1,5-PDO from cheap carbohydrates. This biosynthesis route expands natural lysine pathways and employs two artificial metabolic modules to sequentially convert lysine into 5-hydroxyvalerate (5-HV) and 1,5-PDO via 5-hydroxyvaleryl-CoA. Theoretically, the 5-hydroxyvaleryl-CoA-based pathway is more energy-efficient than a recently published carboxylic acid reductase-based pathway for 1,5-PDO production. By combining strategies of systematic enzyme screening, pathway balancing, and transporter engineering, we successfully constructed a minimally engineered Escherichia coli strain capable of producing 3.19 g/L of 5-HV and 0.35 g/L of 1,5-PDO in a medium containing 20 g/L of glucose and 5 g/L lysine. Introducing the synthetic modules into a lysine producer and enhancing NADPH supply enabled the strain to accumulate 1.04 g/L of 5-HV and 0.12 g/L of 1,5-PDO using glucose as the main carbon source. This work lays the basis for the development of a biological route for 1,5-PDO production from renewable bioresources.

Thermal Behavior and Morphology of Thermoplastic Polyurethane Derived from Different Chain Extenders of 1,3- and 1,4-Butanediol

In this study, when deriving thermoplastic polyurethane (TPU), the researchers replaced 1,4-butanediol (1,4-BDO) with 1,3-butanediol (1,3-BDO) as a chain extender and examined how the structure of the chain extender affected the final polymers. Regarding the raw materials for polymerization, three types of commercial polyols with the same molecular weight (Mn = 1000 g/mol), namely, poly (butyl acrylate) (PBA), poly (tetramethylene ether) glycol (PTMG), and polycarbonate diol (PCDL) were used. These polyols were used in conjunction with butanediol and 4,4’-methylene diphenyl diisocyanate. Three groups of TPUs were successfully synthesized using one-shot solvent-free bulk polymerization. Compared with TPUs polymerized using 1,4-BDO, materials polymerized using 1,3-BDO are more transparent and viscous. Structural analysis revealed that no substantial differences between the final structures of the TPUs were present when different chain extenders were used. Thermal analysis indicated that compared with TPUs polymerized using 1,4-BDO, the glass transition temperature of those with 1,3-BDO was 15 °C higher. Examination of microphase separation in the structure by using morphological analysis revealed that compared with TPUs synthesized using 1,4-BDO, PBA, and PTMG synthesized using 1,3-BDO were relatively separated. PCDL synthesized using 1,3-BDO exhibited no morphological difference. Rheological analysis indicated PCDL synthesized using either 1,4-BDO or 1,3-BDO did not exhibit any obvious differences. In conclusion, TPUs synthesized using PCDL and 1,3-BDO exhibited thermal plasticity at room temperature (15–20 °C). Their basic application could be extended to the development of smart materials. In terms of further application, they could be used in shape memory and temperature-sensitive high molecular polymers.

Synthesis of Dicarboxylic Acids from Aqueous Solutions of Diols with Hydrogen Evolution Catalyzed by an Iridium Complex

A catalytic system for the synthesis of dicarboxylic acids from aqueous solutions of diols accompanied by the evolution of hydrogen was developed. An iridium complex bearing a functional bipyridonate ligand with N,N-dimethylamino substituents exhibited a high catalytic performance for this type of dehydrogenative reaction. For example, adipic acid was synthesized from an aqueous solution of 1,6-hexanediol in 97 % yield accompanied by the evolution of four equivalents of hydrogen by the present catalytic system. It should be noted that the simultaneous production of industrially important dicarboxylic acids and hydrogen, which is useful as an energy carrier, was achieved. In addition, the selective dehydrogenative oxidation of vicinal diols to give α-hydroxycarboxylic acids was also accomplished.

Bacterial synthesis of C3-C5 diols via extending amino acid catabolism

Amino acids are naturally occurring and structurally diverse metabolites in biological system, whose potentials for chemical expansion, however, have not been fully explored. Here, we devise a metabolic platform capable of producing industrially important C3-C5 diols from amino acids. The presented platform combines the natural catabolism of charged amino acids with a catalytically efficient and thermodynamically favorable diol formation pathway, created by expanding the substrate scope of the carboxylic acid reductase toward noncognate ω-hydroxylic acids. Using the established platform as gateways, seven different diol-convertible amino acids are converted to diols including 1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol. Particularly, we afford to optimize the production of 1,4-butanediol and demonstrate the de novo production of 1,5-pentanediol from glucose, with titers reaching 1.41 and 0.97 g l^-1, respectively. Our work presents a metabolic platform that enriches the pathway repertoire for nonnatural diols with feedstock flexibility to both sugar and protein hydrolysates.

Ethylene glycol and glycolic acid production from xylonic acid by Enterobacter cloacae

BACKGROUND

Biological routes for ethylene glycol production have been developed in recent years by constructing the synthesis pathways in different microorganisms. However, no microorganisms have been reported yet to produce ethylene glycol naturally.

RESULTS

Xylonic acid utilizing microorganisms were screened from natural environments, and an Enterobacter cloacae strain was isolated. The major metabolites of this strain were ethylene glycol and glycolic acid. However, the metabolites were switched to 2,3-butanediol, acetoin or acetic acid when this strain was cultured with other carbon sources. The metabolic pathway of ethylene glycol synthesis from xylonic acid in this bacterium was identified. Xylonic acid was converted to 2-dehydro-3-deoxy-D-pentonate catalyzed by D-xylonic acid dehydratase. 2-Dehydro-3-deoxy-D-pentonate was converted to form pyruvate and glycolaldehyde, and this reaction was catalyzed by an aldolase. D-Xylonic acid dehydratase and 2-dehydro-3-deoxy-D-pentonate aldolase were encoded by yjhG and yjhH, respectively. The two genes are part of the same operon and are located adjacent on the chromosome. Besides yjhG and yjhH, this operon contains four other genes. However, individually inactivation of these four genes had no effect on either ethylene glycol or glycolic acid production; both formed from glycolaldehyde. YqhD exhibits ethylene glycol dehydrogenase activity in vitro. However, a low level of ethylene glycol was still synthesized by E. cloacae ΔyqhD. Fermentation parameters for ethylene glycol and glycolic acid production by the E. cloacae strain were optimized, and aerobic cultivation at neutral pH were found to be optimal. In fed batch culture, 34 g/L of ethylene glycol and 13 g/L of glycolic acid were produced in 46 h, with a total conversion ratio of 0.99 mol/mol xylonic acid.

CONCLUSIONS

A novel route of xylose biorefinery via xylonic acid as an intermediate has been established.

Efficient bioproduction of 5-aminolevulinic acid, a promising biostimulant and nutrient, from renewable bioresources by engineered Corynebacterium glutamicum

BACKGROUND

5-Aminolevulinic acid (5-ALA) is a promising biostimulant, feed nutrient, and photodynamic drug with wide applications in modern agriculture and therapy. Considering the complexity and low yield of chemical synthesis methods, bioproduction of 5-ALA has drawn intensive attention recently. However, the present bioproduction processes use refined glucose as the main carbon source and the production level still needs further enhancement.

RESULTS

To lay a solid technological foundation for large-scale commercialized bioproduction of 5-ALA, an industrial workhorse Corynebacterium glutamicum was metabolically engineered for high-level 5-ALA biosynthesis from cheap renewable bioresources. After evaluation of 5-ALA synthetases from different sources, the 5-ALA biosynthetic pathway and anaplerotic pathway were rebalanced by regulating intracellular activities of 5-ALA synthetase and phosphoenolpyruvate carboxylase. The engineered biocatalyst produced 5.5 g/L 5-ALA in shake flasks and 16.3 g/L in 5-L bioreactors with a one-step fermentation process from glucose. To lower the cost of feedstock, cheap raw materials were used to replace glucose. Enzymatically hydrolyzed cassava bagasse was proven to be a perfect alternative to refined sugars since the final 5-ALA titer further increased to 18.5 g/L. Use of corn starch hydrolysate resulted in a similar 5-ALA production level (16.0 g/L) with glucose, whereas use of beet molasses caused seriously inhibition. The results obtained here represent a new record of 5-ALA bioproduction. It is estimated that replacing glucose with cassava bagasse will reduce the carbon source cost by 90.1%.

CONCLUSIONS

The high-level biosynthesis of 5-ALA from cheap bioresources will brighten the prospects for industrialization of this sustainable and environment-friendly process. The strategy for balancing metabolic flux developed in this study can also be used for improving the bioproduction of other value-added chemicals.

Acetogenic Fermentation From Oxygen Containing Waste Gas

The microbial production of bulk chemicals from waste gas is becoming a pertinent alternative to industrial strategies that rely on fossil fuels as substrate. Acetogens can use waste gas substrates or syngas (CO, CO₂, H₂) to produce chemicals, such as acetate or ethanol, but as the feed gas often contains oxygen, which inhibits acetogen growth and product formation, a cost-prohibitive chemical oxygen removal step is necessary. Here, we have developed a two-phase microbial system to facilitate acetate production using a gas mixture containing CO and O₂. In the first phase the facultative anaerobic carboxydotroph Parageobacillus thermoglucosidasius was used to consume residual O₂ and produce H₂ and CO₂, which was subsequently utilized by the acetogen Clostridium ljungdahlii for the production of acetate. From a starting amount of 3.3 mmol of CO, 0.52 mmol acetate was produced in the second phase by C. ljungdahlii. In this set-up, the yield achieved was 0.16 mol acetate/mol CO, a 63% of the theoretical maximum. This system has the potential to be developed for the production of a broad range of bulk chemicals from oxygen-containing waste gas by using P. thermoglucosidasius as an oxygen scrubbing tool.

Engineering microbial pathways for production of bio-based chemicals from lignocellulosic sugars: current status and perspectives

Lignocellulose is the most abundant biomass on earth with an annual production of about 2 × 10¹¹ tons. It is an inedible renewable carbonaceous resource that is very rich in pentose and hexose sugars. The ability of microorganisms to use lignocellulosic sugars can be exploited for the production of biofuels and chemicals, and their concurrent biotechnological processes could advantageously replace petrochemicals' processes in a medium to long term, sustaining the emerging of a new economy based on bio-based products from renewable carbon sources. One of the major issues to reach this objective is to rewire the microbial metabolism to optimally configure conversion of these lignocellulosic-derived sugars into bio-based products in a sustainable and competitive manner. Systems' metabolic engineering encompassing synthetic biology and evolutionary engineering appears to be the most promising scientific and technological approaches to meet this challenge. In this review, we examine the most recent advances and strategies to redesign natural and to implement non-natural pathways in microbial metabolic framework for the assimilation and conversion of pentose and hexose sugars derived from lignocellulosic material into industrial relevant chemical compounds leading to maximal yield, titer and productivity. These include glycolic, glutaric, mesaconic and 3,4-dihydroxybutyric acid as organic acids, monoethylene glycol, 1,4-butanediol and 1,2,4-butanetriol, as alcohols. We also discuss the big challenges that still remain to enable microbial processes to become industrially attractive and economically profitable.

Comprehensive analysis of metabolic sensitivity of 1,4-butanediol producing Escherichia coli toward substrate and oxygen availability

Nowadays, chemical production of 1,4-butanediol is supplemented by biotechnological processes using a genetically modified Escherichia coli strain, which is an industrial showcase of successful application of metabolic engineering. However, large scale bioprocess performance can be affected by presence of physical and chemical gradients in bioreactors which are a consequence of imperfect mixing and limited oxygen transfer. Hence, upscaling comes along with local and time dependent fluctuations of cultivation conditions. This study emphasizes on scale-up related effects of microbial 1,4-butanediol production by comprehensive bioprocess characterization in lab scale. Due to metabolic network constraints 1,4-butanediol formation takes place under oxygen limited microaerobic conditions, which can be hardly realized in large scale bioreactor. The purpose of this study was to assess the extent to which substrate and oxygen availability influence the productivity. It was found, that the substrate specific product yield and the production rate are higher under substrate excess than under substrate limitation. Furthermore, the level of oxygen supply within microaerobic conditions revealed strong effects on product and by-product formation. Under strong oxygen deprivation nearly 30% of the consumed carbon is converted into 1,4-butanediol, whereas an increase in oxygen supply results in 1,4-butanediol reduction of 77%. Strikingly, increasing oxygen availability leads to strong increase of main by-product acetate as well as doubled carbon dioxide formation. The study provides clear evidence that scale-up of microaerobic bioprocesses constitute a substantial challenge. Although oxygen is strictly required for product formation, the data give clear evidence that terms of anaerobic and especially aerobic conditions strongly interfere with 1,4-butanediol production.

Metabolic Engineering of a Homoserine-Derived Non-Natural Pathway for the De Novo Production of 1,3-Propanediol from Glucose

Engineering a homoserine-derived non-natural pathway allows heterologous production of 1,3-propanediol (1,3-PDO) from glucose without adding expensive vitamin B₁₂. Due to the lack of efficient enzymes to catalyze the deamination of homoserine and the decarboxylation of 4-hydroxy-2-ketobutyrate, the previously engineered strain can only produce 51.5 mg/L 1,3-PDO using homoserine and glucose as cosubstrates. In this study, we systematically screened the enzymes from different protein families to catalyze the two corresponding reactions and further optimized the selected enzymes by protein engineering. Together with the improvement of homoserine supply by systematic metabolic engineering, an engineered Escherichia coli strain with an optimal combination of aspartate transaminase ( aspC) from E. coli, pyruvate decarboxylase ( pdc) from Zymomonas mobiliz, and alcohol dehydrogenase yqhD from E. coli can produce 0.32 g/L 1,3-PDO from glucose in shake flask cultivation. The titer of 1,3-PDO was further increased to 0.49 g/L or 0.63 g/L by introducing a point mutation of I472A into pdc gene or constructing a fusion protein between aspC and pdc. This study lays the basis for developing a potential process for 1,3-PDO production from sugars without using expensive coenzyme B₁₂.

Engineering a Microbial Consortium Based Whole-Cell System for Efficient Production of Glutarate From L-Lysine

Glutarate is an important C5 platform chemical produced during the catabolism of L-lysine through 5-aminovalerate (5-AMV) pathway. Here, we first established a whole-cell biocatalysis system for the glutarate production from L-lysine with the engineered Escherichia coli (E. coli) that co-expressed DavAB and GabDT. However, the accumulation of intermediate 5-AMV was identified as one important factor limiting glutarate production. Meanwhile, the negative interaction of co-expressing DavAB and GabDT in a single cell was also confirmed. Here, we solved these problems through engineering a microbial consortium composed of two engineered E. coli strains, BL21-22AB and BL21-YDT, as the whole-cell biocatalysts, each of which contains a part of the glutarate pathway. After the optimization of bioconversion conditions, including temperature, metal ion additives, pH, and cell ratio, 17.2 g/L glutarate was obtained from 20 g/L L-lysine with a yield of 95.1%, which was improved by 19.2% compared with that in a single cell. Little accumulation of 5-AMV was detected. Even at the high substrate concentration, the reduced 5-AMV accumulation and increased glutarate production were achieved. This synthetic consortium produced 43.8 g/L glutarate via a fed-batch strategy, the highest titer reported to date.

Biotechnological production of glycolic acid and ethylene glycol: current state and perspectives

Glycolic acid (GA) and ethylene glycol (EG) are versatile two-carbon organic chemicals used in multiple daily applications. GA and EG are currently produced by chemical synthesis, but their biotechnological production from renewable resources has received a substantial interest. Several different metabolic pathways by using genetically modified microorganisms, such as Escherichia coli, Corynebacterium glutamicum and yeast have been established for their production. As a result, the yield of GA and EG produced from sugars has been significantly improved. Here, we describe the recent advancement in metabolic engineering efforts focusing on metabolic pathways and engineering strategies used for GA and EG production.

Production of 2,3-butanediol from glucose and cassava hydrolysates by metabolically engineered industrial polyploid Saccharomyces cerevisiae

BACKGROUND

2,3-Butanediol (2,3-BDO) is a valuable chemical for industrial applications. Bacteria can produce 2,3-BDO with a high productivity, though most of their classification as pathogens makes them undesirable for the industrial-scale production. Though Saccharomyces cerevisiae (GRAS microorganism) was engineered to produce 2,3-BDO efficiently in the previous studies, their 2,3-BDO productivity, yield, and titer were still uncompetitive compared to those of bacteria production. Thus, we propose an industrial polyploid S. cerevisiae as a host for efficient production of 2,3-BDO with high growth rate, rapid sugar consumption rate, and resistance to harsh conditions. Genetic manipulation tools for polyploid yeast had been limited; therefore, we engineered an industrial polyploid S. cerevisiae strain based on the CRISPR-Cas9 genome-editing system to produce 2,3-BDO instead of ethanol.

RESULTS

Endogenous genes coding for pyruvate decarboxylase and alcohol dehydrogenase were partially disrupted to prevent declined growth rate and C₂-compound limitation. A bacterial 2,3-BDO-producing pathway was also introduced in engineered polyploid S. cerevisiae. A fatal redox imbalance was controlled through the heterologous NADH oxidase from Lactococcus lactis during the 2,3-BDO production. The resulting strain (YG01_SDBN) still retained the beneficial traits as polyploid strains for the large-scale fermentation. The combination of partially disrupted PDC (pyruvate decarboxylase) and ADH (alcohol dehydrogenase) did not cause the severe growth defects typically found in all pdc- or adh-deficient yeast. The YG01_SDBN strain produced 178 g/L of 2,3-BDO from glucose with an impressive productivity (2.64 g/L h). When a cassava hydrolysate was used as a sole carbon source, this strain produced 132 g/L of 2,3-BDO with a productivity of 1.92 g/L h.

CONCLUSIONS

The microbial production of 2,3-BDO has been limited to bacteria and haploid laboratorial S. cerevisiae strains. This study suggests that an industrial polyploid S. cerevisiae (YG01_SDBN) can produce high concentration of 2,3-BDO with various advantages. Integration of metabolic engineering of the industrial yeast at the gene level with optimization of fed-batch fermentation at the process scale resulted in a remarkable achievement of 2,3-BDO production at 178 g/L of 2,3-BDO concentration and 2.64 g/L h of productivity. Furthermore, this strain could make a bioconversion of a cassava hydrolysate to 2,3-BDO with economic and environmental benefits. The engineered industrial polyploid strain could be applicable to production of biofuels and biochemicals in large-scale fermentations particularly when using modified CRISPR-Cas9 tools.

The viscous consequence of different trends in clustering of 1,2-diol and 1,n-diol molecules

This paper presents the molecular basis for the quite different behavior of the viscosity of 1,2- and 1,n-diols in dependence of the length of the alkyl part of the molecules of these compounds. The experimental data on the dipolar orientational effects revealed a decidedly different role of that part of the molecules in creating a microstructure of both the hydrogen-bonded liquids. In the case of 1,n-diols, an increase in the alkyl radical length, i.e. an increasing of the distance between the OH groups within the molecule, highly stimulates molecular self-assembly in form of gradually longer and wider ribbon-like clusters. This effect yields a quite important increase in the viscosity of 1,n-diols as n increases. In the case of 1,2-diols, due to gradual separation of the hydrophilic and hydrophobic parts of the molecules, the situation is quite different. Two OH groups situated on one of the ends of the hydrocarbon radical form the clusters of a micelle-like shape, however, the dipole moment is not compensated. Along with an increase in the hydrocarbon part in 1,2-diol molecules, one only observes an increase in the intermolecular consolidation within the micelle-like entities. This manifests as a gradual decrease in the polarity of these clusters. So, actually, there are no relevant reasons for essential differences of viscosities in the series of 1,2-diols.