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As climate change is an important environmental issue, the conventional petrochemical-based processes to produce valuable chemicals are being shifted toward eco-friendly biological-based processes. In this study, 3-hydroxypropionic acid (3-HP), an industrially important three carbon (C3) chemical, was overproduced by metabolically engineered Escherichia coli using glycerol as a sole carbon source. As the first step to construct a glycerol-dependent 3-HP biosynthetic pathway, the dhaB1234 and gdrAB genes from Klebsiella pneumoniae encoding glycerol dehydratase and glycerol reactivase, respectively, were introduced into E. coli to convert glycerol into 3-hydroxypropionaldehyde (3-HPA). In addition, the ydcW gene from K. pneumoniae encoding γ-aminobutyraldehyde dehydrogenase, among five aldehyde dehydrogenases examined, was selected to further convert 3-HPA to 3-HP. Increasing the expression level of the ydcW gene enhanced 3-HP production titer and reduced 1,3-propanediol production. To enhance 3-HP production, fed-batch fermentation conditions were optimized by controlling dissolved oxygen (DO) level and employing different feeding strategies including intermittent feeding, pH-stat feeding, and continuous feeding strategies. Fed-batch culture of the final engineered E. coli strain with DO control and continuous feeding strategy produced 76.2 g/L of 3-HP with the yield and productivity of 0.457 g/g glycerol and 1.89 g/L/h, respectively. To the best of our knowledge, this is the highest 3-HP productivity achieved by any microorganism reported to date.

Acrylic acid (AA) is an important industrial chemical used for several applications including superabsorbent polymers and acrylate esters. Here, we report the development of a new biosynthetic pathway for the production of AA from glucose in metabolically engineered Escherichia coli through the β-alanine (BA) route. The AA production pathway was partitioned into two modules: an AA forming downstream pathway and a BA forming upstream pathway. We first validated the operation of the downstream pathway in vitro and in vivo, and then constructed the downstream pathway by introducing efficient enzymes (Act, Acl2, and YciA) screened out of various microbial sources and optimizing the expression levels. For the direct fermentative production of AA from glucose, the downstream pathway was introduced into the BA producing E. coli strain. The resulting strain could successfully produce AA from glucose in flask cultivation. AA production was further enhanced by expressing the upstream genes (panD and aspA) under the constitutive BBa_J23100 promoter. Replacement of the native promoter of the acs gene with the BBa_J23100 promoter in the genome increased AA production to 55.7 mg/L in flask. Fed-batch fermentation of the final engineered strain allowed production of 237 mg/L of AA in 57.5 h, representing the highest AA titer reported to date.

Microbial production of chemicals and materials from renewable carbon sources is becoming increasingly important to help establish sustainable chemical industry. In this paper, we review current status of metabolic engineering for the bio-based production of linear and saturated dicarboxylic acids and diamines, important platform chemicals used in various industrial applications, especially as monomers for polymer synthesis. Strategies for the bio-based production of various dicarboxylic acids having different carbon numbers including malonic acid (C3), succinic acid (C4), glutaric acid (C5), adipic acid (C6), pimelic acid (C7), suberic acid (C8), azelaic acid (C9), sebacic acid (C10), undecanedioic acid (C11), dodecanedioic acid (C12), brassylic acid (C13), tetradecanedioic acid (C14), and pentadecanedioic acid (C15) are reviewed. Also, strategies for the bio-based production of diamines of different carbon numbers including 1,3-diaminopropane (C3), putrescine (1,4-diaminobutane; C4), cadaverine (1,5-diaminopentane; C5), 1,6-diaminohexane (C6), 1,8-diaminoctane (C8), 1,10-diaminodecane (C10), 1,12-diaminododecane (C12), and 1,14-diaminotetradecane (C14) are revisited. Finally, future challenges are discussed towards more efficient production and commercialization of bio-based dicarboxylic acids and diamines.

Production of free fatty acids (FFAs) and derivatives from renewable non-food biomass by microbial fermentation is of great interest. Here, we report the development of engineered Rhodococcus opacus strains producing FFAs, fatty acid ethyl esters (FAEEs) and long-chain hydrocarbons (LCHCs). Culture conditions were optimized to produce 82.9 g l−1 of triacylglycerols from glucose, and an engineered strain with acyl-coenzyme A (CoA) synthetases deleted, overexpressing three lipases with lipase-specific foldase produced 50.2 g l−1 of FFAs. Another engineered strain with acyl-CoA dehydrogenases deleted, overexpressing lipases, foldase, acyl-CoA synthetase and heterologous aldehyde/alcohol dehydrogenase and wax ester synthase produced 21.3 g l−1 of FAEEs. A third engineered strain with acyl-CoA dehydrogenases and alkane-1 monooxygenase deleted, overexpressing lipases, foldase, acyl-CoA synthetase and heterologous acyl-CoA reductase, acyl-ACP reductase and aldehyde deformylating oxygenase produced 5.2 g l−1 of LCHCs. Metabolic engineering strategies and engineered strains developed here may help establish oleaginous biorefinery platforms for the sustainable production of chemicals and fuels. Optimization of triacylglycerol production in the oleaginous bacterium Rhodococcus opacus followed by pathway engineering enables the enhanced production of free fatty acids, fatty acid ethyl esters and long-chain hydrocarbons from glucose.

Production of industrial chemicals using renewable biomass feedstock is becoming increasingly important to address limited fossil resources, climate change and other environmental problems. To develop high-performance microbial cell factories, equivalent to chemical plants, microorganisms undergo systematic metabolic engineering to efficiently convert biomass-derived carbon sources into target chemicals. Over the past two decades, many engineered microorganisms capable of producing natural and non-natural chemicals have been developed. This Review details the current status of representative industrial chemicals that are produced through biological and/or chemical reactions. We present a comprehensive bio-based chemicals map that highlights the strategies and pathways of single or multiple biological reactions, chemical reactions and combinations thereof towards production of particular chemicals of interest. Future challenges are also discussed to enable production of even more diverse chemicals and more efficient production of chemicals from renewable feedstocks.

4-Amino-1-butanol (4AB) serves as an important intermediate compound for drugs and a precursor of biodegradable polymers used for gene delivery. Here, we report for the first time the fermentative production of 4AB from glucose by metabolically engineered Corynebacterium glutamicum harboring a newly designed pathway comprising a putrescine aminotransferase (encoded by ygjG) and an aldehyde dehydrogenase (encoded by yqhD) from Escherichia coli, which convert putrescine to 4AB. Application of several metabolic engineering strategies such as fine-tuning the expression levels of ygjG and yqhD, eliminating competing pathways, and optimizing culture condition further improved 4AB production. Fed-batch culture of the final metabolically engineered C. glutamicum strain produced 24.7 g/L of 4AB. The strategies reported here should be useful for the microbial production of primary amino alcohols from renewable resources. This article is protected by copyright. All rights reserved.

Acrylic acid (AA) is an important industrial chemical used for several applications including superabsorbent polymers and acrylate esters. Here, we report the development of a new biosynthetic pathway for the production of AA from glucose in metabolically engineered

Poly(d-lactate-co-glycolate-co-4-hydroxybutyrate) [poly(d-LA-co-GA-co-4HB)] and poly(d-lactate-co-glycolate-co-4-hydroxybutyrate-co-d-2-hydroxybutyrate) [poly(d-LA-co-GA-co-4HB-co-d-2HB)] are of interest for their potential applications as new biomedical polymers. Here we report their enhanced production by metabolically engineered Escherichia coli. To examine the polymer properties, poly(d-LA-co-GA-co-4HB) polymers having various monomer compositions (3.4-41.0mol% of 4HB) were produced by culturing the engineered E. coli strain expressing xylBC from Caulobacter crescentus, evolved phaC1 from Pseudomonas sp. MBEL 6-19 (phaC1437), and evolved pct from Clostridium propionicum (pct540) in a medium supplemented with sodium 4HB at various concentrations. To produce these polymers without 4HB feeding, the 4HB biosynthetic pathway was additionally constructed by expressing Clostridium kluyveri sucD and 4hbD. The engineered E. coli expressing xylBC, phaC1437, pct540, sucD, and 4hbD successfully produced poly(d-LA-co-GA-co-4HB-co-d-2HB) and poly(d-LA-co-GA-co-4HB) from glucose and xylose. Through modulating the expression levels of the heterologous genes and performing fed-batch cultures, the polymer content and titer could be increased to 65.76wt% and 6.19g/L, respectively, while the monomer fractions in the polymers could be altered as desired. The polymers produced, in particular, the 4HB-rich polymers showed viscous and sticky properties suggesting that they might be used as medical adhesives.

Ethylene glycol (EG) is an important chemical used for several industrial applications including poly(ethylene terephthalate) synthesis. In this study, Escherichia coli was metabolically engineered to efficiently produce EG from xylose. To biosynthesize EG, the Dahms pathway was introduced by expressing xylBC genes from Caulobacter crescentus (xylBC). Various E. coli strains and glycolaldehyde reductases were screened to find E. coli W3110 strain and glycolaldehyde reductase (yqhD) as optimal combination for EG production. In silico genome-scale metabolic simulation suggested that increasing the native xylose pathway flux, in the presence of the overexpressed Dahms pathway, is beneficial for EG production. This was achieved by reducing the Dahms pathway flux by employing a synthetic small regulatory RNA targeting xylB. Fed-batch culture of the final engineered E. coli strain produced 108.2g/L of EG in a xylose minimal medium. The yield on xylose and EG productivity were 0.36g/g (0.87mol/mol) and 2.25g/L/h, respectively.

Recent metabolic engineering strategies for bio-based production of monomers and polymers are reviewed. In the case of monomers, we describe strategies for producing polyamide precursors, namely diamines (putrescine, cadaverine, 1,6-diaminohexane), dicarboxylic acids (succinic, glutaric, adipic, and sebacic acids), and ω-amino acids (γ-aminobutyric, 5-aminovaleric, and 6-aminocaproic acids). Also, strategies for producing diols (monoethylene glycol, 1,3-propanediol, and 1,4-butanediol) and hydroxy acids (3-hydroxypropionic and 4-hydroxybutyric acids) used for polyesters are reviewed. Furthermore, we review strategies for producing aromatic monomers, including styrene, p-hydroxystyrene, p-hydroxybenzoic acid, and phenol, and propose pathways to aromatic polyurethane precursors. Finally, in vivo production of polyhydroxyalkanoates and recombinant structural proteins having interesting applications are showcased.

Microbial production of chemicals and materials from renewable sources is becoming increasingly important for sustainable chemical industry. Here, we report construction of a new and efficient platform metabolic pathway for the production of four-carbon (butyrolactam), five-carbon (valerolactam) and six-carbon (caprolactam) lactams. This pathway uses ω-amino acids as precursors and comprises two steps. Activation of ω-amino acids catalyzed by the Clostridium propionicum β-alanine CoA transferase (Act) followed by spontaneous cyclization. The pathway operation was validated both in vitro and in vivo. Three metabolically engineered Escherichia coli strains were developed by introducing the newly constructed metabolic pathway followed by systems-level optimization, which resulted in the production of butyrolactam, valerolactam and caprolactam from renewable carbon source. In particular, fed-batch fermentation of the final engineered E. coli strain produced 54.14 g/L of butyrolactam in a glucose minimal medium. These results demonstrate the high efficiency of the novel lactam pathway developed in this study.

Metabolic engineering has been playing increasingly important roles in developing microbial cell factories for the production of various chemicals and materials to achieve sustainable chemical industry. Nowadays, many tools and strategies are available for performing systems metabolic engineering that allows systems-level metabolic engineering in more sophisticated and diverse ways by adopting rapidly advancing methodologies and tools of systems biology, synthetic biology and evolutionary engineering. As an outcome, development of more efficient microbial cell factories has become possible. Here, we review recent advances in systems metabolic engineering tools and strategies together with accompanying application examples. In addition, we describe how these tools and strategies work together in simultaneous and synergistic ways to develop novel microbial cell factories.

We have previously analyzed the proteome of recombinant Escherichia coli producing poly(3-hydroxybutyrate) [P(3HB)] and revealed that the expression level of several enzymes in central metabolism are proportional to the amount of P(3HB) accumulated in the cells. Based on these results, the amplification effects of triosephosphate isomerase (TpiA) and fructose-bisphosphate aldolase (FbaA) on P(3HB) synthesis were examined in recombinant E. coli W3110, XL1-Blue, and W lacI mutant strains using glucose, sucrose and xylose as carbon sources. Amplification of TpiA and FbaA significantly increased the P(3HB) contents and concentrations in the three E. coli strains. TpiA amplification in E. coli XL1-Blue lacI increased P(3HB) from 0.4 to 1.6 to g/l from glucose. Thus amplification of glycolytic pathway enzymes is a good strategy for efficient production of P(3HB) by allowing increased glycolytic pathway flux to make more acetyl-CoA available for P(3HB) biosynthesis.