Metabolic Engineering of Escherichia coli for High-Yield Production of (R)-1,3-Butanediol

Advances in biosynthesis and downstream processing of diols

Diols are important platform chemicals with a wide range of applications in the fields of chemical and pharmaceutical industries, food, feed and cosmetics. In particular, 1,3-propanediol (PDO), 1,4-butanediol (1,4-BDO) and 1,3-butanediol (1,3-BDO) are appealing monomers for producing industrially important polymers and plastics. Therefore, the commercialization of bio-based diols is highly important for supporting the growth of biomanufacturing for the fiber industry. This review focuses primarily on the microbial production of PDO, 1,4-BDO and 1,3-BDO with respect to different microbial strains and biological routes. In addition, metabolic platforms which are designed to produce various diols using generic bioconversion strategies are reviewed for the first time. Finally, we also summarize and discuss recent developments in the downstream processing of PDO according to their advantages and drawbacks, which is taken as an example to present the prospects and challenges for industrial separation and purification of diols from microbial fermentation broth.

Metabolic Engineering of High L-Lysine-Producing Escherichia coli for de Novo Production of L-Lysine-Derived Compounds

5-Aminovalerate (5-AVA), 5-hydroxyvalerate (5-HV), and 1,5-pentanediol (1,5-PDO) are l-lysine derivatives with extensive applications in the production of materials such as polyesters, polyurethane, plasticizers, inks, and coatings. However, their large-scale production is limited by the lack of efficient synthetic pathways. Here, we aimed to construct multiple synthetic pathways by screening the key enzymes involved in the synthesis of these compounds in Escherichia coli. The engineered pathway utilizing RaiP demonstrated a superior catalytic efficiency. The LER strain that overexpressed only raiP successfully synthesized 9.70 g/L 5-HV and 8.31 g/L 5-AVA, whereas the strain LERGY that overexpressed raiP, gabT, and yahK accumulated 9.72 g/L 5-HV and 7.95 g/L 5-AVA from 20 g/L glucose. The introduction of exogenous transaminases and dehydrogenases enhanced cell growth and fermentation efficiency with respect to 5-HV synthesis, albeit without significantly impacting the yield. Strain LE05, incorporating only two exogenous enzymes, RaiP and CaR, produced 1.87 g/L 1,5-PDO, 3.85 g/L 5-HV, and 4.78 g/L 5-hydroxyglutaraldehyde from 20 g/L glucose after 6 days. The strain LE02G, fortified with transaminase, dehydrogenase, and NADPH regeneration system, accumulated 7.82 g/L 1,5-PDO, whereas the aldp-knock out LE02G2 synthesized 10.98 g/L 1,5-PDO from 50 g/L glucose in fed-batch fermentation after 6 days, yielding 0.22 g/g glucose (0.37 mol/mol). Introducing the NADPH regeneration pathway and deleting the NADPH-consuming pathways increased the 1,5-PDO yield and decreased the precursor concentration. The proposed pathways and engineering strategies presented in this study can prove instrumental in developing biological routes for the practical production of 5-AVA, 5-HV, and 1,5-PDO.

Heterologous Expression of Ketoreductase ChKRED20 Mutant in Pichia pastoris and Bioreductive Production of (R)-1, 3-Butanediol

(R)-1, 3-Butanediol (1, 3-BDO) is an important intermediate in the synthesis of aromatics, pheromones, insecticides, and beta-lactam antibiotics. The ChKRED20 is a robust NADH-dependent ketoreductase identified from Chryseobacterium sp. CA49. We obtained a ChKRED20 mutant (M12) through directed evolutionary screening of ChKRED20, the mutant with significantly improved activity to asymmetrically reduce 4-hydroxy-2-butanone (4H2B) to (R)-1, 3-BDO. So far, both ChKRED20 and its mutants have been expressed in intracellular in E. coli, the process of purification after intracellular expression is complicated, which leads to high cost. Here, we expressed M12 by constructing multicopy expression strains in P. pastoris, and the target protein yield was 302 mg/L in shake-flask fermentation and approximately 3.5 g/L in high-density fermentation. The recombinant M12 showed optimal enzyme activity at 30 °C and had high activity within a broad pH range of 6.0-8.0, and also showed high thermal stability. The recombinant M12 was further used for the reduction of 4H2B to (R)-1, 3-BDO, and 98.9% yield was achieved at 4540 mM 4H2B. The crude M12 enzyme extract was found to catalyze the bioreductive production of (R)-1, 3-BDO with excellent stereoselectivity (ee > 99%) and meet the production requirements. Our research shows that the M12 mutant can be used for the synthesis of (R)-1, 3-BDO, and the P. pastoris expression system is an ideal platform for the large-scale, low-cost preparation of ChKRED20 or its mutants, which may have applications in industrial settings.

Metabolic engineering of Escherichia coli to utilize methanol as a co-substrate for the production of (R)-1,3-butanediol

Due to its abundance, cost-effectiveness, and high reducibility, methanol has gained considerable attention in the biomanufacturing industry as a nonfood feedstock for the production of value-added chemicals. The range of chemicals that can be derived from methanol, however, remains constrained and is currently in the concept validation phase. This study aimed to develop and evaluate a hybrid methanol assimilation pathway in Escherichia coli to improve the production of (R)-1,3-butanediol ((R)-1,3-BDO) by utilizing methanol and sugars as co-substrates. By combining the methanol dehydrogenase (MDH) from the prokaryotes with the dihydroxyacetone synthase (DAS) from the eukaryotes, the hybrid pathway facilitates methanol conversion into the central metabolism while generating NADH at the same time. Through pathway optimization and targeted gene deletions, we have successfully developed an E. coli strain capable of producing 5.79 g/L (R)-1,3-BDO in shake flask experiments and 13.71 g/L (R)-1,3-BDO with a yield of 0.35 C-mol/C-mol in batch fermentation using methanol and glucose as co-substrates. Our study also showed the incorporation of 13C-methanol into cellular intermediates and an increase in NAD(P)H concentration, confirming the role of methanol as a co-substrate and supplier of NADH. In addition, our study also demonstrated the co-utilization of methanol with xylose for the production of (R)-1,3-BDO, expanding the substrate spectrum for sustainable 1,3-BDO production.

Metabolic engineering of Escherichia coli for enhanced production of 1,3-butanediol from glucose

1,3-Butanediol (1,3-BDO) finds versatile applications in the cosmetic, chemical, and food industries. This study focuses on the metabolic engineering of Escherichia coli K12 to achieve efficient production of 1,3-BDO from glucose via acetoacetyl-CoA, 3-hydroxybutyryl-CoA, and 3-hydroxybutyraldehyde. The accumulation of an intermediary metabolite (pyruvate) and a byproduct (3-hydroxybutyric acid) was reduced by disruption of the negative transcription factor (PdhR) for pyruvate dehydrogenase complex (PDHc) and employing an efficient alcohol dehydrogenase (YjgB), respectively. Additionally, to improve NADPH availability, carbon flux was redirected from the Embden-Meyerhof-Parnas (EMP) pathway to the Entner-Doudoroff (ED) pathway. One resulting strain achieved a record-high titer of 790 mM (∼71.1 g/L) with a yield of 0.65 mol/mol for optically pure (R)-1,3-BDO, with an enantiomeric excess (e.e.) value of 98.5 %. These findings are useful in the commercial production of 1,3-BDO and provide valuable insights into the development of an efficient cell factory for other acetyl-CoA derivatives.

Application of cofactors in the regulation of microbial metabolism: A state of the art review

Cofactors are crucial chemicals that maintain cellular redox balance and drive the cell to do synthetic and catabolic reactions. They are involved in practically all enzymatic activities that occur in live cells. It has been a hot research topic in recent years to manage their concentrations and forms in microbial cells by using appropriate techniques to obtain more high-quality target products. In this review, we first summarize the physiological functions of common cofactors, and give a brief overview of common cofactors acetyl coenzyme A, NAD(P)H/NAD(P)+, and ATP/ADP; then we provide a detailed introduction of intracellular cofactor regeneration pathways, review the regulation of cofactor forms and concentrations by molecular biological means, and review the existing regulatory strategies of microbial cellular cofactors and their application progress, to maximize and rapidly direct the metabolic flux to target metabolites. Finally, we speculate on the future of cofactor engineering applications in cell factories. Graphical Abstract.

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.

Engineering polyester monomer diversity through novel pathway design

Polyesters composed of hydroxy acids (HAs) and diols serve many material niches and are invaluable to our daily lives. However, their traditional synthesis from petrochemicals creates many environmental concerns. Metabolically engineered microorganisms have been leveraged for the industrially competitive production of a few polyesters with properties that limit their application. Designing new metabolic pathways to polyester building blocks is essential to broadening material property diversity and improving carbon and energy usage of current bioproduction schemes. This review focuses on recently developed pathways to HAs and diols. Specifically, new pathways to 2,3- and ω-Hydroxy acids, as well as C3-C4 and medium-chain-length diols, are discussed. Pathways to the same compound are compared on the basis of criteria such as energy usage, number of pathway steps, and titer. Finally, suggestions for improvements and next steps for each pathway are also discussed.

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.

Metabolic Engineering and Regulation of Diol Biosynthesis from Renewable Biomass in Escherichia coli

As bulk chemicals, diols have wide applications in many fields, such as clothing, biofuels, food, surfactant and cosmetics. The traditional chemical synthesis of diols consumes numerous non-renewable energy resources and leads to environmental pollution. Green biosynthesis has emerged as an alternative method to produce diols. Escherichia coli as an ideal microbial factory has been engineered to biosynthesize diols from carbon sources. Here, we comprehensively summarized the biosynthetic pathways of diols from renewable biomass in E. coli and discussed the metabolic-engineering strategies that could enhance the production of diols, including the optimization of biosynthetic pathways, improvement of cofactor supplementation, and reprogramming of the metabolic network. We then investigated the dynamic regulation by multiple control modules to balance the growth and production, so as to direct carbon sources for diol production. Finally, we proposed the challenges in the diol-biosynthesis process and suggested some potential methods to improve the diol-producing ability of the host.