One-step fermentative production of poly(lactate-co-glycolate) from carbohydrates in Escherichia coli

Biosynthesis of organic photosensitizer Zn-porphyrin by diphtheria toxin repressor (DtxR)-mediated global upregulation of engineered heme biosynthesis pathway in Corynebacterium glutamicum

Zn-porphyrin is a promising organic photosensitizer in various fields including solar cells, interface and biomedical research, but the biosynthesis study has been limited, probably due to the difficulty of understanding complex biosynthesis pathways. In this study, we developed a Corynebacterium glutamicum platform strain for the biosynthesis of Zn-coproporphyrin III (Zn-CP III), in which the heme biosynthesis pathway was efficiently upregulated. The pathway was activated and reinforced by strong promoter-induced expression of hemA^M (encoding mutated glutamyl-tRNA reductase) and hemL (encoding glutamate-1-semialdehyde aminotransferase) genes. This engineered strain produced 33.54 ± 3.44 mg/l of Zn-CP III, while the control strain produced none. For efficient global regulation of the complex pathway, the dtxR gene encoding the transcriptional regulator diphtheria toxin repressor (DtxR) was first overexpressed in C. glutamicum with hemA^M and hemL genes, and its combinatorial expression was improved by using effective genetic tools. This engineered strain biosynthesized 68.31 ± 2.15 mg/l of Zn-CP III. Finally, fed-batch fermentation allowed for the production of 132.09 mg/l of Zn-CP III. This titer represents the highest in bacterial production of Zn-CP III reported to date, to our knowledge. This study demonstrates that engineered C. glutamicum can be a robust biotechnological model for the production of photosensitizer Zn-porphyrin.

Deciphering bacterial xylose metabolism and metabolic engineering of industrial microorganisms for use as efficient microbial cell factories

The goal of sustainable production of biochemicals and biofuels has driven the engineering of microbial cell as factories that convert low-value substrates to high-value products. Xylose is the second most abundant sugar substrate in lignocellulosic hydrolysates. We analyzed the mechanisms of xylose metabolism using genome sequencing data of 492 industrially relevant bacterial species in the mini-review. The analysis revealed the xylose isomerase and Weimberg pathways as the major routes across diverse routes of bacterial xylose metabolism. In addition, we discuss recent developments in metabolic engineering of xylose metabolism in industrial microorganisms. Genome-scale analyses have revealed xylose pathway-specific flux landscapes. Overall, a comprehensive understanding of bacterial xylose metabolism could be useful for the feasible development of microbial cell factories.

Enhanced production of gamma-aminobutyrate (GABA) in recombinant Corynebacterium glutamicum strains from empty fruit bunch biosugar solution

BACKGROUND

Recent interest has been focused on the production of platform chemicals from renewable biomass due to increasing concerns on global warming and depletion of fossil fuel reserves. Microbial production of platform chemicals in biorefineries has been suggested to be a promising solution for these problems. Gamma-aminobutyrate (GABA), a versatile bulk chemical used in food and pharmaceutical industry, is also used as a key monomer for nylon 4. GABA can be biologically produced by decarboxylation of glutamate.

RESULTS

In this study, we examined high glutamate-producing Corynebacterium glutamicum strains as hosts for enhanced production of GABA from glucose and xylose as carbon sources. An Escherichia coli gadB mutant with a broad pH range of activity and E. coli xylAB genes were expressed under the control of a synthetic H36 promoter. When empty fruit bunch (EFB) solution was used as carbon source (45 g/L glucose and 5 g/L xylose), 12.54 ± 0.07 g/L GABA was produced by recombinant C. glutamicum H36GD1852 expressing E. coli gadB mutant gene and xylAB genes. Batch fermentation of the same strain resulted in the production of 35.47 g/L of GABA when EFB solution was added to support 90 g/L glucose and 10 g/L xylose.

CONCLUSIONS

This is the first report of GABA production by recombinant C. glutamicum strains from co-utilization of glucose and xylose from EFB solution. Recombinant C. glutamicum strains developed in this study should be useful for an efficient and sustainable production of GABA from lignocellulosic biomasses.

Structural Insights into Polyhydroxyalkanoates Biosynthesis

Polyhydroxyalkanoates (PHAs) are diverse biopolyesters produced by numerous microorganisms and have attracted much attention as a substitute for petroleum-based polymers. Despite several decades of study, the detailed molecular mechanisms of PHA biosynthesis have remained unknown due to the lack of structural information on the key PHA biosynthetic enzyme PHA synthase. The recently determined crystal structure of PHA synthase, together with the structures of acetyl-coenzyme A (CoA) acetyltransferase and reductase, have changed this situation. Structural and biochemical studies provided important clues for the molecular mechanisms of each enzyme as well as the overall mechanism of PHA biosynthesis from acetyl-CoA. This new information and knowledge is expected to facilitate production of designed novel PHAs and also enhanced production of PHAs.

A synthetic pathway for the production of 2-hydroxyisovaleric acid in Escherichia coli

Synthetic biology, encompassing the design and construction of novel artificial biological pathways and organisms and the redesign of existing natural biological systems, is rapidly expanding the number of applications for which biological systems can play an integral role. In the context of chemical production, the combination of synthetic biology and metabolic engineering approaches continues to unlock the ability to biologically produce novel and complex molecules from a variety of feedstocks. Here, we utilize a synthetic approach to design and build a pathway to produce 2-hydroxyisovaleric acid in Escherichia coli and demonstrate how pathway design can be supplemented with metabolic engineering approaches to improve pathway performance from various carbon sources. Drawing inspiration from the native pathway for the synthesis of the 5-carbon amino acid l-valine, we exploit the decarboxylative condensation of two molecules of pyruvate, with subsequent reduction and dehydration reactions enabling the synthesis of 2-hydroxyisovaleric acid. Key to our approach was the utilization of an acetolactate synthase which minimized kinetic and regulatory constraints to ensure sufficient flux entering the pathway. Critical host modifications enabling maximum product synthesis from either glycerol or glucose were then examined, with the varying degree of reduction of these carbons sources playing a major role in the required host background. Through these engineering efforts, the designed pathway produced 6.2 g/L 2-hydroxyisovaleric acid from glycerol at 58% of maximum theoretical yield and 7.8 g/L 2-hydroxyisovaleric acid from glucose at 73% of maximum theoretical yield. These results demonstrate how the combination of synthetic biology and metabolic engineering approaches can facilitate bio-based chemical production.

Biochemical characterization of Caulobacter crescentus xylose dehydrogenase

d-Xylose sugar is a common component of hemicellulose, the second largest fraction of biomass. Many groups have developed biological conversions of d-xylose to value-added products by recombinant expression of the xylose dehydrogenase enzyme from Caulobacter crescentus. This enzyme uses NAD⁺ as a cofactor to oxidize d-xylose to d-xylono-1,4-lactone. A detailed understanding of the mechanism of this enzyme could be useful in engineering more efficient versions. Therefore, we have conducted kinetic studies including both the forward and reverse physiological reactions of this enzyme. We demonstrate that the enzyme's substrate binding mode follows a sequential steady state ordered mechanism with NAD⁺ or NADH binding first. Furthermore, the k_cat of the reaction in the direction of NAD⁺ reduction is 10-fold higher than that of the reverse reaction. From rapid reaction studies, we demonstrate the binding of NAD⁺ and NADH to the free enzyme and that hydride transfer occurs in a fast step followed by a much slower steady state. We calculate that the dissociations of the sugar products from the enzyme complexes are the major rate limiting steps in both directions.

RapidRIP quantifies the intracellular metabolome of 7 industrial strains of E. coli

Fast metabolite quantification methods are required for high throughput screening of microbial strains obtained by combinatorial or evolutionary engineering approaches. In this study, a rapid RIP-LC-MS/MS (RapidRIP) method for high-throughput quantitative metabolomics was developed and validated that was capable of quantifying 102 metabolites from central, amino acid, energy, nucleotide, and cofactor metabolism in less than 5 minutes. The method was shown to have comparable sensitivity and resolving capability as compared to a full length RIP-LC-MS/MS method (FullRIP). The RapidRIP method was used to quantify the metabolome of seven industrial strains of E. coli revealing significant differences in glycolytic, pentose phosphate, TCA cycle, amino acid, and energy and cofactor metabolites were found. These differences translated to statistically and biologically significant differences in thermodynamics of biochemical reactions between strains that could have implications when choosing a host for bioprocessing.

Enhancement of malate production through engineering of the periplasmic rTCA pathway in Escherichia coli

The compartmentalization of enzymes into organelles is a promising strategy for limiting metabolic crosstalk and improving pathway efficiency; however, prokaryotes are unicellular organisms that lack membrane-bound organelles. To mimic this natural compartmentalization, we present here the targeting of the reductive tricarboxylic acid (rTCA) pathway to the periplasm to enhance the production of malate. A multigene combination knockout strategy was used to construct a phosphoenolpyruvate (PEP) pool. Then, the genes encoding phosphoenolpyruvate carboxykinase and malate dehydrogenase were combinatorially overexpressed to construct a cytoplasmic rTCA pathway for malate biosynthesis; however, the efficiency of malate production was low. To further enhance malate production, the rTCA pathway was targeted to the periplasm, which led to a 100% increase in malate production to 18.8 mM. Next, dual metabolic engineering regulation was adopted to balance the cytoplasmic and periplasmic pathways, leading to an increase in malate production to 58.8 mM. The final engineered strain, GL2306, produced 193 mM malate with a yield of 0.53 mol/mol in 5 L of pH-stat fed-batch culture. The strategy described here paves the way for the development of metabolic engineering and synthetic biology in the microbial production of chemicals.

Investigation of the Synergetic Effect of Xylose Metabolic Pathways on the Production of Glutaric Acid

Efficient utilization of lignocellulose is pivotal for economically converting renewable feedstocks into value-added products. Xylose is the second most abundant sugar in lignocellulose, but it is quite challenging to ferment xylose as efficiently as glucose by microorganisms. Here, we investigated the metabolic potential of three xylose catabolic pathways (isomerase, Weimberg, and Dahms pathways) and illustrated the synergetic effect between the isomerase pathway and Weimberg pathway for the synthesis of chemicals derived from 2-ketoglutarate and acetyl-CoA. When using glutaric acid as the target product, employment of such synergetic pathways in combination resulted in an increased glutaric acid titer (602 mg/L) compared with using each pathway alone (104 or 209 mg/L), and this titer even outcompetes that obtained from the glucose catabolic pathway for glutaric acid synthesis (420 mg/L). This work validates a novel and powerful strategy for xylose metabolic utilization to overcome the inefficiency of using a single xylose metabolic pathway for the synthesis of TCA cycle derived chemicals.

Synthetic Biology: Immunotherapy by Design

Cellular immunotherapy holds great promise for the treatment of human disease. Clinical evidence suggests that T cell immunotherapies have the potential to combat cancers that evade traditional immunotherapy. Despite promising results, adverse effects leading to fatalities have left scientists seeking tighter control over these therapies, which is reflected in the growing body of synthetic biology literature focused on developing tightly controlled, context-independent parts. In addition, researchers are adapting these tools for other uses, such as for the treatment of autoimmune disease, HIV infection, and fungal interactions. We review this body of work and devote special attention to approaches that may lend themselves to the development of an "ideal" therapy: one that is safe, efficient, and easy to manufacture. We conclude with a look toward the future of immunotherapy: how synthetic biology can shift the paradigm from the treatment of disease to a focus on wellness and human health as a whole.

Microbial synthesis of pyrogallol using genetically engineered Escherichia coli

Pyrogallol is a simple phenolic compound that serves as an attractive chemical with broad applications in food, agricultural, dyeing, printing, cosmetic, photography, chemical and pharmaceutical industries owing to its antioxidant, antibacterial, antiseptic, anticancer, oxygen-absorbing and strong reducing properties. Currently, pyrogallol is commercially produced by thermal decarboxylation of gallic acid under high temperature and pressure. However, this process is limited by the inaccessible raw material, the strict reaction conditions and the relatively low product yield. Here, we report establishment of a novel and efficient biosynthetic pathway for the synthesis of pyrogallol. First, we identified and characterized an efficient 2,3-dihydroxybenzoic acid (2,3-DHBA) 1-monoxygenase from a series of oxygenases and hydroxylases based on the structural similarity in the substrates and products, which enabled non-natural production of pyrogallol from 2,3-DHBA. Then, over-expression of 2,3-DHBA synthase and 2,3-DHBA 1-monoxygenase achieved synthesis of pyrogallol in Escherichia coli, with a titer of 201.52mg/L at 24h. Further optimizations by enhancement of the carbon flux through the shikimate pathway, modular optimization of the pathway and alleviation of the pyrogallol autoxidation boosted pyrogallol titer to 1035.75mg/L in shake flask experiments. This work constructed an efficient microbial platform for gram per liter level production of pyrogallol, indicating the great potential for industrial biomanufacturing of pyrogallol.

Production of 1,3-diols in Escherichia coli

To expand the diversity of chemical compounds produced through microbial conversion, a platform pathway for the production of widely used industrial chemicals, 1,3-diols, was engineered in Escherichia coli. The pathway was designed by modifying the previously reported (R)-1,3-butanediol synthetic pathway to consist of pct (propionate CoA-transferase) from Megasphaera elsdenii, bktB (thiolase), phaB (NADPH-dependent acetoacetyl-CoA reductase) from Ralstonia eutropha, bld (butyraldehyde dehydrogenase) from Clostridium saccharoperbutylacetonicum, and the endogenous alcohol dehydrogenase(s) of E. coli. The recombinant E. coli strains produced 1,3-pentanediol, 4-methyl-1,3-pentanediol, and 1,2,4-butanetriol, together with 1,3-butanediol, from mixtures of glucose and propionate, isobutyrate, and glycolate, respectively, in shake flask cultures. To the best of our knowledge, this is the first report of microbial production of 1,3-pentanediol and 4-methyl-1,3-pentanediol.

Engineering Escherichia coli for malate production by integrating modular pathway characterization with CRISPRi-guided multiplexed metabolic tuning

The application of rational design in reallocating metabolic flux to overproduce desired chemicals is always restricted by the native regulatory network. Here, we demonstrated that in vitro modular pathway optimization combined with in vivo multiplexed combinatorial engineering enables effective characterization of the bottleneck of a complex biosynthetic cascade and improves the output of the engineered pathway. As a proof of concept, we systematically identified the rate-limiting step of a five-gene malate biosynthetic pathway by combinatorially tuning the enzyme loads of a reconstituted biocatalytic reaction in a cell-free system. Using multiplexed CRISPR interference, we subsequently eliminated the metabolic constraints by rationally assigning an optimal gene expression pattern for each pathway module. The present engineered strain Escherichia coli B0013-47 exhibited a 2.3-fold increase in malate titer compared with that of the parental strain, with a yield of 0.85 mol/mol glucose in shake-flask culture and titer of 269 mM (36 g/L) in fed-batch cultivation. The strategy reported herein represents a powerful method for improving the efficiency of multi-gene pathways and advancing the success of metabolic engineering.

Engineering 'cell robots' for parallel and highly sensitive screening of biomolecules under in vivo conditions

Cells are capable of rapid replication and performing tasks adaptively and ultra-sensitively and can be considered as cheap "biological-robots". Here we propose to engineer cells for screening biomolecules in parallel and with high sensitivity. Specifically, we place the biomolecule variants (library) on the bacterial phage M13. We then design cells to screen the library based on cell-phage interactions mediated by a specific intracellular signal change caused by the biomolecule of interest. For proof of concept, we used intracellular lysine concentration in E. coli as a signal to successfully screen variants of functional aspartate kinase III (AK-III) under in vivo conditions, a key enzyme in L-lysine biosynthesis which is strictly inhibited by L-lysine. Comparative studies with flow cytometry method failed to distinguish the wild-type from lysine resistance variants of AK-III, confirming a higher sensitivity of the method. It opens up a new and effective way of in vivo high-throughput screening for functional molecules and can be easily implemented at low costs.

Production of ethylene glycol or glycolic acid from D-xylose in Saccharomyces cerevisiae

The important platform chemicals ethylene glycol and glycolic acid were produced via the oxidative D-xylose pathway in the yeast Saccharomyces cerevisiae. The expression of genes encoding D-xylose dehydrogenase (XylB) and D-xylonate dehydratase (XylD) from Caulobacter crescentus and YagE or YjhH aldolase and aldehyde dehydrogenase AldA from Escherichia coli enabled glycolic acid production from D-xylose up to 150 mg/L. In strains expressing only xylB and xylD, 29 mg/L 2-keto-3-deoxyxylonic acid [(S)-4,5-dihydroxy-2-oxopentanoic acid] (2K3DXA) was produced and D-xylonic acid accumulated to ca. 9 g/L. A significant amount of D-xylonic acid (ca. 14%) was converted to 3-deoxypentonic acid (3DPA), and also, 3,4-dihydroxybutyric acid was formed. 2K3DXA was further converted to glycolaldehyde when genes encoding by either YagE or YjhH aldolase from E. coli were expressed. Reduction of glycolaldehyde to ethylene glycol by an endogenous aldo-keto reductase activity resulted further in accumulation of ethylene glycol of 14 mg/L. The possibility of simultaneous production of lactic and glycolic acids was evaluated by expression of gene encoding lactate dehydrogenase ldhL from Lactobacillus helveticus together with aldA. Interestingly, this increased the accumulation of glycolic acid to 1 g/L. The D-xylonate dehydratase activity in yeast was notably low, possibly due to inefficient Fe-S cluster synthesis in the yeast cytosol, and leading to D-xylonic acid accumulation. The dehydratase activity was significantly improved by targeting its expression to mitochondria or by altering the Fe-S cluster metabolism of the cells with FRA2 deletion.

Biosynthesis of 2-Hydroxyacid-Containing Polyhydroxyalkanoates by Employing butyryl-CoA Transferases in Metabolically Engineered Escherichia coli

The authors previously reported the production of polyhydroxyalkanoates (PHAs) containing 2-hydroxyacid monomers by expressing evolved Pseudomonas sp. 6-19 PHA synthase and Clostridium propionicum propionyl-CoA transferase in engineered microorganisms. Here, the authors examined four butyryl-CoA transferases from Roseburia sp., Eubacterium hallii, Faecalibacterium prausnitzii, and Anaerostipes caccae as potential CoA-transferases to support synthesis of polymers having 2HA monomer. In vitro activity analyses of the four butyryl-CoA transferases suggested that each butyryl-CoA transferase has different activities towards 2-hydroxybutyrate (2HB), 3-hydroxybutyrate (3HB), and lactate (LA). When Escherichia coli XL1-Blue expressing Pseudomonas sp. 6-19 PhaC1437 along with one butyryl-CoA transferase is cultured in chemically defined MR medium containing 20 g L^-1 of glucose, 2 g L^-1 of sodium 3-hydroxybutyrate, and various concentrations of sodium 2-hydroxybutyrate, PHAs consisting of 3HB, 2HB, and LA are produced. The monomer composition of PHAs agreed well with the substrate specificities of butyryl-CoA transferases from E. hallii, F. prausnitzii, and A. caccae, but not Roseburia sp. When E. coli XL1-Blue expressing PhaC1437 and E. hallii butyryl-CoA transferase is cultured in MR medium containing 20 g L^-1 of glucose and 2 g L^-1 of sodium 2-hydroxybutyrate, P(65.7 mol% 2HB-co-34.3 mol% LA) is produced with the highest PHA content of 30 wt%. Butyryl-CoA transferases also supported the production of P(3HB-co-2HB-co-LA) from glucose as the sole carbon source in E. coli XL1-Blue strains when one of these bct genes is expressed with phaC1437, cimA3.7, leuBCD, panE, and phaAB genes. Butyryl-CoA transferases characterized in this study can be used for engineering of microorganisms that produce PHAs containing novel 2-hydroxyacid monomers.

Reversible bacterial immobilization based on the salt-dependent adhesion of the bacterionanofiber protein AtaA

Background

Immobilization of microbial cells is an important strategy for the efficient use of whole-cell catalysts because it simplifies product separation, enables the cell concentration to be increased, stabilizes enzymatic activity, and permits repeated or continuous biocatalyst use. However, conventional immobilization methods have practical limitations, such as limited mass transfer in the inner part of a gel, gel fragility, cell leakage from the support matrix, and adverse effects on cell viability and catalytic activity. We previously showed a new method for bacterial cell immobilization using AtaA, a member of the trimeric autotransporter adhesin family found in Acinetobacter sp. Tol 5. This approach is expected to solve the drawbacks of conventional immobilization methods. However, similar to all other immobilization methods, the use of support materials increases the cost of bioprocesses and subsequent waste materials.

Results

We found that the stickiness of the AtaA molecule isolated from Tol 5 cells is drastically diminished at ionic strengths lower than 10 mM and that it cannot adhere in deionized water, which also inhibits cell adhesion mediated by AtaA. Cells immobilized on well plates and polyurethane foam in a salt solution were detached in deionized water by rinsing and shaking, respectively. The detached cells regained their adhesiveness in a salt solution and could rapidly be re-immobilized. The cells expressing the ataA gene maintained their adhesiveness throughout four repeated immobilization and detachment cycles and could be repeatedly immobilized to polyurethane foam by a 10-min shake in a flask. We also demonstrated that both bacterial cells and a support used in a reaction could be reused for a different type of reaction after detachment of the initially immobilized cells from the support and a subsequent immobilization step.

Conclusions

We invented a unique reversible immobilization method based on the salt-dependent adhesion of the AtaA molecule that allows us to reuse bacterial cells and supports by a simple manipulation involving a deionized water wash. This mitigates problems caused by the use of support materials and greatly helps to enhance the efficiency and productivity of microbial production processes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-017-0740-7) contains supplementary material, which is available to authorized users.

Systems metabolic engineering as an enabling technology in accomplishing sustainable development goals

With pressing issues arising in recent years, the United Nations proposed 17 Sustainable Development Goals (SDGs) as an agenda urging international cooperations for sustainable development. In this perspective, we examine the roles of systems metabolic engineering (SysME) and its contribution to improving the quality of life and protecting our environment, presenting how this field of study offers resolutions to the SDGs with relevant examples. We conclude with offering our opinion on the current state of SysME and the direction it should move forward in the generations to come, explicitly focusing on addressing the SDGs.

Bacterial cellulose as an example product for sustainable production and consumption

Life cycle of bacterial cellulose. Sustainable production and consumption of bio-based products are showcased using bacterial cellulose as an example.

Engineering the xylose-catabolizing Dahms pathway for production of poly(d-lactate-co-glycolate) and poly(d-lactate-co-glycolate-co-d-2-hydroxybutyrate) in Escherichia coli

Poly(lactate-co-glycolate), PLGA, is a representative synthetic biopolymer widely used in medical applications. Recently, we reported one-step direct fermentative production of PLGA and its copolymers by metabolically engineered Escherichia coli from xylose and glucose. In this study, we report development of metabolically engineered E. coli strains for the production of PLGA and poly(d-lactate-co-glycolate-co-d-2-hydroxybutyrate) having various monomer compositions from xylose as a sole carbon source. To achieve this, the metabolic flux towards Dahms pathway was modulated using five different synthetic promoters for the expression of Caulobacter crescentus XylBC. Further metabolic engineering to concentrate the metabolic flux towards d-lactate and glycolate resulted in production of PLGA and poly(d-lactate-co-glycolate-co-d-2-hydroxybutyrate) with various monomer fractions from xylose. The engineered E. coli strains produced polymers containing 8.8-60.9 mol% of glycolate up to 6.93 g l^-1 by fed-batch cultivation in a chemically defined medium containing xylose. Finally, the biocompatibility of poly(d-lactate-co-glycolate-co-d-2-hydroxybutyrate) was confirmed by live/dead assay using human mesenchymal stem cells.

Engineered microbial biosensors based on bacterial two-component systems as synthetic biotechnology platforms in bioremediation and biorefinery

Two-component regulatory systems (TCRSs) mediate cellular response by coupling sensing and regulatory mechanisms. TCRSs are comprised of a histidine kinase (HK), which serves as a sensor, and a response regulator, which regulates expression of the effector gene after being phosphorylated by HK. Using these attributes, bacterial TCRSs can be engineered to design microbial systems for different applications. This review focuses on the current advances in TCRS-based biosensors and on the design of microbial systems for bioremediation and their potential application in biorefinery.

Synthetic metabolism: metabolic engineering meets enzyme design

Metabolic engineering aims at modifying the endogenous metabolic network of an organism to harness it for a useful biotechnological task, for example, production of a value-added compound. Several levels of metabolic engineering can be defined and are the topic of this review. Basic 'copy, paste and fine-tuning' approaches are limited to the structure of naturally existing pathways. 'Mix and match' approaches freely recombine the repertoire of existing enzymes to create synthetic metabolic networks that are able to outcompete naturally evolved pathways or redirect flux toward non-natural products. The space of possible metabolic solution can be further increased through approaches including 'new enzyme reactions', which are engineered on the basis of known enzyme mechanisms. Finally, by considering completely 'novel enzyme chemistries' with de novo enzyme design, the limits of nature can be breached to derive the most advanced form of synthetic pathways. We discuss the challenges and promises associated with these different metabolic engineering approaches and illuminate how enzyme engineering is expected to take a prime role in synthetic metabolic engineering for biotechnology, chemical industry and agriculture of the future.

Modular pathway engineering of Corynebacterium glutamicum to improve xylose utilization and succinate production

Xylose-negative Corynebacterium glutamicum has been engineered to utilize xylose as the sole carbon source via either the xylose isomerase (XI) pathway or the Weimberg pathway. Heterologous expression of xylose isomerase and overexpression of a gene encoding for xylulose kinase enabled efficient xylose utilization. In this study, we show that two functionally-redundant transcriptional regulators (GntR1 and GntR2) present on xylose repress the pentose phosphate pathway genes. For efficient xylose utilization, pentose phosphate pathway genes and a phosphoketolase gene were overexpressed with the XI pathway in C. glutamicum. Overexpression of the genes encoding for transaldolase (Tal), 6-phosphogluconate dehydrogenase (Gnd), or phosphoketolase (XpkA) enhanced the growth and xylose consumption rates compared to the wild-type with the XI pathway alone. However, co-expression of these genes did not have a synergetic effect on xylose utilization. For the succinate production from xylose, overexpression of the tal gene with the XI pathway in a succinate-producing strain improved xylose utilization and increased the specific succinate production rate by 2.5-fold compared to wild-type with the XI pathway alone. Thus, overexpression of the tal, gnd, or xpkA gene could be helpful for engineering C. glutamicum toward production of value-added chemicals with efficient xylose utilization.

Microbial production of vitamin B12: a review and future perspectives

Vitamin B12 is an essential vitamin that is widely used in medical and food industries. Vitamin B12 biosynthesis is confined to few bacteria and archaea, and as such its production relies on microbial fermentation. Rational strain engineering is dependent on efficient genetic tools and a detailed knowledge of metabolic pathways, regulation of which can be applied to improve product yield. Recent advances in synthetic biology and metabolic engineering have been used to efficiently construct many microbial chemical factories. Many published reviews have probed the vitamin B12 biosynthetic pathway. To maximize the potential of microbes for vitamin B12 production, new strategies and tools are required. In this review, we provide a comprehensive understanding of advances in the microbial production of vitamin B12, with a particular focus on establishing a heterologous host for the vitamin B12 production, as well as on strategies and tools that have been applied to increase microbial cobalamin production. Several worthy strategies employed for other products are also included.

Crystal structure of Ralstonia eutropha polyhydroxyalkanoate synthase C-terminal domain and reaction mechanisms

Polyhydroxyalkanoates (PHAs) are natural polyesters synthesized by numerous microorganisms as energy and reducing power storage materials, and have attracted much attention as substitutes for petroleum-based plastics. Here, we report the first crystal structure of Ralstonia eutropha PHA synthase at 1.8 Å resolution and structure-based mechanisms for PHA polymerization. RePhaC1 contains two distinct domains, the N-terminal (RePhaC1_ND ) and C-terminal domains (RePhaC1_CD ), and exists as a dimer. RePhaC1_CD catalyzes polymerization via non-processive ping-pong mechanism using a Cys-His-Asp catalytic triad. Molecular docking simulation of 3-hydroxybutyryl-CoA to the active site of RePhaC1_CD reveals residues involved in the formation of 3-hydroxybutyryl-CoA binding pocket and substrate binding tunnel. Comparative analysis with other polymerases elucidates how different classes of PHA synthases show different substrate specificities. Furthermore, we attempted structure-based protein engineering and developed a RePhaC1 mutant with enhanced PHA synthase activity.

Microbial synthesis of a novel terpolyester P(LA-co-3HB-co-3HP) from low-cost substrates

Polylactide (PLA) is a bio-based plastic commonly synthesized by chemical catalytic reaction using lactic acid (LA) as a substrate. Here, novel LA-containing terpolyesters, namely, P[LA-co-3-hydroxybutyrate (3HB)-co-3-hydroxypropionate (3HP)], short as PLBP, were successfully synthesized for the first time by a recombinant Escherichia coli harbouring polyhydroxyalkanoate (PHA) synthase from Pseudomonas stutzeri (PhaC1_Ps ) with 4-point mutations at E130D, S325T, S477G and Q481K, and 3-hydroxypropionyl-CoA (3HP-CoA) synthesis pathway from glycerol, 3-hydroxybutyryl-CoA (3HB-CoA) as well as lactyl-CoA (LA-CoA) pathways from glucose. Combining these pathways with the PHA synthase mutant phaC1_Ps (E130D S325T S477G Q481K), the random terpolyester P(LA-co-3HB-co-3HP), or PLBP, was structurally confirmed by nuclear magnetic resonance to consist of 2 mol% LA, 90 mol% 3HB, and 8 mol% 3HP respectively. Remarkably, the PLBP terpolyester was produced from low-cost sustainable glycerol and glucose. Monomer ratios of PLBP could be regulated by ratios of glycerol to glucose. Other terpolyester thermal and mechanical properties can be manipulated by adjusting the monomer ratios. More PLBP applications are to be expected.

Tracing metabolism from lignocellulosic biomass and gaseous substrates to products with stable-isotopes

Engineered microbes offer a practical and sustainable alternative to traditional industrial approaches. To increase the economic feasibility of biological processes, microbial isolates are engineered to take up inexpensive feedstocks (including lignocellulosic biomass, syngas, methane, and carbon dioxide), and convert them into substrates of central metabolism and further into value-added products. To trace the metabolism of these feedstocks into products, isotopic tracers are applied together with isotopomer analysis techniques such as ¹³C-metabolic flux analysis to provide a detailed picture of pathway utilization. Flux data is then integrated with kinetic models and constraint-based approaches to identify metabolic bottlenecks, propose novel metabolic engineering strategies, and improve process performance.

Metabolic engineering of Corynebacterium glutamicum for enhanced production of 5-aminovaleric acid

Background

5-Aminovaleric acid (5AVA) is an important five-carbon platform chemical that can be used for the synthesis of polymers and other chemicals of industrial interest. Enzymatic conversion of l-lysine to 5AVA has been achieved by employing lysine 2-monooxygenase encoded by the davB gene and 5-aminovaleramidase encoded by the davA gene. Additionally, a recombinant Escherichia coli strain expressing the davB and davA genes has been developed for bioconversion of l-lysine to 5AVA. To use glucose and xylose derived from lignocellulosic biomass as substrates, rather than l-lysine as a substrate, we previously examined direct fermentative production of 5AVA from glucose by metabolically engineered E. coli strains. However, the yield and productivity of 5AVA achieved by recombinant E. coli strains remain very low. Thus, Corynebacterium glutamicum, a highly efficient l-lysine producing microorganism, should be useful in the development of direct fermentative production of 5AVA using l-lysine as a precursor for 5AVA. Here, we report the development of metabolically engineered C. glutamicum strains for enhanced fermentative production of 5AVA from glucose.

Results

Various expression vectors containing different promoters and origins of replication were examined for optimal expression of Pseudomonas putida davB and davA genes encoding lysine 2-monooxygenase and delta-aminovaleramidase, respectively. Among them, expression of the C. glutamicum codon-optimized davA gene fused with His₆-Tag at its N-Terminal and the davB gene as an operon under a strong synthetic H₃₆ promoter (plasmid p36davAB3) in C. glutamicum enabled the most efficient production of 5AVA. Flask culture and fed-batch culture of this strain produced 6.9 and 19.7 g/L (together with 11.9 g/L glutaric acid as major byproduct) of 5AVA, respectively. Homology modeling suggested that endogenous gamma-aminobutyrate aminotransferase encoded by the gabT gene might be responsible for the conversion of 5AVA to glutaric acid in recombinant C. glutamicum. Fed-batch culture of a C. glutamicum gabT mutant-harboring p36davAB3 produced 33.1 g/L 5AVA with much reduced (2.0 g/L) production of glutaric acid.

Conclusions

Corynebacterium glutamicum was successfully engineered to produce 5AVA from glucose by optimizing the expression of two key enzymes, lysine 2-monooxygenase and delta-aminovaleramidase. In addition, production of glutaric acid, a major byproduct, was significantly reduced by employing C. glutamicum gabT mutant as a host strain. The metabolically engineered C. glutamicum strains developed in this study should be useful for enhanced fermentative production of the novel C5 platform chemical 5AVA from renewable resources.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0566-8) contains supplementary material, which is available to authorized users.

Central metabolic nodes for diverse biochemical production

Central carbon metabolism is conserved among all organisms for cellular function and energy generation. The connectivity of this metabolic map gives rises to key metabolite nodes. Five of these nodes in particular, pyruvate, citric acid, tyrosine and aspartate, acetyl-CoA, serve as critical starting points for the generation of a broad class of relevant chemical molecules with ranging applications from fuels, pharmaceuticals and polymer precursors. This review highlights recent progress in converting these metabolite nodes into valuable products. In particular, acetyl-CoA, the most well-connected node, serves as the building block for several classes of molecules including fatty acids and terpenes. Systematic metabolic engineering efforts focused on these metabolic building blocks has enabled the production of industrially-relevant, biobased compounds.

Prospects of microbial cell factories developed through systems metabolic engineering

While academic‐level studies on metabolic engineering of microorganisms for production of chemicals and fuels are ever growing, a significantly lower number of such production processes have reached commercial‐scale. In this work, we review the challenges associated with moving from laboratory‐scale demonstration of microbial chemical or fuel production to actual commercialization, focusing on key requirements on the production organism that need to be considered during the metabolic engineering process. Metabolic engineering strategies should take into account techno‐economic factors such as the choice of feedstock, the product yield, productivity and titre, and the cost effectiveness of midstream and downstream processes. Also, it is important to develop an industrial strain through metabolic engineering for pathway construction and flux optimization together with increasing tolerance to products and inhibitors present in the feedstock, and ensuring genetic stability and strain robustness under actual fermentation conditions.

Recombinant Ralstonia eutropha engineered to utilize xylose and its use for the production of poly(3-hydroxybutyrate) from sunflower stalk hydrolysate solution

Background

Lignocellulosic raw materials have extensively been examined for the production of bio-based fuels, chemicals, and polymers using microbial platforms. Since xylose is one of the major components of the hydrolyzed lignocelluloses, it is being considered a promising substrate in lignocelluloses based fermentation process. Ralstonia eutropha, one of the most powerful and natural producers of polyhydroxyalkanoates (PHAs), has extensively been examined for the production of bio-based chemicals, fuels, and polymers. However, to the best of our knowledge, lignocellulosic feedstock has not been employed for R. eutropha probably due to its narrow spectrum of substrate utilization. Thus, R. eutropha engineered to utilize xylose should be useful in the development of microbial process for bio-based products from lignocellulosic feedstock.

Results

Recombinant R. eutropha NCIMB11599 expressing the E. coli xylAB genes encoding xylose isomerase and xylulokinase respectively, was constructed and examined for the synthesis of poly(3-hydroxybutyrate) [P(3HB)] using xylose as a sole carbon source. It could produce 2.31 g/L of P(3HB) with a P(3HB) content of 30.95 wt% when it was cultured in a nitrogen limited chemically defined medium containing 20.18 g/L of xylose in a batch fermentation. Also, recombinant R. eutropha NCIMB11599 expressing the E. coli xylAB genes produced 5.71 g/L of P(3HB) with a P(3HB) content of 78.11 wt% from a mixture of 10.05 g/L of glucose and 10.91 g/L of xylose in the same culture condition. The P(3HB) concentration and content could be increased to 8.79 g/L and 88.69 wt%, respectively, when it was cultured in the medium containing 16.74 g/L of glucose and 6.15 g/L of xylose. Further examination of recombinant R. eutropha NCIMB11599 expressing the E. coli xylAB genes by fed-batch fermentation resulted in the production of 33.70 g/L of P(3HB) in 108 h with a P(3HB) content of 79.02 wt%. The concentration of xylose could be maintained as high as 6 g/L, which is similar to the initial concentration of xylose during the fed-batch fermentation suggesting that xylose consumption is not inhibited during fermentation. Finally, recombinant R. eutorpha NCIMB11599 expressing the E. coli xylAB gene was examined for the production of P(3HB) from the hydrolysate solution of sunflower stalk. The hydrolysate solution of sunflower stalk was prepared as a model lignocellulosic biomass, which contains 78.8 g/L of glucose, 26.9 g/L of xylose, and small amount of 4.8 g/L of galactose and mannose. When recombinant R. eutropha NCIMB11599 expressing the E. coli xylAB genes was cultured in a nitrogen limited chemically defined medium containing 23.1 g/L of hydrolysate solution of sunflower stalk, which corresponds to 16.8 g/L of glucose and 5.9 g/L of xylose, it completely consumed glucose and xylose in the sunflower stalk based medium resulting in the production of 7.86 g/L of P(3HB) with a P(3HB) content of 72.53 wt%.

Conclusions

Ralstonia eutropha was successfully engineered to utilize xylose as a sole carbon source as well as to co-utilize it in the presence of glucose for the synthesis of P(3HB). In addition, R. eutropha engineered to utilized xylose could synthesize P(3HB) from the sunflower stalk hydrolysate solution containing glucose and xylose as major sugars, which suggests that xylose utilizing R. eutropha developed in this study should be useful for development of lignocellulose based microbial processes.