Fed-batch culture of a metabolically engineered Escherichia coli strain designed for high-level succinate production and yield under aerobic conditions

Advances in succinic acid production: the enhancement of CO2 fixation for the carbon sequestration benefits

Succinic acid (SA), one of the 12 top platform chemicals produced from biomass, is a precursor of various high value-added derivatives. Specially, 1 mol CO2 is assimilated in 1 mol SA biosynthetic route under anaerobic conditions, which helps to achieve carbon reduction goals. In this review, methods for enhanced CO2 fixation in SA production and utilization of waste biomass for SA production are reviewed. Bioelectrochemical and bioreactor coupling systems constructed with off-gas reutilization to capture CO2 more efficiently were highlighted. In addition, the techno-economic analysis and carbon sequestration benefits for the synthesis of bio-based SA from CO2 and waste biomass are analyzed. Finally, a droplet microfluidics-based high-throughput screening technique applied to the future bioproduction of SA is proposed as a promising approach.

Biosynthetic Pathway and Metabolic Engineering of Succinic Acid

Succinic acid, a dicarboxylic acid produced as an intermediate of the tricarboxylic acid (TCA) cycle, is one of the most important platform chemicals for the production of various high value-added derivatives. As traditional chemical synthesis processes suffer from nonrenewable resources and environment pollution, succinic acid biosynthesis has drawn increasing attention as a viable, more environmentally friendly alternative. To date, several metabolic engineering approaches have been utilized for constructing and optimizing succinic acid cell factories. In this review, different succinic acid biosynthesis pathways are summarized, with a focus on the key enzymes and metabolic engineering approaches, which mainly include redirecting carbon flux, balancing NADH/NAD⁺ ratios, and optimizing CO₂ supplementation. Finally, future perspectives on the microbial production of succinic acid are discussed.

Synthetic Formatotrophs for One-Carbon Biorefinery

The use of CO2 as a carbon source in biorefinery is of great interest, but the low solubility of CO2 in water and the lack of efficient CO2 assimilation pathways are challenges to overcome. Formic acid (FA), which can be easily produced from CO2 and more conveniently stored and transported than CO2, is an attractive CO2-equivalent carbon source as it can be assimilated more efficiently than CO2 by microorganisms and also provides reducing power. Although there are native formatotrophs, they grow slowly and are difficult to metabolically engineer due to the lack of genetic manipulation tools. Thus, much effort is exerted to develop efficient FA assimilation pathways and synthetic microorganisms capable of growing solely on FA (and CO2). Several innovative strategies are suggested to develop synthetic formatotrophs through rational metabolic engineering involving new enzymes and reconstructed FA assimilation pathways, and/or adaptive laboratory evolution (ALE). In this paper, recent advances in development of synthetic formatotrophs are reviewed, focusing on biological FA and CO2 utilization pathways, enzymes involved and newly developed, and metabolic engineering and ALE strategies employed. Also, future challenges in cultivating formatotrophs to higher cell densities and producing chemicals from FA and CO2 are discussed.

Promising advancement in fermentative succinic acid production by yeast hosts

As a platform chemical with various applications, succinic acid (SA) is currently produced by petrochemical processing from oil-derived substrates such as maleic acid. In order to replace the environmental unsustainable hydrocarbon economy with a renewable environmentally sound carbohydrate economy, bio-based SA production process has been developed during the past two decades. In this review, recent advances in the valorization of solid organic wastes including mixed food waste, agricultural waste and textile waste for efficient, green and sustainable SA production have been reviewed. Firstly, the application, market and key global players of bio-SA are summarized. Then achievements in SA production by several promising yeasts including Saccharomyces cerevisiae and Yarrowia lipolytica are detailed, followed by calculation and comparison of SA production costs between oil-based substrates and raw materials. Lastly, challenges in engineered microorganisms and fermentation processes are presented together with perspectives on the development of robust yeast SA producers via genome-scale metabolic optimization and application of low-cost raw materials as fermentation substrates. This review provides valuable insights for identifying useful directions for future bio-SA production improvement.

The Role of Selected Lactic Acid Bacteria on Organic Acid Accumulation during Wet and Spray-Dried Fish-based Silages. Contributions to the Winning Combination of Microbial Food Safety and Environmental Sustainability

Organic acid contents of acidified and fermented fish silages made from gibel carp (Caracius gibelio) and klunzinger’s ponyfish (Equulites klunzingeri) fishes, and from fish processing residues or by-products, were determined and studied. The silages were undertaken in wet and spray-dried fish-based raw-materials for 3 weeks at room temperature (ca. 25 °C). Selected lactic acid bacteria (LAB) of Enterococcus gallinarum, Lactobacillus brevis, Lactobacillus plantarum, Pediococcus acidilactici, and Streptococcus spp. were employed to produce fermented fish-based silages, while acidified silage was prepared resorting to the addition of formic acid (3%, v/v). Lactic and propionic acids were the dominant produced organic acids, while succinic acid was formed at the smallest amounts in fermented silages. In the acidified silage, lactic and formic acids were produced in amounts higher than 800 and 1000 mg _{organic acid}/100 g _sample, respectively. Among the fermented fish-based silages, LAB strains unfolded considerable ability to presumptively produce propionic acid in gibel carp silage (>2370 mg _{organic acid}/100 g _sample). Spray-dried fermented silages displayed significantly higher organic acid content than wet silages. Propionic acid accumulation was found at the highest levels in gibel carp silage fermented with L. plantarum (6335.40 mg _{propionic acid}/100 g _sample). This research effort pointed out the good capability of various selected lactic acid bacteria strains to produce significant amounts of organic acids—especially lactic, acetic, and propionic acids—during the fermentation of fish-based silages. In terms of food safety and quality, such a production of relatively high amounts of organic acids in wet and spray-dried fish-based silages clearly indicated their suitableness to be used for animal feed.

Reconstruction of a genome-scale metabolic model for Actinobacillus succinogenes 130Z

BACKGROUND

Actinobacillus succinogenes is a promising bacterial catalyst for the bioproduction of succinic acid from low-cost raw materials. In this work, a genome-scale metabolic model was reconstructed and used to assess the metabolic capabilities of this microorganism under producing conditions.

RESULTS

The model, iBP722, was reconstructed based on the functional reannotation of the complete genome sequence of A. succinogenes 130Z and manual inspection of metabolic pathways, covering 1072 enzymatic reactions associated with 722 metabolic genes that involve 713 metabolites. The highly curated model was effective in capturing the growth of A. succinogenes on various carbon sources, as well as the SA production under various growth conditions with fair agreement between experimental and predicted data. Calculated flux distributions under different conditions show that a number of metabolic pathways are affected by the activity of some metabolic enzymes at key nodes in metabolism, including the transport mechanism of carbon sources and the ability to fix carbon dioxide.

CONCLUSIONS

The established genome-scale metabolic model can be used for model-driven strain design and medium alteration to improve succinic acid yields.

Advancement of Biotechnology by Genetic Modifications

One of the greatest sources of metabolic and enzymatic diversity are microorganisms. In recent years, emerging recombinant DNA and genomic techniques have facilitated the development of new efficient expression systems, modification of biosynthetic pathways leading to new metabolites by metabolic engineering, and enhancement of catalytic properties of enzymes by directed evolution. Complete sequencing of industrially important microbial genomes is taking place very rapidly, and there are already hundreds of genomes sequenced. Functional genomics and proteomics are major tools used in the search for new molecules and development of higher-producing strains.

Holistic bioengineering: rewiring central metabolism for enhanced bioproduction

What does it take to convert a living organism into a truly productive biofactory? Apart from optimizing biosynthesis pathways as standalone units, a successful bioengineering approach must bend the endogenous metabolic network of the host, and especially its central metabolism, to support the bioproduction process. In practice, this usually involves three complementary strategies which include tuning-down or abolishing competing metabolic pathways, increasing the availability of precursors of the desired biosynthesis pathway, and ensuring high availability of energetic resources such as ATP and NADPH. In this review, we explore these strategies, focusing on key metabolic pathways and processes, such as glycolysis, anaplerosis, the TCA (tricarboxylic acid) cycle, and NADPH production. We show that only a holistic approach for bioengineering — considering the metabolic network of the host organism as a whole, rather than focusing on the production pathway alone — can truly mold microorganisms into efficient biofactories.

Re-engineering Escherichia coli KJ122 to enhance the utilization of xylose and xylose/glucose mixture for efficient succinate production in mineral salt medium

Escherichia coli KJ122 was previously engineered to produce high concentration and yield of succinate in mineral salt medium containing glucose and sucrose under anaerobic conditions. However, this strain does not efficiently utilize xylose. To improve the xylose uptake and utilization in the strain KJ122, xylFGH and xylE genes were individually and simultaneously deleted. E. coli KJ12201 (KJ122::ΔxylFGH) exhibited superior abilities in growth, xylose consumption, and succinate production compared to those of the parental strain KJ122. However, E. coli KJ12202 (KJ122::ΔxylE) lessened xylose consumption due to an ATP deficit for metabolizing xylose thus making succinate production from xylose not preferable. Moreover, E. coli KJ12203 (KJ122::ΔxylFGHΔxylE) exhibited an impaired growth on xylose due to lacking of xylose transporters. After performing metabolic evolution, the evolved KJ12201-14T strain exhibited a great improvement in succinate production from pure xylose with higher concentration and productivity about 18 and 21%, respectively, compared to KJ12201 strain. During fed-batch fermentation, KJ12201-14T also produced succinate from xylose at a concentration, yield, and overall productivity of 84.6 ± 0.7 g/L, 0.86 ± 0.01 g/g and 1.01 ± 0.01 g/L/h, respectively. KJ12201 and KJ12201-14T strains co-utilized glucose/xylose mixture without catabolite repression. Both strains produced succinate from glucose/xylose mixture at concentration, yield, and overall and specific productivities of about 85 g/L, 0.85 g/g, 0.70 g/L/h, and 0.44 g/gCDW/h, respectively. Based on our results, KJ12201 and KJ12201-14T strains exhibited a greater performance in succinate production from xylose containing medium than those of other published works. They would be potential strains for the economic bio-based succinate production from xylose.

Production of succinic acid by metabolically engineered microorganisms

Succinic acid (SA) has been recognized as one of the most important bio-based building block chemicals due to its numerous potential applications. For the economical bio-based production of SA, extensive research works have been performed on developing microbial strains by metabolic engineering as well as fermentation and downstream processes. Here we review metabolic engineering strategies applied for bio-based production of SA using representative microorganisms, including Saccharomyces cerevisiae, Pichia kudriavzevii, Escherichia coli, Mannheimia succiniciproducens, Basfia succiniciproducens, Actinobacillus succinogenes, and Corynebacterium glutamicum. In particular, strategies employed for developing engineered strains of these microorganisms leading to the best performance indices (titer, yield, and productivity) are showcased based on the published papers as well as patents. Those processes currently under commercialization are also analyzed and future perspectives are provided.

Succinate Overproduction: A Case Study of Computational Strain Design Using a Comprehensive Escherichia coli Kinetic Model

Computational strain-design prediction accuracy has been the focus for many recent efforts through the selective integration of kinetic information into metabolic models. In general, kinetic model prediction quality is determined by the range and scope of genetic and/or environmental perturbations used during parameterization. In this effort, we apply the k-OptForce procedure on a kinetic model of E. coli core metabolism constructed using the Ensemble Modeling (EM) method and parameterized using multiple mutant strains data under aerobic respiration with glucose as the carbon source. Minimal interventions are identified that improve succinate yield under both aerobic and anaerobic conditions to test the fidelity of model predictions under both genetic and environmental perturbations. Under aerobic condition, k-OptForce identifies interventions that match existing experimental strategies while pointing at a number of unexplored flux re-directions such as routing glyoxylate flux through the glycerate metabolism to improve succinate yield. Many of the identified interventions rely on the kinetic descriptions that would not be discoverable by a purely stoichiometric description. In contrast, under fermentative (anaerobic) condition, k-OptForce fails to identify key interventions including up-regulation of anaplerotic reactions and elimination of competitive fermentative products. This is due to the fact that the pathways activated under anaerobic condition were not properly parameterized as only aerobic flux data were used in the model construction. This study shed light on the importance of condition-specific model parameterization and provides insight on how to augment kinetic models so as to correctly respond to multiple environmental perturbations.

Succinic acid production from hemicellulose hydrolysate by an Escherichia coli mutant obtained by atmospheric and room temperature plasma and adaptive evolution

Atmospheric and room temperature plasma and adaptive evolution were combined to generate Escherichia coli mutants, which can simultaneously and efficiently utilize glucose and xylose to produce succinic acid in chemically defined medium under exclusively anaerobic condition. Compared to the parent strain BA305, a pflB, ldhA, ppc, and ptsG deletion strain overexpressing ATP-forming phosphoenolpyruvate (PEP) carboxykinase (PEPCK), the sugar consumption rate and succinic acid productivity of mutant BA408 were significantly improved with a marked increase in the key enzyme activities. Subsequent anaerobic fermentation of BA408 with corn stalk hydrolysate produced a final succinic acid concentration of 23.1 g L(-1) with a yield of 0.85 g g(-1) sugar mixture. The observed synthesis of succinic acid from the corn stalk hydrolysate showed a great potential usage of renewable biomass as a feedstock for an economical succinic acid production using E. coli.

Fermentative succinate production: an emerging technology to replace the traditional petrochemical processes

Succinate is a valuable platform chemical for multiple applications. Confronted with the exhaustion of fossil energy resources, fermentative succinate production from renewable biomass to replace the traditional petrochemical process is receiving an increasing amount of attention. During the past few years, the succinate-producing process using microbial fermentation has been made commercially available by the joint efforts of researchers in different fields. In this review, recent attempts and experiences devoted to reduce the production cost of biobased succinate are summarized, including strain improvement, fermentation engineering, and downstream processing. The key limitations and challenges faced in current microbial production systems are also proposed.

Directed pathway evolution of the glyoxylate shunt in Escherichia coli for improved aerobic succinate production from glycerol

α-Ketoglutarate is accumulated as the main byproduct during the aerobic succinate production from glycerol by Escherichia coli BL21(DE3) in minimal medium. To address this issue, here a strategy of directed pathway evolution was developed to enhance the alternative succinate production route-the glyoxylate shunt. Via the directed pathway evolution, the glyoxylate shunt was recruited as the primary anaplerotic pathway in a ppc mutant, which restored its viability in glycerol minimal medium. Subsequently, the operon sdhCDAB was deleted and the gene ppc was reverted in the evolved strain for succinate production. The resulting strain E2-Δsdh-ppc produced 30 % more succinate and 46 % less α-ketoglutarate than the control strain. A G583T mutation in gene icdA, which significantly decreased the activity of isocitrate dehydrogenase, was identified in the evolved strain as the main mutation responsible for the observed phenotype. Overexpression of α-ketoglutarate dehydrogenase complex in E2-Δsdh-ppc further reduced the amount of byproduct and improved succinate production. The final strain E2-Δsdh-ppc-sucAB produced 366 mM succinate from 1.3 M glycerol in minimal medium in fed-batch fermentation. The maximum and average succinate volumetric productivities were 19.2 and 6.55 mM h(-1), respectively, exhibiting potential industrial production capacity from the low-priced substrate.

Enhanced succinic acid production by Actinobacillus succinogenes after genome shuffling

Succinic acid is an important platform chemical for synthesis of C4 compounds. We applied genome shuffling to improve fermentative production of succinic acid by A. succinogenes. Using a screening strategy composed of selection in fermentation broth, cultured in 96-deep-well plates, and condensed HPLC screening, a starting population of 11 mutants producing a higher succinic acid concentration was selected and subjected to recursive protoplasts fusion. After three rounds of genome shuffling, strain F3-II-3-F was obtained, producing succinic acid at 1.99 g/l/h with a yield of 95.6 g/l. The genome shuffled strain had about a 73 % improvement in succinic acid production compared to the parent strain after 48 h in fed-batch fermentation. The genomic variability of F3-II-3-F was confirmed by amplified fragment-length polymorphism. The activity levels of key enzymes involved in end-product formation from glucose and metabolic flux distribution during succinic acid production were compared between A. succinogenes CGMCC 1593 and F3-II-3-F. Increased activity of glucokinase, fructose-1,6-bisphosphate aldolase, PEP carboxykinase and fumarase, as well as decreased activity of pyruvate kinase, pyruvate formate-lyase, and acetate kinase explained the enhanced succinic acid production and decreased acetic acid formation. Metabolic flux analysis suggested that increased flux to NADH was the main reason for increased activity of the C4 pathway resulting in increased yields of succinic acid. The present work will be propitious to the development of a bio-succinic acid fermentation industry.

Activation of glyoxylate pathway without the activation of its related gene in succinate-producing engineered Escherichia coli

For the first time, glyoxylate pathway in the biosynthesis of succinate was activated without the genetic manipulations of any gene related with glyoxylate pathway. Furthermore, the inactivation of succinate biosynthesis by-products genes encoding acetate kinase (ackA) and phosphotransacetylase (pta) was proven to be the key factor to activate glyoxylate pathway in the metabolically engineered Escherichia coli under anaerobic conditions. In order to enhance the succinate biosynthesis specifically, the genes (i.e., ldhA, ptsG, ackA-pta, focA-pflB, adhE) that disrupt by-products biosynthesis pathways were combinatorially deleted, while the E. coli malate dehydrogenase (MDH) was overexpression. The highest succinate production of 150.78 mM was obtained with YJ003 (ΔldhA, ptsG, ackA-pta), which were 5-folds higher than that obtained with wild type control strain DY329 (25.13 mM). For further understand the metabolic response as a result of several genetic manipulations, an anaerobic stoichiometric model that takes into account the glyoxylate pathway have successfully been implemented to estimate the intracellular fluxes in various recombinant E. coli. The fraction to the glyoxylate pathway from OAA in DY329 was 0 and 31% in YJ003, which indicated that even without the absence of the iclR mutation; the glyoxylate pathway was also activated by deleting the by-products biosynthetic genes, and to be responsible for the higher succinate yields. For further strengthen glyoxylate pathway, a two-stage fed-batch fermentation process was developed by using a 600 g l(-1) glucose feed to achieve a cell growth rate of 0.07 h(-1) in aerobic fermentation, and using a 750 g l(-1) glucose feed to maintain the residual glucose concentration around 40 g l(-1) when its residual level decreased to 10gl(-1) in anaerobic fermentation. The best mutant strain YJ003/pTrc99A-mdh produces final succinate concentration of 274 mM by fed-batch culture, which was 10-folds higher than that obtained with wild type control strain DY329. This study discovered that glyoxylate pathway could be activated by deleting glyoxylate pathway irrelevant genes (i.e., genes encoding acetate and lactate) and consequently the succinate biosynthesis was effectively improved. This work provides useful information for the modification of metabolic pathway to improve succinate production.

Activating phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase in combination for improvement of succinate production

Phosphoenolpyruvate (PEP) carboxylation is an important step in the production of succinate by Escherichia coli. Two enzymes, PEP carboxylase (PPC) and PEP carboxykinase (PCK), are responsible for PEP carboxylation. PPC has high substrate affinity and catalytic velocity but wastes the high energy of PEP. PCK has low substrate affinity and catalytic velocity but can conserve the high energy of PEP for ATP formation. In this work, the expression of both the ppc and pck genes was modulated, with multiple regulatory parts of different strengths, in order to investigate the relationship between PPC or PCK activity and succinate production. There was a positive correlation between PCK activity and succinate production. In contrast, there was a positive correlation between PPC activity and succinate production only when PPC activity was within a certain range; excessive PPC activity decreased the rates of both cell growth and succinate formation. These two enzymes were also activated in combination in order to recruit the advantages of each for the improvement of succinate production. It was demonstrated that PPC and PCK had a synergistic effect in improving succinate production.

Enhancement of succinate production by metabolically engineered Escherichia coli with co-expression of nicotinic acid phosphoribosyltransferase and pyruvate carboxylase

Escherichia coli BA002, in which the ldhA and pflB genes are deleted, cannot utilize glucose anaerobically due to the inability to regenerate NAD(+). To restore glucose utilization, overexpression of nicotinic acid phosphoribosyltransferase (NAPRTase) encoded by the pncB gene, a rate-limiting enzyme of NAD(H) synthesis pathway, resulted in a significant increase in cell mass and succinate production under anaerobic conditions. However, a high concentration of pyruvate accumulated. Thus, co-expression of NAPRTase and the heterologous pyruvate carboxylase (PYC) of Lactococcus lactis subsp. cremoris NZ9000 in recombinant E. coli BA016 was investigated. The total concentration of NAD(H) was 9.8-fold higher in BA016 than in BA002, and the NADH/NAD(+) ratio decreased from 0.60 to 0.04. Under anaerobic conditions, BA016 consumed 17.50 g l(-1) glucose and produced 14.08 g l(-1) succinate with a small quantity of pyruvate. Furthermore, when the reducing agent dithiothreitol or reduced carbon source sorbitol was added, the cell growth and carbon source consumption rate of BA016 was reasonably enhanced and succinate productivity increased.

Production of succinic acid by engineered E. coli strains using soybean carbohydrates as feedstock under aerobic fermentation conditions

Escherichia coli strains HL2765 and HL27659k harboring pRU600 and pKK313 were examined for succinate production under aerobic conditions using galactose, sucrose, raffinose, stachyose, and mixtures of these sugars extracted from soybean meal and soy solubles. HL2765(pKK313)(pRU600) and HL27659k(pKK313)(pRU600) consumed 87mM and 98mM hexose of soybean meal extract and produced 83mM and 95mM succinate, respectively. While using soy solubles extract, HL2765(pKK313)(pRU600) and HL27659k(pKK313)(pRU600) consumed 160mM and 187mM hexose and produced 158mM and 183mM succinate, respectively. Succinate yield of HL2765(pKK313)(pRU600) was low as compared to that of HL27659k(pKK313)(pRU600) while using acid hydrolysate of soybean meal or soy solubles extracts. Maximum succinate production of 312mM with a molar yield of 0.82mol/mol hexose was obtained using soy solubles hydrolysate by HL27659k(pKK313)(pRU600). This study demonstrated the use of soluble carbohydrates of the renewable feedstock, soybean as an inexpensive carbon source to produce succinate by fermentation.

Essential role of genetics in the advancement of biotechnology

Microorganisms are one of the greatest sources of metabolic and enzymatic diversity. In recent years, emerging recombinant DNA and genomic techniques have facilitated the development of new efficient expression systems, modification of biosynthetic pathways leading to new metabolites by metabolic engineering, and enhancement of catalytic properties of enzymes by directed evolution. Complete sequencing of industrially important microbial genomes is taking place very rapidly and there are already hundreds of genomes sequenced. Functional genomics and proteomics are major tools used in the search for new molecules and development of higher-producing strains.

Recruiting alternative glucose utilization pathways for improving succinate production

The phosphoenolpyruvate (PEP): carbohydrate phosphotransferase system (PTS) of Escherichia coli was usually inactivated to increase PEP supply for succinate production. However, cell growth and glucose utilization rate decreased significantly with PTS inactivation. In this work, two glucose transport proteins and two glucokinases (Glk) from E. coli and Zymomonas mobilis were recruited in PTS(-) strains, and their impacts on glucose utilization and succinate production were compared. All PTS(-) strains recruiting Z. mobilis glucose facilitator Glf had higher glucose utilization rates than PTS(-) strains using E. coli galactose permease (GalP), which was suggested to be caused by higher glucose transport velocity and lower energetic cost of Glf. The highest rate obtained by combinatorial modulation of glf and glk E. coli (2.13 g/L•h) was 81 % higher than the wild-type E. coli and 30 % higher than the highest rate obtained by combinatorial modulation of galP and glk E. coli . On the other hand, although glucokinase activities increased after replacing E. coli Glk with isoenzyme of Z. mobilis, glucose utilization rate decreased to 0.58 g/L•h, which was assumed due to tight regulation of Z. mobilis Glk by energy status of the cells. For succinate production, using GalP led to a 20 % increase in succinate productivity, while recruiting Glf led to a 41 % increase. These efficient alternative glucose utilization pathways obtained in this work can also be used for production of many other PEP-derived chemicals, such as malate, fumarate, and aromatic compounds.

Fermentation of xylose to succinate by enhancement of ATP supply in metabolically engineered Escherichia coli

In Escherichia coli K12, succinate was not the dominant fermentation product from xylose. To reduce byproduct formation and increase succinate accumulation,pyruvate formate lyase and lactate dehydrogenase, encoded by pflB and ldhA genes, were inactivated. However, these mutations eliminated cell growth and xylose utilization. During anaerobic growth of bacteria, organic intermediates,such as pyruvate, serve as electron acceptors to maintain the overall redox balance. Under these conditions, the ATP needed for cell growth is derived from substrate level phosphorylation. In E. coli K12, conversion of xylose to pyruvate only yielded 0.67 net ATP per xylose during anaerobic fermentation. However, E. coli produces equimolar amounts of acetate and ethanol from two pyruvates, and these reactions generate one additional ATP. Conversion of xylose to acetate and ethanol increases the net ATP yield from 0.67 to 1.5 per xylose, which could meet the ATP needed for xylose metabolism. A pflB deletion strain cannot convert pyruvate to acetyl coenzyme A, the precursor for acetate and ethanol production, and could not produce the additional ATP. Thus,the double mutations eliminated cell growth and xylose utilization. To supply the sufficient ATPs, overexpression of ATP-forming phosphoenolpyruvate-carboxykinase from Bacillus subtilis 168 in an ldhA, pflB, and ppc deletion strain resulted in a significant increase in cell mass and succinate production. In addition, fermentation of corn stalk hydrolysate containing a high percentage of xylose and glucose produced a final succinate concentration of 11.13 g l−1 with a yield of1.02 g g−1 total sugars during anaerobic fermentation.

Succinate production in Escherichia coli

Succinate has been recognized as an important platform chemical that can be produced from biomass. While a number of organisms are capable of succinate production naturally, this review focuses on the engineering of Escherichia coli for the production of four-carbon dicarboxylic acid. Important features of a succinate production system are to achieve an optimal balance of reducing equivalents generated by consumption of the feedstock, while maximizing the amount of carbon channeled into the product. Aerobic and anaerobic production strains have been developed and applied to production from glucose and other abundant carbon sources. Metabolic engineering methods and strain evolution have been used and supplemented by the recent application of systems biology and in silico modeling tools to construct optimal production strains. The metabolic capacity of the production strain, the requirement for efficient recovery of succinate, and the reliability of the performance under scaleup are important in the overall process. The costs of the overall biorefinery-compatible process will determine the economic commercialization of succinate and its impact in larger chemical markets.

Production of succinate and polyhydroxyalkanoate from substrate mixture by metabolically engineered Escherichia coli

The strategic design of this study aimed at producing succinate and polyhydroxyalkanoate (PHA) from substrate mixture of glycerol/glucose and fatty acid in Escherichia coli. To accomplish this, an E. coli KNSP1 strain derived from E. coli LR1110 was constructed by deletions of ptsG, sdhA and pta genes and overexpression of phaC1 from Pseudomonas aeruginosa. Cultivation of E. coli KNSP1 showed that this strain was able to produce 21.07 g/L succinate and 0.54 g/L PHA (5.62 wt.% of cell dry weight) from glycerol and fatty acid mixture. The generated PHA composed of 58.7 mol% 3-hydroxyoctanoate (3HO) and 41.3 mol% 3-hydroxydecanoate (3HD). This strain would be useful for complete utilization of byproducts glycerol and fatty acid of biodiesel production process.

Metabolic engineering of Escherichia coli for biotechnological production of high-value organic acids and alcohols

Confronted with the gradual and inescapable exhaustion of the earth's fossil energy resources, the bio-based process to produce platform chemicals from renewable carbohydrates is attracting growing interest. Escherichia coli has been chosen as a workhouse for the production of many valuable chemicals due to its clear genetic background, convenient to be genetically modified and good growth properties with low nutrient requirements. Rational strain development of E. coli achieved by metabolic engineering strategies has provided new processes for efficiently biotechnological production of various high-value chemical building blocks. Compared to previous reviews, this review focuses on recent advances in metabolic engineering of the industrial model bacteria E. coli that lead to efficient recombinant biocatalysts for the production of high-value organic acids like succinic acid, lactic acid, 3-hydroxypropanoic acid and glucaric acid as well as alcohols like 1,3-propanediol, xylitol, mannitol, and glycerol with the discussion of the future research in this area. Besides, this review also discusses several platform chemicals, including fumaric acid, aspartic acid, glutamic acid, sorbitol, itaconic acid, and 2,5-furan dicarboxylic acid, which have not been produced by E. coli until now.

High-level succinic acid production and yield by lactose-induced expression of phosphoenolpyruvate carboxylase in ptsG mutant Escherichia coli

Escherichia coli strains with foreign genes under the isopropyl-beta-D: -thiogalactopyranoside-inducible promoters such as lac, tac, and trc were engineered and considered as the promising succinic acid-producing bacteria in many reports. The promoters mentioned above could also be induced by lactose, which had not been attempted for succinic acid production before. Here, the efficient utilization of lactose as inducer was demonstrated in cultures of the ptsG, ldhA, and pflB mutant strain DC1515 with ppc overexpression. A fermentative process for succinic acid production at high level by this strain was developed. In flask anaerobic culture, 14.86 g l(-1) succinic acid was produced from 15 g l(-1) glucose with a yield of 1.51 mol mol(-1) glucose. In two-stage culture carried out in a 3-l bioreactor, the overall yield and concentration of succinic acid reached to 1.67 mol mol(-1) glucose and 99.7 g l(-1), respectively, with a productivity of 1.7 g l(-1) h(-1) in the anaerobic stage. The efficient utilization of lactose as inducer made recombinant E. coli a more capable strain for succinic acid production at large scale.