Efficient succinic acid production from lignocellulosic biomass by simultaneous utilization of glucose and xylose in engineered Escherichia coli

Biosynthesis of D-1,2,4-butanetriol promoted by a glucose-xylose dual metabolic channel system in engineered Escherichia coli

D-1,2,4-butanetriol (BT) is a widely used fine chemical that can be manufactured by engineered Escherichia coli expressing heterologous pathways and using xylose as a substrate. The current study developed a glucose-xylose dual metabolic channel system in an engineered E. coli and Combinatorially optimized it using multiple strategies to promote BT production. The carbon catabolite repression effects were alleviated by deleting the gene ptsG that encodes the major glucose transporter IICBGlc and mutating the gene crp that encodes the catabolite repressor protein, thereby allowing C-fluxes of both glucose and xylose into their respective metabolic channels separately and simultaneously, which increased BT production by 33% compared with that of the original MJ133K-1 strain. Then, the branch metabolic pathways of intermediates in the BT channel were investigated, the transaminase HisC, the ketoreductases DlD, OLD, and IlvC, and the aldolase MhpE and YfaU were identified as the enzymes for the branched metabolism of 2-keto-3-deoxy-xylonate, deletion of the gene hisC increased BT titer by 21.7%. Furthermore, the relationship between BT synthesis and the intracellular NADPH level was examined, and deletion of the gene pntAB that encodes a transhydrogenase resulted in an 18.1% increase in BT production. The combination of the above approaches to optimize the metabolic network increased BT production by 47.5%, resulting in 2.67 g/L BT in 24 deep-well plates. This study provides insights into the BT biosynthesis pathway and demonstrates effective strategies to increase BT production, which will promote the industrialization of the biosynthesis of BT.

An efficient production of bio-succinate in a novel metabolically engineered Klebsiella oxytoca by rational metabolic engineering and evolutionary adaptation

Klebsiella oxytoca KC004 (ΔadhEΔpta-ackAΔldhAΔbudABΔpflB) was engineered to enhance succinate production. The strain exhibited poor growth without succinate production due to its deficiencies in ATP production and NADH reoxidation. To overcome obstacles, evolutionary adaptation with over 6,000 generations of growth-based selection was conducted. Under anaerobic conditions, enhanced productions of ATP for growth and succinate for NADH reoxidation by the evolved KC004-TF160 strain were coupled to an increased transcript of PEP carboxykinase (pck) while those of genes in the oxidative branch of TCA cycle (gltA, acnAB, and icd), and pyruvate and acetate metabolisms (pykA, acs, poxB and tdcD) were alleviated. The expression of pyruvate dehydrogenase repressor (pdhR) decreased whereas threonine decarboxylase (tdcE) increased. KC004-TF160 produced succinate at 84 g/L (0.84 g/g, 79 % theoretical maximum). KC004-TF160 produced succinate at 0.87 g/g non-pretreated sugarcane molasses without addition of nutrients and buffers. KC004-TF160 may be a microbial platform for commercial production of bio-succinate.

Progress in research on the biosynthesis of 1,2,4-butanetriol by engineered microbes

1,2,4-butanetriol (BT) is a polyol with unique chemical properties, which has a stereocenter and can be divided into D-BT (the S-enantiomer) and L-BT (the R-enantiomer). BT can be used for the synthesis of 1,2,4-butanetriol trinitrate, 3-hydroxytetrahydrofuran, polyurethane, and other chemicals. It is widely used in the military industry, medicine, tobacco, polymer. At present, the BT is mainly synthesized by chemical methods, which are accompanied by harsh reaction conditions, poor selectivity, many by-products, and environmental pollution. Therefore, BT biosynthesis methods with the advantages of mild reaction conditions and green sustainability have become a current research hotspot. In this paper, the research status of microbial synthesis of BT was summarized from the following three aspects: (1) the biosynthetic pathway establishment for BT from xylose; (2) metabolic engineering strategies employed for improving BT production from xylose; (3) other substrates for BT production. Finally, the challenges and prospects of biosynthetic BT were discussed for future methods to improve competitiveness for industrial production.

Sustainable production of 4-hydroxyisoleucine with minimised carbon loss by simultaneously utilising glucose and xylose in engineered Escherichia coli

4-Hydroxyisoleucine is a promising drug for diabetes therapy; however, microbial production of 4-hydroxyisoleucine is not economically efficient because of the carbon loss in the form of CO₂. This study aims to achieve de novo synthesis of 4-hydroxyisoleucine with minimised carbon loss in engineered Escherichia coli. Initially, an L-isoleucine-producing strain, ILE-5, was established, and the 4-hydroxyisoleucine synthesis pathway was introduced. The flux toward α-ketoglutarate was enhanced by reinforcing the anaplerotic pathway and disrupting competitive pathways. Subsequently, the metabolic flux for 4-hydroxyisoleucine synthesis was redistributed by dynamically modulating the α-ketoglutarate dehydrogenase complex activity, achieving a 4-hydroxyisoleucine production of 16.53 g/L. Finally, carbon loss was minimised by employing the Weimberg pathway, resulting in a 24.5% decrease in sugar consumption and a 31.6% yield increase. The 4-hydroxyisoleucine production by strain IEOH-11 reached 29.16 g/L in a 5-L fermenter. The 4-hydroxyisoleucine yield (0.29 mol/mol sugar) and productivity (0.91 g/(L⋅h)) were higher than those previously reported.

A paradigm shift towards production of sustainable bioenergy and advanced products from Cannabis/hemp biomass in Canada

The global cannabis (Cannabis sativa) market was 17.7 billion in 2019 and is expected to reach up to 40.6 billion by 2024. Canada is the 2nd nation to legalize cannabis with a massive sale of $246.9 million in the year 2021. Waste cannabis biomass is managed using disposal strategies (i.e., incineration, aerobic/anaerobic digestion, composting, and shredding) that are not good enough for long-term environmental sustainability. On the other hand, greenhouse gas emissions and the rising demand for petroleum-based fuels pose a severe threat to the environment and the circular economy. Cannabis biomass can be used as a feedstock to produce various biofuels and biochemicals. Various research groups have reported production of ethanol 9.2-20.2 g/L, hydrogen 13.5 mmol/L, lipids 53.3%, biogas 12%, and biochar 34.6% from cannabis biomass. This review summarizes its legal and market status (production and consumption), the recent advancements in the lignocellulosic biomass (LCB) pre-treatment (deep eutectic solvents (DES), and ionic liquids (ILs) known as "green solvents") followed by enzymatic hydrolysis using glycosyl hydrolases (GHs) for the efficient conversion efficiency of pre-treated biomass. Recent advances in the bioconversion of hemp into oleochemicals, their challenges, and future perspectives are outlined. A comprehensive insight is provided on the trends and developments of metabolic engineering strategies to improve product yield. The thermochemical processing of disposed-off hemp lignin into bio-oil, bio-char, synthesis gas, and phenol is also discussed. Despite some progress, barricades still need to be met to commercialize advanced biofuels and compete with traditional fuels.

Valorisation of xylose to renewable fuels and chemicals, an essential step in augmenting the commercial viability of lignocellulosic biorefineries

Biologists and engineers are making tremendous efforts in contributing to a sustainable and green society. To that end, there is growing interest in waste management and valorisation. Lignocellulosic biomass (LCB) is the most abundant material on the earth and an inevitable waste predominantly originating from agricultural residues, forest biomass and municipal solid waste streams. LCB serves as the renewable feedstock for clean and sustainable processes and products with low carbon emission. Cellulose and hemicellulose constitute the polymeric structure of LCB, which on depolymerisation liberates oligomeric or monomeric glucose and xylose, respectively. The preferential utilization of glucose and/or absence of the xylose metabolic pathway in microbial systems cause xylose valorization to be alienated and abandoned, a major bottleneck in the commercial viability of LCB-based biorefineries. Xylose is the second most abundant sugar in LCB, but a non-conventional industrial substrate unlike glucose. The current review seeks to summarize the recent developments in the biological conversion of xylose into a myriad of sustainable products and associated challenges. The review discusses the microbiology, genetics, and biochemistry of xylose metabolism with hurdles requiring debottlenecking for efficient xylose assimilation. It further describes the product formation by microbial cell factories which can assimilate xylose naturally and rewiring of metabolic networks to ameliorate xylose-based bioproduction in native as well as non-native strains. The review also includes a case study that provides an argument on a suitable pathway for optimal cell growth and succinic acid (SA) production from xylose through elementary flux mode analysis. Finally, a product portfolio from xylose bioconversion has been evaluated along with significant developments made through enzyme, metabolic and process engineering approaches, to maximize the product titers and yield, eventually empowering LCB-based biorefineries. Towards the end, the review is wrapped up with current challenges, concluding remarks, and prospects with an argument for intense future research into xylose-based biorefineries.

The Escherichia coli CitT transporter can be used as a succinate exporter for succinate production

Escherichia coli strain, whose gene is one of the subunits of succinate dehydrogenase (sdhA), and gene of the transcriptional repressor of isocitrate lyase (iclR) were disrupted, accumulated 6.6 times as much intracellular succinate as the wild-type MG1655 strain in aerobic growth, but succinate was not found in the culture medium. E. coli citT gene that encodes a citrate transporter was cloned under the control of the lacI promoter in pBR322-based plasmid and the above strain was transformed. This transformant, grown under aerobic condition in M9-tryptone medium with citrate, accumulated succinate in the medium while no succinate was found in the medium without citrate. CitT was active as a succinate transporter for 168 h by changing the culture medium or for 24 h in fed-batch culture. This study suggests that the CitT transporter functions as a succinate exporter in E. coli for succinate production in the presence of citrate.

Recent progress on bio-succinic acid production from lignocellulosic biomass

Succinic acid is a valuable bulk chemical, which has been extensively applied in food, medicine, surfactants and biodegradable plastics industries. As a substitute for chemical raw material, bio-based succinic acid production has received increasing attention due to the depletion of fossil fuels and environmental issues. Meanwhile, the effective bioconversion of lignocellulosic biomass has always been a hot spot of interest owning to the advantages of low expense, abundance and renewability. Consolidated bioprocessing (CBP) is considered to be an alternative approach with outstanding potential, as CBP can not only improve the product yield and productivity, but also reduce the equipment and operating costs. In addition, the current emerging microbial co-cultivation systems provide strong competitiveness for lignocellulose utilization through CBP. This article comprehensively discusses different strategies for the bioconversion of lignocellulose to succinic acid. Based on the principles and technical concepts of CBP, this review focuses on the progress of succinic acid production under different CBP strategies (metabolic engineering based and microbial co-cultivation based). Moreover, the main challenges faced by CBP-based succinic acid fermentation are analyzed, and the future direction of CBP production is prospected.

Improved 2,3-butanediol yield and productivity from lignocellulose biomass hydrolysate in metabolically engineered Enterobacter aerogenes

We previously engineered Enterobacter aerogenesfor glucose and xylose co-utilization and 2,3-butanediol production. Here, strain EMY-22 was further engineered to improve the 2,3-butanediol titer, productivity, and yield by reducing the production of byproducts. To reduce succinate production, the budABC operon and galP gene were overexpressed, which increased 2,3-butanediol production. For further reduction of succinate and 2-ketogluconate production, maeA was selected and overexpressed in EMY-22. The optimally engineered strain produced 2,3-butanediol for a longer time and showed reduced byproduct formation from sugarcane bagasse hydrolysate under flask cultivation conditions. The engineered strain displayed 66.6, 13.4, and 16.8% improvements in titer, yield, productivity of 2,3-butanediol, respectively, compared to its parental strain under fed-batch fermentation conditions. The data demonstrate that the metabolic engineering to reduce byproduct formation is a promising strategy to improve 2,3-butanediol production from lignocellulosic biomass.

Production and characterization of functional wheat bran hydrolysate rich in reducing sugars, xylooligosaccharides and phenolic acids

The aim was to enhance production of functional hydrolysate from wheat bran (WB). WB was hydrolyzed with 3000 U/mL ɑ-amylase and 1200 U/mL alkaline protease to prepare WB insoluble dietary fibre (WBIDF). Functional hydrolysate production from the extract containing crude xylan of WBIDF by xylanase was optimized by Taguchi method. The optimal condition for xylan degradation and functional substances production was 78.50 U/mL xylanase, pH 10.0, 50 °C, and reaction time 6 h. The maximum yield of reducing sugars was 614.0 μg/mL, xylobiose increased from 12.9 μg/mL to 213.3 μg/mL, xylotriose increased from 34.9 μg/mL to 174.0 μg/mL, ferulic acid 13.1 μg/mL made up 57.5 % of the total identifiable phenolic pool in the hydrolysate. The total antioxidant activity of hydrolysate was 141.8 mg ascorbic acid equivalents g^-1 crude xylan, and the highest 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity reached 92.7 %. The hydrolysate exhibited great potential in agricultural and food industry application.

Novel Routes in Transformation of Lignocellulosic Biomass to Furan Platform Chemicals: From Pretreatment to Enzyme Catalysis

The constant depletion of fossil fuels along with the increasing need for novel materials, necessitate the development of alternative routes for polymer synthesis. Lignocellulosic biomass, the most abundant carbon source on the planet, can serve as a renewable starting material for the design of environmentally-friendly processes for the synthesis of polyesters, polyamides and other polymers with significant value. The present review provides an overview of the main processes that have been reported throughout the literature for the production of bio-based monomers from lignocellulose, focusing on physicochemical procedures and biocatalysis. An extensive description of all different stages for the production of furans is presented, starting from physicochemical pretreatment of biomass and biocatalytic decomposition to monomeric sugars, coupled with isomerization by enzymes prior to chemical dehydration by acid Lewis catalysts. A summary of all biotransformations of furans carried out by enzymes is also described, focusing on galactose, glyoxal and aryl-alcohol oxidases, monooxygenases and transaminases for the production of oxidized derivatives and amines. The increased interest in these products in polymer chemistry can lead to a redirection of biomass valorization from second generation biofuels to chemical synthesis, by creating novel pathways to produce bio-based polymers.

Pretreatment and Detoxification of Acid-Treated Wood Hydrolysates for Pyruvate Production by an Engineered Consortium of Escherichia coli

The biorefinery concept makes use of renewable lignocellulosic biomass to produce commodities sustainably. A synthetic microbial consortium can enable the simultaneous utilization of sugars such as glucose and xylose to produce biochemicals, where each consortium member converts one sugar into the target product. In this study, woody biomass was used to generate glucose and xylose after pretreatment with 20% (w/v) sulfuric acid and 60-min reaction time. We compared several strategies for detoxification with charcoal and sodium borohydride treatments to improve the fermentability of this hydrolysate in a defined medium for the production of the growth-associated product pyruvate. In shake flask culture, the highest pyruvate yield on xylose of 0.8 g/g was found using pH 6 charcoal-treated hydrolysate. In bioreactor studies, a consortium of two engineered E. coli strains converted the mixture of glucose and xylose in batch studies to 12.8 ± 2.7 g/L pyruvate in 13 h. These results demonstrate that lignocellulosic biomass as the sole carbon source can be used to produce growth-related products after employing suitable detoxification strategies.

Metabolic engineering of Escherichia coli for shikimate pathway derivative production from glucose-xylose co-substrate

Glucose and xylose are the major components of lignocellulose. Effective utilization of both sugars can improve the efficiency of bioproduction. Here, we report a method termed parallel metabolic pathway engineering (PMPE) for producing shikimate pathway derivatives from glucose–xylose co-substrate. In this method, we seek to use glucose mainly for target chemical production, and xylose for supplying essential metabolites for cell growth. Glycolysis and the pentose phosphate pathway are completely separated from the tricarboxylic acid (TCA) cycle. To recover cell growth, we introduce a xylose catabolic pathway that directly flows into the TCA cycle. As a result, we can produce 4.09 g L⁻¹cis,cis-muconic acid using the PMPE Escherichia coli strain with high yield (0.31 g g⁻¹ of glucose) and produce l-tyrosine with 64% of the theoretical yield. The PMPE strategy can contribute to the development of clean processes for producing various valuable chemicals from lignocellulosic resources.

In lignocellulose biomass, microbes prefer consuming glucose over xylose, which affects target compound production. Here, the authors achieve simultaneous utilization of glucose and xylose for target chemical production and cell growth, respectively, and realize high-level production of shikimate pathway derivatives.

Efficient Biosynthesis of High-Value Succinic Acid and 5-Hydroxyleucine Using a Multienzyme Cascade and Whole-Cell Catalysis

Succinic acid (SA) is applied in the food, chemical, and pharmaceutical industries. 5-Hydroxyleucine (5-HLeu) is a promising precursor for the biosynthesis of antituberculosis drugs. Here, we designed a promising synthetic route for the simultaneous production of SA and 5-HLeu by combining l-leucine dioxygenase (NpLDO), l-glutamate oxidase (LGOX), and catalase (CAT). Two bioconversion systems: "a multienzyme cascade catalysis in vitro" (MECCS) and "whole-cell catalysis system" (WCCS) were constructed. A high-activity NpLDO mutant was screened by error-prone polymerase chain reaction (PCR) and showed 6.1-fold improvement of catalytic activity. After optimization of reaction conditions, MECSS yielded 3.15 g/L SA and 3.92 g/L 5-HLeu, while the production of SA and 5-HLeu by the most effective WCSS reached 15.12 and 18.83 g/L, respectively. This is the first attempt to use ferrous iron/α-ketoglutarate-dependent dioxygenases for the simultaneous production of SA and hydroxy-amino-acid. This research provides a tool for industrial production of food of high-value products from low-cost raw materials.

Co-production of 1,2,4-butantriol and ethanol from lignocellulose hydrolysates

The aim of this work was to realize 1,2,4-butantriol (BT) production from sugarcane bagasse hydrolysates by microbial fermentation, and obtain co-production of BT and ethanol. Candida glycerinogenes UG21 was utilized to reduce the effect of osmolality resulting from high glucose concentration and furfural in hydrolysates on cell growth of BT-producing strains, and produced 54.1 g/L ethanol from glucose. After ethanol recovering, xylose containing stillage was obtained and used for BT production. 1.3 g/L BT was generated by a BT-producing strain. By the deletion of the crr gene and process optimization, BT titer reached 4.9 g/L. Meanwhile, the efficient utilization of sugarcane bagasse was achieved by a two-stage fermentation for co-production of BT and ethanol. This study provided a novel strategy for BT production from sugarcane bagasse, and demonstrated the potential for making full use of sugarcane bagasse hydrolysates to co-production value-added products.

Systems Metabolic Engineering Strategies for Non-Natural Microbial Polyester Production

Plastics, used everyday, are mostly synthetic polymers derived from fossil resources, and their accumulation is becoming a serious concern worldwide. Polyhydroxyalkanoates (PHAs) are naturally produced polyesters synthesized and intracellularly accumulated by many different microorganisms. PHAs are good alternatives to petroleum-based plastics because they possess a wide range of material properties depending on monomer types and molecular weights. In addition, PHAs are biodegradable and can be produced from renewable biomass. Thus, producing PHAs through the development of high-performance engineered microorganisms and efficient bioprocesses gained much interest. In addition, non-natural polyesters comprising 2-hydroxycarboxylic acids as monomers have been produced by fermentation of metabolically engineered bacteria. For example, poly(lactic acid) and poly(lactic acid-co-glycolic acid), which have been chemically synthesized using the corresponding monomers either fermentatively or chemically produced, can be produced by metabolically engineered bacteria by one-step fermentation. Recently, PHAs containing aromatic monomers could be produced by fermentation of metabolically engineered bacteria. Here, metabolic engineering strategies applied in developing microbial strains capable of producing non-natural polyesters in a stepwise manner are reviewed. It is hoped that the detailed strategies described will be helpful for designing metabolic engineering strategies for developing diverse microbial strains capable of producing various polymers that can replace petroleum-derived polymers.

GH-10 and GH-11 Endo-1,4-β-xylanase enzymes from Kitasatospora sp. produce xylose and xylooligosaccharides from sugarcane bagasse with no xylose inhibition

A novel strategy for the low-cost, high-yield co-production of xylose and xylooligosaccharides together with no xylose inhibition was developed using a novel heterologous expression of XYN10Ks_480 endo-1,4-β-xylanase with a ricin-type β-trefoil type of domain and XYN11Ks_480 endo-1,4-β-xylanase with a CBM 2 superfamily from the Kitasatospora sp in an actinomycetes expression system. Xylose is the main building block for hemicellulose xylan. Our findings demonstrated high levels of expression and catalytic activity for XYN10Ks_480 during hydrolysis of the extracted xylan of bagasse, and three types of xylan-based substrates were used to produce xylose and xylooligosaccharides. However, hydrolysis by XYN11Ks_480 produced xylooligosaccharides without xylose formation. This study demonstrated how integrating sodium hypochlorite-extracted xylan and enzymatic hydrolysis could provide an alternative strategy for the generation of XOS from lignocellulosic material.

Metabolic Engineering of Escherichia coli for High Yield Production of Succinic Acid Driven by Methanol

Methanol is increasingly becoming an attractive carbon feedstock for the production of various biochemicals due to its high abundance and low price. In this study, when methanol assimilation module was introduced into succinic acid producing Escherichia coli by employing the NAD-dependent methanol dehydrogenase from Bacillus methanolicus and ribulose monophosphate pathway from different donor organisms, succinic acid yield was increased from 0.91 ± 0.08 g/g to 0.98 ± 0.11 g/g with methanol as an auxiliary substrate under the anaerobic fermentation. Further ¹³C-labeling experiments showed that the recombinant strain successfully converted methanol into succinic acid, as the carbon atom of carboxyl group in succinic acid was labeled by ¹³C. It was found that the NADH generated by methanol oxidation would benefit succinate production, as the NADH/NAD⁺ ratio in vivo was decreased from 0.67 to 0.45 in the engineered strain, indicating that the efficiency of succinic acid synthesis was significantly improved when driven by methanol. This study represents a successful case for the development of reducing chemical production using methanol as an auxiliary substrate.

Genome shuffling improved acid-tolerance and succinic acid production of Actinobacillus succinogenes

Succinic acid is widely applied to chemical, pharmaceutical, food, and agricultural industries. With the rapid development of these industries, a great demand of succinic acid is required. The acid-tolerance and succinic acid production of Actinobacillus succinogenes strain were improved by using genome shuffling. Results showed that one modified strain AS-F32, with the best acid resistance and the highest succinic acid production, was obtained after 3 cycles of genome shuffling. The minimum growth pH of AS-F32 was 3.5, and the acid production and cell dry weight were 5.1 and 4.8 g/L in flask, improved 2.6 and 1.85 times over the start strain As-R2. Furthermore, the succinic acid yield of As-32 was 31.2 g/L and the dry cell weight was increased 44.4% by maintaining pH 4.8 with 7.0 M NH₄OH in 5 L bioreactor, increased 1.1 times than the original strain As-R2.

Enhanced production of succinic acid from methanol-organosolv pretreated Strophanthus preussii by recombinant Escherichia coli

A biorefinery process for high yield production of succinic acid from biomass sugars was investigated using recombinant Escherichia coli. The major problem been addressed is utilization of waste biomass for the production of succinic acid using metabolic engineering strategy. Here, methanol extract of Strophanthus preussii was used for fermentation. The process parameters were optimized. Glucose (9 g/L), galactose (4 g/L), xylose (6 g/L) and arabinose (0.5 g/L) were the major sugars present in the methanol extract of S. preussii. E. coli K3OS with overexpression of soluble nucleotide pyridine transhydrogenase sthA and mutation of lactate dehydrogenase A (ldhA), phosphotransacetylase acetate kinase A (pta-ackA), pyruvate formate lyase B (pflB), pyruvate oxidase B (poxB), produced a final succinic acid concentration of 14.40 g/L and yield of 1.10 mol/mol total sugars after 72 h dual-phase fermentation in M9 medium. Here, we show that the maximum theoretical yield using methanol extracts of S. preussii was 64%. Hence, methanol extract of S. preussii could be used for the production of biochemicals such as succinate, malate and pyruvate.

Metabolic engineering of Corynebacterium glutamicum for efficient production of succinate from lignocellulosic hydrolysate

BACKGROUND

Succinate has been recognized as one of the most important bio-based building block chemicals due to its numerous potential applications. However, efficient methods for the production of succinate from lignocellulosic feedstock were rarely reported. Nevertheless, Corynebacterium glutamicum was engineered to efficiently produce succinate from glucose in our previous study.

RESULTS

In this work, C. glutamicum was engineered for efficient succinate production from lignocellulosic hydrolysate. First, xylose utilization of C. glutamicum was optimized by heterologous expression of xylA and xylB genes from different sources. Next, xylA and xylB from Xanthomonas campestris were selected among four candidates to accelerate xylose consumption and cell growth. Subsequently, the optimal xylA and xylB were co-expressed in C. glutamicum strain SAZ3 (ΔldhAΔptaΔpqoΔcatP_sod-ppcP_sod-pyc) along with genes encoding pyruvate carboxylase, citrate synthase, and a succinate exporter to achieve succinate production from xylose in a two-stage fermentation process. Xylose utilization and succinate production were further improved by overexpressing the endogenous tkt and tal genes and introducing araE from Bacillus subtilis. The final strain C. glutamicum CGS5 showed an excellent ability to produce succinate in two-stage fermentations by co-utilizing a glucose-xylose mixture under anaerobic conditions. A succinate titer of 98.6 g L^-1 was produced from corn stalk hydrolysate with a yield of 0.87 g/g total substrates and a productivity of 4.29 g L^-1 h^-1 during the anaerobic stage.

CONCLUSION

This work introduces an efficient process for the bioconversion of biomass into succinate using a thoroughly engineered strain of C. glutamicum. To the best of our knowledge, this is the highest titer of succinate produced from non-food lignocellulosic feedstock, which highlights that the biosafety level 1 microorganism C. glutamicum is a promising platform for the envisioned lignocellulosic biorefinery.

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.

Characterization of a β-glucosidase from Paenibacillus species and its application for succinic acid production from sugarcane bagasse hydrolysate

In this study, a β-glucosidase from Paenibacillus sp. M1 was expressed in E. coli BL21(DE3), purified and characterized. The specific activity of purified BglA was 137.64U·mg^-1 protein with optimal temperature and pH of 50°C and 6.0. Furthermore, BglA shows excellent adaption to various environmental factors such as temperature, pH and metal ions. Engineered E. coli Suc260 was further reconstructed by overexpressing the β-glucosidase for achieving direct cellobiose utilization, which could efficiently utilize the pretreated sugarcane bagasses hydrolysate (SBH) consisting of 25.30g·L^-1 cellobiose, 9.70g·L^-1 glucose, 5.90g·L^-1 arabinose and 7.10g·L^-1 xylose. As a result, 26.50g·L^-1 and 24.30g·L^-1 succinic acid were produced by strain Suc260(pTbglA) from cellobiose and SBH with corresponding yields of 88.30% and 89.20% using dual-phase fermentation, respectively. This study indicated that incomplete enzymatic hydrolysate of SCB will be a potential feedstock for succinic acid production.

Protein and metabolic engineering for the production of organic acids

Organic acids are natural metabolites of living organisms. They have been widely applied in the food, pharmaceutical, and bio-based materials industries. In recent years, biotechnological routes to organic acids production from renewable raw materials have been regarded as very promising approaches. In this review, we provide an overview of current developments in the production of organic acids using protein and metabolic engineering strategies. The organic acids include propionic acid, pyruvate, itaconic acid, succinic acid, fumaric acid, malic acid and citric acid. We also expect that rapid developments in the fields of systems biology and synthetic biology will accelerate protein and metabolic engineering for microbial organic acid production in the future.

Biotechnological route for sustainable succinate production utilizing oil palm frond and kenaf as potential carbon sources

Due to the world's dwindling energy supplies, greater thrust has been placed on the utilization of renewable resources for global succinate production. Exploration of such biotechnological route could be seen as an act of counterbalance to the continued fossil fuel dominance. Malaysia being a tropical country stands out among many other nations for its plenty of resources in the form of lignocellulosic biomass. To date, oil palm frond (OPF) contributes to the largest fraction of agricultural residues in Malaysia, while kenaf, a newly introduced fiber crop with relatively high growth rate, holds great potential for developing sustainable succinate production, apart from OPF. Utilization of non-food, inexhaustible, and low-cost derived biomass in the form of OPF and kenaf for bio-based succinate production remains largely untapped. Owing to the richness of carbohydrates in OPF and kenaf, bio-succinate commercialization using these sources appears as an attractive proposition for future sustainable developments. The aim of this paper was to review some research efforts in developing a biorefinery system based on OPF and kenaf as processing inputs. It presents the importance of the current progress in bio-succinate commercialization, in addition to describing the potential use of different succinate production hosts and various pretreatments-saccharifications under development for OPF and kenaf. Evaluations on the feasibility of OPF and kenaf as fermentation substrates are also discussed.

Enhancement of succinate yield by manipulating NADH/NAD+ ratio and ATP generation

We previously engineered Escherichia coli YL104 to efficiently produce succinate from glucose. In this study, we investigated the relationships between the NADH/NAD⁺ ratio, ATP level, and overall yield of succinate production by using glucose as the carbon source in YL104. First, the use of sole NADH dehydrogenases increased the overall yield of succinate by 7% and substantially decreased the NADH/NAD⁺ ratio. Second, the soluble fumarate reductase from Saccharomyces cerevisiae was overexpressed to manipulate the anaerobic NADH/NAD⁺ ratio and ATP level. Third, another strategy for reducing the ATP level was applied by introducing ATP futile cycling for improving succinate production. Finally, a combination of these methods exerted a synergistic effect on improving the overall yield of succinate, which was 39% higher than that of the previously engineered strain YL104. The study results indicated that regulation of the NADH/NAD⁺ ratio and ATP level is an efficient strategy for succinate production.

Engineering Bacillus licheniformis as a thermophilic platform for the production of l-lactic acid from lignocellulose-derived sugars

BACKGROUND

Bacillus licheniformis MW3 as a GRAS and thermophilic strain is a promising microorganism for chemical and biofuel production. However, its capacity to co-utilize glucose and xylose, the major sugars found in lignocellulosic biomass, is severely impaired by glucose-mediated carbon catabolite repression (CCR). In this study, a "dual-channel" process was implemented to engineer strain MW3 for simultaneous utilization of glucose and xylose, using l-lactic acid as a target product.

RESULTS

A non-phosphotransferase system (PTS) glucose uptake route was activated via deletion of the glucose transporter gene ptsG and introduction of the galactose permease gene galP. After replacing the promoter of glucokinase gene glck with the strong promoter P_als, the engineered strain recovered glucose consumption and utilized glucose and xylose simultaneously. Meanwhile, to improve the consumption rate of xylose in this strain, several measures were undertaken, such as relieving the regulation of the xylose repressor XylR, reducing the catabolite-responsive element, and optimizing the rate-limiting step. Knockout of ethanol and acetic acid pathway genes further increased lactic acid yield by 6.2%. The resultant strain, RH15, was capable of producing 121.9 g/L l-lactic acid at high yield (95.3%) after 40 h of fermentation from a mixture of glucose and xylose. When a lignocellulosic hydrolysate was used as the substrate, 99.3 g/L l-lactic acid was produced within 40 h, with a specific productivity of 2.48 g/[L h] and a yield of 94.6%.

CONCLUSIONS

Our engineered strain B. licheniformis RH15 could thermophilically produced l-lactic acid from lignocellulosic hydrolysate with relatively high concentration and productivity at levels that were competitive with most reported cases of l-lactic acid-producers. Thus, the engineered strain might be used as a platform for the production of other chemicals. In addition to engineering the B. licheniformis strain, the "dual-channel" process might serve as an alternative method for engineering a variety of other strains.

Production of Succinate from Acetate by Metabolically Engineered Escherichia coli

Acetate, a major component of industrial biological wastewater and of lignocellulosic biomass hydrolysate, could potentially be a less costly alternative carbon source. Here we engineered Escherichia coli MG1655 strain for succinate production from acetate as the sole carbon source. Strategies of metabolic engineering included the blockage of the TCA cycle, redirection of the gluconeogenesis pathway, and enhancement of the glyoxylate shunt. The engineered strain MG03 featuring the deletion of genes: succinate dehydrogenase (sdhAB), isocitrate lyase regulator (iclR), and malic enzymes (maeB) accumulated 6.86 mM of succinate in 72 h. MG03(pTrc99a-gltA) overexpressing citrate synthase (gltA) accumulated 16.45 mM of succinate and the yield reached 0.46 mol/mol, about 92% of the maximum theoretical yield. Resting-cell was adopted for the conversion of acetate to succinate, and the highest concentration of succinate achieved 61.7 mM.

ATP-Based Ratio Regulation of Glucose and Xylose Improved Succinate Production

We previously engineered E. coli YL104H to efficiently produce succinate from glucose. Furthermore, the present study proved that YL104H could also co-utilize xylose and glucose for succinate production. However, anaerobic succinate accumulation using xylose as the sole carbon source failed, probably because of an insufficient supply of energy. By analyzing the ATP generation under anaerobic conditions in the presence of glucose or xylose, we indicated that succinate production was affected by the intracellular ATP level, which can be simply regulated by the substrate ratio of xylose to glucose. This finding was confirmed by succinate production using an artificial mixture containing different xylose to glucose ratios. Using xylose mother liquor, a waste containing both glucose and xylose derived from xylitol production, a final succinate titer of 61.66 g/L with an overall productivity of 0.95 g/L/h was achieved, indicating that the regulation of the intracellular ATP level may be a useful and efficient strategy for succinate production and can be extended to other anaerobic processes.

Enhanced succinic acid productivity by expression of mgtCB gene in Escherichia coli mutant

In this study, a novel engineering Escherichia coli strain (CBMG111) with the expression of mgtCB gene was constructed for the enhanced fermentative production of succinic acid by utilizing the synergetic effect of mgtC gene to improve the growth of strains at the environment of low Mg2+ concentration and mgtB to enhance the transport of Mg2+ into cells. After the effect of the expression of the individual genes (mgtA, mgtB, mgtC) on the growth of E. coli was clarified, the fermentative production of succinic acid by CBMG111 was studied with the low-price mixture of Mg(OH)2 and NH3·H2O as the alkaline neutralizer and the biomass hydrolysates as the carbon sources, which demonstrated that the expression of mgtCB gene can significantly increase the productivity of succinic acid (2.97 g L−1 h−1) compared with that by using the engineering strain with the overexpression of mgtA gene.

Robust succinic acid production from crude glycerol using engineered Yarrowia lipolytica

BACKGROUND

Integrating waste management with fuels and chemical production is considered to address the food waste problem and oil crisis. Approximately, 600 million tonnes crude glycerol is produced from the biodiesel industry annually, which is a top renewable feedstock for succinic acid production. To meet the increasing demand for succinic acid production, the development of more efficient and cost-effective production methods is urgently needed. Herein, we have proposed a new strategy for integration of both biodiesel and SA production in a biorefinery unit by construction of an aerobic yeast Yarrowia lipolytica with a deletion in the gene coding succinate dehydrogenase subunit 5.

RESULTS

Robust succinic acid production by an engineered yeast Y. lipolytica from crude glycerol without pre-treatment was demonstrated. Diversion of metabolic flow from tricarboxylic acid cycle led to the success in generating a succinic acid producer Y. lipolytica PGC01003. The fermentation media and conditions were optimized, which resulted in 43 g L(-1) succinic acid production from crude glycerol. Using the fed-batch strategy in 2.5 L fermenter, up to 160 g L(-1) SA was yielded, indicating the great industrial potential.

CONCLUSIONS

Inactivation of SDH5 in Y. lipolytica Po1f led to succinic acid accumulation and secretion significantly. To our best knowledge, this is the highest titer obtained in fermentation on succinic acid production. In addition, the performance of batch and fed-batch fermentation showed high tolerance and yield on biodiesel by-product crude glycerol. All these results indicated that PGC01003 is a promising microbial factorial cell for the highly efficient strategy solving the environmental problem in connection with the production of value-added product.

Mutation in galP improved fermentation of mixed sugars to succinate using engineered Escherichia coli AS1600a and AM1 mineral salts medium

Escherichia coli KJ122 was engineered to produce succinate from glucose using the wild type GalP for glucose uptake instead of the native phosphotransferase system (ptsI mutation). This strain now ferments 10% xylose poorly. Mutants were selected by serial transfers in AM1 mineral salts medium with 10% xylose. Clones from this population all exhibited a similar improvement, co-fermentation of an equal mixture of xylose and glucose. One of these, AS1600a, produced 84.26 ± 1.37 g/L succinate, equivalent to that produced by the parent (KJ122) from 10% glucose (85.46 ± 1.78 g/L). AS1600a was sequenced and found to contain a mutation in galactose permease (GalP, G236D). This mutation was shown to be responsible for the improvement in fermentation using KJΔgalP as the host and expression vectors with native galP and with mutant galP(∗). Strain AS1600a and KJΔgalP(pLOI5746; galP(∗)) also co-fermented a mixture of glucose, xylose, arabinose, and galactose in sugarcane bagasse hydrolysate using mineral salts medium.

Efficient production of succinic acid from macroalgae hydrolysate by metabolically engineered Escherichia coli

In this study, microbial production of succinic acid from macroalgae (i.e., Laminaria japonica) was investigated for the first time. The engineered Escherichia coli BS002 exhibited higher molar yield of succinic acid on mannitol (1.39±0.01mol/mol) than glucose (1.01±0.05mol/mol). After pretreatment and enzymatic hydrolysis, L. japonica hydrolysate was mainly glucose (10.31±0.32g/L) and mannitol (10.12±0.17g/L), which was used as the substrate for succinic acid fermentation with the recombinant BS002. A final 17.44±0.54g/L succinic acid was obtained from the hydrolysate after 72h dual-phase fermentation. The yield was as high as 1.24±0.08mol/mol total sugar, which reached 73% of the maximum theoretical yield. The results demonstrate that macroalgae biomass represents a novelty and economical alternative feedstock for biochemicals production.

Ionic liquid pretreatment to increase succinic acid production from lignocellulosic biomass

In this study, pinewood and corn stover pretreated with the ionic liquid (IL) 1-allyl-3-methylimidazolium chloride (AmimCl) were used as a feedstock for succinic acid production. Results reveal that 5% (v/v) AmimCl inhibited bacterial growth, whereas 0.01% (v/v) AmimCl inhibited succinic acid production. AmimCl was effective in extracting cellulose from pinewood and in degrading pinewood into a uniform pulp, as revealed by scanning electron microscopy (SEM). The rate of enzymatic hydrolysis of pinewood extract reached 72.16%. The combinations of AmimCl pretreatment with steam explosion or with hot compressed water were effective in treating corn stover, whereas AmimCl treatment alone did not result in a significant improvement. Pinewood extract produced 20.7g/L succinic acid with an average yield of 0.37g per gram of biomass. Workflow calculations indicated pine wood pretreated with IL has a theoretical yield of succinic acid of 57.1%. IL pretreatment led to increase in succinic acid yields.

Enhanced succinic acid production and magnesium utilization by overexpression of magnesium transporter mgtA in Escherichia coli mutant

In this study, a novel engineered Escherichia coli strain KMG111 was constructed by overexpression of mgtA in E. coli mutant DC1515. By adopting KMG111, nearly a concentration of succinic acid (32.41gL(-1)) with a yield of 0.81gg(-1) glucose, could be obtained in a batch fermentation by using the low-cost mixture of Mg(OH)2 and NH3·H2O to replace MgCO3 as the alkaline neutralizer. Moreover, the effect of the inhibitory compounds in lignocellulosic hydrolyzates on cell growth and succinic acid production could be relieved. In a 3-L bioreactor, the overall productivity and yield of succinic acid in the whole anaerobic stage were 2.15gL(-1)h(-1) and 0.86gg(-1) total sugar, respectively. This study was the first to report decreased alkaline neutralizer cost via genetic manipulation for succinic acid production, which contributed to the industrialization of this microbial synthesis process.

Efficient free fatty acid production from woody biomass hydrolysate using metabolically engineered Escherichia coli

Four engineered Escherichia coli strains, ML103(pXZ18), ML103(pXZ18Z), ML190(pXZ18) and ML190(pXZ18Z), were constructed to investigate free fatty acid production using hydrolysate as carbon source. These strains exhibited efficient fatty acid production when xylose was used as the sole carbon source. For mixed sugars, ML103 based strains utilized glucose and xylose sequentially under the carbon catabolite repression (CCR) regulation, while ML190 based strains, with ptsG mutation, used glucose and xylose simultaneously. The total free fatty acid concentration and yield of the strain ML190(pXZ18Z) based on the mixed sugar reached 3.64 g/L and 24.88%, respectively. Furthermore, when hydrolysate from a commercial plant was used as the carbon source, the strain ML190(pXZ18Z) can produce 3.79 g/L FFAs with a high yield of 21.42%.

Construction of reductive pathway in Saccharomyces cerevisiae for effective succinic acid fermentation at low pH value

Succinic acid is an important precursor for the synthesis of high-value-added products. Saccharomyces cerevisiae is a suitable platform for succinic acid production because of its high tolerance towards acidity. In this study, a modified pathway for succinate production was established and investigated in S. cerevisiae. The engineered strain could produce up to 6.17±0.34g/L of succinate through the constructed pathway. The succinate titer was further improved to 8.09±0.28g/L by the deletion of GPD1 and even higher to 9.98±0.23g/L with a yield of 0.32mol/mol glucose through regulation of biotin and urea levels. Under optimal supplemental CO2 conditions in a bioreactor, the engineered strain produced 12.97±0.42g/L succinate with a yield of 0.21mol/mol glucose at pH 3.8. These results demonstrated that the proposed engineering strategy was efficient for succinic acid production at low pH value.