Elimination of by-product formation during production of 1,3-propanediol in Klebsiella pneumoniae by inactivation of glycerol oxidative pathway

Transcriptome Analysis of Glycerin Regulating Reuterin Production of Lactobacillus reuteri

Reuterin can be produced from glycerol dehydration catalyzed by glycerol dehydratase (GDHt) in Lactobacillus reuteri and has broad application prospects in industry, agriculture, food, and other fields as it is active against prokaryotic and eukaryotic organisms and is resistant to proteases and lipases. However, high concentrations of glycerin inhibit reuterin production, and the mechanism behind this phenomenon is not clear. To elucidate the inhibitory mechanism of glycerol on reuterin synthesis in L. reuteri and provide reference data for constructing an L. reuteri culture system for highly effective 3-hydroxypropionaldehyde synthesis, we used transcriptome-sequencing technology to compare the morphologies and transcriptomes of L. reuteri cultured in a medium with or without 600 mM of glycerol. Our results showed that after the addition of 600 mM of glycerol to the culture medium and incubation for 10 h at 37 °C, the culture medium of L. reuteri LR301 exhibited the best bacteriostatic effect, and the morphology of L. reuteri cells had significantly changed. The addition of 600 mM of glycerol to the culture medium significantly altered the transcriptome and significantly downregulated the transcription of genes involved in glycol metabolism, such as gldA, dhaT, glpK, plsX, and plsY, but significantly upregulated the transcription of genes related to D-glucose synthesis.

Enhancing 1,3-Propanediol Productivity in the Non-Model Chassis Clostridium beijerinckii through Genetic Manipulation

Biotechnological processes at biorefineries are considered one of the most attractive alternatives for valorizing biomasses by converting them into bioproducts, biofuels, and bioenergy. For example, biodiesel can be obtained from oils and grease but generates glycerol as a byproduct. Glycerol recycling has been studied in several bioprocesses, with one of them being its conversion to 1,3-propanediol (1,3-PDO) by Clostridium. Clostridium beijerinckii is particularly interesting because it can produce a range of industrially relevant chemicals, including solvents and organic acids, and it is non-pathogenic. However, while Clostridium species have many potential advantages as chassis for synthetic biology applications, there are significant limitations when considering their use, such as their limited genetic tools, slow growth rate, and oxygen sensitivity. In this work, we carried out the overexpression of the genes involved in the synthesis of 1,3-PDO in C. beijerinckii Br21, which allowed us to increase the 1,3-PDO productivity in this strain. Thus, this study contributed to a better understanding of the metabolic pathways of glycerol conversion to 1,3-PDO by a C. beijerinckii isolate. Also, it made it possible to establish a transformation method of a modular vector in this strain, therefore expanding the limited genetic tools available for this bacterium, which is highly relevant in biotechnological applications.

Polymers without Petrochemicals: Sustainable Routes to Conventional Monomers

Access to a wide range of plastic materials has been rationalized by the increased demand from growing populations and the development of high-throughput production systems. Plastic materials at low costs with reliable properties have been utilized in many everyday products. Multibillion-dollar companies are established around these plastic materials, and each polymer takes years to optimize, secure intellectual property, comply with the regulatory bodies such as the Registration, Evaluation, Authorisation and Restriction of Chemicals and the Environmental Protection Agency and develop consumer confidence. Therefore, developing a fully sustainable new plastic material with even a slightly different chemical structure is a costly and long process. Hence, the production of the common plastic materials with exactly the same chemical structures that does not require any new registration processes better reflects the reality of how to address the critical future of sustainable plastics. In this review, we have highlighted the very recent examples on the synthesis of common monomers using chemicals from sustainable feedstocks that can be used as a like-for-like substitute to prepare conventional petrochemical-free thermoplastics.

Production of 1,2-propanediol from glycerol in Klebsiella pneumoniae GEM167 with flux enhancement of the oxidative pathway

BACKGROUND

To support the sustainability of biodiesel production, by-products, such as crude glycerol, should be converted into high-value chemical products. 1,2-propanediol (1,2-PDO) has been widely used as a building block in the chemical and pharmaceutical industries. Recently, the microbial bioconversion of lactic acid into 1,2-PDO is attracting attention to overcome limitations of previous biosynthetic pathways for production of 1,2-PDO. In this study, we examined the effect of genetic engineering, metabolic engineering, and control of bioprocess factors on the production of 1,2-PDO from lactic acid by K. pneumoniae GEM167 with flux enhancement of the oxidative pathway, using glycerol as carbon source.

RESULTS

We developed K. pneumoniae GEM167ΔadhE/pBR-1,2PDO, a novel bacterial strain that has blockage of ethanol biosynthesis and biosynthesized 1,2-PDO from lactic acid when glycerol is carbon source. Increasing the agitation speed from 200 to 400 rpm not only increased 1,2-PDO production by 2.24-fold to 731.0 ± 24.7 mg/L at 48 h but also increased the amount of a by-product, 2,3-butanediol. We attempted to inhibit 2,3-butanediol biosynthesis using the approaches of pH control and metabolic engineering. Control of pH at 7.0 successfully increased 1,2-PDO production (1016.5 ± 37.3 mg/L at 48 h), but the metabolic engineering approach was not successful. The plasmid in this strain maintained 100% stability for 72 h.

CONCLUSIONS

This study is the first to report the biosynthesis of 1,2-PDO from lactic acid in K. pneumoniae when glycerol was carbon source. The 1,2-PDO production was enhanced by blocking the synthesis of 2,3-butanediol through pH control. Our results indicate that K. pneumoniae GEM167 has potential for the production of additional valuable chemical products from metabolites produced through oxidative pathways.

In Vitro One-Pot 3-Hydroxypropanal Production from Cheap C1 and C2 Compounds

One- or two-carbon (C1 or C2) compounds have been considered attractive substrates because they are inexpensive and abundant. Methanol and ethanol are representative C1 and C2 compounds, which can be used as bio-renewable platform feedstocks for the biotechnological production of value-added natural chemicals. Methanol-derived formaldehyde and ethanol-derived acetaldehyde can be converted to 3-hydroxypropanal (3-HPA) via aldol condensation. 3-HPA is used in food preservation and as a precursor for 3-hydroxypropionic acid and 1,3-propanediol that are starting materials for manufacturing biocompatible plastic and polytrimethylene terephthalate. In this study, 3-HPA was biosynthesized from formaldehyde and acetaldehyde using deoxyribose-5-phosphate aldolase from Thermotoga maritima (DERA_Tma) and cloned and expressed in Escherichia coli for 3-HPA production. Under optimum conditions, DERA_Tma produced 7 mM 3-HPA from 25 mM substrate (formaldehyde and acetaldehyde) for 60 min with 520 mg/L/h productivity. To demonstrate the one-pot 3-HPA production from methanol and ethanol, we used methanol dehydrogenase from Lysinibacillus xylanilyticus (MDH_Lx) and DERA_Tma. One-pot 3-HPA production via aldol condensation of formaldehyde and acetaldehyde from methanol and ethanol, respectively, was investigated under optimized reaction conditions. This is the first report on 3-HPA production from inexpensive alcohol substrates (methanol and ethanol) by cascade reaction using DERA_Tma and MDH_Lx.

Gut microbial beta-glucuronidase and glycerol/diol dehydratase activity contribute to dietary heterocyclic amine biotransformation

BACKGROUND

Consuming red and processed meat has been associated with an increased risk of colorectal cancer (CRC), which is partly attributed to exposure to carcinogens such as heterocyclic amines (HCA) formed during cooking and preservation processes. The interaction of gut microbes and HCA can result in altered bioactivities and it has been shown previously that human gut microbiota can transform mutagenic HCA to a glycerol conjugate with reduced mutagenic potential. However, the major form of HCA in the colon are glucuronides (HCA-G) and it is not known whether these metabolites, via stepwise microbial hydrolysis and acrolein conjugation, are viable precursors for glycerol conjugated metabolites. We hypothesized that such a process could be concurrently catalyzed by bacterial beta-glucuronidase (B-GUS) and glycerol/diol dehydratase (GDH) activity. We therefore investigated how the HCA-G PhIP-N2-β-D-glucuronide (PhIP-G), a representative liver metabolite of PhIP (2-Amino-1-methyl-6-phenylimidazo [4,5-b] pyridine), which is the most abundant carcinogenic HCA in well-cooked meat, is transformed by enzymatic activity of human gut microbial representatives of the phyla Firmicutes, Bacteroidetes, and Proteobacteria.

RESULTS

We employed a combination of growth and enzymatic assays, and a bioanalysis approach combined with metagenomics. B-GUS of Faecalibacterium prausnitzii converted PhIP-G to PhIP and GDH of Flavonifractor plautii, Blautia obeum, Eubacterium hallii, and Lactobacillus reuteri converted PhIP to PhIP-M1 in the presence of glycerol. In addition, B-GUS- and GDH-positive bacteria cooperatively converted PhIP-G to PhIP-M1. A screen of genes encoding B-GUS and GDH was performed for fecal microbiome data from healthy individuals (n = 103) and from CRC patients (n = 53), which revealed a decrease in abundance of taxa with confirmed GDH and HCA transformation activity in CRC patients.

CONCLUSIONS

This study for the first time demonstrates that gut microbes mediate the stepwise transformation of PhIP-G to PhIP-M1 via the intermediate production of PhIP. Findings from this study suggest that targeted manipulation with gut microbes bearing specific functions, or dietary glycerol supplementation might modify gut microbial activity to reduce HCA-induced CRC risk.

An integrated process for the production of 1,3-propanediol, lactate and 3-hydroxypropionic acid by an engineered Lactobacillus reuteri

The process economy of food grade 1,3-propanediol (1,3-PD) production by GRAS organisms like Lactobacillus reuteri (L. reuteri), is negatively impacted by the low yield and use of expensive feedstocks. In order to improve the process economy, we have developed a multiproduct process involving the production of three commercially important chemicals, namely, 1,3-PD, lactate and 3-Hydroxypropionic acid (3-HP), by engineered L. reuteri. The maximum 1,3-PD and lactate titer of 41 g/L and 31 g/L, with a volumetric productivity of 1.69 g/L/h and 0.67 g/L/h were achieved, respectively. The maximum 3-HP titer of 5.2 g/L with a volumetric productivity of 1.3 g/L/h, was obtained by biotransformation using cells recovered from the repeated fed-batch process. The volumetric productivity of 1,3-PD obtained in this study is the highest ever reported for this organism. Further cost reduction can be achieved by using waste feedstocks like milk whey, biomass hydrolysate, and crude glycerol.

Inactivating NADH:quinone oxidoreductases affects the growth and metabolism of Klebsiella pneumoniae

NADH:quinone oxidoreductases (NQOs) act as the electron entry sites in bacterial respiration and oxidize intracellular NADH that is essential for the synthesis of numerous molecules. Klebsiella pneumoniae contains three NQOs (NDH-1, NDH-2, and NQR). The effects of inactivating these NQOs, separately and together, on cell metabolism were investigated under different culture conditions. Defective growth was evident in NDH-1-NDH-2 double and NDH-1-NDH-2-NQR triple deficient mutants, which was probably due to damage to the respiratory chain. The results also showed that K. pneumoniae can flexibly use NQOs to maintain normal growth in single NQO-deficient mutants. And more interestingly, under aerobic conditions, inactivating NDH-1 resulted in a high intracellular NADH:NAD⁺ ratio, which was proven to be beneficial for 2,3-butanediol production. Compared with the parent strain, 2,3-butanediol production by the NDH-1-deficient mutant was increased by 46% and 62% in glycerol- and glucose-based media, respectively. Thus, our findings provide a practical strategy for metabolic engineering of respiratory chains to promote the biosynthesis of 2,3-butanediol in K. pneumoniae.

Coordination of metabolic pathways: Enhanced carbon conservation in 1,3-propanediol production by coupling with optically pure lactate biosynthesis

Metabolic engineering has emerged as a powerful tool for bioproduction of both fine and bulk chemicals. The natural coordination among different metabolic pathways contributes to the complexity of metabolic modification, which hampers the development of biorefineries. Herein, the coordination between the oxidative and reductive branches of glycerol metabolism was rearranged in Klebsiella oxytoca to improve the 1,3-propanediol production. After deliberating on the product value, carbon conservation, redox balance, biological compatibility and downstream processing, the lactate-producing pathway was chosen for coupling with the 1,3-propanediol-producing pathway. Then, the other pathways of 2,3-butanediol, ethanol, acetate, and succinate were blocked in sequence, leading to improved d-lactate biosynthesis, which as return drove the 1,3-propanediol production. Meanwhile, efficient co-production of 1,3-propanediol and l-lactate was also achieved by replacing ldhD with ldhL from Bacillus coagulans. The engineered strains PDL-5 and PLL co-produced over 70g/L 1,3-propanediol and over 100g/L optically pure d-lactate and l-lactate, respectively, with high conversion yields of over 0.95mol/mol from glycerol.

Production of C2-C4 diols from renewable bioresources: new metabolic pathways and metabolic engineering strategies

C2-C4 diols classically derived from fossil resource are very important bulk chemicals which have been used in a wide range of areas, including solvents, fuels, polymers, cosmetics, and pharmaceuticals. Production of C2-C4 diols from renewable resources has received significant interest in consideration of the reducing fossil resource and the increasing environmental issues. While bioproduction of certain diols like 1,3-propanediol has been commercialized in recent years, biosynthesis of many other important C2-C4 diol isomers is highly challenging due to the lack of natural synthesis pathways. Recent advances in synthetic biology have enabled the de novo design of completely new pathways to non-natural molecules from renewable feedstocks. In this study, we review recent advances in bioproduction of C2-C4 diols, focusing on new metabolic pathways and metabolic engineering strategies being developed. We also discuss the challenges and future trends toward the development of economically competitive processes for bio-based diol production.

Engineering Escherichia coli for Microbial Production of Butanone

To expand the chemical and molecular diversity of biotransformation using whole-cell biocatalysts, we genetically engineered a pathway in Escherichia coli for heterologous production of butanone, an important commodity ketone. First, a 1-propanol-producing E. coli host strain with its sleeping beauty mutase (Sbm) operon being activated was used to increase the pool of propionyl-coenzyme A (propionyl-CoA). Subsequently, molecular heterofusion of propionyl-CoA and acetyl-CoA was conducted to yield 3-ketovaleryl-CoA via a CoA-dependent elongation pathway. Lastly, 3-ketovaleryl-CoA was channeled into the clostridial acetone formation pathway for thioester hydrolysis and subsequent decarboxylation to form butanone. Biochemical, genetic, and metabolic factors affecting relative levels of ketogenesis, acidogenesis, and alcohol genesis under selected fermentative culture conditions were investigated. Using the engineered E. coli strain for batch cultivation with 30 g liter(-1)glycerol as the carbon source, we achieved coproduction of 1.3 g liter(-1)butanone and 2.9 g liter(-1)acetone. The results suggest that approximately 42% of spent glycerol was utilized for ketone biosynthesis, and thus they demonstrate potential industrial applicability of this microbial platform.

Utilization of excess NADH in 2,3-butanediol-deficient Klebsiella pneumoniae for 1,3-propanediol production

AIMS

To utilize excess NADH for 1,3-propanediol production by 2,3-butanediol-deficient mutants, the effect of dhaT overexpression in two distinct 2,3-butanediol-deficient mutants was investigated.

METHODS AND RESULTS

Two 2,3-butanediol-deficient mutants, KG1-3 (blocking of the 2,3-butanediol pathway only) and KG1-5 (blocking of both of 2,3-butanediol and lactate pathways) were constructed. Our results showed that although the intracellular redox balance (NADH/NAD(+)) was extremely high at the end of fermentation for both mutants, the status of intracellular redox in KG1-5 was maintained at a normal level following the first stage of fermentation. Analysis of cell growth and metabolite formation confirmed the inhibition of excess lactate in 2,3-butanediol pathway-deficient mutants. Furthermore, dhaT was overexpressed in two 2,3-butanediol-deficient mutants (KG1-3T and KG1-5T). In KG1-5T, the intracellular redox balance was restored to normal and 1,3-propanediol production increased. The yield of 1,3-propanediol from glycerol in KG1-5T was also restored to a normal level of 0·6.

CONCLUSIONS

The excess NADH in both the 2,3-butanediol- and lactate-deficient mutants can be used by overexpresstion of dhaT.

SIGNIFICANCE AND IMPACT OF STUDY

The metabolic flux tended to increase lactate production by the abolishment of the 2,3-butanediol pathway in Klebsiella pneumoniae, and the high accumulation of lactate prevented the cell from using excess NADH, thereby inhibiting cell growth and 1,3-propanediol production.

1,3-propanediol production with Citrobacter werkmanii DSM17579: effect of a dhaD knock-out

Background

1,3-propanediol (PDO) is a substantially industrial metabolite used in the polymer industry. Although several natural PDO production hosts exist, e.g. Klebsiella sp., Citrobacter sp. and Clostridium sp., the PDO yield on glycerol is insufficient for an economically viable bio-process. Enhancing this yield via strain improvement can be achieved by disconnecting the production and growth pathways. In the case of PDO formation, this approach results in a microorganism metabolizing glycerol strictly for PDO production, while catabolizing a co-substrate for growth and maintenance. We applied this strategy to improve the PDO production with Citrobacter werkmanii DSM17579.

Results

Genetic tools were developed and used to create Citrobacter werkmanii DSM17579 ∆dhaD in which dhaD, encoding for glycerol dehydrogenase, was deleted. Since this strain was unable to grow on glycerol anaerobically, both pathways were disconnected. The knock-out strain was perturbed with 13 different co-substrates for growth and maintenance. Glucose was the most promising, although a competition between NADH-consuming enzymes and 1,3-propanediol dehydrogenase emerged.

Conclusion

Due to the deletion of dhaD in Citrobacter werkmanii DSM17579, the PDO production and growth pathway were split. As a consequence, the PDO yield on glycerol was improved 1,5 times, strengthening the idea that Citrobacter werkmanii DSM17579 could become an industrially interesting host for PDO production.

Effect of the inactivation of lactate dehydrogenase, ethanol dehydrogenase, and phosphotransacetylase on 2,3-butanediol production in Klebsiella pneumoniae strain

BACKGROUND

2,3-Butanediol (2,3-BD) is a high-value chemical usually produced petrochemically but which can also be synthesized by some bacteria. To date, Klebsiella pneumoniae is the most powerful 2,3-BD producer which can utilize a wide range of substrates. However, many by-products are also produced by K. pneumoniae, such as ethanol, lactate, and acetate, which negatively regulate the 2,3-BD yield and increase the costs of downstream separation and purification.

RESULTS

In this study, we constructed K. pneumoniae mutants with lactate dehydrogenase (LDH), acetaldehyde dehydrogenase (ADH), and phosphotransacetylase (PTA) deletion individually by suicide vector conjugation.These mutants showed different behavior of production formation. Knock out of ldhA had little influence on the yield of 2,3-BD, whereas knock out of adhE or pta significantly improved the formation of 2,3-BD. The accumulation of the intermediate of 2,3-BD biosynthesis, acetoin, was decreased in all the mutants. The mutants were then tested in five different carbon sources and increased 2,3-BD was observed. Also a double mutant strain with deletion of adhE and ldhA was constructed which resulted in accelerated fermentation and higher 2,3-BD production. In fed-batch culture this strain achieved more than 100 g/L 2,3-BD from glucose with a relatively high yield of 0.49 g/g.

CONCLUSION

2,3-BD production was dramatically improved with the inactivation of adhE and pta. The inactivation of ldhA could advance faster cell growth and shorter fermentation time. The double mutant strain with deletion of adhE and ldhA resulted in accelerated fermentation and higher 2,3-BD production. These results provide new insights for industrial production of 2,3-BD by K. pneumoniae.

Glycerol dehydrogenase plays a dual role in glycerol metabolism and 2,3-butanediol formation in Klebsiella pneumoniae

Glycerol dehydrogenase (GDH) is an important polyol dehydrogenase for glycerol metabolism in diverse microorganisms and for value-added utilization of glycerol in the industry. Two GDHs from Klebsiella pneumoniae, DhaD and GldA, were expressed in Escherichia coli, purified and characterized for substrate specificity and kinetic parameters. Both DhaD and GldA could catalyze the interconversion of (3R)-acetoin/(2R,3R)-2,3-butanediol or (3S)-acetoin/meso-2,3-butanediol, in addition to glycerol oxidation. Although purified GldA appeared more active than DhaD, in vivo inactivation and quantitation of their respective mRNAs indicate that dhaD is highly induced by glycerol and plays a dual role in glycerol metabolism and 2,3-butanediol formation. Complementation in K. pneumoniae further confirmed the dual role of DhaD. Promiscuity of DhaD may have vital physiological consequences for K. pneumoniae growing on glycerol, which include balancing the intracellular NADH/NAD(+) ratio, preventing acidification, and storing carbon and energy. According to the kinetic response of DhaD to modified NADH concentrations, DhaD appears to show positive homotropic interaction with NADH, suggesting that the physiological role could be regulated by intracellular NADH levels. The co-existence of two functional GDH enzymes might be due to a gene duplication event. We propose that whereas DhaD is specialized for glycerol utilization, GldA plays a role in backup compensation and can turn into a more proficient catalyst to promote a survival advantage to the organism. Revelation of the dual role of DhaD could further the understanding of mechanisms responsible for enzyme evolution through promiscuity, and guide metabolic engineering methods of glycerol metabolism.

Inhibition Effect of Glycerol on the Corrosion of Copper in NaCl Solutions at Different pH Values

The inhibitory effect of glycerol on copper corrosion in aerated NaCl (0.5 M) solutions at three pH values (4, 7, and 10) was evaluated. Inhibition efficiency was assessed with conventional electrochemical techniques: open circuit potential, potentiodynamic polarization, and electrochemical impedance analysis. Glycerol reduced the corrosion rate of copper in NaCl solutions. The best inhibition effect (η≈83%) was produced in alkaline (pH 10) chloride media. This effect can be ascribed to increased viscosity and the presence of copper-glycerol complexes.

Enhanced aldehyde dehydrogenase activity by regenerating NAD+ in Klebsiella pneumoniae and implications for the glycerol dissimilation pathways

In Klebsiella pneumoniae, 3-hydroxypropaldehyde is converted to 3-hydroxypropionic acid (3-HP) by aldehyde dehydrogenase (ALDH) with NAD(+) as a cofactor. Although ALDH overexpression stimulates the formation of 3-HP, it ceases to accumulate when NAD(+) is exhausted. Here we show that NAD(+) regeneration, together with ALDH overexpression, facilitates 3-HP production and benefits cell growth. Three distinct NAD(+)-regenerating enzymes: NADH oxidase and NADH dehydrogenase from K. pneumoniae, and glycerol-3-phosphate dehydrogenase (GPD1) from Saccharomyces cerevisiae, were individually expressed in K. pneumoniae. In vitro assay showed their higher activities than that of the control, indicating their capacities to regenerate NAD(+). When they were respectively co-expressed with ALD4, an ALDH from S. cerevisiae, the activities of ALD4 were significantly elevated compared with that expressing ALD4 alone, suggesting that the regenerated NAD(+) enhanced the activity of ALD4. More interestingly, the growth rates of all NAD(+)-regenerating strains were prolonged in comparison with the control, indicating that NAD(+) regeneration stimulated cell proliferation. This study not only reveals the reliance of ALD4 activity on NAD(+) availability but also provides a method for regulating the dha regulon.

The role of aldehyde/alcohol dehydrogenase (AdhE) in ethanol production from glycerol by Klebsiella pneumoniae

Transcriptome analysis of a K. pneumoniae GEM167 mutant strain derived by irradiation with gamma rays, which exhibited high-level production of ethanol from glycerol, showed that the mutant expressed AdhE at a high level. Ethanol production decreased significantly, from 8.8 to 0.5 g l−1, when an adhE-deficient derivative of that strain was grown on glycerol. Bacterial growth was also reduced under such conditions, showing that AdhE plays a critical role in maintenance of redox balance by catalyzing ethanol production. Overexpression of AdhE enhanced ethanol production, from pure or crude glycerol, to a maximal level of 31.9 g l−1 under fed-batch fermentation conditions; this is the highest level of ethanol production from glycerol reported to date.

Identification and characterization of Klebsiella pneumoniae aldehyde dehydrogenases increasing production of 3-hydroxypropionic acid from glycerol

Klebsiella pneumoniae produces 3-hydroxypropionic acid (3-HP) from glycerol with oxidation of 3-hydroxypropionaldehyde (3-HPA) to 3-HP in a reaction catalyzed by aldehyde dehydrogenase (ALDH). In the present study, two putative ALDHs of K. pneumoniae, YneI and YdcW were identified and characterized. Recombinant YneI was specifically active on 3-HPA and preferred NAD(+) as a cofactor, whereas YdcW exhibited broad substrate specificity and preferred NADP(+) as a cofactor. Overexpression of ALDHs in the glycerol oxidative pathway-deficient mutant K. pneumoniae AK resulted in a significant increase in 3-HP production upon shake-flask culture. The final titers of 3-HP were 2.4 and 1.8 g L(-1) by recombinants overexpressing YneI and YdcW, respectively. Deletion of the ALDH gene from K. pneumoniae did not affect the extent of 3-HP synthesis, implying non-specific activity of ALDHs on 3-HPA. The ALDHs might play major roles in detoxifying the aldehyde generated in glycerol metabolism.

Red recombinase assisted gene replacement in Klebsiella pneumoniae

The Red recombinase system, the most convenient genetic tool applied in Escherichia coli and other bacteria, was introduced for gene replacement in Klebsiella pneumoniae. The novel K. pneumoniae gene replacement system comprised the Red and FLP recombinases expression vector pDK6-red and pDK6-flp, and linear DNA fragments which encompassed a selective marker gene with target gene flanking extensions; the latter were PCR amplified using a plasmid DNA template obtained by in vivo recombination in E. coli. In this study, dhak1 gene, encoding a subunit of dihydroxyacetone kinase II, was deleted markerlessly at a transformation ratio of 260 CFU/μg DNA, i.e., 1,000-fold higher than that achieved in the native way. Our studies provide an efficient method with detailed protocol to perform gene replacement in K. pneumoniae and has great potential to be developed as a routine genetic approach for this important industrial microorganism.

Production of 3-hydroxypropionic acid through propionaldehyde dehydrogenase PduP mediated biosynthetic pathway in Klebsiella pneumoniae

The pduP gene encodes a propionaldehyde dehydrogenase (PduP) was investigated for the role in 3-hydroxypropionic acid (3-HP) glycerol metabolism in Klebsiella pneumoniae. The enzyme assay showed that cell extracts from a pduP mutant strain lacked measurable dehydrogenase activity. Additionally, the mutant strain accumulated the cytotoxic intermediate metabolite 3-hydroxypropionaldehyde (3-HPA), causing both cell death and a lower final 3-HP titer. Ectopic expression of pduP restored normal cell growth to mutant. The enzymatic property of recombinant protein from Escherichia coli was examined, exhibiting a broad substrate specificity, being active on 3-HPA. The present work is thus the first to demonstrate the role of PduP in glycerol metabolism and biosynthesis of 3-HP.

Fermentation strategies for 1,3-propanediol production from glycerol using a genetically engineered Klebsiella pneumoniae strain to eliminate by-product formation

We generated a genetically engineered Klebsiella pneumoniae strain (AK-VOT) to eliminate by-product formation during production of 1,3-propanediol (1,3-PD) from glycerol. In the present study, the glycerol-metabolizing properties of the recombinant strain were examined during fermentation in a 5 L bioreactor. As expected, by-product formation was completely absent (except for acetate) when the AK-VOT strain fermented glycerol. However, 1,3-PD productivity was severely reduced owing to a delay in cell growth attributable to a low rate of glycerol consumption. This problem was solved by establishing a two-stage process separating cell growth from 1,3-PD production. In addition, nutrient co-supplementation, especially with starch, significantly increased 1,3-PD production from glycerol during fed-batch fermentation by AK-VOT in the absence of by-product formation.

Influence of blocking of 2,3-butanediol pathway on glycerol metabolism for 1,3-propanediol production by Klebsiella oxytoca

Glycerol metabolism is a typical biological oxidoreductive reaction. 1,3-Propanediol (1,3-PD) is the final product of the reductive branch, while acetate, succinate, lactate, 2,3-butanediol (2,3-BD), and ethanol were produced in the oxidative branch. 2,3-BD, which has similar properties of high boiling point and water solubility with 1,3-PD, not only contests the carbon flow and NADH with 1,3-PD but also serves as an obstacle for obtaining high purity 1,3-PD in downstream processes. In this study, a 2,3-BD pathway-deficient mutant of Klebsiella oxytoca ZG36 was constructed by knocking out the budA gene of the wild-type strain M5al. The results of fed-batch fermentation by ZG36 indicated that the glycerol flux and the distribution of metabolites were altered in the K. oxytoca when the 2,3-BD pathway was blocked. No 2,3-BD was produced, and the activity of α-acetolactate decarboxylase (α-ALDC) can not be detected in the fermentation processes. The indexes of the 1,3-PD titer, the conversion from glycerol to 1,3-PD, and the productivity per cell dry weight (CDW) increased by 42%, 62%, and 46%, respectively, compared with the M5al, and the yield of the byproducts also increased obviously. The assay of the enzyme activities in the oxidative branch and the reductive branch of the glycerol metabolism, as well as the intracellular redox state, exposited the results logically.

Development of recombinant Klebsiella pneumoniae ∆dhaT strain for the co-production of 3-hydroxypropionic acid and 1,3-propanediol from glycerol

Klebsiella pneumoniae converts glycerol to the specialty chemical 1,3-propanediol (1,3-PDO), which is used for the production of polytrimethylene terepthalate (PTT). In this study, an NAD(+)-dependent gamma-glutamyl-gamma-aminobutyraldehyde dehydrogenase (PuuC) of K. pneumoniae DSM 2026, which oxidizes 3-hydroxypropionaldehyde to a platform chemical 3-hydroxypropionic acid (3-HP), was cloned and overexpressed in K. pneumoniae DSM 2026 for the co-production of 3-HP and 1,3-PDO from glycerol. In addition, the gene dhaT, encoding NADH-dependent 1,3-propanediol oxidoreductase (1,3-PDOR), was deleted from the chromosome for the balanced production of 3-HP and 1,3-PDO. The recombinant K. pneumoniae ∆dhaT, expressing puuC, produced 3.6 g 3-HP and 3.0 g 1,3-PDO per liter with an average yield of 81% on glycerol carbon in shake flask culture under microaerobic conditions. When a fed-batch culture was carried out under microaerobic conditions at pH 7.0 in a 5-l bioreactor, the recombinant K. pneumoniae ∆dhaT (puuC) strain produced 16.0 g 3-HP and 16.8 g 1,3-PDO per liter with a cumulative yield of 51% on glycerol carbon in 24 h. The production of 1,3-PDO in the dhaT-deletion mutant was attributed to the expression of NAD(P)H-dependent hypothetical oxidoreductase. This study demonstrates the feasibility of obtaining two commercially valuable chemicals, 3-HP and 1,3-PDO, at a significant scale.

Efficient production of ethanol from crude glycerol by a Klebsiella pneumoniae mutant strain

A mutant strain of Klebsiella pneumoniae, termed GEM167, was obtained by γ irradiation, in which glycerol metabolism was dramatically affected on exposure to γ rays. Levels of metabolites of the glycerol reductive pathway, 1,3-propanediol (1,3-PD) and 3-hydroxypropionic acid (3-HP), were decreased in the GEM167 strain compared to a control strain, whereas the levels of metabolites derived from the oxidative pathway, 2,3-butanediol (2,3-BD), ethanol, lactate, and succinate, were increased. Notably, ethanol production from glycerol was greatly enhanced upon fermentation by the mutant strain, to a maximum production level of 21.5 g/l, with a productivity of 0.93 g/l/h. Ethanol production level was further improved to 25.0 g/l upon overexpression of Zymomonas mobilis pdc and adhII genes encoding pyruvate decarboxylase (Pdc) and aldehyde dehydrogenase (Adh), respectively in the mutant strain GEM167.