2-Butanol and butanone production in Saccharomyces cerevisiae through combination of a B12 dependent dehydratase and a secondary alcohol dehydrogenase using a TEV-based expression system

Recent advances in the microbial production of C4 alcohols by metabolically engineered microorganisms

BACKGROUND

The heavy global dependence on petroleum-based industries has led to serious environmental problems, including climate change and global warming. As a result, there have been calls for a paradigm shift towards the use of biorefineries, which employ natural and engineered microorganisms that can utilize various carbon sources from renewable resources as host strains for the carbon-neutral production of target products.

PURPOSE AND SCOPE

C4 alcohols are versatile chemicals that can be used directly as biofuels and bulk chemicals and in the production of value-added materials such as plastics, cosmetics, and pharmaceuticals. C4 alcohols can be effectively produced by microorganisms using DCEO biotechnology (tools to design, construct, evaluate, and optimize) and metabolic engineering strategies.

SUMMARY OF NEW SYNTHESIS AND CONCLUSIONS

In this review, we summarize the production strategies and various synthetic tools available for the production of C4 alcohols and discuss the potential development of microbial cell factories, including the optimization of fermentation processes, that offer cost competitiveness and potential industrial commercialization.

Process optimization for mass production of 2,3-butanediol by Bacillus subtilis CS13

BACKGROUND

Bacillus subtilis CS13 was previously isolated for 2,3-butanediol (2,3-BD) and poly-γ-glutamic acid (γ-PGA) co-production. When culturing this strain without L-glutamic acid in the medium, 2,3-BD is the main metabolic product. 2,3-BD is an important substance and fuel with applications in the chemical, food, and pharmaceutical industries. However, the yield and productivity for the B. subtilis strain should be improved for more efficient production of 2,3-BD.

RESULTS

The medium composition, which contained 281.1 g/L sucrose, 21.9 g/L ammonium citrate, and 3.6 g/L MgSO₄·7H₂O, was optimized by response surface methodology for 2,3-BD production using B. subtilis CS13. The maximum amount of 2,3-BD (125.5 ± 3.1 g/L) was obtained from the optimized medium after 96 h. The highest concentration and productivity of 2,3-BD were achieved simultaneously at an agitation speed of 500 rpm and aeration rate of 2 L/min in the batch cultures. A total of 132.4 ± 4.4 g/L 2,3-BD was obtained with a productivity of 2.45 ± 0.08 g/L/h and yield of 0.45 g_2,3-BD/g_sucrose by fed-batch fermentation. The meso-2,3-BD/2,3-BD ratio of the 2,3-BD produced by B. subtilis CS13 was 92.1%. Furthermore, 89.6 ± 2.8 g/L 2,3-BD with a productivity of 2.13 ± 0.07 g/L/h and yield of 0.42 g_2,3-BD/g_sugar was achieved using molasses as a carbon source.

CONCLUSIONS

The production of 2,3-BD by B. subtilis CS13 showed a higher concentration, productivity, and yield compared to the reported generally recognized as safe 2,3-BD producers. These results suggest that B. subtilis CS13 is a promising strain for industrial-scale production of 2,3-BD.

Rational design of a new mutant of tobacco etch virus protease in order to increase the in vitro solubility

Background and purpose:

Tobacco etch virus (TEV) protease is a protease with high sequence specificity which is useful for the cleavage of fusion proteins. A major limitation of this enzyme is its relatively poor solubility. This study aimed to investigate the effects of some suggested mutations by online tools and molecular dynamics simulation to improve the solubility of TEV protease in vitro.

Experimental approach:

We designed a rational multi-stage process to determine the solubilizing mutations of TEV protease. At the first stage, all the possible mutations were predicted using online tools such as PoPMuSiC and Eris servers, in which five mutations include N23F, N23L, Q74L, Q74V, and Q74I were suggested for further studies. In the next step, the three dimensional structure of the wild type (WT) and the best mutations were subjected to molecular dynamic simulations to evaluate the dynamic behaviour of the obtained structures. The selected mutation was introduced into the structure using site-directed mutagenesis and expressed in Escherichia coli BL21DE3. After purification, solubility and activity of the purified mutant and WT-TEV proteases were assayed.

Findings /Results:

By considering the analysis of various factors such as structural and solubility properties, one mutant, N23F, was selected for in vitro studies which led to a 1.5 times increase in the solubility compared to the WT while its activity was decreased somewhat.

Conclusion and implications:

We propose N23F mutation, according to computational and experimental analyses for TEV proteases which resulted in a 150% increase in solubility compared to the WT.

A tight cold-inducible switch built by coupling thermosensitive transcriptional and proteolytic regulatory parts

Natural organisms have evolved intricate regulatory mechanisms that sense and respond to fluctuating environmental temperatures in a heat- or cold-inducible fashion. Unlike dominant heat-inducible switches, very few cold-inducible genetic switches are available in either natural or engineered systems. Moreover, the available cold-inducible switches still have many shortcomings, including high leaky gene expression, small dynamic range (<10-fold) or broad transition temperature (>10°C). To address these problems, a high-performance cold-inducible switch that can tightly control target gene expression is highly desired. Here, we introduce a tight and fast cold-inducible switch that couples two evolved thermosensitive variants, TF_ts and TEV_ts, as well as an additional Mycoplasma florum Lon protease (mf-Lon) to effectively turn-off target gene expression via transcriptional and proteolytic mechanisms. We validated the function of the switch in different culture media and various Escherichia coli strains and demonstrated its tightness by regulating two morphogenetic bacterial genes and expressing three heat-unstable recombinant proteins, respectively. Moreover, the additional protease module enabled the cold-inducible switch to actively remove the pre-existing proteins in slow-growing cells. This work establishes a high-performance cold-inducible system for tight and fast control of gene expression which has great potential for basic research, as well as industrial and biomedical applications.

Growth-coupled bioconversion of levulinic acid to butanone

Common strategies for conversion of lignocellulosic biomass to chemical products center on deconstructing biomass polymers into fermentable sugars. Here, we demonstrate an alternative strategy, a growth-coupled, high-yield bioconversion, by feeding cells a non-sugar substrate, by-passing central metabolism, and linking a key metabolic step to generation of acetyl-CoA that is required for biomass and energy generation. Specifically, we converted levulinic acid (LA), an established degradation product of lignocellulosic biomass, to butanone (a.k.a. methyl-ethyl ketone - MEK), a widely used industrial solvent. Our strategy combines a catabolic pathway from Pseudomonas putida that enables conversion of LA to 3-ketovaleryl-CoA, a CoA transferase that generates 3-ketovalerate and acetyl-CoA, and a decarboxylase that generates 2-butanone. By removing the ability of E. coli to consume LA and supplying excess acetate as a carbon source, we built a strain of E. coli that could convert LA to butanone at high yields, but at the cost of significant acetate consumption. Using flux balance analysis as a guide, we built a strain of E. coli that linked acetate assimilation to production of butanone. This strain was capable of complete bioconversion of LA to butanone with a reduced acetate requirement and increased specific productivity. To demonstrate the bioconversion on real world feedstocks, we produced LA from furfuryl alcohol, a compound readily obtained from biomass. These LA feedstocks were found to contain inhibitors that prevented cell growth and bioconversion of LA to butanone. We used a combination of column chromatography and activated carbon to remove the toxic compounds from the feedstock, resulting in LA that could be completely converted to butanone. This work motivates continued collaboration between chemical and biological catalysis researchers to explore alternative conversion pathways and the technical hurdles that prevent their rapid deployment.

Integrated Vacuum Stripping and Adsorption for the Efficient Recovery of (Biobased) 2-Butanol

Biobased 2-butanol offers high potential as biofuel, but its toxicity toward microbial hosts calls for efficient techniques to alleviate product inhibition in fermentation processes. Aiming at the selective recovery of 2-butanol, the feasibility of a process combining in situ vacuum stripping followed by vapor adsorption has been assessed using mimicked fermentation media. The experimental vacuum stripping of model solutions and corn stover hydrolysate closely aligned with mass transfer model predictions. However, the presence of lignocellulosic impurities affected 2-butanol recovery yields resulting from vapor condensation, which decreased from 96 wt % in model solutions to 40 wt % using hydrolysate. For the selective recovery of 2-butanol from a vapor mixture enriched in water and carbon dioxide, silicalite materials were the most efficient, particularly at low alcohol partial pressures. Integrating in situ vacuum stripping with vapor adsorption using HiSiv3000 proved useful to effectively concentrate 2-butanol above its azeotropic composition (>68 wt %), facilitating further product purification.

Microbial 2-butanol production with Lactobacillus diolivorans

BACKGROUND

Biobutanol has great potential as biofuel of the future. However, only a few organisms have the natural ability to produce butanol. Amongst them, Clostridium spp. are the most efficient producers. The high toxicity of biobutanol constitutes one of the bottlenecks within the biobutanol production process which often suffers from low final butanol concentrations and yields. Butanol tolerance is a key driver for process optimisation and, therefore, in the search for alternative butanol production hosts. Many Lactobacillus species show a remarkable tolerance to solvents and some Lactobacillus spp. are known to naturally produce 2-butanol from meso-2,3-butanediol (meso-2,3-BTD) during anaerobic sugar fermentations. Lactobacillus diolivorans showed already to be highly efficient in the production of other bulk chemicals using a simple two-step metabolic pathway. Exactly, the same pathway enables this cell factory for 2-butanol production.

RESULTS

Due to the inability of L. diolivorans to produce meso-2,3-BTD, a two-step cultivation processes with Serratia marcescens has been developed. S. marcescens is a very efficient producer of meso-2,3-BTD from glucose. The process yielded a butanol concentration of 10 g/L relying on wild-type bacterial strains. A further improvement of the maximum butanol titer was achieved using an engineered L. diolivorans strain overexpressing the endogenous alcohol dehydrogenase pduQ. The two-step cultivation process based on the engineered strain led to a maximum 2-butanol titer of 13.4 g/L, which is an increase of 34%.

CONCLUSION

In this study, L. diolivorans is for the first time described as a good natural producer for 2-butanol from meso-2,3-butanediol. Through the application of a two-step cultivation process with S. marcescens, 2-butanol can be produced from glucose in a one-vessel, two-step microbial process.

Discovery and Evaluation of Biosynthetic Pathways for the Production of Five Methyl Ethyl Ketone Precursors

The limited supply of fossil fuels and the establishment of new environmental policies shifted research in industry and academia toward sustainable production of the second generation of biofuels, with methyl ethyl ketone (MEK) being one promising fuel candidate. MEK is a commercially valuable petrochemical with an extensive application as a solvent. However, as of today, a sustainable and economically viable production of MEK has not yet been achieved despite several attempts of introducing biosynthetic pathways in industrial microorganisms. We used BNICE.ch as a retrobiosynthesis tool to discover all novel pathways around MEK. Out of 1325 identified compounds connecting to MEK with one reaction step, we selected 3-oxopentanoate, but-3-en-2-one, but-1-en-2-olate, butylamine, and 2-hydroxy-2-methylbutanenitrile for further study. We reconstructed 3 679 610 novel biosynthetic pathways toward these 5 compounds. We then embedded these pathways into the genome-scale model of E. coli, and a set of 18 622 were found to be the most biologically feasible ones on the basis of thermodynamics and their yields. For each novel reaction in the viable pathways, we proposed the most similar KEGG reactions, with their gene and protein sequences, as candidates for either a direct experimental implementation or as a basis for enzyme engineering. Through pathway similarity analysis we classified the pathways and identified the enzymes and precursors that were indispensable for the production of the target molecules. These retrobiosynthesis studies demonstrate the potential of BNICE.ch for discovery, systematic evaluation, and analysis of novel pathways in synthetic biology and metabolic engineering studies.

Metabolic engineering of Saccharomyces cerevisiae to produce a reduced viscosity oil from lignocellulose

BACKGROUND

Acetyl-triacylglycerols (acetyl-TAGs) are unusual triacylglycerol (TAG) molecules that contain an sn-3 acetate group. Compared to typical triacylglycerol molecules (here referred to as long chain TAGs; lcTAGs), acetyl-TAGs possess reduced viscosity and improved cold temperature properties, which may allow direct use as a drop-in diesel fuel. Their different chemical and physical properties also make acetyl-TAGs useful for other applications such as lubricants and plasticizers. Acetyl-TAGs can be synthesized by EaDAcT, a diacylglycerol acetyltransferase enzyme originally isolated from Euonymus alatus (Burning Bush). The heterologous expression of EaDAcT in different organisms, including Saccharomyces cerevisiae, resulted in the accumulation of acetyl-TAGs in storage lipids. Microbial conversion of lignocellulose into acetyl-TAGs could allow biorefinery production of versatile molecules for biofuel and bioproducts.

RESULTS

In order to produce acetyl-TAGs from abundant lignocellulose feedstocks, we expressed EaDAcT in S. cerevisiae previously engineered to utilize xylose as a carbon source. The resulting strains were capable of producing acetyl-TAGs when grown on different media. The highest levels of acetyl-TAG production were observed with growth on synthetic lab media containing glucose or xylose. Importantly, acetyl-TAGs were also synthesized by this strain in ammonia fiber expansion (AFEX)-pretreated corn stover hydrolysate (ACSH) at higher volumetric titers than previously published strains. The deletion of the four endogenous enzymes known to contribute to lcTAG production increased the proportion of acetyl-TAGs in the total storage lipids beyond that in existing strains, which will make purification of these useful lipids easier. Surprisingly, the strains containing the four deletions were still capable of synthesizing lcTAG, suggesting that the particular strain used in this study possesses additional undetermined diacylglycerol acyltransferase activity. Additionally, the carbon source used for growth influenced the accumulation of these residual lcTAGs, with higher levels in strains cultured on xylose containing media.

CONCLUSION

Our results demonstrate that S. cerevisiae can be metabolically engineered to produce acetyl-TAGs when grown on different carbon sources, including hydrolysate derived from lignocellulose. Deletion of four endogenous acyltransferases enabled a higher purity of acetyl-TAGs to be achieved, but lcTAGs were still synthesized. Longer incubation times also decreased the levels of acetyl-TAGs produced. Therefore, additional work is needed to further manipulate acetyl-TAG production in this strain of S. cerevisiae, including the identification of other TAG biosynthetic and lipolytic enzymes and a better understanding of the regulation of the synthesis and degradation of storage lipids.

Growth inhibition of S. cerevisiae, B. subtilis, and E. coli by lignocellulosic and fermentation products

This paper describes the effect of several inhibiting components on three potential hosts for the bio-based production of methyl propionate, namely, wild-type Escherichia coli and Bacillus subtilis, and evolved Saccharomyces cerevisiae IMS0351. The inhibition by the lignocellulose-derived products 5-hydroxymethyl-2-furaldehyde, vanillin, and syringaldehyde and the fermentation products 2-butanol, 2-butanone, methyl propionate, and ethyl acetate has been assessed for these strains in defined medium. Multiple screenings were performed using small-scale cultures in both shake flasks and microtiter plates. Technical drawbacks revealed the limited applicability of the latter in this study. The microbial growth was characterized by means of a lag-time model, and the inhibitory thresholds were determined using product-inhibition models. The lignocellulose-derived products were found to be highly inhibitory, and none of the strains could grow in the presence of 2.0 g L-1 of product. From the fermentation products tested, methyl propionate had the most severe impact resulting in complete inhibition of all the strains when exposed to concentrations in the range of 12-18 g L-1. In general, S. cerevisiae and B. subtilis were comparatively more tolerant than E. coli to all the fermentation products, despite E. coli's lower sensitivity towards vanillin. The results suggest that, overall, the strains investigated have good potential to be engineered and further established as hosts for the bio-based production of methyl esters.

Recent advances in engineering propionyl-CoA metabolism for microbial production of value-added chemicals and biofuels

Diminishing fossil fuel reserves and mounting environmental concerns associated with petrochemical manufacturing practices have generated significant interests in developing whole-cell biocatalytic systems for the production of value-added chemicals and biofuels. Although acetyl-CoA is a common natural biogenic precursor for the biosynthesis of numerous metabolites, propionyl-CoA is unpopular and non-native to most organisms. Nevertheless, with its C3-acyl moiety as a discrete building block, propionyl-CoA can serve as another key biogenic precursor to several biological products of industrial importance. As a result, engineering propionyl-CoA metabolism, particularly in genetically tractable hosts with the use of inexpensive feedstocks, has paved an avenue for novel biomanufacturing. Herein, we present a systematic review on manipulation of propionyl-CoA metabolism as well as relevant genetic and metabolic engineering strategies for microbial production of value-added chemicals and biofuels, including odd-chain alcohols and organic acids, bio(co)polymers and polyketides. [Formula: see text].

Tobacco Etch Virus protease: A shortcut across biotechnologies

About thirty years ago, studies on the RNA genome of Tobacco Etch Virus revealed the presence of an efficient and specific protease, called Tobacco Etch Virus protease (TEVp), that was part of the Nuclear Inclusion a (NIa) enzyme. TEVp is an efficient and specific protease of 27kDa that has become a valuable biotechnological tool. Nowadays TEVp is a unique endopeptidase largely exploited in biotechnology from industrial applications to in vitro and in vivo cellular studies. A number of TEVp mutants with different rate of cleavage, stability and specificity have been reported. Similarly, a panel of different target cleavage sites, derived from the canonical ENLYFQ-G/S site, has been established. In this review we describe these aspects of TEVp and some of its multiple applications. A particular focus is on the use and molecular biology of TEVp in living cells and organisms.

Post-translational control of genetic circuits using Potyvirus proteases

Genetic engineering projects often require control over when a protein is degraded. To this end, we use a fusion between a degron and an inactivating peptide that can be added to the N-terminus of a protein. When the corresponding protease is expressed, it cleaves the peptide and the protein is degraded. Three protease:cleavage site pairs from Potyvirus are shown to be orthogonal and active in exposing degrons, releasing inhibitory domains and cleaving polyproteins. This toolbox is applied to the design of genetic circuits as a means to control regulator activity and degradation. First, we demonstrate that a gate can be constructed by constitutively expressing an inactivated repressor and having an input promoter drive the expression of the protease. It is also shown that the proteolytic release of an inhibitory domain can improve the dynamic range of a transcriptional gate (200-fold repression). Next, we design polyproteins containing multiple repressors and show that their cleavage can be used to control multiple outputs. Finally, we demonstrate that the dynamic range of an output can be improved (8-fold to 190-fold) with the addition of a protease-cleaved degron. Thus, controllable proteolysis offers a powerful tool for modulating and expanding the function of synthetic gene circuits.

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.

Increased availability of NADH in metabolically engineered baker's yeast improves transaminase-oxidoreductase coupled asymmetric whole-cell bioconversion

BACKGROUND

Saccharomyces cerevisiae can be engineered to perform a multitude of different chemical reactions that are not programmed in its original genetic code. It has a large potential to function as whole-cell biocatalyst for one-pot multistep synthesis of various organic molecules, and it may thus serve as a powerful alternative or complement to traditional organic synthetic routes for new chemical entities (NCEs). However, although the selectivity in many cases is high, the catalytic activity is often low which results in low space-time-yields. In the case for NADH-dependent heterologous reductive reactions, a possible constraint is the availability of cytosolic NADH, which may be limited due to competition with native oxidative enzymes that act to maintain redox homeostasis. In this study, the effect of increasing the availability of cytosolic NADH on the catalytic activity of engineered yeast for transamination-reduction coupled asymmetric one-pot conversion was investigated.

RESULTS

A series of active whole-cell biocatalysts were constructed by over-expressing the (S)-selective ω-transaminase (VAMT) from Capsicum chinense together with the NADH-dependent (S)-selective alcohol dehydrogenase (SADH) originating from Rhodococcus erythropolis in strains with or without deletion of glycerol-3-phosphate dehydrogenases 1 and 2 (GPD1 and GPD2). The yeast strains were evaluated as catalysts for simultaneous: (a) kinetic resolution of the racemic mixture to (R)-1-phenylethylamine, and (b) reduction of the produced acetophenone to (S)-1-phenylethanol. For the gpd1Δgpd2Δ strain, cell metabolism was effectively used for the supply of both amine acceptors and the co-factor pyridoxal-5'-phosphate (PLP) for the ω-transaminase, as well as for regenerating NADH for the reduction. In contrast, there was nearly no formation of (S)-1-phenylethanol when using the control strain with intact GPDs and over-expressing the VAMT-SADH coupling. It was found that a gpd1Δgpd2Δ strain over-expressing SADH had a 3-fold higher reduction rate and a 3-fold lower glucose requirement than the strain with intact GPDs over-expressing SADH.

CONCLUSIONS

Overall the results demonstrate that the deletion of the GPD1 and GPD2 genes significantly increases activity of the whole-cell biocatalyst, and at the same time reduces the co-substrate demand in a process configuration where only yeast and sugar is added to drive the reactions, i.e. without addition of external co-factors or prosthetic groups.

Recent progress in biobutanol tolerance in microbial systems with an emphasis on Clostridium

Biobased production of butanol promises a more sustainable route for industrial production. However, butanol toxicity remains a barrier for achieving high product titers. Investigation into butanol stress has shed some light on its modes of toxicity. Unfortunately, there still remain significant shortfalls in our understanding of the complex interactions of butanol with cells. To address this knowledge gap, a diverse range of tools have been employed to gain a better understanding of the adverse effects of butanol on the cell. These findings have lead to the identification of possible molecular mechanisms associated with butanol tolerance, which can be harnessed for future strain development efforts. This review focuses on recent efforts to address the toxicity of butanol in microbial producers and offers some perspectives on the future direction of this research sector.

Engineering Bacillus licheniformis for the production of meso-2,3-butanediol

BACKGROUND

2,3-Butanediol (2,3-BD) can be used as a liquid fuel additive to replace petroleum oil, and as an important platform chemical in the pharmaceutical and plastic industries. Microbial production of 2,3-BD by Bacillus licheniformis presents potential advantages due to its GRAS status, but previous attempts to use this microorganism as a chassis strain resulted in the production of a mix of D-2,3-BD and meso-2,3-BD isomers.

RESULTS

The aim of this work was to develop an engineered strain of B. licheniformis suited to produce the high titers of the pure meso-2,3-BD isomer. Glycerol dehydrogenase (Gdh) was identified as the catalyst for D-2,3-BD biosynthesis from its precursor acetoin in B. licheniformis. The gdh gene was, therefore, deleted from the wild-type strain WX-02 to inhibit the flux of acetoin to D-2,3-BD biosynthesis. The acoR gene involved in acetoin degradation through AoDH ES was also deleted to provide adequate flux from acetoin towards meso-2,3-BD. By re-directing the carbon flux distribution, the double-deletion mutant WX-02ΔgdhΔacoR produced 28.2 g/L of meso-2,3-BD isomer with >99 % purity. The titer was 50 % higher than that of the wide type. A bench-scale fermentation by the double-deletion mutant was developed to further improve meso-2,3-BD production. In a fed-batch fermentation, meso-2,3-BD titer reached 98.0 g/L with a purity of >99.0 % and a productivity of 0.94 g/L-h.

CONCLUSIONS

This work demonstrates the potential of producing meso-2,3-BD with high titer and purity through metabolic engineering of B. licheniformis.

Snβ-zeolite catalyzed oxido-reduction cascade chemistry with biomass-derived molecules

High activity of post-synthetically synthesized Sn-beta, producing novel caprolactone polymer building blocks, is demonstrated in Meerwein–Ponndorf–Verley (MPV), Oppenauer (OPO), Baeyer–Villiger (BV) and cascade reactions thereof with biomass-derived molecules.

Metabolic engineering of Klebsiella pneumoniae for the de novo production of 2-butanol as a potential biofuel

Butanol isomers are important bulk chemicals and promising fuel substitutes. The inevitable toxicity of n-butanol and isobutanol to microbial cells hinders their final titers. In this study, we attempt to engineer Klebsiella pneumoniae for the de novo production of 2-butanol, another butanol isomer which shows lower toxicity than n-butanol and isobutanol. 2-Butanol synthesis was realized by the extension of the native meso-2,3-butanediol synthesis pathway with the introduction of diol dehydratase and secondary alcohol dehydrogenase. By the screening of different secondary alcohol dehydrogenases and diol dehydratases, 320mg/L of 2-butanol was produced by the best engineered K. pneumoniae. The production was increased to 720mg/L by knocking out the ldhA gene and appropriate addition of coenzyme B12. Further improvement of 2-butanol to 1030mg/L was achieved by protein engineering of diol dehydratase. This work lays the basis for the metabolic engineering of microorganism for the production of 2-butanol as potential biofuel.

Metabolic Engineering of Klebsiella pneumoniae for the Production of 2-Butanone from Glucose

2-Butanone is an important commodity chemical of wide application in different areas. In this study, Klebsiella pneumoniae was engineered to directly produce 2-butanone from glucose by extending its native 2, 3-butanediol synthesis pathway. To identify the potential enzyme for the efficient conversion of 2, 3-butanediol to 2-butanone, we screened different glycerol dehydratases and diol dehydratases. By introducing the diol dehydratase from Lactobacillus brevis and deleting the ldhA gene encoding lactate dehydrogenase, the engineered K. pneumoniae was able to accumulate 246 mg/L of 2-butanone in shake flask. With further optimization of culture condition, the titer of 2-butanone was increased to 450 mg/L. This study lays the basis for developing an efficient biological process for 2-butanone production.

Opportunities for Bio-Based Solvents Created as Petrochemical and Fuel Products Transition towards Renewable Resources

The global bio-based chemical market is growing in size and importance. Bio-based solvents such as glycerol and 2-methyltetrahydrofuran are often discussed as important introductions to the conventional repertoire of solvents. However adoption of new innovations by industry is typically slow. Therefore it might be anticipated that neoteric solvent systems (e.g., ionic liquids) will remain niche, while renewable routes to historically established solvents will continue to grow in importance. This review discusses bio-based solvents from the perspective of their production, identifying suitable feedstocks, platform molecules, and relevant product streams for the sustainable manufacturing of conventional solvents.