In one aspect, the invention provides a recombinant microbial cell modified to exhibit increased biosynthesis of a biosynthetic product compared to a wild-type control. In some cases, the biosynthetic product can be isocaproate; in other cases the biosynthetic product can be isovalerate.
In some embodiments, the recombinant microbial cell can be a fungal cell such as, for example, a member of the Saccharomycetaceae family. In particular embodiments, the fungal cell can be Saccharomyces cerevisiae, Candida rugosa, or Candida albicans.
In other embodiments, the microbial cell can be a bacterial cell such as, for example, a member of the phylum Protobacteria, a member of the phylum Firmicutes, or a member of the phylum Cyanobacteria. In certain of these embodiments, the bacterial cell can be a member of the Enterobacteriaceae family such as, for example, Escherichia coli; a member of the Pseudomonaceae family such as, for example, Pseudomonas putida; a member of the Bacillaceae family such as, for example, Bacillus subtilis; a member of the Streptococcaceae family such as, for example, Lactococcus lactis; or a member of the Clostridiaceae family such as, for example, Clostridium cellulolyticum.
In some embodiments, the recombinant cell can be photosynthetic.
In some embodiments, the recombinant cell can be cellulolytic.
In embodiments in which the biosynthetic product includes isocaproate, the recombinant cell can exhibit an increase in 2-isopropylmalate synthase activity compared to a wild-type control, an increase in ketoleucine elongation activity compared to a wild-type control, an increase in ketoacid decarboxylase activity compared to a wild-type control, an increase in ketoacid decarboxylase selectivity toward a predetermined substrate compared to a wild-type control, or an increase in aldehyde dehydrogenase activity compared to a wild-type control.
In embodiments in which the biosynthetic product includes isovalerate, the recombinant cell can exhibit an increase in leucine biosynthesis compared to a wild-type control, an increase in decarboxylase activity compared to a wild-type control, an increase in aldehyde dehydrogenase activity compared to a wild-type control, or an increase in branched-chain ketoacid dehydrogenase activity compared to a wild-type control.
In some embodiments, the recombinant can exhibit a decrease in activity of at least one native ldhA, adhE, pta, pflB, poxB, yqhD enzyme compared to wild-type, thereby decreasing byproduct synthesis compared to a wild-type control.
In another aspect, the invention provides a method that generally includes incubating a recombinant cell modified to exhibit an increase in the biosynthesis of isocaproate in medium that comprises a carbon source under conditions effective for the recombinant cell to produce isocaproate, wherein the carbon source comprises one or more of: glucose, pyruvate, ketovaline, ketoleucine, ketohomoleucine, CO2, cellulose, xylose, sucrose, arabinose, or glycerol.
In another aspect, the invention provides a method that generally includes incubating a recombinant cell modified to exhibit an increase in the biosynthesis of isovalerate in medium that comprises a carbon source under conditions effective for the recombinant cell to produce isovalerate, wherein the carbon source comprises one or more of: glucose, pyruvate, ketovaline, ketoleucine, isopentanal, CO2, cellulose, xylose, sucrose, arabinose, or glycerol.
In another aspect, the invention provides a method that generally includes introducing into a host cell a heterologous polynucleotide encoding at least one polypeptide that catalyzes conversion of a carbon source to isocaproate, wherein the at least one polypeptide is operably linked to a promoter so that the modified host cell catalyzes conversion of the carbon source to isocaproate. In some embodiments, the carbon source can include one or more of: glucose, pyruvate, ketovaline, ketoleucine, ketohomoleucine, CO2, cellulose, xylose, sucrose, arabinose, or glycerol.
In another aspect, the invention provides a method that generally includes introducing into a host cell a heterologous polynucleotide encoding at least one polypeptide that catalyzes conversion of a carbon source to isovalerate, wherein the at least one polypeptide is operably linked to a promoter so that the modified host cell catalyzes conversion of the carbon source to isovalerate. In some embodiments, the carbon source can include one or more of: glucose, pyruvate, ketovaline, ketoleucine, isopentanal, CO2, cellulose, xylose, sucrose, arabinose, or glycerol. In another aspect, the invention provides a method of harvesting an organic acid from a fermentation broth. Generally, the method includes adjusting the pH of the fermentation broth to about 3.0, adding an organic solvent to the fermentation broth, thereby producing an aqueous phase and a non-aqueous phase, and extracting the organic acid from the aqueous phase. IN some embodiments, the organic solvent can include hexane or oleyl alcohol.
Economical biosynthesis of certain ketones can provide commercial and environmental benefits. For example, methyl isoamyl ketone (MIAK) is one of the most widely used solvents (more than 50 million pounds per year), having broad applications in cellulose esters, acrylics and vinyl copolymers (www.epa.gov/oppt/iur/tools/data/2002-vol.html). In particular, MIRK is an excellent solvent for high-solids coatings due to its high solution activity, low density, low surface tension, slow evaporation rate and high boiling point (www.eastman.com/Literature_Center/M/M285.pdf). As another example, industrial ketones such as methyl ethyl ketone (MEK) and methyl isobutyl ketone (MIBK) are commonly used organic solvents in industry. Oxygenated solvents such as aliphatic ketones constitute a large segment of the industrial solvent market. Currently, aliphatic ketones such as methyl isobutyl ketone (MIBK) and diisobutyl ketone (DIBK) cannot be economically obtained from carbohydrates. Here we report genetically modified organisms that may be used for the biosynthesis of such ketones. In certain aspects, we report methods for the biosynthesis of certain ketones. The ketones may be commercial-ready or may be a component of, an ingredient in the production of, or chemical intermediate in the production of, another product.
Currently, MIRK is manufactured from isobutyraldehyde and acetone via three chemical reaction steps shown in
To manufacture MIAK in a sustainable fashion, we have developed a hybrid biocatalytic conversion approach (FIG. lb) by combining both synthetic biology and chemical synthesis. Based on retrosynthesis analysis, cross-condensation of isocaproic acid with acetic acid to MIAK represents an excellent synthetic route. With the release of only carbon dioxide and water, the one-step ketonization is an environmentally friendly and highly efficient method. To synthesize MIAK, isocaproate is preferably obtained by microbial fermentation from renewable carbon sources such as, for example, glucose. However, no organisms to date have been discovered to accumulate a significant amount of isocaproate although it is a minor metabolite in Clostridium difficile (Kim et al., Appl. Environ. Microbiol. 72:6062-6069 (2006)).
In one aspect of this work, an isocaproate overproducer was created via three steps, by (1) engineering enzymes to generate ketohomoleucine through elongating ketoleucine with one more carbon, (2) screening for optimal ketoacid decarboxylases that convert ketohomoleucine to isocaproaldehyde, and (3) screening for active aldehyde dehydrogenases that oxidize isocaproaldehyde into isocaproate. In some exemplary embodiments, E. coli may be chosen as the host. E. coli naturally can convert glucose to ketoleucine via the branched-chain amino acid biosynthetic pathway (Atsumi et al., Nature 451:86-9 (2008)). E. coli carrying a certain leuA mutation can produce ketohomoleucine (Zhang et al., Proc Natl Acad Sci USA 105:20653-8 (2008)), but no E. coli thus described can biosynthesize isocaproate. Herein we describe building a synthetic metabolic pathway to achieve high-level production of isocaproate directly from glucose. The isocaproate may subsequently be condensed with acetic acid into MIAK with high yield and selectivity (
Similarly, at present, the manufacturing processes of aliphatic ketones can be relatively complicated. As illustrated in
In another aspect of this work, we present a general strategy for synthesizing aliphatic ketones from carbohydrates. This biocatalytic conversion approach exploits engineered biosynthesis to produce ketones that may serve as the starting material for further chemical processes (
We have engineered E. coli to overproduce isovalerate by (1) amplifying the leucine biosynthetic pathway to increase the pool of ketoleucine, (2) screening for 2-ketoacid decarboxylases that convert ketoleucine to isopentanal, and (3) screening for aldehyde dehydrogenases that oxidize isopentanal into isovalerate (
In the description of exemplary embodiments that follow, certain metabolic enzymes—and the natural source of those enzymes—are specified. These are merely examples of suitable enzymes and suitable sources of the specified enzymes. Alternative enzymes with similar catalytic activities are possible, as are homologs that are obtainable from different microbial species or strains. Accordingly, the exemplary embodiments described herein should not be construed as limiting the scope of the microbes or methods that are reflected in the claims.
As shown in
We chose several aldehyde dehydrogenases that were reported to possess decent activity towards aliphatic aldehydes as candidate enzymes (Zhang et al., ChemSusChem 4:1068-1070 (2011)). They are E. coli acetaldehyde dehydrogenase AldB (Ho and Weiner, J. Bacteriol. 187:1067-1073 (2005)), E. coli 3-hydroxypropionaldehyde dehydrogenase AldH (Jo et al., Appl. Microbiol. Biotechnol. 81:51-60 (2008)), E. coli phenylacetaldehyde dehydrogenase PadA (Rodriguez Zavala et al., Protein Sci. 15:1387-1396 (2006)), and Burkholderia ambifaria α-ketoglutaric semialdehyde dehydrogenase KDHba (Watanabe et al., J. Biol. Chem. 282:6685-6695 (2007)). These enzymes (designated DH) were individually cloned after 2-ketoacid decarboxylase (KIVD) from Lactococcus lactis (de la Plaza et al., FEMS Microbiol. Lett. 238:367-74 (2004)).
The BW25113 E. coli strain with yqhD gene deletion (Atsumi et al., Appl. Microbiol. Biotechnol. 85:651-657 (2010)) was used as the fermentation host in order to decrease the reduction activity of endogenous alcohol dehydrogenases. After transformation with the two constructed plasmids, shake flask femientation was performed in M9 minimal medium containing 40 g/L glucose at 30° C. Fermentation broths after 48 hours were analyzed by HPLC. As seen in
While the discovery of KDHba has maximized the selectivity in the oxidation step, the promiscuous nature of KIVD decarboxylase may not necessarily confer optimal selectivity in the decarboxylation step. To increase the production of isocaproate and decrease the formation of byproducts, we investigated the effect of enlarging the binding pocket of KIVD. According to the crystal structure (PDB: 2VBG), amino acid residues Phe-381, Phe-382, Val-461, Met 538, and Phe-542, in combination with the cofactor thiamine diphosphate (ThDP), delineate the active site of KIVD (
We investigated the effect of further engineering another enzyme, indolepyruvate decarboxylase (IPDC) from Salmonella typhimurium, into our biosynthetic system. Since E. coli and S. typhimurium diverged from a common ancestor, S. typhimurium enzymes should express well in E. coli. While E. coli genome does not encode any 2-ketoacid decarboxylase, S. typhimurium has IPDC even though its activity was not fully investigated. Interestingly, the combination of wild-type IPDC and KDHba produced 2.67 g/L isocaproate (
While engineering IPDC improved isocaproate production, there were still some byproducts such as isobutyrate, isobutanol, isovalerate, and isopentanol accumulated in the fermentation broth. Since these byproducts were generated from decarboxylation of ketovaline and ketoleucine, we investigated increasing the chain elongation activity of LeuABCD to maximize the carbon conversion from ketovaline and ketoleucine to ketohomoleucine. An additional copy of wild-type or mutant LeuABCD encoded on either high- or medium- copy plasmid was introduced (
Thus far no microorganism has been discovered to accumulate a significant quantity of isovalerate, although it is a minor metabolite in lactobacillus and yeast (<3 mg/L) (Hazelwood et al., Appl. Environ. Microbiol. 74:2259-2266 (2008; Lambrechts et al., S. Afr. J. Enol. Vitic. 21:97-129 (2000)). As briefly stated above, we have engineered E. coli to overproduce isovalerate. In the Erhlich pathway, 2-keto acids are normally processed to aldehydes by 2-ketoacid decarboxylases, and then to alcohols by alcohol dehydrogenases (Hazelwood et al., Appl. Environ. Microbiol. 74:2259-2266 (2008)). To shift the products from alcohols to acids, we constructed a non-natural metabolic pathway to oxidize rather than reduce aldehydes. We chose several aldehyde dehydrogenases as candidate enzymes: E. coli acetaldehyde dehydrogenase AldB; E. coli 3-hydroxypropionaldehyde dehydrogenase AldH; E. coli phenylacetaldehyde dehydrogenase PadA; and Burkholderia ambifaria α-ketoglutaric semialdehyde dehydrogenase KDHba (Zhang et al., ChemSusChem 4:1068-1070 (2011)). These enzymes (designated DH) were individually cloned after 2-ketoacid decarboxylase (KIVD) from Lactococcus lactis (de la Plaza et al., FEMS Microbiol. Lett. 238, 367-74 (2004)) to build an expression cassette on a high copy plasmid (
Byproduct isobutyrate was present in the fermentation broth at a concentration of approximately 0.7 g/L. This may have been a result KIVD decarboxylating the intermediate metabolite ketovaline as well as ketoleucine. In order to achieve higher selectivity towards ketoleucine, we examined the effect of enlarging the binding pocket of KIVD. Based on the crystal structure, the active site residue V461 was mutated to alanine (Zhang et al., Proc. Natl. Acad. Sci. USA 105:20653-20658 (2008)). A second mutation—either F381L, F382L, M538A, or F542L—was introduced in addition to the V461 mutation (
We next investigated cloning another enzyme, indolepyruvate decarboxylase (IPDC) from Salmonella typhimurium. To our surprise, the combination of IPDC and PadA produced 8.9 g/L isovalerate (DC 6,
To explore the possibility of scale-up production, we translated the fermentation process from shake flasks to a 1-liter bioreactor (
We also explored biosynthetic alternatives for producing isovalerate. One alternative non-natural biosynthetic pathway was achieved by overexpressing a metabolic pathway from pyruvate (
Since isovalerate is protonated under acidic conditions and becomes hydrophobic, we exploited this feature to develop a purification method. The pH of the fermentation broth was adjusted to 3.0, and then solvents were applied to extract isovaleric acid out of the aqueous phase (
Isocaproate produced as described herein may be used as a raw material for the production of industrial chemicals such as, for example, MIAK. Isovalerate herein may be used as a raw material for the production of industrial chemicals such as, for example, MIBK and/or DIBK. The cells and methods described herein may therefore be used to provide renewable sources of raw materials for the production of industrial chemicals in an economical and environmentally advantageous manner.
This research opens a new path for the production of renewable chemicals by exploiting certain benefits of microbial fermentation. The process and raw material requirements are simplified compared to the current chemical manufacturing process. While this approach can be easily extended to other aliphatic ketones, we envision that the concept has broader implications. Currently, to produce industrial chemicals such as plastics, fertilizers, and pharmaceuticals, 99% of the feedstock materials comes from petroleum or natural gas (McFarlane and Robinson, Survey of Alternative Feedstocks for Commodity Chemical Manufacturing, available on the World Wide Web at info.ornl.gov/sites/publications/files/Pub8760.pdf (2007)). The existing chemical synthesis routes are optimized based on the availability and cost of petroleum stock. The cells and methods described herein allow the design of more simplified manufacturing routes based on chemical precursors that can be efficiently biosynthesized from renewable carbohydrates.
Thus, in one aspect, the invention provides recombinant microbial cell modified to exhibit increased biosynthesis of isocaproate compared to a wild-type control. In another aspect, the invention provides a recombinant microbial cell modified to exhibit increased biosynthesis of isovalerate compared to a wild-type control. In some cases, the wild-type control may be unable to produce isocaproate or isovalerate and, therefore, an increase in the biosynthesis of a particular product may reflect any measurable biosynthesis of that product. In certain embodiments, an increase in the biosynthesis of an isocaproate or isovalerate can include biosynthesis sufficient for a culture of the microbial cell to accumulate isocaproate or isovalerate to a predetermine concentration.
The predetermined concentration may be any predetermined concentration of the product suitable for a given application. Thus, a predetermined concentration may be, for example, a concentration of at least 0.1 g/L such as, for example, at least 0.5 g/L, at least 1.0 g/L, at least 2.0 g/L, at least 3.0 g/L, at least 4.0 g/L, at least 5.0 g/L, at least 6.0 g/L, at least 7.0 g/L, at least 8.0 g/L, at least 9.0 g/L, at least 10 g/L, at least 20 g/L, at least 50 g/L, at least 100 g/L, or at least 200 g/L.
The recombinant cell can be, or be derived from, any suitable microbe including, for example, a prokaryotic microbe or a eukaryotic microbe. As used herein, the term “or derived from” in connection with a microbe simply allows for the “host cell” to possess one or more genetic modifications before being modified' to exhibit the indicated increased biosynthetic activity. Thus, the term “recombinant cell” encompasses a “host cell” that may contain nucleic acid material from more than one species before being modified to exhibit the indicated biosynthetic activity.
In some embodiments, the host cell may be selected to possess one or more natural physiological activities. For example, the host cell may be photosynthetic (e.g., cyanobacteria) or may be cellulolytic (e.g., Clostridium cellulolyticum).
In some embodiments, the recombinant cell may be, or be derived from, a eukaryotic microbe such as, for example, a fungal cell. In some of these embodiments, the fungal cell may be, or be derived from, a member of the Saccharomycetaceae family such as, for example, Saccharomyces cerevisiae, Candida rugosa, or Candida albicans.
In other embodiments, the recombinant cell may be, or be derived from, a prokaryotic microbe such as, for example, a bacterium. In some of these embodiments, the bacterium may be a member of the phylum Protobacteria. Exemplary members of the phylum Protobacteria include, for example, members of the Enterobacteriaceae family (e.g., Escherichia coli) and, for example, members of the Pseudomonaceae family (e.g., Pseudomonas putida). In other cases, the bacterium may be a member of the phylum Firmicutes. Exemplary members of the phylum Firmicutes include, for example, members of the Bacillaceae family (e.g., Bacillus subtilis), members of the Clostridiaceae family (e.g., Clostridium cellulolyticum) and, for example, members of the Streptococcaceae family (e.g., Lactococcus lactis). In other cases, the bacterium may be a member of the phylum Cyanobacteria.
In some embodiments, the increased biosynthesis of isocaproate compared to a wild-type control can include an increase in elongating ketoleucine to ketohomoleucine compared to a wild-type control, an increase in ketoacid decarboxylase activity compared to a wild-type control, and/or an increase in aldehyde dehydrogenase activity compared to a wild-type control. In other embodiments, the increased biosynthesis of isocaproate compared to a wild-type control can include an increase in conversion of ketohomoleucine to isocaproaldehyde compared to a wild-type control and/or an increase in conversion of isocaproaldehyde to isocaproate compared to a wild-type control. In some cases, at least a portion of the increase in ketoacid decarboxylase activity can result from modification of the ketoacid decarboxylase enzyme. For example, 2-ketoacid decarboxylase of Lactococcus lactis (or an analog) may be modified to include at least one amino acid substitution selected from: V461A, M538A, or F542L, or an analogous substitution. In some cases, the 2-ketoacid decarboxylase can be modified to include the V461A substitution (or an analogous substitution) in combination with either the M528A substitution (or an analogous substitution) or the V461A substitution (or an analogous substitution).
As used herein, the term “analog” refers to a related enzyme from the same or a different microbial source with similar enzymatic activity. As such, analogs often show significant conservation, it is a trivial matter for a person of ordinary skill in the art to identify a suitably related analog. Also, it is a trivial matter for a person of ordinary skill in the art to identify an “analogous substitution” by aligning the amino acid sequence of the analog with the amino acid sequence of the reference enzyme. Thus, positional differences and/or amino acid residue differences may exist between the recited substitution and an analogous substitution despite conservation between the analog and the reference enzyme. In other embodiments, the increased biosynthesis of isovalerate compared to a wild-type control comprises increased leucine biosynthesis compared to a wild-type control, increased decarboxylase activity compared to a wild-type control, or increased aldehyde dehydrogenase activity compared to a wild-type control.
In some embodiments, the recombinant cell can exhibit an increase in indolepyruvate decarboxylase (IPDC) activity. The increase in IPDC activity can result from expression of an IPDC enzyme. Exemplary IPDC enzymes include, for example, any one of the polypeptides reflected in any one of SEQ ID NO:1-21. Thus, in some embodiments, the recombinant cell can include a heterologous nucleotide sequence that encodes an IPDC decarboxylase such as, for example, any one of the polypeptides reflected in any one SEQ ID NO:1-21.
In some embodiments the recombinant cell can exhibit an increase in aldehyde dehydrogenase activity. The increase in aldehyde dehydrogenase activity can result from expression of an aldehyde dehydrogenase enzyme. Exemplary aldehyde dehydrogenase enzymes include, for example, any one of the polypeptides reflected in any one of SEQ ID NO:22-55. Thus, in some embodiments, the recombinant cell can include a heterologous nucleotide sequence that encodes an aldehyde dehydrogenase such as, for example, any one of the polypeptides reflected in any one SEQ ID NO:22-55.
In some embodiments, the recombinant cell can exhibit an increase in branched-chain ketoacid dehydrogenase (BKDH) activity. The increase in branched-chain ketoacid dehydrogenase activity can result from expression of a branched-chain ketoacid dehydrogenase enzyme. Exemplary branched-chain ketoacid dehydrogenase enzymes include, for example, any one of the polypeptides reflected in any one of SEQ ID NO: 91-93. Thus, in some embodiments, the recombinant cell can include a heterologous nucleotide sequence that encodes a branched-chain ketoacid dehydrogenase enzyme such as, for example, any one of the polypeptides reflected in any one SEQ ID NO:91-93.
In some embodiments, the recombinant cell can include a heterologous nucleotide sequence that encodes a variant of E. coli 2-isopropylmalate synthase that exhibits reduced enzymatic activity compared to wild-type E. coli 2-isopropylmalate synthase. In some cases, the modified E. coli 2-isopropylmalate synthase includes a G462D amino acid substitution or an analogous amino acid substitution.
As used herein, the terms “activity” with regard to particular enzyme refers to the ability of a polypeptide, regardless of its common name or native function, to catalyze the conversion of the enzyme's substrate to a product, regardless of whether the “activity” as less than, equal to, or greater than the native activity of the identified enzyme. Methods for measuring the biosynthetic activities of cells are routine and well known to those of ordinary skill in the art.
As used herein, an increase in catalytic activity can be quantitatively measured and described as a percentage of the catalytic activity of an appropriate wild-type control. The catalytic activity exhibited by a genetically-modified polypeptide can be, for example, at least 110%, at least 125%, at least 150%, at least 175%, at least 200% (two-fold), at least 250%, at least 300% (three-fold), at least 400% (four-fold), at least 500% (five-fold), at least 600% (six-fold), at least 700% (seven-fold), at least 800% (eight-fold), at least 900% (nine-fold), at least 1000% (10-fold), at least 2000% (20-fold), at least 3000% (30-fold), at least 4000% (40-fold), at least 5000% (50-fold), at least 6000% (60-fold), at least 7000% (70-fold), at least 8000% (80-fold), at least 9000% (90-fold), at least 10,000% (100-fold), or at least 100,000% (1000-fold) of the activity of an appropriate wild-type control.
Alternatively, an increase in catalytic activity may be expressed as at an increase in kcat such as, for example, at least a two-fold increase, at least a three-fold increase, at least a four-fold increase, at least a five-fold increase, at least a six-fold increase, at least a seven-fold increase, at least an eight-fold increase, at least a nine-fold increase, at least a 10-fold increase, at least a 15-fold increase, or at least a 20-fold increase in the kcat value of the enzymatic conversion.
An increase in catalytic activity also may be expressed in terms of a decrease in Km such as, for example, at least a two-fold decrease, at least a three-fold decrease, at least a four-fold decrease, at least a five-fold decrease, at least a six-fold decrease, at least a seven-fold decrease, at least an eight-fold decrease, at least a nine-fold decrease, at least a 10-fold decrease, at least a 15-fold decrease, or at least a 20-fold decrease in the Km value of the enzymatic conversion.
A decrease in catalytic activity can be quantitatively measured and described as a percentage of the catalytic activity of an appropriate wild-type control. The catalytic activity exhibited by a genetically-modified polypeptide can be, for example, no more than 95%, no more than 90%, no more than 85%, no more than 80%, no more than 75%, no more than 70%, no more than 65%, no more than 60%, no more than 55%, no more than 50%, no more than 45%, no more than 40%, no more than 35%, no more than 30%, no more than 25%, no more than 20%, no more than 15%, no more than 10%, no more than 5%, no more than 4%, no more than 3%, no more than 2%, no more than 1% of the activity, or 0% of the activity of a suitable wild-type control.
Alternatively, a decrease in catalytic activity can be expressed as an appropriate change in a catalytic constant. For example, a decrease in catalytic activity may be expressed as at a decrease in kcat such as, for example, at least a two-fold decrease, at least a three-fold decrease, at least a four-fold decrease, at least a five-fold decrease, at least a six-fold decrease, at least a seven-fold decrease, at least an eight-fold decrease, at least a nine-fold decrease, at least a 10-fold decrease, at least a 15-fold decrease, or at least a 20-fold decrease in the kat value of the enzymatic conversion.
A decrease in catalytic activity also may be expressed in terms of an increase in Km such as, for example, an increase in Km of at least two-fold, at least three-fold, at least four-fold, at least five-fold, at least six-fold, at least seven-fold, at least an eight-fold, at least nine-fold, at least 10-fold, at least 15-fold, at least 20-fold, at least 25-fold, at least 30-fold, at least 35-fold, at least 40-fold, at least 45-fold, at least 50-fold, at least 75-fold, at least 100-fold, at least 150-fold, at least 200-fold, at least 230-fold, at least 250-fold, at least 300-fold, at least 350-fold, or at least 400-fold.
Thus, in another aspect, we describe herein methods for biosynthesis of isocaproate or isovalerate. Generally, the methods includes incubating a recombinant cell as described herein in medium that includes a carbon source under conditions effective for the recombinant cell to produce isocaproate or isovalerate. For producing isocaproate, the carbon source can include one or more of: glucose, pyruvate, ketovaline, ketoleucine, or ketohomoleucine. For producing isovalerate, the carbon source can include one or more of: glucose, pyruvate, ketovaline, ketoleucine, or isopentanal. In addition, the carbon sources for cell growth can be CO2, cellulose, glucose, xylose, sucrose, arabinose, glycerol, etc. as long as the related carbon assimilation pathways are introduced in the engineered microbe.
In yet another aspect, we describe herein methods for introducing a heterologous polynucleotide into cell so that the host cell exhibits an increased ability to convert a carbon source to isocaproate or isovalerate. For cells to produce isocaproate, the heterologous polynucleotide can encode a polypeptide operably linked to a promoter so that modified cell catalyzes conversion of the carbon source to isocaproate. In some of these embodiments, the carbon source can include one or more of glucose, pyruvate, ketovaline, ketoleucine, or ketohomoleucine. For cells to produce isovalerate, the heterologous polynucleotide can encode a polypeptide operably linked to a promoter so that modified cell catalyzes conversion of the carbon source to isovalerate. In some of these embodiments, the carbon source can include one or more of glucose, pyruvate, ketovaline, ketoleucine, or isopentanal. The host cells for such methods can include, for example, any of the microbial species identified above with regard to the recombinant cells described herein.
In some cases, the recombinant cell can include a genetically-modified version of a polypeptide that catalyzes the conversion of isobutyraldehyde to isobutanol. An exemplary polypeptide of this type can include, for example, can be a genetically-modified version of an alcohol dehydrogenase such as, for example, a polypeptide encoded by a genetically-modified adhE or a genetically-modified adhP. In other embodiments, the genetically-modified polypeptide can be genetically-modified version of an ethanolamine utilization protein such as, for example, a polypeptide encoded by a genetically-modified eutG. In some embodiments, the genetically-modified polypeptide can be a polypeptide encoded by a genetically-modified yiaY, a genetically-modified yqhD, or a genetically-modified yigB.
As used in the preceding description, the term “and/or” means one or all of the listed elements or a combination of any two or more of the listed elements; the term “comprises” and variations thereof do not have a limiting meaning where these terms appear in the description and claims; nnless otherwise specified, “a,” “an,” “the,” and “at least one” are used interchangeably and mean one or more than one; and the recitations of numerical ranges by endpoints include all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).
For any method disclosed herein that includes discrete steps, the steps may be conducted in any feasible order. And, as appropriate, any combination of two or more steps may be conducted simultaneously.
The present invention is illustrated by the following example. It is to be understood that the particular examples, materials, amounts, and procedures are to be interpreted broadly in accordance with the scope and spirit of the invention as set forth herein.
The E. coli strain XL10-gold (Stratagene) was used as the host to conduct all cloning procedures. All the primers (Table 1) were purchased from Eurofms MWG Operon. PCR amplification were carried out with using Phusion® High-Fidelity DNA polymerase (New England Biolabs; Ipswich, Mass.). All the plasmids were sequenced to make sure their corrections.
The 4702 by fragment of leuABCD operon, in which leuA carries G462D/H97L/S139G mutations, was obtained from pZE_LeuABCDKA6 (Zhang et al., Proc Natl Acad Sci USA 105:20653-8 (2008)) plasmid after Acc65I digestion, and inserted into the Acc65I site of pZAlac_ilvD_alsS (pIBA1) (Zhang et al., ChemSusChem 4:1068-1070 (2011)) to create p4MV1 (see
IPDC was amplified from Salmonella typhimurium with primers IPDC_accfwd and IPDC_sphrev, digested with Acc65I and SphI, and then was used to replace the KIVD fragment of plasmid pIBA8 (Zhang et al., ChemSusChem 4:1068-1070 (2011)) to form plasmid of p4MV2. Based on the crystal structure of KIVD active sites (
The strain carrying only one plasmid of p4MV1 was fermented in 125 mL flask for 48 hours. The fermentation product of homoketoleucine was collected the fraction system of Agilent 1260 Infinity HPLC. The pH of the homoketoleucine collection was adjusted to 2.0 and then ethyl acetate was added to extract the homoketoleucine. The top layer carrying the ethyl acetate and homoketoleucine was transferred into 1.5 mL tubes, and then these tubes were evaporated in the vacuum machine. The homoketoleucine was quantified for later use.
The gene fragments of kdhba and the wild-type IPDC were PCR amplified using p4MV2 plasmid as template with primers of kdhba—barnfwd and kdhba—bamrev, IPDC_bamfwd and IPDC_bamrev, respectively. The fragment of IPDC L544A mutation was PCR amplified using p4MV9 plasmid as template with primers of IPDC_banifwd and IPDC_bamrev. After digestion with BamHI, the three gene fragments were inserted into expression plasmid pQE9 (Qiagen; Valencia, Calif.) to yield pQE9-kdhba, pQE9_IPDC and pQE9-IPDCL544A. These plasmids of pQE9-kdhba, pQE9_IPDC and pQE9-IPDCL544A were transformed into E. coli strain BL21. 2 mL overnight pre-cultured BL21 cells were inoculated in 200 ml 2XYT rich medium containing 50 mg/L ampicillin and 25 mg/L kanamycin. 0.1 mM IPTG was added into the medium to induce the recombinant proteins expression. Cell pellets were lysed by sonication in a buffer containing 250 mM NaCl, 2 mM DTT, 5 mM imidazole and 50 mM Tris pH 9.0, and then using Ni-NTA column chromatography to purify the enzymes from crude cell lysates by applying a stepwise gradient of imidazole (up to 250 mM). The Amicon Ultra centrifugal filters (Millipore; Billerica, MA) were used to obtain the fractions of highest purity by exchanged buffer. The concentrated protein solutions, in which kdhba was stored at buffer 1 (50 μM Tris buffer, pH 8.0, 1 mM MgSO4, and 20% glycerol), wild-type IPDC and L544A [PDC was kept at buffer 2 (50 μM Tris buffer, pH 8.0, 1 mM MgSO4, 0.2 mM ThDP, and 20% glycerol), were flash frozen at −80° C. for long term storage.
4. Enzyme Assay of kdhba, wild-type IPDC and IPDC of L544A
Substrates of isobutyraldehyde, isovaleraldehyde, isocaproaldehyde, ketovaline and ketoleucine were purchased from Fisher, and NAD+was from New England Biolabs (Ipswich, Mass.). Substrate of homoketoleucine was collected by HPLC fraction system. Protein concentration was determined by measuring UV absorbance at 280 nm. The reaction mixture contained 0.5 mM NAD+ and 0.2-4 mM aldehyde in assay buffer (50 mM NaH2PO4, pH 8.0, 1 mM DTT) with a total volume of 80 μL. The reactions were started by adding 2 μL kdhba (final enzyme concentration 25 nM), and the generation of NADH was monitored at 340 nm (extinction coefficient, 6.22 mM−1 cm−1). The decarboxylation activity of IPDC and IPDC L544A mutant were measured at 30° C. using a coupled enzymatic assay method. Excess kdhba was used to oxidize aldehyde into acid, and concomitantly, cofactor NAD+ was reduced to NADH. The assay mixture contained 0.5 mM NAD+, 0.1 μM kdhba and 0.2-4 mM 2-keto acids in assay buffer (50 mM NaH2PO4, pH 6.8, 1 mM MgSO4, 0.5 mM ThDP) with a total volume of 80 μL. The reactions were started by adding 2 μL IPDC or IPDC L544A mutation (final concentration 25 nM), and the generation of NADH was monitored at 340 nm. Kinetic parameters (Kcat and Km) were determined by fitting initial velocity data to the Michaelis-Menten equation using Origin software.
Table 2 shows the products produced by the various constructs. The host strain is AKO1. Certain alcohols were produced in addition to the indicated organic acids. Thus, the cells still exhibit some detectable level of alcohol dehydrogenase activity.
E. coli strain AKO1 strain is BW25113 with ΔyqhD. E. coli strain AKO5 is BW25113 with Δpta, ΔpoxB, ΔadhE, ΔldhA, and ΔyqhD. Construction of plasmids is described in detail below.
For
The 4702 by fragment of leuABCD operon was obtained from pZE_LeuABCDKA6 plasmid after Acc65I digestion, and inserted into pZAlac_iIvD_alsS (pIBA1) (Stephanopoulos et al., Metabolic engineering: principles and methodologies, (Academic Press, 1998)) to create pIVA1 (see
Based on the crystal structure of KIVD active site (
To increase the production of isovalerate and decrease the formation of byproducts, the effect of enlarging the binding pocket of KIVD was investigated. According to the crystal structure (PDB: 2VBG), amino acid residues Phe-381, Phe-382, Val-461, Met 538 and Phe-542, in combination with the cofactor thiamine diphosphate (ThDP), delineate the active site of KIVD (de la Plaza et al., FEMS Microbiol. Lett. 238, 367-74 (2004)).
IPDC gene fragment was PCR amplified with primers IPDC_bamfwd and IPDC_bamrev. After digestion with BamHI, the gene fragments were inserted into expression plasmid pQE9 (Qiagen; Valencia, Calif.) to yield pQE9-IPDC. The plasmid was transformed into BL-21 E. coli host harboring the pREP4 plasmid (Qiagen; Valencia, Calif.). Cells were inoculated from an overnight pre-culture at 1/100 dilution and grown in 200 ml 2XYT rich medium containing 50 mg/L ampicillin and 25 mg/L kanamycin. Protein expression was induced with 0.1 mM IPTG when OD600 reached 0.6. After incubation at 30° C. overnight, cell pellets were lysed by sonication in a buffer containing 250 mM NaCl, 2 mM DTT, 5 mM imidazole and 50 mM Tris pH 9.0. The target protein was purified from crude cell lysates with Ni-NTA chromatography. Next, the protein solution was buffer-exchanged into storage buffer (50 μM Tris buffer, pH 8.0, 1 mM MgSO4, 0.2 mM ThDP, and 20% glycerol) using Amicon Ultra centrifugal filters (Millipore). It was aliquoted (100 μl) into PCR tubes and flash frozen for long term storage at −80° C.
Table 4 shows the products produced by the various constructs. The host strain is AKO1. Certain alcohols were produced in addition to the indicated organic acids. Thus, the cells still exhibit some detectable level of alcohol dehydrogenase activity.
Chemicals were obtained from Sigma-Aldrich (St. Louis, Mo.) or Fisher Scientific (Waltham, Mass.). Restriction enzymes, DNA ligation kit and Phusion DNA polymerase were from New England Biolabs (Ipswich, Mass.). Rapid DNA ligation kit was from Roche (Madison, Wis.). Oligonucleotides were from Eurofms MWG Operon (Huntsville, Ala.).
Gene knockouts were introduced in BW25113 by P1 phage transduction using Keio collection. The knockout strain was transformed with plasmid pCP20 to remove the kanamycin resistance marker. The E. coil host was transformed with pIVA1 plasmid and another plasmid from pIBA4 to pIBA8 or pIVA2 to pfVA6 for isovalerate production. Construction of plasmids was carried out with standard molecular biology methods and is described in detail in the Supplementary information.
Overnight culture was diluted 25 fold into 5 ml M9 medium (plus 0.5% yeast extract and 4% glucose) in a 125-ml conical flask. Ampicillin (100 mg/L) and kanamycin (25 mg/L) were added to maintain the transformed plasmids. The culture medium was buffered with CaCO3 powder. Isopropyl-b-D-thiogalactoside (IPTG) was added at a concentration of 0.1 mM to induce protein expression. The fermentation flasks were kept in a 30° C. shaker (250 rpm) and incubated for 48 hours.
Seeding medium has the following composition (g/L): glucose, 10; (NH4)2SO4, 1.8; K2HPO4, 8.76; KH2PO4, 2.4; sodium citrate, 1.32; yeast extract, 15; ampicillin, 0.1; kanamycin, 0.05. Fermentation media for bioreactor cultures contained the following components (g/L): glucose, 30; (NH4)2SO4, 3; K2HPO4, 14.6; KH2PO4, 4; sodium citrate, 2.2; yeast extract, 25; MgSO4.7H2O, 1.25; CaCl2.2H2O, 0.015, calcium pantothenate, 0.001; Thiamine, 0.01; ampicillin, 0.1; kanamycin, 0.05; and 1 mL/L of trace metal solution. Trace metal solution contained (g/L): NaCl, 5; ZnSO4.7H2O, 1; MnCl2.4H2O, 4; CuSO4.5H2O, 0.4; H3BO3, 0.575; Na2MoO4.2H2O, 0.5; FeCl3.6H2O, 4.75; 6N H2SO4, 12.5 mL. The feeding solution contained (g/L): glucose, 600; (NH4.)2SO4, 5; MgSO4.7H2O, 1.25; yeast extract, 5; CaCl2.2H2O, 0.015; calcium pantothenate, 0.001; Thiamine, 0.01; ampicillin, 0.1; kanamycin, 0.05, 0.2 mM IPTG; and 1 mL/L of trace elements.
Cultures of E. coli were performed in a 1.3 L Bioflo 115 fermenter (NBS; Edison, N.J.) using a working volume of 0.6 L. The fermenter was inoculated with 10% of overnight pre-culture with seeding medium and then the cells were grown at 37° C., 30% dissolved oxygen (DO), and pH 7.0. When OD600 was 8.0, 0.2 mM IPTG was added and the temperature was decreased to 30° C. to start isovalerate production. The pH was controlled at 7.0 by automatic addition of 10 M NaOH solution. Air flow rate was maintained at 1 vvm in the whole process. Dissolved oxygen (DO) was maintained about 10% with respect to air saturation by raising stirring speed (from 300 to 800 rpm). The glucose level in the fermenter was kept about 10 g/L by inputting feeding medium continuously. Fermentation samples were collected to determinate concentrations of isovalerate, organic acids and glucose.
Protein concentration was measured by UV absorbance at 280 nm. For PadA characterization, the assay buffer (50 mM NaH2PO4, pH 8.0, 1mM DTT) contained 0.5 mM NAD+ and 0.2-4 mM isopentanal. Enzymatic reactions were initiated by adding 25 nM PadA. The reactions were monitored by measuring the UV absorbance at 340 nm (extinction coefficient, 6.22 mM−1 cm−1) as a consequence of NADH generation.
The decarboxylation activity of IPDC was measured using a coupled enzymatic assay method. Excess PadA was added to the reaction mixture to quickly oxidize the aldehyde product into the corresponding acid, and concomitantly, cofactor NAD+was reduced to NADH. The assay mixture contained 0.5 mM NAD+, 0.1 μM PadA and 0.2-4 mM 2-keto acids in the reaction buffer (50 mM NaH2PO4, pH 6.8, 1 mM MgSO4, 0.5 mM ThDP) with a total volume of 80 The reaction was started with the addition of 25 nM IPDC, and the progress was monitored by determining the generation of NADH (absorbance at 340 nm). Kinetic parameters (kcat and Km) were determined by fitting experimental velocity to Michaelis-Menten equation using Origin software.
BKDH enzyme complex genes are amplified from Pseudomonas Putida KT2440 genomic DNA with primers bkdh_ecofwd (TGCATCGAATTCAGGAGAAATTAACTATGA ACGAGTACGCCCCCCTGCGTTTGC; SEQ ID NO:85) and bkdh_hindrev (TGCATCAAGC TTTCAGATATGCAAGGCGTGGCCCAG; SEQ ID NO:86). The PCR product was then digestion with EcoRI and Hindlll, and inserted into pZE12 to make pIBA16. The tesA gene was amplified from E. coli strain K12 genomic DNA using the primer pair TesAJffind111JF (GGGCCCAAGCTTAGGAGAAATTAACTATGATGAACTTCAACAATGTTTTCCG; SEQ ID NO:87) and TesA_Xbal_R (GGGCCCTCTAGATTATGAGTCATGATTTACTAAAGGCT; SEQ ID NO:88); tesB was amplified with primer pair TesB_Hindlll_F (GGGCCCAAGCTTAG GAGAAATTAACTATGATGAGTCAGGCGCTAAAAAATTTACT; SEQ ID NO:89) and TesB_Xbal_R (GGGCCCTCTAGATTAATTGTGATTACGCATCACCCCTT; SEQ ID NO:90). After PCR, the DNA fragments were purified and digested using the restriction enzymes Hindlll and Xbal. The digested fragments containing tesA are inserted into pIBA16 (
The resulting plasmids were transformed into E. coli BW25113 for fermentation analysis.
Overnight cultures incubated in LB medium were diluted 25-fold into 5 mL M9 medium supplemented with 0.5% yeast extract and 4% glucose in 125-mL conical flasks. Antibiotics were added appropriately (ampicillin 100 mg/L and kanamycin 25 mg/L). 0.1 mM isopropyl-b-D-thiogalactoside (IPTG) was added to induce protein expression. The culture medium was buffered by addition of 0.5 g CaCO3. Cultures were placed in a 30° C. shaker (250 rpm) and incubated for 48 hours.
Fermentation products were quantified by HPLC analysis with refractive index detection using an Agilent 1100 Capillary HPLC. Results are shown in Table 5.
The purpose of this Example is to describe the cloning of genes encoding P. putida branched-chain ketoacid dehydrogenase (BKDH) subunits, co-expressed with an E. coli gene encoding thioesterase II (TesB), under the control of a constitutively active promoter, and to describe the expression of such genes in an S. cerevisiae host strain.
For the cloning of BKDH subunits, genes are amplified using Pseudomonas putida genomic DNA and bkdh_ecofwd (TGCATCGAATTCAGGAGAAATTAACTATGAACGAGT ACGCCCCCCTGCGTTTGC; SEQ ID NO:85) and bkdh_hindrev (TGCATCAAGCTTTCAGA TATGCAAGGCGTGGCCCAG; SEQ ID NO:86). For the cloning of the E.coli tesB gene, genomic DNA of E. coli is amplified using primers TesB_Hindlll_F (GGGCCCAAGCTTAGGA GAAATTAACTATGATGAGTCAGGCGCTAAAAAATTTACT; SEQ ID NO:89) and TesB_Xbal_R (GGGCCCTCTAGATTAATTGTGATTACGCATCACCCCTT; SEQ ID NO:90) After PCR, the DNA fragments are purified and digested using the restriction enzymes Hindlll and Xbal. The digested fragments containing tesB are inserted into pIBA16 (
Saccharomyces cerevisiae (W303a) transformations are done using the lithium acetate method (Gietz, 2002, Methods in Enzymology, 350:87-96). One mL of an overnight yeast culture is diluted into 50 mL of fresh YPD medium and incubated in a 30° C. shaker for approximately six hours. The cells are collected, washed with 50 mL of sterile water, and washed again with 25 mL of sterile water. The cells are then resuspended using 1 mL of 100 mM lithium acetate and transferred to a new microcentrifuge tube. The cells are centrifuged for ten seconds in order to pellet them. The supernatant is then discarded and the cells are resuspended 4× in 100 mM lithium acetate. Fifteen microliters of cells are added to a DNA mix that consists of 72 μL 50% PEG, 10 μL 1M lithium acetate, 3 μL 10g/L denatured salmon sperm DNA, 2 μL of the desired plasmid DNA, and sterile water to a total volume of 100 μL. The samples are incubated at 30° C. for 30 minutes and heat shocked at 42° C. for 22 minutes. The cells are collected through centrifugation for ten seconds, resuspended in 100 μL SOS medium and plated onto appropriate SC selection plates without uracil, tryptophan, leucine, or histidine.
The complete disclosure of all patents, patent applications, and publications, and electronically available material (including, for instance, nucleotide sequence submissions in, e.g., GenBank and RefSeq, and amino acid sequence submissions in, e.g., SwissProt, MR, PRF, PDB, and translations from annotated coding regions in GenBank and RefSeq) cited herein are incorporated by reference in their entirety. In the event that any inconsistency exists between the disclosure of the present application and the disclosure(s) of any document incorporated herein by reference, the disclosure of the present application shall govern. The foregoing detailed description and examples have been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described, for variations obvious to one skilled in the art will be included within the invention defined by the claims.
Unless otherwise indicated, all numbers expressing quantities of components, molecular weights, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless otherwise indicated to the contrary, the numerical parameters set forth in the specification and claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. All numerical values, however, inherently contain a range necessarily resulting from the standard deviation found in their respective testing measurements.
All headings are for the convenience of the reader and should not be used to limit the meaning of the text that follows the heading, unless so specified.
This application claims the benefit of U.S. Provisional Application Ser. No. 61/545,328, filed Oct. 10, 2011, which is incorporated by reference herein.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US2012/059321 | 10/9/2012 | WO | 00 | 4/9/2014 |
Number | Date | Country | |
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61545328 | Oct 2011 | US |