Patent Application
20150218594
This invention pertains to an enzymatic process for the production of chemicals from carbon sources. In particular, a process for the production of a target chemical is disclosed using a cell-free enzyme system that converts carbohydrate sources to the intermediate pyruvate and subsequently the intermediate pyruvate to the target chemical.
The development of sustainable, biomass-based production strategies requires efficient depolymerization into intermediate carbohydrates as well as flexible and efficient technologies to convert such intermediate carbohydrates into chemical products. Presently, biotechnological approaches for conversion of biomass to chemicals focus on well established microbial fermentation processes.
However, these fermentative approaches remain restricted to the physiological limits of cellular production systems. Key barriers for cost effective fermentation processes are their low temperature and solvent tolerance, which result in low conversion efficiencies and yields. Additionally, the multitude of cellular metabolic pathways often leads to unintended substrate redirection into non-productive pathways. Despite advances in genetic engineering, streamlining these metabolic networks for optimal product formation at an organismic level is time-consuming and due to the high complexity continues to be rather unpredictable.
A prominent example is the recombinant fermentative production of isobutanol in E. coli. Titers of 1-2% (v/v) isobutanol already induce toxic effects in the microbial production host, resulting in low product yields (Nature 2008, 451, 86-89). Additionally, depolymerized biomass often contains inhibitory or non-fermentable components that limit microbial growth and product yields. Therefore, strategies are desired to overcome such limitations of cell-based production.
The cell-free production of chemicals has been shown as early as 1897 when Eduard Buchner used a lysate of yeast cells to convert glucose to ethanol (Ser. Dtsch. Chem. Ges. 1901, 34, 1523-1530). Later Welch and Scopes, 1985 demonstrated cell free production of ethanol, a process which, however, was technically not useful (J. Biotechnol. 1985, 2, 257-273). The system lacked specificity and included side reaction of enzymes and unwanted activities in the lysate.
A number of technical processes have been described that use isolated enzymes for the production of chemicals. For example, alcohol dehydrogenases are used in the production of chiral alcohols from ketons requiring cofactor (NAD) regeneration, for example, by adding glucose and glucose dehydrogenase. Such processes have been designed to produce high-value chemicals but not to provide an enzyme system comprising multiple enzyme reactions that convert carbohydrates into chemicals with high energy and carbon efficiency.
Zhang et al. (Biotechnology: Research, Technology and Applications; 2008) describe the idea for cell free enzymatic conversion of glucose to n-butanol. The concept includes a minimum of 18 enzymes, several different cofactors and coenzymes (e.g. ATP, ADP, NADH, NAD, ferredoxin and coenzyme A). In addition the postulated process results in a net-production of ATP so that it requires in addition an ATPase enzyme to remove the ATP. Under practical terms control of ATPase addition while maintaining a balanced ATP level is very difficult to achieve. To manage balanced ATP cycling, the hydrolysis of excess ATP must be adjusted very carefully or eliminated by using highly toxic arsenate. In summary, the described process would be expensive, inefficient and technically instable.
EP2204453 discloses a process for the conversion of glucose to ethanol, n-butanol and/or isobutanol.
There is thus a need for a cost effective process for the production of chemicals from carbohydrates, in particular for the production of chemicals that can be derived from pyruvate such as ethanol, isobutanol and other C4 alcohols.
A general objective of the invention is to provide stable and technically feasible cell-free processes by minimizing the number of added components. The present invention addresses this need through a cell free enzymatic system, using only a limited number of enzymes and a limited set of cofactors. As a solution, the invention eliminates the need for cells.
Consequently, cell-associated process limitations such as higher process temperatures, extreme pH, and substrate or product toxicity are avoided. By limiting the enzyme activities to those required for the reaction cascade and choosing enzymes with sufficient reaction selectivity undesired substrate redirection into alternative reaction pathways is eliminated. Due to their reduced molecular complexity and rapid adaptability to harsh industrial reaction conditions, designed biocatalytic processes are superior to their cellular counterparts.
The invention is thus directed to a process for the bioconversion of a carbon source, which is preferably a carbohydrate, into a target chemical by an enzymatic process, preferably in the absence of productive living cells. The target chemical is preferably a hydrophobic, a hydrophilic or an intermediate chemical compound.
In particular, according to a preferred aspect, the inventive process does not result in a net production of ATP/ADP and does not involve ATPase and/or arsenate. The inventive process preferably does not involve the cofactor ATP/ADP and/or any phosphorylation reaction.
According to a further preferred aspect, the inventive process uses only one redox cofactor in the process. Such a cofactor is preferably selected from NAD/NADH, NADP/NADPH, FAD/FADH2. Preferably, NAD/NADH (i.e. the redox pair NAD+ and NADH+H+) is the only cofactor in the process.
According to a further preferred embodiment, the inventive process comprises the adjustment of the enzyme activity of each enzymatically catalyzed reaction step so that it is the same or greater than the activity of any preceding enzymatically catalyzed reaction step.
According to a further preferred aspect, the inventive process uses an artificial minimized reaction cascade that, preferably, only requires one single cofactor. As a result of the current invention, the cell-free production of target chemical from a carbohydrate, in particular to pyruvate, was achieved. Pyruvate can further be converted to other chemicals, such as ethanol and isobutanol. The invention also includes streamlined cascades which function under conditions where microbial productions cease. The current invention is extendible to an array of industrially relevant molecules. Application of solvent-tolerant biocatalysts allows for high product yields, which significantly simplifies downstream product recovery.
According to another preferred aspect the invention only uses enzymes that withstand the inactivating presence of the produced chemicals.
According to one aspect, the present invention concerns a process for the production of ethanol and all enzymes employed in the process withstand 2% (w/w) concentration of the product, preferably 4% (w/w), more preferably 6% (w/w), more preferably 8% (w/w), more preferably 10% (w/w), more preferably 12% (w/w), even more preferably 14% (w/w).
According to another aspect, the present invention concerns a process for the production of isobutanol and all enzymes employed in the process withstand 2% (w/w) concentration of the product, preferably 4% (w/w), more preferably 6% (w/w), more preferably 8% (w/w), more preferably 10% (w/w), more preferably 12% (w/w), even more preferably 14% (w/w).
According to one aspect, the present invention concerns a process for the production of a target chemical from glucose and/or galactose, or a glucose- and or galactose-containing dimer, oligomer or polymer, by a cell-free enzyme system, comprising the conversion of glucose to pyruvate as an intermediate product; wherein no net production of ATP occurs and, preferably, wherein no phosphorylation reaction occurs; and wherein the conversion of glucose to pyruvate consists of the use of three, four or five enzymes selected from the group of dehydrogenases, dehydratases, and aldolases, wherein one or more enzymes is selected from each group. Preferably the conversion from glucose to pyruvate is achieved by three or four enzymes, most preferably by four enzymes. More preferably, for the conversion of glucose to pyruvate, this process comprises the use of only one redox cofactor and, preferably, one or more of those enzymes requiring such cofactor are optimized for greater activity towards this cofactor as compared to the respective non-optimized enzyme or wildtype enyzme. More preferably such redox cofactor is NAD/NADH and one or more enzymes are optimized for greater activity towards NAD/NADH as compared to the respective non-optimized enzyme or wildtype enyzme.
Pyruvate is a central intermediate from which molecules like ethanol or isobutanol can be produced with few additional enzymatic steps. The novel cell-free engineering approach allows production of pyruvate, or target chemicals derived from pyruvate. Particular target chemicals that can be derived from pyruvate are ethanol, n-butanol, 2-butanol and isobutanol, under reaction conditions that are prohibitive to any cell-based microbial equivalents. As the reaction cascade is designed as a toolbox-system, other products also serve as target compounds.
In general, thermostable enzymes from thermophiles are preferred, as they are prone to tolerate higher process temperatures and higher solvent concentrations. Thus, enhanced thermostability allows for increased reaction rates, substrate diffusion, lower viscosities, better phase separation and decreased bacterial contamination of the reaction medium. As demands for substrate selectivity vary at different reaction stages, enzyme fidelity has to be selected accordingly. For example, in the conversion of glucose to the key intermediate pyruvate, DHAD (dihydroxy acid dehydratase) promiscuity allows for parallel conversion of gluconate and glycerate (
In order to minimize reaction complexity, the designed pathway may be further consolidated to use coenzyme NADH as the only electron carrier. Provided that subsequent reactions maintain redox-neutrality, pyruvate can be converted to an array of industrial platform chemicals without continuous addition of any electron shuttle.
According to one aspect of the invention, engineered enzymes, for example an ALDH variant with a greater activity for NADH, may also be used, for example resulting from a directed evolution approach. Such optimized enzymes reflecting a greater activity for a specific cofactor, for example NADH, can be applied in combination with minimized enzyme usage during conversion of glucose to pyruvate (e.g. the use of three or four or five enzymes for conversion of glucose to pyruvate), allowing for consolidation of enzyme usage and further improved efficiency and productivity for both conversion of glucose to pyruvate and for the overall conversion of the carbon source to the target organic compound.
Molecular optimization of individual enzymes allows for iterative improvements and extension of the presented cell-free production systems with particular focus on activity, thermal stability and solvent tolerance. In addition, resistance to inhibitors that are present when hydrolysed lignocellulosic biomass is used as feedstock and which can be detrimental to cell-based methods, can be addressed by enzyme engineering.
In regard to and as reflected in the invention, the stability and minimized complexity of the cell-free system eliminate the barriers of current cell-based production, which hamper the wider industrial exploitation of bio-based platform chemicals. Pyruvate is a central intermediate, which may serve as a starting point for cell-free biosynthesis of other commodity compounds. The enzymatic approach demonstrated here is minimized in the number of enzymes and required coenzymes and serves as a highly efficient, cost effective bio-production system.
The present invention concerns a process for the production of a target organic compound from glucose, galactose or mixture of glucose and galactose or a glucose-containing oligomer or polymer and/or a galactose-containing oligomer or polymer by a cell-free enzyme system, comprising the conversion of glucose and/or galactose to pyruvate as an intermediate product; wherein said process comprises the following steps:
(1) oxidation of glucose and/or galactose to gluconate and/or galactonate
Preferred embodiments of this process include the following:
The process of the invention, wherein the process is performed at a temperature of at least 40° C. over a period of at least 30 minutes,
The process of the invention, wherein the temperature of the process is maintained in a range from 40-80° C., preferably in a range from 45°-70° C., more preferably in a range from 50°-60° C., and most preferred at or greater than 50°.
The process of the invention, wherein the process is maintained at the given temperature for at least 30 minutes, preferably at least 3 hours, more preferably at least 12 hours, even more preferably for at least 24 hours, most preferred for at least 48 hours, and most highly preferred for at least 72 hours.
The process of the invention, wherein step (1) of the process comprises the use of a single dehydrogenase for the oxidation of glucose and/or galactose to gluconate and/or galactonate.
The process of the invention, wherein step (1) is catalyzed by a single enzyme.
The process of the invention, wherein step (1) comprises the use of a dehydrogenase.
The process of the invention, wherein step (2) comprises the conversion of gluconate to 2-keto-3-deoxygluconate and of 2-keto-3-deoxygluconate to glyceraldehyde and pyruvate.
The process of the invention, wherein step (2) comprises the use of dehydroxy acid dehydratase and keto-3-deoxygluconate aldolase.
The process of the invention, wherein step (3) comprises the use of a dehydrogenase for the oxidation of glyceraldehyde to glycerite.
The process of the invention, wherein steps (1) and (3) are carried out with the use of a single dehydrogenase.
The process of the invention, wherein no net production of ATP occurs.
The process of the invention, wherein no ATPase or arsenate is added.
The process of the invention, wherein said process occurs without ATP and/or ADP as cofactors.
The process of the invention, wherein the single cofactor is selected from the group consisting of NAD/NADH, NADP/NADPH, and FAD/FADH2.
The process of the invention, wherein the single cofactor is NAD/NADH.
The process of the invention, wherein glucose is a preferred starting material.
The process of the invention, wherein the enzyme activity of each enzymatically catalyzed reaction step is adjusted so that it is the same or greater than the activity of any preceding enzymatically catalyzed reaction step.
The process of the invention, wherein the specific enzymatic activity when using glyceraldehyde as a substrate is at least 100 fold greater than using either acetaldehyde or isobutyraldehyde as a substrate, more preferably at least 500 fold greater, even more preferably at least 800 fold greater, most preferably at least 1000 fold greater.
The process of the invention, whereby
Preferred dehydrogenases, dehydratases, and aldolases are dehydrogenases belonging to EC 1.1.1.47 or EC 1.2.1.3, dehydratases belonging to EC 4.2.1.9 or 4.2.1.39, and aldolases belonging to EC 4.1.2.14.
The process of the invention, wherein the conversion of glucose and/or galactose to pyruvate consists of the use of one or two dehydrogenases, one or two dehydratases, and one aldolase.
The process of the invention, wherein the conversion of glucose and/or galactose to pyruvate consists of the use of two dehydrogenases, one dehydratase, and one aldolase.
The process of the invention, wherein the conversion of glucose and/or galactose to pyruvate consists of the use of one dehydrogenases, one dehydratase, and one aldolase.
The process of the invention, wherein one of the enzyme combinations included in any one of the tables P-1, P-2-a, P-2-b, P-2-c, P-2-d, P-3-a, P-3-b, and P-3-c are employed for the conversion of glucose and/or galactose to pyruvate.
The process of the invention, wherein the enzymes used for the conversion of glucose and/or galactose to pyruvate are selected from the group consisting of Glucose dehydrogenase GDH (EC 1.1.1.47) from Sulfolobus solfataricus, NP 344316.1, Seq ID 02; Dihydroxy acid dehydratase DHAD (EC 4.2.1.9) from Sulfolobus solfataricus, NP 344419.1, Seq ID 04; Gluconate dehydratase (EC 4.2.1.39) from Sulfolobus solfataricus, NP—344505; Gluconate dehydratase (EC 4.2.1.39) from Sulfolobus solfataricus, NP—344505, Mutation I9L; Gluconate dehydratase ilvEDD (EC 4.2.1.39) from Achromobacter xylsoxidans; Gluconate dehydratase ilvEDD (EC 4.2.1.39) from Metaliosphaera sedula DSM 5348; Gluconate dehydratase ilvEDD (EC 4.2.1.39) from Thermoplasma acidophilum DSM 1728; Gluconate dehydratase ilvEDD (EC 4.2.1.39) from Thermoplasma acidophilum DSM 1728; 2-Keto-3-deoxyaluconate aldolase KDGA (EC 4.1.2.14) from Sulfolobus solfataricus, NP 344504.1; 2-Keto-3-deoxygluconate aldolase KDGA (EC 4.1.2.14) from Sulfolobus acidocaldaricus, Seq ID 06; Aldehyde Dehydrogenase ALDH (EC 1.2.1.3) from Flavobacterium frigidimaris, BAB96577.1; Aldehyde Dehydrogenase ALDH (EC 1.2.1.3) from Thermoplasma acidophilum, Seq ID 08; Aldehyde Dehydrogenase ALDH (EC 1.2.1.3) from Thermoplasma acidophilum, Mutations F34M+Y399C+S405N, Seq ID 10; Glycerate kinase (EC 2.7.1.) from Sulfolobus solfataricus, NP—342180.1; Glycerate 2-kinase (EC 2.7.1.165) from Sulfolobus tokodaii, Uniprot Q96YZ3.1; Enolase (EC 4.2.1.11) from Sulfoiobus solfataricus, NP 342405.1; Pyruvate Kinase (EC 2.7.1.40) from Sulfolobus solfataricus, NP 342465.1; Glycerate dehydrogenase/hydroxypyruvate reductase (EC 1.1.1.29/1.1.1.81) from Picrophilus torridus, YP—023894.1; Serine-pyruvate transaminase (EC 2.6.1.51) from Sulfolobus solfataricus, NCBI Gen ID: NP—343929.1; L-serine ammonia-lyase (EC 4.3.1.17) from Thermus thermophilus, YP—144295.1 and YP—144005.1; and Alanine dehydrogenase (EC 1.4.1.1) from Thermus thermophilus, NCBI-Gen ID: YP—005739.1.
The process of the invention, wherein the conversion of glucose to pyruvate consists of the conversion of one mole of glucose to two moles of pyruvate.
The process of the invention, wherein pyruvate is further converted to a target chemical, and wherein during such conversion 1 (one) molecule NADH is converted to 1 (one) molecule NAD per molecule pyruvate, and wherein such target chemical is preferably selected from ethanol, isobutanol, n-butanol and 2-butanol.
The process of the invention, wherein one of the enzyme combinations included in any one of the tables E-1, I-1, N-1, T-1, T-2, and T-2a are employed for the conversion of pyruvate to the respective target chemical.
The process of the invention, wherein the target chemical is ethanol and the enzymes used for the conversion of pyruvate to ethanol are selected from the group consisting of Pyruvate decarboxylase PDC (EC 4.1.1.1) from Zymomonas mobilis, Seq ID 20; and Alcohol dehydrogenase ADH (EC 1.1.1.1) from Geobacillus stearothermophilus, Seq ID 18.
The process of the invention, wherein the target chemical is isobutanol and the enzymes used for the conversion of pyruvate to isobutanol are selected from the group consisting of Acetolactate synthase ALS (EC 2.2.1.6) from Bacillus subtilis, Seq ID 12; Acetolactate synthase ALS (EC 2.2.1.6) from Sulfolobus solfataricus, NCBI-GenID: NP—342102.1; Acetolactate synthetase ALS (EC: 2.2.1.6) from Thermotoga maritima, NCBI-GeneID: NP—228358.1; Ketol-acid reductoisomerase KARI (EC 1.1.1.86) from Meiothermus ruber, Seq ID 14; Ketol-acid reductoisomerase KARI (EC 1.1.1.86) from Sulfoiobus solfataricus, NCBI-GenID: NP—342100.1; Ketol-acid reductoisomerase KARI (EC. 1.1.1.86) from Thermotoga maritima, NCBI-GeneID: NP—228360.1; Branched-chain-2-oxo acid decarboxylase KDC (EC 4.1.1.72) from Lactococcus lactis, Seq ID 16; α-Ketoisovalerate decarboxylase KDC, (EC 4.1.1.−) from Lactococcus lactis, NCBI-GeneID: CAG34226.1; Dihydroxy acid dehydratase DHAD (EC 4.2.1.9) from Sulfolobus solfataricus, NP 344419.1, Seq ID 04; Dihydroxy-acid dehydratase DHAD, (EC: 4.2.1.9) from Thermotoga maritime, NCBI-GeneID: NP—228361.1; Alcohol dehydrogenase ADH (EC 1.1.1.1) from Geobacillus stearothermophilus, Seq ID 18; Alcohol dehydrogenase ADH (EC 1.1.1.1) from Flavobacterium frigidimaris, NCBI-GenID: BAB91411.1; and Alcohol dehydrogenase ADH (EC: 1.1.1.1) from Saccharomyces cerevisiae.
The process of the invention, wherein the product is removed from the reaction continuously or in a batch mode, preferably by extraction, perstraction, distillation, adsorption, gas stripping, pervaporation, membrane extraction or reverse osmosis.
The process of the invention, wherein the solvent tolerance of the used enzymes for the respective target chemical is preferably better than 1% (w/w), more preferably better than 4% (w/w), even more preferably better than 6% (w/w) and most preferably better than 10% (w/w).
The process of the invention, wherein the enzyme or enzymes involved in steps (1) and/or (3) is/are optimized for the single cofactor by having a higher specific activity to said cofactor.
The process of the invention, wherein the optimized enzyme has a sequence identity of at least 50%, preferably 70%, more preferably 80%, even more preferably 90%, even more preferably 95%, most preferably 97%, most highly preferred 99% as compared to either SEQ ID NO. 8 or SEQ ID NO. 2 and has an improved specific activity to said cofactor as compared to SEQ ID NO. 8 or SEQ ID NO. 2, respectively.
The process of the invention, wherein the enzyme has a specific activity of 0.4 U/mg or more to said cofactor at 50° C. and pH 7.0 with glyceraldehyde/glycerate as substrates at 1 mM and the said cofactor at 2 mM in a total reaction volume of 0.2 ml. A preferred embodiment is wherein the specific activity is 0.6 U/mg or more, more preferably 0.8 U/mg or more, even more preferably 1.0 U/mg or more, most preferably 1.2 U/mg or more, and most highly preferred 1.5 U/mg or more.
The process of the invention, wherein the specific activity is measured in the presence of 3% isobutanol.
The process of the invention, wherein the enzyme involved in steps (1) and/or (3) is aldehyde dehydrogenase.
The process of the invention, wherein the aldehyde dehydrogenase belongs to the class EC 1.1.1.47,
The process of the invention, wherein the aldehyde dehydrogenase is selected from the group consisting of SEQ ID NO: 10, SEQ ID NO 57, SEQ ID NO 59, SEQ ID NO 61, SEQ ID NO 63, SEQ ID NO 65, SEQ ID NO 67, SEQ ID NO 69.
The above embodiments may be combined to provide further specific embodiments of the invention.
The present invention is directed to a cell-free process for the biotechnological production of target chemicals from carbohydrate sources, in particular of alcohols, including C2 alcohols such as ethanol, and 04 alcohols such as n-butanol, isobutanol or 2-butanol. Products include hydrophobic, hydrohilic, and intermediate chemicals. A most preferred hydrophobic chemical of the current invention is isobutanol. A most preferred hydrophilic and intermediate chemical of the current invention is ethanol.
According to a preferred aspect, the invention discloses a process for the production of a target chemical from a carbohydrate source by a cell-free enzyme system, comprising the conversion of glucose to pyruvate as an intermediate product wherein no phosphorylation reaction and no net production of ATP occurs.
As used herein, the term “enzyme” encompasses also the term “enzyme activity” and may be used interchangeably.
According to a preferred aspect, the invention discloses a process for the production of a target chemical from glucose and/or galactose or a glucose- and/or galactose-containing dimer, oligomer or polymer by a cell-free enzyme system, wherein one molecule glucose or galactose is converted to one molecule pyruvate and one molecule glycerate without net production of ATP, and wherein the glycerate is converted to pyruvate. Preferably such glucose is converted via the intermediate gluconate (or gluconic acid) to 2-keto-3-deoxy-gluconate (or 2-keto-3-deoxy-gluconic acid). Preferably such galactose is converted via the intermediate galactonate (or galactonic acid) to 2-keto-3-deoxy-galactonate (or 2-keto-3-deoxy-galactonic acid). Preferably such 2-keto-3-deoxy-gluconate or 2-keto-3-deoxy-galactonate is converted to one molecule pyruvate and one molecule glyceraldehyde. Preferably such glyceraldehyde is converted to glycerate.
Different options exist for the conversion of glycerate to pyruvate. In one preferred aspect the glycerate is converted via glycerate-2-phosphate and phosphoenolpyruvate to pyruvate. In another preferred aspect glycerate is converted via hydroxypyruvate and serine to pyruvate. In another preferred aspect glycerate is converted directly to pyruvate.
According to a preferred aspect, the invention discloses a process for the production of a target chemical from glucose and/or galactose or a glucose- and/or galactose-containing dimer, oligomer or polymer by a cell-free enzyme system, comprising the conversion of glucose and/or galactose to pyruvate as an intermediate product; wherein no net production of ATP occurs; and wherein the conversion of glucose and/or galactose to pyruvate consists of the use of three, four or five enzymes, preferably three to four and most preferred four enzymes, selected from the group of dehydrogenases, dehydratases, and aldolases, wherein one or more enzymes is be selected from each group,
Different preferred options to minimize the number of enzymes in the conversion of glucose to pyruvate are disclosed:
(1) A particularly preferred aspect of the invention is wherein said enzymes are a dehydrogenase (EC 1.1.1.47) accepting gluconate/glucose and glycerate/glyceraldehyde as substrates, a dihydroxyacid dehydratase (EC 4.2.1.9) accepting gluconate/2-keto-3-deoxygluconate and glycerate/pyruvate as substrates, and a 2-keto-3-deoxy gluconate aldolase (EC 4.1.2.14) accepting 2-keto-3-deoxy gluconate/pyruvate/glyceraldehyde as substrate. (i.e. 3 enzymes)
(2) A particularly preferred aspect of the invention is wherein said enzymes are a glucose dehydrogenase (EC 1.1.1.47) accepting gluconate/glucose as substrate, an aldehyde dehydrogenase (EC 1.2.1.3) accepting glycerate/glyceraldehyde as substrate, a dihydroxyacid dehydratase (EC 4.2.1.9 or 4.2.1.39) accepting gluconate/2-keto-3-deoxygluconate and glycerate/pyruvate as substrates, and a 2-keto-3-deoxy gluconate aldolase (EC 4.1.2.14) accepting 2-keto-3-deoxy gluconate/pyruvate/glyceraldehyde as substrate. (i.e. 4 enzymes)
(3) A particularly preferred aspect of the invention is wherein said enzymes are a dehydrogenase (EC 1.1.1.47 or 1.2.1.3) accepting gluconate/glucose and glycerate/glyceraldehyde as substrates, two different dihydroxyacid dehydratases accepting gluconate/2-keto-3-deoxygluconate (EC 4.2.1.39) and glycerate/pyruvate (EC 4.2.1.9) as substrates, and a 2-keto-3-deoxy gluconate aldolase (EC 4.1.2.14) accepting 2-keto-3-deoxy gluconate/pyruvate/glyceraldehyde as substrate. (i.e. 4 enzymes)
(4) A particularly preferred aspect of the invention is wherein said enzymes are a glucose dehydrogenase (EC 1.1.1.47) accepting gluconate/glucose as substrate, two different dihydroxyacid dehydratases (EC 4.2.1.9) accepting gluconate/2-keto-3-deoxygluconate and glycerate/pyruvate as substrates, an aldehyde dehydrogenase (EC 1.2.1.3) accepting glycerate/glyceraldehyde as substrates, and a 2-keto-3-deoxy gluconate aldolase (EC 4.1.2.14) accepting 2-keto-3-deoxy gluconate/pyruvate and glyceraldehyde as substrate. (i.e. 5 enzymes)
(5) A particularly preferred aspect of the invention is wherein said enzymes are a glucose dehydrogenase (EC 1.1.1.47) accepting gluconate/glucose as substrate, an aldehyde dehydrogenase (EC 1.2.1.3) accepting gluconate/glucose and glycerite/glyceraldehyde as substrates, two different dihydroxyacid dehydratases (EC 4.2.1.9) accepting gluconate/2-keto-3-deoxygluconate and glycerate/pyruvate as substrates, and a 2-keto-3-deoxy gluconate aldolase (EC 4.1.2.14) accepting 2-keto-3-deoxy gluconate/pyruvate/glyceraldehyde as substrate. (i.e. 5 enzymes)
Most enzymes can promiscuously catalyze reactions and act on different substrates. These enzymes will not only catalyze one particular reaction, but they are specific for a particular type of chemical bond or functional group. This promiscuity can be utilized in the reaction cascade of the current invention by converting several substrates to the wanted products by one enzyme.
In order to identify suitable promiscuous enzymes the activity of a relevant enzyme for a substrate or a cofactor may be determined where applicable with a photometrical enzyme assay in micro titer plates to allow a high throughput. Enzyme activity is defined as the amount of enzyme necessary to convert 1 μmol substrate to the desired product per minute. An enzyme having a specific activity per mg enzyme of 0.01 or more, preferably 0.1 or more, more preferably 1 or more, even more preferably 10 or more, most preferably 100 or more, most highly preferred 1000 or more for a tested substrate or cofactor is to be considered as an enzyme accepting a particular substrate and/or cofactor.
Selection of the Carbon Source and/or Carbohydrate Source:
Carbon source can be any material which can be utilized by microorganisms for growth or production of chemicals. These include carbohydrates and derivatives: polyoses such as cellulose, hemicellulose, starch; biases such as sucrose, maltose, lactose; hexoses such as glucose, mannose, galactose; pentoses such as xylose, arabinose; uronic acids, glucosamines etc.; polyols such as sorbitol, glycerol; lipids and derivatives, lignin and derivatives. Particularly preferred carbon sources are carbohydrates, such as glucose, a glucose-containing oligomer or polymer, a non-glucose monomeric hexose, or mixtures thereof.
According to a preferred aspect, the carbohydrate source is glucose, galactose, or a mixture of glucose and galactose, or a hydrolysate of a glucose- and/or galactose containing disaccharide, oligosaccharide, or polysaccharide.
In a particularly preferred aspect, the carbohydrate source is cellulose or lignocellulose, whereby the cellulose is hydrolysed first by the use of cellulases to glucose, and/or whereby cellulase activity is added to the enzyme mixture. For one embodiment of the invention, lignocellulosic material is converted to glucose using only 3 to 6 enzymes (preferably 1 or 2 or 3 endocellulase, 1 or 2 exocellulase, 1 beta-glucosidase).
In a particularly preferred aspect, the carbohydrate source is starch, whereby the starch is hydrolysed first, and/or whereby amylase and glucoamylase activity is added to the enzyme mixture.
In another particularly preferred aspect, the carbohydrate source is sucrose, whereby an invertase is added to the enzyme mixture to hydrolyse the sucrose and an isomerase is added to convert fructose to glucose.
In another particularly preferred aspect, the carbohydrate source is lactose whereby the lactose is hydrolysed with a galactosidase first, and/or whereby a galactosidase is added to the enzyme mixture.
Examples of Products that can be Produced from Pyruvate:
According to a further preferred aspect, the target chemical is ethanol, a four-carbon mono-alcohol, in particular n-butanol, iso-butanol, 2-butanol, or another chemical derivable from pyruvate, preferably by an enzymatic pathway. A most preferred hydrophobic chemical of the current invention is isobutanol. A most preferred target chemical of the current invention is ethanol.
Hydrophobic chemicals according to the invention comprise, without limitation, C4 alcohols such as n-butanol, 2-butanol, and isobutanol, and other chemicals that have a limited miscibility with water. Limited miscibility means that at room temperature not more than 20% (w/w) can be mixed with water without phase separation. Hydrophilic and intermediate chemicals according to the invention comprise, without limitation ethanol and other chemicals.
A hydrophobic chemical is a chemical which is only partially soluble in water and which resides in the solid or liquid state at ambient pressure and temperature. Hydrophobic chemicals have a limited miscibiliy with water of not more than 20% (w/w) without phase separation. Particular examples of hydrophobic chemicals according to the present invention include n-butanol, 2-butanol and isobutanol.
n-Butanol is a colorless, neutral liquid of medium volatility with restricted miscibility (about 7-8% at 20° C. in water. n-Butanol is used as an intermediate in the production of chemicals, as a solvent and as an ingredient in formulated products such as cosmetics. n-Butanol is used in the synthesis of acrylate/methacrylate esters, glycol ethers, n-butyl acetate, amino resins and n-butylamines. n-Butanol can also be used as a fuel in combustion engines due to low vapor pressure, high energy content and the possibility to be blended with gasoline at high concentrations. n-Butanol can be produced using solventogenic Clostridia, such as C. acetobuylicum or C. beijerinckii, typically producing a mixture of n-butanol, acetone and ethanol. Butanol production using solventogenic clostridia has several drawbacks: (i) Product isolation from dilute aqueous solution is very expensive as it is either elaborate (e.g. using membrane processes) or energy consuming (e.g. using distillation), (ii) The yield is low as significant parts of the substrate go into the formation of byproducts such as acetone, ethanol, hydrogen and biomass. (iii) The productivity of butanol production is low due to limited cell titers. (iv) The complex metabolism limits metabolical engineering for higher productivity and yield. (v) Limited process stability often leads to production losses and sterility is difficult to maintain. (vi) The biphasic nature of clostridial growth limits process flexibility and productivity. 2-Butanol is a colorless, neutral liquid of medium volatility with restricted miscibility (about 12% at 20° C.). in water. 2-Butanol is used as solvent for paints and coatings as well as food ingredients or in the production of 1-buten.
Isobutanol is a colorless, neutral liquid of medium volatility with restricted miscibility (about 9-10% at 20° C.) in water. Isobutanol is used as solvent or as plasticizer. It is also used in the production of isobuten which is a precursor for the production of MTBE or ETBE.
According to a preferred embodiment of the invention, the conversion of glucose to pyruvate as an intermediate product works completely without a net production of ATP and/or ADP as cofactors. More preferably, the overall conversion of the carbon source to the target chemical (the reaction pathway) works completely without a net production of ATP and/or ADP as cofactors.
According to a preferred embodiment of the invention, the conversion of glucose to pyruvate as an intermediate product works completely without ATP and/or ADP as cofactors. More preferably, the overall conversion of the carbon source to the target chemical (the reaction pathway) works completely without ATP and/or ADP as cofactors.
According to a preferred embodiment of the invention, the conversion of glucose to pyruvate as an intermediate product works completely without any phosphorylation reaction. More preferably, the overall conversion of the carbon source to the target chemical (the reaction pathway) works completely without any phosphorylation reaction.
According to a preferred embodiment of the invention, the conversion of glucose to pyruvate as an intermediate product works completely without addition of any cofactor except redox cofactors. More preferably, the overall conversion of the carbon source to the target chemical (the reaction pathway) works completely without addition of any cofactor as metabolic intermediate except redox cofactors. Preferred redox cofactors are FMN/FMNH2, FAD/FADH2, NAD/NADH, and/or NADP/NADPH.
According to a preferred embodiment of the invention, the conversion of glucose to pyruvate as an intermediate product works when only a single redox cofactor is added for the redox reactions for either the conversion of glucose to pyruvate and/or for the overall conversion of the carbon source to the target organic compound; a particularly preferred single cofactor is NAD/NADH.
According to one aspect of the invention, for the conversion of glucose to pyruvate, the process of the present invention preferably comprises the use of only one redox cofactor and, preferably, one or more of those enzymes requiring such cofactor are optimized for greater activity towards this cofactor as compared to the respective non-optimized enzyme or wildtype enyzme. More preferably such redox cofactor is NAD/NADH and one or more enzymes are optimized for greater activity towards NAD/NADH as compared to the respective non-optimized enzyme or wildtype enzyme; even more preferable is that the optimized enzyme is one or more dehydrogenases; even more preferred is that the optimized enzyme is one or more of the following:
The combined use of one or more optimized enzymes for greater cofactor activity with a reduced number of enzymes as herein described results in a further improved process for the conversion of glucose to pyruvate and thus ultimately a further improved process for the production of the target chemical.
According to a preferred aspect of the invention, the conversion of glucose to pyruvate comprises the use of one or more enzymes optimized for greater cofactor activity, preferably greater NADH activity, as compared to the respective non-optimized enzyme. A greater cofactor activity includes, for example, improved acceptance of an enzyme of the cofactor as a substrate. An enzyme optimized for greater cofactor activity, preferably greater NADH activity, as compared to the respective non-optimized enzyme is an enzyme which has been engineered as a variant to the non-optimized enzyme resulting in a greater cofactor activity, preferably greater NADH activity. One example of optimization is the use of a directed evolution approach. A variant to the non-optimized enzyme resulting in greater cofactor activity, preferably greater NADH activity, is an enzyme which is encoded by a nucleotide sequence which consists of at least one nucleotide that differs from the nucleotide sequence encoding the enzyme which is non-optimized (i.e. the comparative enzyme), or likewise a variant to the non-optimized enzyme resulting in a greater cofactor activity, preferably greater NADH activity, is an enzyme which consists of at least one amino acid which differs from the amino acid sequence for the enzyme which is non-optimized (i.e. the comparative enzyme). The non-optimized enzyme may be a wildtype enzyme. SEQ ID NO. 8 is a preferred example of a wildtype enzyme enzyme, which is a non-optimized enzyme. Another example is that of SEQ ID NO. 2.
Improved cofactor activity may be measured in absolute amounts, or, alternatively by relative amounts. For example, the specific activity of the purified form of the enzyme may be used. In such a case, in the current invention an improved activity, meaning that which defines an optimized enzyme, to a specific cofactor is when the enzyme has a specific activity of 0.4 U/mg or more to said cofactor at 50° C. and pH 7.0 with glyceraldehyde/glycerate as substrates at 1 mM and the specific cofactor at 2 mM in a total reaction volume of 0.2 ml. A preferred cofactor is NAD/NADH. It is a preferred embodiment for the specific activity to be 0.6 U/mg or more, preferably 0.8 U/mg or more, more preferably 1.0 U/mg or more, most preferably 1.2 U/mg or more, and most highly preferred 1.5 U/mg or more under these conditions. A further preferred embodiment is wherein the specific activity as indicated above is measured in the presence of 3% isobutanol. Alternatively, the specific activity is measured and normalized to the comparative wildtype (e.g. non-optimized enzyme) and an improved activity, meaning that which defines an optimized enzyme, is one which is 2 fold greater or more than the wildtype value, preferably 4 fold greater or more, even more preferably 8 fold greater or more, most preferably 16 fold greater or more, most highly preferred 32 fold greater or more.
Alternatively, relative values can be made using standard protocols and normalizing to the wildtype values. For example, colonies of cells, (e.g. E. coli) containing a library of enzyme variants (e.g. dehydrogenase variants; Ta-ALDH) are induced, grown, harvested, lysed and insoluble cell debris is removed, supernatants containing the variants can be tested for relative activity under standard conditions (e.g. 2 mM NAD, 1 mM D-glyceraldehyde in 50 mM HEPES (pH 7) at 50° C.). After a given period (e.g. 20 min), cofactor formation (e.g. NADH formation) can be detected spectrophotometrically and superior cofactor (e.g. NADH formation) as compared to wild type can be detected and is provided as improved relative activity to the wildtype of the variant of interest. In particular, an improved activity, meaning that which defines an optimized enzyme, is one which is 2 fold greater or more than the wildtype value, preferably 4 fold greater or more, even more preferably 8 fold greater or more, most preferably 16 fold greater or more, most highly preferred 32 fold greater or more.
The optimized enzyme resulting in a greater cofactor activity, preferably greater NADH activity, may be encoded by a nucleotide sequence which consists of substitutions, additions or deletions to the nucleotide sequence encoding the non-optimized enzyme, wherein the substitutions, additions or deletions include 1-60 nucleotides, preferably 1-21 nucleotides, more preferably 1-9 nucleotides, in particularly preferred 1-3 nucleotides, most particularly preferred 1 nucleotide. The substitutions, additions or deletions to the nucleotide sequence encoding the non-optimized enzyme resulting in an optimized enzyme with a greater cofactor activity, preferably greater NADH activity, may be defined as resulting in 80% nucleotide sequence identity to the non-optimized enzyme, preferably 90% nucleotide sequence identity, more preferably 95% nucleotide sequence identity, most preferred 97% nucleotide sequence identity, most highly preferred 99% nucleotide sequence identity. The non-optimized enzyme may be a wildtype enzyme. SEQ ID NO. 8 is a preferred example of a wildtype enzyme enzyme, which is a non-optimized enzyme. Another example is that of SEQ ID NO. 2.
Comparison of sequence identity (alignments) is performed using the ClustalW Algorithm (Larkin M. A., Blackshields G., Brown N. P., Chenna R., McGettigan P. A., McWilliam H., Valentin F., Wallace I. M., Wilm A., Lopez R., Thompson J. D., Gibson T. J. and Higgins D. G. (2007) ClustalW and ClustalX version 2. Bioinformatics 2007 23(21): 2947-2948).
Similarly the optimized enzyme resulting in a greater cofactor activity, preferably greater NADH activity, may be an amino acid sequence which consists of substitutions, additions or deletions to the amino acid sequence of the non-optimized enzyme, where in the substitutions, additions or deletions include 1-20 amino acids, preferably 1-10 amino acids, more preferably 1-5 amino acids, in particularly preferred 1-3 amino acids, most particularly preferred 1 amino acid. The substitutions, additions or deletions to the amino acid sequence of the non-optimized enzyme resulting in an optimized enzyme with a greater cofactor activity, preferably greater NADH activity, may be defined as resulting in an 80% amino acid sequence identity to the non-optimized enzyme, preferably 90% amino acid sequence identity, more preferably 95% amino acid sequence identity, most preferred 97% amino acid sequence identity, most highly preferred 99% amino acid sequence identity. The non-optimized enzyme may be a wildtype enzyme. SEQ ID NO. 8 is a preferred example of a wildtype enzyme enzyme, which is a non-optimized enzyme. Another example is that of SEQ ID NO. 2.
A highly preferred embodiment of the invention is that the optimized enzyme resulting in a greater cofactor activity, preferably greater NADH activity, is an amino acid sequence which consists of 1-10 mutations, preferably 1-8 mutations, more preferably 1-5 mutations, more highly preferred 1-3 mutations, most preferred 1-2 mutations, most highly preferred 1 mutation in the amino acid sequence as compared to the non-optimized enzyme. A mutation is considered as an amino acid which does not correspond to the same amino acid as compared to the non-optimized enzyme. The non-optimized enzyme may be a wildtype enzyme. SEQ ID NO. 8 is a preferred example of a wildtype enzyme enzyme, which is a non-optimized enzyme. Another example is that of SEQ ID NO. 2.
Likewise, the invention includes the nucleotide sequences encoding the optimized aldehyde dehydrogenases with the mutation of F34L as reflected in SEQ ID NO: 56 as well as the amino acid sequences for the optimized aldehyde dehydrogenases with the mutation of F34L as reflected in SEQ ID NO: 57; the nucleotide sequences encoding the optimized aldehyde dehydrogenases with the mutation of S4050 as reflected in SEQ ID NO: 58 as well as the amino acid sequences for the optimized aldehyde dehydrogenases with the mutation of S405C as reflected in SEQ ID NO: 59; the nucleotide sequences encoding the optimized aldehyde dehydrogenases with the mutation of F34L and the mutation of S405C as reflected in SEQ ID NO: 60 as well as the amino acid sequences for the optimized aldehyde dehydrogenases with the mutation of F34L and the mutation of S405C as reflected in SEQ ID NO: 61; the nucleotide sequences encoding the optimized aldehyde dehydrogenases with the mutation of F34M and the mutation of S405N as reflected in SEQ ID NO: 62 as well as the amino acid sequences for the optimized aldehyde dehydrogenases with the mutation of F34M and the mutation of S405N as reflected in SEQ ID NO: 63; the nucleotide sequences encoding the optimized aldehyde dehydrogenases with the mutation of W271S as reflected in SEQ ID NO: 64 as well as the amino acid sequences for the optimized aldehyde dehydrogenases with the mutation of W271S as reflected in SEQ ID NO: 65; the nucleotide sequences encoding the optimized aldehyde dehydrogenases with the mutation of F34M and the mutation of Y399C and the mutation of S405N as reflected in SEQ ID NO: 9 as well as the amino acid sequences for the optimized aldehyde dehydrogenases with the mutation of F34M and the mutation of Y399C and the mutation of S405N as reflected in SEQ ID NO: 10; the nucleotide sequences encoding the optimized aldehyde dehydrogenases w with the mutation of Y399R as reflected in SEQ ID NO: 66 as well as the amino acid sequences for the optimized aldehyde dehydrogenases with the mutation of Y399R as reflected in SEQ ID NO: 67; the nucleotide sequences encoding the optimized aldehyde dehydrogenases with the mutation of F34M and the mutation of W271S and the mutation of Y399C and the mutation of S405N as reflected in SEQ ID NO: 68 as well as the amino acid sequences for the optimized aldehyde dehydrogenases with the mutation of F34M and the mutation of W271S and the mutation of Y399C and the mutation of S405N as reflected in SEQ ID NO: 69. Exemplary technical effects linked with these specific embodiments are included in table 3.
As a preferred embodiments SEQ ID NO: 2 and SEQ ID NO: 8 may be considered as representative of the wildtype enzyme for glucose dehydrogenase and aldehyde dehydrogenase, respectively. SEQ ID NO: 8 is the preferred sequence used as a comparative sequence to identify if an improved activity for a cofactor, in particular NAD/NADH, is present.
As a preferred embodiments SEQ ID NO: 2 and SEQ ID NO: 8 may be considered as representative of the non-optimized enzyme for glucose dehydrogenase and aldehyde dehydrogenase, respectively. SEQ ID NO: 8 is the preferred sequence used as a comparative sequence to identify if an improved activity for a cofactor, in particular NAD/NADH, is present.
As a preferred embodiment the protein sequence encoded by the nucleotide sequence of SEQ ID NO: 1 and SEQ ID NO: 7 may be considered as representative of the wildtype enzyme for glucose dehydrogenase and aldehyde dehydrogenase, respectively. SEQ ID NO: 7 is the preferred nucleotide sequence encoding the protein sequence used as a comparative sequence to identify if an improved activity for a cofactor, in particular NAD/NADH, is present.
As a preferred embodiment the protein sequence encoded by the nucleotide sequence of SEQ ID NO: 1 and SEQ ID NO: 7 may be considered as representative of the non-optimized enzyme for glucose dehydrogenase and aldehyde dehydrogenase, respectively. SEQ ID NO: 7 is the preferred nucleotide sequence encoding the protein sequence used as a comparative sequence to identify if an improved activity for a cofactor, in particular NAD/NADH, is present.
The invention includes both the above mentioned optimized enzymes per se as well as the use of said optimized enzymes in the conversion of glucose to pyruvate in combination with the described process for the production of a target chemical. For example, one or more of the optimized enzymes is used in the process for the production of a target chemical from glucose and/or galactose, or a glucose- and or galactose-containing dimer, oligomer or polymer, by a cell-free enzyme system, comprising the conversion of glucose to pyruvate as an intermediate product; wherein no net production of ATP occurs and, preferably, wherein no phosphorylation reaction occurs; and wherein the conversion of glucose to pyruvate consists of the use of three, four or five enzymes selected from the group of dehydrogenases, dehydratases, and aldolases, wherein one or more enzymes is selected from each group. Preferably the conversion from glucose to pyruvate is achieved by three or four enzymes, most preferably by four enzymes.
According to a preferred embodiment of the invention, the conversion of glucose to pyruvate as an intermediate product consists of the conversion of one mole of glucose to two moles of pyruvate.
According to a further preferred embodiment of the invention and as further described herein, the production process is performed in a liquid system comprising two separate phases, and the target chemical is mainly present in or forms one of the separate phases, and the target chemical is collected from the separate phase. According to a further preferred embodiment of the invention and as further described herein, an organic solvent is added to establish the two separate phases.
According to a further preferred embodiment of the invention and as further described herein, the carbon source compound is continuously fed to the process and the target chemical is continuously removed (fed-batch process).
According to one preferred aspect, the inventive production process comprises the following 4 steps:
Step I: Production of enzymes (the “target enzymes”) for the conversion of a carbon source into a chemical (also herein referred to as the “target chemical” or “target organic compound”) using microbial cells;
Step II: Release of the target enzymes from the microbial cells used in step I, preferably combined with release of cofactors and with inactivation of further, non-target enzyme activities; or purification of the target enzyme from non-target enzyme activities preferably combined with release of cofactors;
Step III: Bringing the target enzymes of step II in contact with the carbon source under conditions suitable for the conversion of the carbon source into the target chemical;
Step IV; Separating the target chemical from the reaction mixture.
In step I the target enzymes are produced using microbial cells. In one embodiment of the invention, enzyme production is done in two or more different microbial cell lines, such that the entire production route or major parts of it are not reconstituted in one microorganism. This avoids the unwanted initiation of substrate conversion towards the chemical and leads to a more efficient enzyme production. Enzyme production can be intracellular or extracellular, recombinant or non-recombinant. If enzyme production is recombinant it can be homologous or heterologous.
In a further embodiment of the invention, the target enzymes are selective for one substrate and one reaction. Preferably, the target enzymes have a substrate selectivity (kcat/kM) of at least 10 fold compared to any other naturally present substance and a reaction selectivity of at least 90%, More preferably, the target enzymes have a substrate selectivity of at least 20 fold and a reaction selectivity of at least 95%. Even more preferably, the target enzymes have a substrate selectivity of at least 100 fold and a reaction selectivity of at least 99%.
In a further embodiment of the invention, the target enzymes show no or low inhibition by the substrate or the product or other intermediates of the multistep reaction (no or low feedback inhibition), Preferably, the inhibition constants (Ki) for any substrate, product or intermediate of the multistep reaction are at least 10 fold higher than the KM value for the respective enzyme and substrate. More preferably, such inhibition constants are 100 fold higher than the respective KM. In a further, particularly preferred embodiment the target enzymes still have 50% of their maximum activity at concentrations of any substrate, intermediate or product of the multistep reaction of 100 mM or more.
Preferably, the target enzymes have Kcat and KM values that are adjusted to the multistep nature of the enzymatic route.
According to one embodiment of the process of the invention, the target enzymes tolerate elevated levels of the target chemical and, optionally, other organic solvents that are optionally added to support segregation of the target chemical into a separate phase.
Preferably the target enzymes tolerate concentrations of the target chemical of more than 2% (w/w), more preferably more than 4% (w/w), more preferably more than 6% (w/w), even more preferably more preferably more than 8% (w/w). In a particularly preferred embodiment, the target enzymes tolerate concentrations of the target chemicals up to the maximum solubility in water.
In a preferred embodiment, the entire cell-free enzyme system tolerates elevated levels of the target chemical, such as isobutanol, butanol, or ethanol. In a particularly preferred embodiment, the entire cell-free enzyme system tolerates isobutanol in concentrations of 2% (w/w) or more, preferably 4%(w/w) or more, more preferably 6% (w/w) or more, even more preferably 8% (w/w) or more. In a particularly preferred embodiment, the entire cell-free enzyme system tolerates concentrations of the target chemicals such as isobutanol at concentrations at which phase separation occurs, i.e. at concentrations corresponding to the maximum solubility of the target chemical in water at the process temperature.
In a preferred embodiment of the invention, the target enzymes tolerate elevated levels of chaotropic substances and elevated temperatures. Preferably, the target enzymes tolerate concentrations of guanidinium chloride of more than 1 M, more preferably more than 3 M, and most preferably more than 6 M. Alternatively or in combination, the target enzymes tolerate preferably temperatures of more than 40-90° C., more preferably more than 50-80° C., and most preferably more than 50-60° C. in such preferred embodiment, target enzyme production is done in a host organism whose endogenous enzyme activities are mostly inactivated at elevated levels of chaotropic substances and/or at elevated temperatures.
Preferably target enzyme production is performed using one or more of the following microbial species: Escherichia coli; Pseudomonas fluorescence; Bacillus subtilis; Saccharomyces cerevisiae; Pichia pastoris; Hansenula polymorpha; Klyuveromyces lactis; Trichoderma reesei; Aspergillus niger. More preferably target enzyme production is performed using Escherichia coli as host organism. Preferably, enzyme production and cell growth are separated into separate growth phases. Thereby, no substrate is used for general metabolic activity.
In a preferred aspect of the invention, E. coli, B. subtilis, S. cerevisae and/or Pichia pastoris are used as the expression hosts and one or more enzymes are recombinantly produced in each individual strain. in another preferred aspect, E. coli is the expression host and one enzyme is expressed per individual strain. Preferably, such enzymes have a half-life of more than 12 hours under temperatures and/or pH values that inactivate, when incubated for at least 10 min, preferably at least 30 min, the expression host (i.e. cell growth) and inactivate at least 90%, preferably 95%, more preferably 99% of the enzyme activities that originate from the expression host and accept any of the intermediates from the carbohydrate source (e.g. glucose or galactose) to the target chemical as substrate. In a particularly preferred aspect, no such enzyme activities can be detected after incubation for 30 min at such temperature. Most preferably the expressed enzymes have a half-life of at least 12 hours when incubated at 50° C. in standard buffer at neutral pH.
In a further preferred embodiment of the invention, the target enzymes tolerate elevated levels of oxygen. Preferably, the target enzymes tolerate oxygen concentrations of more than 1 ppm, more preferably more than 7 ppm. Most preferably the target enzymes are active and stable under aerobic conditions. Preferably, the multistep reaction does not require oxygen and is not inhibited by oxygen. Thereby, no special precautions for oxygen exclusion have to be taken, making the process more stable with less effort on the production environment and/or equipment.
In step II the target enzymes are released from the cells. In a preferred embodiment, the target enzymes tolerate high temperatures and chaotropic conditions, whereas the background enzymes from the producing microorganism do not tolerate these conditions. According to this embodiment, the target enzymes are produced intracellularly in microbial cells, the cells are lysed using high temperature and/or chaotropic conditions, thereby releasing the target enzymes in active form, optionally together with cofactors, while unwanted background enzyme activities are inactivated.
In another preferred embodiment, enzyme production is extracellular, the target enzymes tolerate high temperatures and chaotropic conditions, and background enzyme activities (non-target enzymes) do not tolerate these conditions. According to this embodiment, the supernatant from extracellular production is treated under conditions such as high temperatures and/or chaotropic conditions. This leads to inactivation of unwanted background enzyme activities (non-target enzymes) while the target enzymes remain active.
In one embodiment of the invention, cofactors are required for one or more of the multiple enzymatic conversion steps. In one aspect of the invention such cofactors are added to the enzyme mixture. In another, particularly preferred aspect of this embodiment such cofactors are also produced by the microbial cells intracellularly and are released by the same treatment as to release the target enzymes. In another preferred embodiment of the invention, the microbial cells are engineered in order to optimize the level of cofactors produced. Preferably, the microbial cells are inactivated during step II. Thereby, cell growth and enzyme activity are separated in the process, and no carbon source is consumed by undesirable cell growth.
In step III the carbon source is converted by a mixture of enzymes in a multistep enzymatic reaction to pyruvate, and, optionally, further to the target chemical. According to the inventive process, the enzymes are active under the denaturing activity of the target chemical. Preferably, the microbial cells used in step I are inactive and/or are inactivated under the reaction conditions of step III.
Preferably, the concentration of each enzyme in the target enzyme mixture is adjusted to the optimal level under process conditions. In a particularly preferred embodiment one or more enzyme concentrations are increased above typical intracellular concentrations in order to improve the yield of the process (no limit by the maximal density of the microorganisms as in classical processes). in another particularly preferred embodiment, one or more enzymes are engineered for maximal catalytic efficiency (leading to lower reactor size and running costs compared to classical processes).
The activity of the enzymes in the target enzyme mixture is adjusted to the optimal level under process conditions. In a preferred embodiment the activity of the first enzyme of the enzymatic cascade is adjusted to a lower level compared to the activities of the enzymes later in the cascade. Such enzyme activities prevent accumulation of intermediates in the reaction. in case an enzyme is used in more than one step of the cascade, the activity may be increased accordingly. In a particularly preferred embodiment the activity of each enzyme of the enzymatic cascade is adjusted to a level which is greater than any preceeding enzyme activity in the enzymatic cascade. Such enzyme activities prevent accumulation of intermediates in the reaction.
In a further preferred embodiment, the target chemical is added to or present in the reaction mixture in step III at a concentration at or slightly above the maximum level that can be mixed in a single phase with water under process conditions. According to this embodiment, the target chemical continuously segregates into the second phase during the process. In a particular variant of this embodiment, a water soluble substance is added to the reaction mixture that leads to a phase separation of the target chemical at lower concentration than without the added substance. Examples of such substances are salts and are known to the person skilled in the art. In a particularly preferred variant of this embodiment, sodium chloride is added to lower the solubility of the target chemical in the water phase.
In another preferred embodiment, an additional organic solvent is added to the process that forms its own phase and extracts the produced hydrophobic chemical from the water phase. Preferred examples of such additional solvents comprise: n-hexane, cyclohexane, octanol, ethylacetate, methylbutylketone, or combinations thereof.
In another preferred embodiment, the yield is improved because the formation of side products is decreased by using target enzymes that are specific for the desired reactions. In another preferred embodiment, host enzymes that would catalyse side reactions are inactivated during step II and/or are inactive under the reaction conditions of step III.
In yet another preferred embodiment of the invention, contamination of the process by microorganisms is avoided by adjusting reaction conditions in step III that are toxic for typical microbial contaminants. Such conditions comprise elevated temperature, extreme pH, addition of organic chemicals. In a particularly preferred embodiment of the invention, the target chemical itself is toxic at the concentration achieved in the process (more stable process with less effort on production environment and/or equipment).
According to another preferred embodiment to the invention, no additional redox cofactors are added to the reaction mixture except for those cofactors that are produced by the microorganisms used in step I and that are included in the cell lysate produced in step II. Examples of such cofactors are FAD/FADH2, FMN/FMNH2, NAD/NADH, NADP/NADPH. According to this embodiment, the cofactors that are required are produced by the microbial cells in step I and are regenerated during the process (NADH to NAD and vice versa; NADPH to NADP and vice versa). In one embodiment of the invention, excess reduction equivalents (NADH, NADPH) or energy equivalents (ATP) are regenerated by additional enzymes (e.g. NADH oxidase for NADH; NADPH oxidase for NADPH).
In a particularly preferred embodiment of the invention, neither ATP nor ADP is involved as a cofactor in the conversion from glucose to pyruvate and none of the target enzymes involved in this conversion comprises a phosphorylation step (non-phosphorylative pyruvate production).
In step IV, the one or more target chemicals are separated from the reaction mixture. In a preferred embodiment of the invention the one or more target chemicals are hydrophobic and form a separate phase which preferably contains at least a substantial fraction of the produced chemicals. In a particularly preferred embodiment the one or more target chemicals are continuously removed from the reaction mixture.
In a further preferred embodiment of the invention, the carbon source is continuously fed to the reaction mixture to be converted into the target chemical. Likewise, the target chemical is preferably continuously removed as a separate phase and further purified by methods known in the art. Thereby, product isolation is simplified as the product is collected in a separate phase from which it can be purified further. Thereby, the yield is improved and product purification is simplified.
In a further preferred embodiment, the inventive process does not require ADP or ATP as cofactors. Other processes (Welch and Scopes, 1985; Alger and Scopes, 1985) require cofactors such as ADP/ATP and NAD/NADH. The postulated conversion of glucose to Butanol (Zhang et al, 2008) requires the cofactors ADP/ATP, NAD/NADH, Ferredoxin and Coenzyme A. A major problem of the described cell free enzymatic processes (Zhang et al., 2008, Welch and Scopes 1985) is the accumulation of ATP. In the known processes, the undesired accumulation of ATP is circumvented by the addition of an ATPase. To find the right concentration of ATPase, however, is difficult as it depends on the concentration of the substrate and different intermediates as well as on the activity of the enzymes. With either too much ATPase or too little ATPase, an ATP imbalance results and the conversion completely ceases (Welch and Scopes, 1985). As an alternative to ATPase, arsenate may also be used, with similar disadvantages as for ATPase. In contrast, according to a particularly preferred aspect, the inventive cell-free process converts a carbon source such as glucose to pyruvate, and more preferably glucose to the target chemical, without net production of ATP and without using an ATPase and/or arsenate.
Several preferred embodiments are hereinafter described regarding the production of pyruvate from glucose. Two molecules pyruvate are produced from one molecule glucose. The pyruvate can subsequently be converted to target chemicals such as n-butanol, isobutanol, ethanol and 2-butanol.
According to one aspect of the invention glucose is converted to pyruvate by the use of the following enzymes:
According to another preferred aspect of the invention glucose is converted to pyruvate by the use of the following enzymes:
According to another preferred aspect of the invention glucose is converted to pyruvate by the use of the following enzymes:
According to another preferred aspect of the invention glucose is converted to pyruvate by the use of the following enzymes:
According to another preferred aspect of the invention glucose is converted to pyruvate by the use of the following enzymes:
According to another preferred aspect of the invention glucose is converted to pyruvate by the use of the following enzymes:
According to another preferred aspect of the invention glucose is converted to pyruvate by the use of the following enzymes:
According to another preferred aspect of the invention glucose is converted to pyruvate by the use of the following enzymes:
In all enzyme combinations P-x (i.e. P-1, P-2-a, P-2-b, P-2-c, P-2-d, P-3-a, P-3-b, and P-3-c) one mol glucose is converted into two moles pyruvate, coupled with the reduction of two NAD equivalents. To eliminate phosphorylation and dephosphorylation steps of natural pathways and thus reduce the number of required enzymes, the invention exploits, for example, the substrate promiscuity of an archaeal dihydroxy acid dehydratase (DHAD) which catalyzes both, the transformation of glycerate to pyruvate and of gluconate to 2-keto-3-deoxygluconate. The molecular efficiency of DHAD allows for the consolidated conversion of glucose to pyruvate with just 4 enzymes, comprising glucose dehydrogenase (GDH) (J. Biol. Chem. 2006, 281, 14796-14804), gluconate/glycerate/dihydroxyacid dehydratase (DHAD) (J. Biochem. 2006, 139, 591-596), 2-keto-3-deoxygluconate aldolase (KDGA) (Biochem. J. 2007, 403, 421-430) and glyceraldehyde dehydrogenase (ALDH) (Biochem J. 2006, 397, 131-138). ALDH together with DHAD redirects glyceraldehyde produced via aldol cleavage towards pyruvate formation. Enzymes of the cell-free reaction cascade are chosen based on their stability and selectivity.
A preferred embodiment of the invention is the use of optimized enzymes for improved NADH activity.
Preferably, enzymes for the conversion of glucose to pyruvate are selected from the following list of enzymes (Information: Enzyme name, E.C. number in brackets, Source organism, NCBI/Gene Number if applicable, Mutations if applicable, Seq ID if applicable):
Alanine dehydrogenase (EC 1.4.1.1), Thermus thermophilus, NCBI-Gen ID: YP—005739.1
In a preferred embodiment of the invention, enzymes for the conversion of glucose to pyruvate are selected from the following list of enzymes:
The enzyme combinations listed above for the conversion of glucose to pyruvate can also be employed for the conversion of galactose or a mixture of glucose and galactose to pyruvate. Thereby, galactose is converted via galactonate, and 2-keto-3-deoxy-galactanate, to pyruvate and glycerate. The glycerate is then converted as described above.
The conversion of galactose to galactonate is preferably done by a dehydrogenase accepting galactose, more preferably by a dehydrogenase accepting both, glucose and galactose. The conversion of galactonate to 2-keto-3-deoxy-galactanate is preferably done by a dehydratase accepting galactonate, more preferably by a dehydratase accepting both, gluconate and galactonate. The conversion of 2-keto-3-deoxy-galactanate is preferably done by an aldolase accepting 2-keto-3-deoxy-galactanate, more preferably by an aldolase accepting both, 2-keto-3-deoxy-gluconate and 2-keto-3-deoxy-galactanate.
In a particularly preferred aspect such enzymes are selected from the following enzymes:
In a particularly preferred embodiment, n-butanol is produced from pyruvate.
Various options exist for the conversion of pyruvate to acetyl CoA. In one embodiment of the invention, one or more of the following enzymes is used for the conversion: (i) pyruvate oxidoreductase using ferredoxin as cofactor; (ii) pyruvate dehydrogenase using NAD(P)H as cofactor; (iii) pyruvate formate lyase; (iv) pyruvate dehydrogenase enzyme complex.
In a preferred embodiment, pyruvate dehydrogenase is used as the enzyme for this conversion, using NADH as cofactor. Pyruvate dehydrogenases are usually part of a multi enzyme complex (Pyruvate dehydrogenase complex, PDHC) which consists of three enzymatic activities and has a molecular weight of ca. 1 Mio Da. For application in a cell-free reaction system it is beneficial to have small and robust non-complexed enzymes. It has been found that the pyruvate dehydrogenase from Euglena gracilis can be used therefore. This enzyme is singular and complex-free. Furthermore it uses NADH as cofactor.
Alternatively, pyruvate formate lyase can be combined with a formate dehydrogenase using NADH as cofactor.
Various options exist for the conversion of acetyl CoA to n-butanol, employing enzymes from n-butanol producing bacteria, such as C. acetobutylicum, C. saccharobutylicum, C. saccharoperbutyfacetonicum, C. beijerinckii.
According to a preferred aspect of the invention pyruvate is converted to n-butanol by the use of the following enzymes:
When any of the enzyme combinations for the production of pyruvate (P-1, P-2-a, P-2-b, P-2-c, P-2-d, P-3-a, P-3-b, and P-3-c) is combined with any of the enzyme combinations for the production of n-butanol (N-1) a net conversion of one molecule glucose to two molecules of CO2, one molecule of water and one molecule of n-butanol is achieved.
Preferably, enzymes for the conversion of pyruvate to n-butanol are selected from the following list of enzymes (Information: Enzyme name, E.C. number in brackets, Source organism, NCBI/Gene Number if applicable, Mutations if applicable, Seq ID if applicable):
In another particularly preferred embodiment, isobutanol is produced from pyruvate. Various options exist for the conversion of pyruvate to isobutanol.
According to a preferred aspect of the invention pyruvate is converted to isobutanol by the use of the following enzymes:
When any of the enzyme combinations for the production of pyruvate (P-1, P-2-a, P-2-b, P-2-c, P-2-d, P-3-a, P-3-b, and P-3-c) is combined with any of the enzyme combinations for the production of isobutanol (1-1) a net conversion of one molecule glucose to two molecules of CO2, one molecule of water and one molecule of isobutanol is achieved.
Preferably, enzymes for the conversion of pyruvate to isobutanol are selected from the following list of enzymes (Information: Enzyme name, E.C. number in brackets, Source organism, NCBI/Gene Number if applicable. Mutations if applicable, Seq ID if applicable):
In a preferred embodiment of the invention, enzymes for the conversion of pyruvate to isobutanol are selected from the following list of enzymes:
In a further particularly preferred embodiment of the invention a single dehydratase can be employed for the conversion of gluconate to 2-keto-3-deoxygluconate and glycerate to pyruvate and 2,3-dihydroxyisovalerate to 2-keto-isovalerate. Therefore a single enzyme can be employed for enzyme activity #2 in enzyme combination P-1, P-2-a, P-2-b, P-2-c, P-2-d, P-3-a, P-3-b, and P-3-c and for enzyme activity #3 in enzyme combination I-1.
In another embodiment of the invention, the target chemical is ethanol. Various options exist for the conversion of pyruvate to ethanol.
According to a preferred aspect of the invention pyruvate is converted to ethanol by the use of the following enzymes:
When any of the enzyme combinations for the production of pyruvate (P-1, P-2-a, P-2-b, P-2-c, P-2-d, P-3-a, P-3-b, and P-3-c′ is combined with any of the enzyme combinations for the production of ethanol (E-1) a net conversion of one molecule glucose to two molecules of CO2, and two molecules of ethanol is achieved.
A preferred embodiment of the invention is the use of optimized enzymes for improved NADH activity.
Preferably, enzymes for the conversion of pyruvate to ethanol are selected from the following list of enzymes (Information: Enzyme name, E.C. number in brackets, Source organism, NCBI/Gene Number if applicable, Mutations if applicable, Seq ID if applicable):
In another embodiment of the invention the target chemical is 2-butanol. Various options exist for the conversion of pyruvate to 2-butanol.
According to a preferred aspect of the invention pyruvate is converted to 2-butanol by the use of the following enzymes:
In a further preferred embodiment an alcohol dehydrogenase is used that uses acetoin as well as 2-butanon as substrate. Therefore, pyruvate is converted to 2-butanol by the use of the following enzymes:
Enzyme combination T-2a:
When any of the enzyme combinations for the production of pyruvate (P-1, P-2-a, P-2-b, P-2-c, P-2-d, P-3-a, P-3-b, and P-3-c) is combined with any of the enzyme combinations for the production of 2-butanol (T-1, T-2, T-2a) a net conversion of one molecule glucose to two molecules CO2, one molecule water and one molecule 2-butanol is achieved.
A preferred embodiment of the invention is the use of optimized enzymes for improved NADH activity.
The present invention is further defined in the following examples. It should be understood that these examples are given by way of illustration only and are not limiting the scope of the invention. From the above discussion and these examples, a person skilled in the art can ascertain the essential characteristics of this invention and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various uses and conditions.
Substrate and product concentrations in the herein described experiments are comparably low. For allowing easy product separation, for more economic processes, the product concentration may be increased above the solubility limit, which for example for isobutanol is 1.28 M at 20° C. (ca. 95 g/l). The product solubility can also be lowered by increasing process temperature and adjusting salt concentrations.
In one embodiment, 1 mol glucose or galactose is converted to 1 mol isobutanol in the described system, therefore substrate concentrations are to be chosen according to the desired end concentration (e.g. 230 g/l glucose or galactose) or higher.
Furthermore, a continuously running process comprising constant substrate feed (glucose syrup) and product removal (organic phase) is further advantageous, given that enzymes and cofactors are retained, e.g. by immobilization.
One general example of the feasibility of the cell-free synthesis toolbox, glucose or galactose was converted to pyruvate using the enzyme cascade of conversion of glucose or galactose to pyruvate with four enzymes, comprising glucose dehydrogenase (GDH), gluconate/glycerate/dihydroxyacid dehydratase (DHAD), 2-keto-3-deoxygluconate aldolase (KDGA) and glyceraldehyde dehydrogenase (ALDH). The ALDH used in this example is defined by SEQ ID NO 10 as established in Example 4.
In a subsequent two-step reaction pyruvate was converted to acetaldehyde and then to ethanol by action of pyruvate decarboxylase (PDC) (J. Mol. Catal. B-Enzym. 2009, 61, 30-35) and alcohol dehydrogenase (ADH) (Protein Eng. 1998, 11, 925-930). The PDC from Zymomonas mobilis was selected due to its relatively high thermal tolerance and activity. Despite its mesophilic origin, Z. m. PDC is thermostable up to 50° C. (see table 10) which is in accord with the temperature range of more thermostable enzymes. Consequently, experiments were carried out at 50° C. The six required enzymes were recombinantly expressed in E. coli and subjected to different purification regimes. Using this set of enzymes, together with 5 mM NAD, 25 mM glucose was converted to 28.7 mM ethanol (molar yield of 57.4%) in 19 h (
While residual intermediates generally accumulated at the end, gluconate maximum was measured between 8 and 10 h during the course of the reaction. Notably, undesired side-products such as lactate and acetate were not detected, indicating that the selected enzymes did provide the necessary substrate specificity. Although the enzyme-catalyzed reaction was not completed over the course of the experiment, the cumulative mass of all detectable intermediates and product gives a yield in excess of 80%.
S. solfataricus/
S. solfataricus/
S. acidocaldarius/
T. acidophilum
c/
Z. mobilis/
G. stearothermophilus/
aactivity for natural substrates,
babove solubility,
cenzyme was engineered,
[2]M. Reher, P. Schonheit, FEBS Lett. 2006, 580, 1198-1204;
[3]G. Fiorentino, R. Cannio, M. Rossi, S. Bartolucci, Protein Eng. 1998, 11, 925-930.)
This example demonstrates the successful conversion of pyruvate to isobutanol using only four additional enzymes (see
S. solfataricus/
S. solfataricus/
S. acidocaldarius/
T. acidophilum
c/
G. stearothermophilus/
B. subtilis/
M. ruber/
L. lactis/
aactivity for natural substrates, DHAD for gluconate, glycerate and dihydroxyisovalerate, ADH for acetaldehyde and isobutyraldehyde (resp.) as substrates;
babove solubility,
cenzyme was engineered,
[2]M. Reher, P. Schonheit, FEBS Lett. 2006, 580, 1198-1204;
[3]G. Fiorentino, R. Cannio, M. Rossi, S. Bartolucci, Protein Eng. 1998, 11, 925-930;
[4]F. Wiegeshoff, M. A. Marahiel, FEMS Microbiol. Lett. 2007, 272, 30-34; and
[5]D. Gocke, C. L. Nguyen, M. Pohl, T. Stillger, L. Walter, M. Mueller, Adv. Synth. Catal. 2007, 349, 1425-1435.)
The enzymes 2-ketoacid decarboxylase (KDC) (J. Mol. Catal. B-Enzym. 2009, 61, 30-35) and an ADH (Protein Eng. 1998, 11, 925-930) produce the final product isobutanol via isobutyraldehyde. Again the substrate ambiguity of DHAD is exploited to minimize the total number of enzymes required.
In analogy to ethanol production, the enzymes of the general pyruvate synthesis route differ from the following three biocatalysts with respect to thermal stability, solvent tolerance and activity profiles (Table 2). To allow experimental comparison, reaction conditions remained the same as described previously. The activity of the enzymes was adjusted to to 0.12 U per mM glucose in the reaction for the GDH, to 0.6 U for the DHAD and to 0.2 U for the remaining enzymes. Measurements indicated that 19.1 mM glucose was converted to 10.3 mM isobutanol within 23 h, which corresponds to a molar yield of 53% (
In analogy to isobutanol production with glucose, galactose was used as a substrate. To allow experimental comparison, reaction conditions remained the same. Measurements indicated that galactose was converted to 7.5 mM isobutanol within 23 h, which corresponds to a molar yield of 38%.
A key characteristic of cell-free systems is their pronounced tolerance against higher alcohols. To evaluate solvent tolerance of the artificial enzyme cascade, glucose conversion to ethanol was conducted as in Example 1 in the presence of increasing isobutanol concentrations (
In contrast to microbial cells, where minor isobutanol concentrations (ca. 1% v/v) already result in loss of productivity, presumably through loss of membrane integrity, cell-free ethanol productivity and reaction kinetics were not significantly affected by isobutanol concentrations up to 4% (v/v). Only in the presence of 6% (v/v) isobutanol, ethanol productivity rapidly declined (1.4 mM ethanol in 8 h). This demonstrates that cell-free processes have the potential to tolerate much higher solvent concentrations than equivalent whole-cell systems. Based on the current data ALDH has the lowest solvent tolerance, as 3% (v/v) isobutanol already induce adverse effects on activity. In contrast, KDGA remains completely active even in a two-phase isobutanol/water system, which forms spontaneously at product titers above 12% (v/v) (see table 2). As shown for an engineered transaminase, which remains active in a reaction medium containing 50% DMSO, such short-comings can be addressed by engineering of the respective protein. In comparison, there is neither a successful example nor a straight-forward technology in place to engineer an entire cell for solvent tolerance. It is expected, that all enzymes utilized in the cell-free pathways can be engineered to be as solvent tolerant as KDGA or can be replaced by a stable naturally occurring equivalent, so that isobutanol production is achieved in a two phase system. Product recovery by a simple phase separation would significantly simplify the downstream processing (Ind. Eng. Chem. Res. 2009, 48, 7325-7336) and, while conceivable with a cell-free system, it is highly unlikely to be realized by microbial fermentation.
Random mutations were introduced into Ta-aldh gene by PCR under error prone conditions according to the protocol of Jaeger et al. (Applied Microbiology and Biotechnology, 2001. 55(5): p. 519-530). Mutated Ta-aldh genes were purified, cut, ligated (via Xbal and Bsal) into pCBR-Chis and used to transform E. coli BL21 (DE3) to create expression library. Ta-aldh libraries created by random mutagenesis were calculated to have 1.3-3 base pair changes per Ta-aldh gene. This calculation was based on the reference that a concentration of 0.1 mmol/l MnCl2 in the FOR reaction leads to a mutation rate of about 1-2 bases per 1000 bases. The Ta-aldh gene contains 1515 bases.
The following primers, reaction conditions and temperature program were used in the PCR reaction:
Followed by purification with MN Gelextraction kit.
TaALDH-variants with improved properties were found by screening method described below. Mutations in TaALDH were detected by sequencing Ta-aldh gene (GATC Biotech, Cologne, Germany). Base triplets coding for beneficial amino acid changes were isolated and saturated by quickchange FOR according to the protocol of Wang and Malcolm (Biotechniques, 1999. 26(4): p. 680-68). Mutations were inserted into pCBR-Ta-ALDH-Chis using degenerative primer pairs. Quickchange PCR product was purified from pCBR-Ta-aldh-Chis template and used to transform E. coli BL21 (DE3) to create expression library.
First 10 cycles:
15 μL each from sample using fw-Primer and rev-Primer were pooled.
Second 25 cycles (same conditions as above):
Saturation mutagenesis on amino acid position 34:
Saturation mutagenesis on amino acid position 405:
Saturation mutagenesis on amino acid position 271:
Saturation mutagenesis on amino acid position 399:
Colonies of E. coli BL21 (DE3) containing Ta-ALDH library were transferred into 96-deepwell plates (Gainer BioOne) containing Zym5052 autoinduction medium (Protein Expression and Purification, 2005. 41(1): p. 207-234). Cultures were grown over night at 37° C., 1000 rpm. Cells were harvested by centrifugation at 5000 g at 2° C. for 2 min and lysed with B-Per® protein extraction reagent (Thermo Fisher Scentific, Rockford, USA). After incubation for 60 min at 50° C., insoluble cell debris and was removed by centrifugation at 5000 g at 20° C. for 30 min. Supernatants containing variants of TaALDH were tested for relative activity under standard conditions: 2 mM NAD, 1 mM D-glyceraldehyde in 50 mM HEPES (pH 7) at 50° C. After 20 min NADH formation was detected spectrophotometrically at 340 nm. Superior NADH formation compared to wild type TaALDH indicated an improved relative activity of TaALDH variant.
Activity in solvents of Ta-ALDH variants was tested under standard assay conditions containing additional solvent. After 20 min NADH formation was detected and compared to relative activity.
Change in cofactor acceptance of Ta-ALDH variants was tested under standard assay conditions but with 10 mM NAD. After 20 min NADH formation was detected and compared to relative activity.
The Ta-aldh gene was cloned in pCBR-Chis expression vector as described above. Mutations F34L, F34M, Y399C, S405C and S405N were inserted into Ta-aldh gene by Quickchange PCR as described above. For recombinant expression, E. coli BL21 (DE3) was transformed with pCBR-Ta-aldh-Chis, pCBR-Ta-aldh-f34I-s405c-Chis, pCBR-Ta-aldh-f34m-s405n-Chis or pCBR-Ta-aldh-f34m-y399c-s405n-Chis. Each of the four variants was produced using the following protocol:
Large amounts of a TaALDH-Variant were produced with fed-batch fermentation in a 40 L Biostat Cplus bioreactor (Sartorius Stedim, Goettingen, Germany). Defined media was supplemented with 30 μg/ml kanamycin. After inoculation cells were grown at 30° C. for 24 h and induced with 0.3 mM IPTG. Enzyme expression was performed at 30° C. for 3 h. One fermentation produced 300 g cells (wet weight). Cells were harvested and lysed with Basic-Z Cell Disruptor (Constant Systems, Northants, UK) in loading buffer (200 mM NaCl; 20 mM Imidazol; 2.5 mM MgCl2; 50 mM NaPi, pH 6.2). After heat treatment at 50° C. for 30 min, cell debris and protein aggregates were separated from soluble fraction by centrifugation at 30,000 g at 20° C. for 30 min (Sorvall RC6+, SS-34 rotor, Thermo Scientific).
Soluble fraction of His-tagged TaALDH-Variant was further purified by Ni-NTA chromatography using ÄKTA UPC-900 FPLC-system (GE Healthcare, Freiburg, Germany). Supernatant was loaded on HiTrap FF-column and washed with two column volumes of loading buffer. Highly purified and concentrated TaALDH was fractioned after elution in imidazole buffer (200 mM NaCl; 500 mM imidazol; 50 mM NaPi, pH 6.2). Buffer was changed to 20 mM (NH4)HCO3 with HiPrep 26/10 Desalting column and TaALDH was lyophilized with an Alpha 2-4 LD Plus freeze dryer (Martin Christ GmbH, Osterode am Harz, Germany).
TaALDH activity was determined spectrophotometrically at 50° C. by measuring the rate of cofactor reduction at 340 nm in flat bottom microtiter plates (Grainer BioOne) with a Fluorostar Omega Photometer (BMG Labtech GmbH, Ortenberg, Germany). One unit of activity was defined as reduction of 1 μmol of cofactor per minute. Reaction mixtures (total volume 0.2 contained 1 mM D-Glyceraldehyde and 2 mM NAD and appropriate amounts of enzyme in 100 mM HEPES pH 7.
The values “relative to wt” are the mU/mL values normalized to the wildtype control value.
Restriction enzymes, Klenow fragment, T4 ligase and T4 kinase were purchased from New England Biolabs (Frankfurt, Germany). Phusion polymerase was from Finnzymes (Espoo, Finland), desoxynucleotides from Rapidozym (Berlin, Germany). All enzymes were used according to the manufacturers' recommendations, applying the provided buffer solutions. Oligonucleotides were ordered from Thermo Scientific (Ulm, Germany). Full-length genes were synthesized by Geneart (Regensburg, Germany), with optimized E. coli codon usage, and delivered in the company's standard plasmids. Porcine heart lactate dehydrogenase (LDH) was bought from Serve (Heidelberg, Germany), Aspergillus niger glucose oxidase and horseradish peroxidase from Sigma-Aldrich (Munich, Germany). All chemicals were, unless otherwise stated, purchased in analytical grade from Sigma-Aldrich, Carl Roth GmbH (Karlsruhe, Germany), Serve Electrophoresis GmbH and Merck KGaA (Darmstadt, Germany).
E. coli BL21(DE3) (F-ompT hsdSB (rB-mB-) gal dcm (DE3)) was purchased from Novagen (Nottingham, UK), E. coli XL1-Blue (recA1 endA1 gyrA96 thi-1 hsdR17 supE44 relA1 lac [F′ proAB laclqZΔM15 Tn10 (Tetr)]) from Stratagene (Waldbronn, Germany). pET28a-DNA was provided by Novagen.
Plasmids pCBR, pCBRHisN and pCBRHisC were constructed on the basis of pET28a (Novagen). DNA-sequences (see Table 6) for the corresponding new multiple cloning sites were synthesized (Geneart, Regensburg, Germany) and cloned into pET28a via Xbal/BamHI (pCBR), NdeI/EcoRI (pCBRHisN) or XbaI/Bpu1102I (pCBRHisC), thereby replacing the existing multiple cloning site with a new restriction site containing a BfuAI- and a BsaI-sequence and, in case of pCBR and pCBRHisN, a stop codon.
The three new vectors allow the simultaneous cloning of any gene using the same restriction sites, enabling the user to express the respective gene without or with an N- or C-terminal His-tag, whereby a stop codon must not be attached at the 3′-end of the gene. Vector-DNA was first restricted with Bsal, followed by blunt end generation with Klenow fragment. Afterwards, the linearized plasmids were digested with BfuAI, generating a 5-overhang. Genes were amplified using the Geneart vectors as templates and the corresponding oligonucleotides (Table 7).
S. solfataricus Glucose
S. solfataricus Glucose
S. solfataricus
S. solfataricus
S. acidocaldarius KDG
S. acidocaldarius KDG
T. acidophilum
T. acidophilum
M. ruber Ketolacid
M. ruber Ketolacid
After PCR, DNA fragments were digested with Bsal, 3′-phosphorylated (T4 kinase) and subsequently ligated into the appropriate vectors. In some cases, phosphorylation could be replaced by digestion using Psil. Plasmids were transformed into E. coli as described elsewhere (Molecular Cloning: A Laboratory Manual Cold Spring Harbor Laboratory Press Cold Spring Habor, N.Y., 1989). Sequence analysis was performed by GATC Biotech (Konstanz, Germany). pET28a-HisN-LIKdcA was cloned according to Gocke et al. (Adv. Synth. Catal. 2007, 349, 1425-1435)
Enzyme expression was performed using E. coli BL21(DE3) or BL21 Rosetta(DE3)-pLysS as host strains, either in shaking flask cultures or in a 10 L Biostat Cplus bioreactor (Sartorius Stedim, Goettingen, Germany). All media were supplemented with 30-50 μg/ml kanamycin. GDH and DHAD were expressed in LB medium, acetolactate synthase in TB medium, After inoculation cells were grown at 37° C. to an optical density at 600 nm of 0.6, induced with 1 mM IPTG and the temperature lowered to 16-20° C. for 16-24 h expression. KDGA and ALDH were expressed according to the fed-batch cultivation method of Neubauer et al. (Biotechnol. Bioeng. 1995, 47, 139-146) at 37° C. After inoculation cells were grown for 24 h and induced with 1 mM IPTG. Enzyme expression was performed for 24 or 30 h, respectively. KDC expression was performed for 22 h at 30° C. in batch mode using Zyp-5052 (Protein Expression Purif. 2005, 41, 207-234) as a medium. KARI was expressed in a batch fermentation using TB medium. Cells were grown at 37° C. to an optical density of 5.2 and induced by the addition of 0.5 mM IPTG. Afterwards, expression was performed for 24 h at 20° C.
All protein purification steps were performed using an ÄKTA UPC-900 FPLC-system (GE Healthcare, Freiburg, Germany), equipped with HiTrap FF-, HiPrep 26/10 Desalting- and HiTrap Q-Sepharose FF-columns (GE Healthcare). Cell lysates were prepared with a Basic-Z Cell Disruptor (Constant Systems, Northants, UK), cell debris was removed by centrifugation at 35.000 g and 4° C. for 30 min (Sorvall RC6+, SS-34 rotor, Thermo Scientific). For lyophilization an Alpha 2-4 LD Plus freeze dryer (Martin Christ GmbH, Osterode am Harz, Germany) was used. GDH and DHAD were purified by heat denaturation (30 minutes at 70° C., respectively). GDH was subsequently freeze-dried (SpeedVac Plus, Thermo Scientific), DHAD concentrated using a stirred Amicon cell (Milipore, Darmstadt, Germany) and either stored at −80° C. or directly applied to experiments. KDGA, ALDH and KDC were purified as previously described (Biochem. J. 2007, 403, 421-430; FEBS Lett. 2006, 580, 1198-1204; Adv. Synth. Catal. 2007, 349, 1425-1435) and stored as lyophilisates. ALS and KARI were purified via IMAC using 25 or 50 mM HEPES, pH 7. Elution was achieved with 500 mM imidazol. Enzymes were desalted and stored as a liquid stock (ALS) or lyophilisate (KARI).
Protein concentration was determined with the Roti-Nanoquant reagent (Carl Roth GmbH) according to the manufacturer's recommendations using bovine serum albumin as a standard.
Protein samples were analyzed as described by Laemmli (Nature 1970, 227, 680-685) using a Mini-Protean system from Biorad (Munich, Germany).
All photometrical enzyme assays were performed in microtiter plate format using a Thermo Scientific Multiskan or Varioskan photometer. When necessary, reaction mixtures were incubated in a waterbath (Julabo, Seelbach, Germany) for accurate temperature control. Buffers were prepared according to Stoll (Guide to Protein Purification, Vol. 466, Elsevier Academic Press Inc, San Diego, 2009, pp. 43-56), adjusting the pH to the corresponding temperature. Reactions using NAD or NADH as coenzymes were followed at 340 nm (molar extinction coefficient NADH=6.22 L mmol-1 cm-1) Reaction mixtures (total volume 0.2 mL) contained 1 mM D-Glyceraldehyde and 2 mM NAD and appropriate amounts of enzyme in 100 mM HEPES pH 7. (J. Mol. Catal. B-Enzym. 2009, 61, 30-35). One unit of enzyme activity is defined as the amount of enzyme necessary to convert 1 μmol substrate per minute. In addition to the standard reaction conditions described below, enzyme activity was tested under reaction conditions (100 mM HEPES, pH 7, 2.5 mM MgCl2, 0.1 mM thiamine pyrophosphate) prior to alcohol synthesis experiments.
GDH activity: GDH activity was assayed at 50° C. by oxidizing D-glucose to gluconate, whereby the coenzyme NAD is reduced to NADH. Assay mixture contained 50 mM HEPES (pH 7), 2 mM NAD and 50 mM D-glucose. (J. Biol. Chem. 2006, 281, 14796-14804)
DHAD activity: DHAD activity was measured by an indirect assay. The assay mixture containing DHAD, 20 mM substrate and 100 mM HEPES (pH 7) was incubated at 50° C. Afterwards the conversion of glycerate to pyruvate, gluconate to 2-keto-3-deoxygluconate or 2,3-dihydroxy-isovalerate to 2-ketoisovalerate, respectively, was determined via HPLC as described below.
KDGA activity: KDGA activity was followed in cleavage direction at 50° C. Reaction mixture contained 50 mM HEPES (pH 7), 0.1 mM thiamine pyrophosphate, 2.5 mM MgCl2, 20 U PDC and 10 mM KDG. KDG cleavage was followed by HPLC as described below.
ALDH activity: ALDH activity was assayed at 50° C. by oxidizing D-glyceraldeyde to glycerate, whereby the coenzyme NAD is reduced to NADH. Assay mixture contained 50 mM HEPES (pH 7), 2.5 mM MgCl2, 2 mM NAD and 1 mM glyceraldehyde. (FEBS Lett. 2006, 580, 1198-1204)
ALS activity: ALS activity was determined by following pyruvate consumption at 50° C. Reaction mixtures contained 25 mM HEPES (pH 7), 0.1 mM thiamine-pyrophosphate, 2.5 mM MgCl2, 15 mM sodium pyruvate. Pyruvate concentration in the samples was determined via lactate dehydrogenase as described elsewhere. (Biochem. J. 2007, 403, 421-430)
KARL activity: KARI activity was assayed by following the NADH consumption connected to the conversion of acetolactate to 2,3-dihydroxy isovalerate at 50° C. Assay mixture contained 5 mM acetolactate, 0.3 mM NADH, 10 mM MgCl2 and 50 mM HEPES, pH 7.
KDC activity: KDC activity was assayed by following the decarboxylation of 2-ketoisovalerate to isobutyraldehyde at 50° C. and 340 nm. Assay mixture contained 50 mM HEPES (pH 7), 0.1 mM thiamine-pyrophosphate, 2.5 mM MgCl2 and 60 mM 2-ketoisovalerate. Decarboxylation rate was calculated using the molar extinction coefficient of 2-ketoisovalerate (s=0.017 L mmol-1 cm-1). (J. Mol. Catal. B-Enzym. 2009, 61, 30-35)
ADH activity: ADH activity was determined by following the NADH-dependent reduction of isobutyraldehyde to isobutanol at 50° C. Assay mixture contained 10 mM HEPES (pH 7.2), 5 mM isobutanol and 0.3 mM NADH.
Glucose analysis: Glucose oxidase was used for the quantification of glucose. Assay mixture contained 20 mM potassium phosphate (pH 6), 0.75 mM 2,2-azino-bis(3-ethylbenzthiazoline)-6-sulfonic acid (ABTS), 2 U glucose oxidase and 0.1 U peroxidase. After the addition of samples the reaction mixture was incubated for 30 min at 30° C. and the extinction at 418 and 480 nm measured. Assay calibration was performed using defined glucose standard solutions. (J. Olin. Chem. Olin. Biochem. 1979, 17, 1-7)
Isobutyraldehyde and isobutanol or acetaldehyde and ethanol were quantified by GC-FID using a Thermo Scientific Trace GC Ultra, equipped with a flame ionization detector and a Headspace Tri Plus autosampler. Alcohol and aldehyde compounds were separated by a StabilWax column (30 m, 0.25 mm internal diameter, 0.25 μm film thickness; Restek, Bellefonte, USA), whereby helium (0.8 or 1.2 ml min-1) was used as the carrier gas. The oven temperature was programmed to be held at 50° C. for 2 min, raised with a gradient 10° C. min-1 to 150° C. and held for 1 min. Injector and detector were kept at 200° C. Samples were incubated prior to injection at 40° C. for 15 min. Injection was done in the split mode with a flow of 10 ml min-1, injecting 700 μl using headspace mode.
Gluconate, 2-keto-3-deoxygluconate, pyruvate, glycerate, 2,3-dihydroxylsovalerate and 2-ketoisovalerate were separated and quantified by HPLC, using an Ultimate-3000 HPLC system (Dionex, Idstein, Germany), equipped with autosampler and a diode-array detector. Chromatographic separation of gluconate, 2-keto-3-deoxygluconate, pyruvate and glycerate was achieved on a Metrosep A Supp10-250/40 column (250 mm, particle size 4.6 μm; Metrohm, Filderstadt, Germany) at 65° C. by isocratic elution with 12 mM ammonium bicarbonate (pH 10), followed by a washing step with 30 mM sodium carbonate (pH 10.4). Mobile phase flow was adjusted to 0.2 ml min-1. 2,3-dihydroxyisovalerate and 2-ketoisovalerate were separated using a Nucleogel Sugar 810H column (300 mm, 7.8 mm internal diameter; Macherey-Nagel, Dueren, Germany) at 60° C. by isocratic elution with 3 mM H2SO4 (pH 2.2). Mobile phase flow was adjusted to 0.6 ml min-1. Sample volume was 10 μl in each case. System calibration was performed using external standards of each of the abovementioned intermediates. Samples were prepared by filtration (10 kDa MWCO, modified PES; VWR, Darmstadt, Germany) and diluted.
All reactions were set up in 20 ml GC vials. Reaction mixtures contained 100 mM HEPES (pH 7 at 50° C.), 0.1 mM thiamine-pyrophosphate, 2.5 mM MgCl2, 25 mM D-glucose and 5 mM NAD. Enzymes were added as follows: GDH: 6 U, DHAD: 20 U for ethanol synthesis and 30 U for isobutanol synthesis, all other enzymes: 10 U. Control reactions were performed either without enzymes or without D-glucose. Reaction mixtures were placed in a water bath at 50° C. and gently stirred at 100 rpm.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12181085.7 | Aug 2012 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/067291 | 8/20/2013 | WO | 00 |
