Process and composition for preparing D-aspartic acid

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for preparing D-aspartic acid from racemic aspartic acid. More particularly, the invention relates to a method of preparing D-aspartic acid and .beta.-alanine by enzymatic decarboxylation of racemic aspartic acid.
2. Description of the Related Art
The L-aspartate-.alpha.-decarboxylase enzyme catalyzes the production of .beta.-alanine from L-aspartic acid. Nakano et al. demonstrated the activity of the enzyme in the form of a crude extract from E. coli, B. Nakano, Y. et al., J. Biochem. Vol. 70, pages 327-334 (1971). Williamson et al. further investigated the activity of the enzyme in E. coli purified to apparent homogeneity. Williamson, J. et al., J. Biol. Chem., Vol. 254(16), pages 8074-8082 (August 1979). These disclosures do not demonstrate the production of the enzyme in any useful amount beyond experimental curiosity. As discussed more fully below, this invention utilizes a composition having between 100 and 2000 times the L-aspartate-.alpha.-decarboxylase activity of that disclosed in the Nakano et al. and Williamson et al. references.
U.S. Pat. No. 5,019,509 to Rozzell discloses methods and compositions for the production of L-alanine and derivatives thereof. Rozzell, however, discloses the gene encoding the aspartate-.beta.-decarboxylase and its use to produce L-alanine. U.S. Pat. Nos. 5,552,317 and 5,552,318 to Houng, et. al. discloses methods for preparing optically active amino acids and esters thereof using lipase from wheat germ or Candida lipolytica. In contrast, the process of this invention is not a common resolution such as the Houng et al. methods, where it is common that resolution of amino acids is actually performed on derivatives of the amino acids, thus requiring additional chemical steps. Most often, racemic esters are treated with esterases/lipases or N-acetyl-amino acids are treated with acylases.
Other resolution procedures are known that use an amidase acting on racemic amino acid amides where the enzyme is just specific for one isomer. See, U.S. Pat. No. 4,880,737. However, the present invention provides a much more simple process because there are no additional chemical modifications to prepare the substrate.
This invention provides a facile route to produce two highly useful compounds from an abundant source, D,L-aspartate. D-aspartic acid and its derivatives have been widely documented in pharmaceuticals. These uses include, but are not limited to, the inhibition of arginiosuccinate synthetase activity which is useful to prevent or treat sepsis or cytokine-induced systemic hypotension and as immunosuppressive agents. Other uses of D-aspartic acid include taste modifying compositions for foods and a new sweetener, Alitame.TM., under development by Pfizer. In addition, .beta.-alanine is used in the synthesis of pantothenic acid, which is necessary for coenzyme A synthesis. .beta.-alanine is also known to be used as a chiral synthon for the synthesis of .alpha.-substituted .beta.-amino acids. Derivatives of .beta.-alanine have been used as a buffer in the electroplating industry.
SUMMARY OF THE INVENTION
A racemic mixture of aspartic acid isomers (D and L) are incubated with a microorganism containing the panD gene encoding the enzyme L-aspartate-.alpha.-decarboxylase (E.C. 4.1.1.11) or a suitable source of enzyme. The enzyme catalyzes the following reaction: ##STR1##
The enzyme stereospecifically decarboxylates L-aspartate to .beta.-alanine and CO.sub.2 whereas D-aspartate remains unreacted. Because this reaction is essentially irreversible with the release of CO.sub.2, a high conversion of product is realized. This reaction can be carried out under mild conditions, i.e., room temperature and neutral pH, thus making this process very environmentally friendly and highly desirable.

BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 shows the production of Plasmid pIF309.
FIG. 2 shows the bioconversion of D,L-aspartate substrate solution to D-aspartate and .beta.-alanine, as demonstrated by the reduction in the amount of L-aspartate remaining in solution, catalyzed by the L-aspartate-.alpha.-decarboxylase enzyme preparation of the invention.
FIG. 3 shows the activity of fresh cells of the L-aspartate-.alpha.-decarboxylase enzyme preparation on the supernatant fluid left after completion of the reaction demonstrated in FIG. 2.
FIG. 4 shows the activity of previously used L-aspartate-.alpha.-decarboxylase enzyme on a fresh D,L-aspartate substrate solution.
FIG. 5 shows the activity of L-aspartate-.alpha.-decarboxylase enzyme different loads, 50 g/l and 100 g/l, on a 1.0M solution of D,L-aspartate.
FIG. 6 shows the activity of L-aspartate-.alpha.-decarboxylase enzyme on NH.sub.4 OH-neutralized D,L-aspartate substrate.
FIG. 7 compares the activity of L-aspartate-.alpha.-decarboxylase enzyme on various amino acid substrates, D,L-aspartate, D,L-glutamate and D,L-alanine.
FIG. 8 compares the results of bioconversion of D,L-aspartate at temperatures of 37.degree. C., 45.degree. C. and 50.degree. C.

DETAILED DESCRIPTION
The present invention provides a process for preparing D-aspartic acid from D,L-aspartic acid which comprises incubation of a solution of D,L-aspartic acid or a salt thereof with a composition comprising cells of a microorganism or extracts thereof, wherein the composition has an L-aspartate-.alpha.-decarboxylase activity of greater than 100 .mu.mol L-aspartate used per hour per gram of cells, under appropriate conditions to produce D-aspartic acid and .beta.-alanine.
The enzyme L-aspartate-.alpha.-decarboxylase, also referred to herein as L-aspartate-1-decarboxylase, aspartate-1-decarboxylase or E.C. 4.1.1.11, is used in the present invention to prepare D-aspartic acid from racemic aspartic aid according to the following reaction scheme: ##STR2##
The substrate and reaction products are alternatively referred to herein by their acid and salt forms. For example, the substrate D,L-aspartic acid is also referred to herein as D,L-aspartate. Those of ordinary skill in the art recognize that the form, and therefore the nomenclature, of an acid differs depending on the pH of the solution in which it is found. As will be discussed more fully below, the process of this invention is useful over a wide pH range which provides for the use of both acid and salt forms of substrate. For the purposes of the present disclosure, the acid and salt-based nomenclature used herein is intended to be synonymous.
The panD gene of E. coli K12, encoding the enzyme was cloned into E. coli on a multi copy plasmid vector which is used in a whole cell biotransformation system to produce .beta.-alanine and D-aspartic acid. "Whole cell biotransformation system" refers to the use of cells containing the enzyme which have not been extracted to purify the enzyme but which may be ultrafiltered or otherwise separated from the fermentation media and added directly to the solution to catalyze the reaction to produce .beta.-alanine and D-aspartic acid. Other "Systems" can be used to practice the claimed invention without departing from the scope of the invention. Such methods include, but are not limited to, the use of immobilized cell fractions, immobilized whole enzyme or enzyme fraction. Methods of forming cell fractions and purified enzyme and enzyme fractions are well known to those of ordinary skill in the art. Preferred methods of immobilization are discussed below.
The enzyme L-aspartate-.alpha.-decarboxylase, also referred to herein as L-aspartate-1-decarboxylase or EC 4.1.1.11, catalyzes the removal of the alpha carboxyl group from L-aspartate to produce .beta.-alanine.
The present invention also provides a composition comprising cells of a microorganism or extracts thereof, wherein the composition has an L-aspartate-.alpha.-decarboxylase activity of greater than 100 .mu.mol L-aspartate used per hour per gram of cells.
In one embodiment the composition comprises cells or extracts of cells of a microorganism transformed with a vector comprising the panD gene which encodes for the L-aspartate-.alpha.-decarboxylase enzyme. For example the vector can be a plasmid, such as pIF309 described herein. The vector is then used to transform a microorganism such as a bacterium, for example E. coli. In a preferred embodiment the composition comprises cells or extracts of cells of E. coli NS3291 ATCC Accession NO. 98675.
The composition of this invention has an L-aspartate-.alpha.-decarboxylase activity of greater than 100 .mu.mol L-aspartate used per hour per gram of cells, and preferably, between about 100 and 2000 .mu.mol L-aspartate used per hour per gram of cells. The enzyme activity of the claimed compositions is calculated using the activity demonstrated in Williamson et al. noted above as a reference point. In that reference, the crude extract showed an activity of 6700 U from 400 grams of cells where 1 U was equal to 1 nmol of CO.sub.2 liberated per minute at 42.degree. C. Thus, 16.75 U/g cells is equal to 16.75 nmol CO.sub.2 liberated per minute per gram of cells. Assuming complete conversion of one mol of L-aspartate to one mol of .beta.-alanine and one mol of CO.sub.2, 16.75 nmol L-aspartate used per minute per gram of cells would result in approximately 1 .mu.mol L-aspartate utilized per hour per gram cells. As demonstrated in the Examples herein, the composition of the present invention provides an activity of greater than 100 times the activity of the enzyme preparations known in the prior art. From Table III of the Williamson paper the specific activity of the purified enzyme was 650 U/mg protein. Assuming that 50% of the total cell protein has this activity, the present process and composition provide L-aspartate-.alpha.-decarboxylase activity up to 2000 .mu.mol L-aspartate used per hour per gram of cells. In a preferred embodiment the composition has an L-aspartate-.alpha.-decarboxylase activity of between about 100 and 2000 .mu.mol L-aspartate used per hour per gram of cells. In another embodiment, the composition has an L-aspartate-.alpha.-decarboxylase activity of between about 100 and 1000 .mu.mol L-aspartate used per hour per gram of cells.
Cloning of the L-aspartate-.alpha.-decarboxylase gene can be accomplished by the isolation of a DNA fragment encoding the polypeptide carrying the L-aspartate-.alpha.-decarboxylase activity from a suitable donor microorganism, and incorporating the DNA fragment into a suitable vector, known to those of ordinary skill in the art. Suitable donor microorganisms include all microorganisms carrying a gene encoding a polypeptide which catalyzes the .alpha.-decarboxylation of L-aspartate to .beta.-alanine. Examples include, but are not limited to, bacteria from the genus Escherichia coli ("E. coli"), including E. coli B, E. coli K12, E. coli NIHJ and E. coli Tennessee; Proteus vulgaris; Bacterium cadaveris; Azotobacter vinelandii; Rhizobium leguminosarum; R. trifolii; or bacterium from the genus Bacillus.
Suitable cloning vectors useful in the practice of this invention normally contain an origin of replication and a selectable marker to maintain the stability of the vector in the host cell and to facilitate the identification of transformants. A description of some methods and materials useful in the cloning of the L-aspartate-.alpha.-decarboxylase gene can be found in Molecular Cloning: A Laboratory Manual �T. Maniatis, E. Fritsch, and J. Sambrook, Cold Spring Harbor Laboratory (1982)! and references therein, which are hereby incorporated by reference.
The DNA fragment encoding the polypeptide displaying L-aspartate-.alpha.-decarboxylase activity is then expressed. The general strategy for expression of the L-aspartate-.alpha.-decarboxylase gene involves the ligation of the DNA fragment encoding the polypeptide displaying L-aspartate-.alpha.-decarboxylase activity into an expression vector suitable for the desired host cell, many examples of which are well known in the art. Suitable expression vectors are normally characterized by the presence of an origin of replication, a promoter or other transcription enhancing sequences, a ribosome binding site, and a transcriptional termination sequence. The expression vector may also include a gene conferring resistance to an antibiotic as a selectable marker; however, plasmids containing any other gene encoding a protein required by the host microorganism for growth and viability may be used as a selectable marker, if desired. Examples of promoters useful in recombinant DNA constructions include, but are not limited to, tryptophan, alkaline phosphatase, beta-galactosidase, beta-lactamase, and P.sub.L promoter systems; and hybrid promoter systems composed of components or structures from two or more known promoter systems such as tac. While these are the most common promoters used in bacterial host strains, other strains and microbial species amenable to genetic manipulation and other promoters useful in those host strains may also be used.
After the ligation, the resultant vector of this invention contains the L-aspartate-.alpha.-decarboxylase gene in operative association with a promoter such that host cells transformed with the vector are capable of directing the production of L-aspartate-.alpha.-decarboxylase. Suitable hosts for transforming with the vector include any microorganism, for example a bacterium, which allows for the expression of the gene encoding L-aspartate-.alpha.-decarboxylase. Examples include, but are not limited to, bacteria of the genera Escherichia, Bacillus, Klebsiella, Pseudomonas, Salmonella, Proteus, Azotobacter, and Rhizobium.
In a preferred embodiment of this invention, the host strain for expression of the L-aspartate-.alpha.-decarboxylase gene is any strain of E. coli, in particular E. coli W3110, and the promoter system is the modified pheA promoter described in U.S. Pat. No. 5,120,837 (Fotheringham et al.) the contents of which are incorporated herein by reference. Expression of the gene is thus accomplished by ligation of the gene into a vector for gene expression in the chosen host strain. In an especially preferred embodiment of this invention, the vector used for the expression of the L-aspartate-.alpha.-decarboxylase gene in E. coli is pBR322 or a plasmid which is derived from pBR322, such as pIF309. The plasmid pBR322 contains genes encoding ampicillin and tetracycline resistance, allowing the facile identification of transformed cells.
If the preferred embodiment pIF309 is used, expression of the enzyme is constitutive. If a heat sensitive promoter is used, for example the .lambda.cI857 repressor/promoter system, induction of the synthesis of L-aspartate-.alpha.-decarboxylase can be accomplished by a temperature shift, for example, raising the fermentation temperature from 30.degree. C. to 40.degree. C. L-aspartate-.alpha.-decarboxylase enzyme would be expected to be produced at levels of from about 5% to about 40% of the total protein in the cell, thus yielding whole cells containing an enzyme composition with very high L-aspartate-.alpha.-decarboxylase activity.
It is not critical to this invention whether the expression of the gene results in enzyme produced intracellularly or extracellularly, since the desired product may be conveniently recovered and, if desired, further purified in either case by methods known to those of ordinary skill in the art. In a separately preferred embodiment, the gene can be incorporated into a thermostable microorganism to identify mutations in the gene which enhance the thermal stability of the enzyme thus allowing for the operation of this process at elevated temperatures.
In the practice of this invention, the cells producing elevated levels of L-aspartate-.alpha.-decarboxylase enzyme may be contacted with a solution containing D,L-aspartic acid or a salt thereof, with the resulting conversion of at least a portion of the L-aspartic acid in the reaction mixture to .beta.-alanine, liberating CO.sub.2 and leaving D-aspartic acid. The cells may be permeabilized to facilitate diffusion of the substrates and products into and out of the cells. This permeabilization can be accomplished by treating cells with a low concentration of a surfactant, including but not limited to Tween 80.RTM., Triton X-100.RTM., Nonidet P40.RTM., cetylpyridinium chloride, deoxycholic acid, hexadecyltrimethylammonium bromide or benzalkonium chloride. Further, organic solvents, including but not limited to N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol or acetone at low concentrations have also been used to increase permeabilization. L-aspartate-.alpha.-decarboxylase may also be added to the D,L-aspartate-containing reaction mixture in the form of cell extracts containing crude, partially purified, or purified L-aspartate-.alpha.-decarboxylase enzyme. Cell extracts are prepared by methods known to those skilled in the art which provide for cell disruption and recovery of the enzyme. Cell disruption, can be accomplished by mechanical or non-mechanical means. Most often, for bacterial suspensions mechanical devices such as a French pressure cell, ultrasonication, bead mill or Manton-Gaulin homogenizer is used with the specifics of the method known to those of ordinary skill in the art. See, Scopes, R. K. "Protein Purification", (1982) (Springer-Verlag, N.Y.). The reaction using the cell extract is then carried out in similar fashion to the whole cell method discussed above.
The L-aspartate-.alpha.-decarboxylase-containing cells, or extracts thereof or purified enzyme or enzyme fractions, may also be immobilized, if desired. Immobilization methods which may be used in the practice of this invention include well-known methods such as entrapment in polymeric gels, covalent attachment, crosslinking, adsorption, and encapsulation. Some examples of these methods are described by A. M. Klibanov in Science, 219:722-727 (1983) and the references therein and in Methods in Enzymology (1976), Volume 44, (K. Mosbach editor) which are hereby incorporated by reference. In one method of immobilization disclosed in U.S. Pat. No. 5,019,509, a support material containing at least 20% by weight of silica or alumina is contacted with aminoalkyl compound such as an aminoalkyl silane, polyethyleneimine, or a polyalkylamine, followed by activation with glutaraldehyde. The enzyme-containing solution is then contacted with the activated support to produce an immobilized enzyme composition having L-aspartate-.alpha.-decarboxylase activity. Other immobilization supports useful in the practice of this invention include, but are not limited to, porous glass and porous ceramics, bentonite, diatomaceous earth, charcoal Sepharose.RTM. and Sepharose.RTM. derivatives, cellulose and cellulose derivatives, polyacrylamide and polyacrylamide derivatives, polyazetidine, alginate, carrageenan, and Chromosorb.RTM.. Sepharose.RTM. (Pharmacia Fine Chemicals, Uppsala Sweden) is a bead-formed gel prepared from agarose. The manufacturer's product literature reports that in its natural state, agarose occurs as part of the complex mixture of charged and neutral polysaccharides referred to as agar. The agarose used to make Sepharose.RTM. is obtained by a purification process which removes the charged polysaccharides to give a gel with only a very small number of residual charged groups. Those of ordinary skill in the art will appreciate that a number of other materials suitable for the immobilization of cells or extracts derived therefrom may also be useful for the immobilization of the L-aspartate-.alpha.-decarboxylase of the present invention. These supports can be activated, if desired, by techniques well-known in the art.
The reaction to produce D-aspartic acid utilizing cells containing L-aspartate-.alpha.-decarboxylase, or compositions comprising extracts derived from said cells, is carried out by contacting a solution containing D,L-aspartate with the L-aspartate-.alpha.-decarboxylase under conditions permitting the conversion of at least a portion of the L-aspartate to .beta.-alanine. The concentration of D,L-aspartate is not critical in this reaction and concentrations as high as 2M would be anticipated to be useful. In a preferred embodiment of the process the cells or extracts of cells contact an aqueous solution D,L-aspartate having a concentration of 0.5M. In a separately preferred embodiment, the solution of D,L-aspartate has a concentration of 1.0M.
The enzymatic reactions of this invention are carried out at temperatures in the range of from about 4.degree. C. to about 70.degree. C., and preferably at temperatures ranging from about 20.degree. C. to about 60.degree. C. In a preferred embodiment, the temperature of the reaction is ambient temperature. For the purposes of this invention, "ambient temperature" means that no methods or devices are employed to either raise or lower the temperature of the reaction vessel from the temperature of the reaction vessel at rest in its natural environment. The optimal pH for the reaction ranges from about 2.0 to about 12.0, and more preferably from about 4.0 to about 9.0, with a pH of 7.0 being most preferred.
The D-aspartate and .beta.-alanine may be separated and recovered by methods known in the art, or the D-aspartate/.beta.-alanine mixture may be recovered or used directly as a mixture. The L-aspartate-.alpha.-decarboxylase compositions of this invention are further applicable to the production of .beta.-alanine or .beta.-alanine/D-aspartate mixtures containing radioactive or non-radioactive isotopic labels. Such products are readily produced by using an appropriately labelled aspartate precursor, and/or by carrying out the reaction in the presence of isotopically labelled solvent. Examples of isotopic labels which can be incorporated into the .beta.-alanine and/or D-aspartate products include, but are not limited to, .sup.14 C, .sup.13 C, .sup.13 N, .sup.15 N, .sup.2 H, .sup.3 H, .sup.17 O, and .sup.18 O.
The invention will now be further illustrated by the following examples, which are not intended, and should not be interpreted, to limit the scope of the invention which is defined in the claims which follow thereafter.
EXAMPLE 1
Preparation of Plasmid pIF309
The pad gene encoding L-aspartate-1-decarboxylase was isolated from the chromosome of E. coli K12 by PCR using the following methods.
A. Chromosomal DNA Preparation
E. coli strain W3110 was obtained from the Coli Genetic Stock Center (Yale University, Newhaven, Conn.) The strain was recovered as directed in the supplier's instructions and cultured overnight with shaking in a 50 ml volume of Luria Broth at 37.degree. C. in a 500 ml flask. A 10 ml aliquot of culture was centrifuged at 10,000.times.G for 4 minutes. The supernatant fluid was discarded and the pellet resuspended in 1 ml of a solution containing 50 mM Tris/HCl at pH 8.0, 10 mM EDTA and 100 .mu.g/ml RNase A (Sigma). This was incubated for 10 minutes at room temperature. To this was added 2 ml of solution containing 0.4% (w/v) sodium dodecyl sulfate ("SDS") and 100 .mu.g/ml protease K (Sigma). This was incubated for 20 minutes at 37.degree. C. To this was added sodium acetate at pH 5.2 to a final concentration of 300 mM. This solution was then extracted 3 times with an equal volume of phenol at 37.degree. C. and precipitated with 2.5 volumes of ethanol. The DNA was then removed with a sterile loop and redissolved in 400 .mu.l of 300 mM sodium acetate at pH 5.2. The DNA was then re-precipitated in 2.5 volumes of ethanol, removed as before, dried and taken up in 500 .mu.l of 10 mM Tris pH 8.0 and 1 mM EDTA. The final concentration was approximately 200 .mu.g/ml.
B. Amplification of panD by PCR
Amplification of the E. coli panD gene by PCR was accomplished using a 0.2 ml MicroAmp.TM. reaction tube (Perkin Elmer, Norwalk, Conn.) to which was added 100 ng E. coli chromosomal DNA prepared as above, 2 .mu.l each of dATP, dGTP, dCTP and dTTP (10 mM each), 10 .mu.l of buffer comprising 15 mM MgCl.sub.2, 500 mM KCl, 100 mM Tris pH 8.3, and 0.01% (w/v) gelatin, 5 U of Taq DNA polymerase (AmpliTaq.TM.) and 5 .mu.l of a 10 nmol/ml solution of each of the following oligonucleotide primers which were synthesized according to known methods using an Applied Biosystems 380B DNA synthesizer.
(MB1682)
5' CGC GGA TCC ACT ATG ATT CGC ACG ATG CTG CAG GGC 3'
(MG1683)
5' CAG CGT GCA TGC TCA AGC AAC CTG TAC CGG AAT CGC 3'
The reaction was carried out in a Perkin Elmer 9600.TM. PCR Thermal Cycler (Perkin Elmer). Amplification was carried out by pre-heating at 94.degree. C. for 3 minutes followed by 30 cycles of denaturation at 94.degree. C. for 30 seconds, annealing at 50.degree. C. for 30 seconds and extension at 72.degree. C. for 2 minutes. The reaction mixture was then stored at 4.degree. C.
C. Cloning of panD
The amplified panD gene was cloned on a plasmid vector derived from plasmid pBR322. This plasmid was named pIF309. The fragment containing the panD gene was digested to completion with the restriction enzymes BamHI and SphI and ligated between the unique BamHI and SphI sites of the vector which had been similarly cleaved. Between the HindIII and BamHI sites was inserted a synthetic DNA fragment which carried a modified version of the E. coli K12 pheA promoter region. This fragment was completely synthesized on an Applied Biosystems 380B DNA synthesizer and derived from that characterized in co-owned U.S. Pat. No. 5,120,837 to Fotheringham et al. such that the sequence was as follows:
HindIII
AAGCTTTTTTGTTGACAGCGTGAAAACAGTACGGGTATAATACT AAAGTCACAAGGAGGATCC
BamHI
Plasmid pIF309 is shown in FIG. 1. Restriction digests were carried out using enzymes supplied by New England Biolabs and used according to the manufacturer's specifications (NEB, Beverly, Mass.). Ligation of DNA fragments was carried out using a ligation kit supplied by Takara Biochemicals (Panvera Corp., Madison Wis.) according to the manufacturer's specifications.
Expression of panD was determined by biological assay of L-aspartate-1-decarboxylase activity in extracts prepared from cultures of strain NS3291 grown as described below.
EXAMPLE 2
Production and Fermentation of E. coli Strain NS3291
Plasmid pIF309 was used to transform E. coli strain W3110, using conditions described in U.S. Pat. No. 5,354,672 the contents of which are hereby incorporated by reference, resulting in the strain NS3291. Strain NS3291 was deposited with the American Type Culture Collection (ATCC) 12301 Parklawn Drive, Rockville, Md. 20852, USA. The strain is designated ATCC Accession No. 98675.
The strain NS3291 was inoculated into a 2800 ml Fernbach flask containing 1 L of the following growth medium:
______________________________________Potassium phosphate (dibasic) 13 gPotassium phosphate (monobasic) 2 gAmmonium phosphate 4 gFerric ammonium citrate 0.24 gYeast extract 2 gMagnesium sulfate (7 .multidot. H.sub.2 O) 1 gWater 930 mlAfter Sterilization the following were added:Glucose (50% W/V stock) 70 mlAmpicillin 0.2 g______________________________________
The flask was incubated on a shaker incubator at 37.degree. C. with agitation. The strain was grown to 800-1100 Klett units and used to inoculate the fermentor. The fermentor was a Biolafite 78-100 (St. Germain-en Laye, France) 20 L. The fermentor was operated under the following conditions:
______________________________________Agitation 500 rpmTemperature 37.degree. C.Back Pressure 0.7 BarpH (adjusted with NH.sub.3) 7.2Aeration 1 vvmSet Volume 10 LInoculation 100 mlRun Time 16 hr______________________________________
The fermentation medium was comprised of the following, per liter of medium unless otherwise noted:
______________________________________Magnesium Sulfate (7 .multidot. H.sub.2 O) 5.35 gFerric ammonium citrate 0.13 gPotassium phosphate (dibasic) 4.6 gManganese sulfate 0.023 gPotassium iodide 0.74 mgNickel sulfate 0.74 mgAntifoam (Mazur Mazu DF204) 0.4 mlYeast extract 5 gL-aspartic acid 10 gTap water 10 L______________________________________
Prior to inoculation, glucose was added to a concentration of 25 g/l. After the initial glucose was completely depleted, glucose was fed at a variable rate to achieve less than 1 g/l for the remaining time for a total of 290 grams of glucose in 16 hours. The final volume of the tank was 12.2 L. The fermentation reached 10,352 Klett units and a dry cell weight of 24.2 g/l. The fermentor broth was cooled to below 10.degree. C. and ultrafiltered using a hollow fiber cartridge (500,000 Mol Wt. cutoff) to a dry cell weight of 158 g/l. The concentrated cell cream was stored at 4.degree. C.
This strain was used in the following Examples.
EXAMPLE 3
Bioconversion of D/L-aspartate with L-aspartate-.alpha.-decarboxylase
66.5 g (0.5M) D/L-aspartate (Sigma, A-9006) was weighed into a 2 L beaker and the pH adjusted to 7.0 with 10N NaOH. The volume was brought to 963 ml with deionized H.sub.2 O. 137 ml of cells (NS 3291, described above) were added at a packed cell volume ("PCV") of 7.31 g/10 ml cells resulting in a final volume of 1.1 L. Samples (approximately 1 ml each) were removed over time, centrifuged and the supernatant fluid was analyzed by HPLC for D- and L-aspartic acid content.
In all of the Examples herein, D- and L-aspartic acid isomers were quantified by HPLC using the following precolumn derivatization method. Aspartic acid was converted into diastereomers using ortho-phthalaldehyde and N-acetyl-L-cysteine. Separation of the diastereomers was achieved on a Supelcosil.TM. LC-18DB, 3.mu., 150.times.4.6 mm column using a mobile phase of A: 20% methanol/80% triethylamine phosphate and B: methanol, and a step gradient as follows:
______________________________________Time (min) A (%) B (%)______________________________________ 6.5 100 0 7.0 40 6011.5 40 6012.0 100 0______________________________________
The derivatization was carried out automatically using a Hewlett-Packard 1090M chromatograph and detection was at 338 nm (bandwidth 4) and a flow rate of 1.0 ml/minute. Retention times were 4.19 minutes and 4.54 minutes for D-aspartic acid and L-aspartic acid, respectively.
Samples from bioconversions were centrifuged to remove cells in a microcentrifuge and the supernatant fluid diluted appropriately such that the value determined from the HPLC fell within the linear region of the standard curve. These dilutions were usually between 1:100 and 1:200, made with deionized water.
Throughout the Examples the terms "D-Asp" and "L-Asp" are used as short hand notation for the respective isomers of aspartic acid.
TABLE 1______________________________________Time D-Asp D-Asp L-Asp L-Asp(hr) (mg/ml) (%) (mg/ml) (%)______________________________________0 0.0000 29.300 100.00 28.870 100.001 5.5000 29.320 100.07 12.050 41.7392 24.500 29.250 99.829 0.17000 0.588853 26.500 27.740 94.676 0.12000 0.415664 32.500 28.090 95.870 0.060000 0.207835 47.500 28.060 95.768 0.030000 0.103916 52.000 28.390 96.894 0.020000 0.0692767 68.000 25.520 87.099 0.43000 1.4894______________________________________
The data in Table 1 are presented graphically in FIG. 2. At approximately 24 hours, the percent L-aspartate remaining is essentially negligible, about 0.17 mg/ml. The enzyme activity of the composition used in the bioconversion is calculated as follows: ##EQU1## which equals 114.6 .mu.mol L-aspartic acid used per hour per gram cells, or more than 100 times the enzyme activity known in the prior art.
The enantiomeric enrichment at this time is calculated as follows: ##EQU2##
Following the whole-cell bioconversion reaction the cells were removed by centrifugation or ultrafiltration. The pH of the cell-free bioconversion mixture was adjusted with mineral acid, usually sulfuric acid, to a pH where D-aspartic acid precipitates as a solid and the .beta.-alanine remains in solution, typically between pH 2.0 and 2.5. The solid was collected by filtration or other means and washed with cold water to afford D-aspartic acid. Recovery was between 50% and 90%.
EXAMPLE 4
Re-Treatment of Supernatant Fluid with Fresh Cells
In this Example the supernatant fluid from Example 3 was re-treated with fresh cells. The reaction from Example 3, stopped at 68 hours, was centrifuged for 45 minutes in a GS-3 rotor at 7000 rpm. The supernatant fluid, 990 ml, was separated from the cells. 963 ml of this supernatant fluid was mixed with 137 ml fresh cells and placed in a 2 L fermentor. Bioconversion took place with mixing at 300 rpm, without automatic pH control and at a temperature of 37.degree. C. Samples, approximately 1 ml, were removed over time and analyzed as in Example 3.
TABLE 2______________________________________Time D-Asp D-Asp L-Asp L-Asp(hr) (mg/ml) (%) (mg/ml) (%)______________________________________0 0.0000 21.450 100.00 0.43000 100.001 2.7500 21.350 99.534 0.11000 25.5812 13.500 21.190 98.788 0.090000 20.9303 15.000 20.770 96.830 0.080000 18.6054 19.500 21.480 100.14 0.090000 20.9305 23.500 20.880 97.343 0.070000 16.2796 39.000 19.080 88.951 0.10000 23.2567 47.000 17.280 80.559 0.16000 37.2098 51.750 17.010 79.301 0.23000 53.4889 68.500 14.900 69.464 0.55000 127.91______________________________________
The data in Table 2 are represented graphically in FIG. 3. Although the concentration of L-aspartic acid at t=0 was not high (0.43 mg/ml), it was essentially reduced to nothing (0.07 mg/ml) by 23.5 hours. It is believed that the increases shown for concentration of L-aspartic acid after 23.5 hours is due to evaporation, although concomitant synthesis from extraneous enzymes could not be ruled out. At the end of the reaction (68.5 hours) there was only 0.55 mg/ml L-aspartic acid.
EXAMPLE 5
Re-use of Cells
Cells that had already been used in Example 3 were put in fresh substrate to determine if they were still active.
Fresh D/L-aspartate substrate was prepared as previously described in Example 3. The cells from Example 3 were resuspended in deionized H.sub.2 O and made up to 100 ml. The pH of the substrate (900 ml, 0.5M D/L-Asp) was adjusted to 7.0 with NaOH. After placing in the 2 L fermentor, the cells (100 ml) were added. There was no pH control and deionized H.sub.2 O was used for substrate. Samples were removed over time, centrifuged and the supernatant fluid diluted appropriately for both D- and L-aspartic acid determination. Temperature was set at 37.degree. C.
TABLE 3______________________________________Time D-Asp D-Asp L-Asp L-Asp(hr) (mg/ml) (%) (mg/ml) (%)______________________________________0 0.0000 30.850 100.00 24.700 100.001 6.2500 30.820 99.903 17.130 69.3522 15.250 29.870 96.823 15.620 63.2393 25.750 28.250 91.572 10.870 44.0084 28.000 27.980 90.697 9.5300 38.5835 44.250 25.000 81.037 4.4200 17.8956 63.500 21.760 70.535 0.98000 3.96767 65.000 20.340 65.932 0.95000 3.8462______________________________________
The data in Table 3 are also graphically represented in FIG. 4. These results show that even after one cycle, the L-aspartate-.alpha.-decarboxylase still remains active, thus indicating that this biocatalyst can be recycled.
EXAMPLE 6
D/L-aspartate as Substrate for Various Enzyme Cell Loads
1.0M D/L-aspartate was tested as a substrate for two enzyme cell loads, 100 g/l cells, referred to here as reaction "A", and 50 g/l cells, referred to here as reaction "B".
133.1 g of D/L-aspartic acid was dissolved in approximately 600 ml deionized H.sub.2 O and pH was adjusted to 7.0 with NaOH, using 100 ml of 10N NaOH and a small amount of 1.0N NaOH. The substrate for reaction A was brought to 963 ml with deionized H.sub.2 O. The reaction was started with 137 ml cells that were stored in the cold room. For reaction B, the substrate was brought to a volume of 981.5 ml. The reaction was started with 65.0 ml cells.
Both reactions were carried out in 2 L fermentors at pH 7.0 without constant adjustments and at 37.degree. C. Samples were removed over time, centrifuged and the supernatant fluid was diluted and submitted for D- and L-aspartic acid HPLC analysis. Table 4 shows the results of reaction A and Table 5 shows the results of reaction B.
TABLE 4______________________________________Time D-Asp D-Asp L-Asp L-Asp(hr) (mg/ml) (%) (mg/ml) (%)______________________________________0 0.0000 53.600 100.00 54.850 100.001 2.0000 50.680 94.552 49.340 89.9542 4.5000 44.920 83.806 50.710 92.4523 7.0000 42.260 78.843 50.230 91.5774 22.500 35.680 66.567 54.490 99.3445 24.000 34.420 64.216 50.780 92.5806 31.000 32.660 60.933 51.450 93.8017 37.500 27.950 52.146 53.850 98.1778 47.000 20.710 38.638 52.370 95.4799 50.500 18.490 34.496 51.080 93.12710 53.800 13.730 25.616 46.500 84.77711 58.000 13.410 25.019 50.040 91.23112 70.000 6.1300 11.437 52.400 95.53313 72.000 5.3900 10.056 50.430 91.942______________________________________
TABLE 5______________________________________Time D-Asp D-Asp L-Asp L-Asp(hr) (mg/ml) (%) (mg/ml) (%)______________________________________0 0.0000 56.730 100.00 57.440 100.001 2.0000 52.420 .92.403 53.770 93.6112 4.5000 51.340 90.499 52.310 91.0693 7.0000 50.470 88.965 53.860 93.7674 22.500 44.260 78.019 53.650 93.4025 24.000 44.280 78.054 53.460 93.0716 31.000 40.590 71.549 51.740 90.0777 37.500 38.650 68.130 55.000 95.7528 47.000 35.420 62.436 53.410 92.9849 50.500 34.920 61.555 54.310 94.55110 53.800 31.780 56.020 55.060 95.85711 58.000 30.860 54.398 53.420 93.00112 70.000 24.300 42.834 54.060 94.11613 72.000 26.830 47.294 54.230 94.412______________________________________
The data in Tables 4 and 5 are graphically presented in FIG. 5. Results show that the conversion rates for 100 g/l are faster than with 50 g/l. However, the rate at a substrate concentration of 1.0M D/L-aspartate is slower that at 0.5M. (See Example 3 and FIG. 2). Also, at 1.0M substrate, after 72.0 hours, the L-aspartate was reduced to 5.39 mg/ml from a starting concentration of 53.6 mg/ml, a 90% reduction.
EXAMPLE 7
L-aspartate-.alpha.-decarboxylase NH.sub.4 OH Neutralized Substrate
This Example shows the effect of neutralizing the substrate, D/L-aspartate, with NH.sub.4 OH instead of NaOH as done in the prior examples. Cells were stored frozen at -80.degree. C. 133.1 g (1 mol) D/L-aspartic acid was dissolved in approximately 600 ml deionized H.sub.2 O and adjusted to a pH of about 7 with concentrated NH.sub.4 OH. The final volume of substrate was brought to 963 ml to which was added 137 ml cells, freshly thawed. The mixture was placed in the 2 L fermentor and the temperature adjusted to 37.degree. C. No automatic pH control was used, however, the pH was monitored as samples were removed and the pH adjusted with 50% NH.sub.4 OH. Samples, approximately 1 ml, were removed periodically, centrifuged, and the supernatant fluid was stored frozen prior to diluting and HPLC analysis.
TABLE 6______________________________________Time D-Asp D-Asp L-Asp L-Asp(hr) (mg/ml) (%) (mg/ml) (%)______________________________________0 0.0000 55.260 100.00 55.740 100.001 3.0000 54.670 98.932 53.380 95.7662 6.0000 49.100 88.853 56.370 101.133 9.0000 47.810 86.518 56.680 101.694 19.000 43.490 78.701 59.270 106.335 23.500 39.120 70.793 54.930 98.5476 28.000 36.290 65.671 52.620 94.4037 32.500 33.540 60.695 50.900 91.3178 43.000 30.370 54.958 53.520 96.0179 49.000 21.580 39.052 51.i00 91.676______________________________________
The data in Table 6 is presented graphically in FIG. 6. The rate of L-Asp utilization is shown to be slower with the NH.sub.4.sup.+ salt as compared to the Na.sup.+ salt.
EXAMPLE 8
Activity of L-aspartate-.alpha.-decarboxylase towards D/L-glutamate and D/L-alanine
This Example shows the activity of L-aspartate-.alpha.-decarboxylase towards two potential amino acid substrates, D/L-glutamate and D/L-alanine. D/L-aspartate was run as a control.
Substrates were prepared by dissolving solid amino acid racemate in approximately 75 ml deionized H.sub.2 O and adjusting the pH to 7.0 with 10N NaOH. The volume was then brought to 100 ml with deionized H.sub.2 O. The cells used were stored frozen for about 1 month and then thawed and stored in a refrigerator for about 1 month prior to running the experiments.
The reaction was run the same as described in the prior Examples. Samples of supernatant fluid, approximately 0.20 ml, were removed over time, centrifuged and stored frozen before analysis by HPLC as described in the prior Examples. Table 7 shows the results of the reactions.
TABLE 7__________________________________________________________________________Time L-Glu D-Glu L-Ala D-Ala L-Glu D-Glu L-Ala D-Ala L-Asp D-Asp L-Asp D-Asp(hr) (mg/ml) (mg/ml) (mg/ml) (mg/ml) (%) (%) (%) (%) (mg/ml) (mg/ml) (%) (%)__________________________________________________________________________0 0 29.350 29.440 19.320 19.190 100.00 100.00 100.00 100.00 26.810 29.890 100.0 100.01 3.5 28.400 29.010 18.960 18.590 96.763 98.539 98.137 96.873 14.050 29.850 52.406 99.8662 8.5 28.740 29.520 18.730 18.370 97.922 100.27 96.946 95.727 12.660 29.900 47.221 100.033 24 28.200 29.350 18.320 17.930 96.082 99.694 94.824 93.434 9.4700 29.740 35.323 99.4984 55 28.000 29.840 17.830 17.580 95.400 101.36 92.288 91.610 0.3300 30.120 1.2309 100.77__________________________________________________________________________
The data in Table 7 are presented graphically in FIG. 7. The results show the specificity of the claimed method and enzyme preparation in that all isomers of substrates used except L-aspartic acid are stable up to 55 hours in the presence of the enzyme, indicating that L-aspartic acid is the only substrate for the enzyme. L-aspartic acid, as demonstrated throughout the specification, decreased to 1.23% of starting material.
EXAMPLE 9
Temperature Study
This Example shows the activity of the L-aspartate-.alpha.-decarboxylase reaction at 37.degree. C., 45.degree. C. and 50.degree. C.
133.1 g (1 mol) of D/L-aspartic acid was dissolved in deionized H.sub.2 O and the pH was adjusted to 7.0 with 10N NaOH, approximately 100 ml. Cells, freshly thawed from -80.degree. C., were added to achieve a final concentration of 100 g/l. Three 2 L fermentors were each charged with 963 ml of substrate solution and 137 ml of cells and then heated to 37.degree. C. and 45.degree. C. while mixing at 300 rpm or heated to 50.degree. C. while mixing at 450 rpm. Samples, approximately 1 ml, were removed over time and pH adjusted to 7.0 with either NaOH or HCl. After centrifugation to remove cells, the supernatant fluid was analyzed for both D- and L-aspartic acid.
Table 8 shows the results of the reaction at 37.degree. C., Table 9 shows the results at 45.degree. C. and Table 10 shows the results at 50.degree. C.
TABLE 8______________________________________Time D-Asp D-Asp L-Asp L-Asp(hr) (mg/ml) (%) (mg/ml) (%)______________________________________0 0.0000 55.000 100.00 53.700 100.001 2.5000 54.310 98.745 48.940 91.1362 4.0000 54.440 98.982 47.600 88.6413 6.0000 54.470 99.036 46.170 85.9784 8.0000 54.760 99.564 44.810 83.4455 10.000 55.840 101.53 42.120 78.4366 12.000 56.760 103.20 42.120 78.4627 20.000 55.460 100.84 35.960 66.9658 24.000 55.390 100.71 34.840 64.8799 29.000 53.990 98.164 28.040 54.07810 34.000 53.440 97.164 28.040 54.07811 44.000 52.960 96.291 21.670 40.35412 48.000 52.940 96.255 18.440 34.33913 68.000 49.940 90.800 5.9700 11.11714 92.000 50.670 92.127 0.91500 1.7039______________________________________
TABLE 9______________________________________Time D-Asp D-Asp L-Asp L-Asp(hr) (mg/ml) (%) (mg/ml) (%)______________________________________0 0.000 55.280 100.00 54.410 100.001 2.5000 54.820 99.168 50.i00 92.0792 4.0000 53.960 97.612 47.110 86.5833 6.0000 53.460 96.708 43.130 79.2694 8.0000 54.040 97.757 39.310 72.2485 10.000 54.870 99.258 36.800 67.6356 12.000 54.060 97.793 34.540 63.4817 20.000 52.360 94.718 24.690 45.3788 24.000 51.310 92.818 23.210 42.6589 29.000 53.830 97.377 21.700 39.88210 34.000 52.880 95.658 19.310 35.49011 44.000 53.640 97.033 15.800 29.03912 48.000 52.730 95.387 16.700 30.82213 68.000 53.560 96.889 16.050 29.49814 92.000 53.560 96.889 13.370 24.573______________________________________
TABLE 10______________________________________Time D-Asp D-Asp L-Asp L-Asp(hr) (mg/ml) (%) (mg/ml) (%)______________________________________0 0.000 54.89 48.47 100.0 100.01 1.000 54.30 52.78 98.93 108.92 3.000 58.01 40.61 105.7 83.783 5.0000 58.16 36.97 106.0 73.634 7.000 58.51 35.69 106.6 73.635 9.000 58.88 33.99 107.3 70.136 13.50 59.62 34.19 108.4 70.547 23.00 56.71 31.98 103.3 65.988 25.00 59.85 31.86 109.0 65.739 27.00 56.87 26.89 103.6 55.4810 32.00 57.30 25.85 104.4 53.3311 37.50 58.48 27.54 106.5 58.8212 47.00 57.93 25.58 10s.s 52.7713 52.00 58.74 27.28 107.0 56.2814 57.00 65.56 32.42 119.4 66.8915 71.00 57.17 25.52 104.2 52.6516 77.00 57.85 25.86 105.4 53.3517 81.00 57.41 25.54 104.6 52.6918 99.75 59.36 26.14 108.1 53.93______________________________________
The data from Tables 8, 9 and 10 are presented graphically together in FIG. 8. These results show that at all three temperatures, the D-isomer is stable. At 37.degree. C., the L-isomer is almost completely consumed by 99.75 hours. At 45.degree. C. and 50.degree. C. the initial rate is shown to be increased with the higher temperature, but it appears that the enzyme becomes unstable and inactivates after an extended period of time. Although moderate temperatures are preferred for the claimed process and composition, the reaction rate can be increased by raising the reaction temperature.

Process and composition for preparing D-aspartic acid

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Non-Patent Literature Citations (6)

Entry
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