Pergamon 0957-4166(95)00157-3 Tetrahedron: Asymmetry Vol. 6, No. 6, pp. 1257-1260. 1995 Elsevier Science Ltd Printed in Great Britain 0957-4166/95 $9.50+0.00 New Hydantoinases from Thermophilic Microorganisms - Synthesis of Enantiomerically Pure D-Amino acids Oliver Keil, Manfred P. Schneider* and J. Peter l~sor** *FB 9-Bergische Universitt~t-GH-Wuppertal, D-42097 Wuppertal, Germany **Boehringer Mannheim GmbH, Nonnenwald 2, D-82372 Penzberg, Germany Abstract: A series of 14 D-a-amino acids were prepared in high chemical and optical yields from the corresponding racemic hydantoins by employing two novel hydantoinases from thermophilic microorganisms. Enantiomerically pure D-s-amino acids are important building blocks for a variety of biologically active pharmaceuticals like peptides 1, semisynthetic 13-1actam antibiotics 2 and ACE inhibitors 3. On a technical scale several enzymatic processes for their preparation are known 4. Of particular interest is the highly efficient preparation of D-amino acids via the enantioselective hydrolytic ring opening of hydantoins (5-monosubstituted imidazolidin-2,4-dions) by D-specific hydantoinases EC 3.5.2.2 (Fig. 1). R O R O 0 0 L-hydamoin D-hydantoin R chemical or ----COOH enzymatical R D-Hydantoinase transformation l, ~ ~--COOH H20 ~.__ NH 2 H2N o N-earbamoyl-D-amino acid D-amino acid Fig. 1. Enzymatic preparation of enantiomerically pure D-amino acids from racemic 5-monosubstituted hydantoins The thus produced N-carbamoyl-D-amino acids are transformed chemically or enzymatically into the corresponding amino acids with complete retention of the configuration. Due to the spontaneous racemization of the hydantoins under the reaction conditions (pH> 8), the method allows a quantitative conversion of the racemic starting materials into the desired D-amino acids. In contrast to other hydrolases, D-hydantoinases were not readily available, also most of the described hydantoinases are instable at elevated temperatures. In the present paper we report the use of two new, commercially available and thermally highly stable hydantoinases from thermophilic microorganisms. The required racemic 5-substituted hydantoins (+)-1-14 were synthesized according to the procedure of Bueherer and Bergs 5 by simple condensation of the corresponding aliphatie or aromatic aldehydes with KCN and (NH4)2CO 3 . 1257
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`1258 O. KEIL et al. In typical experiments, 7 mmol of (+)-1-14 were added to 70 ml of a thermostated (50°C) buffer solution [0.1 M glycine/NaOH, pH 8.5,1 mM Mn 2+] under nitrogen, followed by the addition of 70 lal D-HYD 1 or 14 pl D-HYD 2 solution. The beginning enzymatic hydrolysis is indicated by a decrease of pH which is kept constant at pH 8.5 by continuous addition of 1 N NaOH-solution using an autotitrator. The specific activity of the enzymes 6 was calculated from the initial rate of the transformation. After completion the obtained reaction mixtures were filtered over Celite ®, concentrated (vacuum) to 10 -20 ml and then acidified with HC1 to pH 2-3. With the exception of the products resulting from (+)-9 and (+)-10, the N-carbamoyl-D-amino acids were isolated by filtration after ice cooling, washed with a small amount of water and then dried. They were further converted into the corresponding D-amino acids by diazotation. For this 1-3 mmol of the corresponding carbamoylate were dissolved in 50 ml 3.5 N HCI to which at 0°C an equimolar quantity of NaNO2, dissolved in 10 ml H20 was added under vigorous stirring. The resulting D- amino acids were purified by ion exchange chromatography (25 g DOWEX ® 50). Their enantiomeric purities were determined via derivatisation with 2,3,4,6-tetra-O-benzoyl-13-D-glucopyranosylisothiocyanate BGIT 7 followed by HPLC analysis [LiChrospher 100 RP-18; acetonitrile/0.1% trifluoroacetic acid] of the resulting diastereomeric thiourethanes. The optical purities of D-23-27 were determined by HPLC using a chiral column [Chrownpack ® CR(+), Daicel]. The results, also showing the broad substrate tolerance, are summarized in the table. Using (+)-2 and (+)-3 as model substrates the dependence of the specific activities from (a) buffer, (b) pH (Fig.2) and temperature (Fig.3) were determined for both enzymes. Glycine/NaOH buffer proved to be superior to TRIS/I-ICI, a pH 8.5 resulting in the highest activity for both D-HYD 1 (85 U/mg protein) and D-HYD 2 (200 U/mg protein) (Fig.2). A ten fold increase was observed in changing the temperature from 37°C to 80°C (Fig.3). Both enzymes display a remarkable temperature stability and can be used for several hours at temperatures as high as 70°C, while at T>80°C increasing loss of activity is observed. A similar stability was reported previously only in one case (hydantoinase from Agrobacterium sp. IP-6718). 200.00 i5000 m 10000
`0.00 t J [ 700 8.00 9.00 1000 II 00 pH D-Hydaatoinase-2 3oo.oo ................. D-Hydantoinu¢-I 1 TRIS/HCI 0.I N ~ GIy/NaOH 0.1 N 0". I I ' I ' T 20.00 40.00 60.00 80.00 100,00 temperature [~l Fig.2: Effect of the pH on the specific activity in 0.1 N glycine/NaOH and 0.1 N TRIS/HCI buffer using DL-n-butylhydantoin (+)-2 as substrate Fig.3: Effect of temperature on the specific activity of hydantoinase catalysed hydrolysis of (:t=)-5-(J3-phenylethyl)hydantoin (+)-3 with D-HYD 2
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`New hydantoinases 1259 Table Specific activity, reaction time, isolated yield, optical rotation of the N-carbamoyl-amino acids and the enantiomeric purities of the isolated amino acids Substratc Enzyme Specific Reaction Isolated [OqD 20 cnanfiomcric (5-substituted hydantoin) Activity time [h] yield of (c=l in excess of the [Ulm~] carbamoylate MeOH) amino acid [%] DL-5-Methyl (+)-1 D-HYD-1 133 22.8 71% -4.0 Alanine 94 D-HYD-2 769 7.6 73% -1.8 D-15 34 DL-5-n-Butyl 0:)-2 D-HYD-1 82 23.3 67% -14.0 Norleucine >99 D-HYD-2 127 21.5 70% -11.5 D-16 83 DL-5 -(]3-Phenylethyl) D-HYD-1 63 41.2 53% -19.0 Homophenyl- >99 (+)-3 DoHYD-2 70 27.2 77% - 17.8 alanine D-17 >99 DL-5-Isobutyl (+)-4 D-HYD-1 94 19.2 89% +5.8 a) Leucine >99 D-HYD-2 156 18.0 93% +4.7 a) D-18 >99 DL-5-Benzyl (+)-5 D-HYD-1 47 39.2 67% -46.7 Phenylalanine >99 D-HYD-2 5 29.2 12% -43.7 D-19 >99 DL-5-Isopropyl (+)-6 D-HYD-1 61 45.0 66% -15.6 Valine >99 D-HYD-2 65 43.7 71% -16.0 D-20 >99 DL-5-sec-Butyl (-1-)-7 D-HYD-1 29 24.2 65% -23.1 Isoleucine >99, >99 b) D-I-IYD-2 23 41.0 71% -23.0 D-21a,b b) >99, >99 b) DL-5-Methylthioethyl D-HYD-1 71 12.7 81% -1.7 Methionine >99 (+)-8 D-I--IYD-2 358 14.1 75% -1.4 D-22 99 DL-5-Hydroxymethyl D-I--IYD- 1 27 16.6 Serine 98 (+)-9 D-I--IYD-2 156 10.2 D-23 97 DL-5 -(2-Hydroxyethyl), D-HYD-1 61 21.0 Threonine >98 (+)-10 (allo-free) D-HYD-2 78 21.2 D-24a,b b) >98 DL-5-(2-Thienyl) D-I--IYD- 1 612 0.5 42% -110.4 (2-Thienyl)glycine 97 (±)-11 D-HYD-2 1351 0.3 50% -111.9 D-25 96 DL-5-Phenyl D-HYD-1 620 0.75 95% -143.3 Phenylglycine 96 (+)-12 D-HYD-2 1221 0.6 90% -152.5 D-26 >99 DL-5-(p-Hydroxyphenyl) D-HYD-1 302 5.8 65% -172.0 (p-Hydroxyphenyl) >99 (±)-13 D-HYD-2 52 11.0 78% -167.5 ~lycine D-27 >99 DL-5-Methyl-5-phenyl D-HYD-1 <1 72 3 2-Phenylalanine >99 (5:)-14 D-HYD-2 <1 72 7 D-28 97 a) c=0.5 in MeOH b) allo-configurafion; enantiomeric purity of a/to-threonJne not determined D-HYD 2 usually displays higher activities with aliphatic substituted hydantoins (_+)-1-7 than D-HYD 1. Considerable activity differences were observed for compounds (_+)-1,2,4 which have two or more protons at C(1) next to the ring system, and also for the aliphatic heterosubstituted hydantoins (_+)-8- 10. The specific activity of both D-hydantoinases seems to decrease with increasing steric bulk in (_+)-1-7. In the hydrolysis of the aromatic substituted compounds (_+)-11-13 remarkably high specific activities were observed, most likely due to the high acidity of the proton on C(5) of the hydantoin ring.
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`1260 O. KE1L et al. In contrast to this, both hydantoinases display only a very small activity towards the 5,5-disubstituted hydantoin (+)-14. The resulting N-carbamoyl-D-amino acids were isolated in high yields (>75%) and further converted into the corresponding D-amino acids, which were obtained with excellent enantiomeric purities. D-HYD 1 usually leads to higher enantiomeric purities for aliphatic amino acids D-15-24 as compared to D- HYD 2, which in turn is the preferred enzyme for the produced D-amino acids with aromatic substituents D- 25-27. Both enzymes display no diastereoselectivity, hydrolysis of(+)-7 and (+)-10 produces diastereomeric product mixtures. In summary, the described thermostable D-hydantoinases catalyze the highly enantioselective hydrolysis of hydantoins with great structural variety. They thus allow the synthesis of numerous a-D-amino acids in high chemical and optical yields. REFERENCES 1. J. Kamphuis, W.H.J. Boesten, B. Kaptein, H.F.M. Hermes, T. Sonke, Q.B. Broxtermann, W.J.J. van den Tweel, H.E. Schoemaker; "Chirality in industry", Eds. A.N. Collins, G.N. Sheldrake, J. Crosby, Wiley & Sons Ltd. (1992) 187-208 2. W. Diarckheimer, J. Blumbach, R. Lattrell, K.H. Scheunemann, Angew. Chem. 97 (1985) 183-205 3. A.A. Patchett, Nature (London) 288 (1980) 280 4. R.M. Williams, "Synthesis of Optically Active ct-Amino Acids", Organic Chemistry Series, Eds. J.E. Baldwin, FRS & P D Magnus, FRS, Vol.7, Pergamon Press (1989) 5. H.T. Bucherer, W. Steiner, J. prakt. Chem. 140 (1934) 291-316 6. D-HYD 1 (14 nag protein/ml); specific activity 2.4 kU/ml; [1% (w/v) 5-phenylhydantoin, 1 mM MnC12, pH 8.5, 37°C] and D-HYD-2 (55 mg protein/ml); specific activity 12 kU/ml [1 °A (w/v) 5-phenylhydantoin, 1 mM MnCI2, pH 8.5, 37°C]. Both enzymes are originally derived from thermophilic microorganisms and are expressed in Escherichia coll. They are commercially available from Boehringer Mannheim. 7. M. Lobell, M.P. Schneider, J Chromatogr. 633 (1993) 287-294 8. S. Runser, E. Ohleyer, Biotechnol. Left. 12(4) (1990) 259-264 (Received in UK 11 May 1995)
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