Diversity and versatility of microbial hydantoin-transforming enzymes

Gene identification and enzymatic characterization of the initial enzyme in pyrimidine oxidative metabolism, uracil-thymine dehydrogenase

Uracil-thymine dehydrogenase (UTDH), which catalyzes the irreversible oxidation of uracil to barbituric acid in oxidative pyrimidine metabolism, was purified from Rhodococcus erythropolis JCM 3132. The finding of unusual stabilizing conditions (pH 11, in the presence of NADP+ or NADPH) enabled the enzyme purification. The purified enzyme was a heteromer consisting of three different subunits. The enzyme catalyzed oxidation of uracil to barbituric acid with artificial electron acceptors such as methylene blue, phenazine methosulfate, benzoquinone, and α-naphthoquinone; however, NAD+, NADP+, flavin adenine dinucleotide, and flavin mononucleotide did not serve as electron acceptors. The enzyme acted not only on uracil and thymine but also on 5-halogen-substituted uracil and hydroxypyrimidine (pyrimidone), while dihydropyrimidine, which is an intermediate in reductive pyrimidine metabolism, and purine did not serve as substrates. The activity of UTDH was enhanced by cerium ions, and this activation was observed with all combinations of substrates and electron acceptors.

Attempt to simultaneously generate three chiral centers in 4-hydroxyisoleucine with microbial carbonyl reductases

A panel of microorganisms was screened for selective reduction ability towards a racemic mixture of prochiral 2-amino-3-methyl-4-ketopentanoate (rac-AMKP). Several of the microorganisms tested produced greater than 0.5mM 4-hydroxyisoleucine (HIL) from rac-AMKP, and the stereoselectivity of HIL formation was found to depend on the taxonomic category to which the microorganism belonged. The enzymes responsible for the AMKP-reducing activity, ApAR and FsAR, were identified from two of these microorganisms, Aureobasidium pullulans NBRC 4466 and Fusarium solani TG-2, respectively. Three AMKP reducing enzymes, ApAR, FsAR, and the previously reported BtHILDH, were reacted with rac-AMKP, and each enzyme selectively produced a specific composition of HIL stereoisomers. The enzymes appeared to have different characteristics in recognition of the stereostructure of the substrate AMKP and in control of the 4-hydroxyl group configuration in the HIL product.

Cloning, overexpression, and characterization of a high enantioselective nitrilase from Sphingomonas wittichii RW1 for asymmetric synthesis of (R)-phenylglycine

In this study, a high (R)-enantioselective nitrilase gene from Sphingomonas wittichii RW1 was cloned and overexpressed in Escherichia coli BL21 (DE3). The recombinant nitrilase was purified to homogeneity with a molecular weight of 40 kDa. The pH and temperature optima were shown to be pH 8.0 and 40 °C, respectively. The purified nitrilase was most active toward succinonitrile, approximately 30-fold higher than that for phenylglycinonitrile. Using the E. coli BL21/ReSWRW1 whole cells as biocatalysts, the kinetic resolution for asymmetric synthesis of (R)-phenylglycine was investigated at pH 6.0. A yield of 46 % was obtained with 95 % enantiomeric excess (ee), which made it a promising biocatalyst for synthesis of (R)-phenylglycine.

New enzymatic method of chiral amino acid synthesis by dynamic kinetic resolution of amino acid amides: use of stereoselective amino acid amidases in the presence of alpha-amino-epsilon-caprolactam racemase

D- and L-amino acids were produced from L- and D-amino acid amides by D-aminopeptidase from Ochrobactrum anthropi C1-38 and L-amino acid amidase from Pseudomonas azotoformans IAM 1603, respectively, in the presence of alpha-amino-epsilon-caprolactam racemase from Achromobacter obae as the catalyst by dynamic kinetic resolution of amino acid amides.

The hydantoin transport protein from Microbacterium liquefaciens

The gene hyuP from Microbacterium liquefaciens AJ 3912 with an added His6 tag was cloned into the expression plasmid pTTQ18 in an Escherichia coli host strain. The transformed E. coli showed transport of radioisotope-labeled 5-substituted hydantoins with apparent K(m) values in the micromolar range. This activity exhibited a pH optimum of 6.6 and was inhibited by dinitrophenol, indicating the requirement of energy for the transport system. 5-Indolyl methyl hydantoin and 5-benzyl hydantoin were the preferred substrates, with selectivity for a hydrophobic substituent in position 5 of hydantoin and for the l isomer over the d isomer. Hydantoins with less hydrophobic substituents, cytosine, thiamine, uracil, allantoin, adenine, and guanine, were not effective ligands. The His-tagged hydantoin transport protein was located in the inner membrane fraction, from which it was solubilized and purified and its identity was authenticated.

Purification and characterization of a novel L-2-amino-Delta2-thiazoline-4-carboxylic acid hydrolase from Pseudomonas sp. strain ON-4a expressed in E. coli

L-2-Amino-Delta2-thiazoline-4-carboxylic acid hydrolase (ATC hydrolase) was purified and characterized from the crude extract of Escherichia coli, in which the gene for ATC hydrolase of Pseudomonas sp. strain ON-4a was expressed. The results of SDS-polyacrylamide gel electrophoresis and gel filtration on Sephacryl S-200 suggested that the ATC hydrolase was a tetrameric enzyme consisted of identical 25-kDa subunits. The optimum pH and temperature of the enzyme activity were pH 7.0 and 30-35 degrees C, respectively. The enzyme did not require divalent cations for the expression of the activity, and Cu2+ and Mn2+ ions strongly inhibited the enzyme activity. An inhibition experiment by diethylpyrocarbonic acid, 2-hydroxy-5-nitrobenzyl bromide, and N-bromosuccinimide suggested that tryptophan, cysteine, or/and histidine residues may be involved in the catalytic site of this enzyme. The enzyme was strictly specific for the L-form of D,L-ATC and exhibited high activity for the hydrolysis of L-ATC with the values of Km (0.35 mM) and Vmax (69.0 U/mg protein). This enzyme could not cleave the ring structure of derivatives of thiazole, thiazoline, and thiazolidine tested, except for D,L- and L-ATC. These results show that the ATC hydrolase is a novel enzyme cleaving the carbon-sulfur bond in a ring structure of L-ATC to produce N-carbamoyl-L-cysteine.

The crystal structures of dihydropyrimidinases reaffirm the close relationship between cyclic amidohydrolases and explain their substrate specificity

In eukaryotes, dihydropyrimidinase catalyzes the second step of the reductive pyrimidine degradation, the reversible hydrolytic ring opening of dihydropyrimidines. Here we describe the three-dimensional structures of dihydropyrimidinase from two eukaryotes, the yeast Saccharomyces kluyveri and the slime mold Dictyostelium discoideum, determined and refined to 2.4 and 2.05 angstroms, respectively. Both enzymes have a (beta/alpha)8-barrel structural core embedding the catalytic di-zinc center, which is accompanied by a smaller beta-sandwich domain. Despite loop-forming insertions in the sequence of the yeast enzyme, the overall structures and architectures of the active sites of the dihydropyrimidinases are strikingly similar to each other, as well as to those of hydantoinases, dihydroorotases, and other members of the amidohydrolase superfamily of enzymes. However, formation of the physiologically relevant tetramer shows subtle but nonetheless significant differences. The extension of one of the sheets of the beta-sandwich domain across a subunit-subunit interface in yeast dihydropyrimidinase underlines its closer evolutionary relationship to hydantoinases, whereas the slime mold enzyme shows higher similarity to the noncatalytic collapsin-response mediator proteins involved in neuron development. Catalysis is expected to follow a dihydroorotase-like mechanism but in the opposite direction and with a different substrate. Complexes with dihydrouracil and N-carbamyl-beta-alanine obtained for the yeast dihydropyrimidinase reveal the mode of substrate and product binding and allow conclusions about what determines substrate specificity, stereoselectivity, and the reaction direction among cyclic amidohydrolases.

Molecular cloning and expression of the hyu genes from Microbacterium liquefaciens AJ 3912, responsible for the conversion of 5-substituted hydantoins to alpha-amino acids, in Escherichia coli

A DNA fragment from Microbacterium liquefaciens AJ 3912, containing the genes responsible for the conversion of 5-substituted-hydantoins to alpha-amino acids, was cloned in Escherichia coli and sequenced. Seven open reading frames (hyuP, hyuA, hyuH, hyuC, ORF1, ORF2, and ORF3) were identified on the 7.5 kb fragment. The deduced amino acid sequence encoded by the hyuA gene included the N-terminal amino acid sequence of the hydantoin racemase from M. liquefaciens AJ 3912. The hyuA, hyuH, and hyuC genes were heterologously expressed in E. coli; their presence corresponded with the detection of hydantoin racemase, hydantoinase, and N-carbamoyl alpha-amino acid amido hydrolase enzymatic activities respectively. The deduced amino acid sequences of hyuP were similar to those of the allantoin (5-ureido-hydantoin) permease from Saccharomyces cerevisiae, suggesting that hyuP protein might function as a hydantoin transporter.

Phylogenetic Analysis and Biochemical Characterization of a Thermostable Dihydropyrimidinase from Alkaliphilic Bacillus sp. TS-23

Two degenerate primers established from the alignment of highly conserved amino acid sequences of bacterial dihydropyrimidinases (DHPs) were used to amplify a 330-bp gene fragment from the genomic DNA of Bacillus sp. TS-23 and the amplified DNA was successfully used as a probe to clone a dhp gene from the strain. The open reading frame of the gene consisted of 1422 bp and was deduced to contain 472 amino acids with a molecular mass of 52 kDa. The deduced amino acid sequence exhibited greater than 45% identity with that of prokaryotic D-hydantoinases and eukaryotic DHPs. Phylogenetic analysis showed that Bacillus sp. TS-23 DHP is grouped together with Bacillus stearothermophilus D-hydantoinase and related to dihydroorotases and allantoinases from various organisms. His6-tagged DHP was over-expressed in Escherichia coli and purified by immobilized metal affinity chromatography to a specific activity of 3.46 U mg(-1) protein. The optimal pH and temperature for the purified enzyme were 8.0 and 60 degrees C, respectively. The half-life of His6-tagged DHP was 25 days at 50 degrees C. The enzyme activity was stimulated by Co2+ and Mn2+ ions. His6-tagged DHP was most active toward dihydrouracil followed by hydantoin derivatives. The catalytic efficiencies (kcat/Km) of the enzyme for dihydrouracil and hydantoin were 2.58 and 0.61 s(-1) mM(-1), respectively.

Practical resolution system for dl-pantoyl lactone using the lactonase from Fusarium oxysporum

We developed an enzymatic resolution system for DL-pantoyl lactone that uses immobilized mycelia of Fusarium oxysporum, which produce a lactone-hydrolyzing enzyme (lactonase). The lactonase catalyzes the stereospecific hydrolysis of D-pantoyl lactone. One hundred eighty repeated batch reactions (total reaction time, 3780 h) were made with mycelia entrapped in calcium alginate gels as the catalyst, in the presence of 90 mM CaCl2. With a 300 gl(-1)DL-pantoyl lactone solution as the substrate, the hydrolysis rate for DL-pantoyl lactone was > 40% and the optical purity of D-pantoic acid was 90% enantiomer excess. Immobilized mycelia retained 70% of their initial lactonase activity, even after 180 batch reactions. The estimated half-life of the lactonase activity of the immobilized mycelia was 6000 h, which is 35 times higher than that of the free mycelia. The process has been exploited commercially since 1999.

Evolutionary relationship and application of a superfamily of cyclic amidohydrolase enzymes

Cyclic amidohydrolases belong to a superfamily of enzymes that catalyze the hydrolysis of cyclic C-N bonds. They are commonly found in nucleotide metabolism of purine and pyrimidine. These enzymes share similar catalytic mechanisms and show considerable structural homologies, suggesting that they might have evolved from a common ancestral protein. Homology searches based on common mechanistic properties and three-dimensional protein structures provide clues to the evolutionary relationships of these enzymes. Among the superfamily of enzymes, hydantoinase has been highlighted by its potential for biotechnological applications in the production of unnatural amino acids. The enzymatic process for the production of optically pure amino acids consists of three enzyme steps: hydantoin racemase, hydantoinase, and N-carbamoylase. For efficient industrial application, some critical catalytic properties such as thermostability, catalytic activity, enantioselectivity, and substrate specificity require further improvement. To this end, isolation of new enzymes with desirable properties from natural sources and the optimization of enzymatic processes were attempted. A combination of directed evolution techniques and rational design approaches has made brilliant progress in the redesign of industrially important catalytic enzymes; this approach is likely to be widely applied to the creation of designer enzymes with desirable catalytic properties.

Mutational analysis of the hydantoin hydrolysis pathway in Pseudomonas putida RU-KM3S

The biocatalytic conversion of 5-mono-substituted hydantoins to the corresponding D-amino acids or L-amino acids involves first the hydrolysis of hydantoin to a N-carbamoylamino acid by an hydantoinase or dihydropyrimidinase, followed by the conversion of the N-carbamoylamino acid to the amino acid by N-carbamylamino acid amidohydrolase ( N-carbamoylase). Pseudomonas putida strain RU-KM3S, with high levels of hydantoin-hydrolysing activity, has been shown to exhibit non-stereoselective hydantoinase and L-selective N-carbamoylase activity. This study focused on identifying the hydantoinase and N-carbamoylase-encoding genes in this strain, using transposon mutagenesis and selection for altered growth phenotypes on minimal medium with hydantoin as a nitrogen source. Insertional inactivation of two genes, dhp and bup, encoding a dihydropyrimidinase and beta-ureidopropionase, respectively, resulted in loss of hydantoinase and N-carbamoylase activity, indicating that these gene products were responsible for hydantoin hydrolysis in this strain. dhp and bup are linked to an open reading frame encoding a putative transport protein, which probably shares a promoter with bup. Two mutant strains were isolated with increased levels of dihydropyrimidinase but not beta-ureidopropionase activity. Transposon mutants in which key elements of the nitrogen regulatory pathway were inactivated were unable to utilize hydantoin or uracil as a nitrogen source. However, these mutations had no effect on either the dihydropyrimidinase or beta-ureidopropionase activity. Disruption of the gene encoding dihydrolipoamide succinyltransferase resulted in a significant reduction in the activity of both enzymes, suggesting a role for carbon catabolite repression in the regulation of hydantoin hydrolysis in P. putida RU-KM3S cells.

Modifying the oligomeric state of cyclic amidase and its effect on enzymatic catalysis

A group of cyclic amidases, including hydantoinase, allantoinase, dihydropyrimidinase, and dihydroorotase, catalyze the reversible hydrolysis of cyclic ureides, such as 5-monosubstituted hydantoins and dihydropyrimidines. These four enzymes carry hydrophobic patches to form dimers. With the exception of dihydroorotase, these enzymes are further dimerized to form tetramers by hydrophobic interactions. This leads us to speculate that the hydrophobic interaction domain may be a significant factor in the catalytic property of these oligomeric cyclic amidases, for which activities are not allosterically regulated. We generated a dimeric D-hydantoinase by mutating five residues in the hydrophobic alpha-helical interface of a tetramer and analyzed the kinetic properties of the dimeric form of D-hydantoinase. The specific activity of the dimeric D-hydantoinase corresponds to 5.3% of the activity of tetrameric D-hydantoinase. This low specific activity of the dimeric D-hydantoinase indicates that the dimeric interaction to form a tetramer has a significant effect on the catalytic activity of this non-allosteric tetramer.

Crystal structure of D-hydantoinase from Bacillus stearothermophilus: insight into the stereochemistry of enantioselectivity

Industrial production of antibiotics, such as semisynthetic penicillins and cephalosporins, requires optically pure D-p-hydroxylphenylglycine and its derivatives as important side-chain precursors. To produce optically pure D-amino acids, microbial D-hydantoinase (E.C. 3.5.2.2) is used for stereospecific hydrolysis of chemically synthesized cyclic hydantoins. We report the apo-crystal structure of D-hydantoinase from B. stearothermophilus SD1 at 3.0 A resolution. The structure has a classic TIM barrel fold. Despite an undetectable similarity in sequence, D-hydantoinase shares a striking structural similarity with the recently solved structure of dihydroorotase. A structural comparison of hydantoinase with dihydroorotase revealed that the catalytic chemistry is conserved, while the substrate recognition is not. This structure provides insight into the stereochemistry of enantioselectivity in hydrolysis and illustrates how the enzyme recognizes stereospecific exocyclic substituents and hydrolyzes hydantoins. It should also provide a rationale for further directed evolution of this enzyme for hydrolysis of new hydantoins with novel exocyclic substituents.

Barbiturase, a novel zinc-containing amidohydrolase involved in oxidative pyrimidine metabolism

Barbiturase, which catalyzes the reversible amidohydrolysis of barbituric acid to ureidomalonic acid in the second step of oxidative pyrimidine degradation, was purified to homogeneity from Rhodococcus erythropolis JCM 3132. The characteristics and gene organization of barbiturase suggested that it is a novel zinc-containing amidohydrolase that should be grouped into a new family of the amidohydrolases superfamily. The amino acid sequence of barbiturase exhibited 48% identity with that of herbicide atrazine-decomposing cyanuric acid amidohydrolase but exhibited no significant homology to other proteins, indicating that cyanuric acid amidohydrolase may have evolved from barbiturase. A putative uracil phosphoribosyltransferase gene was found upstream of the barbiturase gene, suggesting mutual interaction between pyrimidine biosynthesis and oxidative degradation. Metal analysis with an inductively coupled radiofrequency plasma spectrophotometer revealed that barbiturase contains approximately 4.4 mol of zinc per mol of enzyme. The homotetrameric enzyme had K(m) and V(max) values of 1.0 mm and 2.5 micromol/min/mg of protein, respectively, for barbituric acid. The enzyme specifically acted on barbituric acid, and dihydro-l-orotate, alloxan, and cyanuric acid competitively inhibited its activity. The full-length gene encoding the barbiturase (bar) was cloned and overexpressed in Escherichia coli. The kinetic parameters and physicochemical properties of the cloned enzyme were apparently similar to those of the wild-type.

High-level expression and one-step purification of cyclic amidohydrolase family enzymes

The cyclic amidohydrolase family enzymes, including hydantoinase, dihydropyrimidinase, allantoinase and dihydroorotase, are metal-dependent hydrolases and play a crucial role in the metabolism of purine and pyrimidine in prokaryotic and eukaryotic cells. With the increasing demand for the elucidation of enzyme structures and functions, along with industrial applications, the research on the family enzymes has recently been proliferating, but the related enzymes had been purified conventionally by multistep purification procedures. Here, we reported the expression in Escherichia coli cells of maltose-binding protein-fused family enzymes and their one-step purification. The expression levels of the fusion proteins account for 20-35% of the total protein in E. coli, allowing approximately 2-3 mg of the purified proteins by affinity chromatography to be obtained per 0.3 L of bacterial culture. As more promising results, their nascent biochemical properties, after the cleavage of the fusion proteins with Factor Xa, in terms of oligomeric structure, optimal pH, specific activity, and kinetic property, were also conserved as those from the native enzymes. The availability of the family enzymes to fusion strategy shows potential as a convenient procedure to recombinant protein purification and accelerates the structure-function study of the related family enzymes.

Novel amidohydrolytic reactions in oxidative pyrimidine metabolism: analysis of the barbiturase reaction and discovery of a novel enzyme, ureidomalonase

Amidohydrolytic reactions in oxidative pyrimidine metabolism were investigated in detail. Barbiturase has been reported to catalyze the amidohydrolysis of barbituric acid to urea and malonic acid. However, purification of the enzyme revealed that it catalyzes the ring-opening of barbituric acid to ureidomalonic acid. The existence of a consecutive enzyme named ureidomalonase, which hydrolyzes ureidomalonic acid to urea and malonic acid, was also discovered during the purification of barbiturase.

Development of an Escherichia coli whole cell biocatalyst for the production of L-amino acids

A whole cell biocatalyst for the enzymatic production of L-amino acids from hydantoins was created by coexpressing the genes encoding the L-hydantoinase, the L-N-carbamoylase and the hydantoin racemase from Arthrobacter aurescens in Escherichia coli. In order to construct a well balanced reaction system the enzymatic activity in the cells was varied by using vectors with different copy numbers for expression of the genes. Derivatives of pSC101, pACYC184 and pBR322 were employed for the various constructions and in one construct the hydantoinase gene was integrated into the E. coli chromosome. All constructs carried the E. coli rhamnose promoter system enabling gene expression control by transcriptional regulation. The productivity for L-tryptophan from the corresponding hydantoin was more than 6-fold higher than achieved with Arthrobacter aurescens.

Functional expression and characterization of the two cyclic amidohydrolase enzymes, allantoinase and a novel phenylhydantoinase, from Escherichia coli

A superfamily of cyclic amidohydrolases, including dihydropyrimidinase, allantoinase, hydantoinase, and dihydroorotase, all of which are involved in the metabolism of purine and pyrimidine rings, was recently proposed based on the rigidly conserved structural domains in identical positions of the related enzymes. With these conserved domains, two putative cyclic amidohydrolase genes from Escherichia coli, flanked by related genes, were identified and characterized. From the genome sequence of E. coli, the allB gene and a putative open reading frame, tentatively designated as a hyuA (for hydantoin-utilizing enzyme) gene, were predicted to express hydrolases. In contrast to allB, high-level expression of hyuA in E. coli of a single protein was unsuccessful even under various induction conditions. We expressed HyuA as a maltose binding protein fusion protein and AllB in its native form and then purified each of them by conventional procedures. allB was found to encode a tetrameric allantoinase (453 amino acids) which specifically hydrolyzes the purine metabolite allantoin to allantoic acid. Another open reading frame, hyuA, located near 64.4 min on the physical map and known as a UUG start, coded for D-stereospecific phenylhydantoinase (465 amino acids) which is a homotetramer. As a novel enzyme belonging to a cyclic amidohydrolase superfamily, E. coli phenylhydantoinase exhibited a distinct activity toward the hydantoin derivative with an aromatic side chain at the 5' position but did not readily hydrolyze the simple cyclic ureides. The deduced amino acid sequence of the novel phenylhydantoinase shared a significant homology (>45%) with those of allantoinase and dihydropyrimidinase, but its functional role still remains to be elucidated. Despite the unclear physiological function of HyuA, its presence, along with the allantoin-utilizing AllB, strongly suggested that the cyclic ureides might be utilized as nutrient sources in E. coli.

Hydantoin racemase from Arthrobacter aurescens DSM 3747: heterologous expression, purification and characterization

In Arthrobacter aurescens DSM 3747 three enzymes are involved in the complete conversion of slowly racemizing 5'-monosubstituted D,L-hydantoins to L-amino acids, a stereoselective hydantoinase, a stereospecific L-N-carbamoylase and a hydantoin racemase. The gene encoding the hydantoin racemase, designated hyuA, was identified upstream of the previously described L-N-carbamoylase gene in the plasmid pAW16 containing genomic DNA of A. aurescens. The gene hyuA which encodes a polypeptide of 25.1 kDa, was expressed in Escherichia coli and the recombinant protein purified to homogeneity and further characterized. The optimal condition for racemase activity were pH 8.5 and 55 degrees C with L-5-benzylhydantoin as substrate. The enzyme was completely inhibited by HgCL2 and iodoacetamide and stimulated by addition of dithiothreitol. No effect on enzyme activity was seen with EDTA. The enzyme showed preference for hydantoins with arylalkyl side chains. Kinetic studies revealed substrate inhibition towards the aliphatic substrate L-5-methylthioethylhydantoin. Enzymatic racemization of D-5-indolylmethylenehydantoin in D2O and NMR analysis showed that the hydrogen at the chiral center of the hydantoin is exchanged against solvent deuterium during the racemization.

Construction and evaluation of a novel bifunctional N-carbamylase-D-hydantoinase fusion enzyme

A fully enzymatic process employing two sequential enzymes, D-hydantoinase and N-carbamylase, is a typical case requiring combined enzyme activity for the production of D-amino acids. To test the possibility of generating a bifunctional fusion enzyme, we constructed a fusion protein via end-to-end fusion of a whole gene that encodes an intact protein at the N terminus of the D-hydantoinase. Firstly, maltose-binding protein (MBP) gene of E. coli was fused with D-hydantoinase gene from Bacillus stearothermophilus SD1, and the properties of the resulting fusion protein (MBP-HYD) were compared with those of native D-hydantoinase. Gel filtration and kinetic analyses clearly demonstrated that the typical characteristics of D-hydantoinase are maintained even in a fusion state. Based on this result, we constructed an artificial fusion enzyme composed of the whole length of N-carbamylase (304 amino acids [aa]) from Agrobacterim radiobacter NRRL B11291 and D-hydantoinase (471 aa). The fusion enzyme (CAB-HYD) was functionally expressed with an expected molecular mass of 86 kDa and efficiently converted exogenous hydantoin derivatives to the D-amino acids. A related D-hydantoinase (HYD1) gene from Bacillus thermocatenulatus GH2 was also fused with the N-carbamylase gene at its N terminus. The resulting enzyme (CAB-HYD1) was bifunctional as expected and showed better performance than the CAB-HYD fusion enzyme. The conversion of hydantoin derivatives to corresponding amino acids by the fusion enzymes was much higher than that by the separately expressed enzymes, and comparable to that by the coexpressed enzymes. Thus, the fusion enzyme might be useful as a potential biocatalyst for the production of nonnatural amino acids.

Cyclic-imide-hydrolyzing activity of D-hydantoinase from Blastobacter sp. strain A17p-4

The cyclic-imide-hydrolyzing activity of a prokaryotic cyclic-ureide-hydrolyzing enzyme, D-hydantoinase, was investigated. The enzyme hydrolyzed cyclic imides with bulky substituents such as 2-methylsuccinimide, 2-phenylsuccinimide, phthalimide, and 3,4-pyridine dicarboximide to the corresponding half-amides. However, simple cyclic imides without substituents, which are substrates of imidase (ie.g., succinimide, glutarimide, and sulfur-containing cyclic imides such as 2,4-thiazolidinedione and rhodanine), were not hydrolyzed. The combined catalytic actions of bacterial D-hydantoinase and imidase can cover the function of a single mammalian enzyme, dihydropyrimidinase. Prokaryotic D-hydantoinase also catalyzed the dehyrative cyclization of the half-amide phthalamidic acid to the corresponding cyclic imide, phthalimide. The reversible hydrolysis of cyclic imides shown by prokaryotic D-hydantoinase suggested that, in addition to pyrimidine metabolism, it may also function in cyclic-imide metabolism.