The Crystal Structure of a Novel, Latent Dihydroorotase from Aquifex aeolicus at 1.7Å Resolution

Complexed Crystal Structure of the Dihydroorotase Domain of Human CAD Protein with the Anticancer Drug 5-Fluorouracil

Dihydroorotase (DHOase) is the third enzyme in the pathway used for the biosynthesis of pyrimidine nucleotides. In mammals, DHOase is active in a trifunctional enzyme, CAD, which also carries out the activities of carbamoyl phosphate synthetase and aspartate transcarbamoylase. Prior to this study, it was unknown whether the FDA-approved clinical drug 5-fluorouracil (5-FU), which is used as an anticancer therapy, could bind to the DHOase domain of human CAD (huDHOase). Here, we identified huDHOase as a new 5-FU binding protein, thereby extending the 5-FU interactome to this human enzyme. In order to investigate where 5-FU binds to huDHOase, we solved the complexed crystal structure at 1.97 Å (PDB ID 8GVZ). The structure of huDHOase complexed with malate was also determined for the sake of comparison (PDB ID 8GW0). These two nonsubstrate ligands were bound at the active site of huDHOase. It was previously established that the substrate N-carbamoyl-L-aspartate is either bound to or moves away from the active site, but it is the loop that is extended towards (loop-in mode) or moved away (loop-out mode) from the active site. DHOase also binds to nonsubstrate ligands via the loop-out mode. In contrast to the Escherichia coli DHOase model, our complexed structures revealed that huDHOase binds to either 5-FU or malate via the loop-in mode. We further characterized the binding of 5-FU to huDHOase using site-directed mutagenesis and the fluorescence quenching method. Considering the loop-in mode, the dynamic loop in huDHOase should be a suitable drug-targeting site for further designing inhibitors and clinical chemotherapies to suppress pyrimidine biosynthesis in cancer cell lines.

Crystal structure of Methanococcus jannaschii dihydroorotase

In this paper, we report the structural analysis of dihydroorotase (DHOase) from the hyperthermophilic and barophilic archaeon Methanococcus jannaschii. DHOase catalyzes the reversible cyclization of N-carbamoyl-l-aspartate to l-dihydroorotate in the third step of de novo pyrimidine biosynthesis. DHOases form a very diverse family of enzymes and have been classified into types and subtypes with structural similarities and differences among them. This is the first archaeal DHOase studied by x-ray diffraction. Its structure and comparison with known representatives of the other subtypes help define the structural features of the archaeal subtype. The M. jannaschii DHOase is found here to have traits from all subtypes. Contrary to expectations, it has a carboxylated lysine bridging the two Zn ions in the active site, and a long catalytic loop. It is a monomeric protein with a large β sandwich domain adjacent to the TIM barrel. Loop 5 is similar to bacterial type III and the C-terminal extension is long.

Structural Analysis of Saccharomyces cerevisiae Dihydroorotase Reveals Molecular Insights into the Tetramerization Mechanism

Dihydroorotase (DHOase), a dimetalloenzyme containing a carbamylated lysine within the active site, is a member of the cyclic amidohydrolase family, which also includes allantoinase (ALLase), dihydropyrimidinase (DHPase), hydantoinase, and imidase. Unlike most known cyclic amidohydrolases, which are tetrameric, DHOase exists as a monomer or dimer. Here, we report and analyze two crystal structures of the eukaryotic Saccharomyces cerevisiae DHOase (ScDHOase) complexed with malate. The structures of different DHOases were also compared. An asymmetric unit of these crystals contained four crystallographically independent ScDHOase monomers. ScDHOase shares structural similarity with Escherichia coli DHOase (EcDHOase). Unlike EcDHOase, ScDHOase can form tetramers, both in the crystalline state and in solution. In addition, the subunit-interacting residues of ScDHOase for dimerization and tetramerization are significantly different from those of other DHOases. The tetramerization pattern of ScDHOase is also different from those of DHPase and ALLase. Based on sequence analysis and structural evidence, we identify two unique helices (α6 and α10) and a loop (loop 7) for tetramerization, and discuss why the residues for tetramerization in ScDHOase are not necessarily conserved among DHOases.

Pyrimidine biosynthesis in pathogens - Structures and analysis of dihydroorotases from Yersinia pestis and Vibrio cholerae

The de novo pyrimidine biosynthesis pathway is essential for the proliferation of many pathogens. One of the pathway enzymes, dihydroorotase (DHO), catalyzes the reversible interconversion of N-carbamoyl-l-aspartate to 4,5-dihydroorotate. The substantial difference between bacterial and mammalian DHOs makes it a promising drug target for disrupting bacterial growth and thus an important candidate to evaluate as a response to antimicrobial resistance on a molecular level. Here, we present two novel three-dimensional structures of DHOs from Yersinia pestis (YpDHO), the plague-causing pathogen, and Vibrio cholerae (VcDHO), the causative agent of cholera. The evaluations of these two structures led to an analysis of all available DHO structures and their classification into known DHO types. Comparison of all the DHO active sites containing ligands that are listed in DrugBank was facilitated by a new interactive, structure-comparison and presentation platform. In addition, we examined the genetic context of characterized DHOs, which revealed characteristic patterns for different types of DHOs. We also generated a homology model for DHO from Plasmodium falciparum.

Characterization of the Dihydroorotase from Methanococcus jannaschii

The gene that codes for the putative dihydroorotase in the hyperthermophilic archaeon Methanococcus jannaschii was subcloned in pET-21a and expressed in Escherichia coli. A purification protocol was devised. The purity of the protein was evaluated by SDS-PAGE and the protein was confirmed by sequencing using LC-MS. The calculated molecular mass is 48104 Da. SEC-LS suggested that the protein is a monomer in solution. ICP-MS showed that there are two Zn ions per monomer. Kinetic analysis of the recombinant protein gave hyperbolic kinetics with Vmax = 12.2 µmol/min/mg and Km = 0.14 mM at 25 °C. Furthermore the activity of the protein increased with temperature consistent with the hyperthermophilic nature of the organism. A homology model was constructed using the mesophilic Bacillus anthracis protein as the template. Residues known to be critical for Zn and substrate binding were conserved. The activity of the enzyme at 85 and 90 °C was found to be relatively constant over 160 min and this correlates with the temperature of optimal growth of the organism of 85 °C. The amino acid sequences and structures of the two proteins were compared and this gave insight into some of the factors that may confer thermostability-more Lys and Ile, fewer Ala, Thr, Gln and Gly residues, and shorter N- and C-termini. Additional and better insight into the thermostabilization strategies adopted by this enzyme will be provided when its crystal structure is determined.

Activation of Latent Dihydroorotase from Aquifex aeolicus by Pressure

Elevated hydrostatic pressure was used to probe conformational changes of Aquifex aeolicus dihydroorotase (DHO), which catalyzes the third step in de novo pyrimidine biosynthesis. The isolated protein, a 45-kDa monomer, lacks catalytic activity but becomes active upon formation of a dodecameric complex with aspartate transcarbamoylase (ATC). X-ray crystallographic studies of the isolated DHO and of the complex showed that association induces several major conformational changes in the DHO structure. In the isolated DHO, a flexible loop occludes the active site blocking the access of substrates. The loop is mostly disordered but is tethered to the active site region by several electrostatic and hydrogen bonds. This loop becomes ordered and is displaced from the active site upon formation of DHO-ATC complex. The application of pressure to the complex causes its time-dependent dissociation and the loss of both DHO and ATC activities. Pressure induced irreversible dissociation of the obligate ATC trimer, and as a consequence the DHO is also inactivated. However, moderate hydrostatic pressure applied to the isolated DHO subunit mimics the complex formation and reversibly activates the isolated subunit in the absence of ATC, suggesting that the loop has been displaced from the active site. This effect of pressure is explained by the negative volume change associated with the disruption of ionic interactions and exposure of ionized amino acids to the solvent (electrostriction). The interpretation that the loop is relocated by pressure was validated by site-directed mutagenesis and by inhibition by small peptides that mimic the loop residues.

Intersubunit communication in the dihydroorotase-aspartate transcarbamoylase complex of Aquifex aeolicus

Aspartate transcarbamoylase and dihydroorotase, enzymes that catalyze the second and third step in de novo pyrimidine biosynthesis, are associated in dodecameric complexes in Aquifex aeolicus and many other organisms. The architecture of the dodecamer is ideally suited to channel the intermediate, carbamoyl aspartate from its site of synthesis on the ATC subunit to the active site of DHO, which catalyzes the next step in the pathway, because both reactions occur within a large, internal solvent-filled cavity. Channeling usually requires that the reactions of the enzymes are coordinated so that the rate of synthesis of the intermediate matches its rate of utilization. The linkage between the ATC and DHO subunits was demonstrated by showing that the binding of the bisubstrate analog, N-phosphonacetyl-L-aspartate to the ATC subunit inhibits the activity of the distal DHO subunit. Structural studies identified a DHO loop, loop A, interdigitating between the ATC domains that would be expected to interfere with domain closure essential for ATC catalysis. Mutation of the DHO residues in loop A that penetrate deeply between the two ATC domains inhibits the ATC activity by interfering with the normal reciprocal linkage between the two enzymes. Moreover, a synthetic peptide that mimics that part of the DHO loop that binds between the two ATC domains was found to be an allosteric or noncompletive ATC inhibitor (K(i) = 22 μM). A model is proposed suggesting that loop A is an important component of the functional linkage between the enzymes.

Structure, functional characterization, and evolution of the dihydroorotase domain of human CAD

Upregulation of CAD, the multifunctional protein that initiates and controls the de novo biosynthesis of pyrimidines in animals, is essential for cell proliferation. Deciphering the architecture and functioning of CAD is of interest for its potential usage as an antitumoral target. However, there is no detailed structural information about CAD other than that it self-assembles into hexamers of ∼1.5 MDa. Here we report the crystal structure and functional characterization of the dihydroorotase domain of human CAD. Contradicting all assumptions, the structure reveals an active site enclosed by a flexible loop with two Zn²⁺ ions bridged by a carboxylated lysine and a third Zn coordinating a rare histidinate ion. Site-directed mutagenesis and functional assays prove the involvement of the Zn and flexible loop in catalysis. Comparison with homologous bacterial enzymes supports a reclassification of the DHOase family and provides strong evidence against current models of the architecture of CAD.

The mononuclear metal center of type-I dihydroorotase from Aquifex aeolicus

BACKGROUND

Dihydroorotase (DHO) is a zinc metalloenzyme, although the number of active site zinc ions has been controversial. E. coli DHO was initially thought to have a mononuclear metal center, but the subsequent X-ray structure clearly showed two zinc ions, α and β, at the catalytic site. Aquifex aeolicus DHO, is a dodecamer comprised of six DHO and six aspartate transcarbamoylase (ATC) subunits. The isolated DHO monomer, which lacks catalytic activity, has an intact α-site and conserved β-site ligands, but the geometry of the second metal binding site is completely disrupted. However, the putative β-site is restored when the complex with ATC is formed and DHO activity is regained. Nevertheless, the X-ray structure of the complex revealed a single zinc ion at the active site. The structure of DHO from the pathogenic organism, S. aureus showed that it also has a single active site metal ion.

RESULTS

Zinc analysis showed that the enzyme has one zinc/DHO subunit and the addition of excess metal ion did not stimulate catalytic activity, nor alter the kinetic parameters. The metal free apoenzyme was inactive, but the full activity was restored upon the addition of one equivalent of Zn2+ or Co2+. Moreover, deletion of the β-site by replacing the His180 and His232 with alanine had no effect on catalysis in the presence or absence of excess zinc. The 2.2 Å structure of the double mutant confirmed that the β-site was eliminated but that the active site remained otherwise intact.

CONCLUSIONS

Thus, kinetically competent A. aeolicus DHO has a mononuclear metal center. In contrast, elimination of the putative second metal binding site in amidohydrolyases with a binuclear metal center, resulted in the abolition of catalytic activity. The number of active site metal ions may be a consideration in the design of inhibitors that selectively target either the mononuclear or binuclear enzymes.

Chemical rescue of the post-translationally carboxylated lysine mutant of allantoinase and dihydroorotase by metal ions and short-chain carboxylic acids

Bacterial allantoinase (ALLase) and dihydroorotase (DHOase) are members of the cyclic amidohydrolase family. ALLase and DHOase possess similar binuclear metal centers in the active site in which two metals are bridged by a post-translationally carboxylated lysine. In this study, we determined the effects of carboxylated lysine and metal binding on the activities of ALLase and DHOase. Although DHOase is a metalloenzyme, purified DHOase showed high activity without additional metal supplementation in a reaction mixture or bacterial culture. However, unlike DHOase, ALLase had no activity unless some specific metal ions were added to the reaction mixture or culture. Substituting the metal binding sites H59, H61, K146, H186, H242, or D315 with alanine completely abolished the activity of ALLase. However, the K146C, K146D and K146E mutants of ALLase were still active with about 1-6% activity of the wild-type enzyme. These ALLase K146 mutants were found to have 1.4-1.7 mol metal per mole enzyme subunit, which may indicate that they still contained the binuclear metal center in the active site. The activity of the K146A mutant of the ALLase and the K103A mutant of DHOase can be chemically rescued by short-chain carboxylic acids, such as acetic, propionic, and butyric acids, but not by ethanol, propan-1-ol, and imidazole, in the presence of Co2+ or Mn2+ ions. However, the activity was still ~10-fold less than that of wild-type ALLase. Overall, these results indicated that the 20 natural basic amino acid residues were not sufficiently able to play the role of lysine. Accordingly, we proposed that during evolution, the post-translational modification of carboxylated lysine in the cyclic amidohydrolase family was selected for promoting binuclear metal center self-assembly and increasing the nucleophilicity of the hydroxide at the active site for enzyme catalysis. This kind of chemical rescue combined with site-directed mutagenesis may also be used to identify a binuclear metal center in the active site for other metalloenzymes.

Expression, purification, crystallization and preliminary X-ray diffraction analysis of the dihydroorotase domain of human CAD

CAD is a 243 kDa eukaryotic multifunctional polypeptide that catalyzes the first three reactions of de novo pyrimidine biosynthesis: glutamine-dependent carbamyl phosphate synthetase, aspartate transcarbamylase and dihydroorotase (DHO). In prokaryotes, these activities are associated with monofunctional proteins, for which crystal structures are available. However, there is no detailed structural information on the full-length CAD protein or any of its functional domains apart from that it associates to form a homohexamer of ∼1.5 MDa. Here, the expression, purification and crystallization of the DHO domain of human CAD are reported. The DHO domain forms homodimers in solution. Crystallization experiments yielded small crystals that were suitable for X-ray diffraction studies. A diffraction data set was collected to 1.75 Å resolution using synchrotron radiation at the SLS, Villigen, Switzerland. The crystals belonged to the orthorhombic space group C222(1), with unit-cell parameters a=82.1, b=159.3, c=61.5 Å. The Matthews coefficient calculation suggested the presence of one protein molecule per asymmetric unit, with a solvent content of 48%.

Structure of the catalytic chain of Methanococcus jannaschii aspartate transcarbamoylase in a hexagonal crystal form: insights into the path of carbamoyl phosphate to the active site of the enzyme

Crystals of the catalytic chain of Methanococcus jannaschii aspartate transcarbamoylase (ATCase) grew in the presence of the regulatory chain in the hexagonal space group P6(3)22, with one monomer per asymmetric unit. This is the first time that crystals with only one monomer in the asymmetric unit have been obtained; all known structures of the catalytic subunit contain several crystallographically independent monomers. The symmetry-related chains form the staggered dimer of trimers observed in the other known structures of the catalytic subunit. The central channel of the catalytic subunit contains a sulfate ion and a K(+) ion as well as a glycerol molecule at its entrance. It is possible that it is involved in channeling carbamoyl phosphate (CP) to the active site of the enzyme. A second sulfate ion near Arg164 is near the second CP position in the wild-type Escherichia coli ATCase structure complexed with CP. It is suggested that this position may also be in the path that CP takes when binding to the active site in a partial diffusion process at 310 K. Additional biochemical studies of carbamoylation and the molecular organization of this enzyme in M. jannaschii will provide further insight into these points.

Biochemical characterization of allantoinase from Escherichia coli BL21

Bacterial allantoinase (ALLase; EC 3.5.2.5), which catalyzes the conversion of allantoin into allantoate, possesses a binuclear metal center in which two metal ions are bridged by a posttranslationally carboxylated lysine. Here, we characterized ALLase from Escherichia coli BL21. Purified recombinant ALLase exhibited no activity but could be activated when preincubating with some metal ions before analyzing its activity, and was in the order: Mn(2+)- ≫ Co(2+)- > Zn(2+)- > Ni(2+)- > Cd(2+)- ~Mg(2+)-activated enzyme; however, activity of ALLase (Mn(2+)-activated form) was also significantly inhibited with 5 mM Co(2+), Zn(2+), and Cd(2+) ions. Activity of Mn(2+)-activated ALLase was increased by adding the reducing agent dithiothreitol (DTT), but was decreased by treating with the sulfhydryl modifying reagent N-ethylmaleimide (NEM). Inhibition of Mn(2+)-activated ALLase by chelator 8-hydroxy-5-quinolinesulfonic acid (8-HQSA), but not EDTA, was pH-dependent. Analysis of purified ALLase by gel filtration chromatography revealed a mixture of monomers, dimers, and tetramers. Substituting the putative metal binding residues His59, His61, Lys146, His186, His242, and Asp315 with Ala completely abolished the activity of ALLase, even preincubating with Mn(2+) ions. On the basis of these results, as well as the pH-activity profile, the reaction mechanism of ALLase is discussed and compared with those of other cyclic amidohydrolases.

Crystal structures of YwqE from Bacillus subtilis and CpsB from Streptococcus pneumoniae, unique metal-dependent tyrosine phosphatases

Unique metal-dependent protein tyrosine phosphatases that belong to the polymerase and histindinol phosphatase (PHP) family are present in Gram-positive bacteria. They are distinct from the Cys-based, low-molecular-weight phosphotyrosine protein phosphatases (LMPTPs). Two representative members of the PHP family tyrosine phosphatases are YwqE from Bacillus subtilis and CpsB from Streptococcus pneumoniae. YwqE is involved in polysaccharide biosynthesis, bacterial DNA metabolism, and DNA damage response in B. subtilis. CpsB regulates capsular polysaccharide biosynthesis via tyrosine dephosphorylation of CpsD, its cognate tyrosine kinase, in S. pneumoniae. To gain insights into the active site and possible conformational changes of the metal-dependent tyrosine phosphatases from Gram-positive bacteria, we have determined the crystal structures of B. subtilis YwqE (in both the apo form and the phosphate-bound form) and S. pneumoniae CpsB (in the sulfate-bound form). Comparisons of the three structures reveal conformational plasticity of two active site loops. Furthermore, in both structures of the phosphate-bound YwqE and the sulfate-bound CpsB, the phosphate (or sulfate) ion is bound to a cluster of three metal ions in the active site, thus providing insight into the pre-catalytic state.

Identification and characterization of a putative dihydroorotase, KPN01074, from Klebsiella pneumoniae

Dihydroorotase (DHO; EC 3.5.2.3) is an essential metalloenzyme in the biosynthesis of pyrimidine nucleotides. Here, we identified and characterized DHO from the pathogenic bacterium Klebsiella pneumoniae (Kp). The activity of KpDHO toward L: -dihydroorotate was observed with K (m) = 0.04 mM and V (max) = 8.87 mumol/(mg min). Supplementing the standard growth medium with Co2+, Mn2+, Mg2+, or Ni2+ increased enzyme activity. The catalytic activity of KpDHO was inhibited with Co2+, Zn2+, Mn2+, Cd2+, Ni2+, and phosphate ions. Substituting the putative metal binding residues His17, His19, Lys103, His140, His178, and Asp251 with Ala completely abolished KpDHO activity. However, the activity of the mutant D251E was fourfold higher than that of the wild-type protein. On the basis of these biochemical and mutational analyses, KpDHO (KPN01074) was identified as type II DHO.

Dihydroorotase from the hyperthermophile Aquifex aeolicus is activated by stoichiometric association with aspartate transcarbamoylase and forms a one-pot reactor for pyrimidine biosynthesis

In prokaryotes, the first three enzymes in pyrimidine biosynthesis, carbamoyl phosphate synthetase (CPS), aspartate transcarbamoylase (ATC), and dihydroorotase (DHO), are commonly expressed separately and either function independently (Escherichia coli) or associate into multifunctional complexes (Aquifex aeolicus). In mammals the enzymes are expressed as a single polypeptide chain (CAD) in the order CPS-DHO-ATC and associate into a hexamer. This study presents the three-dimensional structure of the noncovalent hexamer of DHO and ATC from the hyperthermophile A. aeolicus at 2.3 A resolution. It is the first structure of any multienzyme complex in pyrimidine biosynthesis and is a possible model for the core of mammalian CAD. The structure has citrate, a near isosteric analogue of carbamoyl aspartate, bound to the active sites of both enzymes. Three active site loops that are intrinsically disordered in the free, inactive DHO are ordered in the complex. The reorganization also changes the peptide bond between Asp153, a ligand of the single zinc atom in DHO, and Gly154, to the rare cis conformation. In the crystal structure, six DHO and six ATC chains form a hollow dodecamer, in which the 12 active sites face an internal reaction chamber that is approximately 60 A in diameter and connected to the cytosol by narrow tunnels. The entrances and the interior of the chamber are both electropositive, which suggests that the architecture of this nanoreactor modifies the kinetics of the bisynthase, not only by steric channeling but also by preferential escape of the product, dihydroorotase, which is less negatively charged than its precursors, carbamoyl phosphate, aspartate, or carbamoyl aspartate.

The crystal structure of enamidase: a bifunctional enzyme of the nicotinate catabolism

The hydrolysis of 1,4,5,6-tetrahydro-6-oxonicotinate to 2-formylglutarate is a central step in the catabolism of nicotinate in several Clostridia and Proteobacteria. This reaction is catalyzed by the novel enzyme enamidase, a new member of the amidohydrolase superfamily as indicated by its unique reaction, sequence relationship, and the stoichiometric binding of iron and zinc. A hallmark of enamidase is its capability to catalyze a two-step reaction: the initial decyclization of 1,4,5,6-tetrahydro-6-oxonicotinate leading to 2-(enamine)glutarate followed by an additional hydrolysis step yielding (S)-2-formylglutarate. Here, we present the crystal structure of enamidase from Eubacterium barkeri at 1.9 A resolution, providing a structural basis for catalysis and suggesting a mechanism for its exceptional activity and enantioselectivity. The enzyme forms a 222-symmetric tetramer built up by a dimer of dimers. Each enamidase monomer consists of a composite beta-sandwich domain and an (alpha/beta)(8)-TIM-barrel domain harboring the active site. With its catalytic binuclear metal center comprising both zinc and iron ions, enamidase represents a special case of subtype II amidohydrolases.

Structures of ligand-free and inhibitor complexes of dihydroorotase from Escherichia coli: implications for loop movement in inhibitor design

Dihydroorotase (DHOase) catalyzes the reversible cyclization of N-carbamyl-L-aspartate (CA-asp) to L-dihydroorotate (DHO) in the de novo biosynthesis of pyrimidine nucleotides. DHOase is a potential anti-malarial drug target as malarial parasites can only synthesize pyrimidines via the de novo pathway and do not possess a salvage pathway. Here we report the structures of Escherichia coli DHOase crystallized without ligand (1.7 A resolution) and in the presence of the inhibitors 2-oxo-1,2,3,6-tetrahydropyrimidine-4,6-dicarboxylate (HDDP; 2.0 A) and 5-fluoroorotate (FOA, 2.2 A). These are the first crystal structures of DHOase-inhibitor complexes, providing structural information on the mode of inhibitor binding. HDDP possesses features of both the substrate and product, and ligates the Zn atoms in the active site. In addition, HDDP forms hydrogen bonds to the flexible loop (residues 105-115) stabilizing the "loop-in" conformation of the flexible loop normally associated with the presence of CA-asp in the active site. By contrast, FOA, a product-like inhibitor, binds to the active site in a similar fashion to DHO but does not ligate the Zn atoms directly nor stabilize the loop-in conformation. These structures define the necessary features for the future design of improved inhibitors of DHOase.

Structure of the T109S mutant of Escherichia coli dihydroorotase complexed with the inhibitor 5-fluoroorotate: catalytic activity is reflected by the crystal form

Crystals of a single-point mutant (T109S) of Escherichia coli dihydroorotase (DHOase) with diminished activity grown in the presence of L-dihydroorotate (L-DHO) are tetragonal, with a monomer in the asymmetric unit. These crystals are extremely unstable and disintegrate shortly after formation, which is followed by the growth of orthorhombic crystals from the remnants of the tetragonal crystals or at new nucleation sites. Orthorhombic crystals, for which a structure has previously been reported [Thoden et al. (2001), Biochemistry, 40, 6989-6997; Lee et al. (2005), J. Mol. Biol. 348, 523-533], contain a dimer of DHOase in the asymmetric unit; the active site of one monomer contains the substrate N-carbamyl-L-aspartate (L-CA-asp) and the active site of the other monomer contains the product of the reaction, L-DHO. In the subunit with L-DHO in the active site, a surface loop (residues 105-115) is 'open'. In the other subunit, with L-CA-asp in the active site, the loop folds inwards, forming specific hydrogen bonds from the loop to the L-CA-asp. The tetragonal crystal form can be stabilized by crystallization in the presence of the inhibitor 5-fluoroorotate (FOA), a product (L-DHO) mimic. Crystals of the complex of T109S DHOase with FOA are tetragonal, space group P4(1)2(1)2, with unit-cell parameters a = b = 72.6, c = 176.1 A. The structure has been refined to R and R(free) values of 0.218 and 0.257, despite severe anisotropy of the diffraction. In this structure, the flexible loops are both in the 'open' conformation, which is consistent with FOA, like L-DHO, binding at both sites. The behaviour of the T109S mutant crystals of DHOase in the presence of L-DHO is explained by initial binding of L-DHO to both subunits, followed by slow conversion to L-CA-asp, with consequent movement of the flexible loop and dissolution of the crystals. Orthorhombic crystals are then able to grow in the presence of L-DHO and L-CA-asp.

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.