Recapture of [S]-allantoin, the product of the two-step degradation of uric acid, by urate oxidase

Signaling pathways in uric acid homeostasis and gout: From pathogenesis to therapeutic interventions

Uric acid is a product of purine degradation, and uric acid may have multiple physiologic roles, including the beneficial effects as an antioxidant and neuroprotector, maintenance of blood pressure during low salt ingestion, and modulation of immunity. However, overproduction of metabolic uric acid, and/or imbalance of renal uric acid secretion and reabsorption, and/or underexcretion of extrarenal uric acid, e.g. gut, will contribute to hyperuricemia, which is a common metabolic disease. Long-lasting hyperuricemia can induce the formation and deposition of monosodium urate (MSU) crystals within the joints and periarticular structures. MSU crystals further induce an acute, intensely painful, and sterile inflammation conditions named as gout by NLRP3 inflammasome-mediated cleavage of pro-IL-1β to bioactive IL-1β. Moreover, hyperuricemia and gout are associated with multiple cardiovascular and renal disorders, e.g., hypertension, myocardial infarction, stroke, obesity, hyperlipidemia, type 2 diabetes mellitus and chronic kidney disease. Although great efforts have been made by scientists of modern medicine, however, modern therapeutic strategies with a single target are difficult to exert long-term positive effects, and even some of these agents have severe adverse effects. The Chinese have used the ancient classic prescriptions of traditional Chinese medicine (TCM) to treat metabolic diseases, including gout, by multiple targets, for more than 2200 years. In this review, we discuss the current understanding of urate homeostasis, the pathogenesis of hyperuricemia and gout, and both modern medicine and TCM strategies for this commonly metabolic disorder. We hope these will provide the good references for treating hyperuricemia and gout.

Cascade enzymatic preparation of carboxymethyl chitosan-based multifunctional hydrogels for promoting cutaneous wound healing

Designing wound dressings with inherent multifunctional therapeutic effects is desirable for clinical applications. Herein, a series of multifunctional carboxymethyl chitosan (CMCS)-based hydrogels were fabricated by the facile urate oxidase (UOX)-horseradish peroxidase (HRP) cascade enzymatic crosslinking system. For the first time, the cascade enzymatic crosslinking system was not only used for preparing hydrogel wound dressings but also for accelerating wound healing due to the activity retention of the self-compartmental enzymes. A CMCS derivative (HCMCS-mF) synthesized by successively grafting 4-hydroxybenzaldehyde (H) and 5-methylfurfural (mF) on CMCS and a quaternary ammonium crosslinker (QMal) with terminal grafting maleimide (Mal) groups were combined with enzymatic system for the facile preparation of hydrogels. The mild Diels-Alder (DA) crosslinking reaction between mF and Mal groups constructed the first network of hydrogels. The cascade UOX-HRP system mediated the oxidative crosslinking of phenols thus forming the second gel network. Self-entrapped UOX maintained its enzymatic activity and could continuously catalyze the oxidation of uric acid, generating therapeutic allantoin. These porous, degradable, mechanically stable hydrogels with excellent antioxidant performance and enhanced antibacterial capacity could effectively accelerate skin wound repair by simultaneously reducing oxidative stress, relieving inflammation, promoting collagen deposition and upregulating the expression level of CD31.

Uricase-deficient rats with similarly stable serum uric acid to human’s are sensitive model animals for studying hyperuricemia

The aim of this study was to provide a sensitive model animal for studying hyperuricemia. Male uricase-deficient rats, named Kunming-DY rats, were raised for 130 days, or orally administered with purines and other chemicals. Serum uric acid (SUA) in the animals was assayed, and the UA level in their organs and their 24-h excretion was determined. Genes in the jejunum, ileum, kidney and liver related to UA synthesis and transportation were detected by quantitative RNA sequencing. Uricase-deficient rats have a high level of SUA and are sensitive to xanthine, adenosine, inosine, allopurinol, and alcohol. Besides, the high level of SUA in male uricase-deficient rats was stable, much higher than that in wild-type rats but similar to that in men. The distribution pattern of UA in uricase-deficient rats’ organs was different from that in wild-type rats. The kidney, liver, and small intestine were the top three organs where UA distributed, but the UA in the small intestine, colon, lung, thymus, and brain was less affected by uricase deficiency, indicating that these organs are constitutive distribution organs in UA. The 24-h UA excreted by a uricase-deficient rat was about five times higher than that excreted by a wild-type rat. However, the 24-h UA excreted through feces was not significantly changed. Both the urine volume and UA in uricase-deficient rats significantly increased, and more than 90% of UA was excreted via urine. The expression of xanthine dehydrogenase was not upregulated. Some genes of transporter associated with uric acid excretion in the kidney were significantly regulated, though not sufficient to explain the increase in SUA. In conclusion, male uricase-deficient rats’ UA metabolism is similar to that of men. The elevation of SUA in uricase-deficient rats is caused by uricase deficiency, and uricase-deficient rats are a sensitive model for studying hyperuricemia.

Structural and biochemical insights into a hyperthermostable urate oxidase from Thermobispora bispora for hyperuricemia and gout therapy

Microbial urate oxidase has emerged as a potential source of therapeutic properties for hyperuricemia in arthritic gout and renal disease. The thermostability and long-term thermal tolerance of the enzyme need to be established to prolong its therapeutic effects. Here, we present the biochemical and structural aspects of a hyperthermostable urate oxidase (TbUox) from the thermophilic microorganism Thermobispora bispora. Enzymatic characterization of TbUox revealed that it was active over a wide range of temperatures, from 30 to 70 °C, with optimal activity at 65 °C and pH 8.0, which suggests its applicability under physiological conditions. Moreover, TbUox exhibits high thermostability from 10 to 65 °C, with Tm of 70.3 °C and near-neutral pH stability from pH 7.0 to 8.0 and high thermal tolerance. The crystal structures of TbUox revealed a distinct feature of the C-terminal loop extensions that may help with protein stability via inter-subunit interactions. In addition, the high thermal tolerance of TbUox may be contributed by the extensive inter-subunit contacts via salt bridges, hydrogen bonds, and hydrophobic interactions. The findings in this study provide a molecular basis for the thermophilic TbUox urate oxidase for application in hyperuricemia and gout therapy.

Molecular Elucidation of a Urate Oxidase from Deinococcus radiodurans for Hyperuricemia and Gout Therapy

Urate oxidase initiates the uric acid degradation pathways and is extensively used for protein drug development for gout therapy and serum uric acid diagnosis. We first present the biochemical and structural elucidation of a urate oxidase from the extremophile microorganism Deinococcus radiodurans (DrUox). From enzyme characterization, DrUox showed optimal catalytic ability at 30 °C and pH 9.0 with high stability under physiological conditions. Only the Mg²⁺ ion moderately elevated its activity, which indicates the characteristic of the cofactor-free urate oxidase family. Of note, DrUox is thermostable in mesophilic conditions. It retains almost 100% activity when incubated at 25 °C and 37 °C for 24 h. In this study, we characterized a thermostable urate oxidase, DrUox with high catalytic efficiency and thermal stability, which strengthens its potential for medical applications.

A new uricase from Bacillus cereus SKIII: Characterization, gene identification and genetic improvement

Twenty-five microbial isolates were investigated for uricase production on uric acid medium. All isolates were obtained from Jeddah, Saudi Arabia. The highest uricase producer was identified as Bacillus cereus SKIII. Using glucose peptone broth at pH 7.5, incubation temperature 30 °C for 3 days with shaking of 150 rpm were the best conditions for maximum enzyme production. Glucose and peptone were the best carbon and nitrogen sources. The molecular weight of the purified enzyme was34.5 KDa, and isoelectric point was 7.9. The optimum pH and temperature were pH 8.0 and 35 °C, respectively. It was stable at 35 °C for 60 min, but thermally inactivated at 60 °C after 60 min Its enzymatic activity was enhanced by Mg²⁺, Ca²⁺,Fe²⁺, Mn^2+, Zn²⁺ions and inhibited by Co²⁺, Na⁺, Hg²⁺, Ag⁺ ions and EDTA at 1 mM. Uricase production was enhanced using UV mutation and the obtained mutant produced six times higher than the original isolate. An amplicon 900 bp of uricase gene (Pucl) was sequenced (accession number MF417635). No remarkable difference was noticed in B. cereus SKIII and SKm mutant nucleotide sequences. In conclusion, SKIII and SKm are promising strains in uricase production for biotechnological applications.

A unique spiro-β-triazinedione-γ-hydantoin type alkaloid with antiviral activity against tobacco mosaic virus from Streptomyces gamaensis

Chemical investigation of a soil actinomycete strain Streptomyces gamaensis NEAU-Gz11 led to the isolation of three alkaloids 1–3 with antiviral activity.

Glassy carbon electrode modified with G‑MoS2‑Nafion acts as an electrochemical biosensor to determine uric acid in human serum

At present, the majority of methods used for uric acid (UA) detection are not able to meet the detection requirements with speed, accuracy, high sensitivity, high specificity, a wide linear range or a low cost. Compared with other methods, the electrochemical method has a high sensitivity and fast detection. The present study aimed to identify an electrochemical sensor with high sensitivity, fast detection and a wide linear range for the detection of UA. A glassy carbon electrode modified with graphene-molybdenum disulfide-Nafion (G-MoS₂-Nafion) composites was prepared for use as the working electrode. The morphologies and elemental compositions of the G-MoS₂ composites were characterized by field emission scanning electron microscopy, elemental distribution spectrometry and X-ray diffraction, respectively. The electrochemical behaviors were investigated by cyclic voltammetry, linear sweep voltammetry and the amperometric i-t curve (i-t). The interference of glucose, ascorbic acid and dopamine, and the accuracy and precision of the electrochemical method were subsequently evaluated. The present study identified the following: (1) Only the reduction peak of UA was detected in human serum, indicating that the method established in the present study has a high specificity for the determination of UA in human serum; (2) UA concentration has a linear correlation with current intensity (y=0.012×+0.998; R²=0.998), wide linear range and high sensitivity (minimum detectability=13.91 µM; signal-to-noise ratio=3); (3) the values of UA content in human serum were positively proportional to the clinical results (y=0.9802×+11.494; R²=0.978); (4) the average recovery rate of UA (95.28%) and the replicability assay of the i-t electrochemical method (coefficient of variation=2.04%), suggest that the method had a high accuracy and good precision for UA detection. Due to its characteristics of good accuracy, high sensitivity, wide linear range, good anti-interference ability and replicability, G-MoS₂-Nafion has good prospects for UA detection in the clinical setting.

Measurement of Rapid Protein Diffusion in the Cytoplasm by Photo-Converted Intensity Profile Expansion

The fluorescence microscopy methods presently used to characterize protein motion in cells infer protein motion from indirect observables, rather than measuring protein motion directly. Operationalizing these methods requires expertise that can constitute a barrier to their broad utilization. Here, we have developed PIPE (photo-converted intensity profile expansion) to directly measure the motion of tagged proteins and quantify it using an effective diffusion coefficient. PIPE works by pulsing photo-convertible fluorescent proteins, generating a peaked fluorescence signal at the pulsed region, and analyzing the spatial expansion of the signal. We demonstrate PIPE's success in measuring accurate diffusion coefficients in silico and in vitro and compare effective diffusion coefficients of native cellular proteins and free fluorophores in vivo. We apply PIPE to measure diffusion anomality in the cell and use it to distinguish free fluorophores from native cellular proteins. PIPE's direct measurement and ease of use make it appealing for cell biologists.

The Recognition of Identical Ligands by Unrelated Proteins

The binding of drugs and reagents to off-targets is well-known. Whereas many off-targets are related to the primary target by sequence and fold, many ligands bind to unrelated pairs of proteins, and these are harder to anticipate. If the binding site in the off-target can be related to that of the primary target, this challenge resolves into aligning the two pockets. However, other cases are possible: the ligand might interact with entirely different residues and environments in the off-target, or wholly different ligand atoms may be implicated in the two complexes. To investigate these scenarios at atomic resolution, the structures of 59 ligands in 116 complexes (62 pairs in total), where the protein pairs were unrelated by fold but bound an identical ligand, were examined. In almost half of the pairs, the ligand interacted with unrelated residues in the two proteins (29 pairs), and in 14 of the pairs wholly different ligand moieties were implicated in each complex. Even in those 19 pairs of complexes that presented similar environments to the ligand, ligand superposition rarely resulted in the overlap of related residues. There appears to be no single pattern-matching "code" for identifying binding sites in unrelated proteins that bind identical ligands, though modeling suggests that there might be a limited number of different patterns that suffice to recognize different ligand functional groups.

Direct evidence for a peroxide intermediate and a reactive enzyme-substrate-dioxygen configuration in a cofactor-free oxidase

Cofactor-free oxidases and oxygenases promote and control the reactivity of O2 with limited chemical tools at their disposal. Their mechanism of action is not completely understood and structural information is not available for any of the reaction intermediates. Near-atomic resolution crystallography supported by in crystallo Raman spectroscopy and QM/MM calculations showed unambiguously that the archetypical cofactor-free uricase catalyzes uric acid degradation via a C5(S)-(hydro)peroxide intermediate. Low X-ray doses break specifically the intermediate C5-OO(H) bond at 100 K, thus releasing O2 in situ, which is trapped above the substrate radical. The dose-dependent rate of bond rupture followed by combined crystallographic and Raman analysis indicates that ionizing radiation kick-starts both peroxide decomposition and its regeneration. Peroxidation can be explained by a mechanism in which the substrate radical recombines with superoxide transiently produced in the active site.

Azide inhibition of urate oxidase

The inhibition of urate oxidase (UOX) by azide was investigated by X-ray diffraction techniques and compared with cyanide inhibition. Two well characterized sites for reagents are present in the enzyme: the dioxygen site and the substrate-binding site. To examine the selectivity of these sites towards azide inhibition, several crystallization conditions were developed. UOX was co-crystallized with azide (N3) in the presence or absence of either uric acid (UA, the natural substrate) or 8-azaxanthine (8AZA, a competitive inhibitor). In a second set of experiments, previously grown orthorhombic crystals of the UOX-UA or UOX-8AZA complexes were soaked in sodium azide solutions. In a third set of experiments, orthorhombic crystals of UOX with the exchangeable ligand 8-nitroxanthine (8NXN) were soaked in a solution containing uric acid and azide simultaneously (competitive soaking). In all assays, the soaking periods were either short (a few hours) or long (one or two months). These different experimental conditions showed that one or other of the sites, or the two sites together, could be inhibited. This also demonstrated that azide not only competes with dioxygen as cyanide does but also competes with the substrate for its enzymatic site. A model in agreement with experimental data would be an azide in equilibrium between two sites, kinetically in favour of the dioxygen site and thermodynamically in favour of the substrate-binding site.

Structure-based characterization of canine-human chimeric uricases and its evolutionary implications

Uricase was lost in hominoids during primate evolution, but the inactivation mechanism remains controversial. To investigate the inactivation process of hominoid uricase, chimeric constructions between canine and human uricase were employed to screen the target regions that may contain labile or inactivated mutations in deduced human uricase. Four chimeric uricases were constructed and showed different enzymatic characteristics. Homology modeling, rational site-directed mutagenesis and DNA alignment were used to analyze the changes. Arg119 is conserved in functional mammalian uricases and its side-chains are crucial in maintaining the stability of the β-barrel core. A single CGT (Arg) to CAT (His) mutation at codon 119 that is shared by the human and great ape clade greatly reduces this stability and could cause the loss of uricase activity. We speculate that this missense mutation occurred first and inactivated the uricase protein in humans and great apes and that later the known nonsense mutation at codon 33 occurred and silenced the uricase gene. A single GTC (Val) to GCC (Ala) mutation at codon 296 in canine uricase is regarded as deleterious structural mutation, but such kinds of deleterious mutations have been widely accumulated in extant mammalian uricases. We speculate that a reduction in uricase activity has been an evolutionary tendency in mammals. Moreover, from structure-activity analysis of helix 2 in ancestral primate uricase, we suggest that before the inactivation of hominoid uricase, deleterious structural evolutionary changes had occurred in ancestral primates. The loss of hominoid uricase should be caused by progressive multistep mutations rather than a single mutation event.

Absolute stereochemistry and preferred conformations of urate degradation intermediates from computed and experimental circular dichroism spectra

The enzymatic oxidation of urate leads to the sequential formation of optically active intermediates with unknown stereochemistry: (-)-5-hydroxyisourate (HIU) and (-)-2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline (OHCU). In accordance with the observation that a defect in HIU hydrolase causes hepatocarcinoma in mouse, a detoxification role has been proposed for the enzymes accelerating the conversion of HIU and OHCU into optically active (+)-allantoin. The enzymatic products of urate oxidation are normally not present in humans, but are formed in patients treated with urate oxidase. We used time-dependent density functional theory (TDDFT) to compute the electronic circular dichroism (ECD) spectra of the chiral compounds of urate degradation (HIU, OHCU, allantoin) and we compared the results with experimentally measured ECD spectra. The calculated ECD spectra for (S)-HIU and (S)-OHCU reproduced well the experimental spectra obtained through the enzymatic degradation of urate. Less conclusive results were obtained with allantoin, although the computed optical rotations in the transparent region supported the original assignment of the (+)-S configuration. These absolute configuration assignments can facilitate the study of the enzymes involved in urate metabolism and help us to understand the mechanism leading to the toxicity of urate oxidation products.

Near-atomic resolution structures of urate oxidase complexed with its substrate and analogues: the protonation state of the ligand

Urate oxidase (uricase; EC 1.7.3.3; UOX) fromAspergillus flavuscatalyzes the oxidation of uric acid in the presence of molecular oxygen to 5-hydroxyisourate in the degradation cascade of purines; intriguingly, catalysis proceeds using neither a metal ion (Fe, Cuetc.) nor a redox cofactor. UOX is a tetrameric enzyme with four active sites located at the interface of two subunits; its structure was refined at atomic resolution (1 Å) using new crystal data in the presence of xanthine and at near-atomic resolution (1.3–1.7 Å) in complexes with the natural substrate (urate) and two inhibitors: 8-nitroxanthine and 8-thiouric acid. Three new features of the structural and mechanistic behaviour of the enzyme were addressed. Firstly, the high resolution of the UOX–xanthine structure allowed the solution of an old structural problem at a contact zone within the tetramer; secondly, the protonation state of the substrate was determined from both a halochromic inhibitor complex (UOX–8-nitroxanthine) and from the H-atom distribution in the active site, using the structures of the UOX–xanthine and the UOX–uric acid complexes; and thirdly, it was possible to extend the general base system, characterized by the conserved catalytic triad Thr–Lys–His, to a large water network that is able to buffer and shuttle protons back and forth between the substrate and the peroxo hole along the reaction pathway.

Large crystal growth by thermal control allows combined X-ray and neutron crystallographic studies to elucidate the protonation states in Aspergillus flavus urate oxidase

Urate oxidase (Uox) catalyses the oxidation of urate to allantoin and is used to reduce toxic urate accumulation during chemotherapy. X-ray structures of Uox with various inhibitors have been determined and yet the detailed catalytic mechanism remains unclear. Neutron crystallography can provide complementary information to that from X-ray studies and allows direct determination of the protonation states of the active-site residues and substrate analogues, provided that large, well-ordered deuterated crystals can be grown. Here, we describe a method and apparatus used to grow large crystals of Uox (Aspergillus flavus) with its substrate analogues 8-azaxanthine and 9-methyl urate, and with the natural substrate urate, in the presence and absence of cyanide. High-resolution X-ray (1.05-1.20 A) and neutron diffraction data (1.9-2.5 A) have been collected for the Uox complexes at the European Synchrotron Radiation Facility and the Institut Laue-Langevin, respectively. In addition, room temperature X-ray data were also collected in preparation for joint X-ray and neutron refinement. Preliminary results indicate no major structural differences between crystals grown in H(2)O and D(2)O even though the crystallization process is affected. Moreover, initial nuclear scattering density maps reveal the proton positions clearly, eventually providing important information towards unravelling the mechanism of catalysis.

Purine nucleotide catabolism in rat liver. Certain preliminary aspects of uricase reaction

We investigated the mechanism of action of uricase, which oxidizes uric acid to allantoin, in the rat. Allantoin may decompose chemically to urea and hydantoin, containing the carbons in positions 2 and 8 of the purine ring, respectively. These carbons are derived by formylation, catalyzed by formyltransferase, in two reactions of de novo synthesis. Since uric acid and allantoin are represented in equivalent amounts in the liver, we expected to find identical incorporation of radioactivity in C(2) and C(8) of both compounds after administration of (14)C-formate. In the case of (14)C-allantoin, this was true, but not for (14)C-uric acid extracted from rat liver. We interpret these results through a series of experiments and considerations.

Structural analysis of urate oxidase in complex with its natural substrate inhibited by cyanide: mechanistic implications

Background

Urate oxidase (EC 1.7.3.3 or UOX) catalyzes the conversion of uric acid and gaseous molecular oxygen to 5-hydroxyisourate and hydrogen peroxide, in the absence of cofactor or particular metal cation. The functional enzyme is a homo-tetramer with four active sites located at dimeric interfaces.

Results

The catalytic mechanism was investigated through a ternary complex formed between the enzyme, uric acid, and cyanide that stabilizes an intermediate state of the reaction. When uric acid is replaced by a competitive inhibitor, no complex with cyanide is formed.

Conclusion

The X-ray structure of this compulsory ternary complex led to a number of mechanistic evidences that support a sequential mechanism in which the two reagents, dioxygen and a water molecule, process through a common site located 3.3 Å above the mean plane of the ligand. This site is built by the side chains of Asn 254, and Thr 57, two conserved residues belonging to two different subunits of the homo-tetramer. The absence of a ternary complex between the enzyme, a competitive inhibitor, and cyanide suggests that cyanide inhibits the hydroxylation step of the reaction, after the initial formation of a hydroperoxyde type intermediate.

Oxygen pressurized X-ray crystallography: probing the dioxygen binding site in cofactorless urate oxidase and implications for its catalytic mechanism

The localization of dioxygen sites in oxygen-binding proteins is a nontrivial experimental task and is often suggested through indirect methods such as using xenon or halide anions as oxygen probes. In this study, a straightforward method based on x-ray crystallography under high pressure of pure oxygen has been developed. An application is given on urate oxidase (UOX), a cofactorless enzyme that catalyzes the oxidation of uric acid to 5-hydroxyisourate in the presence of dioxygen. UOX crystals in complex with a competitive inhibitor of its natural substrate are submitted to an increasing pressure of 1.0, 2.5, or 4.0 MPa of gaseous oxygen. The results clearly show that dioxygen binds within the active site at a location where a water molecule is usually observed but does not bind in the already characterized specific hydrophobic pocket of xenon. Moreover, crystallizing UOX in the presence of a large excess of chloride (NaCl) shows that one chloride ion goes at the same location as the oxygen. The dioxygen hydrophilic environment (an asparagine, a histidine, and a threonine residues), its absence within the xenon binding site, and its location identical to a water molecule or a chloride ion suggest that the dioxygen site is mainly polar. The implication of the dioxygen location on the mechanism is discussed with respect to the experimentally suggested transient intermediates during the reaction cascade.

Structural and Functional Basis for (S)-Allantoin Formation in the Ureide Pathway

The ureide pathway, which mediates the oxidative degradation of uric acid to (S)-allantoin, represents the late stage of purine catabolism in most organisms. The details of uric acid metabolism remained elusive until the complete pathway involving three enzymes was recently identified and characterized. However, the molecular details of the exclusive production of one enantiomer of allantoin in this pathway are still undefined. Here we report the crystal structure of 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline (OHCU) decarboxylase, which catalyzes the last reaction of the pathway, in a complex with the product, (S)-allantoin, at 2.5-A resolution. The homodimeric helical protein represents a novel structural motif and reveals that the active site in each monomer contains no cofactors, distinguishing this enzyme mechanistically from other cofactor-dependent decarboxylases. On the basis of structural analysis, along with site-directed mutagenesis, a mechanism for the enzyme is proposed in which a decarboxylation reaction occurs directly, and the invariant histidine residue in the OHCU decarboxylase family plays an essential role in producing (S)-allantoin through a proton transfer from the hydroxyl group at C4 to C5 at the re-face of OHCU. These results provide molecular details that address a longstanding question of how living organisms selectively produce (S)-allantoin.

The structure of 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline decarboxylase provides insights into the mechanism of uric acid degradation

The complete degradation of uric acid to (S)-allantoin, as recently elucidated, involves three enzymatic reactions. Inactivation by pseudogenization of the genes of the pathway occurred during hominoid evolution, resulting in a high concentration of urate in the blood and susceptibility to gout. Here, we describe the 1.8A resolution crystal structure of the homodimeric 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline decarboxylase, which catalyzes the last step in the urate degradation pathway, for both ligand-free enzyme and enzyme in complex with the substrate analogs (R)-allantoin and guanine. Each monomer comprises ten alpha-helices, grouped into two domains and assembled in a novel fold. The structure and the mutational analysis of the active site have allowed us to identify some residues that are essential for catalysis, among which His-67 and Glu-87 appear to play a particularly significant role. Glu-87 may facilitate the exit of the carboxylate group because of electrostatic repulsion that destabilizes the ground state of the substrate, whereas His-67 is likely to be involved in a protonation step leading to the stereoselective formation of the (S)-allantoin enantiomer as reaction product. The structural and functional characterization of 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline decarboxylase can provide useful information in view of the potential use of this enzyme in the enzymatic therapy of gout.