Short-chain Dehydrogenase Research Articles

Substrate flexibility and reaction specificity of tropinone reductase-like short-chain dehydrogenases

Annotations of protein or gene sequences from large scale sequencing projects are based on protein size, characteristic binding motifs, and conserved catalytic amino acids, but biochemical functions are often uncertain. In the large family of short-chain dehydrogenases/reductases (SDRs), functional predictions often fail. Putative tropinone reductases, named tropinone reductase-like (TRL), are SDRs annotated in many genomes of organisms that do not contain tropane alkaloids. SDRs in vitro often accept several substrates complicating functional assignments. Cochlearia officinalis, a Brassicaceae, contains tropane alkaloids, in contrast to the closely related Arabidopsis thaliana. TRLs from Arabidopsis and the tropinone reductase isolated from Cochlearia (CoTR) were investigated for their catalytic capacity. In contrast to CoTR, none of the Arabidopsis TRLs reduced tropinone in vitro. NAD(H) and NADP(H) preferences were relaxed in two TRLs, and protein homology models revealed flexibility of amino acid residues in the active site allowing binding of both cofactors. TRLs reduced various carbonyl compounds, among them terpene ketones. The reduction was stereospecific for most of TRLs investigated, and the corresponding terpene alcohol oxidation was stereoselective. Carbonyl compounds that were identified to serve as substrates were applied for modeling pharmacophores of each TRL. A database of commercially available compounds was screened using the pharmacophores. Compounds identified as potential substrates were confirmed by turnover in vitro. Thus pharmacophores may contribute to better predictability of biochemical functions of SDR enzymes.

Isolation and Biochemical Characterization of a Glucose Dehydrogenase from a Hay Infusion Metagenome

Glucose hydrolyzing enzymes are essential to determine blood glucose level. A high-throughput screening approach was established to identify NAD(P)-dependent glucose dehydrogenases for the application in test stripes and the respective blood glucose meters. In the current report a glucose hydrolyzing enzyme, derived from a metagenomic library by expressing recombinant DNA fragments isolated from hay infusion, was characterized. The recombinant clone showing activity on glucose as substrate exhibited an open reading frame of 987 bp encoding for a peptide of 328 amino acids. The isolated enzyme showed typical sequence motifs of short-chain-dehydrogenases using NAD(P) as a co-factor and had a sequence similarity between 33 and 35% to characterized glucose dehydrogenases from different Bacillus species. The identified glucose dehydrogenase gene was expressed in E. coli, purified and subsequently characterized. The enzyme, belonging to the superfamily of short-chain dehydrogenases, shows a broad substrate range with a high affinity to glucose, xylose and glucose-6-phosphate. Due to its ability to be strongly associated with its cofactor NAD(P), the enzyme is able to directly transfer electrons from glucose oxidation to external electron acceptors by regenerating the cofactor while being still associated to the protein.

Comparative Study on Sequence-Structure-Function Relationship of the Human Short-chain Dehydrogenases/Reductases Protein Family.

Human short-chain dehydrogenases/reductases (SDRs) protein family has been the subject of recent studies for its critical role in human metabolism. Studies also found that single nucleotide polymorphisms of the SDR protein family were responsible for a variety of genetic diseases, including type II diabetes. This study reports the effect of sequence variation on the structural and functional integrities of human SDR protein family using phylogenetics and correlated mutation analysis tools. Our results indicated that (i) tyrosine, serine, and lysine are signature protein residues that have direct contribution to the structural and functional stabilities of the SDR protein family, (ii) subgroups of SDR protein family have their own signature protein combination that represent their unique functionality, and (iii) mutations of the human SDR protein family showed high correlation in terms of evolutionary history. In combination, the results inferred that over evolutionary history, the SDR protein family was able to diverge itself in order to adapt with the changes in human nutritional demands. Our study reveals understanding of structural and functional scaffolds of specific SDR subgroups that may facilitate the design of specific inhibitor.

Common Chromosomal Fragile Site Gene WWOX in Metabolic Disorders and Tumors.

WWOX, a gene that spans the second most common chromosomal fragile site (FRA16D), often exhibits homozygous deletions and translocation breakpoints under multiple cellular stresses induced by extrinsic or intrinsic factors, such as hypoxia, UV, and DNA damage regents. Loss of WWOX is closely related to genomic instability, tumorigenesis, cancer progression and therapy resistance. WWOX heterozygous knockout mice show an increased incidence of spontaneous or induced tumors. WWOX can interact via the WW domain with proteins that possess proline PPxY motifs and is involved in a variety of cellular processes. Accumulating evidence has shown that WWOX that contains a short-chain dehydrogenase/reductase (SDR) domain is involved in steroid metabolism and bone development. Reduced or lost expression of WWOX will lead to development of metabolic disease. In this review, we focus on the roles of WWOX in metabolic disorders and tumors.

Comparative Metabolomics Reveals Endogenous Ligands of DAF-12, a Nuclear Hormone Receptor, Regulating C. elegans Development and Lifespan

Small-molecule ligands of nuclear hormone receptors (NHRs) govern the transcriptional regulation of metazoan development, cell differentiation, and metabolism. However, the physiological ligands of many NHRs remain poorly characterized, primarily due to lack of robust analytical techniques. Using comparative metabolomics, we identified endogenous steroids that act as ligands of the C. elegans NHR, DAF-12, a vitamin D and liver X receptor homolog regulating larval development, fat metabolism, and lifespan. The identified molecules feature unexpected chemical modifications and include only one of two DAF-12 ligands reported earlier, necessitating a revision of previously proposed ligand biosynthetic pathways. We further show that ligand profiles are regulated by a complex enzymatic network, including the Rieske oxygenase DAF-36, the short-chain dehydrogenase DHS-16, and the hydroxysteroid dehydrogenase HSD-1. Our results demonstrate the advantages of comparative metabolomics over traditional candidate-based approaches and provide a blueprint for the identification of ligands for other C. elegans and mammalian NHRs.

Short‐chain dehydrogenase/reductase catalyzing the final step of noscapine biosynthesis is localized to laticifers in opium poppy

The final step in the biosynthesis of the phthalideisoquinoline alkaloid noscapine involves a purported dehydrogenation of the narcotinehemiacetal keto moiety. A short-chain dehydrogenase/reductase (SDR), designated noscapine synthase (NOS), that catalyzes dehydrogenation of narcotinehemiacetal to noscapine was identified in opium poppy and functionally characterized. The NOS gene was isolated using an integrated transcript and metabolite profiling strategy and subsequently expressed in Escherichia coli. Noscapine synthase is highly divergent from other characterized members of the NADPH-dependent SDR superfamily involved in benzylisoquinoline alkaloid metabolism, and it exhibits exclusive substrate specificity for narcotinehemiacetal. Kinetic analyses showed that NOS exhibits higher catalytic efficiency with NAD+ as the cofactor compared with NADP+. Suppression of NOS transcript levels in opium poppy plants subjected to virus-induced gene silencing resulted in a corresponding reduction in the accumulation of noscapine and an increase in narcotinehemiacetal levels in the latex. Noscapine and NOS transcripts were detected in all opium poppy organs, but both were most abundant in stems. Unlike other putative biosynthetic genes clustered in the opium poppy genome, and their corresponding proteins, NOS transcripts and the cognate enzyme were abundant in latex, indicating that noscapine metabolism is completed in a distinct cell type compared with the rest of the pathway.

Biochemical properties of human dehydrogenase/reductase (SDR family) member 7

Dehydrogenase/reductase (SDR family) member 7 (DHRS7, retSDR4, SDR34C1) is a previously uncharacterized member of the short-chain dehydrogenase/reductase (SDR) superfamily. While human SDR members are known to play an important role in various (patho)biochemical pathways including intermediary metabolism and biotransformation of xenobiotics, only 20% of them are considered to be well characterized. Based on phylogenetic tree and SDR sequence clusters analysis DHRS7 is a close relative to well-known SDR member 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) that participates in metabolism of endogenous and xenobiotic substances with carbonyl group. The aim of present study is to determine the basic biochemical properties of DHRS7 and its possible involvement in metabolism of substrates with carbonyl group. For the first time the computational predictions of this membrane protein and membrane topology were experimentally confirmed. DHRS7 has been demonstrated to be an integral protein facing the lumen of the endoplasmic reticulum with lack of posttranscriptional glycosylation modification. Subsequently, NADP(H) cofactor preference and enzymatic reducing activity of DHRS7 was determined towards endogenous substrates with a steroid structure (cortisone, 4-androstene-3,17-dion) and also toward relevant exogenous substances bearing a carbonyl group harmful to human health (1,2-naphtoquinone, 9,10-phenantrenequinone). In addition to 11β-HSD1, DHRS7 is another enzyme from SDR superfamily that have been proved, at least in vitro, to contribute to the metabolism of xenobiotics with carbonyl group.

Molecular characterization and homology modeling of a short-chain reductase/dehydrogenase from Gracilaria changii (Rhodophyta)

Short-chain dehydrogenases (SDRs) which catalyze the dehydrogenation/reduction reaction have a wide range of substrate specificities. In this study, we report the molecular characterization of a transcript encoding SDR from the red seaweed, Gracilaria changii (Rhodophyta) as part of our effort to elucidate the functions of novel transcripts from this marine alga. The transcript, denoted as GcSDR, encodes a protein of 282 amino acids with a predicted size of approximately 31 kDa. The GxxxGxG coenzyme binding motif and YxxxK active-site motif of SDRs are well conserved in GcSDR. The coding sequence of GcSDR was cloned and expressed as recombinant protein in Escherichia coli BL21 (DE3) pLysS. Kinetic analysis of recombinant GcSDR using pyruvaldehyde dimethyl acetal as a substrate has a Km value of 116.52 mM and a Vmax value of 720 nmol product formed per minute per milligram. It has a higher affinity towards NADPH compared to NADP+ as a cofactor. Homology modeling showed that the three-dimensional structure of GcSDR has 34.5 % sequence identity to the SDR from a soil bacterium. Virtual screening of the possible substrates for GcSDR revealed that CMP-N-acetyl-beta-neuraminate (2-) which belongs to a group of amino sugars has the lowest binding energy among the compounds examined. The predicted cis-acting regulatory elements (CREs) at the 5′-flanking genomic sequence of GcSDR include CREs associated with abscisic acid, methyl jasmonic acid, light, anoxia, endosperm, low-temperature stress, and heat stress. The transcripts of GcSDR were found to accumulate in seaweed samples treated under low-salinity stress, thus suggesting its involvement during saline deprivation.

Cloning, expression and characterization of a putative 7alpha-hydroxysteroid dehydrogenase in Comamonas testosteroni

The short-chain dehydrogenase/reductase (SDR) superfamily is a large and diverse group of genes with members found in all forms of life. Comamonas testosteroni (C. testosterone) ATCC11996 is a Gram-negative bacterium which can use steroids as carbon and energy source. In the present investigation, we found a novel SDR gene 7alpha-hydroxysteroid dehydrogenase (7α-HSD) which is located 11.9kb upstream from hsdA with the same transcription orientation in the C. testosteroni genome. The open reading frame of this putative 7alpha-hydroxysteroid dehydrogenase gene consists of 771bp and translates into a protein of 256 amino acids. Two consensus sequences of the SDR superfamily were found, an N-terminal Gly-X-X-X-Gly-X-Gly cofactor-binding motif and a Tyr-X-X-X-Lys segment (residues 161–165 in the 7α-HSD sequence) essential for catalytic activity of SDR proteins. To produce purified 7α-HSD protein, the 7α-HSD gene was cloned into plasmid pET-15b and the over expressed protein was purified by His-tag sequence on metal chelate chromatography. To prove that 7α-HSD is involved in the metabolic pathway of steroid compounds, we constructed a 7α-HSD knock-out mutant of C. testosteroni. Compared to the wild type C. testosteroni, degradation of testosterone, estradiol and cholesterol were decreased in the 7α-HSD knock-out mutant.Furthermore, growth in the medium with testosterone, estradiol and cholesterol was impaired in 7α-HSD knock-out mutant. The results showed that 7α-HSD is involved in steroid degradation.

VEP1- encoded enone 1,4 - reductases from Brassicaceae: cloning, expression, molecular phylogeny and modelling

The mustard family (Brassicaceae) supplies the world with many crop species and medicinal plants. In the Brassicaceae only few genera contain cardenolides (Syrenia, Draba, Erysimum incl. Cheiranthus) whereas all of them contain glucosinolates. One may ask whether cardenolides originated independently several times in the Brassicaceae or whether they were lost during evolution in most of the genera. In order to shed light on the conservation and/or the multiple evolution of genes coding for cardenolide-biosynthetic enzymes, we isolated cDNA encoding progesterone 5β-reductases (P5βR) from several Brassicaceae species (Armoracia rusticana, Aethionema grandiflora, Barbarea vulgaris, various Brassica species, Cakile maritima, Cardamine pratensis, Draba aizoides, Lunaria annua, Lepidium sativum, Matthiola tricuspidata, Nasturtium officinale, Raphanus sativus and Sisymbrium officinale). P5βRs, are members of the short chain dehydrogenases/reductases (SDR) superfamily of proteins and are supposed to be involved in the biosynthesis of 5β-cardenolides. These enzymes have been characterized as substrate-promiscuous enone 1,4-reductases and are encoded by Vein Patterning 1 (VEP1) genes, which occur in a wide range of plant species independent of their ability to produce cardenolides or not. The nucleotide and deduced amino acid sequences of the new P5βRs were aligned with known and putative VEP1-encoded P5βRs and a cladistic tree was constructed which fitted well to the accepted Brassicaceae phylogenetic relationship, indicating a common ancestor for all Brassicaceae P5βRs. The new P5βR cDNAs were over-expressed in E. coli and the respective P5βR proteins tested for their catalytic function. Their kinetic constants were determined using progesterone and small enones as substrates. Molecular modelling was applied to elucidate structural and functional relationship between VEP1-encoded P5βRs.

VEP1- encoded enone 1,4-reductases from Brassicaceae: cloning, expression, molecular phylogeny and modelling

Progesterone 5β-reductases (P5βR), which are members of the short chain dehydrogenases/reductases (SDR) superfamily are involved in the biosynthesis of 5β-cardenolides. These enzymes have been characterized as substrate-promiscuous enone 1,4-reductases and are encoded by Vein Patterning1-like (VEP1) genes, which occur in cardenolide producing plants as well as in non-cardenolide producing plants. Since cardenolides have been reported for a few Brassicaceae genera only, one may ask whether cardenolides originated several times in Brassicaceae or whether they were lost during evolution in most of the genera. In order to shed light on the conservation and/or the multiple evolution of genes coding for cardenolide-biosynthetic enzymes, we isolated P5βRs from several Brassicaceae. The nucleotide and deduced amino acid sequences of the new P5βRs were aligned with known and putative VEP1-encoded P5βRs. The new genes were over-expressed in E. coli and the respective P5βR proteins tested for their catalytic function. Molecular modelling was applied to elucidate structural and functional relationship between VEP1-encoded P5βRs.

The biocatalytic capacity of tropinone reductase-like enzymes from plants

Tropinone reductases (TRs) are short-chain dehydrogenases/reductases (SDRs) involved in tropane alkaloid biosynthesis. TRs reduce tropinone stereospecifically either to tropine (3a-tropanol), precursor of classical tropane alkaloids like hyoscyamine and scopolamine, or to pseudotropine (3b-tropanol), precursor of nortropane alkaloids e.g. calystegines. TRs were cloned and expressed from several tropane and nortropane alkaloid containing Solanaceae. In addition, one tropane alkaloid forming species of the Brassicaceae, Cochlearia officinalis, contains a special tropinone reductase (CoTR), which reduces tropinone to pseudotropine and tropine in vitro.

L-Arabinose degradation pathway in the haloarchaeon Haloferax volcanii involves a novel type of l-arabinose dehydrogenase

The pathway of L-arabinose degradation was studied in the haloarchaeon Haloferax volcanii. It is shown that L-arabinose is oxidatively degraded to α-ketoglutarate. During growth on L-arabinose, L-arabinose dehydrogenase (L-AraDH) was induced. The enzyme was purified as a 130kDa homotetrameric protein catalyzing the oxidation of L-arabinose with both NADP(+) and NAD(+). The gene encoding L-AraDH was identified as HVO_B0032 and recombinant L-AraDH showed similar properties as the native enzyme. The L-AraDH deletion mutant did not grow on L-arabinose, but grew unaffected on glucose and D-xylose, indicating a specific involvement in L-arabinose degradation. Phylogenetic analyses attribute the first archaeal L-AraDH to the extended short-chain dehydrogenase/reductase (SDRe) family, where it is part of a novel cluster and thus differs from known archaeal and bacterial pentose dehydrogenases. Further, cell extracts of H. volcanii catalyzed the NADP(+)-dependent conversion of L-arabinoate to α-ketoglutarate. The genes involved in that conversion were identified by analyses of transcripts and deletion mutants as HVO_B0038A, HVO_B0027 and HVO_B0039 recently reported to be involved in D-xylonate conversion to α-ketoglutarate in H. volcanii (Johnsen et al. 2009).

Enantiocomplementary Yarrowia lipolytica Oxidoreductases: Alcohol Dehydrogenase 2 and Short Chain Dehydrogenase/Reductase.

Enzymes of the non-conventional yeast Yarrowia lipolytica seem to be tailor-made for the conversion of lipophilic substrates. Herein, we cloned and overexpressed the Zn-dependent alcohol dehydrogenase ADH2 from Yarrowia lipolytica in Escherichia coli. The purified enzyme was characterized in vitro. The substrate scope for YlADH2 mediated oxidation and reduction was investigated spectrophotometrically and the enzyme showed a broader substrate range than its homolog from Saccharomyces cerevisiae. A preference for secondary compared to primary alcohols in oxidation direction was observed for YlADH2. 2-Octanone was investigated in reduction mode in detail. Remarkably, YlADH2 displays perfect (S)-selectivity and together with a highly (R)-selective short chain dehydrogenase/ reductase from Yarrowia lipolytica it is possible to access both enantiomers of 2-octanol in >99% ee with Yarrowia lipolytica oxidoreductases.

Computational design of short-chain dehydrogenase Gox2181 for altered coenzyme specificity

Short-chain dehydrogenase Gox2181 from Gluconobacter oxydans catalyzes the reduction of 2,3-pentanedione by using NADH as the physiological electron donor. To realize its synthetic biological application for coenzyme recycling use, computational design and site-directed mutagenesis have been used to engineer Gox2181 to utilize not only NADH but also NADPH as the electron donor. Single and double mutations at residues Q20 and D43 were made in a recombinant expression system that corresponded to Gox2181-D43Q and Gox2181-Q20R&D43Q, respectively. The design of mutant Q20R not only resolved the hydrogen bond interaction and electrostatic interaction between R and 2′-phosphate of NADPH, but also could enhance the binding with 2′-phophated of NADPH by combining with D43Q. Molecular dynamics simulation has been carried out to testify the hydrogen bond interactions between mutation sites and 2′-phosphate of NADPH. Steady-state turnover measurement results indicated that Gox2181-D43Q could use both NADH and NADPH as its coenzyme, and so could Gox2181-Q20R&D43Q. Meanwhile, compared to the wild-type enzyme, Gox2181-D43Q exhibited dramatically reduced enzymatic activity while Gox2181-Q20R&D43Q successfully retained the majority of enzymatic activity.

Adaptive and Specialised Transcriptional Responses to Xenobiotic Stress in Caenorhabditis elegans Are Regulated by Nuclear Hormone Receptors

Characterisation of the pathways by which xenobiotics are metabolised and excreted in both target and non-target organisms is crucial for the rational design of effective and specific novel bioactive molecules. Consequently, we have investigated the induced responses of the model nematode Caenorhabditis elegans to a variety of xenobiotics which represent a range of putative modes of action. The majority of genes that were specifically induced in preliminary microarray analyses encoded enzymes from Phase I and II metabolism, including cytochrome P450s, short chain dehydrogenases, UDP-glucuronosyl transferases and glutathione transferases. Changes in gene expression were confirmed by quantitative PCR and GFP induction in reporter strains driven by promoters for transcription of twelve induced enzymes was investigated. The particular complement of metabolic genes induced was found to be highly contingent on the xenobiotic applied. The known regulators of responses to applied chemicals ahr-1, hif-1, mdt-15 and nhr-8 were not required for any of these inducible responses and skn-1 regulated GFP expression from only two of the promoters. Reporter strains were used in conjunction with systematic RNAi screens to identify transcription factors which drive expression of these genes under xenobiotic exposure. These transcription factors appeared to regulate specific xenobiotic responses and have no reported phenotypes under standard conditions. Focussing on nhr-176 we demonstrate the role of this transcription factor in mediating the resistance to thiabendazole.

Crystallographic analysis and structure‐guided engineering of NADPH‐dependent Ralstonia sp. Alcohol dehydrogenase toward NADH cosubstrate specificity

The NADP⁺-dependent alcohol dehydrogenase from Ralstonia sp. (RasADH) belongs to the protein superfamily of short-chain dehydrogenases/reductases (SDRs). As an enzyme that accepts different types of substrates--including bulky-bulky as well as small-bulky secondary alcohols or ketones--with high stereoselectivity, it offers potential as a biocatalyst for industrial biotechnology. To understand substrate and cosubstrate specificities of RasADH we determined the crystal structure of the apo-enzyme as well as its NADP⁺-bound state with resolutions down to 2.8 Å. RasADH displays a homotetrameric quaternary structure that can be described as a dimer of homodimers while in each subunit a seven-stranded parallel β-sheet, flanked by three α-helices on each side, forms a Rossmann fold-type dinucleotide binding domain. Docking of the well-known substrate (S)-1-phenylethanol clearly revealed the structural determinants of stereospecificity. To favor practical RasADH application in the context of established cofactor recycling systems, for example, those involving an NADH-dependent amino acid dehydrogenase, we attempted to rationally change its cosubstrate specificity from NADP⁺ to NAD⁺ utilizing the structural information that NADP⁺ specificity is largely governed by the residues Asn15, Gly37, Arg38, and Arg39. Furthermore, an extensive sequence alignment with homologous dehydrogenases that have different cosubstrate specificities revealed a modified general SDR motif ASNG (instead of NNAG) at positions 86-89 of RasADH. Consequently, we constructed mutant enzymes with one (G37D), four (N15G/G37D/R38V/R39S), and six (N15G/G37D/R38V/R39S/A86N/S88A) amino acid exchanges. RasADH (N15G/G37D/R38V/R39S) was better able to accept NAD⁺ while showing much reduced catalytic efficiency with NADP⁺, leading to a change in NADH/NADPH specificity by a factor of ∼3.6 million.

Synthetic Strategy toward γ-Keto Nitriles and Their Biocatalytic Conversion to Asymmetric γ-Lactones

Asymmetric γ-hydroxy nitriles are valuable intermediates because hydrolysis of the nitriles can result in an intramolecular cyclization to chiral γ-lactones, which have a variety of biological uses. Starting with an assortment of different aldehydes (alkyl and aryl) a 4-step synthesis of γ-keto nitriles was developed. These prochiral substrates were then screened against a library of ketoreductases for their ability to stereoselectively reduce the carbonyl. Enzymes from the short chain dehydrogenase family showed activity and these enzymatic reactions were scaled up to produce a diverse set of chiral γ-lactones.

Crystal structure of the capsular polysaccharide synthesizing protein CapE of Staphylococcus aureus

Enzymes synthesizing the bacterial CP (capsular polysaccharide) are attractive antimicrobial targets. However, we lack critical information about the structure and mechanism of many of them. In an effort to reduce that gap, we have determined three different crystal structures of the enzyme CapE of the human pathogen Staphylococcus aureus. The structure reveals that CapE is a member of the SDR (short-chain dehydrogenase/reductase) super-family of proteins. CapE assembles in a hexameric complex stabilized by three major contact surfaces between protein subunits. Turnover of substrate and/or coenzyme induces major conformational changes at the contact interface between protein subunits, and a displacement of the substrate-binding domain with respect to the Rossmann domain. A novel dynamic element that we called the latch is essential for remodelling of the protein–protein interface. Structural and primary sequence alignment identifies a group of SDR proteins involved in polysaccharide synthesis that share the two salient features of CapE: the mobile loop (latch) and a distinctive catalytic site (MxxxK). The relevance of these structural elements was evaluated by site-directed mutagenesis.

Human UDP-α-d-xylose Synthase Forms a Catalytically Important Tetramer That Has Not Been Observed in Crystal Structures

Human UDP-α-d-xylose synthase (hUXS) is a member of the extended short chain dehydrogenase/reductase (SDR) family of enzymes. Previous crystallographic studies have shown that hUXS conserves the same dimeric quaternary structure observed in other SDR enzymes. Here, we present evidence that hUXS also forms a tetramer in solution that is important for activity. Sedimentation velocity studies show that two hUXS dimers undergo a concentration-dependent association to form a tetramer with a Kd of 2.9 μM. The tetrameric complex is also observed in small-angle X-ray scattering (SAXS). The specific activity for the production of the reaction intermediate UDP-α-d-4-keto-xylose displays a hyperbolic dependence on protein concentration that is well modeled by an isotherm using the 2.9 μM Kd of the tetramer. Likewise, the rate of UDP-α-d-xylose production in the presence of increasing concentrations of the small molecule crowder trimethylamine N-oxide is consistent with the formation of a higher activity tetramer. We present several possible structural models of the hUXS tetramer based on (i) hUXS crystal packing, (ii) homology modeling, or (iii) ab initio simulated annealing of dimers. We analyze the models in terms of packing quality and agreement with SAXS data. The higher activity of the tetramer coupled with the relative instability of the complex suggests that an association-dissociation mechanism may regulate hUXS activity.