Molecular structures of the S124A, S124T, and S124V site-directed mutants of UDP-galactose 4-epimerase from Escherichia coli

Molecular evolution and functional divergence of UDP-hexose 4-epimerases

UDP-glucose 4-epimerase (GalE) catalyzes the interconversion of UDP-glucose (UDP-Glc) and UDP-galactose (UDP-Gal) and/or the interconversion of UDP-N-acetylglucosamine (UDP-GlcNAc) and UDP-N-acetylgalactosamine (UDP-GalNAc) in sugar metabolism. GalEs belong to the short-chain dehydrogenase/reductase superfamily, use a conserved 'transient keto intermediate' mechanism and have variable substrate specificity. GalEs have been classified into three groups based on substrate specificity: group 1 prefers UDP-Glc/Gal, group 3 prefers UDP-GlcNAc/GalNAc, and group 2 has comparable activities for both types of the substrates. The phylogenetic relationship and structural basis for the specificities of GalEs revealed possible molecular evolution of UDP-hexose 4-epimerases in various organisms. Based on the recent advances in studies on GalEs and related enzymes, an updated view of their evolutional diversification is presented.

Structural basis for broad substrate specificity of UDP-glucose 4-epimerase in the human milk oligosaccharide catabolic pathway of Bifidobacterium longum

Infant gut-associated bifidobacteria has a metabolic pathway that specifically utilizes lacto-N-biose I (Gal-β1,3-GlcNAc) and galacto-N-biose (Gal-β1,3-GalNAc) from human milk and mucin glycans. UDP-glucose 4-epimerase (GalE) from Bifidobacterium longum (bGalE) catalyzes epimerization reactions of UDP-Gal into UDP-Glc and UDP-GalNAc into UDP-GlcNAc with the same level of activity that is required to send galacto-hexoses into glycolysis. Here, we determined the crystal structures of bGalE in three ternary complex forms: NAD⁺/UDP, NAD⁺/UDP-GlcNAc, and NAD⁺/UDP-Glc. The broad specificity of bGalE was explained by structural features of the binding pocket for the N-acetyl or C2 hydroxy group of the substrate. Asn200 is located in a pocket of the C2 group, and its side chain adopts different conformations in the complex structures with UDP-Glc and UDP-GlcNAc. On the other side, Cys299 forms a large pocket for the C5 sugar ring atom. The flexible C2 pocket and the large C5 pocket of bGalE are suitable for accommodating both the hydroxy and N-acetyl groups of the substrate during sugar ring rotation in the catalytic cycle. The substrate specificity and active site structure of bGalE were distinct from those of Esherichia coli GalE but similar to those of human GalE.

UDP-Glucose 4-Epimerase and β-1,4-Galactosyltransferase from the Oyster Magallana gigas as Valuable Biocatalysts for the Production of Galactosylated Products

Uridine diphosphate galactose (UDP-galactose) is a valuable building block in the enzymatic synthesis of galactose-containing glycoconjugates. UDP-glucose 4-epimerase (UGE) is an enzyme which catalyzes the reversible conversion of abundantly available UDP-glucose to UDP-galactose. Herein, we described the cloning, expression, purification, and biochemical characterization of an unstudied UGE from the oyster Magallana gigas (MgUGE). Activity tests of recombinantly expressed MgUGE, using HPLC (high-performance liquid chromatography), mass spectrometry, and photometric assays, showed an optimal temperature of 16 °C, and reasonable thermal stability up to 37 °C. No metal ions were required for enzymatic activity. The simple nickel-affinity-purification procedure makes MgUGE a valuable biocatalyst for the synthesis of UDP-galactose from UDP-glucose. The biosynthetic potential of MgUGE was further exemplified in a coupled enzymatic reaction with an oyster-derived β-1,4-galactosyltransferase (MgGalT7), allowing the galactosylation of the model substrate para-nitrophenol xylose (pNP-xylose) using UDP-glucose as the starting material.

Functional characterization and transcriptional analysis of galE gene encoding a UDP-galactose 4-epimerase in Xanthomonas campestris pv. campestris

The Gram-negative plant pathogen Xanthomonas campestris pv. campestris (Xcc) is the causative agent of black rot in crucifers, a disease that causes tremendous agricultural loss. In this study, the Xcc galE gene was characterized. Sequence and mutational analysis demonstrated that the Xcc galE encodes a UDP-galactose 4-epimerase (EC 5.1.3.2), which catalyzes the interconversion of UDP-galactose and UDP-glucose. Alanine substitution of the putative catalytic residues (Ser124, Tyr147, and Lys151) of GalE caused loss of epimerase activity. Further study showed that the Xcc galE mutant had reduced biofilm formation ability. Furthermore, reporter assays revealed that galE transcription exhibits a distinct expression profile under different culture conditions, is subject to catabolite repression, and is positively regulated by Clp and RpfF. In addition, the galE transcription initiation site was mapped. This is the first time that UDP-galactose 4-epimerase has been characterized in the crucifer pathogen Xcc.

Elucidation of substrate specificity in Aspergillus nidulans UDP-galactose-4-epimerase

The frequency of invasive fungal infections has rapidly increased in recent years. Current clinical treatments are experiencing decreased potency due to severe host toxicity and the emergence of fungal drug resistance. As such, new targets and their corresponding synthetic pathways need to be explored for drug development purposes. In this context, galactofuranose residues, which are employed in fungal cell wall construction, but are notably absent in animals, represent an appealing target. Herein we present the structural and biochemical characterization of UDP-galactose-4-epimerase from Aspergillus nidulans which produces the precursor UDP-galactopyranose required for galactofuranose synthesis. Examination of the structural model revealed both NAD⁺ and UDP-glucopyranose were bound within the active site cleft in a near identical fashion to that found in the Human epimerase. Mutational studies on the conserved catalytic motif support a similar mechanism to that established for the Human counterpart is likely operational within the A. nidulans epimerase. While the K_m and k_cat for the enzyme were determined to be 0.11 mM and 12.8 s^-1, respectively, a single point mutation, namely L320C, activated the enzyme towards larger N-acetylated substrates. Docking studies designed to probe active site affinity corroborate the experimentally determined activity profiles and support the kinetic inhibition results.

Chemical and stereochemical actions of UDP-galactose 4-epimerase

Uridine(5')diphospho(1)α-D-galactose (UDP-gal) provides all galactosyl units in biologically synthesized carbohydrates. All healthy cells produce UDP-gal from uridine(5')diphospho(1)α-D-glucose (UDP-glc) by the action of UDP-galactose 4-epimerase (GalE). This Account provides our recent results describing unusual mechanistic features of this enzyme. Fully active GalE is dimeric and contains one tightly bound nicotinamide adenine dinucleotide (NAD) per subunit. The NAD undergoes reversible reduction to NADH in the chemical mechanism. GalE displays unusual enzymological, chemical, and stereochemical properties. These include practically irreversible binding of NAD, nonstereospecific hydride transfer, uridine nucleotide-induced activation of NAD, Tyr149 as a base catalyst, and [GalE-NADH]-oxidation in one-electron steps by one-electron acceptors. Early studies revealed that uridine(5')diphospho(1)α-D-4-ketopyranose (UDP-4-ketopyranose) and NADH are reaction intermediates. Weak binding of the 4-ketopyranosyl moiety and strong binding of the UDP-moiety allowed either face of the 4-ketopyranosyl moiety to accept hydride from NADH. In crystal structures of GalE, NAD bound within a Rossmann-type fold and uridine nucleotides within a substrate domain. Structures of [GalE-NADH] in complex with UDP-glc show Lys153, Tyr149, and Ser124 in contact with NAD or glucosyl-C4(OH). Lys153 forms hydrogen bonds to the ribosyl-OH groups of NAD. The phenolate of Tyr149 is associated with both the nicotinamide ring of NAD and glucosyl-C4(OH). Ser124 is hydrogen-bonded to glucosyl-C4(OH). Spectrophotometry studies show a pH-dependent charge transfer (CT) complex between Tyr149 and NAD. The CT-complex has a pKa of 6.1, which results in bleaching of the CT-band. The CT-band also bleaches upon binding of a uridine nucleotide. Kinetic experiments with wild-type GalE and Ser124Ala-GalE show the same kinetic pKa values as the corresponding CT-band pKa, which point to Tyr149 as the base catalyst for hydride transfer. We used NMR studies to verify that uridine nucleotide binding polarizes nicotinamide π-electrons. The binding of uridine(5')-diphosphate (UDP) to GalE-[nicotinamide-1-¹⁵N]NAD shifts the ¹⁵N-signal upfield 3 ppm, whereas UDP-binding to GalE-[nicotinamide-4-¹³C]NAD shifts the ¹³C-signal downfield by 3.4 ppm. Electrochemical and ¹³C NMR data for a series of N-alkylnicotinamides show that the 3.4 ppm downfield ¹³C-perturbation in GalE corresponds to an elevation of the NAD reduction potential by 150 mV. These results account for the uridine nucleotide-dependence in the reduction of [GalE-NAD] by glucose or NaBH₃CN. Kinetics in the reduction of Tyr149Phe- and Lys153Met-GalE-NAD implicate Tyr149 and Lys153 in the nucleotide-dependent reduction of NAD. They further implicate electrostatic repulsion between N1 of NAD and the ε-aminium group of Lys153 in nucleotide-induced activation of NAD. In an O₂-dependent reaction, [GalE-NADH] reduces the stable radical UDP-TEMPO with production of superoxide radical. The reaction must proceed by way of a NAD-pyridinyl radical intermediate.

Insights into role of the hydrogen bond networks in substrate recognition by UDP-GalNAc 4-epimerases

UDP-hexose 4-epimerases are critical in galactose metabolism and often important in lipopolysaccharide biosynthesis as well. Three groups of these enzymes have been reported based on their substrate specificity towards non-acetylated substrates (group 1), dual specificity towards N-acetylated and non-acetylated substrates (group 2) and specificity towards N-acetylated substrates (group 3). We recently reported the structure of a novel UDP-GalNAc 4-epimerase called WbgU and based on the structure proposed a model of specific substrate recognition by UDP-GalNAc 4-epimerases. In this work, we present an analysis of the proposed model of substrate recognition using site-directed mutagenesis of WbgU and crystal structure of the His305Ala mutant. This investigation reveals that the wild-type activity of WbgU is retained in most single-point mutants targeting the active site. However, a graded loss in activity is observed in double-and triple-point mutants with the quadruple-point mutant being completely inactive corroborating the proposed rationale of substrate recognition. Furthermore, crystal structure of the His305Ala mutant shows that the structure is significantly similar to the wild-type WbgU, albeit a loss in the critical hydrogen bond network seated at His305 and ensuing minor conformational changes. It is inferred that the specific and non-specific interactions throughout the active site confer it sufficient elasticity to sustain wild-type activity for several of the single-point mutations.

Crystal structure of UDP-galactose 4-epimerase from the hyperthermophilic archaeon Pyrobaculum calidifontis

The crystal structure of a highly thermostable UDP-galactose 4-epimerase (GalE) from the hyperthermophilic archaeon Pyrobaculum calidifontis was determined at a resolution of 1.8Å. The asymmetric unit contained one subunit, and the functional dimer was generated by a crystallographic two-fold axis. Each monomer consisted of a Rossmann-fold domain with NAD bound and a carboxyl terminal domain. The overall structure of P. calidifontis GalE showed significant similarity to the structures of the GalEs from Escherichia coli, human and Trypanosoma brucei. However, the sizes of several surface loops were markedly smaller in P. calidifontis GalE than the corresponding loops in the other enzymes. Structural comparison revealed that the presence of an extensive hydrophobic interaction at the subunit interface is likely the main factor contributing to the hyperthermostability of the P. calidifontis enzyme. Within the NAD-binding site of P. calidifontis GalE, a loop (NAD-binding loop) tightly holds the adenine ribose moiety of NAD. Moreover, a deletion mutant lacking this loop bound NAD in a loose, reversible manner. Thus the presence of the NAD-binding loop in GalE is largely responsible for preventing the release of the cofactor from the holoenzyme.

Widespread head-to-head hydrocarbon biosynthesis in bacteria and role of OleA

Previous studies identified the oleABCD genes involved in head-to-head olefinic hydrocarbon biosynthesis. The present study more fully defined the OleABCD protein families within the thiolase, alpha/beta-hydrolase, AMP-dependent ligase/synthase, and short-chain dehydrogenase superfamilies, respectively. Only 0.1 to 1% of each superfamily represents likely Ole proteins. Sequence analysis based on structural alignments and gene context was used to identify highly likely ole genes. Selected microorganisms from the phyla Verucomicrobia, Planctomyces, Chloroflexi, Proteobacteria, and Actinobacteria were tested experimentally and shown to produce long-chain olefinic hydrocarbons. However, different species from the same genera sometimes lack the ole genes and fail to produce olefinic hydrocarbons. Overall, only 1.9% of 3,558 genomes analyzed showed clear evidence for containing ole genes. The type of olefins produced by different bacteria differed greatly with respect to the number of carbon-carbon double bonds. The greatest number of organisms surveyed biosynthesized a single long-chain olefin, 3,6,9,12,15,19,22,25,28-hentriacontanonaene, that contains nine double bonds. Xanthomonas campestris produced the greatest number of distinct olefin products, 15 compounds ranging in length from C(28) to C(31) and containing one to three double bonds. The type of long-chain product formed was shown to be dependent on the oleA gene in experiments with Shewanella oneidensis MR-1 ole gene deletion mutants containing native or heterologous oleA genes expressed in trans. A strain deleted in oleABCD and containing oleA in trans produced only ketones. Based on these observations, it was proposed that OleA catalyzes a nondecarboxylative thiolytic condensation of fatty acyl chains to generate a beta-ketoacyl intermediate that can decarboxylate spontaneously to generate ketones.

Medium- and short-chain dehydrogenase/reductase gene and protein families : the SDR superfamily: functional and structural diversity within a family of metabolic and regulatory enzymes

Short-chain dehydrogenases/reductases (SDRs) constitute a large family of NAD(P)(H)-dependent oxidoreductases, sharing sequence motifs and displaying similar mechanisms. SDR enzymes have critical roles in lipid, amino acid, carbohydrate, cofactor, hormone and xenobiotic metabolism as well as in redox sensor mechanisms. Sequence identities are low, and the most conserved feature is an α/β folding pattern with a central beta sheet flanked by 2–3 α-helices from each side, thus a classical Rossmannfold motif for nucleotide binding. The conservation of this element and an active site, often with an Asn-Ser-Tyr-Lys tetrad, provides a platform for enzymatic activities encompassing several EC classes, including oxidoreductases, epimerases and lyases. The common mechanism is an underlying hydride and proton transfer involving the nicotinamide and typically an active site tyrosine residue, whereas substrate specificity is determined by a variable C-terminal segment. Relationships exist with bacterial haloalcohol dehalogenases, which lack cofactor binding but have the active site architecture, emphasizing the versatility of the basic fold in also generating hydride transfer-independent lyases. The conserved fold and nucleotide binding emphasize the role of SDRs as scaffolds for an NAD(P)(H) redox sensor system, of importance to control metabolic routes, transcription and signalling.

Crystal structure of isoflavone reductase from alfalfa (Medicago sativa L.)

Isoflavonoids play important roles in plant defense and exhibit a range of mammalian health-promoting activities. Isoflavone reductase (IFR) specifically recognizes isoflavones and catalyzes a stereospecific NADPH-dependent reduction to (3R)-isoflavanone. The crystal structure of Medicago sativa IFR with deletion of residues 39-47 has been determined at 1.6A resolution. Structural analysis, molecular modeling and docking, and comparison with the structures of other NADPH-dependent enzymes, defined the putative binding sites for co-factor and substrate and potential key residues for enzyme activity and substrate specificity. Further mutagenesis has confirmed the role of Lys144 as a catalytic residue. This study provides a structural basis for understanding the enzymatic mechanism and substrate specificity of IFRs as well as the functions of IFR-like proteins.

Towards a better understanding of the substrate specificity of the UDP-N-acetylglucosamine C4 epimerase WbpP

WbpP is the only genuine UDP-GlcNAc (UDP-N-acetylglucosamine) C4 epimerase for which both biochemical and structural data are available. This represents a golden opportunity to elucidate the molecular basis for its specificity for N-acetylated substrates. Based on the comparison of the substrate binding site of WbpP with that of other C4 epimerases that convert preferentially non-acetylated substrates, or that are able to convert both acetylated and non-acetylated substrates equally well, specific residues of WbpP were mutated, and the substrate specificity of the mutants was determined by direct biochemical assays and kinetic analyses. Most of the mutations tested were anticipated to trigger a significant switch in substrate specificity, mostly towards a preference for non-acetylated substrates. However, only one of the mutations (A209H) had the expected effect, and most others resulted in enhanced specificity of WbpP for N-acetylated substrates (Q201E, G102K, Q201E/G102K, A209N and S143A). One mutation (S144K) totally abolished enzyme activity. These data indicate that, although all residues targeted in the present study turned out to be important for catalysis, determinants of substrate specificity are not confined to the substrate-binding pocket and that longer range interactions are essential in allowing proper positioning of various ligands in the binding pocket. Hence prediction or engineering of substrate specificity solely based on sequence analysis, or even on modelling of the binding pocket, might lead to incorrect functional assignments.

Structure and mechanism of ArnA: conformational change implies ordered dehydrogenase mechanism in key enzyme for polymyxin resistance

The modification of lipid A with 4-amino-4-deoxy-L-arabinose (Ara4N) allows gram-negative bacteria to resist the antimicrobial activity of cationic antimicrobial peptides and antibiotics such as polymyxin. ArnA is the first enzyme specific to the lipid A-Ara4N pathway. It contains two functionally and physically separable domains: a dehydrogenase domain (ArnA_DH) catalyzing the NAD+-dependent oxidative decarboxylation of UDP-Glucuronic acid (UDP-GlcA), and a transformylase domain that formylates UDP-Ara4N. Here, we describe the crystal structure of the full-length bifunctional ArnA with UDP-GlcA and ATP bound to the dehydrogenase domain. Binding of UDP-GlcA triggers a 17 A conformational change in ArnA_DH that opens the NAD+ binding site while trapping UDP-GlcA. We propose an ordered mechanism of substrate binding and product release. Mutation of residues R619 and S433 demonstrates their importance in catalysis and suggests that R619 functions as a general acid in catalysis. The proposed mechanism for ArnA_DH has important implications for the design of selective inhibitors.

Crystal structure of Escherichia coli ArnA (PmrI) decarboxylase domain. A key enzyme for lipid A modification with 4-amino-4-deoxy-L-arabinose and polymyxin resistance

Gram-negative bacteria including Escherichia coli, Salmonella typhimurium, and Pseudomonas aeruginosa can modify the structure of lipid A in their outer membrane with 4-amino-4-deoxy-l-arabinose (Ara4N). Such modification results in resistance to cationic antimicrobial peptides of the innate immune system and antibiotics such as polymyxin. ArnA is a key enzyme in the lipid A modification pathway, and its deletion abolishes both the Ara4N-lipid A modification and polymyxin resistance. ArnA is a bifunctional enzyme. It can catalyze (i) the NAD(+)-dependent decarboxylation of UDP-glucuronic acid to UDP-4-keto-arabinose and (ii) the N-10-formyltetrahydrofolate-dependent formylation of UDP-4-amino-4-deoxy-l-arabinose. We show that the NAD(+)-dependent decarboxylating activity is contained in the 360 amino acid C-terminal domain of ArnA. This domain is separable from the N-terminal fragment, and its activity is identical to that of the full-length enzyme. The crystal structure of the ArnA decarboxylase domain from E. coli is presented here. The structure confirms that the enzyme belongs to the short-chain dehydrogenase/reductase (SDR) family. On the basis of sequence and structure comparisons of the ArnA decarboxylase domain with other members of the short-chain dehydrogenase/reductase (SDR) family, we propose a binding model for NAD(+) and UDP-glucuronic acid and the involvement of residues T(432), Y(463), K(467), R(619), and S(433) in the mechanism of NAD(+)-dependent oxidation of the 4''-OH of the UDP-glucuronic acid and decarboxylation of the UDP-4-keto-glucuronic acid intermediate.

Crystal structure of a tetrameric GDP-D-mannose 4,6-dehydratase from a bacterial GDP-D-rhamnose biosynthetic pathway

d-Rhamnose is a rare 6-deoxy monosaccharide primarily found in the lipopolysaccharide of pathogenic bacteria, where it is involved in host-bacterium interactions and the establishment of infection. The biosynthesis of d-rhamnose proceeds through the conversion of GDP-d-mannose by GDP-d-mannose 4,6-dehydratase (GMD) to GDP-4-keto-6-deoxymannose, which is subsequently reduced to GDP-d-rhamnose by a reductase. We have determined the crystal structure of GMD from Pseudomonas aeruginosa in complex with NADPH and GDP. GMD belongs to the NDP-sugar modifying subfamily of the short-chain dehydrogenase/reductase (SDR) enzymes, all of which exhibit bidomain structures and a conserved catalytic triad (Tyr-XXX-Lys and Ser/Thr). Although most members of this enzyme subfamily display homodimeric structures, this bacterial GMD forms a tetramer in the same fashion as the plant MUR1 from Arabidopsis thaliana. The cofactor binding sites are adjoined across the tetramer interface, which brings the adenosyl phosphate moieties of the adjacent NADPH molecules to within 7 A of each other. A short peptide segment (Arg35-Arg43) stretches into the neighboring monomer, making not only protein-protein interactions but also hydrogen bonding interactions with the neighboring cofactor. The interface hydrogen bonds made by the Arg35-Arg43 segment are generally conserved in GMD and MUR1, and the interacting residues are highly conserved among the sequences of bacterial and eukaryotic GMDs. Outside of the Arg35-Arg43 segment, residues involved in tetrameric contacts are also quite conserved across different species. These observations suggest that a tetramer is the preferred, and perhaps functionally relevant, oligomeric state for most bacterial and eukaryotic GMDs.

Determinants of function and substrate specificity in human UDP-galactose 4'-epimerase

UDP-galactose 4'-epimerase (GALE) interconverts UDP-galactose and UDP-glucose in the final step of the Leloir pathway. Unlike the Escherichia coli enzyme, human GALE (hGALE) also efficiently interconverts a larger pair of substrates: UDP-N-acetylgalactosamine and UDP-N-acetylglucosamine. The basis of this differential substrate specificity has remained obscure. Recently, however, x-ray crystallographic data have both predicted essential active site residues and suggested that differential active site cleft volume may be a key factor in determining GALE substrate selectivity. We report here a direct test of this hypothesis. In brief, we have created four substituted alleles: S132A, Y157F, S132A/Y157F, and C307Y-hGALE. While the first three substitutions were predicted to disrupt catalytic activity, the fourth was predicted to reduce active site cleft volume, thereby limiting entry or rotation of the larger but not the smaller substrate. All four alleles were expressed in a null-background strain of Saccharomyces cerevisiae and characterized in terms of activity with regard to both UDP-galactose and UDP-N-acetylgalactosamine. The S132A/Y157F and C307Y-hGALE proteins were also overexpressed in Pichia pastoris and purified for analysis. In all forms tested, the Y157F, S132A, and Y157F/S132A-hGALE proteins each demonstrated a complete loss of activity with respect to both substrates. In contrast, the C307Y-hGALE demonstrated normal activity with respect to UDP-galactose but complete loss of activity with respect to UDP-N-acetylgalactosamine. Together, these results serve to validate the wild-type hGALE crystal structure and fully support the hypothesis that residue 307 acts as a gatekeeper mediating substrate access to the hGALE active site.

Crystal structure of WbpP, a genuine UDP-N-acetylglucosamine 4-epimerase from Pseudomonas aeruginosa: substrate specificity in udp-hexose 4-epimerases

The O antigen of lipopolysaccharide in Gram-negative bacteria plays a critical role in bacterium-host interactions, and for pathogenic bacteria it is a major virulence factor. In Pseudomonas aeruginosa serotype O6 one of the initial steps in O-antigen biosynthesis is catalyzed by a saccharide epimerase, WbpP. WbpP is a member of the UDP-hexose 4-epimerase family of enzymes and exists as a homo-dimer. This enzyme preferentially catalyzes the conversion between UDP-GlcNAc and UDPGalNAc above UDP-Glc and UDP-Gal, using NAD(+) as a cofactor. The crystal structures of WbpP in complex with cofactor and either UDP-Glc or UDP-GalNAc were determined at 2.5 and 2.1 A, respectively, which represents the first structural studies of a genuine UDP-GlcNAc 4-epimerase. These structures in combination with complementary mutagenesis studies suggest that the basis for the differential substrate specificity of WbpP is a consequence of the presence of a pliable solvent network in the active site. This information allows for a comprehensive analysis of the relationship between sequence and substrate specificity for UDP-hexose 4-epimerases and enables the formulation of consensus sequences that predict substrate specificity of UDP-hexose 4-epimerases yet to be biochemically characterized. Furthermore, the examination indicates that as little as one residue can dictate substrate specificity. Nonetheless, phylogenetic analysis suggests that this substrate specificity is an evolutionary and highly conserved property within UDP-hexose 4-epimerases.

The Hypocrea jecorina gal10 (uridine 5'-diphosphate-glucose 4-epimerase-encoding) gene differs from yeast homologues in structure, genomic organization and expression

As part of a comprehensive study on lactose metabolism in Hypocrea jecorina (anamorph: Trichoderma reesei), a genomic clone of the gal10 gene encoding H. jecorina uridine 5'-diphosphate (UDP)-glucose 4-epimerase has been cloned and sequenced. It contains an open reading frame of 1548-base pair, interrupted by three introns, and encoding a 370-amino acids protein with similarity to pro- and eukaryotic UDP-glucose-4-epimerases. H. jecorina Gal10 does not contain the C-terminal mutarotase domain which is present in yeast Gal10 proteins but is able to functionally complement a corresponding Saccharomyces cerevisiae gal10 mutant. gal10 is not clustered with other H. jecorina gal genes (gal7, gene encoding galactose-1-phosphate uridylyltransferase and gal1, gene encoding galactokinase). The genomic location of H. jecorina gal10 and gal7 was syntenic with that in Neurospora crassa and colinear over an area of 6 and 3.5-kilobase. gal10 is constitutively expressed, and--unlike H. jecorina gal7--not further stimulated by D-galactose or L-arabinose or its corresponding polyols.

Variation on a theme of SDR. dTDP-6-deoxy-L- lyxo-4-hexulose reductase (RmlD) shows a new Mg2+-dependent dimerization mode

dTDP-6-deoxy-L-lyxo-4-hexulose reductase (RmlD) catalyzes the final step in the conversion of dTDP-D-glucose to dTDP-L-rhamnose in an NAD(P)H- and Mg2+-dependent reaction. L-rhamnose biosynthesis is an antibacterial target. The structure of RmlD from Salmonella enterica serovar Typhimurium has been determined, and complexes with NADH, NADPH, and dTDP-L-rhamnose are reported. RmlD differs from other short chain dehydrogenases in that it has a novel dimer interface that contains Mg2+. Enzyme catalysis involves hydride transfer from the nicotinamide ring of the cofactor to the C4'-carbonyl group of the substrate. The substrate is activated through protonation by a conserved tyrosine. NAD(P)H is bound in a solvent-exposed cleft, allowing facile replacement. We suggest a novel role for the conserved serine/threonine residue of the catalytic triad of SDR enzymes.

Toward a structural understanding of the dehydratase mechanism

dTDP-D-glucose 4,6-dehydratase (RmlB) was first identified in the L-rhamnose biosynthetic pathway, where it catalyzes the conversion of dTDP-D-glucose into dTDP-4-keto-6-deoxy-D-glucose. The structures of RmlB from Salmonella enterica serovar Typhimurium in complex with substrate deoxythymidine 5'-diphospho-D-glucose (dTDP-D-glucose) and deoxythymidine 5'-diphosphate (dTDP), and RmlB from Streptococcus suis serotype 2 in complex with dTDP-D-glucose, dTDP, and deoxythymidine 5'-diphospho-D-pyrano-xylose (dTDP-xylose) have all been solved at resolutions between 1.8 A and 2.4 A. The structures show that the active sites are highly conserved. Importantly, the structures show that the active site tyrosine functions directly as the active site base, and an aspartic and glutamic acid pairing accomplishes the dehydration step of the enzyme mechanism. We conclude that the substrate is required to move within the active site to complete the catalytic cycle and that this movement is driven by the elimination of water. The results provide insight into members of the SDR superfamily.

Human UDP-galactose 4-epimerase. Accommodation of UDP-N-acetylglucosamine within the active site

UDP-galactose 4-epimerase catalyzes the interconversion of UDP-galactose and UDP-glucose during normal galactose metabolism. One of the key structural features in the proposed reaction mechanism for the enzyme is the rotation of a 4'-ketopyranose intermediate within the active site pocket. Recently, the three-dimensional structure of the human enzyme with bound NADH and UDP-glucose was determined. Unlike that observed for the protein isolated from Escherichia coli, the human enzyme can also turn over UDP-GlcNAc to UDP-GalNAc and vice versa. Here we describe the three-dimensional structure of human epimerase complexed with NADH and UDP-GlcNAc. To accommodate the additional N-acetyl group at the C2 position of the sugar, the side chain of Asn-207 rotates toward the interior of the protein and interacts with Glu-199. Strikingly, in the human enzyme, the structural equivalent of Tyr-299 in the E. coli protein is replaced with a cysteine residue (Cys-307) and the active site volume for the human protein is calculated to be approximately 15% larger than that observed for the bacterial epimerase. This combination of a larger active site cavity and amino acid residue replacement most likely accounts for the inability of the E. coli enzyme to interconvert UDP-GlcNAc and UDP-GalNAc.

The crystal structure of dTDP-D-Glucose 4,6-dehydratase (RmlB) from Salmonella enterica serovar Typhimurium, the second enzyme in the dTDP-l-rhamnose pathway

l-Rhamnose is a 6-deoxyhexose that is found in a variety of different glycoconjugates in the cell walls of pathogenic bacteria. The precursor of l-rhamnose is dTDP-l-rhamnose, which is synthesised from glucose- 1-phosphate and deoxythymidine triphosphate (dTTP) via a pathway requiring four enzymes. Significantly this pathway does not exist in humans and all four enzymes therefore represent potential therapeutic targets. dTDP-D-glucose 4,6-dehydratase (RmlB; EC 4.2.1.46) is the second enzyme in the dTDP-L-rhamnose biosynthetic pathway. The structure of Salmonella enterica serovar Typhimurium RmlB had been determined to 2.47 A resolution with its cofactor NAD(+) bound. The structure has been refined to a crystallographic R-factor of 20.4 % and an R-free value of 24.9 % with good stereochemistry.RmlB functions as a homodimer with monomer association occurring principally through hydrophobic interactions via a four-helix bundle. Each monomer exhibits an alpha/beta structure that can be divided into two domains. The larger N-terminal domain binds the nucleotide cofactor NAD(+) and consists of a seven-stranded beta-sheet surrounded by alpha-helices. The smaller C-terminal domain is responsible for binding the sugar substrate dTDP-d-glucose and contains four beta-strands and six alpha-helices. The two domains meet to form a cavity in the enzyme. The highly conserved active site Tyr(167)XXXLys(171) catalytic couple and the GlyXGlyXXGly motif at the N terminus characterise RmlB as a member of the short-chain dehydrogenase/reductase extended family. The quaternary structure of RmlB and its similarity to a number of other closely related short-chain dehydrogenase/reductase enzymes have enabled us to propose a mechanism of catalysis for this important enzyme.

Identification of two GDP-6-deoxy-D-lyxo-4-hexulose reductases synthesizing GDP-D-rhamnose in Aneurinibacillus thermoaerophilus L420-91T

The glycan repeats of the surface layer glycoprotein of Aneurinibacillus thermoaerophilus L420-91T contain d-rhamnose and 3-acetamido-3,6-dideoxy-d-galactose, both of which are also constituents of lipopolysaccharides of Gram-negative plant and human pathogenic bacteria. The two genes required for biosynthesis of the nucleotide-activated precursor GDP-d-rhamnose, gmd and rmd, were cloned, sequenced, and overexpressed in Escherichia coli. The corresponding enzymes Gmd and Rmd were purified to homogeneity, and functional studies were performed. GDP-d-mannose dehydratase (Gmd) converted GDP-d-mannose to GDP-6-deoxy-d-lyxo-4-hexulose, with NADP+ as cofactor. The reductase Rmd catalyzed the second step in the pathway, namely the reduction of the keto-intermediate to the final product GDP-d-rhamnose using both NADH and NADPH as hydride donor. The elution behavior of the intermediate and end product was analyzed by high performance liquid chromatography. Nuclear magnetic resonance spectroscopy was used to identify the structure of the final product of the reaction sequence as GDP-alpha-d-rhamnose. This is the first characterization of a GDP-6-deoxy-d-lyxo-4-hexulose reductase. In addition, Gmd has been shown to be a bifunctional enzyme with both dehydratase and reductase activities. So far, no enzyme catalyzing these two types of reactions has been identified. Both Gmd and Rmd are members of the SDR (short chain dehydrogenase/reductase) protein family.

The rhamnose pathway

L-Rhamnose is a deoxy sugar found widely in bacteria and plants. Evidence continues to emerge about its essential role in many pathogenic bacteria. The crystal structures of two of the four enzymes involved in its biosynthetic pathway have been reported and the other two have been submitted for publication. This pathway does not exist in humans, making enzymes of this pathway very attractive targets for therapeutic intervention.

Crystal structure of SQD1, an enzyme involved in the biosynthesis of the plant sulfolipid headgroup donor UDP-sulfoquinovose

The SQD1 enzyme is believed to be involved in the biosynthesis of the sulfoquinovosyl headgroup of plant sulfolipids, catalyzing the transfer of SO(3)(-) to UDP-glucose. We have determined the structure of the complex of SQD1 from Arabidopsis thaliana with NAD(+) and the putative substrate UDP-glucose at 1.6-A resolution. Both bound ligands are completely buried within the binding cleft, along with an internal solvent cavity which is the likely binding site for the, as yet, unidentified sulfur-donor substrate. SQD1 is a member of the short-chain dehydrogenase/reductase (SDR) family of enzymes, and its structure shows a conservation of the SDR catalytic residues. Among several highly conserved catalytic residues, Thr-145 forms unusually short hydrogen bonds with both susceptible hydroxyls of UDP-glucose. A His side chain may also be catalytically important in the sulfonation.

Prediction of the active-site structure and NAD(+) binding in SQD1, a protein essential for sulfolipid biosynthesis in Arabidopsis

Sulfolipids of photosynthetic bacteria and plants are characterized by their unique sulfoquinovose headgroup, a derivative of glucose in which the 6-hydroxyl group is replaced by a sulfonate group. These sulfolipids have been discussed as promising anti-tumor and anti-HIV therapeutics based on their inhibition of DNA polymerase and reverse transcriptase. To study sulfolipid biosynthesis, in particular the formation of UDP-sulfoquinovose, we have combined computational modeling with biochemical methods. A database search was performed employing the derived amino acid sequence from SQD1, a gene involved in sulfolipid biosynthesis of Arabidopsis thaliana. This sequence shows high similarity to other sulfolipid biosynthetic proteins of different organisms and also to sugar nucleotide modifying enzymes, including UDP-glucose epimerase and dTDP-glucose dehydratase. Additional biochemical data on the purified SQD1 protein suggest that it is involved in the formation of UDP-sulfoquinovose, the first step of sulfolipid biosynthesis. To understand which aspects of epimerase catalysis may be shared by SQD1, we built a three-dimensional model of SQD1 using the 1.8 A crystallographic structure of UDP-glucose 4-epimerase as a template. This model predicted an NAD(+) binding site, and the binding of NAD(+) was subsequently confirmed by enzymatic assay and mass spectrometry. The active-site interactions together with biochemical data provide the basis for proposing a reaction mechanism for UDP-sulfoquinovose formation.

Identification and characterization of a mutation, in the human UDP-galactose-4-epimerase gene, associated with generalized epimerase-deficiency galactosemia

Epimerase-deficiency galactosemia results from impairment of the human enzyme UDP-galactose-4-epimerase (hGALE). We and others have identified substitution mutations in the hGALE alleles of patients with the clinically mild, peripheral form of epimerase deficiency. We report here the first identification of an hGALE mutation in a patient with the clinically severe, generalized form of epimerase deficiency. The mutation, V94M, was found on both GALE alleles of this patient. This same mutation also was found in the homozygous state in two additional patients with generalized epimerase deficiency. The specific activity of the V94M-hGALE protein expressed in yeast was severely reduced with regard to UDP-galactose and partially reduced with regard to UDP-N-acetylgalactosamine. In contrast, two GALE-variant proteins associated with peripheral epimerase deficiency, L313M-hGALE and D103G-hGALE, demonstrated near-normal levels of activity with regard to both substrates, but a third allele, G90E-hGALE, demonstrated little, if any, detectable activity, despite near-normal abundance. G90E originally was identified in a heterozygous patient whose other allele remains uncharacterized. Thermal lability and protease-sensitivity studies demonstrated compromised stability in all of the partially active mutant enzymes.

First molecular characterization of a uridine diphosphate galacturonate 4-epimerase: an enzyme required for capsular biosynthesis in Streptococcus pneumoniae type 1

Uridine diphosphate galacturonate 4-epimerases (UDPGLEs) are enzymes that convert UDP-glucuronate into UDP-galacturonate. Although the presence of UDPGLEs has been reported in prokaryoic and eukaryotic organisms, the genes coding for these enzymes are completely unknown. The galacturonic acid-containing capsular polysaccharide of Streptococcus pneumoniae type 1 is synthesized through the action of a specific UDPGLE. We have constructed a defined deletion mutant in the cap1J gene (one of the 15 cap1 genes responsible for the synthesis of the type 1 capsule) that exhibited an unencapsulated phenotype. This mutant was unable to synthesize UDPGLE, suggesting that Cap1J was the type 1-specific UDPGLE of S. pneumoniae. Escherichia coli cells harbouring the recombinant plasmid pRMM38 (cap1J) overproduced a 40 kDa protein, characterized as Cap1J on the basis of the N-terminal amino acid sequence analysis, and expressed high levels of enzymatically active Cap1J epimerase. Cap1J was partially purified, although purification to electrophoretic homogeneity inactivated the enzyme irreversibly. The enzyme has the following characteristics: K(m) for UDP-glucuronate, 0.24 mM; pH optimum, 7.5; equilibrium constant (in the direction of UDP-galacturonate formation), 1.3; and an approximate M(r) of 80,000 for the active form. The Cap1J protein exhibited a fluorescence emission spectrum similar to that of NADH. Upon inactivation with p-hydroxymercuribenzoate, the addition of NAD+ and 2-mercaptoethanol were sufficient to reactivate the enzyme. Among several compounds tested, UDP-galactose and UDP-xylose exhibited the highest inhibition of the UDPGLE activity. Inactivation of UDPGLE activity was also observed in the presence of UMP and several reducing sugars. To our knowledge, this is the first example of a thoroughly molecular characterization of a UDPGLE.

GDP-4-keto-6-deoxy-D-mannose epimerase/reductase from Escherichia coli, a key enzyme in the biosynthesis of GDP-L-fucose, displays the structural characteristics of the RED protein homology superfamily

BACKGROUND

The process of guanosine 5'-diphosphate L-fucose (GDP-L-fucose) biosynthesis is conserved throughout evolution from prokaryotes to man. In animals, GDP-L-fucose is the substrate of fucosyltransferases that participate in the biosynthesis and remodeling of glycoconjugates, including ABH blood group and Lewis-system antigens. The 'de novo' pathway of GDP-L-fucose biosynthesis from GDP-D-mannose involves a GDP-D-mannose 4,6 dehydratase (GMD) and a GDP-4-keto-6-deoxy-D-mannose epimerase/reductase (GMER). Neither of the catalytic mechanisms nor the three-dimensional structures of the two enzymes has been elucidated yet. The severe leukocyte adhesion deficiency (LAD) type II genetic syndrome is known to result from deficiencies in this de novo pathway.

RESULTS

The crystal structures of apo- and holo-GMER have been determined at 2.1 A and 2.2 A resolution, respectively. Each subunit of the homodimeric (2 x 34 kDa) enzyme is composed of two domains. The N-terminal domain, a six-stranded Rossmann fold, binds NADP+; the C-terminal domain (about 100 residues) displays an alpha/beta topology. NADP+ interacts with residues Arg12 and Arg36 at the adenylic ribose phosphate; moreover, a protein loop based on the Gly-X-X-Gly-X-X-Gly motif (where X is any amino acid) stabilizes binding of the coenzyme diphosphate bridge. The nicotinamide and the connected ribose ring are located close to residues Ser107, Tyr136 and Lys140, the putative GMER active-site center.

CONCLUSIONS

The GMER fold is reminiscent of that observed for UDP-galactose epimerase (UGE) from Escherichia coli. Consideration of the enzyme fold and of its main structural features allows assignment of GMER to the reductase-epimerase-dehydrogenase (RED) enzyme homology superfamily, to which short-chain dehydrogenase/reductases (SDRs) also belong. The location of the NADP+ nicotinamide ring at an interdomain cleft is compatible with substrate binding in the C-terminal domain.