Short-chain dehydrogenases/reductases

The mushroom body development and learning ability of adult honeybees are influenced by cold exposure during their early pupal stage

The honeybees are the most important pollinator in the production of crops and fresh produce. Temperature affects the survival of honeybees, and determines the quality of their development, which is of great significance for beekeeping production. Yet, little was known about how does low temperature stress during development stage cause bee death and any sub-lethal effect on subsequent. Early pupal stage is the most sensitive stage to low temperature in pupal stage. In this study, early pupal broods were exposed to 20°C for 12, 16, 24, and 48 h, followed by incubation at 35°C until emergence. We found that 48 h of low temperature duration cause 70% of individual bees to die. Although the mortality at 12 and 16 h seems not very high, the association learning ability of the surviving individuals was greatly affected. The brain slices of honeybees showed that low temperature treatment could cause the brain development of honeybees to almost stop. Gene expression profiles between low temperature treatment groups (T24, T48) and the control revealed that 1,267 and 1,174 genes were differentially expressed respectively. Functional enrichment analysis of differentially expressed genes showed that the differential expression of Map3k9, Dhrs4, and Sod-2 genes on MAPK and peroxisome signaling pathway caused oxidative damage to the honeybee head. On the FoxO signal pathway, InsR and FoxO were upregulated, and JNK, Akt, and Bsk were downregulated; and on the insect hormone synthesis signal pathway, Phm and Spo genes were downregulated. Therefore, we speculate that low temperature stress affects hormone regulation. It was detected that the pathways related to the nervous system were Cholinergic synapse, Dopaminergic synapse, GABAergic synapse, Glutamatergic synapse, Serotonergic synapse, Neurotrophin signaling pathway, and Synaptic vesicle cycle. This implies that the synaptic development of honeybees is quite possibly greatly affected by low temperature stress. Understanding how low temperature stress affects the physiology of bee brain development and how it affects bee behavior provide a theoretical foundation for a deeper comprehension of the temperature adaptation mechanism that underlies the "stenothermic" development of social insects, and help to improve honeybee management strategies to ensure the healthy of colony.

Analysis of leaf morphology, secondary metabolites and proteins related to the resistance to Tetranychus cinnabarinus in cassava (Manihot esculenta Crantz)

Constitutive resistance of plant can be divided into physical and chemical barriers. Cassava (Manihot esculenta Crantz) is susceptible to mites, especially Tetranychus cinnabarinus. Although significant differences in the resistance to T. cinnabarinus are observed in different cassava cultivars, limited research has been done on the mechanism accounting for the resistance. The aim of this study was to explore the mechanism of resistance to T. cinnabarinus by comparing morphology, secondary metabolites and proteins in different cassava cultivars. The anatomical structure of leaves showed that the cassava cultivar Xinxuan 048 (XX048), which showed a stronger resistance to T. cinnabarinus in both greenhouse testing and three years field evaluation tests (2016–2018), had thicker palisade tissue, spongy tissue, lower epidermis and leaf midrib tissue compared to cultivar Guire 4 (GR4). Greenhouse evaluation demonstrated that originally these cultivars were different, leading to differences in constitutive levels of metabolites. The proteomic analysis of protected leaves in XX048 and GR4 revealed that up-regulated differentially expressed proteins (DEPs) were highly enriched in secondary metabolic pathways, especially in the biosynthesis of flavonoids. This study not only provides a comprehensive data set for overall proteomic changes of leaves in resistant and susceptible cassava, but also sheds light on the morphological characteristics of cassava-mite interaction, secondary metabolite defense responses, and molecular breeding of mite-resistant cassava for effective pest control.

Structural and functional characterization of a novel acidophilic 7α-hydroxysteroid dehydrogenase

7α-Hydroxysteroid dehydrogenase (7α-HSDH) is an NAD(P)H-dependent oxidoreductase belonging to the short-chain dehydrogenases/reductases. In vitro, 7α-HSDH is involved in the efficient biotransformation of taurochenodeoxycholic acid (TCDCA) to tauroursodeoxycholic acid (TUDCA). In this study, a gene encoding novel 7α-HSDH (named as St-2-1) from fecal samples of black bear was cloned and heterologously expressed in Escherichia coli. The protein has subunits of 28.3 kDa and a native size of 56.6 kDa, which suggested a homodimer. We studied the relevant properties of the enzyme, including the optimum pH, optimum temperature, thermal stability, activators, and inhibitors. Interestingly, the data showed that St-2-1 differs from the 7α-HSDHs reported in the literature, as it functions under acidic conditions. The enzyme displayed its optimal activity at pH 5.5 (TCDCA). The acidophilic nature of 7α-HSDH expands its application environment and the natural enzyme bank of HSDHs, providing a promising candidate enzyme for the biosynthesis of TUDCA or other related chemical entities.

Genome-wide association study unravels the genetic control of the apple volatilome and its interplay with fruit texture

Fruit quality represents a fundamental factor guiding consumers' preferences. Among apple quality traits, volatile organic compounds and texture features play a major role. Proton Transfer Reaction-Time of Flight-Mass Spectrometry (PTR-ToF-MS), coupled with an artificial chewing device, was used to profile the entire apple volatilome of 162 apple accessions, while the fruit texture was dissected with a TAXT-AED texture analyzer. The array of volatile compounds was classed into seven major groups and used in a genome-wide association analysis carried out with 9142 single nucleotide polymorphisms (SNPs). Marker-trait associations were identified on seven chromosomes co-locating with important candidate genes for aroma, such as MdAAT1 and MdIGS. The integration of volatilome and fruit texture data conducted with a multiple factor analysis unraveled contrasting behavior, underlying opposite regulation of the two fruit quality aspects. The association analysis using the first two principal components identified two QTLs located on chromosomes 10 and 2, respectively. The distinction of the apple accessions on the basis of the allelic configuration of two functional markers, MdPG1 and MdACO1, shed light on the type of interplay existing between fruit texture and the production of volatile organic compounds.

Structural Insights into the Drosophila melanogaster Retinol Dehydrogenase, a Member of the Short-Chain Dehydrogenase/Reductase Family

The 11-cis-retinylidene chromophore of visual pigments isomerizes upon interaction with a photon, initiating a downstream cascade of signaling events that ultimately lead to visual perception. 11-cis-Retinylidene is regenerated through enzymatic transformations collectively called the visual cycle. The first and rate-limiting enzymatic reaction within this cycle, i.e., the reduction of all-trans-retinal to all-trans-retinol, is catalyzed by retinol dehydrogenases. Here, we determined the structure of Drosophila melanogaster photoreceptor retinol dehydrogenase (PDH) isoform C that belongs to the short-chain dehydrogenase/reductase (SDR) family. This is the first reported structure of a SDR that possesses this biologically important activity. Two crystal structures of the same enzyme grown under different conditions revealed a novel conformational change of the NAD⁺ cofactor, likely representing a change during catalysis. Amide hydrogen-deuterium exchange of PDH demonstrated changes in the structure of the enzyme upon dinucleotide binding. In D. melanogaster, loss of PDH activity leads to photoreceptor degeneration that can be partially rescued by transgenic expression of human RDH12. Based on the structure of PDH, we analyzed mutations causing Leber congenital amaurosis 13 in a homology model of human RDH12 to obtain insights into the molecular basis of RDH12 disease-causing mutations.

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.

Identification of the hRDH-E2 gene, a novel member of the SDR family, and its increased expression in psoriatic lesion

To identify novel psoriasis-associated genes, we focused on several ESTs (expressed sequence tags) whose expression was predominantly increased in the affected skin in patients with psoriasis vulgaris, as assessed by microarray assay. In this paper, a full-length cDNA corresponding to one of those ESTs (AI440266) was isolated by screening of cultured human keratinocyte cDNA libraries. This cDNA has an open reading frame of a 309-amino-acid protein, sharing significant homology to one of the short-chain alcohol dehydrogenase/reductase (SDR) families that can catalyze the first and rate-limiting step that generates retinaldehyde from retinol. So, this gene was designated as hRDH-E2 (human epidermal retinal dehydrogenase 2). The hRDH-E2 gene has a single functional copy on chromosome 8q12.1, spanning approximately 20kb with seven exons. The deduced amino acid sequence contains three motifs that are conserved in the SDR family. Qualitative RT-PCR demonstrated that the mRNA levels of hRDH-E2 were significantly elevated in the affected skin in psoriasis patients as compared to the unaffected skin in patients and the normal skin in healthy individual. These results suggest that hRDH-E2 may be involved in the pathogenesis of psoriasis through its critical role in retinol metabolism in keratinocyte proliferation.

Short-chain dehydrogenase/reductase (SDR) relationships: a large family with eight clusters common to human, animal, and plant genomes

The progress in genome characterizations has opened new routes for studying enzyme families. The availability of the human genome enabled us to delineate the large family of short-chain dehydrogenase/reductase (SDR) members. Although the human genome releases are not yet final, we have already found 63 members. We have also compared these SDR forms with those of three model organisms: Caenorhabditis elegans, Drosophila melanogaster, and Arabidopsis thaliana. We detect eight SDR ortholog clusters in a cross-genome comparison. Four of these clusters represent extended SDR forms, a subgroup found in all life forms. The other four are classical SDRs with activities involved in cellular differentiation and signalling. We also find 18 SDR genes that are present only in the human genome of the four genomes studied, reflecting enzyme forms specific to mammals. Close to half of these gene products represent steroid dehydrogenases, emphasizing the regulatory importance of these enzymes.

Characterization of a novel airway epithelial cell-specific short chain alcohol dehydrogenase/reductase gene whose expression is up-regulated by retinoids and is involved in the metabolism of retinol

Multiple retinoic acid responsive cDNAs were isolated from a high density cDNA microarray membrane, which was developed from a cDNA library of human tracheobronchial epithelial cells. Five selected cDNA clones encoded the sequence of the same novel gene. The predicted open reading frame of the novel gene encoded a protein of 319 amino acids. The deduced amino acid sequence contains four motifs that are conserved in the short-chain alcohol dehydrogenase/reductase (SDR) family of proteins. The novel gene shows the greatest homology to a group of dehydrogenases that can oxidize retinol (retinol dehydrogenases). The mRNA of the novel gene was found in trachea, colon, tongue, and esophagus. In situ hybridization of airway tissue sections demonstrated epithelial cell-specific gene expression, especially in the ciliated cell type. Both all-trans-retinoic acid and 9-cis-retinoic acid were able to elevate the expression of the novel gene in primary human tracheobronchial epithelial cells in vitro. This elevation coincided with an enhanced retinol metabolism in these cultures. COS cells transfected with an expression construct of the novel gene were also elevated in the metabolism of retinol. The results suggested that the novel gene represents a new member of the SDR family that may play a critical role in retinol metabolism in airway epithelia as well as in other epithelia of colon, tongue, and esophagus.

Zinc supplementation might potentiate the effect of vitamin A in restoring night vision in pregnant Nepalese women

BACKGROUND

Zinc deficiency may result in abnormal dark adaptation or night blindness, a symptom primarily of vitamin A deficiency. During a placebo-controlled trial in Nepal, weekly vitamin A supplementation of women reduced but failed to eliminate the incidence of night blindness during pregnancy, suggesting a role for zinc.

OBJECTIVE

The study examined the efficacy of daily zinc supplementation in restoring night vision of pregnant women who developed night blindness while routinely receiving either vitamin A, beta-carotene, or placebo in a field trial.

DESIGN

Women (n = 202) who reported to be night blind during pregnancy were randomly assigned in a double-blind manner, stratified on vitamin A, beta-carotene, or placebo receipt, to receive 25 mg Zn or placebo daily for 3 wk. Thus, the 6 groups studied were as follows: beta-carotene + zinc, beta-carotene alone, vitamin A + zinc, vitamin A alone (vitamin A + placebo), zinc alone (zinc + placebo), and placebo (2 placebos: one for the vitamin A or beta-carotene study and one for the zinc study). Women underwent a clinic-based assessment that included pupillary threshold testing and phlebotomy before and after supplementation. Supplement use and daily history of night blindness were obtained at home twice every week.

RESULTS

Zinc treatment increased serum zinc concentrations, but alone (zinc alone group), failed to restore night vision or to improve dark adaptation. However, women in the vitamin A + zinc group who had baseline serum zinc concentrations <9.9 micromol/L were 4 times more likely to have their night vision restored (95% CI: 1.1, 17.3) than were women in the placebo group and tended to have a small improvement in pupillary threshold scores (by 0.21 log candela/m2; P = 0.09).

CONCLUSION

These data suggest that zinc potentiated the effect of vitamin A in restoring night vision among night-blind pregnant women with low initial serum zinc concentrations.

Structural and kinetic analysis of Escherichia coli GDP-mannose 4,6 dehydratase provides insights into the enzyme's catalytic mechanism and regulation by GDP-fucose

BACKGROUND

GDP-mannose 4,6 dehydratase (GMD) catalyzes the conversion of GDP-(D)-mannose to GDP-4-keto, 6-deoxy-(D)-mannose. This is the first and regulatory step in the de novo biosynthesis of GDP-(L)-fucose. Fucose forms part of a number of glycoconjugates, including the ABO blood groups and the selectin ligand sialyl Lewis X. Defects in GDP-fucose metabolism have been linked to leukocyte adhesion deficiency type II (LADII).

RESULTS

The structure of the GDP-mannose 4,6 dehydratase apo enzyme has been determined and refined using data to 2.3 A resolution. GMD is a homodimeric protein with each monomer composed of two domains. The larger N-terminal domain binds the NADP(H) cofactor in a classical Rossmann fold and the C-terminal domain harbors the sugar-nucleotide binding site. We have determined the GMD dissociation constants for NADP, NADPH and GDP-mannose. Each GMD monomer binds one cofactor and one substrate molecule, suggesting that both subunits are catalytically competent. GDP-fucose acts as a competitive inhibitor, suggesting that it binds to the same site as GDP-mannose, providing a mechanism for the feedback inhibition of fucose biosynthesis.

CONCLUSIONS

The X-ray structure of GMD reveals that it is a member of the short-chain dehydrogenase/reductase (SDR) family of proteins. We have modeled the binding of NADP and GDP-mannose to the enzyme and mutated four of the active-site residues to determine their function. The combined modeling and mutagenesis data suggests that at position 133 threonine substitutes serine as part of the serine-tyrosine-lysine catalytic triad common to the SDR family and Glu 135 functions as an active-site base.

Molecular cloning and characterization of a new fasting-inducible short-chain dehydrogenase/reductase from rat liver(1)

We have isolated a 668 bp cDNA from fasted rat liver, designated RLF98, by suppression subtractive hybridization (SSH). The full-length RLF98 cDNA, cloned by rapid amplification of cDNA ends (RACE), is 1113 bp long with an open reading frame of 912 bp. This cDNA encodes a protein of 303 amino acid residues with a calculated molecular weight of 32433 Da. In vitro transcription and translation of the full-length RLF98 cDNA produced a protein of about 33 kDa. The RLF98 protein shares strong amino acid sequence homology with members of the short-chain dehydrogenase/reductase (SDR) family. Northern analysis of RNA from rat liver revealed a transcript of 1.1 kb. Fasting increased this mRNA 2.7-fold. While the RLF98 gene is widely expressed in rat tissues, its level of expression is highly variable. Expression in liver and kidney is abundant and is more than 10 times that observed in other tissues. Our data indicate that the RLF98 is a new member of the SDR family that is upregulated by fasting. Additional experiments including purification of recombinant RLF98 protein are in progress to define the specific function of this protein and the role it plays during fasting-induced catabolism.

Stereochemical course and steady state mechanism of the reaction catalyzed by the GDP-fucose synthetase from Escherichia coli

Recently the genes encoding the human and Escherichia coli GDP-mannose dehydratase and GDP-fucose synthetase (GFS) protein have been cloned and it has been shown that these two proteins alone are sufficient to convert GDP mannose to GDP fucose in vitro. GDP-fucose synthetase from E. coli is a novel dual function enzyme in that it catalyzes epimerizations and a reduction reaction at the same active site. This aspect separates fucose biosynthesis from that of other deoxy and dideoxy sugars in which the epimerase and reductase activities are present on separate enzymes encoded by separate genes. By NMR spectroscopy we have shown that GFS catalyzes the stereospecific hydride transfer of the ProS hydrogen from NADPH to carbon 4 of the mannose sugar. This is consistent with the stereospecificity observed for other members of the short chain dehydrogenase reductase family of enzymes of which GFS is a member. Additionally the enzyme is able to catalyze the epimerization reaction in the absence of NADP or NADPH. The kinetic mechanism of GFS as determined by product inhibition and fluorescence binding studies is consistent with a random mechanism. The dissociation constants determined from fluorescence studies indicate that the enzyme displays a 40-fold stronger affinity for the substrate NADPH as compared with the product NADP and utilizes NADPH preferentially as compared with NADH. This study on GFS, a unique member of the short chain dehydrogenase reductase family, coupled with that of its recently published crystal structure should aid in the development of antimicrobial or anti-inflammatory compounds that act by blocking selectin-mediated cell adhesion.

Recommended nomenclature for the vertebrate alcohol dehydrogenase gene family

The alcohol dehydrogenase (ADH) gene family encodes enzymes that metabolize a wide variety of substrates, including ethanol, retinol, other aliphatic alcohols, hydroxysteroids, and lipid peroxidation products. Studies on 19 vertebrate animals have identified ADH orthologs across several species, and this has now led to questions of how best to name ADH proteins and genes. Seven distinct classes of vertebrate ADH encoded by non-orthologous genes have been defined based upon sequence homology as well as unique catalytic properties or gene expression patterns. Each class of vertebrate ADH shares <70% sequence identity with other classes of ADH in the same species. Classes may be further divided into multiple closely related isoenzymes sharing >80% sequence identity such as the case for class I ADH where humans have three class I ADH genes, horses have two, and mice have only one. Presented here is a nomenclature that uses the widely accepted vertebrate ADH class system as its basis. It follows the guidelines of human and mouse gene nomenclature committees, which recommend coordinating names across species boundaries and eliminating Roman numerals and Greek symbols. We recommend that enzyme subunits be referred to by the symbol "ADH" (alcohol dehydrogenase) followed by an Arabic number denoting the class; i.e. ADH1 for class I ADH. For genes we recommend the italicized root symbol "ADH" for human and "Adh" for mouse, followed by the appropriate Arabic number for the class; i.e. ADH1 or Adh1 for class I ADH genes. For organisms where multiple species-specific isoenzymes exist within a class, we recommend adding a capital letter after the Arabic number; i.e. ADH1A, ADH1B, and ADH1C for human alpha, beta, and gamma class I ADHs, respectively. This nomenclature will accommodate newly discovered members of the vertebrate ADH family, and will facilitate functional and evolutionary studies.

GDP-fucose synthetase from Escherichia coli: structure of a unique member of the short-chain dehydrogenase/reductase family that catalyzes two distinct reactions at the same active site

BACKGROUND

. In all species examined, GDP-fucose is synthesized from GDP-mannose in a three-step reaction catalyzed by two enzymes, GDP-mannose 4,6 dehydratase and a dual function 3, 5-epimerase-4-reductase named GDP-fucose synthetase. In this latter aspect fucose biosynthesis differs from that of other deoxy and dideoxy sugars, in which the epimerase and reductase activities are present as separate enzymes. Defects in GDP-fucose biosynthesis have been shown to affect nodulation in bacteria, stem development in plants, and are associated with the immune defect leukocyte adhesion deficiency type II in humans.

RESULTS

. We have determined the structure of GDP-fucose synthetase from Escherichia coli at 2.2 A resolution. The structure of GDP-fucose synthetase is closely related to that of UDP-galactose 4-epimerase and more distantly to other members of the short-chain dehydrogenase/reductase family. We have also determined the structures of the binary complexes of GDP-fucose synthetase with its substrate NADPH and its product NADP+. The nicotinamide cofactors bind in the syn and anti conformations, respectively.

CONCLUSIONS

. GDP-fucose synthetase binds its substrate, NADPH, in the proper orientation (syn) for transferring the 4-pro-S hydride of the nicotinamide. We have observed a single binding site in GDP-fucose synthetase for the second substrate, GDP-4-keto,6-deoxy-mannose. This implies that both the epimerization and reduction reactions occur at the same site in the enzyme. As is the case for all members of the short-chain family of dehydrogenase/reductases, GDP-fucose synthetase retains the Ser-Tyr-Lys catalytic triad. We propose that this catalytic triad functions in a mechanistically equivalent manner in both the epimerization and reduction reactions. Additionally, the X-ray structure has allowed us to identify other residues that are potentially required for substrate binding and catalysis.

High-resolution X-ray structure of UDP-galactose 4-epimerase complexed with UDP-phenol

UDP-galactose 4-epimerase from Escherichia coli catalyzes the interconversion of UDP-glucose and UDP-galactose. In recent years, the enzyme has been the subject of intensive investigation due in part to its ability to facilitate nonstereospecific hydride transfer between beta-NADH and a 4-keto hexopyranose intermediate. The first molecular model of the epimerase from E. coli was solved to 2.5 A resolution with crystals grown in the presence of a substrate analogue, UDP-phenol (Bauer AJ, Rayment I, Frey PA, Holden HM, 1992, Proteins Struct Funct Genet 12:372-381). There were concerns at the time that the inhibitor did not adequately mimic the sugar moiety of a true substrate. Here we describe the high-resolution X-ray crystal structure of the ternary complex of UDP-galactose 4-epimerase with NADH and UDP-phenol. The model was refined to 1.8 A resolution with a final overall R-factor of 18.6%. This high-resolution structural analysis demonstrates that the original concerns were unfounded and that, in fact, UDP-phenol and UDP-glucose bind similarly. The carboxamide groups of the dinucleotides, in both subunits, are displaced significantly from the planes of the nicotinamide rings by hydrogen bonding interactions with Ser 124 and Tyr 149. UDP-galactose 4-epimerase belongs to a family of enzymes known as the short-chain dehydrogenases, which contain a characteristic Tyr-Lys couple thought to be important for catalysis. The epimerase/NADH/UDP-phenol model presented here represents a well-defined ternary complex for this family of proteins and, as such, provides important information regarding the possible role of the Tyr-Lys couple in the reaction mechanism.