Characterization of a novel type III alcohol dehydrogenase from Thermococcus barophilus Ch5.

The genome of the hyperthermophilic and piezophilic euryarchaeaon Thermococcus barophilus Ch5 encodes three putative alcohol dehydrogenases (Tba ADHs). Herein, we characterized Tba ADH547 biochemically and probed its catalytic mechanism by mutational studies. Our data demonstrate that Tba ADH547 can oxidize ethanol and reduce acetaldehyde at high temperature with the same optimal temperature (75 °C) and exhibit similar thermostability for oxidization and reduction reactions. However, Tba ADH547 has different optimal pH for oxidation and reduction: 8.5 for oxidation and 7.0 for reduction. Tba ADH547 is dependent on a divalent ion for its oxidation activity, among which Mn2+ is optimal. However, Tba ADH547 displays about 20% reduction activity without a divalent ion, and the maximal activity with Fe2+. Furthermore, Tba ADH547 showcases a strong substrate preference for 1-butanol and 1-hexanol over ethanol and other alcohols. Similarly, Tba ADH547 prefers butylaldehyde to acetaldehyde as its reduction substrate. Mutational studies showed that the mutations of residues D195, H199, H262 and H274 to Ala result in the significant activity loss of Tba ADH547, suggesting that residues D195, H199, H262 and H274 are responsible for catalysis. Overall, Tba ADH547 is a thermoactive ADH with novel biochemical characteristics, thereby allowing this enzyme to be a potential biocatalyst.

Aldehyde-alcohol dehydrogenase forms a high-order spirosome architecture critical for its activity.

Aldehyde-alcohol dehydrogenase (AdhE) is a key enzyme in bacterial fermentation, converting acetyl-CoA to ethanol, via two consecutive catalytic reactions. Here, we present a 3.5 Å resolution cryo-EM structure of full-length AdhE revealing a high-order spirosome architecture. The structure shows that the aldehyde dehydrogenase (ALDH) and alcohol dehydrogenase (ADH) active sites reside at the outer surface and the inner surface of the spirosome respectively, thus topologically separating these two activities. Furthermore, mutations disrupting the helical structure abrogate enzymatic activity, implying that formation of the spirosome structure is critical for AdhE activity. In addition, we show that this spirosome structure undergoes conformational change in the presence of cofactors. This work presents the atomic resolution structure of AdhE and suggests that the high-order helical structure regulates its enzymatic activity.

Chloroacetaldehyde dehydrogenase from Ancylobacter aquaticus UV5: Cloning, expression, characterization and molecular modeling.

1,2-Dichloroethane (1,2-DCE) is oxidatively converted to a carcinogenic intermediate compound, chloroacetaldehyde by chloroacetaldehyde dehydrogenase (CAldA) during its biodegradation by many bacterial strains, including Xanthobacter autotrophicus and Ancylobacter aquaticus. In this study, a 55kDa NAD-dependent CAldA expressed by chromosomally encoded aldA gene, is reported in an indigenous Ancylobacter aquaticus UV5. A. aquaticus UV5 aldA gene was found to be 99% homologous to the plasmid (pXAU1) encoded aldA gene reported in X. autotrophicus GJ10. Pulse-field gel electrophoresis (PFGE) and PCR experiments revealed the absence of pXAU1 in A. aquaticus UV5 and that aldA was chromosomal encoded. A 6× His-tag fused CAldA cloned in pET15b, overexpressed and purified on Co-agarose affinity column using AKTA purification system showed Mr of 57,526. CAldA was active optimally at pH9 and 30°C. The Km and vmax for the substrate, acetaldehyde were found to be 115µM and 650mU/mg, respectively. CAldA substrate specificity was found to be low for chloroacetaldehyde, formaldehyde, propionaldehyde, butyraldehyde, benzaldehyde and glutaraldehyde as compared to acetaldehyde. Computational modeling revealed a predicted structure of CAldA consisting of five ß-sheets that comprise seven antiparallel ß-strands and 11 mix strands. The Molecular Dynamics and Docking studies showed that acetaldehyde bind to CaldA more tightly as compared to chloroacetaldehyde.

Immobilization of formaldehyde dehydrogenase in tailored siliceous mesostructured cellular foams and evaluation of its activity for conversion of formate to formaldehyde.

Formaldehyde dehydrogenase (FaldDH) is used as a catalyst to reduce formate to formaldehyde in a cascade reaction to convert CO2 to methanol. This enzyme, however, has low activity and is sensitive to substrate/product concentration and pH. To improve the performance of FaldDH, it can be immobilized through physical adsorption in siliceous mesostructured cellular foams (MCF), which physical properties are suitable for the immobilization of large molecules as FaldDH (molecular size of 8.6 × 8.6 × 19 nm). In this work two MCF materials were synthesized: MCF1 with a pore size of 26.8 nm and window size of 10.5 nm; and MCF2 with a pore size of 32.9 nm and window size of 13.0 nm. The surfaces of the materials were functionalized with octyl, mercaptopropyl or chloromethyl groups. FaldDH was successfully immobilized inside all the materials, yielding enzyme loadings of about 300 mg g-1 in MCF1 and more than 750 mg g-1 in MCF2. However, the enzyme was inactive upon immobilization on MCF1, whereas on MCF2 the enzyme retained its catalytic activity presumably owing to the larger pores of this material and the need for the enzyme to undergo configurational changes during the reaction. Using MCF2 functionalized with mercaptopropyl groups the activity of FaldDH was enhanced beyond that of the free enzyme. Additionally, low leakage of the enzyme from the MCF2 was observed during the reactions. Thus, tailored MCF is a highly attractive material for employment of the FaldDH enzyme.

Biochemical characterization of a bifunctional acetaldehyde-alcohol dehydrogenase purified from a facultative anaerobic bacterium Citrobacter sp. S-77.

Acetaldehyde-alcohol dehydrogenase (ADHE) is a bifunctional enzyme consisting of two domains of an N-terminal acetaldehyde dehydrogenase (ALDH) and a C-terminal alcohol dehydrogenase (ADH). The enzyme is known to be important in the cellular alcohol metabolism. However, the role of coenzyme A-acylating ADHE responsible for ethanol production from acetyl-CoA remains uncertain. Here, we present the purification and biochemical characterization of an ADHE from Citrobacter sp. S-77 (ADHE(S77)). Interestingly, the ADHE(S77) was unable to be solubilized from membrane with detergents either 1% Triton X-100 or 1% Sulfobetaine 3-12. However, the enzyme was easily dissociated from membrane by high-salt buffers containing either 1.0 M NaCl or (NH(4))(2)SO(4) without detergents. The molecular weight of a native protein was estimated as approximately 400 kDa, consisting of four identical subunits of 96.3 kDa. Based on the specific activity and kinetic analysis, the ADHES77 tended to have catalytic reaction towards acetaldehyde elimination rather than acetaldehyde formation. Our experimental observation suggests that the ADHES77 may play a pivotal role in modulating intracellular acetaldehyde concentration.

Over-expression of carbon monoxide dehydrogenase-I with an accessory protein co-expression: a key enzyme for carbon dioxide reduction.

Carbon monoxide dehydrogenase-I (CODH-I) from the CO-utilizing bacterium Carboxydothermus hydrogenoformans are expected to be utilized as a part of reproducible carbon dioxide photoreduction system. However, the over-expression system for CODH-I remains to be constructed. CODH-I constitutes a hydrogenase/CODH gene cluster including a gene encoding a Ni-insertion accessory protein, CooC (cooC3). Through co-expression of CooC3, we found an over-expression system with higher activity. The Rec-CODH-I with the co-expression exhibits 8060 U/mg which was approximately threefold than that without co-expression (2270 U/mg). In addition, co-expression resulted in Ni(2+) content increase; the amount of Ni atoms of Rec-CODH-I was approximately thrice than that without co-expression.

High-level soluble expression of the hemA gene from Rhodobacter capsulatus and comparative study of its enzymatic properties.

The Rhodobacter capsulatus hemA gene, which encodes 5-aminolevulinic acid synthase (ALAS), was expressed in Escherichia coli Rosetta (DE3) and the enzymatic properties of the purified recombinant ALAS (RC-ALAS) were studied. Compared with ALASs encoded by hemA genes from Agrobacterium radiobacter (AR-ALAS) and Rhodobacter sphaeroides (RS-ALAS), the specific activity of RC-ALAS reached 198.2 U/mg, which was about 31.2% and 69.5% higher than those of AR-ALAS (151.1 U/mg) and RS-ALAS (116.9 U/mg), respectively. The optimum pH values and temperatures of the three above mentioned enzymes were all pH 7.5 and 37 °C, respectively. Moreover, RC-ALAS was more sensitive to pH, while the other two were sensitive to temperature. The effects of metals, ethylene diamine tetraacetic acid (EDTA), and sodium dodecyl sulfate (SDS) on the three ALASs were also investigated. The results indicate that they had the same effects on the activities of the three ALASs. SDS and metal ions such as Co(2+), Zn(2+), and Cu(2+) strongly inhibited the activities of the ALASs, while Mn(2+) exerted slight inhibition, and K(+), Ca(2+), Ba(2+), Mg(2+), or EDTA had no significant effect. The specificity constant of succinyl coenzyme A [(kcat/Km)(S-CoA)] of RC-ALAS was 1.4989, which was higher than those of AR-ALAS (0.7456) and RS-ALAS (1.1699), showing its high catalytic efficiency. The fed-batch fermentation was conducted using the recombinant strain containing the R. capsulatus hemA gene, and the yield of 5-aminolevulinic acid (ALA) achieved was 8.8 g/L (67 mmol/L) under the appropriate conditions.

Identification of glutathione (GSH)-independent glyoxalase III from Schizosaccharomyces pombe.

BACKGROUND: Reactive carbonyl species (RCS), such as methylglyoxal (MG) and glyoxal (GO), are synthesized as toxic metabolites in living systems. Mechanisms of RCS detoxification include the glutathione (GSH)-dependent system consisting of glyoxalase I (GLO1) and glyoxalase II (GLO2), and GSH-independent system involving glyoxalase III (GLO3). Hsp31 and DJ-1 proteins are weakly homologous to each other and belong to two different subfamilies of the DJ-1/Hsp31/PfpI superfamily. Recently, the Escherichia coli Hsp31 protein and the DJ-1 proteins from Arabidopsis thaliana and metazoans have been demonstrated to have GLO3 activity. RESULTS: We performed a systematic survey of homologs of DJ-1 and Hsp31 in fungi. We found that DJ-1 proteins have a very limited distribution in fungi, whereas Hsp31 proteins are widely distributed among different fungal groups. Phylogenetic analysis revealed that fungal and metazoan DJ-1 proteins and bacterial YajL proteins are most closely related and together form a sister clade to bacterial and fungal Hsp31 proteins. We showed that two Schizosaccharomyces pombe Hsp31 proteins (Hsp3101 and Hsp3102) and one Saccharomyces cerevisiae Hsp31 protein (ScHsp31) displayed significantly higher in vitro GLO3 activity than S. pombe DJ-1 (SpDJ-1). Overexpression of hsp3101, hsp3102 and ScHSP31 could confer MG and GO resistance on either wild-type S. pombe cells or GLO1 deletion of S. pombe. S. pombe DJ-1 and Hsp31 proteins exhibit different patterns of subcellular localization. CONCLUSIONS: Our results suggest that fungal Hsp31 proteins are the major GLO3 that may have some role in protecting cells from RCS toxicity in fungi. Our results also support the view that the GLO3 activity of Hsp31 proteins may have evolved independently from that of DJ-1 proteins.

Purification and characterization of NAD+ -dependent salicylaldehyde dehydrogenase from carbaryl-degrading Pseudomonas sp. strain C6.

NAD+-dependent salicylaldehyde dehydrogenase (SALDH) which catalyzes the oxidation of salicylaldehyde to salicylate was purified form carbaryl-degrading Pseudomonas sp. strain C6. The enzyme was found to be a functional homotrimer (150 kDa) with subunit molecular mass of 50 kDa and contained calcium (1.8 mol/mol of enzyme). These properties were found to be unique. External addition of metal ions showed no effect on the activity and addition of chelators showed moderate inhibition of the activity. Potassium ions were found to enhance the activity significantly. SALDH showed higher affinity for salicylaldehyde (Km = 4.5 µM) and accepts mono- as well as di-aromatic aldehydes; however it showed poor activity on aliphatic aldehydes. Chloro-/nitro-substituted benzaldehydes were potent substrate inhibitors as compared to benzaldehyde and 3-hydroxybenzaldehyde, while 2-naphthaldehyde and salicylaldehyde were moderate. The kinetic data revealed that SALDH, though having broad specificity, is more efficient for the oxidation of salicylaldehyde as compared to other aromatic aldehyde dehydrogenases which gives an advantage for Pseudomonas sp. strain C6 to bioremediate carbaryl and other aromatic aldehydes efficiently.

Expression, purification, and characterization of formaldehyde dehydrogenase from Pseudomonas aeruginosa.

As a member of zinc-containing medium-chain alcohol dehydrogenase family, formaldehyde dehydrogenase (FDH) can oxidize toxic formaldehyde to less active formate with NAD(+) as a cofactor and exists in both prokaryotes and eukaryotes. Most FDHs are well known to be glutathione-dependent in the catalysis of formaldehyde oxidation, but the enzyme from Pseudomonas putida is an exception, which is independent of glutathione. To identify novel glutathione-independent FDHs from other bacterial strains and facilitate the corresponding structural and enzymatic studies, high-level soluble expression and efficient purification of these enzymes need to be achieved. Here, we present molecular cloning, expression, and purification of the FDH from Pseudomonas aeruginosa, which is a Gram-negative pathogenic bacterium causing opportunistic human infection. The FDH of P. aeruginosa shows high sequence identity (87.97%) with that of P. putida. Our results indicated that coexpression with molecular chaperones GroES, GroEL, and Tig has significantly attenuated inclusion body formation and improved the solubility of the recombinant FDH in Escherichiacoli cells. A purification protocol including three chromatographic steps was also established to isolate the recombinant FDH to homogeneity with a yield of ∼3.2 mg from 1L of cell culture. The recombinant P. aeruginosa FDH was properly folded and biologically functional, as demonstrated by the mass spectrometric, crystallographic, and enzymatic characterizations of the purified proteins.

An electron-bifurcating caffeyl-CoA reductase.

A low potential electron carrier ferredoxin (E0' ≈ -500 mV) is used to fuel the only bioenergetic coupling site, a sodium-motive ferredoxin:NAD(+) oxidoreductase (Rnf) in the acetogenic bacterium Acetobacterium woodii. Because ferredoxin reduction with physiological electron donors is highly endergonic, it must be coupled to an exergonic reaction. One candidate is NADH-dependent caffeyl-CoA reduction. We have purified a complex from A. woodii that contains a caffeyl-CoA reductase and an electron transfer flavoprotein. The enzyme contains three subunits encoded by the carCDE genes and is predicted to have, in addition to FAD, two [4Fe-4S] clusters as cofactor, which is consistent with the experimental determination of 4 mol of FAD, 9 mol of iron, and 9 mol of acid-labile sulfur. The enzyme complex catalyzed caffeyl-CoA-dependent oxidation of reduced methyl viologen. With NADH as donor, it catalyzed caffeyl-CoA reduction, but this reaction was highly stimulated by the addition of ferredoxin. Spectroscopic analyses revealed that ferredoxin and caffeyl-CoA were reduced simultaneously, and a stoichiometry of 1.3:1 was determined. Apparently, the caffeyl-CoA reductase-Etf complex of A. woodii uses the novel mechanism of flavin-dependent electron bifurcation to drive the endergonic ferredoxin reduction with NADH as reductant by coupling it to the exergonic NADH-dependent reduction of caffeyl-CoA.

Gene cloning and biochemical characterization of 4-N-trimethylaminobutyraldehyde dehydrogenase II from Pseudomonas sp. 13CM.

The gene encoding 4-N-trimethylaminobutyraldehyde dehydrogenase (TMABaldehyde-DH) from Pseudomonas sp. 13CM, responsible for the conversion of 4-N-trimethylaminobutyraldehyde (TMABaldehyde) to γ-butyrobetaine in the carnitine biosynthesis pathway, isolated by shotgun cloning and expressed in Escherichia coli DH5α. The recombinant TMABaldehyde-DH was purified 19.5 fold to apparent homogeneity by hydrophobic and affinity chromatography and biochemically characterized. The enzyme was found to be a trimer with identical 52 kDa subunits. The isoelectric point was found to be 4.5. Optimum pH and temperature were found respectively as pH 9.5 and 40 °C. The Km values for TMABaldehyde, 4-dimethylaminobutyraldehyde, and NAD+ were respectively, 0.31, 0.62, and 1.16 mM. The molecular and catalytic properties differed from those of TMABaldehyde-DH I, which was discovered initially in Pseudomonas sp. 13CM. The new enzyme, designated TMABaldehyde-DH II, structural gene was inserted into an expression vector pET24b (+) and over-expressed in E. coli BL21 (DE3) under the control of a T7 promoter. The recombinant TMABaldehyde-DH from Pseudomonas sp. 13CM can now be obtained in large quantity necessary for further biochemical characterization and applications.

Expression, purification and preliminary crystallographic studies of NahF, a salicylaldehyde dehydrogenase from Pseudomonas putida G7 involved in naphthalene degradation.

Pseudomonas putida G7 is one of the most studied naphthalene-degrading species. The nah operon in P. putida, which is present on the 83 kb metabolic plasmid NAH7, codes for enzymes involved in the conversion of naphthalene to salicylate. The enzyme NahF (salicylaldehyde dehydrogenase) catalyzes the last reaction in this pathway. The nahF gene was subcloned into the pET28a(TEV) vector and the recombinant protein was overexpressed in Escherichia coli Arctic Express at 285 K. The soluble protein was purified by affinity chromatography followed by gel filtration. Crystals of recombinant NahF (6×His-NahF) were obtained at 291 K and diffracted to 2.42 Å resolution. They belonged to the hexagonal space group P6(4)22, with unit-cell parameters a = b = 169.47, c = 157.94 Å. The asymmetric unit contained a monomer and a crystallographic twofold axis generated the dimeric biological unit.

A simple, large-scale overexpression method of deriving carbon monoxide dehydrogenase II from thermophilic bacterium Carboxydothermus hydrogenoformans.

We established an Na(2)S-free, large-scale overexpression system of deriving CODH II from thermophilic bacterium Carboxydothermus hydrogenoformans in Escherichia coli using a large-scale fermentor. Recombinant-CODH II showed a CO oxidation activity of 9,600 U/mg. In addition, recombinant-CODH II exhibited considerable CO(2) reduction activity, of 16.9 U/mg.

Carboxylate and aromatic active-site residues are determinants of high-affinity binding of ω-aminoaldehydes to plant aminoaldehyde dehydrogenases.

The crystal structures of both isoforms of the aminoaldehyde dehydrogenase from pea (PsAMADH) have been solved recently [Tylichováet al. (2010) J Mol Biol396, 870-882]. The characterization of the PsAMADH2 proteins, altered here by site-directed mutagenesis, suggests that the D110 and D113 residues at the entrance to the substrate channel are required for high-affinity binding of ω-aminoaldehydes to PsAMADH2 and for enzyme activity, whereas N162, near catalytic C294, contributes mainly to the enzyme's catalytic rate. Inside the substrate cavity, W170 and Y163, and, to a certain extent, L166 and M167 probably preserve the optimal overall geometry of the substrate channel that allows for the appropriate orientation of the substrate. Unconserved W288 appears to affect the affinity of the enzyme for the substrate amino group through control of the substrate channel diameter without affecting the reaction rate. Therefore, W288 may be a key determinant of the differences in substrate specificity found among plant AMADH isoforms when they interact with naturally occurring substrates such as 3-aminopropionaldehyde and 4-aminobutyraldehyde.

Characterization of new medium-chain alcohol dehydrogenases adds resolution to duplications of the class I/III and the sub-class I genes.

Four additional variants of alcohol and aldehyde dehydrogenases have been purified and functionally characterized, and their primary structures have been determined. The results allow conclusions about the structural and evolutionary relationships within the large family of MDR alcohol dehydrogenases from characterizations of the pigeon (Columba livia) and dogfish (Scyliorhinus canicula) major liver alcohol dehydrogenases. The pigeon enzyme turns out to be of class I type and the dogfish enzyme of class III type. This result gives a third type of evidence, based on purifications and enzyme characterization in lower vertebrates, that the classical liver alcohol dehydrogenase originated by a gene duplication early in the evolution of vertebrates. It is discernable as the major liver form at about the level in-between cartilaginous and osseous fish. The results also show early divergence within the avian orders. Structures were determined by Edman degradations, making it appropriate to acknowledge the methodological contributions of Pehr Edman during the 65 years since his thesis at Karolinska Institutet, where also the present analyses were performed.

Acetaldehyde/alcohol dehydrogenase-2 (EhADH2) and clathrin are involved in internalization of human transferrin by Entamoeba histolytica.

Transferrin (Tf) is a host glycoprotein capable of binding two ferric-iron ions to become holotransferrin (holoTf), which transports iron in to all cells. Entamoeba histolytica is a parasitic protozoan able to use holoTf as a sole iron source in vitro. The mechanism by which this parasite scavenges iron from holoTf is unknown. An E. histolytica holoTf-binding protein (EhTfbp) was purified by using an anti-human transferrin receptor (TfR) monoclonal antibody. EhTfbp was identified by MS/MS analysis and database searches as E. histolytica acetaldehyde/alcohol dehydrogenase-2 (EhADH2), an iron-dependent enzyme. Both EhTfbp and EhADH2 bound holoTf and were recognized by the anti-human TfR antibody, indicating that they correspond to the same protein. It was found that the amoebae internalized holoTf through clathrin-coated pits, suggesting that holoTf endocytosis could be important for the parasite during colonization and invasion of the intestinal mucosa and liver.

Enzymes of naphthalene metabolism by Pseudomonas fluorescens 26K strain.

The ability of Pseudomonas fluorescens 26K strain to utilize naphthalene at concentrations up to 600 mg/liter as the sole source of carbon and energy in mineral liquid media was shown. Using HPLC, TLC, and mass-spectrometry, the intermediates of naphthalene transformation by this strain were identified as naphthalene cis-1,2-dihydrodiol, salicylaldehyde, salicylate, catechol, 2-hydroxymuconic semialdehyde, and 1-naphthol. Catechol 2,3-dioxygenase (a homotetramer with native molecular mass 125 kDa) and NAD+-dependent homohexameric naphthalene cis-1,2-dihydrodiol dehydrogenase with native molecular mass 160 kDa were purified from crude extract of the strain and characterized. NAD+-dependent homodimeric salicylaldehyde dehydrogenase with molecular mass 110 kDa was purified and characterized for the first time. Based on the data, a pathway of naphthalene degradation by P. fluorescens 26K is suggested.

A purified mutant HemA protein from Salmonella enterica serovar Typhimurium lacks bound heme and is defective for heme-mediated regulation in vivo.

Archaea, plants, and most bacteria synthesize heme using the C5 pathway, in which the first committed step is catalyzed by the enzyme glutamyl-tRNA reductase (GluTR or HemA). In some cases, an overproduced and purified HemA enzyme contains noncovalently bound heme. The enteric bacteria Salmonella enterica and Escherichia coli also synthesize heme by the C5 pathway, and the HemA protein in these bacteria is regulated by proteolysis. The enzyme is unstable during normal growth due to the action of Lon and ClpAP, but becomes stable when heme is limiting for growth. We describe a method for the overproduction of S. enterica HemA that yields a purified enzyme containing bound heme, identified as a b-type heme by spectroscopy. A mutant of HemA (C170A) does not contain heme when similarly purified. The mutant was used to test whether heme is directly involved in HemA regulation. When expressed from the S. enterica chromosome in a wild-type background, the C170A mutant allele of hemA is shown to confer an unregulated phenotype, with high levels of HemA regardless of the heme status. These results strongly suggest that the presence of bound heme targets the HemA enzyme for degradation and is required for normal regulation.

Identification in Agrobacterium tumefaciens of the D-galacturonic acid dehydrogenase gene.

There are at least three different pathways for the catabolism of D-galacturonate in microorganisms. In the oxidative pathway, which was described in some prokaryotic species, D-galacturonate is first oxidised to meso-galactarate (mucate) by a nicotinamide adenine dinucleotide (NAD)-dependent dehydrogenase (EC 1.1.1.203). In the following steps of the pathway mucate is converted to 2-keto-glutarate. The enzyme activities of this catabolic pathway have been described while the corresponding gene sequences are still unidentified. The D-galacturonate dehydrogenase was purified from Agrobacterium tumefaciens, and the mass of its tryptic peptides was determined using MALDI-TOF mass spectrometry. This enabled the identification of the corresponding gene udh. It codes for a protein with 267 amino acids having homology to the protein family of NAD(P)-binding Rossmann-fold proteins. The open reading frame was functionally expressed in Saccharomyces cerevisiae. The N-terminally tagged protein was not compromised in its activity and was used after purification for a kinetic characterization. The enzyme was specific for NAD and accepted D-galacturonic acid and D-glucuronic acid as substrates with similar affinities. NMR analysis showed that in water solution the substrate D-galacturonic acid is predominantly in pyranosic form which is converted by the enzyme to 1,4 lactone of galactaric acid. This lactone seems stable under intracellular conditions and does not spontaneously open to the linear meso-galactaric acid.