Cloning of the aldehyde reductase gene from a red yeast, Sporobolomyces salmonicolor, and characterization of the gene and its product

Biotransformation of vanillin into vanillyl alcohol by a novel strain of Cystobasidium laryngis isolated from decaying wood

Vanillin is an aromatic aldehyde found as a component of lignocellulosic material, and in the cured pods of orchidaceae plants. Like other phenolic substances, vanillin has antimicrobial activity and can be extracted from lignin either by a thermo-chemical process or through microbial degradation. Vanillin, can serve as a model monomer in biodegradation studies of lignin. In the present study, a yeast isolated from decaying wood on the Faroe Islands, was identified as Cystobasidium laryngis strain FMYD002, based on internal transcribed spacer sequence analysis. It demonstrated the ability to convert vanillin to vanillyl alcohol, as detected by ultra-high performance liquid chromatography-quadrupole-time-of-flight. Structural analysis of vanillyl alcohol was carried out by using gas chromatography-mass spectrometry and 1H NMR spectroscopy, and further verified by synthesis. The reduction of vanillin to vanillyl alcohol has been documented for only a few species of fungi. However, to our knowledge, this biotransformation has not yet been reported for basidiomycetous yeast species, nor for any representative of the subphylum Pucciniomycotina. The biotransformation capability of the present strain might prove useful in the industrial utilisation of lignocellulosic residues.

Discovery of a reductase-producing strain recombinant E. coli CCZU-A13 using colorimetric screening and its whole cell-catalyzed biosynthesis of ethyl (R)-4-chloro-3-hydroxybutanoate

An NADH-dependent reductase (SsCR) was discovered by genome data mining. After SsCR was overexpressed in E. coli BL21, recombinant E. coli CCZU-A13 with high reductase activity and excellent stereoselectivity for the reduction of ethyl 4-chloro-3-oxobutanoate (COBE) into ethyl (R)-4-chloro-3-hydroxybutanoate ((R)-CHBE) was screened using one high-throughput colorimetric screening strategy. After the reaction optimization, a highly stereoselective bioreduction of COBE into (R)-CHBE (>99% ee) with the resting cells of E. coli CCZU-A13 was successfully demonstrated in n-butyl acetate-water (10:90, v/v) biphasic system. Biotransformation of 600mM COBE for 8h in the biphasic system, (R)-CHBE (>99% ee) could be obtained in the high yield of 100%. Moreover, the broad substrate specificity in the reduction of aliphatic and aromatic carbonyl compounds was also found. Significantly, E. coli CCZU-A13 shows high potential in the industrial production of (R)-CHBE (>99% ee) and its derivatives.

Primary Structure Analysis and Functional Expression of Erythrose Reductases from Erythritol-Producing Fungi (Trichosporonoides megachiliensisSNG-42)

NADPH-dependent erythrose reductases (ERs) in erythritol-producing fungi, Trichosporonoides megachiliensis SNG-42, catalyze the reduction of D-erythrose. We previously characterized the biochemical properties of three isozymes of ERs (ER-I, ER-II, and ER-III). Using internal amino acid sequences of ER-III and ER-I with peptide mapping, we cloned three cDNAs (er1, 1121-bp (AB191474); er2, 1077-bp (AB191475); er3, 1119-bp (AB191476)). The er3 cDNA encoded a polypeptide 36,044 Da, and its deduced amino acid sequence was same as that of the native ER-III. The three recombinant enzymes expressed in Escherichia coli were purified to homogeneity. The recombinant enzymes of ER1, ER2, and ER3 showed similar electrophoretic properties to that of the native ER-I, ER-II, and ER-III on SDS- and Native- but not on IEF-PAGE. All three recombinant enzymes showed substrate specificity towards C-4 and C-3 aldehydes similar to that of the native ER-III. These results strongly suggest that cloned er1, er2, and er3 cDNAs encode erythrose reductases.

Cloning and Overexpression of theExiguobacteriumsp. F42 Gene Encoding a New Short Chain Dehydrogenase, Which Catalyzes the Stereoselective Reduction of Ethyl 3-Oxo-3-(2-thienyl)propanoate to Ethyl (S)-3-Hydroxy-3-(2-thienyl)propanoate

Exiguobacterium sp. F42 was screened as a producer of an enzyme catalyzing the NADPH-dependent stereoselective reduction of ethyl 3-oxo-3-(2-thienyl)propanoate (KEES) to ethyl (S)-3-hydroxy-3-(2-thienyl)propanoate ((S)-HEES). (S)-HEES is a key intermediate for the synthesis of (S)-duloxetine, a potent inhibitor of the serotonin and norepinephrine uptake carriers. The responsible enzyme (KEES reductase) was partially purified, and the gene encoding KEES reductase was cloned and sequenced via an inverse PCR approach. Sequence analysis of the gene for KEES reductase revealed that the enzyme was a member of the short chain dehydrogenase/reductase family. The probable NADPH-interacting site and 3 catalytic residues (Ser-Tyr-Lys) were fully conserved. The gene was highly expressed in Escherichia coli, and the gene product was purified to homogeneity from the recombinant E. coli by simpler procedures than from the original host. The molecular mass of the purified enzyme was 27,500 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and 55,000 as determined by gel filtration chromatography. Our results show that this enzyme can be used for the practical production of (S)-HEES.

Characterization of a novel NADPH-dependent oxidoreductase from Gluconobacter oxydans

A novel protein from Gluconobacter oxydans DSM2003 which shows 60-70% similarity with members of aldo-keto reductase (AKR) superfamily was overexpressed in Escherichia coli BL21 (DE3) and purified by one step affinity chromatography with a Ni-NTA agarose column. The recombinant protein (named GOX0644) consists of 279 amino acids with an apparent molecular mass of 32 kDa in the soluble fraction, and the gene sequence encoding the protein GOX0644 is 100% identical to the ORF of gox0644 in G. oxydans 621H (DSM2343). For a detailed analysis of its enzymatic activity, the substrate specificity of the recombinant protein GOX0644 was determined. With NADPH as a cofactor, GOX0644 exhibited better activity to aromatic aldehydes, especially o-chlorobenzaldehyde, compared to aliphatic aldehydes. It showed almost no activity toward glyceraldehyde, xylose, glucose, and ketones. The protein was unable to oxidize primary- or secondary alcohols. Based on these results, GOX0644 was defined as a novel NADPH-dependent aldehyde reductase. Kinetic parameters of the protein and the dependence of its activity on temperature and pH were also determined.

The intra- and extracellular proteome of Aspergillus niger growing on defined medium with xylose or maltose as carbon substrate

Background

The filamentous fungus Aspergillus niger is well-known as a producer of primary metabolites and extracellular proteins. For example, glucoamylase is the most efficiently secreted protein of Aspergillus niger, thus the homologous glucoamylase (glaA) promoter as well as the glaA signal sequence are widely used for heterologous protein production. Xylose is known to strongly repress glaA expression while maltose is a potent inducer of glaA promoter controlled genes. For a more profound understanding of A. niger physiology, a comprehensive analysis of the intra- and extracellular proteome of Aspergillus niger AB1.13 growing on defined medium with xylose or maltose as carbon substrate was carried out using 2-D gel electrophoresis/Maldi-ToF and nano-HPLC MS/MS.

Results

The intracellular proteome of A. niger growing either on xylose or maltose in well-aerated controlled bioreactor cultures revealed striking similarities. In both cultures the most abundant intracellular protein was the TCA cycle enzyme malate-dehydrogenase. Moreover, the glycolytic enzymes fructose-bis-phosphate aldolase and glyceraldehyde-3-phosphate-dehydrogenase and the flavohemoglobin FhbA were identified as major proteins in both cultures. On the other hand, enzymes involved in the removal of reactive oxygen species, such as superoxide dismutase and peroxiredoxin, were present at elevated levels in the culture growing on maltose but only in minor amounts in the xylose culture. The composition of the extracellular proteome differed considerably depending on the carbon substrate. In the secretome of the xylose-grown culture, a variety of plant cell wall degrading enzymes were identified, mostly under the control of the xylanolytic transcriptional activator XlnR, with xylanase B and ferulic acid esterase as the most abundant ones. The secretome of the maltose-grown culture did not contain xylanolytic enzymes, instead high levels of catalases were found and glucoamylase (multiple spots) was identified as the most abundant extracellular protein. Surprisingly, the intracellular proteome of A. niger growing on xylose in bioreactor cultures differed more from a culture growing in shake flasks using the same medium than from the bioreactor culture growing on maltose. For example, in shake flask cultures with xylose as carbon source the most abundant intracellular proteins were not the glycolytic and the TCA cycle enzymes and the flavohemoglobin, but CipC, a protein of yet unknown function, superoxide dismutase and an NADPH dependent aldehyde reductase. Moreover, vacuolar proteases accumulated to higher and ER-resident chaperones and foldases to lower levels in shake flask compared to the bioreactor cultures.

Conclusions

The utilization of xylose or maltose was strongly affecting the composition of the secretome but of minor influence on the composition of the intracellular proteome. On the other hand, differences in culture conditions (pH control versus no pH control, aeration versus no aeration and stirring versus shaking) have a profound effect on the intracellular proteome. For example, lower levels of ER-resident chaperones and foldases and higher levels of vacuolar proteases render shake flask conditions less favorable for protein production compared to controlled bioreactor cultures.

Regeneration of nicotinamide coenzymes: principles and applications for the synthesis of chiral compounds

Dehydrogenases which depend on nicotinamide coenzymes are of increasing interest for the preparation of chiral compounds, either by reduction of a prochiral precursor or by oxidative resolution of their racemate. The regeneration of oxidized and reduced nicotinamide cofactors is a very crucial step because the use of these cofactors in stoichiometric amounts is too expensive for application. There are several possibilities to regenerate nicotinamide cofactors: established methods such as formate/formate dehydrogenase (FDH) for the regeneration of NADH, recently developed electrochemical methods based on new mediator structures, or the application of gene cloning methods for the construction of "designed" cells by heterologous expression of appropriate genes.A very promising approach is enzymatic cofactor regeneration. Only a few enzymes are suitable for the regeneration of oxidized nicotinamide cofactors. Glutamate dehydrogenase can be used for the oxidation of NADH as well as NADPH while L: -lactate dehydrogenase is able to oxidize NADH only. The reduction of NAD(+) is carried out by formate and FDH. Glucose-6-phosphate dehydrogenase and glucose dehydrogenase are able to reduce both NAD(+) and NADP(+). Alcohol dehydrogenases (ADHs) are either NAD(+)- or NADP(+)-specific. ADH from horse liver, for example, reduces NAD(+) while ADHs from Lactobacillus strains catalyze the reduction of NADP(+). These enzymes can be applied by their inclusion in whole cell biotransformations with an NAD(P)(+)-dependent primary reaction to achieve in situ the regeneration of the consumed cofactor.Another efficient method for the regeneration of nicotinamide cofactors is the electrochemical approach. Cofactors can be regenerated directly, for example at a carbon anode, or indirectly involving mediators such as redox catalysts based on transition-metal complexes.An increasing number of examples in technical scale applications are known where nicotinamide dependent enzymes were used together with cofactor regenerating enzymes.

A new member of the short-chain dehydrogenases/reductases superfamily: purification, characterization and substrate specificity of a recombinant carbonyl reductase from Pichia stipitis

A novel short-chain dehydrogenases/reductases superfamily (SDRs) reductase (PsCR) from Pichia stipitis that produced ethyl (S)-4-chloro-3-hydroxybutanoate with greater than 99% enantiomeric excess, was purified to homogeneity using fractional ammonium sulfate precipitation followed by DEAE-Sepharose chromatography. The enzyme purified from recombinant Escherichia coli had a molecular mass of about 35 kDa on SDS-PAGE and only required NADPH as an electron donor. The K(m) value of PsCR for ethyl 4-chloro-3-oxobutanoate was 4.9 mg/mL and the corresponding V(max) was 337 micromol/mg protein/min. The catalytic efficiency value was the highest ever reported for reductases from yeasts. Moreover, PsCR exhibited a medium-range substrate spectrum toward various keto and aldehyde compounds, i.e., ethyl-3-oxobutanoate with a chlorine substitution at the 2 or 4-position, or alpha,beta-diketones. In addition, the activity of the enzyme was strongly inhibited by SDS and beta-mercaptoethanol, but not by ethylene diamine tetra acetic acid.

Diversity of 4-chloroacetoacetate ethyl ester-reducing enzymes in yeasts and their application to chiral alcohol synthesis

Enzymes which reduce 4-chloroacetoacetate ethyl ester (CAAE) to (R)- or (S)-4-chloro-3-hydroxybutanoate ethyl ester (CHBE) were investigated. Several microorganisms which can reduce CAAE with high yields were discovered. An NADPH-dependent aldehyde reductase, ARI, and an NADPH-dependent carbonyl reductase, S1, were isolated from Sporobolomyces salmonicolor and Candida magnoliae, respectively, and enzymatic synthesis of chiral CHBE was performed through the reduction of CAAE. When ARI-overproducing Escherichia coli transformant cells or C. magnoliae cells were incubated in an organic solvent-water diphasic system. CAAE was stoichiometrically converted to (R)- or (S)-CHBE (> 92% enantiomeric excess), respectively. Multiple CAAE-reducing enzymes were present in S. salmonicolor, C. magnoliae and bakers' yeast. Comparison of the primary structures of these CAAE-reducing enzymes with other protein sequences showed that CAAE-reducing enzymes are widely distributed in various protein families, and various physiological roles of these enzymes in the cell were speculated.

Purification and cDNA cloning of NADPH-dependent aldoketoreductase, involved in asymmetric reduction of methyl 4-bromo-3-oxobutyrate, from Penicillium citrinum IFO4631

Penicillium citrinum was found to catalyze the reduction of methyl 4-bromo-3-oxobutyrate to methyl (S)-4-bromo-3-hydroxybutyrate [(S)-BHBM] with high optical purity. From the strain, a cDNA clone encoding a novel NADPH-dependent alkyl 4-halo-3-oxobutyrate reductase (KER) was isolated. Escherichia coli cells overexpressing KER produced (S)-BHBM in the presence of an NADPH-regeneration system.

Purification and characterization of NADPH-dependent aldo?keto reductase specific for ?-keto esters from Penicillium citrinum, and production of methyl (S)-4-bromo-3-hydroxybutyrate

A novel beta-keto ester reductase (KER) was purified to homogeneity from recombinant Escherichia coli (pTrcKER) cells, which efficiently expressed the ker gene cloned from Penicillium citrinum IFO4631. The enzyme was monomeric and had a molecular mass of 37 kDa. It catalyzed the reduction of some beta-keto esters, especially alkyl 4-halo-3-oxobutyrates. However, it did not catalyze the reverse reaction, the dehydrogenation of alkyl 4-halo-3-hydroxybutyrates and other alcohols. The enzyme required NADPH as a cofactor and showed no activity with NADH. Therefore, it was defined as a NADPH-dependent aldo-keto reductase (AKR3E1), belonging to the AKR superfamily. The enzyme stereospecifically produced methyl (S)-4-bromo-3-hydroxybutyrate from its keto derivative with high stereospecificity (97.9% enantiomer excess). E. coli cells expressing KER and glucose dehydrogenase in the water/butyl acetate two-phase system achieved a high productivity of (S)-4-bromo-3-hydroxybutyrate (277 mM, 54 mg/ml) in the organic solvent layer.

Gene cloning and overexpression of two conjugated polyketone reductases, novel aldo-keto reductase family enzymes, of Candida parapsilosis

The genes encoding two conjugated polyketone reductases (CPR-C1, CPR-C2) of Candida parapsilosis IFO 0708 were cloned and sequenced. The genes encoded a total of 304 and 307 amino acid residues for CPR-C1 and CPR-C2, respectively. The deduced amino acid sequences of the two enzymes showed high similarity to each other and to several proteins of the aldo-keto reductase (AKR) superfamily. However, several amino acid residues in putative active sites of AKRs were not conserved in CPR-C1 and CPR-C2. The two CPR genes were overexpressed in Escherichia coli. The E. coli transformant bearing the CPR-C2 gene almost stoichiometrically reduced 30 mg ketopantoyl lactone/ml to D-pantoyl lactone.

Purification, characterization and identification of cysteine desulfhydrase of Corynebacterium glutamicum, and its relationship to cysteine production

We highly purified the enzyme having L-cysteine desulfhydrase activity from Corynebacterium glutamicum. According to its partial amino acid sequence, the enzyme was identified as the aecD gene product, a C-S lyase with alpha, beta-elimination activity [I. Rossol and A. Pühler (1992) J. Bacteriol. 174, 2968-2977]. To produce L-cysteine in C. glutamicum, the Escherichia coli-altered cysE gene encoding Met256Ile mutant serine acetyltransferase, which is desensitized to feedback inhibition by L-cysteine, was introduced into C. glutamicum. When the altered cysE gene was expressed in the aecD-disrupted strain, the transformants produced approximately 290 mg of L-cysteine plus L-cystine per liter from glucose. The produced amount of these amino acids was about two-fold higher than that of the wild-type strain.

Microbial aldo-keto reductases

The aldo-keto reductases (AKR) are a superfamily of enzymes with diverse functions in the reduction of aldehydes and ketones. AKR enzymes are found in a wide range of microorganisms, and many open reading frames encoding related putative enzymes have been identified through genome sequencing projects. Established microbial members of the superfamily include the xylose reductases, 2,5-diketo-D-gluconic acid reductases and beta-keto ester reductases. The AKR enzymes share a common (alpha/beta)(8) structure, and conserved catalytic mechanism, although there is considerable variation in the substrate-binding pocket. The physiological function of many of these enzymes is unknown, but a variety of methods including gene disruptions, heterologous expression systems and expression profiling are being employed to deduce the roles of these enzymes in cell metabolism. Several microbial AKR are already being exploited in biotransformation reactions and there is potential for other novel members of this important superfamily to be identified, studied and utilized in this way.

C.EcoO109I, a regulatory protein for production of EcoO109I restriction endonuclease, specifically binds to and bends DNA upstream of its translational start site

The EcoO109I restriction-modification system, which recognizes 5'-(A/G)GGNCC(C/T)-3', has been cloned, and contains convergently transcribed endonuclease and methylase. The role and action mechanism of the gene product, C.EcoO109I, of a small open reading frame located upstream of ecoO109IR were investigated in vivo and in vitro. The results of deletion analysis suggested that C.EcoO109I acts as a positive regulator of ecoO109IR expression but has little effect on ecoO109IM expression. Assaying of promoter activity showed that the expression of ecoO109IC was regulated by its own gene product, C.EcoO109I. C.EcoO109I was overproduced as a His-tag fusion protein in recombinant Escherichia coli HB101 and purified to homogeneity. C.EcoO109I exists as a homodimer, and recognizes and binds to the DNA sequence 5'-CTAAG(N)(5)CTTAG-3' upstream of the ecoO109IC translational start site. It was also shown that C.EcoO109I bent the target DNA by 54 +/- 4 degrees.

Isolation and primary structural analysis of two conjugated polyketone reductases from Candida parapsilosis

Two conjugated polyketone reductases (CPRs) were isolated from Candida parapsilosis IFO 0708. The primary structures of CPRs (C1 and C2) were analyzed by amino acid sequencing. The amino acid sequences of both enzymes had high similarity to those of several proteins of the aldo-keto-reductase (AKR) superfamily. However, several amino acid residues in the putative active sites of AKRs were not conserved in CPRs-C1 and -C2.

Dehydrogenases and transaminases in asymmetric synthesis

Improved stereoselectivity in dehydrogenase-mediated reductions has been achieved by rationally designed gene overexpression and knockouts in Saccharomyces cerevisiae cells and by isolating and characterizing novel dehydrogenases from other organisms. Transaminases have been used to prepare unnatural amines and amino acids in good yields, particularly when the equilibria are shifted by selective product removal.

Three aldo-keto reductases of the yeast Saccharomyces cerevisiae

Saccharomyces cerevisiae is an industrially important yeast, which is also used extensively as a model eukaryote. The S. cerevisiae genome has been sequenced in its entirety and therefore represents an ideal organism in which to carry out functional analysis of genes. We have identified several open reading frames in the S. cerevisiae genome which show significant similarity to members of the aldo-keto reductase superfamily. The physiological roles of these gene products have not been previously determined, but their similarity to other enzymes suggests they may perform roles in carbohydrate metabolism and detoxification pathways. Cloning and expression of three of these enzymes has allowed their substrate specificities to be determined. Expression profiling and gene disruption analysis will allow potential roles for these enzymes within the cell to be examined.

Organic transformations catalyzed by engineered yeast cells and related systems

Asymmetric ketone reductions remain the most popular application of baker's yeast (Saccharomyces cerevisiae) in organic synthesis and data from the genome sequencing project is beginning to have an impact on improving the stereoselectivities of these reactions, augmenting traditional approaches based on selective inhibition. In addition, the catalytic repertoire of yeast has been expanded to include chiral ketone oxidations by overexpression of a bacterial Baeyer-Villiger monooxygenase.

Molecular cloning and overexpression of the gene encoding an NADPH-dependent carbonyl reductase from Candida magnoliae, involved in stereoselective reduction of ethyl 4-chloro-3-oxobutanoate

An NADPH-dependent carbonyl reductase (S1) isolated from Candida magnoliae catalyzed the reduction of ethyl 4-chloro-3-oxobutanoate (COBE) to ethyl (S)-4-chloro-3-hydroxybutanoate (CHBE), with a 100% enantiomeric excess, which is a useful chiral building block for the synthesis of pharmaceuticals. The gene encoding the enzyme was cloned and sequenced. The S1 gene comprises 849 bp and encodes a polypeptide of 30,420 Da. The deduced amino acid sequence showed a high degree of similarity to those of the other members of the short-chain alcohol dehydrogenase superfamily. The S1 gene was overexpressed in Escherichia coli under the control of the lac promoter. The enzyme expressed in E. coli was purified to homogeneity and had the same catalytic properties as the enzyme from C. magnoliae did. An E. coli transformant reduced COBE to 125 g/l of (S)-CHBE, with an optical purity of 100% enantiomeric excess, in an organic solvent two-phase system.

Cloning, overexpression, and mutagenesis of the Sporobolomyces salmonicolor AKU4429 gene encoding a new aldehyde reductase, which catalyzes the stereoselective reduction of ethyl 4-chloro-3-oxobutanoate to ethyl (S)-4-chloro-3-hydroxybutanoate

We cloned and sequenced the gene encoding an NADPH-dependent aldehyde reductase (ARII) in Sporobolomyces salmonicolor AKU4429, which reduces ethyl 4-chloro-3-oxobutanoate (4-COBE) to ethyl (S)-4-chloro-3-hydroxybutanoate. The ARII gene is 1,032 bp long, is interrupted by four introns, and encodes a 37,315-Da polypeptide. The deduced amino acid sequence exhibited significant levels of similarity to the amino acid sequences of members of the mammalian 3beta-hydroxysteroid dehydrogenase-plant dihydroflavonol 4-reductase superfamily but not to the amino acid sequences of members of the aldo-keto reductase superfamily or to the amino acid sequence of an aldehyde reductase previously isolated from the same organism (K. Kita, K. Matsuzaki, T. Hashimoto, H. Yanase, N. Kato, M. C.-M. Chung, M. Kataoka, and S. Shimizu, Appl. Environ. Microbiol. 62:2303-2310, 1996). The ARII protein was overproduced in Escherichia coli about 2, 000-fold compared to the production in the original yeast cells. The enzyme expressed in E. coli was purified to homogeneity and had the same catalytic properties as ARII purified from S. salmonicolor. To examine the contribution of the dinucleotide-binding motif G(19)-X-X-G(22)-X-X-A(25), which is located in the N-terminal region, during ARII catalysis, we replaced three amino acid residues in the motif and purified the resulting mutant enzymes. Substrate inhibition of the G(19)-->A and G(22)-->A mutant enzymes by 4-COBE did not occur. The A(25)-->G mutant enzyme could reduce 4-COBE when NADPH was replaced by an equimolar concentration of NADH.

Purification and characterization of monovalent cation-activated levodione reductase from Corynebacterium aquaticum M-13

(6R)-2,2,6-Trimethyl-1,4-cyclohexanedione (levodione) reductase was isolated from a cell extract of the soil isolate Corynebacterium aquaticum M-13. This enzyme catalyzed regio- and stereoselective reduction of levodione to (4R,6R)-4-hydroxy-2,2, 6-trimethylcyclohexanone (actinol). The relative molecular mass of the enzyme was estimated to be 142,000 Da by high-performance gel permeation chromatography and 36,000 Da by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme required NAD(+) or NADH as a cofactor, and it catalyzed reversible oxidoreduction between actinol and levodione. The enzyme was highly activated by monovalent cations, such as K(+), Na(+), and NH(4)(+). The NH(2)-terminal and partial amino acid sequences of the enzyme showed that it belongs to the short-chain alcohol dehydrogenase/reductase family. This is the first report of levodione reductase.

Production of chiral C3- and C4-units by microbial enzymes

Enzyme (biocatalysis) reactions display far greater specificities, such as substrate specificity, stereospecificity, regiospecificity and so on, than more conventional forms of organic reactions. Using these specificities of the enzymes, many useful compounds have been enzymatically produced. Compounds possessing C3- and C4-units with additional functional groups are promising materials for the synthesis of various useful compounds. In particular, optically active C3- and C4-synthetic units are quite important intermediates for the preparation of pharmaceuticals and fine chemicals. Microbial transformation with enzymes showing stereo-specificities have been applied to the asymmetric synthesis of optically active substances. In this article the recent works on the practical production of chiral C3- and C4-synthetic units with microbial enzymes are described.

Occurrence of multiple ethyl 4-chloro-3-oxobutanoate-reducing enzymes in Candida magnoliae

Multiple ethyl 4-chloro-3-oxobutanoate (COBE)-reducing enzymes were isolated from a cell-free extract of Candida magnoliae. A NADPH-dependent COBE-reducing enzyme, distinct from the carbonyl reductase and aldehyde reductase previously reported, was purified to homogeneity using five steps, including polyethylene glycol treatment. The relative molecular mass of the enzyme was estimated to be 86,000 on high performance gel-permeation chromatography and 29,000 on sodium dodecyl sulfate polyacrylamide gel electrophoresis. The enzyme catalyzed the reduction of COBE to the corresponding (S)-alcohol with a 51% enantiomeric excess. The substrate specificity of the enzyme was different from those of the other COBE-reducing enzymes of the same strain. The partial amino acid sequences of the enzyme showed that it belongs to the short chain alcohol dehydrogenase/reductase (SDR) family. This is the first report of multiple COBE-reducing enzymes with various stereoselectivities occurring in the same strain but belonging to different (super)families.

Chiral alcohol synthesis with microbial carbonyl reductases in a water-organic solvent two-phase system

Production of chiral 4-chloro-3-hydroxybutanoate ethyl esters (CHBE) was performed through microbial asymmetric reduction of 4-chloroacetoacetate ethyl ester (CAAE). The enzymes reducing CAAE to (R)- and (S)-CHBE were found to be produced by Sporobolomyces salmonicolor and Candida magnoliae, respectively. The enzyme of S. salmonicolor is a novel NADPH-dependent aldehyde reductase (AR) belonging to the aldo-keto reductase superfamily. When AR-overproducing Escherichia coli transformant cells or C. magnoliae cells were incubated in an organic solvent-water two-phase system, 300 or 90 mg/mL of CAAE was almost stoichiometrically converted to (R)- or (S)-CHBE (> 92% ee), respectively.

Comparative anatomy of the aldo-keto reductase superfamily

The aldo-keto reductases metabolize a wide range of substrates and are potential drug targets. This protein superfamily includes aldose reductases, aldehyde reductases, hydroxysteroid dehydrogenases and dihydrodiol dehydrogenases. By combining multiple sequence alignments with known three-dimensional structures and the results of site-directed mutagenesis studies, we have developed a structure/function analysis of this superfamily. Our studies suggest that the (alpha/beta)8-barrel fold provides a common scaffold for an NAD(P)(H)-dependent catalytic activity, with substrate specificity determined by variation of loops on the C-terminal side of the barrel. All the aldo-keto reductases are dependent on nicotinamide cofactors for catalysis and retain a similar cofactor binding site, even among proteins with less than 30% amino acid sequence identity. Likewise, the aldo-keto reductase active site is highly conserved. However, our alignments indicate that variation ofa single residue in the active site may alter the reaction mechanism from carbonyl oxidoreduction to carbon-carbon double-bond reduction, as in the 3-oxo-5beta-steroid 4-dehydrogenases (Delta4-3-ketosteroid 5beta-reductases) of the superfamily. Comparison of the proposed substrate binding pocket suggests residues 54 and 118, near the active site, as possible discriminators between sugar and steroid substrates. In addition, sequence alignment and subsequent homology modelling of mouse liver 17beta-hydroxysteroid dehydrogenase and rat ovary 20alpha-hydroxysteroid dehydrogenase indicate that three loops on the C-terminal side of the barrel play potential roles in determining the positional and stereo-specificity of the hydroxysteroid dehydrogenases. Finally, we propose that the aldo-keto reductase superfamily may represent an example of divergent evolution from an ancestral multifunctional oxidoreductase and an example of convergent evolution to the same active-site constellation as the short-chain dehydrogenase/reductase superfamily.

A new nomenclature for the aldo-keto reductase superfamily

The aldo-keto reductases (AKRs) represent a growing oxidoreductase superfamily. Forty proteins have been identified and characterized as AKRs, and an additional fourteen genes may encode proteins related to the superfamily. Found in eukaryotes and prokaryotes, the AKRs metabolize a wide range of substrates, including aliphatic aldehydes, monosaccharides, steroids, prostaglandins, and xenobiotics. This broad substrate specificity has caused problems in naming these proteins. Enzymes capable of these reactions have been referred to as aldehyde reductase (ALR1), aldose reductase (ALR2), and carbonyl reductase (ALR3); however, ALR3 is not a member of the AKR superfamily. Also, some AKRs have multiple names based upon substrate specificity. For example, human 3alpha-hydroxysteroid dehydrogenase (3apha-HSD) type I is also known as dihydrodiol dehydrogenase 4 and chlordecone reductase. To address these issues, we propose a new nomenclature system for the AKR superfamily based on amino acid sequence identities. Cluster analysis of the AKRs shows seven distinct families at the 40% amino acid identity level. The largest family (AKR1) contains the aldose reductases, aldehyde reductases, and HSDs. Other families include the prokaryotic AKRs, the plant chalcone reductases, the Shaker channels, and the ethoxyquin-inducible aflatoxin B1 aldehyde reductase. At the level of 60% amino acid identity, subfamilies are discernible. For example, the AKR1 family includes five subfamilies: (A) aldehyde reductases (mammalian); (B) aldose reductases; (C) HSDs; (D) delta4-3-ketosteroid-5beta-reductases; and (E) aldehyde reductases (plant). This cluster analysis forms the basis for our nomenclature system. Recommendations for naming an aldo-keto reductase include the root symbol "AKR," an Arabic number designating the family, a letter indicating the subfamily when multiple subfamilies exist, and an Arabic numeral representing the unique protein sequence. For example, human aldehyde reductase would be assigned as AKR1A1. Our nomenclature is both systematic and expandable, thereby allowing assignment of consistent designations for newly identified members of the superfamily.

Screening of novel microbial enzymes for the production of biologically and chemically useful compounds

Enzymes have been generally accepted as superior catalysts in organic synthesis. Micro-organisms in particular have been regarded as treasure sources of useful enzymes. The synthetic technology using microbial enzymes or micro-organisms themselves is called microbial transformation. In designing a microbial transformation process, one of the most important points is to find a suitable enzyme for the reaction of interest. Various kinds of novel enzymes for specific transformations have been discovered in micro-organisms and their potential characteristics revealed. This article reviews our current results on the discovery of novel enzymes for the production of biologically and chemically useful compounds, and emphasizes the importance of screening enzymes in a diverse microbial world.