Dehydrogenases for the synthesis of chiral compounds

Effect of thermal and chemical denaturants on Thermoanaerobacter ethanolicus secondary-alcohol dehydrogenase stability and activity

Thermoanaerobacter ethanolicus 39E secondary-alcohol dehydrogenase (2 degrees ADH) was optimally active near 90 degrees C displaying thermostability half-lives of 1.2 days, 1.7 h, 19 min, 9.0 min, and 1.3 min at 80 degrees C, 90 degrees C, 92 degrees C, 95 degrees C, and 99 degrees C, respectively. Enzyme activity loss upon heating (90-100 degrees C) was accompanied by precipitation, but the soluble enzyme remaining after partial inactivation retained complete activity. Enzyme thermoinactivation was modeled by a pseudo-first order rate equation suggesting that the rate determining step was unimolecular with respect to protein and thermoinactivation preceded aggregation. The apparent 2 degrees ADH melting temperature (T(m)) occurred at approximately 115 degrees C, 20 degrees C higher than the temperature for maximal activity, suggesting that it is completely folded in its active temperature range. Thermodynamic calculations indicated that the active folded structure of the 2 degrees ADH is stabilized by a relatively small Gibbs energy (triangle upG(stab.)(double dagger) = 110 kJ mol(-1)). 2 degrees ADH catalytic activities at 37 degrees C to 75 degrees C, were 2-fold enhanced by guanidine hydrochloride (GuHCl) concentrations between 120 mM and 190 mM. These results demonstrate the extreme resistance of this thermophilic 2 degrees ADH to thermal or chemical denaturation; and suggest increased temperature or GuHCl levels seem to enhance protein fixability and activity.

Immobilized horse liver alcohol dehydrogenase as an on-line high-performance liquid chromatographic enzyme reactor for stereoselective synthesis

Horse liver alcohol dehydrogenase (HLADH) has been non-covalently immobilized on an immobilized artificial membrane (IAM) high-performance liquid chromatography (HPLC) stationary phase. The resulting IAM-HLADH retained the reductive activity of native HLADH as well as the enzyme's enantioselectivity and enantiospecificity. HLADH was also immobilized in an IAM HPLC stationary phase prepacked in a 13 x 4.1 mm ID column to create an immobilized enzyme reactor (HLADH-IMER). The reactor was connected through a switching valve to a column containing a chiral stationary phase (CSP) based upon p-methylphenylcarbamate derivatized cellulose (Chiralcel OJR-CSP). The results from the combined HLADH-IMER/CSP and chromatographic system demonstrate that the enzyme retained its activity and stereoselectivity after immobilization in the column and that the substrate and products from the enzymatic reduction could be transferred to a second column for analytical or preparative separation. The combined HLADH-IMER/CSP system is a prototype for the preparative on-line use of cofactor-dependent enzymes in large-scale chiral syntheses.

Stereoselective carveol dehydrogenase from Rhodococcus erythropolis DCL14. A novel nicotinoprotein belonging to the short chain dehydrogenase/reductase superfamily

A novel nicotinoprotein, catalyzing the dichlorophenolindophenol-dependent oxidation of carveol to carvone, was purified to homogeneity from Rhodococcus erythropolis DCL14. The enzyme is specifically induced after growth on limonene and carveol. Dichlorophenolindophenol-dependent carveol dehydrogenase (CDH) is a homotetramer of 120 kDa with each subunit containing a tightly bound NAD(H) molecule. The enzyme is optimally active at pH 5.5 and 50 degrees C and displays a broad substrate specificity with a preference for substituted cyclohexanols. When incubated with a diastereomeric mixture of (4R)- or (4S)-carveol, CDH stereoselectively catalyzes the conversion of the (6S)-carveol stereoisomers only. Kinetic studies with pure stereoisomers showed that this is due to large differences in V(max)/K(m) values and simultaneous product inhibition by (R)- or (S)-carvone. The R. erythropolis CDH gene (limC) was identified in an operon encoding the enzymes involved in limonene degradation. The CDH nucleotide sequence revealed an open reading frame of 831 base pairs encoding a 277-amino acid protein with a deduced mass of 29,531 Da. The CDH primary structure shares 10-30% sequence identity with members of the short chain dehydrogenase/reductase superfamily. Structure homology modeling with trihydroxynaphthalene reductase from Magnaporthe grisea suggests that CDH from R. erythropolis DCL14 is an alpha/beta one-domain protein with an extra loop insertion involved in NAD binding and a flexible C-terminal part involved in monoterpene binding.

Cold-adapted alanine dehydrogenases from two antarctic bacterial strains: gene cloning, protein characterization, and comparison with mesophilic and thermophilic counterparts

The genes encoding NAD(+)-dependent alanine dehydrogenases (AlaDHs) (EC 1.4.1.1) from the Antarctic bacterial organisms Shewanella sp. strain Ac10 (SheAlaDH) and Carnobacterium sp. strain St2 (CarAlaDH) were cloned and expressed in Escherichia coli. Of all of the AlaDHs that have been sequenced, SheAlaDH exhibited the highest level of sequence similarity to the AlaDH from the gram-negative bacterium Vibrio proteolyticus (VprAlaDH). CarAlaDH was most similar to AlaDHs from mesophilic and thermophilic Bacillus strains. SheAlaDH and CarAlaDH had features typical of cold-adapted enzymes; both the optimal temperature for catalytic activity and the temperature limit for retaining thermostability were lower than the values obtained for the mesophilic counterparts. The k(cat)/K(m) value for the SheAlaDH reaction was about three times higher than the k(cat)/K(m) value for VprAlaDH, but it was much lower than the k(cat)/K(m) value for the AlaDH from Bacillus subtilis. Homology-based structural models of various AlaDHs, including the two psychotropic AlaDHs, were constructed. The thermal instability of SheAlaDH and CarAlaDH may result from relatively low numbers of salt bridges in these proteins.

Synthesis of probes for the active site of leucine dehydrogenase

A series of novel 3-substituted 2-oxobutanoic acids were prepared and incubated with leucine dehydrogenase giving in one case both a kinetic resolution at C-3 and reductive amination of the ketone. This is the first example of an amino acid dehydrogenase catalysed kinetic resolution and reductive amination.

Laboratory evolution of peroxide-mediated cytochrome P450 hydroxylation

Enzyme-based chemical transformations typically proceed with high selectivity under mild conditions, and are becoming increasingly important in the pharmaceutical and chemical industries. Cytochrome P450 monooxygenases (P450s) constitute a large family of enzymes of particular interest in this regard. Their biological functions, such as detoxification of xenobiotics and steroidogenesis, are based on the ability to catalyse the insertion of oxygen into a wide variety of compounds. Such a catalytic transformation might find technological applications in areas ranging from gene therapy and environmental remediation to the selective synthesis of pharmaceuticals and chemicals. But relatively low turnover rates (particularly towards non-natural substrates), low stability and the need for electron-donating cofactors prohibit the practical use of P450s as isolated enzymes. Here we report the directed evolution of the P450 from Pseudomonas putida to create mutants that hydroxylate naphthalene in the absence of cofactors through the 'peroxide shunt' pathway with more than 20-fold higher activity than the native enzyme. We are able to screen efficiently for improved mutants by coexpressing them with horseradish peroxidase, which converts the products of the P450 reaction into fluorescent compounds amenable to digital imaging screening. This system should allow us to select and develop mono- and di-oxygenases into practically useful biocatalysts for the hydroxylation of a wide range of aromatic compounds.

Thermodynamic and kinetic aspects on water vs. organic solvent as reaction media in the enzyme-catalysed reduction of ketones

The stereoselective reduction of ketones catalysed by alcohol dehydrogenase from Thermoanaerobium brockii was studied in different reaction media, hexane at controlled water activities, hexane with 2. 5% water (biphasic) and pure water. The reactions were studied in the temperature range from -1 to 50 degrees C. Increasing the water activity from 0.53 to 0.97 increased the reaction rate 16-fold. The rate was further enhanced in hexane when exceeding the water solubility and in pure water the rates were even higher. This was general for all ketones studied. At controlled water activity the entropy of activation (DeltaSdouble dagger) was the dominating factor. Large negative DeltaSdouble dagger values caused low reaction rates at low aw. When increasing the carbon chain length of the substrate, for reactions in hexane, the decrease of reaction rate was mainly due to a decrease in DeltaSdouble dagger. In the comparison between hexane and pure water, DeltaGdouble dagger values were higher in hexane due to higher DeltaHdouble dagger (activation enthalpy) values. The enantioselectivity (E value) increased from 2.6 at water activity 0. 53 to 4.6 at water activity 0.97. Changing media from hexane (2.5%, v/v water) to pure water was not affecting the enantioselectivity or the specificity for different ketones.

Bacterial formate dehydrogenase. Increasing the enzyme thermal stability by hydrophobization of alpha-helices

NAD+-dependent formate dehydrogenase (EC 1.2.1.2, FDH) from methylotrophic bacterium Pseudomonas sp.101 exhibits the highest stability among the similar type enzymes studied. To obtain further increase in the thermal stability of FDH we used one of general approaches based on hydrophobization of protein alpha-helices. Five serine residues in positions 131, 160, 168, 184 and 228 were selected for mutagenesis on the basis of (i) comparative studies of nine FDH amino acid sequences from different sources and (ii) with the analysis of the ternary structure of the enzyme from Pseudomonas sp.101. Residues Ser-131 and Ser-160 were replaced by Ala, Val and Leu. Residues Ser-168, Ser-184 and Ser-228 were changed into Ala. Only Ser/Ala mutations in positions 131, 160, 184 and 228 resulted in an increase of the FDH stability. Mutant S168A was 1.7 times less stable than the wild-type FDH. Double mutants S(131,160)A and S(184,228)A and the four-point mutant S(131,160,184,228)A were also prepared and studied. All FDH mutants with a positive stabilization effect had the same kinetic parameters as wild-type enzyme. Depending on the position of the replaced residue, the single point mutation Ser/Ala increased the FDH stability by 5-24%. Combination of mutations shows near additive effect of each mutation to the total FDH stabilization. Four-point mutant S(131,160,184,228)A FDH had 1.5 times higher thermal stability compared to the wild-type enzyme.

Synthesis and properties of new coenzyme mimics based on the artificial coenzyme CL4

We have previously shown that a range of nicotinamide containing 'biomimetic coenzymes' function as active analogues of NAD+ in the oxidation of alcohols by horse liver alcohol dehydrogenase (HLADH), despite their apparently astonishing lack of structural similarity to the natural coenzyme. The simplest structure as yet shown to exhibit activity is the biomimetic coenzyme CL4. To investigate the effect of the structure of this truncated artificial coenzyme on its activity, a range of close structural analogues of CL4 were designed, synthesized and characterized. The electrochemical reduction potentials of the analogues were strongly influenced by the nature of the groups attached to the pyridine ring. All of the analogues could be chemically reduced using sodium borohydride, to give compounds with altered UV-visible absorption and fluorescence properties. An HPLC-based assay suggested that two of the new analogues were coenzymically active in the oxidation of butan-1-ol by HLADH, with one displaying a significantly higher activity than CL4. The results demonstrate which features of the structures of the coenzymes lead to desirable electrochemical and spectroscopic properties, but suggest that the structural requirements for a functional coenzyme are quite stringent. These observations may be used to design an artificial coenzyme which combines the best features of those studied so far.

Synthesis of 2-hydroxy acid from 2-amino acid by Clostridium butyricum

Cultures of Clostridium butyricum type strain in synthetic medium supplemented with various L-2-amino acids revealed the presence of the corresponding 2-hydroxy acid. This metabolite is able to produce the polyester poly(2-hydroxyalkanoic acid). The bioconversion is not stereoselective since D-2-amino acids were also converted. Chiral GC analysis demonstrated that only D-enantiomer is formed from L-leucine.

A pH-controlled fed-batch process can overcome inhibition by formate in NADH-dependent enzymatic reductions using formate dehydrogenase-catalyzed coenzyme regeneration

The NAD-dependent, formate dehydrogenase-catalyzed oxidation of formate anion into CO2 is known as the method for the regeneration of NADH in reductive enzymatic syntheses. Inhibition by formate and inactivation by alkaline pH-shift that occurs when oxidation of formate is carried out at pH approximately 7.0 may, however, hamper the efficient application of this NADH recycling reaction. Here, we have devised a fed-batch process using pH-controlled feeding of formic acid that can overcome enzyme inhibition and inactivation. The reaction pH is thus kept constant by addition of acid, and formate dehydrogenase is supplied continuously with substrate as required, but the concentration of formate is maintained at a constant, non- or weakly inhibitory level throughout the enzymatic conversion, thus enabling a particular NADH-dependent dehydrogenase to operate stably and at high reaction rates. For xylitol production from xylose using yeast xylose reductase (Ki,Formate 182 mM), a fed-batch conversion of 0.5M xylose yielded productivities of 2.8 g (L h)-1 that are three-fold improved when contrasted to a conventional batch reaction that employed equal initial concentrations of xylose and formate.

Rapid SDS-Gel capillary electrophoresis for the analysis of recombinant NADP(+)-dependent formate dehydrogenase during expression in Escherichia coli cells and its purification

The level of expression in Escherichia coli cells and different steps of purification of the recombinant NADP(+)-dependent formate dehydrogenase (EC 1.2.1.2, FDH) from bacterium Pseudomonas sp.101 was analyzed by rapid SDS-Gel capillary electrophoresis (SDS-Gel CE) and compared with SDS polyacrylamide gel electrophoresis (SDS PAGE). First standard proteins were separated in the short capillary and the calibration curve generated, then fractions taken during the fermentation and purification process were analysed. The main advantages of SDS-Gel CE are short analysis time, high sensitivity, the possibility to quantify proteins at different ultraviolet wavelength, and small injection volumes. The data for each step of the fermentation process and during the purification were controlled by spectrophotometric analysis of enzyme activity and protein concentration as well as standard SDS PAGE. The molecular mass of the purified FDH was determined as 44,078 Da by matrix-assisted laser desorption/ ionisation time of flight mass spectrometry.

Synthesis of optically active amino acids from alpha-keto acids with Escherichia coli cells expressing heterologous genes

We describe a simple method for enzymatic synthesis of L and D amino acids from alpha-keto acids with Escherichia coli cells which express heterologous genes. L-amino acids were produced with thermostable L-amino acid dehydrogenase and formate dehydrogenase (FDH) from alpha-keto acids and ammonium formate with only an intracellular pool of NAD+ for the regeneration of NADH. We constructed plasmids containing, in addition to the FDH gene, the genes for amino acid dehydrogenases, including i.e., leucine dehydrogenase, alanine dehydrogenase, and phenylalanine dehydrogenase. L-Leucine, L-valine, L-norvaline, L-methionine, L-phenylalanine, and L-tyrosine were synthesized with the recombinant E. coli cells with high chemical yields (> 80%) and high optical yields (up to 100% enantiomeric excess). Stereospecific conversion of various alpha-keto acids to D amino acids was also examined with recombinant E. coli cells containing a plasmid coding for the four heterologous genes of the thermostable enzymes D-amino acid aminotransferase, alanine racemase, L-alanine dehydrogenase, and FDH. Optically pure D enantiomers of glutamate and leucine were obtained.

Purification and characterization of phenylacetaldehyde reductase from a styrene-assimilating Corynebacterium strain, ST-10

A novel phenylacetaldehyde reductase was purified about 50-fold to homogeneity from Corynebacterium sp. strain ST-10, which can assimilate gaseous styrene as the sole carbon and energy source. The enzyme was inductively synthesized when grown on gaseous styrene and had an important role in styrene metabolism in vivo. The enzyme had a molecular weight of 155,000 and was composed of four identical subunits (molecular weight, 42,000). The enzyme catalyzed the reduction of not only phenylacetaldehyde but also various aldehydes and ketones; however, it did not catalyze the reverse reaction, the dehydrogenation of 2-phenylethanol. The enzyme required NADH as a cofactor and showed no activity with NADPH; therefore, it was defined as an NADH-dependent phenylacetaldehyde reductase. The enzyme stereospecifically produced (S)-(-)-1-phenylethanol from acetophenone; therefore, it would be useful as a biocatalyst.

The enantiomeric purity of alcohols formed by enzymatic reduction of ketones can be improved by optimisation of the temperature and by using a high co-substrate concentration

The stereoselective reduction of ketones by alcohol dehydrogenase from Thermoanaerobium brockii was studied in organic reaction media. 2-Propanol was used as co-substrate to regenerate the coenzyme NADPH. The enantiomeric excess of the alcohol formed from the ketone decreased during the course of the reaction (from 53 to 0% e.e. in the formation of (R)-2-butanol). This was interpreted as being due to the reversibility of all the reactions involved. By using a large excess of 2-propanol this effect was suppressed. In the reduction of 2-butanone to (R)-2-butanol, the enantiomeric excess increased with increasing temperature, but in the reduction of 2-pentanone to (S)-2-pentanol the enantiomeric excess decreased with increasing temperature. The data were evaluated in terms of free energy of activation of the reaction pathways leading to the different possible products.

Crystal structure of a ternary complex of D-2-hydroxyisocaproate dehydrogenase from Lactobacillus casei, NAD+ and 2-oxoisocaproate at 1.9 A resolution

D-2-hydroxyisocaproate dehydrogenase (D-HicDH) from Lactobacillus casei is a homodimer with 333 amino acids and a molecular mass of 37 kDa per subunit. The enzyme belongs to the protein family of NAD+-dependent D-2-hydroxycarboxylate dehydrogenases and within this family to the subgroup of D-lactate dehydrogenases (D-LDHs). Compared with other D-LDHs D-HicDH is characterized by a very low specificity regarding size and chemical constitution of the accepted D-2-hydroxycarboxylates. Hexagonal crystals of recombinant D-HicDH in the presence of NAD+ and 2-oxoisocaproate (4-methyl-2-oxopentanoate) were grown with ammonium sulphate as precipitating agent. The structure of these crystals was solved by molecular replacement and refined to a final R-factor of 19.6% for all measured X-ray reflections in the resolution range (infinity to 1.86 A). Both NAD+ and 2-oxoisocaproate were identified in the electron density map; binding of the latter in the active site, however, competes with a sulphate ion, which is also defined by electron density. Additionally the final model contains 182 water molecules and a second sulphate ion. The binding of both an in vitro substrate and the natural cosubstrate in the active site provides substantial insight into the catalytic mechanism and allows us to assess previously published active site models for this enzyme family, in particular the two most controversial points, the role of the conserved Arg234 and substrate binding. Furthermore the overall topology and details of the D-HicDH structure are described, discussed against the background of homologous structures and compared with one closely and one distantly related protein.

D-2-hydroxy-4-methylvalerate dehydrogenase from Lactobacillus delbrueckii subsp. bulgaricus. I. Kinetic mechanism and pH dependence of kinetic parameters, coenzyme binding and substrate inhibition

The steady-state kinetics of D-2-hydroxy-4-methylvalerate dehydrogenase have been studied at pH 8.0 by initial velocity, product inhibition, and dead-end inhibition techniques. The mechanism is rapid-equilibrium ordered in the NAD+ plus D-2-hydroxy-4-methylvalerate direction, and steady-state ordered in the other direction. In both cases coenzyme is the first substrate added and both the E-NADH-D-2-hydroxy-4-methylvalerate and E-NAD+-2-oxo-4-methylvalerate give rise to abortive complexes which cause excess substrate inhibition. Steady-state measurements show that the rate-limiting step in both directions at pH 8.0 is between formation of the enzyme-coenzyme-substrate ternary complex and the release of the first product of the reaction. Transient kinetics combined with primary kinetic deuterium isotope effects show that in the NADH-->NAD+ direction there is a slow, rate-limiting rearrangement of the E-NADH-oxoacid complex while hydride transfer is very fast. The release of NAD+ at pH 8.0 is 200-times faster than Kcat (NADH-->NAD+) whereas the release of NADH is only 5-times faster than Kcat (NAD+-->NADH). The pH dependence of NADH binding depends upon the presence of two ionizable residues with a pKa of about 5.9. The pH dependence of kinetic parameters is explained by a third ionizable residue with pKa values 7.2 (in the E-NADH complex) and < or = 6.4 (in the E-NAD+ complex) which may be the proton donor and acceptor for the chemical reaction. At pH 6.5 the mechanism changes in the NADH-->NAD+ direction to be partly limited by the chemical step with a measured primary kinetic isotope effect of 5.7 and partly by an only slightly faster dissociation of NAD+. In addition the inhibition by excess oxo-4-methylvalerate is more pronounced. The mechanism implies that removing the positive charges created by the two groups which control coenzyme affinity could both enhance the catalytic rate at pH 6.5 and diminish excess substrate inhibition to provide an enzyme better suited to the bulk synthesis of D-2-hydroxyacids.

New alcohol dehydrogenases for the synthesis of chiral compounds

The enantioselective reduction of carbonyl groups is of interest for the production of various chiral compounds such as hydroxy acids, amino acids, hydroxy esters, or alcohols. Such products have high economic value and are most interesting as additives for food and feed or as building blocks for organic synthesis. Enzymatic reactions or biotransformations with whole cells (growing or resting) for this purpose are described. Although conversions with whole cells are advantageous with respect to saving expensive isolation of the desired enzymes, the products often lack high enantiomeric excess and the process results in low time-space-yield. For the synthesis of chiral alcohols, only lab-scale syntheses with commercially available alcohol dehydrogenases have been described yet. However, most of these enzymes are of limited use for technical applications because they lack substrate specificity, stability (yeast ADH) or enantioselectivity (Thermoanaerobium brockii ADH). Furthermore, all enzymes so far described are forming (S)-alcohols. Quite recently, we found and characterized several new bacterial alcohol dehydrogenases, which are suited for the preparation of chiral alcohols as well as for hydroxy esters in technical scale. Remarkably, of all these novel ADHs the (R)-specific enzymes were found in strains of the genus Lactobacillus. Meanwhile, these new enzymes were characterized extensively. Protein data (amino acid sequence, bound cations) confirm that these catalysts are novel enzymes. (R)-specific as well as (S)-specific ADHs accept a broad variety of ketones and ketoesters as substrates. The applicability of alcohol dehydrogenases for chiral syntheses as an example for the technical use of coenzyme-dependent enzymes is demonstrated and discussed in this contribution. In particular NAD-dependent enzymes coupled with the coenzyme regeneration by formate dehydrogenase proved to be economically feasible for the production of fine chemicals.

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.

13C nuclear magnetic resonance analysis of glucose and citrate end products in an ldhL-ldhD double-knockout strain of Lactobacillus plantarum

We have examined the metabolic consequences of knocking out the two ldh genes in Lactobacillus plantarum using 13C nuclear magnetic resonance. Unlike its wild-type isogenic progenitor, which produced lactate as the major metabolite under all conditions tested, ldh null strain TF103 mainly produced acetoin. A variety of secondary end products were also found, including organic acids (acetate, succinate, pyruvate, and lactate), ethanol, 2,3-butanediol, and mannitol.

Knockout of the two ldh genes has a major impact on peptidoglycan precursor synthesis in Lactobacillus plantarum

Most bacteria synthesize muramyl-pentapeptide peptidoglycan precursors ending with a D-alanyl residue (e.g., UDP-N-acetylmuramyl-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Ala). However, it was recently demonstrated that other types of precursors, notably D-lactate-ending molecules, could be synthesized by several lactic acid bacteria. This particular feature leads to vancomycin resistance. Vancomycin is a glycopeptide antibiotic that blocks cell wall synthesis by the formation of a complex with the extremity of peptidoglycan precursors. Substitution of the terminal D-alanine by D-lactate reduces the affinity of the antibiotic for its target. Lactobacillus plantarum is a lactic acid bacterium naturally resistant to vancomycin. It converts most of the glycolytic pyruvate to L- and D-lactate by using stereospecific enzymes designated L- and D-lactate dehydrogenases, respectively. In the present study, we show that L. plantarum actually synthesizes D-lactate-ending peptidoglycan precursors. We also report the construction of a strain which is deficient for both D- and L-lactate dehydrogenase activities and which produces only trace amounts of D- and L-lactate. As a consequence, the peptidoglycan synthesis pathway is drastically affected. The wild-type precursor is still present, but a new type of D-alanine-ending precursor is also synthesized in large quantities, which results in a highly enhanced sensitivity to vancomycin.

Cloning and expression of the gene encoding the Thermoanaerobacter ethanolicus 39E secondary-alcohol dehydrogenase and biochemical characterization of the enzyme

The adhB gene encoding Thermoanaerobacter ethanolicus 39E secondary-alcohol dehydrogenase (S-ADH) was cloned, sequenced and expressed in Escherichia coli. The 1056 bp gene encodes a homotetrameric recombinant enzyme consisting of 37.7 kDa subunits. The purified recombinant enzyme is optimally active above 90 degrees C with a half-life of approx. 1.7 h at 90 degrees C. An NADP(H)-dependent enzyme, the recombinant S-ADH has 1400-fold greater catalytic efficiency in propan-2-ol oxidation than in ethanol oxidation. The enzyme was inactivated by chemical modification with dithionitrobenzoate (DTNB) and diethylpyrocarbonate, indicating that Cys and His residues are involved in catalysis. Zinc was the only metal enhancing S-ADH reactivation after DTNB modification, implicating the involvement of bound zinc in catalysis. Arrhenius plots for the oxidation of propan-2-ol by the native and recombinant S-ADHs were linear from 25 to 90 degrees C when the enzymes were incubated at 55 degrees C before assay. Discontinuities in the Arrhenius plots for propan-2-ol and ethanol oxidations were observed, however, when the enzymes were preincubated at 0 or 25 degrees C. The observed Arrhenius discontinuity therefore resulted from a temperature-dependent, catalytically significant S-ADH structural change. Hydrophobic cluster analysis comparisons of both mesophilic and thermophilic S-ADH and primary- versus S-ADH amino acid sequences were performed. These comparisons predicted that specific proline residues might contribute to S-ADH thermostability and thermophilicity, and that the catalytic Zn ligands are different in primary-alcohol dehydrogenases (two Cys and a His) and S-ADHs (Cys, His, and Asp).

Purification and characterization of the alcohol dehydrogenase from a novel strain of Bacillus stearothermophilus growing at 70 degrees C

The biocatalysts isolated from thermophilic microorganisms are the object of ever-growing scientific interest for (i) the comprehension of the molecular basis of their thermal tolerance, and (ii) their use in different bio-industrial fields. Here we report the purification and characterization of an alcohol dehydrogenase (designated ADH-hT) from the novel strain LLD-R of Bacillus stearothermophilus which grows at 70 degrees C. ADH-hT was obtained in pure form by anion exchange chromatography and two affinity chromatographies, with a final yield of about 30%. ADH-hT was found to be a tetramer of 37 kDa-subunits, and to have a pI of 4.9. ADH-hT displayed a broad substrate specificity; its activity was highest for aldehydes, and decreased progressively for alcohols and ketones. ADH-hT was endowed with catalytic activity and resistance in the presence of several denaturing agents (organic solvents, detergents, chaotropic agents). ADH-hT shared with ADH 1503 (the alcohol dehydrogenase from B. stearothermophilus strain NCA 1503 which grows at 55 degrees C) the optimal temperature of 65 degrees C, but it was more resistant than ADH 1503 towards heating. In conclusion, due to its stability and broad substrate specificity ADH-hT could be utilized in bio-industrial processes. Furthermore, we believe that ADH-hT could represent a good model system for studying the mechanism(s) which proteins exploit to gain heat resistance.