Dehydrogenases for the synthesis of chiral compounds

NAD(+)-dependent D-2-hydroxyisocaproate dehydrogenase of Lactobacillus delbrueckii subsp. bulgaricus. Gene cloning and enzyme characterization

A genomic library from Lactobacillus delbrueckii subsp. bulgaricus was used to complement an Escherichia coli mutant strain deficient for both lactate dehydrogenase and pyruvate formate lyase, and thus unable to grow anaerobically. One recombinant clone was found to display a broad specificity NAD(+)-dependent D-2-hydroxyacid dehydrogenase activity. The corresponding gene (named hdhD) was subcloned and sequenced. The deduced amino acid sequence of the encoded enzyme indicates a 333-residue protein closely related to D-2-hydroxyisocaproate (i.e. 2-hydroxy-4-methyl-pentanoate) dehydrogenase (D-HO-HxoDH) of Lactobacillus casei and other NAD(+)-dependent D-lactate dehydrogenases (D-LDH) from several other bacterial species. The hdhD gene was overexpressed under the control of the lambda phage PL promoter and the enzyme was purified with a two-step method. The L. delbrueckii subsp. bulgaricus enzyme, like that of L. casei, was shown to be active on a wide variety of 2-oxoacid substrates except those having a branched beta-carbon.

Phosphoenolpyruvate as a dual purpose reagent for integrated nucleotide/nicotinamide cofactor recycling

An efficient technique is presented which integrates cofactor dependent enzymic phosphorylation and dehydrogenation into a single, closed-loop system by employing phosphoenolpyruvate as the sacrificial reagent for sequential ATP and NAD+ recycling steps. Exemplary applications are developed for the synthesis of 6-phosphogluconate from glucose, and that of dihydroxyacetone phosphate from glycerol. The latter system is combined with exergonic diastereoselective aldol additions for the one-flask synthesis of a ketosugar (D-sorbose), thiosugar (L-threo-5-thiopentulose), or a sugar acid (L-threo-pent-4-ulosonic acid) starting from a mixture of glycerol and simple aldehydes.

A kinetic study and application of a novel carbonyl reductase isolated from Rhodococcus erythropolis

The newly described carbonyl reductase from Rhodococcus erythropolis (RECR) accepts a broad range of substrates. Based on the kinetic constants of a variety of methyl and ethyl ketones a hypothetical model of the substrate-binding site is proposed. Whether a substrate of interest may be reduced by the RECR can be predicted from this model together with the kinetic data. A study of initial velocities and product inhibition is presented, which shows that the kinetics of the RECR follow a Theorell-Chance mechanism. The pro-R hydride of NADH is transferred by the enzyme to the re face of the carbonyl compounds yielding (S)-alcohols. The reduction of methyl 3-oxobutanoate and ethyl 4-chloro-3-oxobutanoate catalyzed by the oxidoreductase lead to the corresponding hydroxy compounds with high enantiomeric purity [enantiomeric excess (e.e.) > or = 99%]. The synthesis of ethyl (2R,3S)-3-hydroxy-2-methylbutanoate was accomplished with high diastereoselectivity (diastereomeric excess = 95%) and enantioselectivity (e.e. > or = 95%).

Proteus mirabilis dehydrogenates aldonates and aldarates with an (R)-configured alpha-carbon atom to the corresponding 2-oxocarboxylates

Resting cells of Proteus mirabilis effectively dehydrogenate aldonates and aldarates to the corresponding 2-oxocarboxylates (Figure 2). The prerequisite is an (R)-configured alpha-carbon atom next to the carboxylate group. The oxidation reagent is dimethylsulfoxide and the electron mediator is anthraquinone-2,6-disulfonate (Figure 1). The reactions mostly proceed quantitatively in concentrations up to 0.5 M. The two enzymes necessary for the dehydrogenation, (2R)-hydroxycarboxylate viologen oxidoreductase and dimethylsulfoxide reductase, are present in P. mirabilis in high activities. Nine aldonates have been dehydrogenated to 2-glyculosonates (2-oxoaldonates) and two aldarates to alpha-oxo aldarates. As shown with lactobionate and 6-phospho-D-gluconate, derivatives of aldonates can be dehydrogenated too. The apparent Km values of the substrates are often < 1 mM. The products were isolated as sodium or potassium salts with yields between 65 and 98% and characterized. D-xylo-Hex-2-ulosonate obtained from D-gulonate was converted to D-ascorbic acid.

Purification and characterization of a novel carbonyl reductase isolated from Rhodococcus erythropolis

During growth on n-tetradecane a novel NADH-dependent carbonyl reductase is induced in the Gram-positive bacterium Rhodococcus erythropolis (Peters, P., Zelinski, T. and Kula, M.R. (1992) Appl. Microbiol. Biotechnol. 38, 334-340). The enzyme has been purified to homogeneity using fractional pH precipitation, anion exchange chromatography and affinity chromatography. The isoelectric point of the oxidoreductase is 4.4. The apparent molecular mass of the native enzyme is 161 kDa, that of the subunits 40 kDa as determined by SDS gel electrophoresis. A tetrameric structure of the carbonyl reductase is consistent with these results. Important biochemical data concerning the application of the reductase are: a broad pH-optimum, temperature optimum at 40 degrees C and stability at room temperature for more than 5 days. The oxidoreductase accepted as substrate aliphatic and aromatic ketones, keto esters (esters of keto carboxylic acids) and halogenated carbonyl compounds and reduced them to the corresponding hydroxyl compounds with (S)-configuration with more than 98% enantiomeric excess. The NAD(+)-dependent oxidation of primary alcohols was not catalyzed by the carbonyl reductase, whereas secondary alcohols and hydroxy acid esters were oxidized to the corresponding carbonyl compounds at about 10-fold slower reaction rates compared to the reduction.

The biochemistry and enzymology of amino acid dehydrogenases

This review is an exhaustive description of the biochemistry and enzymology of all 17 known NAD(P)(+)-amino acid dehydrogenases. These enzymes catalyze the oxidative deamination of an amino acid to its keto acid and ammonia, with the concomitant reduction of either NAD+ or NADP+. These enzymes have many important applications in industrial and medical settings and have been the object of prodigious enzymological research. This article describes all that is known about the poorly characterized members of the family and contains detailed information on the better characterized enzymes, including valine, phenylalanine, leucine, alanine, and glutamate dehydrogenases. The latter three enzymes have been the subject of extensive enzymological experimentation, and, consequently, their chemical mechanisms are discussed. The three-dimensional structure of the Clostridium symbiosum glutamate dehydrogenase has been determined recently and remains the only structure known of any amino acid dehydrogenase. The three-dimensional structure and its implications to the chemical mechanisms and rate-limiting steps of the amino acid dehydrogenase family are discussed.

A novel NADH-dependent carbonyl reductase with an extremely broad substrate range from Candida parapsilosis: purification and characterization

A novel oxidoreductase catalyzing the NADH-dependent reduction of a variety of carbonyl compounds, especially keto esters, was found in Candida parapsilosis DSM 70125. The enzyme was purified by fractional poly(ethylene glycol) precipitation, anion exchange, and affinity chromatography. The enzyme was enriched about 3100-fold and appeared to be homogeneous as judged by native and sodium dodecyl sulfate gel electrophoresis. The carbonyl reductase from C. parapsilosis is a dimeric enzyme with an apparent molecular mass of about 135 kDa. Important properties concerning the application of the enzyme are the relatively broad pH optimum between pH 6.5 and 9.0, temperature optimum between 36 and 42 degrees C, and good stability. Besides keto esters, the new enzyme reduces other aliphatic, aromatic, and cyclic ketones, as well as aldehydes and ketoacetals with high reaction rates. 4-Halo-3-hydroxybutanoates, which are promising chiral intermediates for the chemical synthesis of L-carnitine, alkaloids and pharmaceuticals, are now accessible by enzymatic reduction, as well as several phenyl-ethanol derivatives, which are important for the synthesis of pharmaceuticals and agrochemicals. The preparative applicability of the enzyme was demonstrated in a coupled enzyme system with regeneration of coenzyme. Methyl 3-oxobutanoate was converted into methyl (S)-(+)-3-hydroxybutanoate (98.5% ee), a versatile chiral building block for the synthesis of pheromones and different antibiotics.

Prospects for the increased application of biocatalysts in organic transformations

Predicting the future use of biocatalysts for the transformation of organic compounds--both natural substrates and other compounds--needs to take many factors into account. To date, relatively few biotransformations have been developed to the industrial scale, primarily because there has been little economic incentive to replace existing successful processes with biocatalysts, with their inherent problems of instability, lack of selectivity and narrow operational range. However, advances that improve biocatalyst performance, coupled with the increasing emphasis on 'chirotechnology', are driving the development of biocatalysis as a complementary, if not a rival technology to existing chemical approaches.

NADP(+)-dependent D-threonine dehydrogenase from Pseudomonas cruciviae IFO 12047

NADP(+)-dependent D-threonine dehydrogenase (EC 1.1.1.-), which catalyzes the oxidation of the 3-hydroxyl group of D-threonine, was purified to homogeneity from a crude extract of Pseudomonas cruciviae IFO 12047. The enzyme had a molecular mass of about 60,000 Da and consisted of two identical subunits. In addition to D-threonine, D-threo-3-phenylserine, D-threo-3-thienylserine, and D-threo-3-hydroxynorvaline were also substrates. However, the other isomers of threonine and 3-phenylserine were inert. The enzyme showed maximal activity at pH 10.5 for the oxidation of D-threonine. The enzyme required NADP+. NAD+ showed only slight activity. The enzyme was not inhibited by EDTA, o-phenanthroline, alpha,alpha'-dipyridyl, HgCl2, or p-chloromercuribenzoate but was inhibited by tartronate, malonate, pyruvate, and DL-2-hydroxybutyrate. The inhibition by these organic acids was competitive against D-threonine. Initial-velocity and product inhibition studies suggested that the oxidation proceeded through a sequential ordered Bi Bi mechanism. The Michaelis constants for D-threonine and NADP+ were 13 and 0.12 mM, respectively.

Controlling chemical reactivity with antibodies

The remarkable specificity of an antibody molecule has been used to accomplish highly selective functional group transformations not attainable by current chemical methods. An antibody raised against an amine-oxide hapten catalyzes the reduction of a diketone to a hydroxyketone with greater than 75:1 regioselectivity for one of two nearly equivalent ketone moieties. The antibody-catalyzed reaction is highly stereoselective, affording the hydroxyketone in high enantiomeric excess. Similarly, the reduction of ketones containing branched and aryl substituents, including the highly symmetrical 1-nitrophenyl-3-phenyl-2-propanone, was enantioselective. The simple strategy presented herein may find general applicability to the regio- and stereoselective reduction of a broad range of compounds.

Enzymes in the synthesis of chiral drugs

The majority of synthetic chiral drugs are now marketed as racemates. This situation is rapidly changing due to the recent advances in asymmetric chemical synthesis and biocatalytic methods. This article reviews the use of enzymes in the synthesis and modification of optically pure drugs. Special attention is focused on the synthesis of new pharmaceuticals which may require efficient procedures for large-scale synthesis in the future.

Monitoring of enzymes during chromatographic separations

An on-line enzyme assay is presented based on flow injection techniques combined with fluorimetric detection. It allows to monitor NAD-dependent oxidoreductases during the purification of microbial crude extracts or partially purified enzymes by fast protein liquid chromatography (FPLC) in a near real-time mode. The arrangement is simple and can be easily integrated in the chromatographic system avoiding dead volumes. A high measuring frequency (up to 180 samples h-1) and a short response time (10-30 s) are achieved. The method has a low limit of detection (approximately 0.01 U ml-1), and a good reproducibility (1-4%), the injected sample volume is only 2 microliters.

New developments in the chemo-enzymatic production of amino acids

Recent progress in the chemo-enzymatic production of amino acids is reviewed. Both recently developed commercial processes and potentially important new developments are discussed. Emphasis is placed on the use of acylases, aminopeptidases and hydantoinases. The discovery of D-specific enzymes in combination with racemases is an exciting and promising new area. Also, a goal-orientated approach towards the selective generation of these novel enzyme activities using in vivo protein engineering techniques is highlighted. The interest in dipeptide sweeteners has triggered a major research effort towards the production of L-phenylalanine and D-alanine. A number of methods for the production of these amino acids is briefly discussed. Finally, chemo-enzymatic methods for the synthesis of enantiomerically pure alpha-alkyl-alpha-amino acids are reviewed.