Factors Mediating Activity, Selectivity, and Substrate Specificity for the Thiamin Diphosphate-Dependent Enzymes Benzaldehyde Lyase and Benzoylformate Decarboxylase

Enhanced chaotrope tolerance and (S)-2-hydroxypropiophenone production by recombinant Pseudomonas putida engineered with Pprl from Deinococcus radiodurans

Pseudomonas putida is a soil bacterium with multiple uses in fermentation and biotransformation processes. P. putida ATCC 12633 can biotransform benzaldehyde and other aldehydes into valuable α-hydroxyketones, such as (S)-2-hydroxypropiophenone. However, poor tolerance of this strain toward chaotropic aldehydes hampers efficient biotransformation processes. To circumvent this problem, we expressed the gene encoding the global regulator PprI from Deinococcus radiodurans, an inducer of pleiotropic proteins promoting DNA repair, in P. putida. Fine-tuned gene expression was achieved using an expression plasmid under the control of the LacIQ /Ptrc system, and the cross-protective role of PprI was assessed against multiple stress treatments. Moreover, the stress-tolerant P. putida strain was tested for 2-hydroxypropiophenone production using whole resting cells in the presence of relevant aldehyde substrates. P. putida cells harbouring the global transcriptional regulator exhibited high tolerance toward benzaldehyde, acetaldehyde, ethanol, butanol, NaCl, H2 O2 and thermal stress, thereby reflecting the multistress protection profile conferred by PprI. Additionally, the engineered cells converted aldehydes to 2-hydroxypropiophenone more efficiently than the parental P. putida strain. 2-Hydroxypropiophenone concentration reached 1.6 g L-1 upon a 3-h incubation under optimized conditions, at a cell concentration of 0.033 g wet cell weight mL-1 in the presence of 20 mM benzaldehyde and 600 mM acetaldehyde. Product yield and productivity were 0.74 g 2-HPP g-1 benzaldehyde and 0.089 g 2-HPP g cell dry weight-1  h-1 , respectively, 35% higher than the control experiments. Taken together, these results demonstrate that introducing PprI from D. radiodurans enhances chaotrope tolerance and 2-HPP production in P. putida ATCC 12633.

Optimized enantioselective (S)-2-hydroxypropiophenone synthesis by free- and encapsulated-resting cells of Pseudomonas putida

BACKGROUND

Aromatic α-hydroxy ketones, such as S-2-hydroxypropiophenone (2-HPP), are highly valuable chiral building blocks useful for the synthesis of various pharmaceuticals and natural products. In the present study, enantioselective synthesis of 2-HPP was investigated by free and immobilized whole cells of Pseudomonas putida ATCC 12633 starting from readily-available aldehyde substrates. Whole resting cells of P. putida, previously grown in a culture medium containing ammonium mandelate, are a source of native benzoylformate decarboxylase (BFD) activity. BFD produced by induced P. putida resting cells is a highly active biocatalyst without any further treatment in comparison with partially purified enzyme preparations. These cells can convert benzaldehyde and acetaldehyde into the acyloin compound 2-HPP by BFD-catalyzed enantioselective cross-coupling reaction.

RESULTS

The reaction was carried out in the presence of exogenous benzaldehyde (20 mM) and acetaldehyde (600 mM) as substrates in 6 mL of 200 mM phosphate buffer (pH 7) for 3 h. The optimal biomass concentration was assessed to be 0.006 g dry cell weight (DCW) mL^- 1. 2-HPP titer, yield and productivity using the free cells were 1.2 g L^- 1, 0.56 g 2-HPP/g benzaldehyde (0.4 mol 2-HPP/mol benzaldehyde), 0.067 g 2-HPP g^- 1 DCW h^- 1, respectively, under optimized biotransformation conditions (30 °C, 200 rpm). Calcium alginate (CA)-polyvinyl alcohol (PVA)-boric acid (BA)-beads were used for cell entrapment. Encapsulated whole-cells were successfully employed in four consecutive cycles for 2-HPP production under aerobic conditions without any noticeable beads degradation. Moreover, there was no production of benzyl alcohol as an unwanted by-product.

CONCLUSIONS

Bioconversion by whole P. putida resting cells is an efficient strategy for the production of 2-HPP and other α-hydroxyketones.

Bacterial mandelic acid degradation pathway and its application in biotechnology

Mandelic acid and its derivatives are an important class of chemical synthetic blocks, which is widely used in drug synthesis and stereochemistry research. In nature, mandelic acid degradation pathway has been widely identified and analyzed as a representative pathway of aromatic compounds degradation. The most studied mandelic acid degradation pathway from Pseudomonas putida consists of mandelate racemase, S-mandelate dehydrogenase, benzoylformate decarboxylase, benzaldehyde dehydrogenase and downstream benzoic acid degradation pathways. Because of the ability to catalyze various reactions of aromatic substrates, pathway enzymes have been widely used in biocatalysis, kinetic resolution, chiral compounds synthesis or construction of new metabolic pathways. In this paper, the physiological significance and the existing range of the mandelic acid degradation pathway were introduced first. Then each of the enzymes in the pathway is reviewed one by one, including the researches on enzymatic properties and the applications in biotechnology as well as efforts that have been made to modify the substrate specificity or improving catalytic activity by enzyme engineering to adapt different applications. The composition of the important metabolic pathway of bacterial mandelic acid degradation pathway as well as the researches and applications of pathway enzymes is summarized in this review for the first time.

Modeling-Assisted Design of Thermostable Benzaldehyde Lyases from Rhodococcus erythropolis for Continuous Production of α-Hydroxy Ketones

Enantiopure α-hydroxy ketones are important building blocks of active pharmaceutical ingredients (APIs), which can be produced by thiamine-diphosphate-dependent lyases, such as benzaldehyde lyase. Here we report the discovery of a novel thermostable benzaldehyde lyase from Rhodococcus erythropolis R138 (ReBAL). While the overall sequence identity to the only experimentally confirmed benzaldehyde lyase from Pseudomonas fluorescens Biovar I (PfBAL) was only 65 %, comparison of a structural model of ReBAL with the crystal structure of PfBAL revealed only four divergent amino acids in the substrate binding cavity. Based on rational design, we generated two ReBAL variants, which were characterized along with the wild-type enzyme in terms of their substrate spectrum, thermostability and biocatalytic performance in the presence of different co-solvents. We found that the new enzyme variants have a significantly higher thermostability (up to 22 °C increase in T₅₀ ) and a different co-solvent-dependent activity. Using the most stable variant immobilized in packed-bed reactors via the SpyCatcher/SpyTag system, (R)-benzoin was synthesized from benzaldehyde over a period of seven days with a stable space-time-yield of 9.3 mmol ⋅ L^-1  ⋅ d^-1 . Our work expands the important class of benzaldehyde lyases and therefore contributes to the development of continuous biocatalytic processes for the production of α-hydroxy ketones and APIs.

Synthesis of α-hydroxy ketones and vicinal diols with the Bacillus licheniformis DSM 13T butane-2,3-diol dehydrogenase

The enantioselective synthesis of α-hydroxy ketones and vicinal diols is an intriguing field because of the broad applicability of these molecules. Although, butandiol dehydrogenases are known to play a key role in the production of 2,3-butandiol, their potential as biocatalysts is still not well studied. Here, we investigate the biocatalytic properties of the meso-butanediol dehydrogenase from Bacillus licheniformis DSM 13^T (BlBDH). The encoding gene was cloned with an N-terminal StrepII-tag and recombinantly overexpressed in E. coli. BlBDH is highly active towards several non-physiological diketones and α-hydroxyketones with varying aliphatic chain lengths or even containing phenyl moieties. By adjusting the reaction parameters in biotransformations the formation of either the α-hydroxyketone intermediate or the diol can be controlled.

Synthesis of α-hydroxy ketones and vicinal (R,R)-diols by Bacillus clausii DSM 8716T butanediol dehydrogenase

α-hydroxy ketones (HK) and 1,2-diols are important building blocks for fine chemical synthesis. Here, we describe the R-selective 2,3-butanediol dehydrogenase from B. clausii DSM 8716^T (BcBDH) that belongs to the metal-dependent medium chain dehydrogenases/reductases family (MDR) and catalyzes the selective asymmetric reduction of prochiral 1,2-diketones to the corresponding HK and, in some cases, the reduction of the same to the corresponding 1,2-diols. Aliphatic diketones, like 2,3-pentanedione, 2,3-hexanedione, 5-methyl-2,3-hexanedione, 3,4-hexanedione and 2,3-heptanedione are well transformed. In addition, surprisingly alkyl phenyl dicarbonyls, like 2-hydroxy-1-phenylpropan-1-one and phenylglyoxal are accepted, whereas their derivatives with two phenyl groups are not substrates. Supplementation of Mn²⁺ (1 mM) increases BcBDH's activity in biotransformations. Furthermore, the biocatalytic reduction of 5-methyl-2,3-hexanedione to mainly 5-methyl-3-hydroxy-2-hexanone with only small amounts of 5-methyl-2-hydroxy-3-hexanone within an enzyme membrane reactor is demonstrated.

Reduction of symmetric or asymmetric vicinal diketones with BcBDH leads to the synthesis of either α-hydroxyketones or vicinal diols.

Navigating within thiamine diphosphate-dependent decarboxylases: Sequences, structures, functional positions, and binding sites

Thiamine diphosphate-dependent decarboxylases catalyze both cleavage and formation of CC bonds in various reactions, which have been assigned to different homologous sequence families. This work compares 53 ThDP-dependent decarboxylases with known crystal structures. Both sequence and structural information were analyzed synergistically and data were analyzed for global and local properties by means of statistical approaches (principle component analysis and principal coordinate analysis) enabling complexity reduction. The different results obtained both locally and globally, that is, individual positions compared with the overall protein sequence or structure, revealed challenges in the assignment of separated homologous families. The methods applied herein support the comparison of enzyme families and the identification of functionally relevant positions. The findings for the family of ThDP-dependent decarboxylases underline that global sequence identity alone is not sufficient to distinguish enzyme function. Instead, local sequence similarity, defined by comparisons of structurally equivalent positions, allows for a better navigation within several groups of homologous enzymes. The differentiation between homologous sequences is further enhanced by taking structural information into account, such as BioGPS analysis of the active site properties or pairwise structural superimpositions. The methods applied herein are expected to be transferrable to other enzyme families, to facilitate family assignments for homologous protein sequences.

Intramolecular Benzoin Reaction Catalyzed by Benzaldehyde Lyase from Pseudomonas Fluorescens Biovar I

Intramolecular benzoin reactions catalyzed by benzaldehyde lyase from Pseudomonas fluorescens biovar I (BAL) are reported. The structure of the substrates envisaged for this reaction consists of two benzaldehyde derivatives linked by an alkyl chain. The structural requirements needed to achieve the intramolecular carbon-carbon bond reaction catalyzed by BAL were established. Thus, a linker consisting of a linear alkyl chain of three carbon atoms connected through ether-type bonds to the 2 and 2' positions of two benzaldehyde moieties, which could be substituted with either Cl, Br, or OCH₃ at either the 3 and 3' or 5 and 5' positions, were suitable substrates for BAL. Reactions with 61-84 % yields of the intramolecular product and ee values between 64 and 98 %, were achieved.

Reversibility of asymmetric catalyzed C–C bond formation by benzoylformate decarboxylase

Benzoylformate decarboxylase (BFD) fromPseudomonas putidacatalyzed the formation of 2-hydroxy-1-phenylpropanone (2-HPP), a 2-hydroxy ketone, from the kinetic resolution ofrac-benzoin in the presence of acetaldehyde.

A standard numbering scheme for thiamine diphosphate-dependent decarboxylases

Background

Standard numbering schemes for families of homologous proteins allow for the unambiguous identification of functionally and structurally relevant residues, to communicate results on mutations, and to systematically analyse sequence-function relationships in protein families. Standard numbering schemes have been successfully implemented for several protein families, including lactamases and antibodies, whereas a numbering scheme for the structural family of thiamine-diphosphate (ThDP) -dependent decarboxylases, a large subfamily of the class of ThDP-dependent enzymes encompassing pyruvate-, benzoylformate-, 2-oxo acid-, indolpyruvate- and phenylpyruvate decarboxylases, benzaldehyde lyase, acetohydroxyacid synthases and 2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexadiene-1-carboxylate synthase (MenD) is still missing.

Despite a high structural similarity between the members of the ThDP-dependent decarboxylases, their sequences are diverse and make a pairwise sequence comparison of protein family members difficult.

Results

We developed and validated a standard numbering scheme for the family of ThDP-dependent decarboxylases. A profile hidden Markov model (HMM) was created using a set of representative sequences from the family of ThDP-dependent decarboxylases. The pyruvate decarboxylase from S. cerevisiae (PDB: 2VK8) was chosen as a reference because it is a well characterized enzyme. The crystal structure with the PDB identifier 2VK8 encompasses the structure of the ScPDC mutant E477Q, the cofactors ThDP and Mg²⁺ as well as the substrate analogue (2S)-2-hydroxypropanoic acid. The absolute numbering of this reference sequence was transferred to all members of the ThDP-dependent decarboxylase protein family. Subsequently, the numbering scheme was integrated into the already established Thiamine-diphosphate dependent Enzyme Engineering Database (TEED) and was used to systematically analyze functionally and structurally relevant positions in the superfamily of ThDP-dependent decarboxylases.

Conclusions

The numbering scheme serves as a tool for the reliable sequence alignment of ThDP-dependent decarboxylases and the unambiguous identification and communication of corresponding positions. Thus, it is the basis for the systematic and automated analysis of sequence-encoded properties such as structural and functional relevance of amino acid positions, because the analysis of conserved positions, the identification of correlated mutations and the determination of subfamily specific amino acid distributions depend on reliable multisequence alignments and the unambiguous identification of the alignment columns. The method is reliable and robust and can easily be adapted to further protein families.

Influence of Organic Solvents on Enzymatic Asymmetric Carboligations

The asymmetric mixed carboligation of aldehydes with thiamine diphosphate (ThDP)-dependent enzymes is an excellent example where activity as well as changes in chemo- and stereoselectivity can be followed sensitively. To elucidate the influence of organic additives in enzymatic carboligation reactions of mixed 2-hydroxy ketones, we present a comparative study of six ThDP-dependent enzymes in 13 water-miscible organic solvents under equivalent reaction conditions. The influence of the additives on the stereoselectivity is most pronounced and follows a general trend. If the enzyme stereoselectivity in aqueous buffer is already >99.9% ee, none of the solvents reduces this high selectivity. In contrast, both stereoselectivity and chemoselectivity are strongly influenced if the enzyme is rather unselective in aqueous buffer. For the S-selective enzyme with the largest active site, we were able to prove a general correlation of the solvent-excluded volume of the additives with the effect on selectivity changes: the smaller the organic solvent molecule, the higher the impact of this additive. Further, a correlation to log P of the additives on selectivity was detected if two additives have almost the same solvent-excluded volume. The observed results are discussed in terms of structural, biochemical and energetic effects. This work demonstrates the potential of medium engineering as a powerful additional tool for varying enzyme selectivity and thus engineering the product range of biotransformations. It further demonstrates that the use of cosolvents should be carefully planned, as the solvents may compete with the substrate(s) for binding sites in the enzyme active site.

Substrate specificity in thiamin diphosphate-dependent decarboxylases

Thiamin diphosphate (ThDP) is the biologically active form of vitamin B(1), and ThDP-dependent enzymes are found in all forms of life. The catalytic mechanism of this family requires the formation of a common intermediate, the 2α-carbanion-enamine, regardless of whether the enzyme is involved in C-C bond formation or breakdown, or even formation of C-N, C-O and C-S bonds. This demands that the enzymes must screen substrates prior to, and/or after, formation of the common intermediate. This review is focused on the group for which the second step is the protonation of the 2α-carbanion, i.e., the ThDP-dependent decarboxylases. Based on kinetic data, sequence/structure alignments and mutagenesis studies the factors involved in substrate specificity have been identified.

Immobilization and characterization of benzoylformate decarboxylase from Pseudomonas putida on spherical silica carrier

If an adequate biocatalyst is identified for a specific reaction, immobilization is one possibility to further improve its properties. The immobilization allows easy recycling, improves the enzyme performance, and it often enhances the stability of the enzyme. In this work, the immobilization of the benzoylformate decarboxylase (BFD) variant, BFD A460I-F464I, from Pseudomonas putida was accomplished on spherical silica. Silicagel is characterized by its high mechanical stability, which allows its application in different reactor types without restrictions. The covalently bound enzyme was characterized in terms of its activity, stability, and kinetics for the formation of chiral 2-hydroxypropiophenone (2-HPP) from benzaldehyde and acetaldehyde. Moreover, temperature as well as pressure dependency of immobilized BFD A460I-F464I activity and enantioselectivity were analyzed. The used wide-pore silicagel shows a good accessibility of the immobilized enzyme. The activity of the immobilized BFD A460I-F464I variant was determined to be 70% related to the activity of the free enzyme. Thereby, the enantioselectivity of the enzyme was not influenced by the immobilization. In addition, a pressure-induced change in stereoselectivity was found both for the free and for the immobilized enzyme. With increasing pressure, the enantiomeric excess (ee) of (R)-2-HPP can be increased from 44% (0.1 MPa) to 76% (200 MPa) for the free enzyme and from 43% (0.1 MPa) to 66% (200 MPa) for the immobilized enzyme.

Influence of the hydrostatic pressure and pH on the asymmetric 2-hydroxyketone formation catalyzed by Pseudomonas putida benzoylformate decarboxylase and variants thereof

Benzoylformate decarboxylase (BFD) from Pseudomonas putida is a thiamine diphosphate-dependent (ThDP) enzyme that catalyzes the asymmetric C--C bond formation to (S)-2-hydroxypropiophenone [(S)-HPP] starting from benzaldehyde and acetaldehyde. The enantioselectivity of BFD was shown to be a function of temperature and substrate concentration. It can additionally be changed by site-directed mutagenesis on hot spot positions in the active site. In this article, we present the effect of hydrostatic pressure up to 250 MPa on the enantioselectivity for the recombinant wtBFD as well as for the variants BFD F464I, BFD A460I, and BFD A460I-F464I. A general tendency toward lower amounts of (S)-HPP could be observed at increasing pressures. For two of these variants an increase in pressure even caused an inversion in the enantioselectivity and thus increasing enantiomeric excesses, respectively. A pressure-induced increase in enantioselectivity could therefore be observed for the first time in biocatalysis to the best of our knowledge. Furthermore, the pH is shown to be a parameter that also significantly influences the enantioselectivity of the reaction mentioned above.

Biocatalytic strategies for the asymmetric synthesis of alpha-hydroxy ketones

The development of efficient syntheses for enantiomerically enriched alpha-hydroxy ketones is an important research focus in the pharmaceutical industry. For example, alpha-hydroxy ketones are found in antidepressants, in selective inhibitors of amyloid-beta protein production (used in the treatment of Alzheimer's), in farnesyl transferase inhibitors (Kurasoin A and B), and in antitumor antibiotics (Olivomycin A and Chromomycin A3). Moreover, alpha-hydroxy ketones are of particular value as fine chemicals because of their utility as building blocks for the production of larger molecules. They can also be used in preparing many other important structures, such as amino alcohols, diols, and so forth. Several purely chemical synthetic approaches have been proposed to afford these compounds, together with some organocatalytic strategies (thiazolium-based carboligations, proline alpha-hydroxylations, and so forth). However, many of these chemical approaches are not straightforward, lack selectivity, or are economically unattractive because of the large number of chemical steps required (usually combined with low enantioselectivities). In this Account, we describe three different biocatalytic approaches that have been developed to efficiently produce alpha-hydroxy ketones: (i) The use of thiamine diphosphate-dependent lyases (ThDP-lyases) to catalyze the umpolung carboligation of aldehydes. Enantiopure alpha-hydroxy ketones are formed from inexpensive aldehydes with this method. Some lyases with a broad substrate spectrum have been successfully characterized. Furthermore, the use of biphasic media with recombinant whole cells overexpressing lyases leads to productivities of approximately 80-100 g/L with high enantiomeric excesses (up to >99%). (ii) The use of hydrolases to produce alpha-hydroxy ketones by means of (in situ) dynamic kinetic resolutions (DKRs). Lipases are able to successfully resolve racemates, and many outstanding examples have been reported. However, this approach leads to a maximum theoretical yield of 50%. As a means of overcoming this problem, these traditional lipase-catalyzed kinetic resolutions are combined with racemization of remnant substrate, which can be done in situ or in separate compartments. Examples showing high conversions (>90%) and enantiomeric excesses (>99%) are described. (iii) Whole-cell redox processes, catalyzed by several microorganisms, either by means of free enzymes (applying a cofactor regeneration system) or by whole cells. Through the use of redox machineries, different strategies can lead to high yields and enantiomeric excesses. Some enantiopure alpha-hydroxy ketones can be formed by reductions of diketones and by selective oxidations of vicinal diols. Likewise, some redox processes involving sugar chemistry (involving alpha-hydroxy ketones) have been developed on the industrial scale. Finally, the redox whole-cell concept allows racemizations (and deracemizations) as well. These three strategies provide a useful and environmentally friendly synthetic toolbox. Likewise, the field represents an illustrative example of how biocatalysis can assist practical synthetic processes, and how problems derived from the integration of natural tools in synthetic pathways can be efficiently tackled to afford high yields and enantioselectivities.

Understanding the mode of action of ThDP in benzoylformate decarboxylase

The mechanism of all elementary steps involved in the catalytic cycle of benzoylformate decarboxylase (BFD, E.C. 4.1.1.7) to generate the acyloin linkage is investigated by extensive molecular dynamics simulations. Models involving different charge states of amino acids and/or mutants of critical residues were constructed to understand the involvement of the catalytically active residues and the reactivity differences between different substrates in this reaction. Our calculations confirm that H70, S26, and H281 are catalytically active amino acids. H281 functions as a base to accept H(donor) in the first nucleophilic attack and as an acid in the second, to donate the proton back to O(acceptor). S26 assists H281 in deprotonation of the donor aldehyde and protonation of the acceptor aldehyde. In both the first and second nucleophilic attacks, H70 interacts with O(aldehyde) and aligns it toward the nucleophilic center. H70 has been found to have an electrostatic effect on the approaching aldehyde whose absence would block the initiation of the reaction. The reactivity difference between benzaldehyde (BA) and acetaldehyde (AA) is mainly explained by the steric interactions of the acceptor aldehyde with the surrounding amino acids in the active center of the enzyme.

Snapshot of a reaction intermediate: analysis of benzoylformate decarboxylase in complex with a benzoylphosphonate inhibitor

Benzoylformate decarboxylase (BFDC) is a thiamin diphosphate- (ThDP-) dependent enzyme acting on aromatic substrates. In addition to its metabolic role in the mandelate pathway, BFDC shows broad substrate specificity coupled with tight stereo control in the carbon-carbon bond-forming reverse reaction, making it a useful biocatalyst for the production of chiral alpha-hydroxy ketones. The reaction of methyl benzoylphosphonate (MBP), an analogue of the natural substrate benzoylformate, with BFDC results in the formation of a stable analogue (C2alpha-phosphonomandelyl-ThDP) of the covalent ThDP-substrate adduct C2alpha-mandelyl-ThDP. Formation of the stable adduct is confirmed both by formation of a circular dichroism band characteristic of the 1',4'-iminopyrimidine tautomeric form of ThDP (commonly observed when ThDP forms tetrahedral complexes with its substrates) and by high-resolution mass spectrometry of the reaction mixture. In addition, the structure of BFDC with the MBP inhibitor was solved by X-ray crystallography to a spatial resolution of 1.37 A (PDB ID 3FSJ). The electron density clearly shows formation of a tetrahedral adduct between the C2 atom of ThDP and the carbonyl carbon atom of the MBP. This adduct resembles the intermediate from the penultimate step of the carboligation reaction between benzaldehyde and acetaldehyde. The combination of real-time kinetic information via stopped-flow circular dichroism with steady-state data from equilibrium circular dichroism measurements and X-ray crystallography reveals details of the first step of the reaction catalyzed by BFDC. The MBP-ThDP adduct on BFDC is compared to the recently solved structure of the same adduct on benzaldehyde lyase, another ThDP-dependent enzyme capable of catalyzing aldehyde condensation with high stereospecificity.

Propioin synthesis using thiamine diphosphate-dependent enzymes

Benzaldehyde lyase (BAL, EC 4.1.2.38) from Pseudomonas fluorescens and benzoylformate decarboxylase (BFD, EC 4.1.1.7) from Pseudomonas putida are thiamine diphosphate-dependent enzymes. These enzymes share a common tetrameric structure and catalyze various C--C-bond forming and breaking reactions. Here we describe a detailed study of the asymmetric synthesis of propioin from propanal catalyzed by BAL or BFD in aqueous solution in a batch reactor. Both enzymes are deactivated in the presence of high concentration of propanal. Compared to BAL, BFD is more stable under reaction conditions as well as during storage. The kinetic studies showed a typical Michaelis-Menten kinetic for BAL with a maximal specific reaction rate of 26.2 U/mg and an unusually high K(M) of 415 mM, whereas the v/[S]-plot for BFD is almost linear in the concentration range (100-1500 mM) investigated. Both enzymes produce propioin with opposite enantiomeric excess: BAL produced the (S)-propioin (ee of 35%), whereas BFD yielded the (R)-enantiomer (ee of 67%).