Discovery of a novel styrene monooxygenase originating from the metagenome

Oxygenases form an interesting class of biocatalysts, as they typically perform oxygenations with exquisite chemo-, regio-, and/or enantioselectivity. It has been observed that, once heterologously expressed in Escherichia coli, some oxygenases are able to form the blue pigment indigo. We have exploited this characteristic to screen a metagenomic library derived from loam soil and identified a novel oxygenase. This oxygenase shows 50% sequence identity with styrene monooxygenases from pseudomonads (StyA). Only a limited number of homologs can be found in the genome sequence database, indicating that this biocatalyst is a member of a relatively small family of bacterial monooxygenases. The newly identified monooxygenase catalyzes the epoxidation of styrene and styrene derivatives and forms the corresponding (S)-epoxides with excellent enantiomeric excess [e.g., (S)-styrene oxide is formed with >99% enantiomeric excess, ee] and therefore is named styrene monooxgenase subunit A (SmoA). SmoA shows high enantioselectivity towards aromatic sulfides [e.g., (R)-ethyl phenyl sulfoxide is formed with 92% ee]. This excellent enantioselectivity in combination with the moderate sequence identity forms a clear indication that SmoA from a metagenomic origin represents a new enzyme within the small family of styrene monooxygenases.     

Structure-mechanism relationships in hemoproteins. Oxygenations catalyzed by chloroperoxidase and horseradish peroxidase

Chloroperoxidase and H2O2 oxidize styrene to styrene oxide and phenylacetaldehyde but not benzaldehyde. The epoxide oxygen is shown by studies with H2(18)O2 to derive quantitatively from the peroxide. The epoxidation of trans-[1-2H]styrene by chloroperoxidase proceeds without detectable loss of stereochemistry, as does the epoxidation of styrene by rat liver cytochrome P-450, although much more phenylacetaldehyde is produced by chloroperoxidase than cytochrome P-450. Chloroperoxidase and cytochrome P-450 thus oxidize styrene by closely related oxygen-transfer mechanisms. Horseradish peroxidase does not oxidize styrene but does oxidize 2,4,6-trimethylphenol to 2,6-dimethyl-4-hydroxymethylphenol. The new hydroxyl group is partially labeled in incubations with H2(18)O but not H2(18)O2. The hydroxyl group thus appears to be introduced by addition of oxygen to the benzylic radical and water to the quinone methide intermediate but not by a cytochrome P-450-like oxene transfer mechanism. The results support the thesis that substrates primarily or exclusively react with the heme edge of horseradish peroxidase but are able to react with the ferryl oxygen of chloroperoxidase.     

Toluene and ethylbenzene oxidation by purified naphthalene dioxygenase from Pseudomonas sp. strain NCIB 9816-4.

Purified naphthalene dioxygenase (NDO) from Pseudomonas sp. strain NCIB 9816-4 oxidized toluene to benzyl alcohol and benzaldehyde by reactions involving benzylic monooxygenation and dioxygen-dependent alcohol oxidation, respectively. Xylene and nitrotoluene isomers were also oxidized to substituted benzyl alcohol and benzaldehyde derivatives. NDO oxidized ethylbenzene sequentially through (S)-1-phenethyl alcohol (77% enantiomeric excess) and acetophenone to 2-hydroxyacetophenone. In addition, NDO also oxidized ethylbenzene through styrene to (R)-1-phenyl-1,2-ethanediol (74% enantiomeric excess) by reactions involving desaturation and dihydroxylation, respectively. Isotope experiments with 18O2, H2 18O, and D2O suggest that 1-phenethyl alcohol is oxidized to acetophenone by a minor reaction involving desaturation followed by tautomerization. The major reaction in the conversion of 1-phenethyl alcohol and benzyl alcohol to acetophenone and benzaldehyde, respectively, probably involves monohydroxylation to form a gem-diol intermediate which stereospecifically loses the incoming hydroxyl group to leave the carbonyl product. These results are compared with similar reactions catalyzed by cytochrome P-450.     

Novel interface-binding chloroperoxidase for interfacial epoxidation of styrene

A unique interface-binding chloroperoxidase (CPO) was developed and examined for interfacial biocatalysis. Native CPO was conjugated with polystyrene (PS) to form a surfactant-like structure that self assembled at oil-water interfaces. While enantioselectivity of the enzyme was maintained, the interfacial assembly of the enzyme improved its overall catalytic efficiency as compared to that observed with the enzyme contained in the bulk aqueous phase. The PS conjugated CPO (PS-CPO) showed a 2.5-fold enhancement of enzyme productivity versus native CPO under batch reaction conditions for the epoxidation of styrene, whereas a 25-fold improvement was realized in a continuous feeding reaction to reach a productivity of 10 micromol h-1 mg protein-1. The interface-binding enzyme also demonstrated several other advantages such as suppressing unwanted side reactions including the hydrolysis of styrene epoxide products, stabilizing the enzyme by limiting its exposure to both the oxidant H2O2 and epoxide products, and alleviating the deactivating effect of interfacial stress on enzymes by functioning as a surfactant.     

Enantioselectivity in the Rabbit Liver Microsomal Biotransformation of Styrene and Phenylpropenes

The rabbit liver microsomal P-450 catalyzed oxidation of styrene (1a) and isomeric phenylpropenes, trans-1-phenylpropene (1b), cis-1-phenylpropene (1c) and 3-phenylpropene (1d), was investigated and the enantioselectivity of the epoxidation of the olefinic double bond was determined by checking the enantiomeric excesses of the corresponding first formed epoxides (2). These enantiomeric excesses were always modest, ranging between 7% of (1S,2S)-(2b) and 22% of (1R,2R)-(2c). In the case of (1d) a nonenantioselective hydroxylation at the benzylic-allylic C(3) was also oberved. The ratio between this hydroxylation and olefin epoxidation of (Id) was 1:2.     

Expression of the Caldariomyces fumago chloroperoxidase in Aspergillus niger and characterization of the recombinant enzyme

The Caldariomyces fumago chloroperoxidase was successfully expressed in Aspergillus niger. The recombinant enzyme was produced in the culture medium as an active protein and could be purified by a three-step purification procedure. The catalytic behavior of recombinant chloroperoxidase (rCPO) was studied and compared with that of native CPO. The specific chlorination activity (47 units/nmol) of rCPO and its pH optimum (pH 2.75) were very similar to those of native CPO. rCPO catalyzes the oxidation of various substrates in comparable yields and selectivities to native CPO. Indole was oxidized to 2-oxindole with 99% selectivity and thioanisole to the corresponding R-sulfoxide (enantiomeric excess >98%). Incorporation of (18)O from labeled H(2)18O(2) into the oxidized products was 100% in both cases.     

Enantioselectivity in the Rabbit Liver Microsomal Biotransformation of Styrene and Phenylpropenes

The rabbit liver microsomal P-450 catalyzed oxidation of styrene (1a) and isomeric phenylpropenes, trans-1-phenylpropene (1b), cis-1-phenylpropene (1c) and 3-phenylpropene (1d), was investigated and the enantioselectivity of the epoxidation of the olefinic double bond was determined by checking the enantiomeric excesses of the corresponding first formed epoxides (2). These enantiomeric excesses were always modest, ranging between 7% of (1S,2S)-(2b) and 22% of (1R,2R)-(2c). In the case of (1d) a nonenantioselective hydroxylation at the benzylic-allylic C(3) was also oberved. The ratio between this hydroxylation and olefin epoxidation of (Id) was 1:2.     

Chloroperoxidase-catalyzed Epoxidation of Styrene in Aqueous and Nonaqueous Media

To overcome poor product yields and stability in aqueous solution, we have examined the chloroperoxidase (CPO from Caldariomyces fumago ) catalyzed oxidation of styrene in organic media using tert -butyl hydroperoxide as external oxidant. CPO's intrinsic catalytic activity in tert -butanol , as reflected in its k cat value, was ca. one-fourth of that in aqueous buffer, indicating that the enzyme remains highly active in the organic solvent. Styrene epoxidation reactions were modeled in both aqueous and nonaqueous media to provide global kinetic information, which dominates non-initial rate conditions and is heavily influenced by continuous deactivation of the CPO. Deactivation studies revealed that the enzyme is deactivated quickly by the combination of the tert -butyl hydroperoxide and styrene, possibly due to the styrenic free radicals generated during the enzymatic reaction. These results may enable catalyst-engineering strategies to be initiated to improve the prospects of using CPO in nonaqueous media for large-scale epoxidation reactions.     

Chloroperoxidase-catalyzed Epoxidation of Styrene in Aqueous and Nonaqueous Media

To overcome poor product yields and stability in aqueous solution, we have examined the chloroperoxidase (CPO from Caldariomyces fumago ) catalyzed oxidation of styrene in organic media using tert -butyl hydroperoxide as external oxidant. CPO's intrinsic catalytic activity in tert -butanol, as reflected in its k cat value, was ca. one-fourth of that in aqueous buffer, indicating that the enzyme remains highly active in the organic solvent. Styrene epoxidation reactions were modeled in both aqueous and nonaqueous media to provide global kinetic information, which dominates non-initial rate conditions and is heavily influenced by continuous deactivation of the CPO. Deactivation studies revealed that the enzyme is deactivated quickly by the combination of the tert -butyl hydroperoxide and styrene, possibly due to the styrenic free radicals generated during the enzymatic reaction. These results may enable catalyst-engineering strategies to be initiated to improve the prospects of using CPO in nonaqueous media for large-scale epoxidation reactions.     

One electron oxidation of benzyltrialkylsilanes catalysed by lignin peroxidase: comparison with the oxidation induced by chemical oxidants

Lignin peroxidase catalyses the H(2)O(2)-induced oxidation of 4-methoxybenzyltrimethylsilane by an electron transfer mechanism. The intermediate radical cation undergoes preferentially C(alpha)[bond]H deprotonation to give 4-methoxybenzaldehyde whereas C(alpha)[bond]Si bond cleavage is a minor fragmentation pathway and leads to 4-methoxybenzyl alcohol. Similar results are obtained in the oxidation catalysed by the water soluble model compound 5,10,15,20-tetra(N-methyl-4-pyridyl)porphyrinatoiron(III) pentachloride. Instead, in the oxidation promoted by the genuine one-electron transfer oxidant potassium dodecatungstocobalt(III)ate C(alpha)[bond]Si bond cleavage is the exclusive fragmentation process of the intermediate radical cation. It is suggested that in the enzymatic and biomimetic oxidations of 4-methoxybenzyltrimethylsilane the deprotonation of the intermediate radical cation is promoted by the reduced form [PorFe(IV)[double bond]O] of the active oxidant, which is an iron-oxo porphyrin radical cation.     

Unique reactivity of alpha,beta-unsaturated carboxylic acid imidazolides: catalytic asymmetric synthesis of alpha,beta-epoxy esters and alpha,beta-epoxy carboxylic acid derivatives

Catalytic asymmetric synthesis of alpha,beta-epoxy esters and alpha,beta-epoxy carboxylic acid derivatives is described. Catalytic asymmetric epoxidation of alpha,beta-unsaturated carboxylic acid imidazolides using La-BINOL-Ph(3)As=O complex gave the corresponding alpha,beta-epoxy peroxy tert-butyl esters, which were directly converted to the alpha,beta-epoxy methyl esters by adding methanol to the reaction. This catalytic system had broad generality for epoxidation of various substrates. With the use of 5-10 mol% of the catalyst, both beta-aryl and beta-alkyl-substituted-alpha,beta-epoxy methyl esters were obtained in up to 91% yield and in up to 93% enantiomeric excess. In addition, efficient transformations of alpha,beta-epoxy peroxy tert-butyl esters into the alpha,beta-epoxy amides, alpha,beta-epoxy aldehydes, and gamma,delta-epoxy beta-keto esters are also reported.     

pH effects on the haem iron co-ordination state in the nitric oxide and deoxy derivatives of ferrous horseradish peroxidase and cytochrome c peroxidase.

The spectral (e.p.r. and absorbance) properties of the NO and deoxy derivatives of ferrous horseradish peroxidase (HRP; EC 1.11.1.7) and baker's-yeast cytochrome c peroxidase (CCP; EC 1.11.1.5) were investigated between pH 7 and pH 2; over the same pH range the kinetics for CO binding were also determined. At neutral pH the e.p.r. and absorption spectra of the NO and deoxy derivatives of HRP and CCP are typical of systems in which the haem iron is in the hexaco-ordinated state and the pentaco-ordinated state respectively. By lowering pH, the e.p.r. and absorption spectra of HRP and CCP undergo reversible transitions, with pKa values of 4.1 for the NO derivatives and less than or equal to 3 for the deoxy derivatives of the ferrous forms. By analogy with O2-carrying proteins and haem model compounds, the pH-dependent spectral changes of HRP and CCP were interpreted as indicative of the protonation of the N(epsilon) atom of the proximal histidine residue and of the cleavage of the Fe-N(epsilon) bond. However, the slow second-order rate constant (0.003 microM-1.s-1) for CO binding to deoxy ferrous HRP and CCP does not increase substantially even at pH 2.6, suggesting that changes in the Fe-haem plane geometry, presumably associated with the cleavage of the Fe-N(epsilon) bond, do not affect appreciably the observed ligand association rate constant.     

Monooxygenase activity of cytochrome c peroxidase.

Recombinant cytochrome c peroxidase (CcP) and a W51A mutant of CcP, in contrast to other classical peroxidases, react with phenylhydrazine to give sigma-bonded phenyl-iron complexes. The conclusion that the heme iron is accessible to substrates is supported by the observation that CcP and W51A CcP oxidize thioanisole to the racemic sulfoxide with quantitative incorporation of oxygen from H2O2. Definitive evidence for an open active site is provided by stereoselective epoxidation by both enzymes of styrene, cis-beta-methylstyrene, and trans-beta-methylstyrene. trans-beta-methylstyrene yields exclusively the trans-epoxide, but styrene yields the epoxide and phenylacetaldehyde, and cis-beta-methylstyrene yields both the cis- and trans-epoxides and 1-phenyl-2-propanone. The sulfoxide, stereoretentive epoxides, and 1-phenyl-2-propanone are formed by ferryl oxygen transfer mechanisms because their oxygen atom derives from H2O2. In contrast, the oxygen in the trans-epoxide from the cis-olefin derives primarily from molecular oxygen and is probably introduced by a protein cooxidation mechanism. cis-[1,2-2H]-1-Phenyl-1-propene is oxidized to [1,1-2H]-1-phenyl-2-propanone without a detectable isotope effect on the epoxide:ketone product ratio. The phenyl-iron complex is not formed and substrate oxidation is not observed when the prosthetic group is replaced by delta-meso-ethylheme. CcP thus has a sufficiently open active site to form a phenyl-iron complex, to oxidize thioanisole to the sulfoxide, and to epoxidize styrene and beta-methylstyrene. The results indicate that a ferryl (Fe(IV) = O)/protein radical pair can be coupled to achieve two-electron oxidations. The unique ability of CcP to catalyze monooxygenation reactions does not conflict with its peroxidase function because cytochrome c is oxidized at a distinct surface site (DePillis, G. D., Sishta, B. P., Mauk, A. G., and Ortiz de Montellano, P. R. (1991) J. Biol. Chem. 266, 19334-19341).     

Stereoselectivity of lipases. I. Hydrolysis of enantiomeric glyceride analogues by gastric and pancreatic lipases, a kinetic study using the monomolecular film technique

In the present study, porcine pancreatic lipase, rabbit gastric lipase, and human gastric lipase stereospecificity toward enantiomeric glyceride derivatives was kinetically investigated using the monomolecular film technique. Pseudoglycerides such as enantiomeric 1(3)-alkyl-2,3(1,2)-diacyl-sn-glycerol, enantiomeric 1(3)-alkyl-2-acyl-sn-glycerol, or enantiomeric 1(3)-acyl-2-acylamino-2-deoxy-sn-glycerol were synthesized in order to assess the lipase stereoselectivity during the hydrolysis of either the primary or the secondary ester position of these glycerides analogues. The cleaved acyl moiety was the same in both enantiomers, thereby excluding the possibility of effects occurring due to fatty acid specificity. We observed a porcine pancreatic lipase sn-3 stereoselectivity when using the enantiomeric 1(3)-alkyl-2-acylamino-2-deoxy-sn-glycerol (diglyceride analogue) which contrasted with the lack of stereoselectivity observed when using the enantiomeric 1(3)-alkyl-2,3(1,2)-diacyl-sn-glycerol (triglyceride analogue). The gastric lipases, in contrast to the pancreatic lipase, preferentially catalyze the hydrolysis of the primary sn-3 ester bond of the enantiomeric monoakyl-diacyl pair tested. From these kinetic data, high hydrolysis rates and no chiral discrimination were observed in the case of rabbit gastric lipase, whereas low rates and a clear chiral discrimination was noticed in the case of human gastric lipase during hydrolysis of the acyl chain from the secondary ester bond of 1(3)-alkyl-2-acyl enantiomers. It is particularly obvious that in the case of human gastric lipase decreasing the lipid packing increases the lipase sn-3 stereopreference during hydrolysis of the primary ester bond of the enantiomeric 2-acylamino derivatives (diglyceride analogue).     

Identification and characterization of epoxide hydrolase activity of polycyclic aromatic hydrocarbon-degrading bacteria for biocatalytic resolution of racemic styrene oxide and styrene oxide derivatives

A novel epoxide hydrolase (EHase) from polycyclic aromatic hydrocarbon (PAH)-degrading bacteria was identified and characterized. EHase activity was identified in four strains of PAH-degrading bacteria isolated from commercial gasoline and oil-contaminated sediment based on their growth on styrene oxide and its derivatives, such as 2,3- and 4-chlorostyrene oxides, as a sole carbon source. Gordonia sp. H37 exhibited high enantioselective hydrolysis activity for 4-chlorostyrene oxide with an enantiomeric ratio of 27. Gordonia sp. H37 preferentially hydrolyzed the (R)-enantiomer of styrene oxide derivatives resulting in the preparation of a (S)-enantiomer with enantiomeric excess greater than 99.9 %. The enantioselective EHase activity was identified and characterized in various PAH-degrading bacteria, and whole cell Gordonia sp. H37 was employed as a biocatalyst for preparing enantiopure (S)-styrene oxide derivatives.     

Enzymatic and Bactericidal Activity of Monomeric and Dimeric Forms of Myeloperoxidase

Enzymatic and bactericidal activities of mature, dimeric myeloperoxidase (MPO) and its monomeric form have been compared. Dimeric MPO was isolated from HL-60 cells. Hemi-MPO obtained from dimeric MPO by reductive cleavage of a disulfide bond between protomeric subunits was used as the monomeric form. Both peroxidase and halogenating (chlorinating) activities of MPO were assayed, each by two methods. Bactericidal activity of the MPO/Н₂О₂/Cl￣ system was tested using the Escherichia coli laboratory strain DH5α. No difference in the enzymatic and bactericidal activity between dimeric MPO and hemi- MPO was found. Both forms of the enzyme also did not differ in the resistance to HOCl, the main product of MPO. HOCl caused a dose-dependent decrease in peroxidase and chlorinating activity, and the pattern of this decrease was identical for dimeric MPO and hemi-MPO. At the equal heme concentration, the hemi- MPO/Н₂О₂/Cl￣ system demonstrated a somewhat higher bactericidal effect than the dimeric MPO/Н₂О₂/Cl￣ system. This is most likely explained by higher probability of contacts between the bacterial surface and hemi-MPO molecules, since at the same heme concentration the number of hemi-MPO molecules is 2-fold higher than that of dimeric MPO molecules. Using Western-blotting with antibodies to MPO, we have shown, for the first time, that the dimeric molecule of MPO could be cleaved into two monomeric subunits by HOCl, most probably due to oxidation of the disulfide bond between these subunits. This suggests that appearance in blood of MPO with mass corresponding to its monomer may result from the damage of dimeric MPO by reactive halogen species, especially upon their overproduction inducing oxidative/halogenative stress in inflammatory diseases.     

Kinetics and mechanism of epoxidation of olefins by a novel ruthenium(IV)-oxo complex

[Ru^IV(tpy)(pic)(O)]⁺ (1) was synthesized by chemical oxidation of the corresponding aqua-complex [Ru^II(tpy)(pic)(H₂O)]⁺ (2) and characterized by analytical, spectroscopic (UV-vis and IR) and magnetic moment studies. Complex 1 effected epoxidation of styrene and substituted styrenes, cis- and trans-stilbenes and cyclohexene, in CH₃CN at room temperature. Epoxides were found to be the major product for styrenes and stilbenes, whereas, the oxidation of cyclohexene yielded allylic oxidation product. Detailed kinetic studies were performed under pseudo-first order conditions of excess alkene concentrations. A working mechanism in agreement with the rate and activation parameters is presented, and the results are discussed in reference to the data reported for the alkene oxidation by relevant Ru^IVO system in CH₃CN.     

Catalytic reaction profile for alcohol oxidation by galactose oxidase

Galactose oxidase is a remarkable enzyme containing a metalloradical redox cofactor capable of oxidizing a variety of primary alcohols during enzyme turnover. Recent studies using 1-O-methyl alpha-D-galactopyranoside have revealed an unusually large kinetic isotope effect (KIE) for oxidation of the alpha-deuterated alcohol (kH/kD = 22), demonstrating that cleavage of the 6,6'-di[2H]hydroxymethylene C-H bond is fully rate-limiting for oxidation of the canonical substrate. This step is believed to involve hydrogen atom transfer to the tyrosyl phenoxyl in a radical redox mechanism for catalysis [Whittaker, M. M., Ballou, D. P., and Whittaker, J. W. (1998) Biochemistry 37, 8426-8436]. In the work presented here, the enzyme's unusually broad substrate specificity has allowed us to extend these investigations to a homologous series of benzyl alcohol derivatives, in which remote (meta or para) substituents are used to systematically perturb the properties of the hydroxyl group undergoing oxidation. Quantitative structure-activity relationship (QSAR) correlations over the steady state rate data reveal a shift in the character of the transition state for substrate oxidation over this series, reflected in a change in the magnitude of the observed KIE for these reactions. The observed KIE values have been shown to obey a log-linear correlation over the substituent parameter, Hammett sigma. For the relatively difficult to oxidize nitro derivative, the KIE is large (kH/kD = 12.3), implying rate-limiting C-H bond cleavage for the oxidation reaction. This contribution becomes less important for more easily oxidized substrates (e.g., methoxy derivatives) where a much smaller KIE is observed (kH/kD = 3.6). Conversely, the solvent deuterium KIE is vanishingly small for 4-nitrobenzyl alcohol, but becomes significant for the 4-methoxy derivative (kH2O/kD2O = 1.2). These experiments have allowed us to develop a reaction profile for substrate oxidation by galactose oxidase, consisting of three components (hydroxylic proton transfer, electron transfer, and hydrogen atom transfer) comprising a single-step proton-coupled electron transfer mechanism. Each component exhibits a distinct substituent and isotope sensitivity, allowing them to be identified kinetically. The proton transfer component is unique in being sensitive to the isotopic character of the solvent (H2O or D2O), while hydrogen atom transfer (C-H bond cleavage) is independent of solvent composition but is sensitive to substrate labeling. In contrast, electron transfer processes will in general be less sensitive to isotopic substitution. Our results support a mechanism in which initial proton abstraction from a coordinated substrate activates the alcohol toward inner sphere electron transfer to the Cu(II) metal center in an unfavorable redox equilibrium, forming an alkoxy radical which undergoes hydrogen atom abstraction by the tyrosine-cysteine phenoxyl free radical ligand to form the product aldehyde.     

Mutations at the putative active cavity of styrene monooxygenase: Enhanced activity and reversed enantioselectivity

Styrene monooxygenase (SMO) catalyzes the first step of styrene degradation, and also serves as an important enzyme for the synthesis of enantiopure epoxides. To enhance its activity, molecular docking of styrene was performed based on the X-ray crystal structure of the oxygenase subunit of SMO to identify three amino acid residues (Tyr73, His76 and Ser96) being adjacent to the phenyl ring of styrene. Variants at those positions were constructed and their enzymatic activities were analyzed. Three mutants (Y73V, Y73F, and S96A) were found to exhibit higher enzymatic activities than the wild-type in the epoxidation of styrene, while retaining excellent stereoselectivity. The specific epoxidation activity of the most active mutant S96A toward styrene and trans-β-methyl styrene were 2.6 and 2.3-fold of the wild-type, respectively. In addition, the Y73V mutant showed an unexpected reversal of enantiomeric preference toward 1-phenylcyclohexene.     

Expression and characterization of styrene monooxygenases of Rhodococcus sp. ST-5 and ST-10 for synthesizing enantiopure (S)-epoxides

Styrene monooxygenase (StyA, SMOA)- and flavin oxidoreductase (StyB, SMOB)-coding genes of styrene-assimilating bacteria Rhodococcus sp. ST-5 and ST-10 were successfully expressed in Escherichia coli. Determined amino acid sequences of StyAs and StyBs of ST-5 and ST-10 showed more similarity with those of Pseudomonas than with self-sufficient styrene monooxygenase (StyA2B) of Rhodococcus. Recombinant enzymes were purified from E. coli cells as functional proteins, and their properties were characterized in detail. StyBs (flavin oxidoreductase) of strains ST-5 and ST-10 have similar enzymatic properties to those of Pseudomonas, but StyB of strain ST-10 exhibited higher temperature stability than that of strain ST-5. StyAs of strains ST-5 and ST-10 catalyzed the epoxidation of vinyl side-chain of styrene and its derivatives and produced (S)-epoxides from styrene derivatives and showed high stereoselectivity. Both StyAs showed higher specific activity on halogenated styrene derivatives than on styrene itself. Additionally, the enzymes could catalyze the epoxidation of short-chain 1-alkenes to the corresponding (S)-epoxides. Aromatic compounds including styrene, 3-chlorostyrene, styrene oxide, and benzene exhibited marked inhibition of SMO reaction, although linear 1-alkene showed no inhibition of SMO activity at any concentration.