StyA1 and StyA2B from Rhodococcus opacus 1CP: a multifunctional styrene monooxygenase system

An In Vitro Hybrid Biocatalytic System Enabled by a Combination of Surface-Displayed, Purified, and Cell-Free Expressed Enzymes

Enzymatic cascades have become a green and sustainable approach for the synthesis of valuable chemicals and pharmaceuticals. Using sequential enzymes to construct a multienzyme complex is an effective way to enhance the overall performance of biosynthetic routes. Here we report the design of an efficient in vitro hybrid biocatalytic system by assembling three enzymes that can convert styrene to (S)-1-phenyl-1,2-ethanediol. Specifically, we prepared the three enzymes in different ways, which were cell surface-displayed, purified, and cell-free expressed. To assemble them, we fused two orthogonal peptide-protein pairs (i.e., SpyTag/SpyCatcher and SnoopTag/SnoopCatcher) to the three enzymes, allowing their spatial organization by covalent assembly. By doing this, we constructed a multienzyme complex, which could enhance the production of (S)-1-phenyl-1,2-ethanediol by 3 times compared to the free-floating enzyme system without assembly. After optimization of the reaction system, the final product yield reached 234.6 μM with a substrate conversion rate of 46.9% (based on 0.5 mM styrene). Taken together, our strategy integrates the merits of advanced biochemical engineering techniques, including cellular surface display, spatial enzyme organization, and cell-free expression, which offers a new solution for chemical biosynthesis by enzymatic cascade biotransformation. We, therefore, anticipate that our approach will hold great potential for designing and constructing highly efficient systems to synthesize chemicals of agricultural, industrial, and pharmaceutical significance.

Structural and Mechanistic Studies on Substrate and Stereoselectivity of the Indole Monooxygenase VpIndA1: New Avenues for Biocatalytic Epoxidations and Sulfoxidations

Flavoprotein monooxygenases are a versatile group of enzymes for biocatalytic transformations. Among these, group E monooxygenases (GEMs) catalyze enantioselective epoxidation and sulfoxidation reactions. Here, we describe the crystal structure of an indole monooxygenase from the bacterium Variovorax paradoxus EPS, a GEM designated as VpIndA1. Complex structures with substrates reveal productive binding modes that, in conjunction with force-field calculations and rapid mixing kinetics, reveal the structural basis of substrate and stereoselectivity. Structure-based redesign of the substrate cavity yielded variants with new substrate selectivity (for sulfoxidation of benzyl phenyl sulfide) or with greatly enhanced stereoselectivity (from 35.1 % to 99.8 % ee for production of (1S,2R)-indene oxide). This first determination of the substrate binding mode of GEMs combined with structure-function relationships opens the door for structure-based design of these powerful biocatalysts.

Improving biocatalytic properties of an azoreductase via the N-terminal fusion of formate dehydrogenase

Azoreductases require NAD(P)H to reduce azo dyes but the costly price of NAD(P)H limits its application. Formate dehydrogenase (FDH) allows NAD(P)+ recycling and therefore, the fusion of these two biocatalysts seems promising. This study investigated the changes to the fusion protein involving azoreductase (AzoRo) of Rhodococcus opacus 1CP and FDH (FDHC23S and FDHC23SD195QY196H) of Candida boidinii in different positions with His-tag as the linker. The position affected enzyme activities as AzoRo activity decreased by 20-fold when it is in the N-terminus of the fusion protein. FDHC23S+AzoRo was the most active construct and was further characterized. Enzymatic activities of FDHC23S+AzoRo decreased compared to parental enzymes but showed improved substrate scope - accepting bulkier dyes. Moreover, pH has an influence on the stability and activity of the fusion protein because at pH 6 (pH that is suboptimal for FDH), the dye reduction decreased to more than 50% and this could be attributed to the impaired NADH supply for the AzoRo part.

Asymmetric azidohydroxylation of styrene derivatives mediated by a biomimetic styrene monooxygenase enzymatic cascade

Enantioenriched azido alcohols are precursors for valuable chiral aziridines and 1,2-amino alcohols, however their chiral substituted analogues are difficult to access. We established a cascade for the asymmetric azidohydroxylation of styrene derivatives leading to chiral substituted 1,2-azido alcohols via enzymatic asymmetric epoxidation, followed by regioselective azidolysis, affording the azido alcohols with up to two contiguous stereogenic centers. A newly isolated two-component flavoprotein styrene monooxygenase StyA proved to be highly selective for epoxidation with a nicotinamide coenzyme biomimetic as a practical reductant. Coupled with azide as a nucleophile for regioselective ring opening, this chemo-enzymatic cascade produced highly enantioenriched aromatic α-azido alcohols with up to >99% conversion. A bi-enzymatic counterpart with halohydrin dehalogenase-catalyzed azidolysis afforded the alternative β-azido alcohol isomers with up to 94% diastereomeric excess. We anticipate our biocatalytic cascade to be a starting point for more practical production of these chiral compounds with two-component flavoprotein monooxygenases.

Biochemical Characterization of Phenylacetaldehyde Dehydrogenases from Styrene-degrading Soil Bacteria

Four phenylacetaldehyde dehydrogenases (designated as FeaB or StyD) originating from styrene-degrading soil bacteria were biochemically investigated. In this study, we focused on the Michaelis-Menten kinetics towards the presumed native substrate phenylacetaldehyde and the obviously preferred co-substrate NAD+. Furthermore, the substrate specificity on four substituted phenylacetaldehydes and the co-substrate preference were studied. Moreover, these enzymes were characterized with respect to their temperature as well as long-term stability. Since aldehyde dehydrogenases are known to show often dehydrogenase as well as esterase activity, we tested this capacity, too. Almost all results showed clearly different characteristics between the FeaB and StyD enzymes. Furthermore, FeaB from Sphingopyxis fribergensis Kp5.2 turned out to be the most active enzyme with an apparent specific activity of 17.8 ± 2.1 U mg-1. Compared with that, both StyDs showed only activities less than 0.2 U mg-1 except the overwhelming esterase activity of StyD-CWB2 (1.4 ± 0.1 U mg-1). The clustering of both FeaB and StyD enzymes with respect to their characteristics could also be mirrored in the phylogenetic analysis of twelve dehydrogenases originating from different soil bacteria.

Flavoprotein monooxygenases: Versatile biocatalysts

Flavoprotein monooxygenases (FPMOs) are single- or two-component enzymes that catalyze a diverse set of chemo-, regio- and enantioselective oxyfunctionalization reactions. In this review, we describe how FPMOs have evolved from model enzymes in mechanistic flavoprotein research to biotechnologically relevant catalysts that can be applied for the sustainable production of valuable chemicals. After a historical account of the development of the FPMO field, we explain the FPMO classification system, which is primarily based on protein structural properties and electron donor specificities. We then summarize the most appealing reactions catalyzed by each group with a focus on the different types of oxygenation chemistries. Wherever relevant, we report engineering strategies that have been used to improve the robustness and applicability of FPMOs.

Divorce in the two-component BVMO family: the single oxygenase for enantioselective chemo-enzymatic Baeyer-Villiger oxidations

Two-component flavoprotein monooxygenases consist of a reductase and an oxygenase enzyme. The proof of functionality of the latter without its counterpart as well as the mechanism of flavin transfer remains unanswered beyond doubt. To tackle this question, we utilized a reductase-free reaction system applying purified 2,5-diketocamphane-monooxygenase I (2,5-DKCMO), a FMN-dependent type II Baeyer-Villiger monooxygenase, and synthetic nicotinamide analogues (NCBs) as dihydropyridine derivatives for FMN reduction. This system demonstrated the stand-alone quality of the oxygenase, as well as the mechanism of FMNH2 transport by free diffusion. The efficiency of this reductase-free system strongly relies on the balance of FMN reduction and enzymatic (re)oxidation, since reduced FMN in solution causes undesired side reactions, such as hydrogen peroxide formation. Design of experiments allowed us to (i) investigate the effect of various reaction parameters, underlining the importance to balance the FMN/FMNH2 cycle, (ii) optimize the reaction system for the enzymatic Baeyer-Villiger oxidation of rac-bicyclo[3.2.0]hept-2-en-6-one, rac-camphor, and rac-norcamphor. Finally, this study not only demonstrates the reductase-independence of 2,5-DKCMO, but also revisits the terminology of two-component flavoprotein monooxygenases for this specific case.

Asymmetric Epoxidation and Sulfoxidation Catalyzed by a New Styrene Monooxygenase from Bradyrhizobium

Asymmetric epoxidation catalyzed with styrene monooxygenase (SMO) is a powerful enzymatic process producing enantiopure styrene epoxide derivatives. To establish a more diversified reservoir of SMOs, a new SMO from Bradyrhizobium sp. ORS 375, named BrSMO, was mined from the database and characterized. BrSMO was constituted of an epoxygenase component of 415 amino acid residues and an NADH-dependent flavin reductase component of 175 residues. BrSMO catalyzed the epoxidation of styrene and 7 more styrene derivatives, yielding the corresponding (S)-epoxides with excellent enantiomeric excesses (95- > 99% ee), with the highest activity achieved for styrene. BrSMO also catalyzed the asymmetric sulfoxidation of 7 sulfides, producing the corresponding (R)-sulfoxides (20-90% ee) with good yields.

Biodiversity of Microorganisms Colonizing the Surface of Polystyrene Samples Exposed to Different Aqueous Environments

The contamination of marine and freshwater ecosystems with the items from thermoplastics, including polystyrene (PS), necessitates the search for efficient microbial degraders of these polymers. In the present study, the composition of prokaryotes in biofilms formed on PS samples incubated in seawater and the industrial water of a petrochemical plant were investigated. Using a high-throughput sequencing of the V3–V4 region of the 16S rRNA gene, the predominance of Alphaproteobacteria (Blastomonas), Bacteroidetes (Chryseolinea), and Gammaproteobacteria (Arenimonas and Pseudomonas) in the biofilms on PS samples exposed to industrial water was revealed. Alphaproteobacteria (Erythrobacter) predominated on seawater-incubated PS samples. The local degradation of the PS samples was confirmed by scanning microscopy. The PS-colonizing microbial communities in industrial water differed significantly from the PS communities in seawater. Both communities have a high potential ability to carry out the carbohydrates and amino acids metabolism, but the potential for xenobiotic degradation, including styrene degradation, was relatively higher in the biofilms in industrial water. Bacteria of the genera Erythrobacter, Maribacter, and Mycobacterium were potential styrene-degraders in seawater, and Pseudomonas and Arenimonas in industrial water. Our results suggest that marine and industrial waters contain microbial populations potentially capable of degrading PS, and these populations may be used for the isolation of efficient PS degraders.

A Minimized Chemoenzymatic Cascade for Bacterial Luciferase in Bioreporter Applications

Bacterial luciferase (Lux) catalyzes a bioluminescence reaction by using long-chain aldehyde, reduced flavin and molecular oxygen as substrates. The reaction can be applied in reporter gene systems for biomolecular detection in both prokaryotic and eukaryotic organisms. Because reduced flavin is unstable under aerobic conditions, another enzyme, flavin reductase, is needed to supply reduced flavin to the Lux-catalyzed reaction. To create a minimized cascade for Lux that would have greater ease of use, a chemoenzymatic reaction with a biomimetic nicotinamide (BNAH) was used in place of the flavin reductase reaction in the Lux system. The results showed that the minimized cascade reaction can be applied to monitor bioluminescence of the Lux reporter in eukaryotic cells effectively, and that it can achieve higher efficiencies than the system with flavin reductase. This development is useful for future applications as high-throughput detection tools for drug screening applications.

Indigoid dyes by group E monooxygenases: mechanism and biocatalysis

Since ancient times, people have been attracted by dyes and they were a symbol of power. Some of the oldest dyes are indigo and its derivative Tyrian purple, which were extracted from plants and snails, respectively. These 'indigoid dyes' were and still are used for coloration of textiles and as a food additive. Traditional Chinese medicine also knows indigoid dyes as pharmacologically active compounds and several studies support their effects. Further, they are interesting for future technologies like organic electronics. In these cases, especially the indigo derivatives are of interest but unfortunately hardly accessible by chemical synthesis. In recent decades, more and more enzymes have been discovered that are able to produce these indigoid dyes and therefore have gained attention from the scientific community. In this study, group E monooxygenases (styrene monooxygenase and indole monooxygenase) were used for the selective oxygenation of indole (derivatives). It was possible for the first time to show that the product of the enzymatic reaction is an epoxide. Further, we synthesized and extracted indigoid dyes and could show that there is only minor by-product formation (e.g. indirubin or isoindigo). Thus, group E monooxygenase can be an alternative biocatalyst for the biosynthesis of indigoid dyes.

Rhodococcus as A Versatile Biocatalyst in Organic Synthesis

The application of purified enzymes as well as whole-cell biocatalysts in synthetic organic chemistry is becoming more and more popular, and both academia and industry are keen on finding and developing novel enzymes capable of performing otherwise impossible or challenging reactions. The diverse genus Rhodococcus offers a multitude of promising enzymes, which therefore makes it one of the key bacterial hosts in many areas of research. This review focused on the broad utilization potential of the genus Rhodococcus in organic chemistry, thereby particularly highlighting the specific enzyme classes exploited and the reactions they catalyze. Additionally, close attention was paid to the substrate scope that each enzyme class covers. Overall, a comprehensive overview of the applicability of the genus Rhodococcus is provided, which puts this versatile microorganism in the spotlight of further research.

Hyperthermophilic flavin reductase from Sulfolobus solfataricus P2: Production and biochemical characterization

Nicotinamide adenine dinucleotide phosphate (NAD(P)H)-flavin oxidoreductases (flavin reductases) catalyze the reduction of flavin by NAD(P)H and provide the reduced form of flavin mononucleotide (FMN) to flavin-dependent monooxygenases. Based on bioinformatics analysis, we identified a putative flavin reductase gene, sso2055, in the genome of hyperthermophilic archaeon Sulfolobus solfataricus P2, and further cloned this target sequence into an expression vector. The cloned flavin reductase (EC. 1.5.1.30) was purified to homogeneity and characterized further. The purified enzyme exists as a monomer of 17.8 kDa, free of chromogenic cofactors. Homology modeling revealed this enzyme as a TIM barrel, which is also supported by circular dichroism measurements revealing a beta-sheet rich content. The optimal pH for SSO2055 activity was pH 6.5 in phosphate buffer and the highest activity observed was at 120 °C within the measurable temperature. We showed that this enzyme can use FMN and flavin adenine dinucleotide (FAD) as a substrate to generate their reduced forms. The purified enzyme is predicted to be a potential flavin reductase of flavin-dependent monooxygenases that could be involved in the biodesulfurization process of S. solfataricus P2.

The styrene monooxygenase system

Styrene monooxygenases are soluble two-component flavoproteins that catalyze the NADH and FAD-dependent enantioselective epoxidation of styrene to styrene oxide in the aqueous phase. These enzymes present interesting mechanistic features and potential as catalysts in biotechnological applications ranging from green chemical synthesis to bioremediation. This chapter presents approaches for the expression of the reductase (SMOB, StyB) and epoxidase (SMOA, StyA) components of SMO from pET-vectors as native or N-terminally histidine-tagged proteins in commercial strains of E. coli. The two-component structure of SMO and hydrophobic nature of styrene substrate requires some special consideration in evaluating the mechanism of this enzyme. The modular composition of the enzyme allows the flavin-reduction reaction of SMOB and styrene epoxidation reaction of SMOA to be evaluated both independently and as a composite catalytic system. The freedom to independently study the reductase and epoxidase components of SMO significantly simplifies studies of equilibrium-binding and the coupling of the free energy of ligand binding to the electrochemical potential of bound FAD. In this chapter, methods of steady-state and pre-steady-state kinetic assay, experimental approaches to equilibrium-binding reactions of flavin and substrate, and determination of the electrochemical midpoint potential of FAD bound to the reductase and epoxidase components of SMO are presented. This presentation focuses on approaches that have been successfully used in the study of the wild-type styrene monooxygenase system recovered from Pseudomonas putida (S12), but similar approaches may be effective in the characterization of related two-component enzyme systems.

Photoenzymatic epoxidation of styrenes

Photochemical reduction of flavin adenine dinucleotide (FAD) enables the direct, non-enzymatic regeneration of styrene monooxygenase for enantiospecific epoxidation reactions.

Characterised Flavin-Dependent Two-Component Monooxygenases from the CAM Plasmid of Pseudomonas putida ATCC 17453 (NCIMB 10007): ketolactonases by Another Name

The CAM plasmid-coded isoenzymic diketocamphane monooxygenases induced in Pseudomonas putida ATCC 17453 (NCIMB 10007) by growth of the bacterium on the bicyclic monoterpene (rac)-camphor are notable both for their interesting history, and their strategic importance in chemoenzymatic syntheses. Originally named 'ketolactonase-an enzyme system for cyclic lactonization' because of its characterised mode of action, (+)-camphor-induced 2,5-diketocamphane 1,2-monooxygenase was the first example of a Baeyer-Villiger monooxygenase activity to be confirmed in vitro. Both this enzyme and the enantiocomplementary (-)-camphor-induced 3,6-diketocamphane 1,6-monooxygenase were mistakenly classified and studied as coenzyme-containing flavoproteins for nearly 40 years before being correctly recognised and reinvestigated as FMN-dependent two-component monooxygenases. As has subsequently become evident, both the nature and number of flavin reductases able to supply the requisite reduced flavin co-substrate for the monooxygenases changes progressively throughout the different phases of camphor-dependent growth. Highly purified preparations of the enantiocomplementary monooxygenases have been exploited successfully for undertaking both nucleophilic and electrophilic biooxidations generating various enantiopure lactones and sulfoxides of value as chiral synthons and auxiliaries, respectively. In this review the chequered history, current functional understanding, and scope and value as biocatalysts of the diketocamphane monooxygenases are discussed.

Two-Component FAD-Dependent Monooxygenases: Current Knowledge and Biotechnological Opportunities

Flavoprotein monooxygenases create valuable compounds that are of high interest for the chemical, pharmaceutical, and agrochemical industries, among others. Monooxygenases that use flavin as cofactor are either single- or two-component systems. Here we summarize the current knowledge about two-component flavin adenine dinucleotide (FAD)-dependent monooxygenases and describe their biotechnological relevance. Two-component FAD-dependent monooxygenases catalyze hydroxylation, epoxidation, and halogenation reactions and are physiologically involved in amino acid metabolism, mineralization of aromatic compounds, and biosynthesis of secondary metabolites. The monooxygenase component of these enzymes is strictly dependent on reduced FAD, which is supplied by the reductase component. More and more representatives of two-component FAD-dependent monooxygenases have been discovered and characterized in recent years, which has resulted in the identification of novel physiological roles, functional properties, and a variety of biocatalytic opportunities.

A Chimeric Styrene Monooxygenase with Increased Efficiency in Asymmetric Biocatalytic Epoxidation

The styrene monooxygenase (SMO) system from Pseudomonas sp. consists of two enzymes (StyA and StyB). StyB catalyses the reduction of FAD at the expense of NADH. After the transfer of FADH2 from StyB to StyA, reaction with O2 generates FAD-OOH, which is the epoxidising agent. The wastage of redox equivalents due to partial diffusive transfer of FADH2 , the insolubility of recombinant StyB and the impossibility of expressing StyA and StyB in a 1:1 molar ratio reduce the catalytic efficiency of the natural system. Herein we present a chimeric SMO (Fus-SMO) that was obtained by genetic fusion of StyA and StyB through a flexible linker. Thanks to a combination of: 1) balanced and improved expression levels of reductase and epoxidase units, and 2) intrinsically higher specific epoxidation activity of Fus-SMO in some cases, Escherichia coli cells expressing Fus-SMO possess about 50 % higher activity for the epoxidation of styrene derivatives than E. coli cells coexpressing StyA and StyB as discrete enzymes. The epoxidation activity of purified Fus-SMO was up to three times higher than that of the two-component StyA/StyB (1:1, molar ratio) system and up to 110 times higher than that of the natural fused SMO. Determination of coupling efficiency and study of the influence of O2 pressure were also performed. Finally, Fus-SMO and formate dehydrogenase were coexpressed in E. coli and applied as a self-sufficient biocatalytic system for epoxidation on greater than 500 mg scale.

VpStyA1/VpStyA2B of Variovorax paradoxus EPS: An Aryl Alkyl Sulfoxidase Rather than a Styrene Epoxidizing Monooxygenase

Herein we describe the first representative of an E2-type two-component styrene monooxygenase of proteobacteria. It comprises a single epoxidase protein (VpStyA1) and a two domain protein (VpStyA2B) harboring an epoxidase (A2) and a FAD-reductase (B) domain. It was annotated as VpStyA1/VpStyA2B of Variovorax paradoxus EPS. VpStyA2B serves mainly as NADH:FAD-oxidoreductase. A Km of 33.6 ± 4.0 µM for FAD and a kcat of 22.3 ± 1.1 s-1 were determined and resulted in a catalytic efficiency (kcatKm-1) of 0.64 s-1 μM-1. To investigate its NADH:FAD-oxidoreductase function the linker between A2- and B-domain (AREAV) was mutated. One mutant (AAAAA) showed 18.7-fold higher affinity for FAD (kcatKm-1 of 5.21 s-1 μM-1) while keeping wildtype NADH-affinity and -oxidation activity. Both components, VpStyA2B and VpStyA1, showed monooxygenase activity on styrene of 0.14 U mg-1 and 0.46 U mg-1, as well as on benzyl methyl sulfide of 1.62 U mg-1 and 3.11 U mg-1, respectively. The high sulfoxidase activity was the reason to test several thioanisole-like substrates in biotransformations. VpStyA1 showed high substrate conversions (up to 95% in 2 h) and produced dominantly (S)-enantiomeric sulfoxides of all tested substrates. The AAAAA-mutant showed a 1.6-fold increased monooxygenase activity. In comparison, the GQWCSQY-mutant did neither show monooxygenase nor efficient FAD-reductase activity. Hence, the linker between the two domains of VpStyA2B has effects on the reductase as well as on the monooxygenase performance. Overall, this monooxygenase represents a promising candidate for biocatalyst development and studying natural fusion proteins.

A Review: The Styrene Metabolizing Cascade of Side-Chain Oxygenation as Biotechnological Basis to Gain Various Valuable Compounds

Styrene is one of the most produced and processed chemicals worldwide and is released into the environment during widespread processing. But, it is also produced from plants and microorganisms. The natural occurrence of styrene led to several microbiological strategies to form and also to degrade styrene. One pathway designated as side-chain oxygenation has been reported as a specific route for the styrene degradation among microorganisms. It comprises the following enzymes: styrene monooxygenase (SMO; NADH-consuming and FAD-dependent, two-component system), styrene oxide isomerase (SOI; cofactor independent, membrane-bound protein) and phenylacetaldehyde dehydrogenase (PAD; NAD+-consuming) and allows an intrinsic cofactor regeneration. This specific way harbors a high potential for biotechnological use. Based on the enzymatic steps involved in this degradation route, important reactions can be realized from a large number of substrates which gain access to different interesting precursors for further applications. Furthermore, stereochemical transformations are possible, offering chiral products at high enantiomeric excess. This review provides an actual view on the microbiological styrene degradation followed by a detailed discussion on the enzymes of the side-chain oxygenation. Furthermore, the potential of the single enzyme reactions as well as the respective multi-step syntheses using the complete enzyme cascade are discussed in order to gain styrene oxides, phenylacetaldehydes, or phenylacetic acids (e.g., ibuprofen). Altered routes combining these putative biocatalysts with other enzymes are additionally described. Thus, the substrates spectrum can be enhanced and additional products as phenylethanols or phenylethylamines are reachable. Finally, additional enzymes with similar activities toward styrene and its metabolic intermediates are shown in order to modify the cascade described above or to use these enzyme independently for biotechnological application.

N-terminus determines activity and specificity of styrene monooxygenase reductases

Styrene monooxygenases (SMOs) are two-enzyme systems that catalyze the enantioselective epoxidation of styrene to (S)-styrene oxide. The FADH₂ co-substrate of the epoxidase component (StyA) is supplied by an NADH-dependent flavin reductase (StyB). The genome of Rhodococcus opacus 1CP encodes two SMO systems. One system, which we define as E1-type, displays homology to the SMO from Pseudomonas taiwanensis VLB120. The other system, originally reported as a fused system (RoStyA2B), is defined as E2-type. Here we found that E1-type RoStyB is inhibited by FMN, while RoStyA2B is known to be active with FMN. To rationalize the observed specificity of RoStyB for FAD, we generated an artificial reductase, designated as RoStyBart, in which the first 22 amino acid residues of RoStyB were joined to the reductase part of RoStyA2B, while the oxygenase part (A2) was removed. RoStyBart mainly purified as apo-protein and mimicked RoStyB in being inhibited by FMN. Pre-incubation with FAD yielded a turnover number at 30°C of 133.9±3.5s^-1, one of the highest rates observed for StyB reductases. RoStyBart holo-enzyme switches to a ping-pong mechanism and fluorescence analysis indicated for unproductive binding of FMN to the second (co-substrate) binding site. In summary, it is shown for the first time that optimization of the N-termini of StyB reductases allows the evolution of their activity and specificity.

Engineering Styrene Monooxygenase for Biocatalysis: Reductase-Epoxidase Fusion Proteins

The enantioselective epoxidation of styrene and related compounds by two-component styrene monooxygenases (SMOs) has targeted these enzymes for development as biocatalysts. In the present work, we prepare genetically engineered fusion proteins that join the C-terminus of the epoxidase (StyA) to the N-terminus of the reductase (StyB) through a linker peptide and demonstrate their utility as biocatalysts in the synthesis of Tyrain purple and other indigoid dyes. A single-vector expression system offers a simplified platform for transformation and expansion of the catalytic function of styrene monooxygenases, and the resulting fusion proteins are self-regulated and couple efficiently NADH oxidation to styrene epoxidation. We find that the reductase domain proceeds through a sequential ternary-complex mechanism at low FAD concentration and a double-displacement mechanism at higher concentrations of FAD. Single-turnover studies indicate an observed rate constant for FAD-to-FAD hydride transfer of ~8 s^-1. This step is rate limiting in the styrene epoxidation reaction and helps to ensure that flavin reduction and styrene epoxidation reactions proceed without wasteful side reactions. Comparison of the reductase activity of the fusion proteins with the naturally occurring reductase, SMOB, and N-terminally histidine-tagged reductase, NSMOB, suggests that the observed changes in catalytic mechanism are due in part to an increase in flavin-binding affinity associated with the N-terminal extension of the reductase.

The Oxygen Dilemma: A Severe Challenge for the Application of Monooxygenases?

Monooxygenases are promising catalysts because they in principle enable the organic chemist to perform highly selective oxyfunctionalisation reactions that are otherwise difficult to achieve. For this, monooxygenases require reducing equivalents, to allow reductive activation of molecular oxygen at the enzymes' active sites. However, these reducing equivalents are often delivered to O2 either directly or via a reduced intermediate (uncoupling), yielding hazardous reactive oxygen species and wasting valuable reducing equivalents. The oxygen dilemma arises from monooxygenases' dependency on O2 and the undesired uncoupling reaction. With this contribution we hope to generate a general awareness of the oxygen dilemma and to discuss its nature and some promising solutions.

Enhancing Indigo Production by Over-Expression of the Styrene Monooxygenase in Pseudomonas putida

As an important traditional blue dye, indigo has been used in food and textile industry for centuries, which can be produced via the styrene oxygenation pathway in Pseudomonas putida. Hence, the styrene monooxygenase gene styAB and oxide isomerase gene styC are over-expressed in P. putida to investigate their roles in indigo biosynthesis. RT-qPCR analysis indicated that transcriptions of styA and styB were increased by 2500- and 750-folds in the styAB over-expressed strain B4-01, compared with the wild-type strain B4, consequently significantly enhancing the indole monooxygenase activity. Transcription of styC was also increased by 100-folds in the styC over-expressed strain B4-02. Besides, styAB over-expression slightly up-regulated the transcription of styC in B4-01, while styC over-expression hardly exerted an effect on the transcriptional levels of styA and styB and indole monooxygenase activity in B4-02. Furthermore, shaking flask experiments showed that indigo production in B4-01 reached 52.13 mg L(-1) after 24 h, which was sevenfold higher than that in B4. But no obvious increase in indigo yield was observed in B4-02. Over-expression of styAB significantly enhanced the indigo production, revealing that the monooxygenase STYAB rather than oxide isomerase STYC probably acted as the key rate-limiting enzyme in the indigo biosynthesis pathway in P. putida. This work provided a new strategy for enhancing indigo production in Pseudomonas.

Catalytic and hydrodynamic properties of styrene monooxygenases from Rhodococcus opacus 1CP are modulated by cofactor binding

Styrene monooxygenases (SMOs) are flavoenzymes catalyzing the epoxidation of styrene into styrene oxide. SMOs are composed of a monooxygenase (StyA) and a reductase (StyB). The latter delivers reduced FAD to StyA on the expense of NADH. We identified Rhodococcus opacus 1CP as the first microorganism to possess three different StyA isoforms occurring in two systems StyA1/StyA2B and StyA/StyB, respectively. The hydrodynamic properties of StyA isozymes were found to be modulated by the binding of the (reduced) FAD cofactor. StyA1 and SyA2B mainly occur as dimers in their active forms while StyA is a monomer. StyA1 showed the highest epoxidation activity and excellent enantioselectivity in aromatic sulfoxidation. The hydrodynamic and biocatalytic properties of SMOs from strain 1CP are of relevance for investigation of possible industrial applications.

Functional characterization and stability improvement of a 'thermophilic-like' ene-reductase from Rhodococcus opacus 1CP

Ene-reductases (ERs) are widely applied for the asymmetric synthesis of relevant industrial chemicals. A novel ER OYERo2 was found within a set of 14 putative old yellow enzymes (OYEs) obtained by genome mining of the actinobacterium Rhodococcus opacus 1CP. Multiple sequence alignment suggested that the enzyme belongs to the group of 'thermophilic-like' OYEs. OYERo2 was produced in Escherichia coli and biochemically characterized. The enzyme is strongly NADPH dependent and uses non-covalently bound FMNH2 for the reduction of activated α,β-unsaturated alkenes. In the active form OYERo2 is a dimer. Optimal catalysis occurs at pH 7.3 and 37°C. OYERo2 showed highest specific activities (45-50 U mg(-1)) on maleimides, which are efficiently converted to the corresponding succinimides. The OYERo2-mediated reduction of prochiral alkenes afforded the (R)-products with excellent optical purity (ee > 99%). OYERo2 is not as thermo-resistant as related OYEs. Introduction of a characteristic intermolecular salt bridge by site-specific mutagenesis raised the half-life of enzyme inactivation at 32°C from 28 to 87 min and improved the tolerance toward organic co-solvents. The suitability of OYERo2 for application in industrial biocatalysis is discussed.

The taming of oxygen: biocatalytic oxyfunctionalisations

The scope and limitations of oxygenases as catalysts for preparative organic synthesis is discussed.

Gene redundancy of two-component (chloro)phenol hydroxylases in Rhodococcus opacus 1CP

Among other factors, a distinct gene redundancy is discussed to facilitate high metabolic versatility of rhodococci. Rhodococcus opacus 1CP is a typical member in that respect and degrades a multitude of (chlorinated) aromatic compounds. In contrast to the central pathways of aromatic degradation in strain 1CP, little is known about the degree of gene redundancy and to what extent this is reflected on protein level within the steps of peripheral degradation. By means of degenerated primers deduced from tryptic peptides of a purified phenol hydroxylase component and using the amplified fragment as a labelled probe against genomic 1CP-DNA, three gene sets encoding three different two-component phenol hydroxylases pheA1/pheA2(1-3) could be identified. One of them was found to be located on the megaplasmid p1CP, which confirms the role of these elements for metabolic versatility. Protein chromatography of phenol- and 4-chlorophenol-grown 1CP-biomass gave first evidences on a functional expression of these oxygenases, which could be initially characterised in respect of their substrate specificity.

A mechanistic study on SMOB-ADP1: an NADH:flavin oxidoreductase of the two-component styrene monooxygenase of Acinetobacter baylyi ADP1

Two styrene monooxygenase types, StyA/StyB and StyA1/StyA2B, have been described each consisting of an epoxidase and a reductase. A gene fusion which led to the chimeric reductase StyA2B and the occurrence in different phyla are major differences. Identification of SMOA/SMOB-ADP1 of Acinetobacter baylyi ADP1 may enlighten the gene fusion event since phylogenetic analysis indicated both proteins to be more related to StyA2B than to StyA/StyB. SMOB-ADP1 is classified like StyB and StyA2B as HpaC-like reductase. Substrate affinity and turnover number of the homo-dimer SMOB-ADP1 were determined for NADH (24 µM, 64 s(-1)) and FAD (4.4 µM, 56 s(-1)). SMOB-ADP1 catalysis follows a random sequential mechanism, and FAD fluorescence is quenched upon binding to SMOB-ADP1 (K d = 1.8 µM), which clearly distinguishes that reductase from StyB of Pseudomonas. In summary, this study confirmes made assumptions and provides phylogenetic and biochemical data for the differentiation of styrene monooxygenase-related flavin reductases.

Flavoprotein monooxygenases for oxidative biocatalysis: recombinant expression in microbial hosts and applications

External flavoprotein monooxygenases comprise a group of flavin-dependent oxidoreductases that catalyze the insertion of one atom of molecular oxygen into an organic substrate and the second atom is reduced to water. These enzymes are involved in a great number of metabolic pathways both in prokaryotes and eukaryotes. Flavoprotein monooxygenases have attracted the attention of researchers for several decades and the advent of recombinant DNA technology caused a great progress in the field. These enzymes are subjected to detailed biochemical and structural characterization and some of them are also regarded as appealing oxidative biocatalysts for the production of fine chemicals and valuable intermediates toward active pharmaceutical ingredients due to their high chemo-, stereo-, and regioselectivity. Here, we review the most representative reactions catalyzed both in vivo and in vitro by prototype flavoprotein monooxygenases, highlighting the strategies employed to produce them recombinantly, to enhance the yield of soluble proteins, and to improve cofactor regeneration in order to obtain versatile biocatalysts. Although we describe the most outstanding features of flavoprotein monooxygenases, we mainly focus on enzymes that were cloned, expressed and used for biocatalysis during the last years.

Flavin dependent monooxygenases

Flavin-dependent monooxygenases catalyze a wide variety of chemo-, regio- and enantioselective oxygenation reactions. As such, they are involved in key biological processes ranging from catabolism, detoxification and biosynthesis, to light emission and axon guidance. Based on fold and function, flavin-dependent monooxygenases can be distributed into eight groups. Groups A and B comprise enzymes that rely on NAD(P)H as external electron donor. Groups C-F are two-protein systems, composed of a monooxygenase and a flavin reductase. Groups G and H comprise internal monooxygenases that reduce the flavin cofactor through substrate oxidation. Recently, many new flavin-dependent monooxygenases have been discovered. In addition to posing basic enzymological questions, these proteins attract attention of pharmaceutical and fine-chemical industries, given their importance as regio- and enantioselective biocatalysts. In this review we present an update of the classification of flavin-dependent monooxygenases and summarize the latest advances in our understanding of their catalytic and structural properties.

FAD C(4a)-hydroxide stabilized in a naturally fused styrene monooxygenase

StyA2B represents a new class of styrene monooxygenases that integrates flavin-reductase and styrene-epoxidase activities into a single polypeptide. This naturally-occurring fusion protein offers new avenues for studying and engineering biotechnologically relevant enantioselective biochemical epoxidation reactions. Stopped-flow kinetic studies of StyA2B reported here identify reaction intermediates similar to those reported for the separate reductase and epoxidase components of related two-component systems. Our studies identify substrate epoxidation and elimination of water from the FAD C(4a)-hydroxide as rate-limiting steps in the styrene epoxidation reaction. Efforts directed at accelerating these reaction steps are expected to greatly increase catalytic efficiency and the value of StyA2B as biocatalyst.

Characterization of a self-sufficient trans-anethole oxygenase from Pseudomonas putida JYR-1

A novel flavoprotein monooxygenase, trans-anethole oxygenase (TAO), from Pseudomonas putida JYR-1, which is capable of catalyzing the oxidation of trans-anethole to p-anisaldehyde, was heterologously expressed in E. coli and purified. Enzymatic kinetics of diverse substrates and cofactors revealed that TAO is likely to be a novel self-sufficient flavoprotein monooxygenase. Enzyme assays of GST-TAO demonstrated that TAO catalyzed a trans-anethole oxidation reaction without auxiliary component enzyme-like electron-transfer flavin reductases. The single component TAO had the ability to reduce flavin cofactors and simultaneously oxidize trans-anthole to p-anisaldehyde. In the processes of reduction of flavin and oxidation of trans-anethole, TAO accepted various flavin and NAD(P)H cofactors. TAO also catalyzed oxidation of isoeugenol, O-methyl isoeugenol, and isosafrole, all of which contain the 2-propenyl functional group on the aromatic ring structure with different catalytic efficiency. TAO had the greatest catalytic efficiency (k_cat/K_m) with the original substrate, trans-anethole. Investigation about partially deleted mutants of TAO indicated that reductase active sites appeared to be located near the N terminal. Site directed mutagenesis studies also proved that the proposed flavin binding sites, Trp-38, Thr-43, Tyr-55, were critical for flavin reduction. However, disruption of any portion of TAO eliminated the oxygenase activity.

Localization and orientation of heavy-atom cluster compounds in protein crystals using molecular replacement

Heavy-atom clusters (HA clusters) containing a large number of specifically arranged electron-dense scatterers are especially useful for experimental phase determination of large complex structures, weakly diffracting crystals or structures with large unit cells. Often, the determination of the exact orientation of the HA cluster and hence of the individual heavy-atom positions proves to be the critical step in successful phasing and subsequent structure solution. Here, it is demonstrated that molecular replacement (MR) with either anomalous or isomorphous differences is a useful strategy for the correct placement of HA cluster compounds. The polyoxometallate cluster hexasodium α-metatungstate (HMT) was applied in phasing the structure of death receptor 6. Even though the HA cluster is bound in alternate partially occupied orientations and is located at a special position, its correct localization and orientation could be determined at resolutions as low as 4.9 Å. The broad applicability of this approach was demonstrated for five different derivative crystals that included the compounds tantalum tetradecabromide and trisodium phosphotungstate in addition to HMT. The correct placement of the HA cluster depends on the length of the intramolecular vectors chosen for MR, such that both a larger cluster size and the optimal choice of the wavelength used for anomalous data collection strongly affect the outcome.

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.

High pressure refolding, purification, and crystallization of flavin reductase from Sulfolobus tokodaii strain 7

Flavin reductase HpaC(St) catalyzes the reduction of free flavins using NADH or NADPH. High hydrostatic pressure was used for the solubilization and refolding of HpaC(St), which was expressed as inclusion bodies in Escherichia coli to achieve high yield in a flavin-free form. The refolded HpaC(St) was purified using Ni-affinity chromatography followed by a heat treatment, which gave a single band on SDS-PAGE. The purified refolded HpaC(St) did not contain FMN, unlike the same enzyme expressed as a soluble protein. After the addition of FMN to the protein solution, the refolded enzyme showed a higher activity than the enzyme expressed as the soluble protein. Crystals of the refolded enzyme were obtained by adding FMN, FAD, or riboflavin to the protein solution and without the addition of flavin compound.

One-component styrene monooxygenases: an evolutionary view on a rare class of flavoproteins

Styrene monooxygenases (SMOs) are catalysts for the enantioselective epoxidation of terminal alkenes. Most representatives comprise a reductase and a monooxygenase which are encoded by separate genes (styA, styB). Only six presumed self-sufficient one-component SMOs (styA2B) have previously been submitted to databases, and one has so far been characterized. StyA2B can be supported by another epoxidase (StyA1) encoded by styA1, a gene in direct neighborhood of styA2B. The present report describes the identification of a further styA1/styA2B-like SMO, which was detected in Rhodococcus opacus MR11. Based on the initially available sequences of styA2B-type SMOs, primers directed at conserved sequences were designed and a 7,012-bp genomic fragment from strain MR11 was obtained after PCRs and subsequent genome walking. Six open reading frames (ORFs) were detected and compared to genomic fragments of strains comprising either two- or one-component SMOs. Among the proteins encoded by the ORFs, the monooxygenase StyA1/StyA2B showed the highest divergence on amino acid level when comparing proteins from different sources. That finding, a rare distribution of styA2B genes among bacteria, and the general observation of evolution from simple to complex systems indicate that one-component SMOs evolved from two-component ancestors. Analysis of gene products from styA/styB- and styA1/styA2B-like SMOs revealed that a fusion of styA/styB to styA2B might have happened at least twice among microorganisms. This points to a convergent evolution of one-component SMOs.