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

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.

Monooxygenation of aromatic compounds by flavin-dependent monooxygenases

Many flavoenzymes catalyze hydroxylation of aromatic compounds especially phenolic compounds have been isolated and characterized. These enzymes can be classified as either single-component or two-component flavin-dependent hydroxylases (monooxygenases). The hydroxylation reactions catalyzed by the enzymes in this group are useful for modifying the biological properties of phenolic compounds. This review aims to provide an in-depth discussion of the current mechanistic understanding of representative flavin-dependent monooxygenases including 3-hydroxy-benzoate 4-hydroxylase (PHBH, a single-component hydroxylase), 3-hydroxyphenylacetate 4-hydroxylase (HPAH, a two-component hydroxylase), and other monooxygenases which catalyze reactions in addition to hydroxylation, including 2-methyl-3-hydroxypyridine-5-carboxylate oxygenase (MHPCO, a single-component enzyme that catalyzes aromatic-ring cleavage), and HadA monooxygenase (a two-component enzyme that catalyzes additional group elimination reaction). These enzymes have different unique structural features which dictate their reactivity toward various substrates and influence their ability to stabilize flavin intermediates such as C4a-hydroperoxyflavin. Understanding the key catalytic residues and the active site environments important for governing enzyme reactivity will undoubtedly facilitate future work in enzyme engineering or enzyme redesign for the development of biocatalytic methods for the synthesis of valuable compounds.

Structural and Functional Characterization of 4-Hydroxyphenylacetate 3-Hydroxylase from Escherichia coli

The hydroxylation of phenols into polyphenols, which are valuable chemicals and pharmaceutical products, is a challenging reaction. The search for green synthetic processes has led to considering microorganisms and pure hydroxylases as catalysts for phenol hydroxylation. Herein, we report the structural and functional characterization of the flavin adenine dinucleotide (FAD)-dependent 4-hydroxyphenylacetate 3-monooxygenase from Escherichia coli, named HpaB. It is shown that this enzyme enjoys a relatively broad substrate specificity, which allows the conversion of a number of non-natural phenolic compounds, such as tyrosol, hydroxymandelic acid, coumaric acid, hydroxybenzoic acid and its methyl ester, and phenol, into the corresponding catechols. The reaction can be performed by using a simple chemical assay based on formate as the electron donor and the organometallic complex [Rh(bpy)Cp*(H₂ O)]²⁺ (Cp*: 1,2,3,4,5-pentamethylcyclopentadiene, bpy: 2,2'-bipyridyl) as the catalyst for FAD reduction. The availability of a crystal structure of HpaB in complex with FAD at 1.8 Å resolution opens up the possibility of the rational tuning of the substrate specificity and activity of this interesting class of phenol hydroxylases.

Glutathione: A powerful but rare cofactor among Actinobacteria

Glutathione (γ-l-glutamyl-l-cysteinylglycine, GSH) is a powerful cellular redox agent. In nature only the l,l-form is common among the tree of life. It serves as antioxidant or redox buffer system, protein regeneration and activation by interaction with thiol groups, unspecific reagent for conjugation during detoxification, marker for amino acid or peptide transport even through membranes, activation or solubilization of compounds during degradative pathways or just as redox shuttle. However, the role of GSH production and utilization in bacteria is more complex and especially little is known for the Actinobacteria. Some recent reports on GSH use in degradative pathways came across and this is described herein. GSH is used by transferases to activate and solubilize epoxides. It allows funneling epoxides as isoprene oxide or styrene oxide into central metabolism. Thus, the distribution of GSH synthesis, recycling and application among bacteria and especially Actinobacteria are highlighted including the pathways and contributing enzymes.

Structure and Activity of the Thermophilic Tryptophan-6 Halogenase BorH

Flavin-dependent halogenases carry out regioselective aryl halide synthesis in aqueous solution at ambient temperature and neutral pH using benign halide salts, making them attractive catalysts for green chemistry. BorH and BorF, two proteins encoded by the biosynthetic gene cluster for the chlorinated bisindole alkaloid borregomycin A, are the halogenase and flavin reductase subunits of a tryptophan-6-halogenase. Quantitative conversion of l-tryptophan (Trp) to 6-chlorotryptophan could be achieved using 1.2 mol % BorH and 2 mol % BorF. The optimal reaction temperature for Trp chlorination is 45 °C, and the melting temperatures of BorH and BorF are 48 and 50 °C respectively, which are higher than the thermal parameters for most other halogenases previously studied. Steady-state kinetic analysis of Trp chlorination by BorH determined parameters of k_cat =4.42 min^-1 , and K_M of 9.78 μm at 45 °C. BorH exhibits a broad substrate scope, chlorinating and brominating a variety of aromatic substrates with and without indole groups. Chlorination of Trp at a 100 mg scale with 52 % crude yield, using 0.2 mol % BorH indicates that industrial scale biotransformations using BorH/BorF are feasible. The X-ray crystal structure of BorH with bound Trp provides additional evidence for the model that regioselectivity is determined by substrate positioning in the active site, showing C6 of Trp juxtaposed with the catalytic Lys79 in the same binding pose previously observed in the structure of Thal.

An overview of microbial indigo-forming enzymes

Indigo is one of the oldest textile dyes and was originally prepared from plant material. Nowadays, indigo is chemically synthesized at a large scale to satisfy the demand for dyeing jeans. The current indigo production processes are based on fossil feedstocks; therefore, it is highly attractive to develop a more sustainable and environmentally friendly biotechnological process for the production of this popular dye. In the past decades, a number of natural and engineered enzymes have been identified that can be used for the synthesis of indigo. This mini-review provides an overview of the various microbial enzymes which are able to produce indigo and discusses the advantages and disadvantages of each biocatalytic system.

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.

Peroxygenase-Catalysed Epoxidation of Styrene Derivatives in Neat Reaction Media

Biocatalytic oxyfunctionalisation reactions are traditionally conducted in aqueous media limiting their production yield. Here we report the application of a peroxygenase in neat reaction conditions reaching product concentrations of up to 360 mM.

Isolation and characterization of Gram-negative and Gram-positive bacteria capable of producing piceatannol from resveratrol

Piceatannol is a valuable natural polyphenol with therapeutic potential in cardiovascular and metabolic disease treatment. In this study, we screened for microorganisms capable of producing piceatannol from resveratrol via regioselective hydroxylation. In the first screening, we isolated microorganisms utilizing resveratrol, phenol, or 4-hydroxyphenylacetic acid as a carbon source for growth. In the second screening, we assayed the isolated microorganisms for hydroxylation of resveratrol. Using this screening procedure, a variety of resveratrol-converting microorganisms were obtained. One Gram-negative bacterium, Ensifer sp. KSH1, and one Gram-positive bacterium, Arthrobacter sp. KSH3, utilized 4-hydroxyphenylacetic acid as a carbon source for growth and efficiently hydroxylated resveratrol to piceatannol without producing any detectable by-products. The hydroxylation activity of strains KSH1 and KSH3 was strongly induced by cultivation with 4-hydroxyphenylacetic acid as a carbon source during stationary growth phase. Using the 4-hydroxyphenylacetic acid-induced cells as a biocatalyst under optimal conditions, production of piceatannol by strains KSH1 and KSH3 reached 3.6 mM (0.88 g/L) and 2.6 mM (0.64 g/L), respectively. We also cloned genes homologous to the monooxygenase gene hpaBC from strains KSH1 and KSH3. Introduction of either hpaBC homolog into Escherichia coli endowed the host with resveratrol-hydroxylating activity.

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.

Monooxygenase-catalyzed regioselective hydroxylation for the synthesis of hydroxyequols

A one-step product-selective approach for synthesizing hydroxyequols from equol using oxidation biocatalysts was developed.

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.

Biotechnological Methods of Sulfoxidation: Yesterday, Today, Tomorrow

The production of chiral sulphoxides is an important part of the chemical industry since they have been used not only as pharmaceuticals and pesticides, but also as catalysts or functional materials. The main purpose of this review is to present biotechnological methods for the oxidation of sulfides. The work consists of two parts. In the first part, examples of biosyntransformation of prochiral sulfides using whole cells of bacteria and fungi are discussed. They have more historical significance due to the low predictability of positive results in relation to the workload. In the second part, the main enzymes responsible for sulfoxidation have been characterized such as chloroperoxidase, dioxygenases, cytochrome flavin-dependent monooxygenases, and P450 monooxygenases. Particular emphasis has been placed on the huge variety of cytochrome P450 monooxygenases, and flavin-dependent monooxygenases, which allows for pure sulfoxides enantiomers effectively to be obtained. In the summary, further directions of research on the optimization of enzymatic sulfoxidation are indicated.

4-Chlorophenol Oxidation Depends on the Activation of an AraC-Type Transcriptional Regulator, CphR, in Rhodococcus sp. Strain YH-5B

4-Chlorophenol (4-CP) oxidation plays an essential role in the detoxification of 4-CP. However, oxidative regulation of 4-CP at the genetic and biochemical levels has not yet been studied. To explore the regulation mechanism of 4-CP oxidation, a novel gene cluster, cphRA2A1, involved in biodegradation of 4-CP was identified and cloned from Rhodococcus sp. strain YH-5B by genome walking. The sequence analysis showed that the cphRA2A1 gene cluster encoded an AraC-type transcriptional regulator and a two-component monooxygenase enzyme, while quantitative real-time PCR analysis further revealed that cphR was constitutively expressed and positively regulated the transcription of cphA2A1 genes in response to 4-CP or phenol, as evidenced by gene knockout and complementation experiments. Through the transcriptional fusion of the mutated cphA2A1 promoter with the lacZ gene, it was found that the CphR regulator binding sites had two 15-bp imperfect direct repeats (TGCA-N6-GGNTA) at -35 to -69 upstream of the cphA2A1 transcriptional start site. Notably, the sub-motifs at the -46 to -49 positions played a critical role in the appropriate interaction with the CphR dimer. In addition, it was confirmed that the monooxygenase subunits CphA1 and CphA2, which were purified by His-tag affinity chromatography, were able to catalyze the conversion of 4-CP to 4-chlorocatechol, suggesting that strain YH-5B could degrade 4-CP via the 4-chlorocatechol pathway. This study enhances our understanding of the genetic and biochemical diversity in the transcriptional regulation of 4-CP oxidation in Gram-positive bacteria.