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

Evaluation of Differentially Expressed Candidate Genes in Benzo[a]pyrene Degradation by Burkholderia vietnamiensis G4

Bacteria-mediated bioremediation is widely employed for its environmental benefits. The genus Burkholderia can degrade persistent organic compounds, however, little is known about its mechanisms. To increase this knowledge, Burkholderia vietnamiensis G4 bacteria were exposed to benzo[a]pyrene, a recalcitrant compound, and the expression of twelve genes of interest was analyzed at 1, 12 and 24 h. In addition, benzo[a]pyrene degradation, evaluation of cell viability and fluorescence emission of assimilated benzo[a]pyrene was performed over 28 days. The up-regulated genes were xre, paaE, livG and pckA at the three times, ACAD, atoB, bmoA and proV at 1 h and AstB at 12 h. These genes are important for bacterial survival in stress situations, breakdown and metabolization of organic compounds, and nutrient transport and uptake. Furthermore, a 52% reduction of the pollutant was observed, there was no significant variation in the viability rate of the cells, and fluorescence indicated an accumulation of benzo[a]pyrene after 24 h. Our study demonstrates the bacteria adaptability and ability to modulate the expression of genes at different times and as needed. This increases our understanding of biodegradation processes and opens new possibilities for using this bacterial strain as a tool for the bioremediation of contaminated areas.

Effects of riboflavin in the treatment of brain damage caused by oxygen deprivation: an integrative systematic review

Brain oxygen deprivation causes morphological damage involved in the formation of serious pathological conditions such as stroke and cerebral palsy. Therapeutic methods for post-hypoxia/anoxia injuries are limited and still have deficiencies in terms of safety and efficacy. Recently, clinical studies of stroke have reported the use of drugs containing riboflavin for post-injury clinical rehabilitation, however, the effects of vitamin B2 on exposure to cerebral oxygen deprivation are not completely elucidated. This review aimed to investigate the potential antioxidant, anti-inflammatory and neuroprotective effects of riboflavin in cerebral hypoxia/anoxia. After a systematic search, 21 articles were selected, 8 preclinical and 12 clinical studies, and 1 translational study. Most preclinical studies used B2 alone in models of hypoxia in rodents, with doses of 1-20 mg/kg (in vivo) and 0.5-5 µM (in vitro). Together, these works suggested greater regulation of lipid peroxidation and apoptosis and an increase in neurotrophins, locomotion, and cognition after treatment. In contrast, several human studies have administered riboflavin (5 mg) in combination with other Krebs cycle metabolites, except one study, which used only B2 (20 mg). A reduction in lactic acidosis and recovery of sensorimotor functions was observed in children after treatment with B2, while adults and the elderly showed a reduction in infarct volume and cognitive rehabilitation. Based on findings from preclinical and clinical studies, we conclude that the use of riboflavin alone or in combination acts beneficially in correcting the underlying brain damage caused by hypoxia/anoxia and its inflammatory, oxidative, and behavioral impairments.

Functional and structural characterization of a thermostable flavin reductase from Geobacillus mahadii Geo-05

Flavin reductases play a vital role in catalyzing the reduction of flavin through NADH or NADPH oxidation. The gene encoding flavin reductase from the thermophilic bacterium Geobacillus mahadii Geo-05 (GMHpaC) was cloned, overexpressed in Escherichia coli BL21 (DE3) pLysS, and purified to homogeneity. The purified recombinant GMHpaC (Class II) contains chromogenic cofactors, evidenced by maximal absorbance peaks at 370 nm and 460 nm. GMHpaC stands out as the most thermostable and pH-tolerant flavin reductase reported to date, retaining up to 95 % catalytic activity after incubation at 70 °C for 30 min and maintaining over 80 % activity within a pH range of 2-12 for 30 min. Furthermore, GMHpaC's catalytic activity increases by 52 % with FMN as a co-factor compared to FAD and riboflavin. GMHpaC, coupled with 4-hydroxyphenylacetate-3-monooxygenase (GMHpaB) from G. mahadii Geo-05, enhances the hydroxylation of 4-hydroxyphenylacetate (HPA) by 85 %. The modeled structure of GMHpaC reveals relatively conserved flavin and NADH binding sites. Modeling and docking studies shed light on structural features and amino acid substitutions that determine GMHpaC's co-factor specificity. The remarkable thermostability, high catalytic activity, and general stability exhibited by GMHpaC position it as a promising enzyme candidate for various industrial applications.

Flavin-N5OOH Functions as both a Powerful Nucleophile and a Base in the Superfamily of Flavoenzymes

Flavoenzymes can mediate a large variety of oxidation reactions through the activation of oxygen. However, the O2 activation chemistry of flavin enzymes is not yet fully exploited. Normally, the O2 activation occurs at the C4a site of the flavin cofactor, yielding the flavin C4a-(hydro)hydroperoxyl species in monooxygenases or oxidases. Using extensive MD simulations, QM/MM calculations and QM calculations, our studies reveal the formation of the common nucleophilic species, Flavin-N5OOH, in two distinct flavoenzymes (RutA and EncM). Our studies show that Flavin-N5OOH acts as a powerful nucleophile that promotes C-N cleavage of uracil in RutA, and a powerful base in the deprotonation of substrates in EncM. We reason that Flavin-N5OOH can be a common reactive species in the superfamily of flavoenzymes, which accomplish generally selective general base catalysis and C-X (X=N, S, Cl, O) cleavage reactions that are otherwise challenging with solvated hydroxide ion base. These results expand our understanding of the chemistry and catalysis of flavoenzymes.

Exploring the role of flavin-dependent monooxygenases in the biosynthesis of aromatic compounds

Hydroxylated aromatic compounds exhibit exceptional biological activities. In the biosynthesis of these compounds, three types of hydroxylases are commonly employed: cytochrome P450 (CYP450), pterin-dependent monooxygenase (PDM), and flavin-dependent monooxygenase (FDM). Among these, FDM is a preferred choice due to its small molecular weight, stable expression in both prokaryotic and eukaryotic fermentation systems, and a relatively high concentration of necessary cofactors. However, the catalytic efficiency of many FDMs falls short of meeting the demands of large-scale production. Additionally, challenges arise from the limited availability of cofactors and compatibility issues among enzyme components. Recently, significant progress has been achieved in improving its catalytic efficiency, but have not yet detailed and informative viewed so far. Therefore, this review emphasizes the advancements in FDMs for the biosynthesis of hydroxylated aromatic compounds and presents a summary of three strategies aimed at enhancing their catalytic efficiency: (a) Developing efficient enzyme mutants through protein engineering; (b) enhancing the supply and rapid circulation of critical cofactors; (c) facilitating cofactors delivery for enhancing FDMs catalytic efficiency. Furthermore, the current challenges and further perspectives on improving catalytic efficiency of FDMs are also discussed.

4-Hydroxyphenylacetate 3-Hydroxylase (4HPA3H): A Vigorous Monooxygenase for Versatile O-Hydroxylation Applications in the Biosynthesis of Phenolic Derivatives

4-Hydroxyphenylacetate 3-hydroxylase (4HPA3H) is a long-known class of two-component flavin-dependent monooxygenases from bacteria, including an oxygenase component (EC 1.14.14.9) and a reductase component (EC 1.5.1.36), with the latter being accountable for delivering the cofactor (reduced flavin) essential for o-hydroxylation. 4HPA3H has a broad substrate spectrum involved in key biological processes, including cellular catabolism, detoxification, and the biosynthesis of bioactive molecules. Additionally, it specifically hydroxylates the o-position of the C4 position of the benzene ring in phenolic compounds, generating high-value polyhydroxyphenols. As a non-P450 o-hydroxylase, 4HPA3H offers a viable alternative for the de novo synthesis of valuable natural products. The enzyme holds the potential to replace plant-derived P450s in the o-hydroxylation of plant polyphenols, addressing the current significant challenge in engineering specific microbial strains with P450s. This review summarizes the source distribution, structural properties, and mechanism of 4HPA3Hs and their application in the biosynthesis of natural products in recent years. The potential industrial applications and prospects of 4HPA3H biocatalysts are also presented.

Indigo production goes green: a review on opportunities and challenges of fermentative production

Indigo is a widely used dye in various industries, such as textile, cosmetics, and food. However, traditional methods of indigo extraction and processing are associated with environmental and economic challenges. Fermentative production of indigo using microbial strains has emerged as a promising alternative that offers sustainability and cost-effectiveness. This review article provides a critical overview of microbial diversity, metabolic pathways, fermentation strategies, and genetic engineering approaches for fermentative indigo production. The advantages and limitations of different indigo production systems and a critique of the current understanding of indigo biosynthesis are discussed. Finally, the potential application of indigo in other sectors is also discussed. Overall, fermentative production of indigo offers a sustainable and bio-based alternative to synthetic methods and has the potential to contribute to the development of sustainable and circular biomanufacturing.

Extended Biocatalytic Halogenation Cascades Involving a Single-Polypeptide Regeneration System for Diffusible FADH2

Flavin-dependent halogenases have attracted increasing interest for aryl halogenation at unactivated C-H positions because they are characterised by high regioselectivity, while requiring only FADH2 , halide salts, and O2 . Their use in combined crosslinked enzyme aggregates (combiCLEAs) together with an NADH-dependent flavin reductase and an NADH-regeneration system for the preparative halogenation of tryptophan and indole derivatives has been previously described. However, multiple cultivations and protein purification steps are necessary for their production. We present a bifunctional regeneration enzyme for two-step catalytic flavin regeneration using phosphite as an inexpensive sacrificial substrate. This fusion protein proved amenable to co-expression with various flavin-dependent Trp-halogenases and enables carrier-free immobilisation as combiCLEAs from a single cultivation for protein production and the preparative synthesis of halotryptophan. The scalability of this system was demonstrated by fed-batch fermentation in bench-top bioreactors on a 2.5 L scale. Furthermore, the inclusion of a 6-halotryptophan-specific dioxygenase into the co-expression strain further converts the halogenation product to the kynurenine derivative. This reaction cascade enables the one-pot synthesis of l-4-Cl-kynurenine and its brominated analogue on a preparative scale.

Advances in 4-Hydroxyphenylacetate-3-hydroxylase Monooxygenase

Catechols have important applications in the pharmaceutical, food, cosmetic, and functional material industries. 4-hydroxyphenylacetate-3-hydroxylase (4HPA3H), a two-component enzyme system comprising HpaB (monooxygenase) and HpaC (FAD oxidoreductase), demonstrates significant potential for catechol production because it can be easily expressed, is highly active, and exhibits ortho-hydroxylation activity toward a broad spectrum of phenol substrates. HpaB determines the ortho-hydroxylation efficiency and substrate spectrum of the enzyme; therefore, studying its structure-activity relationship, improving its properties, and developing a robust HpaB-conducting system are of significance and value; indeed, considerable efforts have been made in these areas in recent decades. Here, we review the classification, molecular structure, catalytic mechanism, primary efforts in protein engineering, and industrial applications of HpaB in catechol synthesis. Current trends in the further investigation of HpaB are also discussed.

Cloning of two gene clusters involved in the catabolism of 2,4-dinitrophenol by Paraburkholderia sp. strain KU-46 and characterization of the initial DnpAB enzymes and a two-component monooxygenases DnpC1C2

Little is currently known about the metabolism of the industrial pollutant 2,4-dinitrophenol (DNP), particularly among gram-negative bacteria. In this study, we identified two non-contiguous genetic loci spanning 22 kb of Paraburkholderia (formerly Burkholderia) sp. strain KU-46. Additionally, we characterized four key initial genes (dnpA, dnpB, and dnpC1C2) responsible for DNP degradation, providing molecular and biochemical evidence for the degradation of DNP via the formation of 4-nitrophenol (NP), a pathway that is unique among DNP utilizing bacteria. Reverse transcription polymerase chain reaction (PCR) analysis indicated that dnpA, which encodes the initial hydride transferase, and dnpB which encodes a nitrite-eliminating enzyme, were induced by DNP and organized in an operon. Moreover, we purified DnpA and DnpB from recombinant Escherichia coli to demonstrate their effect on the transformation of DNP to NP through the formation of a hydride-Meisenheimer complex of DNP, designated as H--DNP. The function of DnpB appears new since all homologs of the DnpB sequences in the protein database are annotated as putative nitrate ABC transporter substrate-binding proteins. The gene cluster responsible for the degradation of DNP after NP formation was designated dnpC1C2DXFER, and DnpC1 and DnpC2 were functionally characterized as the FAD reductase and oxygenase components of the two-component DNP monooxygenase, respectively. By elucidating the hqdA1A2BCD gene cluster, we are now able to delineate the final degradation pathway of hydroquinone to β-ketoadipate before it enters the tricarboxylic acid cycle.

A Novel Estrone Degradation Gene Cluster and Catabolic Mechanism in Microbacterium oxydans ML-6

Global-scale estrone (E1) contamination of soil and aquatic environments results from the widespread use of animal manure as fertilizer, threatening both human health and environmental security. A detailed understanding of the degradation of E1 by microorganisms and the associated catabolic mechanism remains a key challenge for the bioremediation of E1-contaminated soil. Here, Microbacterium oxydans ML-6, isolated from estrogen-contaminated soil, was shown to efficiently degrade E1. A complete catabolic pathway for E1 was proposed via liquid chromatography-tandem mass spectrometry (LC-MS/MS), genome sequencing, transcriptomic analysis, and quantitative reverse transcription-PCR (qRT-PCR). In particular, a novel gene cluster (moc) associated with E1 catabolism was predicted. The combination of heterologous expression, gene knockout, and complementation experiments demonstrated that the 3-hydroxybenzoate 4-monooxygenase (MocA; a single-component flavoprotein monooxygenase) encoded by the mocA gene was responsible for the initial hydroxylation of E1. Furthermore, to demonstrate the detoxification of E1 by strain ML-6, phytotoxicity tests were performed. Overall, our findings provide new insight into the molecular mechanism underlying the diversity of E1 catabolism in microorganisms and suggest that M. oxydans ML-6 and its enzymes have potential applications in E1 bioremediation to reduce or eliminate E1-related environmental pollution. IMPORTANCE Steroidal estrogens (SEs) are mainly produced by animals, while bacteria are major consumers of SEs in the biosphere. However, the understanding of the gene clusters that participate in E1 degradation is still limited, and the enzymes involved in the biodegradation of E1 have not been well characterized. The present study reports that M. oxydans ML-6 has effective SE degradation capacity, which facilitates the development of strain ML-6 as a broad-spectrum biocatalyst for the production of certain desired compounds. A novel gene cluster (moc) associated with E1 catabolism was predicted. The 3-hydroxybenzoate 4-monooxygenase (MocA; a single-component flavoprotein monooxygenase) identified in the moc cluster was found to be necessary and specific for the initial hydroxylation of E1 to generate 4-OHE1, providing new insight into the biological role of flavoprotein monooxygenase.

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.

Identification of an indole biodegradation gene cluster from Providencia rettgeri and its contribution in selectively biosynthesizing Tyrian purple

Tyrian purple, mainly composed of 6, 6'-dibromoindigo, is a precious dye extracted from sea snails. In this study, we found Tyrian purple can be selectively produced by a bacterial strain GS-2 when fed with 6-bromotryptophan in the presence of tryptophan. This GS-2 strain was then identified as Providencia rettgeri based on bacterial genome sequencing analysis. An indole degradation gene cluster for indole metabolism was identified from this GS-2 strain. The heterologous expression of the indole degradation gene cluster in Escherichia coli BL21 (DE3) and in vitro enzymatic reaction demonstrated that the indole biodegradation gene cluster may contribute to selectively biosynthesizing Tyrian purple. To further explore the underlying mechanism of the selectivity, we explored the intermediates in this indole biodegradation pathway using liquid chromatography electrospray ionization quadrupole time-of-flight mass spectrometry (LC-ESI-QTOF-MS/MS), which indicated that the indole biodegradation pathway in Providencia rettgeri is the catechol pathway. Interestingly, the monooxygenase GS-C co-expressed with its corresponding reductase GS-D in the cluster has better activity for the biosynthesis of Tyrian purple compared with the previously reported monooxygenase from Methylophaga aminisulfidivorans (MaFMO) or Streptomyces cattleya cytochrome P450 enzyme (CYP102G4). This is the first study to show the existence of an indole biodegradation pathway in Providencia rettgeri, and the indole biodegradation gene cluster can contribute to the selective production of Tyrian purple.

Early Transcriptional Response to Monensin in Sensitive and Resistant Strains of Eimeria tenella

Eimeria parasites are the causative agents of coccidiosis, a common parasitic disease in poultry and livestock that causes significant economic losses to the animal husbandry industry. Ionophore coccidiostats, such as monensin and salinomycin, are widely used for prophylaxis of coccidiosis in poultry. Unfortunately, widespread drug resistance has compromised their efficacy. As a result, there is an increasing need to understand the targets and resistance mechanisms to anticoccidials. However, how Eimeria parasite genes respond to ionophores remains unclear. In this study, resistance to monensin was induced in E. tenella through serial generations of selection. Both sensitive and resistant E. tenella sporozoites were treated with 5 μg/ml monensin for 0, 2, and 4 h, respectively. Gene transcription profiles were then compared by high-throughput sequencing. The results showed that protein translation-related genes were significantly downregulated after drug induction. A total of 1,848 DEGs were detected in the sensitive strain after 2 h of exposure, whereas only 31 were detected in the resistant strain. Among these DEGs in the sensitive strain, genes associated with protein degradation were significantly upregulated, supporting the autophagy-like parasite killing theory. Then, 4 h of exposure resulted in additional 626 and 621 DEGs for sensitive and resistant strains, respectively. This result implies that the gene transcription in sensitive strain is more susceptible to monensin treatment. Our results provide gene expression landscapes of E. tenella following monensin treatment. These data will contribute to a better understanding of the mechanism of drug resistance to polyether ionophores in coccidia.

Oxygenating Biocatalysts for Hydroxyl Functionalisation in Drug Discovery and Development

C-H oxyfunctionalisation remains a distinct challenge for synthetic organic chemists. Oxygenases and peroxygenases (grouped here as "oxygenating biocatalysts") catalyse the oxidation of a substrate with molecular oxygen or hydrogen peroxide as oxidant. The application of oxygenating biocatalysts in organic synthesis has dramatically increased over the last decade, producing complex compounds with potential uses in the pharmaceutical industry. This review will focus on hydroxyl functionalisation using oxygenating biocatalysts as a tool for drug discovery and development. Established oxygenating biocatalysts, such as cytochrome P450s and flavin-dependent monooxygenases, have widely been adopted for this purpose, but can suffer from low activity, instability or limited substrate scope. Therefore, emerging oxygenating biocatalysts which offer an alternative will also be covered, as well as considering the ways in which these hydroxylation biotransformations can be applied in drug discovery and development, such as late-stage functionalisation (LSF) and in biocatalytic cascades.

Enantioselectivity and key residue of Herbaspirillum huttiense monooxygenase in asymmetric epoxidation of styrenes

Styrene monooxygenases (SMOs) are powerful enzymes for the synthesis of enantiopure epoxides, but these SMOs have narrow substrate spectra, and the residues in controlling enantioselectivity of SMOs remains unclear. A monooxygenase from Herbaspirillum huttiense (HhMO) was found to have excellent enantioselectivities and diastereoselectivities in the epoxidation of unconjugated terminal alkenes. Here we found that HhMO could also transfer styrene into styrene epoxide with 75% ee, and it could also catalyze the epoxidation of styrene derivatives into the corresponding epoxides with enantioselectivities up to 99% ee. Meanwhile, site 199 in the substrate access channel of HhMO was found to play an important role in the controlling enantioselectivity of the epoxidation. The E199L variant catalyzed the epoxidation of styrene with > 99% ee. The identification of critical residue that affects the enantioselectivity of SMOs would thus be valuable for creating efficient monooxygenases for the preparation of essential enantiopure epoxides. KEY POINTS: • Bioexpoxidation of both conjugated and unconjugated alkenes by HhMO with excellent enantioselectivities. • Gating residue 199 played an essential role in controlling the enantioselectivity of SMO. • HhMO E199L catalyzed the epoxidation of styrenes with up to > 99% ee.

From Tryptophan to Toxin: Nature's Convergent Biosynthetic Strategy to Aetokthonotoxin

Aetokthonotoxin (AETX) is a cyanobacterial neurotoxin that causes vacuolar myelinopathy, a neurological disease that is particularly deadly to bald eagles in the United States. The recently characterized AETX is structurally unique among cyanotoxins and is composed of a pentabrominated biindole nitrile. Herein we report the discovery of an efficient, five-enzyme biosynthetic pathway that the freshwater cyanobacterium Aetokthonos hydrillicola uses to convert two molecules of tryptophan to AETX. We demonstrate that the biosynthetic pathway follows a convergent route in which two functionalized indole monomers are assembled and then reunited by biaryl coupling catalyzed by the cytochrome P450 AetB. Our results revealed enzymes with novel biochemical functions, including the single-component flavin-dependent tryptophan halogenase AetF and the iron-dependent nitrile synthase AetD.

Engineered bacterial flavin-dependent monooxygenases for the regiospecific hydroxylation of polycyclic phenols

4-Hydroxyphenylacetate 3-hydroxylase (4HPA3H), a flavin-dependent monooxygenase from E. coli that catalyzes the hydroxylation of monophenols to catechols, was modified by rational re-design to convert also more bulky substrates, especially phenolic natural products like phenylpropanoids, flavones or coumarins. Selected amino acid positions in the binding pocket of 4HPA3H were exchanged by residues from the homologous protein from Pseudomonas aeruginosa, yielding variants with improved conversion of spacious substrates such as the flavonoid naringenin or the alkaloid mimetic 2-hydroxycarbazole. Reactions were followed by an adapted Fe(III)-catechol chromogenic assay selective for the products. Especially substitution of the residue Y301 facilitated modulation of substrate specificity: introduction of non-aromatic but hydrophobic (iso)leucine resulted in the preference of the substrate ferulic acid (having a guaiacyl (guajacyl) moiety, part of the vanilloid motif) over unsubstituted monophenols. The in vivo (whole-cell biocatalysts) and in vitro (three-enzyme cascade) transformations of substrates by 4HPA3H and its optimized variants was strictly regiospecific and proceeded without generation of by-products.

Meta-analysis and validation of genomic loci governing seedling and reproductive stage salinity tolerance in rice

Identification of concurrent genomic regions contributing tolerance to salinity at the seedling and reproductive stages were done using 45 quantitative trait loci (QTL) mapping studies reporting 915 individual QTLs. The QTL-data were used to perform a meta-analysis to predict, validate and analyze the Meta-QTLs governing component traits contributing to salinity tolerance. We predicted a total of 65 and 49 Meta-QTLs distributed across the genome governing seedling and reproductive stage salinity tolerance, respectively. Salinity stress (EC ~10.0 dSm^-1 ) was evaluated in a set of 32 genotypes grown hydroponically, from these eight extreme (highly tolerant and highly susceptible) genotypes were selected for validation of significant Meta-QTLs. Another set of eight previously known and reported (highly tolerant and highly susceptible) genotypes were evaluated under saline micro plot conditions (EC ~8.0 dSm^-1 ) and used for validation of significant Meta-QTLs for reproductive stage salinity tolerance. The microsatellite marker "RM5635" linked to MSQTL4.2 (~295.43 kb) was able to clearly differentiate contrasting genotypes for seedling stage salinity tolerance, whereas at the reproductive stage, none of the markers were able to validate the predicted Meta-QTL for salinity tolerance. Earlier reported, gene expression studies were used for candidate gene analysis of validated MSQTL4.2, which indicated the down regulation of Os04g0423100, a gene encoding Mono-oxygenase-FAD binding domain containing protein. The traits associated with this Meta-QTL were root and shoot sodium and potassium concentration and leaf chlorophyll content. The identified and validated genomic region assumes a great significant role in seedling stage salinity tolerance in rice, and it can be used for marker-assisted backcross breeding programs.

The Isoenzymic Diketocamphane Monooxygenases of Pseudomonas putida ATCC 17453-An Episodic History and Still Mysterious after 60 Years

Researching the involvement of molecular oxygen in the degradation of the naturally occurring bicyclic terpene camphor has generated a six-decade history of fascinating monooxygenase biochemistry. While an extensive bibliography exists reporting the many varied studies on camphor 5-monooxygenase, the initiating enzyme of the relevant catabolic pathway in Pseudomonas putida ATCC 17453, the equivalent recorded history of the isoenzymic diketocamphane monooxygenases, the enzymes that facilitate the initial ring cleavage of the bicyclic terpene, is both less extensive and more enigmatic. First referred to as ‘ketolactonase—an enzyme for cyclic lactonization’—the enzyme now classified as 2,5-diketocamphane 1,2-monooxygenase (EC 1.14.14.108) holds a special place in the history of oxygen-dependent biochemistry, being the first biocatalyst confirmed to undertake a biooxygenation reaction equivalent to the peracid-catalysed Baeyer–Villiger chemical oxidation first reported in the late 19th century. However, following that auspicious beginning, the biochemistry of EC 1.14.14.108, and its isoenzymic partner 3,6-diketocamphane 1,6-monooxygenase (EC 1.14.14.155) was dogged for many years by the mistaken belief that the enzymes were true flavoproteins that function with a tightly-bound flavin cofactor in the active site. This misconception led to a number of erroneous interpretations of relevant experimental data. It is only in the last decade, initially as the result of pure serendipity, that these enzymes have been confirmed to be members of a relatively recently discovered class of oxygen-dependent enzymes, the flavin-dependent two-component monooxygenases. This has promoted a renaissance of interest in the enzymes, resulting in programmes of research that have significantly expanded current knowledge of both their mode of action and regulation in camphor-grown P. putida ATCC 17453. However, some features of the biochemistry of the isoenzymic diketocamphane monooxygenases remain currently unexplained. It is the episodic history of these enzymes and some of what remains unresolved that are the principal subjects of this review.

Dehydrogenases of acetic acid bacteria

Acetic acid bacteria (AAB) are a group of bacteria that can oxidize many substrates such as alcohols and sugar alcohols and play important roles in industrial biotechnology. A majority of industrial processes that involve AAB are related to their dehydrogenases, including PQQ/FAD-dependent membrane-bound dehydrogenases and NAD(P)⁺-dependent cytoplasmic dehydrogenases. These cofactor-dependent dehydrogenases must effectively regenerate their cofactors in order to function continuously. For PQQ, FAD and NAD(P)⁺ alike, regeneration is directly or indirectly related to the electron transport chain (ETC) of AAB, which plays an important role in energy generation for aerobic cell growth. Furthermore, in changeable natural habitats, ETC components of AAB can be regulated so that the bacteria survive in different environments. Herein, the progressive cascade in an application of AAB, including key dehydrogenases involved in the application, regeneration of dehydrogenase cofactors, ETC coupling with cofactor regeneration and ETC regulation, is systematically reviewed and discussed. As they have great application value, a deep understanding of the mechanisms through which AAB function will not only promote their utilization and development but also provide a reference for engineering of other industrial strains.

Pyrolyzed Substrates Induce Aromatic Compound Metabolism in the Post-fire Fungus, Pyronema domesticum

Wildfires represent a fundamental and profound disturbance in many ecosystems, and their frequency and severity are increasing in many regions of the world. Fire affects soil by removing carbon in the form of CO2 and transforming remaining surface carbon into pyrolyzed organic matter (PyOM). Fires also generate substantial necromass at depths where the heat kills soil organisms but does not catalyze the formation of PyOM. Pyronema species strongly dominate soil fungal communities within weeks to months after fire. However, the carbon pool (i.e., necromass or PyOM) that fuels their rise in abundance is unknown. We used a Pyronema domesticum isolate from the catastrophic 2013 Rim Fire (CA, United States) to ask whether P. domesticum is capable of metabolizing PyOM. Pyronema domesticum grew readily on agar media where the sole carbon source was PyOM (specifically, pine wood PyOM produced at 750°C). Using RNAseq, we investigated the response of P. domesticum to PyOM and observed a comprehensive induction of genes involved in the metabolism and mineralization of aromatic compounds, typical of those found in PyOM. Lastly, we used 13C-labeled 750°C PyOM to demonstrate that P. domesticum is capable of mineralizing PyOM to CO2. Collectively, our results indicate a robust potential for P. domesticum to liberate carbon from PyOM in post-fire ecosystems and return it to the bioavailable carbon pool.

Regiodivergent and stereoselective hydroxyazidation of alkenes by biocatalytic cascades

Asymmetric functionalization of alkenes allows the direct synthesis of a wide range of chiral compounds. Vicinal hydroxyazidation of alkenes provides a desirable path to 1,2-azidoalcohols; however, existing methods are limited by the control of stereoselectivity and regioselectivity. Herein, we describe a dual-enzyme cascade strategy for regiodivergent and stereoselective hydroxyazidation of alkenes, affording various enantiomerically pure 1,2-azidoalcohols. The biocatalytic cascade process is designed by combining styrene monooxygenase-catalyzed asymmetric epoxidation of alkenes and halohydrin dehalogenase-catalyzed regioselective ring opening of epoxides with azide. Additionally, a one-pot chemo-enzymatic route to chiral β-hydroxytriazoles from alkenes is developed via combining the biocatalytic cascades and Cu-catalyzed azide-alkyne cycloaddition.

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.

Two Fusarium copper radical oxidases with high activity on aryl alcohols

BACKGROUND

Biomass valorization has been suggested as a sustainable alternative to petroleum-based energy and commodities. In this context, the copper radical oxidases (CROs) from Auxiliary Activity Family 5/Subfamily 2 (AA5_2) are attractive biocatalysts for the selective oxidation of primary alcohols to aldehydes. Originally defined by the archetypal galactose 6-oxidase from Fusarium graminearum, fungal AA5_2 members have recently been shown to comprise a wide range of specificities for aromatic, aliphatic and furan-based alcohols. This suggests a broader substrate scope of native CROs for applications. However, only 10% of the annotated AA5_2 members have been characterized to date.

RESULTS

Here, we define two homologues from the filamentous fungi Fusarium graminearum and F. oxysporum as predominant aryl alcohol oxidases (AAOs) through recombinant production in Pichia pastoris, detailed kinetic characterization, and enzyme product analysis. Despite possessing generally similar active-site architectures to the archetypal FgrGalOx, FgrAAO and FoxAAO have weak activity on carbohydrates, but instead efficiently oxidize specific aryl alcohols. Notably, both FgrAAO and FoxAAO oxidize hydroxymethyl furfural (HMF) directly to 5-formyl-2-furoic acid (FFCA), and desymmetrize the bioproduct glycerol to the uncommon L-isomer of glyceraldehyde.

CONCLUSIONS

This work expands understanding of the catalytic diversity of CRO from AA5_2 to include unique representatives from Fusarium species that depart from the well-known galactose 6-oxidase activity of this family. Detailed enzymological analysis highlights the potential biotechnological applications of these orthologs in the production of renewable plastic polymer precursors and other chemicals.

E. coli Nickel-Iron Hydrogenase 1 Catalyses Non-native Reduction of Flavins: Demonstration for Alkene Hydrogenation by Old Yellow Enzyme Ene-reductases*

A new activity for the [NiFe] uptake hydrogenase 1 of Escherichia coli (Hyd1) is presented. Direct reduction of biological flavin cofactors FMN and FAD is achieved using H2 as a simple, completely atom-economical reductant. The robust nature of Hyd1 is exploited for flavin reduction across a broad range of temperatures (25-70 °C) and extended reaction times. The utility of this system as a simple, easy to implement FMNH2 or FADH2 regenerating system is then demonstrated by supplying reduced flavin to Old Yellow Enzyme "ene-reductases" to support asymmetric alkene reductions with up to 100 % conversion. Hyd1 turnover frequencies up to 20.4 min-1 and total turnover numbers up to 20 200 were recorded during flavin recycling.

E. coli Nickel-Iron Hydrogenase 1 Catalyses Non-native Reduction of Flavins: Demonstration for Alkene Hydrogenation by Old Yellow Enzyme Ene-reductases

A new activity for the [NiFe] uptake hydrogenase 1 of Escherichia coli (Hyd1) is presented. Direct reduction of biological flavin cofactors FMN and FAD is achieved using H2 as a simple, completely atom-economical reductant. The robust nature of Hyd1 is exploited for flavin reduction across a broad range of temperatures (25-70 °C) and extended reaction times. The utility of this system as a simple, easy to implement FMNH2 or FADH2 regenerating system is then demonstrated by supplying reduced flavin to Old Yellow Enzyme "ene-reductases" to support asymmetric alkene reductions with up to 100 % conversion. Hyd1 turnover frequencies up to 20.4 min-1 and total turnover numbers up to 20 200 were recorded during flavin recycling.

Identification of Compounds with Potential Therapeutic Uses from Sweet Pepper (Capsicum annuum L.) Fruits and Their Modulation by Nitric Oxide (NO)

Plant species are precursors of a wide variety of secondary metabolites that, besides being useful for themselves, can also be used by humans for their consumption and economic benefit. Pepper (Capsicum annuum L.) fruit is not only a common food and spice source, it also stands out for containing high amounts of antioxidants (such as vitamins C and A), polyphenols and capsaicinoids. Particular attention has been paid to capsaicin, whose anti-inflammatory, antiproliferative and analgesic activities have been reported in the literature. Due to the potential interest in pepper metabolites for human use, in this project, we carried out an investigation to identify new bioactive compounds of this crop. To achieve this, we applied a metabolomic approach, using an HPLC (high-performance liquid chromatography) separative technique coupled to metabolite identification by high resolution mass spectrometry (HRMS). After chromatographic analysis and data processing against metabolic databases, 12 differential bioactive compounds were identified in sweet pepper fruits, including quercetin and its derivatives, L-tryptophan, phytosphingosin, FAD, gingerglycolipid A, tetrahydropentoxylin, blumenol C glucoside, colnelenic acid and capsoside A. The abundance of these metabolites varied depending on the ripening stage of the fruits, either immature green or ripe red. We also studied the variation of these 12 metabolites upon treatment with exogenous nitric oxide (NO), a free radical gas involved in a good number of physiological processes in higher plants such as germination, growth, flowering, senescence, and fruit ripening, among others. Overall, it was found that the content of the analyzed metabolites depended on the ripening stage and on the presence of NO. The metabolic pattern followed by quercetin and its derivatives, as a consequence of the ripening stage and NO treatment, was also corroborated by transcriptomic analysis of genes involved in the synthesis of these compounds. This opens new research perspectives on the pepper fruit’s bioactive compounds with nutraceutical potentiality, where biotechnological strategies can be applied for optimizing the level of these beneficial compounds.

Steady-state kinetic analysis of halogenase-supporting flavin reductases BorF and AbeF reveals different kinetic mechanisms

The short-chain flavin reductases BorF and AbeF reduce FAD to FADH₂, which is then used by flavin-dependent halogenases (BorH and AbeH respectively) to regioselectively chlorinate tryptophan in the biosynthesis of indolotryptoline natural products. Recombinant AbeF and BorF were overexpressed and purified as homodimers from E. coli, and copurified with substoichiometric amounts of FAD, which could be easily removed. AbeF and BorF can reduce FAD, FMN, and riboflavin in vitro and are selective for NADH over NADPH. Initial velocity studies in the presence and absence of inhibitors showed that BorF proceeds by a sequential ordered kinetic mechanism in which FAD binds first, while AbeF follows a random-ordered sequence of substrate binding. Fluorescence quenching experiments verified that NADH does not bind BorF in the absence of FAD, and that both AbeF and BorF bind FAD with higher affinity than FADH₂. pH-rate profiles of BorF and AbeF were bell-shaped with maximum k_cat at pH 7.5, and site-directed mutagenesis of BorF implicated His160 and Arg38 as contributing to the catalytic activity and the pH dependence.

Production of Enantiopure Chiral Epoxides with E. coli Expressing Styrene Monooxygenase

Styrene monooxygenases are a group of highly selective enzymes able to catalyse the epoxidation of alkenes to corresponding chiral epoxides in excellent enantiopurity. Chiral compounds containing oxirane ring or products of their hydrolysis represent key building blocks and precursors in organic synthesis in the pharmaceutical industry, and many of them are produced on an industrial scale. Two-component recombinant styrene monooxygenase (SMO) from Marinobacterium litorale was expressed as a fused protein (StyAL2StyB) in Escherichia coli BL21(DE3). By high cell density fermentation, 35 g_DCW/L of biomass with overexpressed SMO was produced. SMO exhibited excellent stability, broad substrate specificity, and enantioselectivity, as it remained active for months and converted a group of alkenes to corresponding chiral epoxides in high enantiomeric excess (˃95–99% ee). Optically pure (S)-4-chlorostyrene oxide, (S)-allylbenzene oxide, (2R,5R)-1,2:5,6-diepoxyhexane, 2-(3-bromopropyl)oxirane, and (S)-4-(oxiran-2-yl)butan-1-ol were prepared by whole-cell SMO.

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.

A new clade of styrene monooxygenases for (R)-selective epoxidation

Styrene monooxygenases (SMOs) are excellent enzymes for the production of (S)-enantiopure epoxides, but so far, only one (R)-selective SMO has been identified with a narrow substrate spectrum.

Biochemical Characterization of the Two-Component Flavin-Dependent Monooxygenase Involved in Valanimycin Biosynthesis

The flavin reductase (FRED) and isobutylamine N-hydroxylase (IBAH) from Streptomyces viridifaciens constitute a two-component, flavin-dependent monooxygenase system that catalyzes the first step in valanimycin biosynthesis. FRED is an oxidoreductase that provides the reduced flavin to IBAH, which then catalyzes the hydroxylation of isobutylamine (IBA) to isobutylhydroxylamine (IBHA). In this work, we used several complementary methods to investigate FAD binding, steady-state and rapid reaction kinetics, and enzyme-enzyme interactions in the FRED:IBAH system. The affinity of FRED for FAD_ox is higher than its affinity for FAD_red, consistent with its function as a flavin reductase. Conversely, IBAH binds FAD_red more tightly than FAD_ox, consistent with its role as a monooxygenase. FRED exhibits a strong preference (28-fold) for NADPH over NADH as the electron source for FAD reduction. Isothermal titration calorimetry was used to study the association of FRED and IBAH. In the presence of FAD, either oxidized or reduced, FRED and IBAH associate with a dissociation constant of 7-8 μM. No interaction was observed in the absence of FAD. These results are consistent with the formation of a protein-protein complex for direct transfer of reduced flavin from the reductase to the monooxygenase in this two-component system.

NfoR: Chromate Reductase or Flavin Mononucleotide Reductase?

Soil bacteria can detoxify Cr(VI) ions by reduction. Within the last 2 decades, numerous reports of chromate reductase enzymes have been published. These reports describe catalytic reduction of chromate ions by specific enzymes. These enzymes each have sequence similarity to known redox-active flavoproteins. We investigated the enzyme NfoR from Staphylococcus aureus, which was reported to be upregulated in chromate-rich soils and to have chromate reductase activity (H. Han, Z. Ling, T. Zhou, R. Xu, et al., Sci Rep 7:15481, 2017, https://doi.org/10.1038/s41598-017-15588-y). We show that NfoR has structural similarity to known flavin mononucleotide (FMN) reductases and reduces FMN as a substrate. NfoR binds FMN with a dissociation constant of 0.4 μM. The enzyme then binds NADPH with a dissociation constant of 140 μM and reduces the flavin at a rate of 1,350 s^-1 Turnover of the enzyme is apparently limited by the rate of product release that occurs, with a net rate constant of 0.45 s^-1 The rate of product release limits the rate of observed chromate reduction, so the net rate of chromate reduction by NfoR is orders of magnitude lower than when this process occurs in solution. We propose that NfoR is an FMN reductase and that the criterion required to define chromate reduction as enzymatic has not been met. That NfoR expression is increased in the presence of chromate suggests that the survival adaption was to increase the net rate of chromate reduction by facile, adventitious redox processes.IMPORTANCE Chromate is a toxic by-product of multiple industrial processes. Chromate reduction is an important biological activity that ameliorates Cr(VI) toxicity. Numerous researchers have identified chromate reductase activity by observing chromate reduction. However, all identified chromate reductase enzymes have flavin as a cofactor or use a flavin as a substrate. We show here that NfoR, an enzyme claimed to be a chromate reductase, is in fact an FMN reductase. In addition, we show that reduction of a flavin is a viable way to transfer electrons to chromate but that it is unlikely to be the native function of enzymes. We propose that upregulation of a redox-active flavoprotein is a viable means to detoxify chromate that relies on adventitious reduction that is not catalyzed.

Regenerable copper anode for the Cu(I)-mediated reduction of FAD in the electroenzymatic styrene epoxidation reaction

Styrene monooxygenase (SMO) is a two-component flavoenzyme composed of NADH-dependent flavin reductase (SMOB) and FAD-specific styrene epoxidase (NSMOA) components. The enantioselective styrene epoxidation reaction catalyzed by this enzyme can be streamlined for chemosynthetic applications by substituting NADH and the reductase with an electrode to supply the epoxidase with reducing equivalents required for catalysis. Slow kinetics of adsorption and desorption of FAD from the electrode surface and unproductive side reactions of the reduced flavin with oxygen limit the efficiency of direct electroenzymatic catalysis. In the present work we develop a miniature spectroelectrochemical cell equipped with a copper electrode for the anodic synthesis of Cu(I) chelates of EDTA, glutamate, and citrate as FAD-reducing agents, and a platinum electrode for the electrolytic generation of oxygen. Copper oxidized in the flavin reduction reaction can be reclaimed subsequently as copper metal at the electrode surface. About 80% transformation of styrene is achieved in a single cell cycle of reduction and oxygenation at pH 7 and 25 °C in good agreement with that predicted by numerical simulation. When the cell is operated in two successive cycles, styrene oxide can be synthesized with an electroenzymatic epoxidation activity of 663U/g in 94% yield. This approach to electroenzymatic catalysis shows promise for the quantitative transformation of styrene to styrene oxide and may be applied more generally to other flavoprotein monooxygenases.

Flavoenzymes for biocatalysis

Flavoenzymes are broadly employed as biocatalysts for a large variety of reactions, owing to the chemical versatility of the flavin cofactor. Oxidases set aside, many flavoenzymes require a source of electrons in form of the biological reductant nicotinamide NAD(P)H in order to initiate catalysis via the reduced flavin. Chemists can take advantage of the reactivity of reduced flavins with oxygen to carry out monooxygenation reactions, while the reduced flavin can also be used for formal hydrogenation reactions. The main advantage of these reactions compared to chemical approaches is the frequent regio-, chemo- and stereo-selectivity of the biocatalysts, which allows the synthesis of chiral molecules in optically active form. This chapter provides an overview of the variety of biocatalytic processes that have been developed with flavoenzymes, with a particular focus on nicotinamide-dependent enzymes. The diversity of molecules obtained is highlighted and in several cases, strategies that allow control of the stereochemical outcome of the reactions are reviewed.

Transcriptome analysis of crude oil degrading Pseudomonas aeruginosa strains for identification of potential genes involved in crude oil degradation

In the microbial world, bacteria are the most effective agents in petroleum hydrocarbons (PHs) degradation, utilization/mineralization and they serve as essential degraders of crude oil contaminated environment. Some genes and traits are involved in the hydrocarbon utilization process for which transcriptome analyses are important to identify differentially expressed genes (DEGs) among different conditions, leading to a new understanding of genes or pathways associated with crude oil degradation. In this work, three crude oil utilizing Pseudomonas aeruginosa strains designated as N002, TP16 and J001 subjected to transcriptome analyses revealed a total of 81, 269 and 137 significant DEGs. Among them are 80 up-regulated genes and one downregulated gene of N002, 121 up- regulated and 148 down-regulated genes of TP16, 97 up-regulated and 40 down-regulated genes of J001 which are involved in various metabolic pathways. TP16 strain has shown more number of DEGs upon crude oil treatment in comparison to the other two strains. Through quantitative real time polymerase chain reaction (qRT-PCR), the selected DEGs of each strain from transcriptome data were substantiated. The results have shown that the up- regulated and down-regulated genes observed by qRT-PCR were consistent with transcriptome data. Taken together, our transcriptome results have revealed that TP16 is a potential P. aeruginosa strain for functional analysis of identified potential DEGs involved in crude oil degradation.

A single amino acid substitution in aromatic hydroxylase (HpaB) of Escherichia coli alters substrate specificity of the structural isomers of hydroxyphenylacetate

BACKGROUND

A broad range of aromatic compounds can be degraded by enteric bacteria, and hydroxyphenylacetic acid (HPA) degrading bacteria are the most widespread. Majority of Escherichia coli strains can use both the structural isomers of HPA, 3HPA and 4HPA, as the sole carbon source, which are catabolized by the same pathway whose associated enzymes are encoded by hpa gene cluster. Previously, we observed that E. coli B REL606 grew only on 4HPA, while E. coli B BL21(DE3) grew on 3HPA as well as 4HPA.

RESULTS

In this study, we report that a single amino acid in 4-hydroxyphenylacetate 3-hydroxylase (HpaB) of E. coli determines the substrate specificity of HPA isomers. Alignment of protein sequences encoded in hpa gene clusters of BL21(DE3) and REL606 showed that there was a difference of only one amino acid (position 379 in HpaB) between the two, viz., Arg in BL21(DE3) and Cys in REL606. REL606 cells expressing HpaB having Arg379 could grow on 3HPA, whereas those expressing HpaB with Gly379 or Ser379 could not. Structural analysis suggested that the amino acid residue at position 379 of HpaB is located not in the active site, but in the vicinity of the 4HPA binding site, and that it plays an important role in mediating the entrance and stable binding of substrates to the active site.

CONCLUSIONS

The arginine residue at position 379 of HpaB is critical for 3HPA recognition. Information regarding the effect of amino acid residues on the substrate specificity of structural isomers can facilitate in designing hydoxylases with high catalytic efficiency and versatility.

Bioconversion of Biologically Active Indole Derivatives with Indole-3-Acetic Acid-Degrading Enzymes from Caballeronia glathei DSM50014

A plant auxin hormone indole-3-acetic acid (IAA) can be assimilated by bacteria as an energy and carbon source, although no degradation has been reported for indole-3-propionic acid and indole-3-butyric acid. While significant efforts have been made to decipher the Iac (indole-3-acetic acid catabolism)-mediated IAA degradation pathway, a lot of questions remain regarding the mechanisms of individual reactions, involvement of specific Iac proteins, and the overall reaction scheme. This work was aimed at providing new experimental evidence regarding the biodegradation of IAA and its derivatives. Here, it was shown that Caballeronia glathei strain DSM50014 possesses a full iac gene cluster and is able to use IAA as a sole source of carbon and energy. Next, IacE was shown to be responsible for the conversion of 2-oxoindole-3-acetic acid (Ox-IAA) intermediate into the central intermediate 3-hydroxy-2-oxindole-3-acetic acid (DOAA) without the requirement for IacB. During this reaction, the oxygen atom incorporated into Ox-IAA was derived from water. Finally, IacA and IacE were shown to convert a wide range of indole derivatives, including indole-3-propionic acid and indole-3-butyric acid, into corresponding DOAA homologs. This work provides novel insights into Iac-mediated IAA degradation and demonstrates the versatility and substrate scope of IacA and IacE enzymes.

Creating Flavin Reductase Variants with Thermostable and Solvent-Tolerant Properties by Rational-Design Engineering

We have employed computational approaches-FireProt and FRESCO-to predict thermostable variants of the reductase component (C₁ ) of (4-hydroxyphenyl)acetate 3-hydroxylase. With the additional aid of experimental results, two C₁ variants, A166L and A58P, were identified as thermotolerant enzymes, with thermostability improvements of 2.6-5.6 °C and increased catalytic efficiency of 2- to 3.5-fold. After heat treatment at 45 °C, both of the thermostable C₁ variants remain active and generate reduced flavin mononucleotide (FMNH^- ) for reactions catalyzed by bacterial luciferase and by the monooxygenase C₂ more efficiently than the wild type (WT). In addition to thermotolerance, the A166L and A58P variants also exhibited solvent tolerance. Molecular dynamics (MD) simulations (6 ns) at 300-500 K indicated that mutation of A166 to L and of A58 to P resulted in structural changes with increased stabilization of hydrophobic interactions, and thus in improved thermostability. Our findings demonstrated that improvements in the thermostability of C₁ enzyme can lead to broad-spectrum uses of C₁ as a redox biocatalyst for future industrial 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.

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.