β-Dicarbonyls Facilitate Engineered Microbial Bromoform Biosynthesis

Ruminant livestock produce around 24% of global anthropogenic methane emissions. Methanogenesis in the animal rumen is significantly inhibited by bromoform, which is abundant in seaweeds of the genus Asparagopsis. This has prompted the development of livestock feed additives based on Asparagopsis to mitigate methane emissions, although this approach alone is unlikely to satisfy global demand. Here we engineer a non-native biosynthesis pathway to produce bromoform in vivo with yeast as an alternative biological source that may enable sustainable, scalable production of bromoform by fermentation. β-dicarbonyl compounds with low pKa values were identified as essential substrates for bromoform production and enabled bromoform synthesis in engineered Saccharomyces cerevisiae expressing a vanadate-dependent haloperoxidase gene. In addition to providing a potential route to the sustainable biological production of bromoform at scale, this work advances the development of novel microbial biosynthetic pathways for halogenation.

Recent Progress on Peroxidase Modification and Application

Peroxdiase is one of the member of oxireductase super family, which has a broad substrate range and a variety of reaction types, including hydroxylation, epoxidation or halogenation of unactivated C-H bonds, and aromatic group or biophenol compounds. Here, we summarized the recently discovered enzymes with peroxidation activity, and focused on the special structures, sites, and corresponding strategies that can change the peroxidase catalytic activity, stability, and substrate range. The comparison of the structural differences between these natural enzymes and the mimic enzymes of binding nanomaterials and polymer materials is helpful to expand the application of peroxidase in industry. In addition, we also reviewed the catalytic application of peroxidase in the synthesis of important organic molecules and the degradation of pollutants.

Decarboxylative halogenation of indoles by vanadium haloperoxidases

Halogenated heteroarenes are key building blocks across numerous chemical industries. Here, we report that vanadium haloperoxidases are capable of producing 3-haloindoles through decarboxylative halogenation of 3-carboxyindoles. This biocatalytic method is applicable to decarboxylative chlorination, bromination, and iodination in moderate to high yields and with excellent chemoselectivity.

Organo-photoredox-Catalyzed Selective Mono- and Bis-C-H Alkylation of Electron-Rich (Hetero)Arenes

Herein, we disclose a simple strategy for the C-H alkylation of electron-rich (hetero)arenes with alkyl bromides employing visible-light-mediated organo-photocatalytic SET processes. The generality of this method has been evidenced by the inclusion of a variety of alkyl radicals (α-alkyl-carbonyl, benzyl, cyanomethyl) as well as diverse biologically active electron-rich arenes and (hetero)arenes under mild conditions. The extent of alkylation with alkyl bromides was found to be controlled by introducing Zn(OAc)2 as a bromide scavenger, ensuring the blocking of potential bromo-arene byproduct formation under photoredox conditions. In addition, a sequential C-H alkylation strategy for selective bis-alkylation has also been developed via chronological incorporation of different alkyl radical precursors in one pot quite efficiently.

Heterologous Expression and Biochemical Characterization of a New Chloroperoxidase Isolated from the Deep-Sea Hydrothermal Vent Black Yeast Hortaea werneckii UBOCC-A-208029

The initiation of this study relies on a targeted genome-mining approach to highlight the presence of a putative vanadium-dependent haloperoxidase-encoding gene in the deep-sea hydrothermal vent fungus Hortaea werneckii UBOCC-A-208029. To date, only three fungal vanadium-dependent haloperoxidases have been described, one from the terrestrial species Curvularia inaequalis, one from the fungal plant pathogen Botrytis cinerea, and one from a marine derived isolate identified as Alternaria didymospora. In this study, we describe a new vanadium chloroperoxidase from the black yeast H. werneckii, successfully cloned and overexpressed in a bacterial host, which possesses higher affinity for bromide (Km = 26 µM) than chloride (Km = 237 mM). The enzyme was biochemically characterized, and we have evaluated its potential for biocatalysis by determining its stability and tolerance in organic solvents. We also describe its potential three-dimensional structure by building a model using the AlphaFold 2 artificial intelligence tool. This model shows some conservation of the 3D structure of the active site compared to the vanadium chloroperoxidase from C. inaequalis but it also highlights some differences in the active site entrance and the volume of the active site pocket, underlining its originality.

How Is Substrate Halogenation Triggered by the Vanadium Haloperoxidase from Curvularia inaequalis?

Vanadium haloperoxidases (VHPOs) are unique enzymes in biology that catalyze a challenging halogen transfer reaction and convert a strong aromatic C-H bond into C-X (X = Cl, Br, I) with the use of a vanadium cofactor and H₂O₂. The VHPO catalytic cycle starts with the conversion of hydrogen peroxide and halide (X = Cl, Br, I) into hypohalide on the vanadate cofactor, and the hypohalide subsequently reacts with a substrate. However, it is unclear whether the hypohalide is released from the enzyme or otherwise trapped within the enzyme structure for the halogenation of organic substrates. A substrate-binding pocket has never been identified for the VHPO enzyme, which questions the role of the protein in the overall reaction mechanism. Probing its role in the halogenation of small molecules will enable further engineering of the enzyme and expand its substrate scope and selectivity further for use in biotechnological applications as an environmentally benign alternative to current organic chemistry synthesis. Using a combined experimental and computational approach, we elucidate the role of the vanadium haloperoxidase protein in substrate halogenation. Activity studies show that binding of the substrate to the enzyme is essential for the reaction of the hypohalide with substrate. Stopped-flow measurements demonstrate that the rate-determining step is not dependent on substrate binding but partially on hypohalide formation. Using a combination of molecular mechanics (MM) and molecular dynamics (MD) simulations, the substrate binding area in the protein is identified and even though the selected substrates (methylphenylindole and 2-phenylindole) have limited hydrogen-bonding abilities, they are found to bind relatively strongly and remain stable in a binding tunnel. A subsequent analysis of the MD snapshots characterizes two small tunnels leading from the vanadate active site to the surface that could fit small molecules such as hypohalide, halide, and hydrogen peroxide. Density functional theory studies using electric field effects show that a polarized environment in a specific direction can substantially lower barriers for halogen transfer. A further analysis of the protein structure indeed shows a large dipole orientation in the substrate-binding pocket that could enable halogen transfer through an applied local electric field. These findings highlight the importance of the enzyme in catalyzing substrate halogenation by providing an optimal environment to lower the energy barrier for this challenging aromatic halide insertion reaction.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

On-Demand Synthesis of Antiseptics at the Site of Infection for Treatment of Otitis Media

Otitis media (OM) is the main reason for pediatric antibiotic prescriptions. The current treatment mandates a rigorous regimen of multidose antibiotics over 5-10 days. The systemic antibiotic exposure and often prematurely terminated treatment due to the challenge of drug administration to young patients are believed to breed antibiotic resistance. To address these challenges, we designed a local treatment that converted a metabolic product (H2O2) of an OM pathogen (Streptococcus pneumoniae) into a potent antiseptic (HOBr), a reaction catalyzed by locally administered vanadium pentoxide nanowires. The therapeutic, HOBr, was only synthesized in the presence of the pathogen, enabling on-demand generation of therapeutics for OM treatment. Hypohalous acids are broad-spectrum and have a long history in general disinfection applications without breeding substantial drug resistance. A single dose of the nanowire formulation eradicated OM in a standard chinchilla model in 7 days with no observable tissue toxicity or negative impact on hearing sensitivity.

Structural Basis of Stereospecific Vanadium-Dependent Haloperoxidase Family Enzymes in Napyradiomycin Biosynthesis

Vanadium-dependent haloperoxidases (VHPOs) from Streptomyces bacteria differ from their counterparts in fungi, macroalgae, and other bacteria by catalyzing organohalogenating reactions with strict regiochemical and stereochemical control. While this group of enzymes collectively uses hydrogen peroxide to oxidize halides for incorporation into electron-rich organic molecules, the mechanism for the controlled transfer of highly reactive chloronium ions in the biosynthesis of napyradiomycin and merochlorin antibiotics sets the Streptomyces vanadium-dependent chloroperoxidases apart. Here we report high-resolution crystal structures of two homologous VHPO family members associated with napyradiomycin biosynthesis, NapH1 and NapH3, that catalyze distinctive chemical reactions in the construction of meroterpenoid natural products. The structures, combined with site-directed mutagenesis and intact protein mass spectrometry studies, afforded a mechanistic model for the asymmetric alkene and arene chlorination reactions catalyzed by NapH1 and the isomerase activity catalyzed by NapH3. A key lysine residue in NapH1 situated between the coordinated vanadate and the putative substrate binding pocket was shown to be essential for catalysis. This observation suggested the involvement of the ε-NH2, possibly through formation of a transient chloramine, as the chlorinating species much as proposed in structurally distinct flavin-dependent halogenases. Unexpectedly, NapH3 is modified post-translationally by phosphorylation of an active site His (τ-pHis) consistent with its repurposed halogenation-independent, α-hydroxyketone isomerase activity. These structural studies deepen our understanding of the mechanistic underpinnings of VHPO enzymes and their evolution as enantioselective biocatalysts.

On-the-Fly Mass Spectrometry in Digital Microfluidics Enabled by a Microspray Hole: Toward Multidimensional Reaction Monitoring in Automated Synthesis Platforms

We report an approach for the online coupling of digital microfluidics (DMF) with mass spectrometry (MS) using a chip-integrated microspray hole (μSH). The technique uses an adapted electrostatic spray ionization (ESTASI) method to spray a portion of a sample droplet through a microhole in the cover plate, allowing its chemical content to be analyzed by MS. This eliminates the need for chip disassembly or the introduction of capillary emitters for MS analysis, as required by state-of-the-art. For the first time, this allows the essential advantage of a DMF device─free droplet movement─to be retained during MS analysis. The broad applicability of the developed seamless coupling of DMF and mass spectrometry was successfully applied to the study of various on-chip organic syntheses as well as protein and peptide analysis. In the case of a Hantzsch synthesis, we were able to show that the method is very well suited for monitoring even rapid chemical reactions that are completed in a few seconds. In addition, the strength of the low resource consumption in such on-chip microsyntheses was demonstrated by the example of enzymatic brominations, for which only a minute amount of a special haloperoxidase is required in the droplet. The unique selling point of this approach is that the analyzed droplet remains completely movable after the MS measurement and is available for subsequent on-DMF chip processes. This is illustrated here for the example of MS analysis of the starting materials in the corresponding droplets before they are combined to investigate the reaction progress by DMF-MS further. This technology enables the ongoing and almost unlimited tracking of multistep chemical processes in a DMF chip and offers exciting prospects for transforming digital microfluidics into automated synthesis platforms.

Synthesis, Anticancer Potential and Comprehensive Toxicity Studies of Novel Brominated Derivatives of Bacterial Biopigment Prodigiosin from Serratia marcescens ATCC 27117

Prodigiosins (prodiginines) are a class of bacterial secondary metabolites with remarkable biological activities and color. In this study, optimized production, purification, and characterization of prodigiosin (PG) from easily accessible Serratia marcescens ATCC 27117 strain has been achieved to levels of 14 mg/L of culture within 24 h. Furthermore, environmentally friendly bromination of produced PG was used to afford both novel mono- and dibrominated derivatives of PG. PG and its Br derivatives showed anticancer potential with IC₅₀ values range 0.62–17.00 µg/mL for all tested cancer cell lines and induction of apoptosis but low selectivity against healthy cell lines. All compounds did not affect Caenorhabditiselegans at concentrations up to 50 µg/mL. However, an improved toxicity profile of Br derivatives in comparison to parent PG was observed in vivo using zebrafish (Danio rerio) model system, when 10 µg/mL applied at 6 h post fertilization caused death rate of 100%, 30% and 0% by PG, PG-Br, and PG-Br_2, respectively, which is a significant finding for further structural optimizations of bacterial prodigiosins. The drug-likeness of PG and its Br derivatives was examined, and the novel Br derivatives obey the Lipinski’s “rule of five”, with an exemption of being more lipophilic than PG, which still makes them good targets for further structural optimization.

Coastal observation of halocarbons in the Yellow Sea and East China Sea during winter: Spatial distribution and influence of different factors on the enzyme-mediated reactions

Volatile brominated compounds are important trace gases for stratospheric ozone chemistry. In this study, the spatial variations of dibromomethane (CH₂Br₂), bromodichloromethane (CHBrCl₂), dibromochloromethane (CHBr₂Cl) and bromoform (CHBr₃) in the seawater and overlying atmosphere were measured in the Yellow Sea (YS) and the East China Sea (ECS) in winter. The air-sea fluxes of CH₂Br₂, CHBrCl₂, CHBr₂Cl and CHBr₃ ranged from -11.46 to 25.33, -4.68 to 7.91, -8.60 to 4.08 and -88.57 to 8.84 nmol m^-2·d^-1, respectively. In order to understand the mechanism of halocarbons production, we measured bromoperoxidase (BrPO) activity (39.18-186.74 μU·L^-1) in the YS and ECS for the first time using an aminophenyl fluorescein (APF) method and performed in-situ incubation experiments in BrPO-treated seawater. The production rates of CH₂Br₂, CHBrCl₂, CHBr₂Cl and CHBr₃ ranged from 14.21 to 94.74, 0.00 to 19.74, 0.00 to 30.62 and 6.18-72.75 pmol L^-1·h^-1, respectively, in BrPO-treated seawater. There were significantly higher production rates in coastal waters compared with the open sea (P = 0.016) because of higher DOC levels near the coast. Moreover, the production rates of halocarbons increased with BrPO activity and H₂O₂ concentration. The results showed that enzyme-mediated reaction was an important source for the production of halocarbons in seawater. The present research is of great significance for understanding the production mechanisms of halocarbons in seawater and global oceanic halocarbons emissions.

Recent developments in the use of peroxygenases - Exploring their high potential in selective oxyfunctionalisations

Peroxygenases are an emerging new class of enzymes allowing selective oxyfunctionalisation reactions in a cofactor-independent way different from well-known P450 monooxygenases. Herein, we focused on recent developments from organic synthesis, molecular biotechnology and reaction engineering viewpoints that are devoted to bring these enzymes in industrial applications. This covers natural diversity from different sources, protein engineering strategies for expression, substrate scope, activity and selectivity, stabilisation of enzymes via immobilisation, and the use of peroxygenases in low water media. We believe that peroxygenases have much to offer for selective oxyfunctionalisations and we have much to study to explore the full potential of these versatile biocatalysts in organic synthesis.

Halogenases: a palette of emerging opportunities for synthetic biology-synthetic chemistry and C-H functionalisation

The enzymatic generation of carbon-halogen bonds is a powerful strategy used by both nature and synthetic chemists to tune the bioactivity, bioavailability and reactivity of compounds, opening up the opportunity for selective C-H functionalisation. Genes encoding halogenase enzymes have recently been shown to transcend all kingdoms of life. These enzymes install halogen atoms into aromatic and less activated aliphatic substrates, achieving selectivities that are often challenging to accomplish using synthetic methodologies. Significant advances in both halogenase discovery and engineering have provided a toolbox of enzymes, enabling the ready use of these catalysts in biotransformations, synthetic biology, and in combination with chemical catalysis to enable late stage C-H functionalisation. With a focus on substrate scope, this review outlines the mechanisms employed by the major classes of halogenases, while in parallel, it highlights key advances in the utilisation of the combination of enzymatic halogenation and chemical catalysis for C-H activation and diversification.

Obligate Brominating Enzymes Underlie Bromoform Production by Marine Cyanobacteria

Marine algae are prolific producers of bromoform (CHBr₃ ). This naturally produced molecule is a potent environmental pollutant as it volatilizes into the atmosphere and contributes to depletion of the ozone layer in a manner akin to, and in magnitude similar to, man-made chlorofluorocarbons. While phototrophs such as seaweeds, diatoms, and dinoflagellates are known sources of bromoform, additional as yet unknown biogenetic sources of bromoform exist in the oceans. Here, using halogenating enzymes as diagnostic genetic elements, we demonstrate that marine cyanobacteria also possess the enzymological potential for bromoform production. Using recombinantly purified vanadium-dependent bromoperoxidases from planktonic and bloom-forming marine cyanobacteria in in vitro biochemical assays, we reconstitute the enzymatic production of bromoform. We find cyanobacterial bromoform synthesizing enzymes to be obligate brominases possessing no chlorinating activities. These results expand the repertoire of marine biotic sources that introduce this pollutant in the atmosphere.

Biocatalysis Fueled by Light: On the Versatile Combination of Photocatalysis and Enzymes

Enzymes catalyze a plethora of highly specific transformations under mild and environmentally benign reaction conditions. Their fascinating performances attest to high synthetic potential that is often hampered by operational obstacles such as in vitro cofactor supply and regeneration. Exploiting light and combining it with biocatalysis not only helps in overcoming these drawbacks, but the fruitful liaison of these two fields of "green chemistry" also offers opportunities to unlock new synthetic reactivities. In this review we provide an overview of the wide variety of photo-biocatalysis, ranging from the photochemical delivery of electrons required in redox biocatalysis and photochemical cofactor and reagent (re)generation to direct photoactivation of enzymes enabling reactions unknown in nature. We highlight synthetically relevant transformations such as asymmetric reactions facilitated by the combination of light as energy source and enzymes' catalytic power.

Haloperoxidases as catalysts in organic synthesis

The current state-of-the-art of haloperoxidase catalysis in organic synthesis for halogenation reactions is presented in this review.

Total Enzyme Syntheses of Napyradiomycins A1 and B1

The biosynthetic route to the napyradiomycin family of bacterial meroterpenoids has been fully described 32 years following their original isolation and 11 years after their gene cluster discovery. The antimicrobial and cytotoxic natural products napyradiomycins A1 and B1 are produced using three organic substrates (1,3,6,8-tetrahydroxynaphthalene, dimethylallyl pyrophosphate, and geranyl pyrophosphate), and catalysis via five enzymes: two aromatic prenyltransferases (NapT8 and T9); and three vanadium dependent haloperoxidase (VHPO) homologues (NapH1, H3, and H4). Building upon the previous characterization of NapH1, H3, and T8, we herein describe the initial (NapT9, H1) and final (NapH4) steps required for napyradiomycin construction. This remarkably streamlined biosynthesis highlights the utility of VHPO enzymology in complex natural product generation, as NapH4 efficiently performs a unique chloronium-induced terpenoid cyclization to establish two stereocenters and a new carbon-carbon bond, and dual-acting NapH1 catalyzes chlorination and etherification reactions at two distinct stages of the pathway. Moreover, we employed recombinant napyradiomycin biosynthetic enzymes to chemoenzymatically synthesize milligram quantities in one pot in 1 day. This method represents a viable enantioselective approach to produce complex halogenated metabolites, like napyradiomycin B1, that have yet to be chemically synthesized.

Biocatalytic Oxidation Reactions: A Chemist's Perspective

Oxidation chemistry using enzymes is approaching maturity and practical applicability in organic synthesis. Oxidoreductases (enzymes catalysing redox reactions) enable chemists to perform highly selective and efficient transformations ranging from simple alcohol oxidations to stereoselective halogenations of non-activated C-H bonds. For many of these reactions, no "classical" chemical counterpart is known. Hence oxidoreductases open up shorter synthesis routes based on a more direct access to the target products. The generally very mild reaction conditions may also reduce the environmental impact of biocatalytic reactions compared to classical counterparts. In this Review, we critically summarise the most important recent developments in the field of biocatalytic oxidation chemistry and identify the most pressing bottlenecks as well as promising solutions.

X-ray Diffraction and Density Functional Theory Provide Insight into Vanadate Binding to Homohexameric Bromoperoxidase II and the Mechanism of Bromide Oxidation

X-ray diffraction of native bromoperoxidase II (EC 1.11.1.18) from the brown alga Ascophyllum nodosum reveals at a resolution of 2.26 Å details of orthovanadate binding and homohexameric protein organization. Three dimers interwoven in contact regions and tightened by hydrogen-bond-clamped guanidinium stacks along with regularly aligned water molecules form the basic structure of the enyzme. Intra- and intermolecular disulfide bridges further stabilize the enzyme preventing altogether the protein from denaturing up to a temperature of 90 °C, as evident from dynamic light scattering and the on-gel ortho-dianisidine assay. Every monomer binds one equivalent of orthovanadate in a cavity formed from side chains of three histidines, two arginines, one lysine, serine, and tryptophan. Protein binding occurs primarily through hydrogen bridges and superimposed by Coulomb attraction according to thermochemical model on density functional level of theory (B3LYP/6-311++G**). The strongest attractor is the arginine side chain mimic N-methylguanidinium, enhancing in positive cooperative manner hydrogen bridges toward weaker acceptors, such as residues from lysine and serine. Activating hydrogen peroxide occurs in the thermochemical model by side-on binding in orthovanadium peroxoic acid, oxidizing bromide with virtually no activation energy to hydrogen bonded hypobromous acid.

Marine Vanadium-Dependent Haloperoxidases, Their Isolation, Characterization, and Application

Vanadium-dependent haloperoxidases in seaweeds, cyanobacteria, fungi, and possibly phytoplankton play an important role in the release of halogenated volatile compounds in the environment. These halocarbons have effects on atmospheric chemistry since they cause ozone depletion. In this chapter, a survey is given of the different sources of these enzymes, some of their properties, the various methods to isolate them, and the bottlenecks in purification. The assays to detect and quantify haloperoxidase activity are described as well as their kinetic properties. Several practical tips and pitfalls are given which have not yet been published explicitly. Recent developments in research on structure and function of these enzymes are reviewed. Finally, the application of vanadium-dependent haloperoxidases in the biosynthesis of brominated and other compounds is discussed.

Halofunctionalization of alkenes by vanadium chloroperoxidase from Curvularia inaequalis

The vanadium-dependent chloroperoxidase from Curvularia inaequalis is a stable and efficient biocatalyst for the hydroxyhalogenation of a broad range of alkenes into halohydrins. Up to 1 200 000 TON with 69 s^-1 TOF were observed for the biocatalyst. A bienzymatic cascade to yield epoxides as reaction products is presented.

Selective aerobic oxidation reactions using a combination of photocatalytic water oxidation and enzymatic oxyfunctionalisations

Peroxygenases offer attractive means to address challenges in selective oxyfunctionalisation chemistry. Despite their attractiveness, the application of peroxygenases in synthetic chemistry remains challenging due to their facile inactivation by the stoichiometric oxidant (H₂O₂). Often atom inefficient peroxide generation systems are required, which show little potential for large scale implementation. Here we show that visible light-driven, catalytic water oxidation can be used for in situ generation of H₂O₂ from water, rendering the peroxygenase catalytically active. In this way the stereoselective oxyfunctionalisation of hydrocarbons can be achieved by simply using the catalytic system, water and visible light.

Microbial Transformation of Iodine: From Radioisotopes to Iodine Deficiency

Iodine is a biophilic element that is important for human health, both as an essential component of several thyroid hormones and, on the other hand, as a potential carcinogen in the form of radioiodine generated by anthropogenic nuclear activity. Iodine exists in multiple oxidation states (-1, 0, +1, +3, +5, and +7), primarily as molecular iodine (I₂), iodide (I^-), iodate [Formula: see text] , or organic iodine (org-I). The mobility of iodine in the environment is dependent on its speciation and a series of redox, complexation, sorption, precipitation, and microbial reactions. Over the last 15years, there have been significant advances in iodine biogeochemistry, largely spurred by renewed interest in the fate of radioiodine in the environment. We review the biogeochemistry of iodine, with particular emphasis on the microbial processes responsible for volatilization, accumulation, oxidation, and reduction of iodine, as well as the exciting technological potential of these fascinating microorganisms and enzymes.