Identification and Characterization of the First Ovothiol Biosynthetic Enzyme

Structural and functional analysis of SAM-dependent N-methyltransferases involved in ovoselenol and ovothiol biosynthesis

Thio/selenoimidazole Nπ-methyltransferases are an emerging family of enzymes catalyzing the final step in the production of the S/Se-containing histidine-derived antioxidants ovothiol and ovoselenol. These enzymes, prevalent in prokaryotes, show minimal sequence similarity to other methyltransferases, and the structural determinants of their reactivities remain poorly understood. Herein, we report ligand-bound crystal structures of OvsM from the ovoselenol pathway as well as a member of a previously unknown clade of standalone ovothiol-biosynthetic Nπ-methyltransferases, which we have designated OvoM. Unlike previously reported ovothiol methyltransferases, which are fused as a C-terminal domain to the sulfoxide synthase OvoA, OvoMs function independently. Comparative structural analyses reveal conserved, ligand-induced conformational changes, suggesting similar behavior in dual-domain OvoA enzymes. Mutagenesis supports a model where OvoA domain rearrangement facilitates substrate recognition via a critical Tyr residue in the domain linker. Biochemical studies identify an essential active-site Asp, likely serving as a catalytic base in the SN2-like nucleophilic substitution reaction.

Molecular evolution of ovothiol biosynthesis in animal life reveals diversity of the natural antioxidant ovothiols in Cnidaria

Sulfoxide synthase OvoA is the key enzyme involved in the biosynthesis of ovothiols (OSHs), secondary metabolites endowed with unique antioxidant properties. Understanding the evolution of such enzymes and the diversity of their metabolites should reveal fundamental mechanisms governing redox signaling and environmental adaptation. "Early-branching" animals such as Cnidaria display unique molecular diversity and symbiotic relationships responsible for the biosynthesis of natural products, however, they have been neglected in previous research on antioxidants and OSHs. In this work, we have integrated genome and transcriptome mining with biochemical analyses to study the evolution and diversification of OSHs biosynthesis in cnidarians. By tracing the history of the ovoA gene, we inferred its loss in the latest common ancestor of Medusozoa, followed by the acquisition of a unique ovoB/ovoA chimaeric gene in Hydrozoa, likely through a horizontal gene transfer from dinoflagellate donors. While Anthozoa (corals and anemones), bearing canonical ovoA genes, produced a striking variety of OSHs (A, B, and C), the multifunctional enzyme in Hydrozoa was related to OSH B biosynthesis, as shown in Clytia hemisphaerica. Surprisingly, the ovoA-lacking jellyfish Aurelia aurita and Pelagia noctiluca also displayed OSHs, and we provided evidence of their incorporation from external sources. Finally, transcriptome mining revealed ovoA conserved expression pattern during larval development from Cnidaria to more evolved organisms and its regulation by external stimuli, such as UV exposure. The results of our study shed light on the origin and diversification of OSH biosynthesis in basal animals and highlight the importance of redox-active molecules from ancient metazoans as cnidarians to vertebrates.

New Frontiers in Nonheme Enzymatic Oxyferryl Species

Non-heme mononuclear iron dependent (NHM-Fe) enzymes exhibit exceedingly diverse catalytic reactivities. Despite their catalytic versatilities, the mononuclear iron centers in these enzymes show a relatively simple architecture, in which an iron atom is ligated with 2-4 amino acid residues, including histidine, aspartic or glutamic acid. In the past two decades, a common high-valent reactive iron intermediate, the S=2 oxyferryl (Fe(IV)-oxo or Fe(IV)=O) species, has been repeatedly discovered in NHM-Fe enzymes containing a 2-His-Fe or 2-His-1-carboxylate-Fe center. However, for 3-His/4-His-Fe enzymes, no common reactive intermediate has been identified. Recently, we have spectroscopically characterized the first S=1 Fe(IV) intermediate in a 3-His-Fe containing enzyme, OvoA, which catalyzes a novel oxidative carbon-sulfur bond formation. In this review, we summarize the broad reactivities demonstrated by S=2 Fe(IV)-oxo intermediates, the discovery of the first S=1 Fe(IV) intermediate in OvoA and the mechanistic implication of such a discovery, and the intrinsic reactivity differences of the S=2 and the S=1 Fe(IV)-oxo species. Finally, we postulate the possible reasons to utilize an S=1 Fe(IV) species in OvoA and their implications to other 3-His/4-His-Fe enzymes.

Structural insights into the convergent evolution of sulfoxide synthase EgtB-IV, an ergothioneine-biosynthetic homolog of ovothiol synthase OvoA

Non-heme iron-dependent sulfoxide/selenoxide synthases (NHISS) constitute a unique metalloenzyme class capable of installing a C-S/Se bond onto histidine to generate thio/selenoimidazole antioxidants, such as ergothioneine and ovothiol. These natural products are increasingly recognized for their health benefits. Among associated ergothioneine-biosynthetic enzymes, type IV EgtBs stand out, as they exhibit low sequence similarity with other EgtB subfamilies due to their recent divergence from the ovothiol-biosynthetic enzyme OvoA. Herein, we present crystal structures of two representative EgtB-IV enzymes, offering insights into the basis for this evolutionary convergence and enhancing our understanding of NHISS active site organization more broadly. The ability to interpret how key residues modulate substrate specificity and regioselectivity has implications for downstream identification of divergent reactivity within the NHISS family. To this end, we identify a previously unclassified clade of OvoA-like enzymes with a seemingly hybrid set of characteristics, suggesting they may represent an evolutionary intermediate between OvoA and EgtB-IV.

Traces of convergent evolution left in the structure of EgtB-IV

The enzymatically regioselective catalyzed incorporation of cysteine sulfoxide into histidine generates physiologically important antioxidants such as ergothioneine and ovothiol. In this issue of Structure, Ireland et al.1 report the crystal structure of EgtB-IV, which provides insights into the convergent evolution of sulfoxide synthase.

Discovery of the selenium-containing antioxidant ovoselenol derived from convergent evolution

Selenium is an essential micronutrient, but its presence in biology has been limited to protein and nucleic acid biopolymers. The recent identification of a biosynthetic pathway for selenium-containing small molecules suggests that there is a larger family of selenometabolites that remains to be discovered. Here we identify a recently evolved branch of abundant and uncharacterized metalloenzymes that we predict are involved in selenometabolite biosynthesis using a bioinformatic search strategy that relies on the mapping of composite active site motifs. Biochemical studies confirm this prediction and show that these enzymes form an unusual C-Se bond onto histidine, thus giving rise to a distinct selenometabolite and potent antioxidant that we have termed ovoselenol. Aside from providing insights into the evolution of this enzyme class and the structural basis of C-Se bond formation, our work offers a blueprint for charting the microbial selenometabolome in the future.

Enzymatic strategies for selenium incorporation into biological molecules

The trace element selenium (Se) is essential to the physiology of most organisms on the planet. The most well documented of Se's biological forms are selenoproteins, where selenocysteine often serves as the catalytic center for crucial redox processes. Se is also found in several other classes of biological molecules, including nucleic acids, sugars, and modified amino acids, although its role in the function of these metabolites is less understood. Despite its prevalence, only a small number of Se-specific biosynthetic pathways have been discovered. Around half of these were first characterized in the past three years, suggesting that the selenometabolome may be more diverse than previously appreciated. Here, we review the recent advances in our understanding of this intriguing biochemical space, and discuss prospects for future discovery efforts.

Solution-Phase Late-Stage Chemoselective Photocatalytic Removal of Sulfonyl and Phenacyl Groups in Peptides

Herein, BPC catalyzed visible-light-triggered target-specific late-stage solution phase desulfonylation from tryptophan in oligopeptides is portrayed by overcoming the isolation issue up to octamers. This robust and mild method is highly predictable and chemoselective, tolerating myriad of functional groups in aza-heteroaromatics and peptides. Interestingly, reductive desulfonylation is also amenable to biologically significant reactive histidine and tyrosine side chains, signifying the versatility of the strategy. Additional efficacy of BPC is demonstrated by solution phase phenacyl deprotection from C-terminal in peptides. Furthermore, excellent catalyst loading of 0.5 mol% and recyclability demonstrate the practical utility and applicability of this strategy.

Ovoselenol, a Selenium-containing Antioxidant Derived from Convergent Evolution

Selenium is an essential micronutrient, but its presence in biology has been limited to protein and nucleic acid biopolymers. The recent identification of the first biosynthetic pathway for selenium-containing small molecules suggests that there is a larger family of selenometabolites that remains to be discovered. Using a bioinformatic search strategy that relies on mapping of composite active site motifs, we identify a recently evolved branch of abundant and uncharacterized metalloenzymes that we predict are involved in selenometabolite biosynthesis. Biochemical studies confirm this prediction and show that these enzymes form an unusual C-Se bond onto histidine, thus giving rise to a novel selenometabolite and potent antioxidant that we have termed ovoselenol. Aside from providing insights into the evolution of this enzyme class and the structural basis of C-Se bond formation, our work offers a blueprint for charting the microbial selenometabolome in the future.

Unusual cysteine modifications in natural product biosynthesis

l-Cysteine is a highly reactive amino acid that is modified into a variety of chemical structures, including cysteine sulfinic acid in human metabolic pathways, and sulfur-containing scaffolds of amino acids, alkaloids, and peptides in natural product biosynthesis. Among the modification enzymes responsible for these cysteine-derived compounds, metalloenzymes constitute an important family of enzymes that catalyze a wide variety of reactions. Therefore, understanding their reaction mechanisms is important for the biosynthetic production of cysteine-derived natural products. This review mainly summarizes recent mechanistic investigations of metalloenzymes, with a particular focus on recently discovered mononuclear non-heme iron (NHI) enzymes, dinuclear NHI enzymes, and radical-SAM enzymes involved in unusual cysteine modifications in natural product biosynthesis.

Ergothioneine and its congeners: anti-ageing mechanisms and pharmacophore biosynthesis

Ergothioneine, Ovothiol, and Selenoneine are sulfur/selenium-containing histidine-derived natural products widely distributed across different organisms. They exhibit significant antioxidant properties, making them as potential lead compounds for promoting health. Increasing evidence suggests that Ergothioneine is positively correlated with healthy ageing and longevity. The mechanisms underlying Ergothioneine's regulation of the ageing process at cellular and molecular levels are beginning to be understood. In this review, we provide an in-depth and extensive coverage of the anti-ageing studies on Ergothioneine and discuss its possible intracellular targeting pathways. In addition, we highlight the recent efforts in elucidating the biosynthetic details for Ergothioneine, Ovothiol, and Selenoneine, with a particular focus on the study of their pharmacophore-forming enzymology.

Biochemical and Structural Characterization of OvoATh2: A Mononuclear Nonheme Iron Enzyme from Hydrogenimonas thermophila for Ovothiol Biosynthesis

Ovothiol A and ergothioneine are thiol-histidine derivatives with sulfur substitutions at the δ-carbon or ε-carbon of the l-histidine imidazole ring, respectively. Both ovothiol A and ergothioneine have protective effects on many aging-related diseases, and the sulfur substitution plays a key role in determining their chemical and biological properties, while factors governing sulfur incorporation regioselectivities in ovothiol and ergothioneine biosynthesis in the corresponding enzymes (OvoA, Egt1, or EgtB) are not yet known. In this study, we have successfully obtained the first OvoA crystal structure, which provides critical information to explain their C-S bond formation regioselectivity. Furthermore, OvoATh2 exhibits several additional activities: (1) ergothioneine sulfoxide synthase activity akin to Egt1 in ergothioneine biosynthesis; (2) cysteine dioxygenase activity using l-cysteine and l-histidine analogues as substrates; (3) cysteine dioxygenase activity upon mutation of an active site tyrosine residue (Y406). The structural insights and diverse chemistries demonstrated by OvoATh2 pave the way for future comprehensive structure-function correlation studies.

An S=1 Iron(IV) Intermediate Revealed in a Non-Heme Iron Enzyme-Catalyzed Oxidative C-S Bond Formation

Ergothioneine (ESH) and ovothiol A (OSHA) are two natural thiol-histidine derivatives. ESH has been implicated as a longevity vitamin and OSHA inhibits the proliferation of hepatocarcinoma. The key biosynthetic step of ESH and OSHA in the aerobic pathways is the O2 -dependent C-S bond formation catalyzed by non-heme iron enzymes (e.g., OvoA in ovothiol biosynthesis), but due to the lack of identification of key reactive intermediate the mechanism of this novel reaction is unresolved. In this study, we report the identification and characterization of a kinetically competent S=1 iron(IV) intermediate supported by a four-histidine ligand environment (three from the protein residues and one from the substrate) in enabling C-S bond formation in OvoA from Methyloversatilis thermotoleran, which represents the first experimentally observed intermediate spin iron(IV) species in non-heme iron enzymes. Results reported in this study thus set the stage to further dissect the mechanism of enzymatic oxidative C-S bond formation in the OSHA biosynthesis pathway. They also afford new opportunities to study the structure-function relationship of high-valent iron intermediates supported by a histidine rich ligand environment.

The first genetic engineered system for ovothiol biosynthesis in diatoms reveals a mitochondrial localization for the sulfoxide synthase OvoA

Diatoms represent one of the most abundant groups of microalgae in the ocean and are responsible for approximately 20% of photosynthetically fixed CO2 on Earth. Due to their complex evolutionary history and ability to adapt to different environments, diatoms are endowed with striking molecular biodiversity and unique metabolic activities. Their high growth rate and the possibility to optimize their biomass make them very promising 'biofactories' for biotechnological applications. Among bioactive compounds, diatoms can produce ovothiols, histidine-derivatives, endowed with unique antioxidant and anti-inflammatory properties, and occurring in many marine invertebrates, bacteria and pathogenic protozoa. However, the functional role of ovothiols biosynthesis in organisms remains almost unexplored. In this work, we have characterized the thiol fraction of Phaeodactylum tricornutum, providing the first evidence of the presence of ovothiol B in pennate diatoms. We have used P. tricornutum to overexpress the 5-histidylcysteine sulfoxide synthase ovoA, the gene encoding the key enzyme involved in ovothiol biosynthesis and we have discovered that OvoA localizes in the mitochondria, a finding that uncovers new concepts in cellular redox biochemistry. We have also obtained engineered biolistic clones that can produce higher amount of ovothiol B compared to wild-type cells, suggesting a new strategy for the eco-sustainable production of these molecules.

Stable Isotope-Guided Metabolomics Reveals Polar-Functionalized Fatty-Acylated RiPPs from Streptomyces

Ribosomally synthesized and posttranslationally modified peptides (RiPPs) with polar-functionalized fatty acyl groups are a rarely found untapped class of natural products. Although polar-functionalized fatty-acylated RiPPs (PFARs) have potential as antimicrobial agents, the repertoire is still limited. Therefore, expanding the chemical space is expected to contribute to the development of pharmaceutical agents. In this study, we performed genome mining and stable isotope-guided comparative metabolomics to discover new PFAR natural products. We focused on the feature that PFARs incorporate l-arginine or l-lysine as the starter unit of the fatty acyl group and fed ¹³C₆,¹⁵N₄-l-arginine or ¹³C₆,¹⁵N₂-l-lysine to bacterial cultures. Metabolites were extracted and compared with those extracted from nonlabeled l-arginine or l-lysine fed cultures. We identified putative PFARs and successfully isolated solabiomycin A and B from Streptomyces lydicus NBRC 13 058 and albopeptin B from Streptomyces nigrescens HEK616, which contained a sulfoxide group in the labionin moiety. The gene disruption experiment indicated that solS, which encodes a putative flavin adenine dinucleotide (FAD)-nicotinamide adenine dinucleotide (phosphate) (NAD(P))-binding protein, is involved in the sulfoxidation of aryl sulfides. The solabiomycins showed antibacterial activity against Gram-positive bacteria, including Mycobacterium tuberculosis H37Rv with a minimum 95% inhibitory concentration (MIC₉₅) of 3.125 μg/mL, suggesting their potential as antituberculosis agents.

Unique thiol metabolism in trypanosomatids: Redox homeostasis and drug resistance

Trypanosomatids are mainly responsible for heterogeneous parasitic diseases: Leishmaniasis, Sleeping sickness, and Chagas disease and control of these diseases implicates serious challenges due to the emergence of drug resistance. Redox-active biomolecules are the endogenous substances in organisms, which play important role in the regulation of redox homeostasis. The redox-active substances like glutathione, trypanothione, cysteine, cysteine persulfides, etc., and other inorganic intermediates (hydrogen peroxide, nitric oxide) are very useful as defence mechanism. In the present review, the suitability of trypanothione and other essential thiol molecules of trypanosomatids as drug targets are described in Leishmania and Trypanosoma. We have explored the role of tryparedoxin, tryparedoxin peroxidase, ascorbate peroxidase, superoxide dismutase, and glutaredoxins in the anti-oxidant mechanism and drug resistance. Up-regulation of some proteins in trypanothione metabolism helps the parasites in survival against drug pressure (sodium stibogluconate, Amphotericin B, etc.) and oxidative stress. These molecules accept electrons from the reduced trypanothione and donate their electrons to other proteins, and these proteins reduce toxic molecules, neutralize reactive oxygen, or nitrogen species; and help parasites to cope with oxidative stress. Thus, a better understanding of the role of these molecules in drug resistance and redox homeostasis will help to target metabolic pathway proteins to combat Leishmaniasis and trypanosomiases.

Charge Maintenance during Catalysis in Nonheme Iron Oxygenases

Here, the choice of the first coordination shell of the metal center is analyzed from the perspective of charge maintenance in a binary enzyme-substrate complex and an O₂-bound ternary complex in the nonheme iron oxygenases. Comparing homogentisate 1,2-dioxygenase and gentisate dioxygenase highlights the significance of charge maintenance after substrate binding as an important factor that drives the reaction coordinate. We then extend the charge analysis to several common types of nonheme iron oxygenases containing either a 2-His-1-carboxylate facial triad or a 3-His or 4-His ligand motif, including extradiol and intradiol ring-cleavage dioxygenases, thiol dioxygenases, α-ketoglutarate-dependent oxygenases, and carotenoid cleavage oxygenases. After forming the productive enzyme-substrate complex, the overall charge of the iron complex at the 0, +1, or +2 state is maintained in the remaining catalytic steps. Hence, maintaining a constant charge is crucial to promote the reaction of the iron center beginning from the formation of the Michaelis or ternary complex. The charge compensation to the iron ion is tuned not only by protein-derived carboxylate ligands but also by substrates. Overall, these analyses indicate that charge maintenance at the iron center is significant when all the necessary components form a productive complex. This charge maintenance concept may apply to most oxygen-activating metalloenzymes systems that do not draw electrons and protons step-by-step from a separate reactant, such as NADH, via a reductase. The charge maintenance perception may also be useful in proposing catalytic pathways or designing prototypical reactions using artificial or engineered enzymes for biotechnological applications.

Mitochondrial Ca2+ and Reactive Oxygen Species in Trypanosomatids

Significance: Millions of people are infected with trypanosomatids and new therapeutic approaches are needed. Trypanosomatids possess one mitochondrion per cell and its study has led to discoveries of general biological interest. These mitochondria, as in their animal counterparts, generate reactive oxygen species (ROS) and have evolved enzymatic and nonenzymatic defenses against them. Mitochondrial calcium ion (Ca2+) overload leads to generation of ROS and its study could lead to relevant information on the biology of trypanosomatids and to novel drug targets. Recent Advances: Mitochondrial Ca2+ is normally involved in maintaining the bioenergetics of trypanosomes, but when Ca2+ overload occurs, it is associated with cell death. Trypanosomes lack key players in the mechanism of cell death described in mammalian cells, although mitochondrial Ca2+ overload results in collapse of their membrane potential, production of ROS, and cytochrome c release. They are also very resistant to mitochondrial permeability transition, and cell death after mitochondrial Ca2+ overload depends on generation of ROS. Critical Issues: In this review, we consider the mechanisms of mitochondrial oxidant generation and removal and the involvement of Ca2+ in trypanosome cell death. Future Directions: More studies are required to determine the reactions involved in generation of ROS by the mitochondria of trypanosomatids, their enzymatic and nonenzymatic defenses against ROS, and the occurrence and composition of a mitochondrial permeability transition pore. Antioxid. Redox Signal. 36, 969-983.

Ergothioneine, Ovothiol A, and Selenoneine-Histidine-Derived, Biologically Significant, Trace Global Alkaloids

The history, chemistry, biology, and biosynthesis of the globally occurring histidine-derived alkaloids ergothioneine (10), ovothiol A (11), and selenoneine (12) are reviewed comparatively and their significance to human well-being is discussed.

A Survey on the Distribution of Ovothiol and ovoA Gene Expression in Different Tissues and Cells: A Comparative Analysis in Sea Urchins and Mussels

Ovothiols are histidine-derived thiols produced by a variety of marine invertebrates, protists and bacteria. These compounds, which are among the strongest natural antioxidants, are involved in controlling the cellular redox balance due to their redox exchange with glutathione. Although ovothiols were initially reported as protective agents against environmental stressors, new evidence suggests that they can also act as pheromones and participate in fundamental biological processes such as embryogenesis. To get further insight into the biological roles of ovothiols, we compared ovothiol biosynthesis in the sea urchin Paracentrotus lividus and in the mussel Mytilus galloprovincialis, the two species that represent the richest sources of these compounds among marine invertebrates. Ovothiol content was measured in different tissues and in the immune cells from both species and the expression levels of ovoA, the gene responsible for ovothiol biosynthesis, was inferred from publicly available transcriptomes. A comparative analysis of ovothiol biosynthesis in the two species allowed the identification of the tissues and cells synthesizing the metabolite and highlighted analogies and differences between sea urchins and mussels. By improving our knowledge on the biological roles of ovothiols and pointing out the existence of sustainable natural sources for their isolation, this study provides the basis for future biotechnological investigations on these valuable compounds.

OvoAMtht from Methyloversatilis thermotolerans ovothiol biosynthesis is a bifunction enzyme: thiol oxygenase and sulfoxide synthase activities

Mononuclear non-heme iron enzymes are a large class of enzymes catalyzing a wide-range of reactions. In this work, we report that a non-heme iron enzyme in Methyloversatilis thermotolerans, OvoA_Mtht, has two different activities, as a thiol oxygenase and a sulfoxide synthase. When cysteine is presented as the only substrate, OvoA_Mtht is a thiol oxygenase. In the presence of both histidine and cysteine as substrates, OvoA_Mtht catalyzes the oxidative coupling between histidine and cysteine (a sulfoxide synthase). Additionally, we demonstrate that both substrates and the active site iron's secondary coordination shell residues exert exquisite control over the dual activities of OvoA_Mtht (sulfoxide synthase vs. thiol oxygenase activities). OvoA_Mtht is an excellent system for future detailed mechanistic investigation on how metal ligands and secondary coordination shell residues fine-tune the iron-center electronic properties to achieve different reactivities.

Modulation of OvoA_Mtht's dual activities: sulfoxide synthase and thiol oxygenase.

First Report of OvoA Gene in Marine Arthropods: A New Candidate Stress Biomarker in Copepods

Ovothiol is one of the most powerful antioxidants acting in marine organisms as a defense against oxidative stress during development and in response to environmental cues. The gene involved in the ovothiol biosynthesis, OvoA, is found in almost all metazoans, but open questions existed on its presence among arthropods. Here, using an in silico workflow, we report a single OvoA gene in marine arthropods including copepods, decapods, and amphipods. Phylogenetic analyses indicated that OvoA from marine arthropods separated from the other marine phyla (e.g., Porifera, Mollusca) and divided into two separate branches, suggesting a possible divergence through evolution. In the copepod Calanus finmarchicus, we suggest that OvoA has a defense role in oxidative stress as shown by its high expression in response to a toxic diet and during the copepodite stage, a developmental stage that includes significant morphological changes. Overall, the results of our study open possibilities for the use of OvoA as a biomarker of stress in copepods and possibly also for other marine holozooplankters. The finding of OvoA in copepods is also promising for the drug discovery field, suggesting the possibility of using copepods as a new source of bioactive compounds to be tested in the marine biotechnological sector.

Kinetic Studies of Antioxidant Properties of Ovothiol A

Ovothiol A (OSH) is one of the strongest natural antioxidants. So far, its presence was found in tissues of marine invertebrates, algae and fish. Due to very low pKa value of the SH group, under physiological conditions, this compound is almost entirely present in chemically active thiolate form and reacts with ROS and radicals significantly faster than other natural thiols. In biological systems, OSH acts in tandem with glutathione GSH, with OSH neutralizing oxidants and GSH maintaining ovothiol in the reduced state. In the present work, we report the rate constants of OSH oxidation by H₂O₂ and of reduction of oxidized ovothiol OSSO by GSH and we estimate the Arrhenius parameters for these rate constants. The absorption spectra of reaction intermediates, adduct OSSG and sulfenic acid OSOH, were obtained. We also found that OSH effectively quenches the triplet state of kynurenic acid with an almost diffusion-controlled rate constant. This finding indicates that OSH may serve as a good photoprotector to inhibit the deleterious effect of solar UV irradiation; this assumption explains the high concentrations of OSH in the fish lens. The unique antioxidant and photoprotecting properties of OSH open promising perspectives for its use in the treatment of human diseases.

Metabolic adaptations to marine environments: molecular diversity and evolution of ovothiol biosynthesis in Bacteria

Ovothiols are sulfur-containing amino acids synthesized by marine invertebrates, protozoans, and bacteria. They act as pleiotropic molecules in signaling and protection against oxidative stress. The discovery of ovothiol biosynthetic enzymes, sulfoxide synthase OvoA and β-lyase OvoB, paves the way for a systematic investigation of ovothiol distribution and molecular diversification in nature. In this work, we conducted genomic and metagenomics data mining to investigate the distribution and diversification of ovothiol biosynthetic enzymes in Bacteria. We identified the bacteria endowed with this secondary metabolic pathway, described their taxonomy, habitat and biotic interactions in order to provide insight into their adaptation to specific environments. We report that OvoA and OvoB are mostly encountered in marine aerobic Proteobacteria, some of them establishing symbiotic or parasitic relationships with other organisms. We identified a horizontal gene transfer event of OvoB from Bacteroidetes living in symbiosis with Hydrozoa. Our search within the Ocean Gene Atlas revealed the occurrence of ovothiol biosynthetic genes in Proteobacteria living in a wide range of pelagic and highly oxygenated environments. Finally, we tracked the evolutionary history of ovothiol biosynthesis from marine bacteria to unicellular eukaryotes and metazoans. Our analysis provides new conceptual elements to unravel the evolutionary and ecological significance of ovothiol biosynthesis.

Proton-Coupled Electron-Transfer Reactivity Controls Iron versus Sulfur Oxidation in Nonheme Iron-Thiolate Complexes

Reaction of the five-coordinate Fe^II(N₄S) complexes, [Fe^II(iPr₃TACN)(abt^X)](OTf) (abt = aminobenzenethiolate, X = H, CF₃), with a one-electron oxidant and an appropriate base leads to net H atom loss, generating new Fe^III(iminobenzenethiolate) complexes that were characterized by single-crystal X-ray diffraction (XRD), as well as UV-vis, EPR, and Mössbauer spectroscopies. The spectroscopic data indicate that the iminobenzenethiolate complexes have S = 3/2 ground states. In the absence of a base, oxidation of the Fe^II(abt) complexes leads to disulfide formation instead of oxidation at the metal center. Bracketing studies with separated proton-coupled electron-transfer (PCET) reagents show that the Fe^II(aminobenzenethiolate) and Fe^III(iminobenzenethiolate) forms are readily interconvertible by H⁺/e^- transfer and provide a measure of the bond dissociation free energy (BDFE) for the coordinated N-H bond between 64 and 69 kcal mol^-1. This work shows that coordination to the iron center causes a dramatic weakening of the N-H bond and that Fe- versus S-oxidation in a nonheme iron complex can be controlled by the protonation state of an ancillary amino donor.

Implications for an imidazol-2-yl carbene intermediate in the rhodanase-catalyzed C-S bond formation reaction of anaerobic ergothioneine biosynthesis

In the anaerobic ergothioneine biosynthetic pathway, a rhodanese domain containing enzyme (EanB) activates tne hercynine's sp² ε-C-H Dona ana replaces it with a C-S bond to produce ergothioneine. The key intermediate for this trans-sulfuration reaction is the Cys412 persulfide. Substitution of the EanB-Cys412 persulfide with a Cys412 perselenide does not yield the selenium analog of ergothioneine, selenoneine. However, in deuterated buffer, the perselenide-modified EanB catalyzes the deuterium exchange between hercynine's sp² ε-C-H bond and D₂O. Results from QM/MM calculations suggest that the reaction involves a carbene intermediate and that Tyr353 plays a key role. We hypothesize that modulating the pK_a of Tyr353 will affect the deuterium-exchange rate. Indeed, the 3,5-difluoro tyrosine containing EanB catalyzes the deuterium exchange reaction with k _ex of ~10-fold greater than the wild-type EanB (EanB_WT). With regards to potential mechanisms, these results support the involvement of a carbene intermediate in EanB-catalysis, rendering EanB as one of the few carbene-intermediate involving enzymatic systems.

Chemical modifications of proteins and their applications in metalloenzyme studies

Protein chemical modifications are important tools for elucidating chemical and biological functions of proteins. Several strategies have been developed to implement these modifications, including enzymatic tailoring reactions, unnatural amino acid incorporation using the expanded genetic codes, and recognition-driven transformations. These technologies have been applied in metalloenzyme studies, specifically in dissecting their mechanisms, improving their enzymatic activities, and creating artificial enzymes with non-natural activities. Herein, we summarize some of the recent efforts in these areas with an emphasis on a few metalloenzyme case studies.

Small-molecule mimicry hunting strategy in the imperial cone snail, Conus imperialis

Venomous animals hunt using bioactive peptides, but relatively little is known about venom small molecules and the resulting complex hunting behaviors. Here, we explored the specialized metabolites from the venom of the worm-hunting cone snail, Conus imperialis Using the model polychaete worm Platynereis dumerilii, we demonstrate that C. imperialis venom contains small molecules that mimic natural polychaete mating pheromones, evoking the mating phenotype in worms. The specialized metabolites from different cone snails are species-specific and structurally diverse, suggesting that the cones may adopt many different prey-hunting strategies enabled by small molecules. Predators sometimes attract prey using the prey's own pheromones, in a strategy known as aggressive mimicry. Instead, C. imperialis uses metabolically stable mimics of those pheromones, indicating that, in biological mimicry, even the molecules themselves may be disguised, providing a twist on fake news in chemical ecology.

Nonheme iron-thiolate complexes as structural models of sulfoxide synthase active sites

Two mononuclear iron(ii)-thiolate complexes have been prepared that represent structural models of the nonheme iron enzymes EgtB and OvoA, which catalyze the O2-dependent formation of carbon-sulfur bonds in the biosynthesis of thiohistidine compounds. The series of Fe(ii) complexes reported here feature tripodal N4 chelates (LA and LB) that contain both pyridyl and imidazolyl donors (LA = (1H-imidazol-4-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine; LB = N,N-bis((1-methylimidazol-2-yl)methyl)-2-pyridylmethylamine). Further coordination with monodentate aromatic or aliphatic thiolate ligands yielded the five-coordinate, high-spin Fe(ii) complexes [FeII(LA)(SMes)]BPh4 (1) and [FeII(LB)(SCy)]BPh4 (2), where SMes = 2,4,6-trimethylthiophenolate and SCy = cyclohexanethiolate. X-ray crystal structures revealed that 1 and 2 possess trigonal bipyramidal geometries formed by the N4S ligand set. In each case, the thiolate ligand is positioned cis to an imidazole donor, replicating the arrangement of Cys- and His-based substrates in the active site of EgtB. The geometric and electronic structures of 1 and 2 were analyzed with UV-vis absorption and Mössbauer spectroscopies in tandem with density functional theory (DFT) calculations. Exposure of 1 and 2 to nitric oxide (NO) yielded six-coordinate FeNO adducts that were characterized with infrared and electron paramagnetic resonance (EPR) spectroscopies, confirming that these complexes are capable of binding diatomic molecules. Reaction of 1 and 2 with O2 causes oxidation of the thiolate ligands to disulfide products. The implications of these results for the development of functional models of EgtB and OvoA are discussed.

Insights into the Light Response of Skeletonema marinoi: Involvement of Ovothiol

Diatoms are one of the most widespread groups of microalgae on Earth. They possess extraordinary metabolic capabilities, including a great ability to adapt to different light conditions. Recently, we have discovered that the diatom Skeletonema marinoi produces the natural antioxidant ovothiol B, until then identified only in clams. In this study, we investigated the light-dependent modulation of ovothiol biosynthesis in S. marinoi. Diatoms were exposed to different light conditions, ranging from prolonged darkness to low or high light, also differing in the velocity of intensity increase (sinusoidal versus square-wave distribution). The expression of the gene encoding the key ovothiol biosynthetic enzyme, ovoA, was upregulated by high sinusoidal light mimicking natural conditions. Under this situation higher levels of reactive oxygen species and nitric oxide as well as ovothiol and glutathione increase were detected. No ovoA modulation was observed under prolonged darkness nor low sinusoidal light. Unnatural conditions such as continuous square-wave light induced a very high oxidative stress leading to a drop in cell growth, without enhancing ovoA gene expression. Only one of the inducible forms of nitric oxide synthase, nos2, was upregulated by light with consequent production of NO under sinusoidal light and darkness conditions. Our data suggest that ovothiol biosynthesis is triggered by a combined light stress caused by natural distribution and increased photon flux density, with no influence from the daily light dose. These results open new perspectives for the biotechnological production of ovothiols, which are receiving a great interest for their biological activities in human model systems.

The catalytic mechanism of sulfoxide synthases

Sulfoxide synthases are non-heme iron enzymes that catalyze oxidative carbonsulfur bond formation in the biosynthesis of thiohistidines such as ergothioneine and ovothiol. The catalytic mechanism of these enzymes has been studied by protein crystallography, steady-state kinetics, non-natural amino acid incorporation and computational modeling. This review discusses the current status of this research and also highlights similarities between the CS bond forming activity of sulfoxide synthases with that of synthetic coordination compounds.

First evidence of ovothiol biosynthesis in marine diatoms

Ovothiols are histidine-derived thiols that are receiving a great interest for their biological activities in human model systems. Thanks to the position of the thiol group on the imidazole ring of histidine, these compounds exhibit unusual antioxidant properties. They have been revealing a very promising pharmacological potential due to their anti-proliferative and anti-inflammatory properties, as well as anti-fibrotic activities not always related to their antioxidant power. Ovothiols occur in three differentially methylated forms (A, B and C), isolated from ovary, eggs and biological fluids of many marine invertebrates, mollusks, microalgae, and pathogenic protozoa. These molecules are synthesized by two enzymes: the sulfoxide synthase OvoA and the sulfoxide lyase OvoB. OvoA catalyzes the insertion of the sulfur atom of cysteine on the imidazole ring of histidine, leading to the formation of a sulfoxide intermediate. This is then cleaved by OvoB, giving 5-thiohistidine, finally methylated on the imidazole ring thanks to the methyltransferase domain of OvoA. Recent studies have shown that OvoA homologs are encoded in a wide variety of genomes suggesting that ovothiol biosynthesis is much more widespread in nature than initially thought. Here we have investigated the OvoA occurrence in diatoms, one of the most abundant group of microalgae, dominating marine and freshwater environments. They are considered a very good model system for both biology/photophysiology studies and for biotechnological applications. We have performed comparative sequence and phylogenetic analyses of OvoA from diatoms, highlighting a high degree of conservation of the canonical domain architecture in the analyzed species, as well as a clear clustering of OvoA in the two different morphological groups, i.e. centric and pennate diatoms. The in silico analyses have also revealed that OvoA gene expression is modulated by growth conditions. More importantly, we have characterized the thiol fraction from cultures of the coastal centric diatom Skeletonema marinoi, providing the first evidence of ovothiol B biosynthesis in diatoms.

Selenocysteine as a Substrate, an Inhibitor and a Mechanistic Probe for Bacterial and Fungal Iron-Dependent Sulfoxide Synthases

Sulfoxide synthases are non-heme iron enzymes that participate in the biosynthesis of thiohistidines, such as ergothioneine and ovothiol A. The sulfoxide synthase EgtB from Chloracidobacterium thermophilum (CthEgtB) catalyzes oxidative coupling between the side chains of N-α-trimethyl histidine (TMH) and cysteine (Cys) in a reaction that entails complete reduction of molecular oxygen, carbon-sulfur (C-S) and sulfur-oxygen (S-O) bond formation as well as carbon-hydrogen (C-H) bond cleavage. In this report, we show that CthEgtB and other bacterial sulfoxide synthases cannot efficiently accept selenocysteine (SeCys) as a substrate in place of cysteine. In contrast, the sulfoxide synthase from the filamentous fungus Chaetomium thermophilum (CthEgt1) catalyzes C-S and C-Se bond formation at almost equal efficiency. We discuss evidence suggesting that this functional difference between bacterial and fungal sulfoxide synthases emerges from different modes of oxygen activation.

Marine Bacteria versus Microalgae: Who Is the Best for Biotechnological Production of Bioactive Compounds with Antioxidant Properties and Other Biological Applications?

Natural bioactive compounds with antioxidant activity play remarkable roles in the prevention of reactive oxygen species (ROS) formation. ROS, which are formed by different pathways, have various pathological influences such as DNA damage, carcinogenesis, and cellular degeneration. Incremental demands have prompted the search for newer and alternative resources of natural bioactive compounds with antioxidant properties. The marine environment encompasses almost three-quarters of our planet and is home to many eukaryotic and prokaryotic microorganisms. Because of extreme physical and chemical conditions, the marine environment is a rich source of chemical and biological diversity, and marine microorganisms have high potential as a source of commercially interesting compounds with various pharmaceutical, nutraceutical, and cosmeceutical applications. Bacteria and microalgae are the most important producers of valuable molecules including antioxidant enzymes (such as superoxide dismutase and catalase) and antioxidant substances (such as carotenoids, exopolysaccharides, and bioactive peptides) with various valuable biological properties and applications. Here, we review the current knowledge of these bioactive compounds while highlighting their antioxidant properties, production yield, health-related benefits, and potential applications in various biological and industrial fields.

Biosynthetic Pathways to Nonproteinogenic α-Amino Acids

Natural nonproteinogenic amino acids vastly outnumber the well-known 22 proteinogenic amino acids. Such amino acids are generated in specialized metabolic pathways. In these pathways, diverse biosynthetic transformations, ranging from isomerizations to the stereospecific functionalization of C-H bonds, are employed to generate structural diversity. The resulting nonproteinogenic amino acids can be integrated into more complex natural products. Here we review recently discovered biosynthetic routes to freestanding nonproteinogenic α-amino acids, with an emphasis on work reported between 2013 and mid-2019.

The complex evolutionary history of sulfoxide synthase in ovothiol biosynthesis

Sulfoxide synthases are enzymes involved in the biosynthesis of small sulfur-containing natural products. Their enzymatic activity represents a unique sulfur transfer strategy in nature that is the insertion of a sulfur atom on the imidazole ring of histidine. To date, only two enzymes are known to carry out this function: the sulfoxide synthase EgtB, involved in the biosynthesis of ergothioneine in fungi and bacteria, and the 5-histidylcysteine sulfoxide synthase OvoA, involved in the biosynthesis of ovothiols, found in the eggs and biological fluids of marine invertebrates, some proteobacteria and protists. In particular, ovothiols, thanks to their unique redox properties, are probably the most intriguing marine sulfur-containing molecules. Although they have long been considered as cellular protective molecules, new evidence suggest that their biological activities and ecological role might be more complex than originally thought. Here, we investigate the evolutionary history of OvoA in Metazoa, reporting its monophyletic ancient origins, which could be traced back to the latest common ancestor of Choanozoa. Nevertheless, we show that OvoA is missing in several major extant taxa and we discuss this patchy distribution in the light of the massive genome reduction events documented in Metazoa. We also highlight two interesting cases of secondary acquisition through horizontal gene transfer, which occurred in hydrozoans and bdelloid rotifers. The evolutionary success of this metabolic pathway is probably ascribable to its role in the maintenance of cellular redox homeostasis, which enables organisms to survive in different environmental niches.

Probing the Interactions of Sulfur-Containing Histidine Compounds with Human Gamma-Glutamyl Transpeptidase

Gamma-glutamyl transpeptidase (GGT) is a cell surface enzyme involved in glutathione metabolism and maintenance of redox homeostasis. High expression of GGT on tumor cells is associated with an increase of cell proliferation and resistance against chemotherapy. GGT inhibitors that have been evaluated in clinical trials are too toxic for human use. We have previously identified ovothiols, 5(Nπ)-methyl-thiohistidines of marine origin, as non-competitive-like inhibitors of GGT that are more potent than the known GGT inhibitor, 6-diazo-5-oxo-l-norleucine (DON), and are not toxic for human embryonic cells. We extended these studies to the desmethylated form of ovothiol, 5-thiohistidine, and confirmed that this ovothiol derivative also acts as a non-competitive-like GGT inhibitor, with a potency comparable to ovothiol. We also found that both 5-thiohistidine derivatives act as reversible GGT inhibitors compared to the irreversible DON. Finally, we probed the interactions of 5-thiohistidines with GGT by docking analysis and compared them with the 2-thiohistidine ergothioneine, the physiological substrate glutathione, and the DON inhibitor. Overall, our results provide new insight for further development of 5-thiohistidine derivatives as therapeutics for GGT-positive tumors.

Natural Sulfur-Containing Compounds: An Alternative Therapeutic Strategy against Liver Fibrosis

Liver fibrosis is a pathophysiologic process involving the accumulation of extracellular matrix proteins as collagen deposition. Advanced liver fibrosis can evolve in cirrhosis, portal hypertension and often requires liver transplantation. At the cellular level, hepatic fibrosis involves the activation of hepatic stellate cells and their transdifferentiation into myofibroblasts. Numerous pro-fibrogenic mediators including the transforming growth factor-β1, the platelet-derived growth factor, endothelin-1, toll-like receptor 4, and reactive oxygen species are key players in this process. Knowledge of the cellular and molecular mechanisms underlying hepatic fibrosis development need to be extended to find novel therapeutic strategies. Antifibrotic therapies aim to inhibit the accumulation of fibrogenic cells and/or prevent the deposition of extracellular matrix proteins. Natural products from terrestrial and marine sources, including sulfur-containing compounds, exhibit promising activities for the treatment of fibrotic pathology. Although many therapeutic interventions are effective in experimental models of liver fibrosis, their efficacy and safety in humans are largely unknown. This review aims to provide a reference collection on experimentally tested natural anti-fibrotic compounds, with particular attention on sulfur-containing molecules. Their chemical structure, sources, mode of action, molecular targets, and pharmacological activity in the treatment of liver disease will be discussed.

The biosynthetic diversity of the animal world

Secondary metabolites are often considered within the remit of bacterial or plant research, but animals also contain a plethora of these molecules with important functional roles. Classical feeding studies demonstrate that, whereas some are derived from diet, many of these compounds are made within the animals. In the past 15 years, the genetic and biochemical origin of several animal natural products has been traced to partnerships with symbiotic bacteria. More recently, a number of animal genome-encoded pathways to microbe-like natural products have come to light. These pathways are sometimes horizontally acquired from bacteria, but more commonly they unveil a new and diverse animal biochemistry. In this review, we highlight recent examples of characterized animal biosynthetic enzymes that reveal an unanticipated breadth and intricacy in animal secondary metabolism. The results so far suggest that there may be an immense diversity of animal small molecules and biosynthetic enzymes awaiting discovery. This biosynthetic dark matter is just beginning to be understood, providing a relatively untapped frontier for discovery.

Non-heme iron enzyme-catalyzed complex transformations: Endoperoxidation, cyclopropanation, orthoester, oxidative C-C and C-S bond formation reactions in natural product biosynthesis

Non-heme iron enzymes catalyze a wide range of chemical transformations, serving as one of the key types of tailoring enzymes in the biosynthesis of natural products. Hydroxylation reaction is the most common type of reactions catalyzed by these enzymes and hydroxylation reactions have been extensively investigated mechanistically. However, the mechanistic details for other types of transformations remain largely unknown or unexplored. In this paper, we present some of the most recently discovered transformations, including endoperoxidation, orthoester formation, cyclopropanation, oxidative C-C and C-S bond formation reactions. In addition, many of them are multi-functional enzymes, which further complicate their mechanistic investigations. In this work, we summarize their biosynthetic pathways, with special emphasis on the mechanistic details available for these newly discovered enzymes.

Sulfur-containing histidine compounds inhibit γ-glutamyl transpeptidase activity in human cancer cells

γ-Glutamyl transpeptidase (GGT) is an enzyme located on the surface of cellular membranes and involved in GSH metabolism and maintenance of redox homeostasis. High GGT expression on tumor cells is associated with increased cell proliferation and resistance against chemotherapy. GGT inhibitors evaluated so far in clinical trials are too toxic for human use. In this study, using enzyme kinetics analyses, we demonstrate that ovothiols, 5(Nπ)-methyl thiohistidines of marine origin, act as noncompetitive inhibitors of GGT, with an apparent K_i of 21 μm, when we fixed the concentrations of the donor substrate. We found that these compounds are more potent than the known GGT inhibitor 6-diazo-5-oxo-l-norleucine and are not toxic toward human embryonic cells. In particular, cellular process-specific fluorescence-based assays revealed that ovothiols induce a mixed cell-death phenotype of apoptosis and autophagy in GGT-overexpressing cell lines, including human liver cancer and chronic B leukemic cells. The findings of our study provide the basis for further development of 5-thiohistidines as therapeutics for GGT-positive tumors and highlight that GGT inhibition is involved in autophagy.

Crystal Structure of the Ergothioneine Sulfoxide Synthase from Candidatus Chloracidobacterium thermophilum and Structure-Guided Engineering To Modulate Its Substrate Selectivity

Ergothioneine is a thiohistidine derivative with potential benefits on many aging-related diseases. The central step of aerobic ergothioneine biosynthesis is the oxidative C-S bond formation reaction catalyzed by mononuclear nonheme iron sulfoxide synthases (EgtB and Egt1). Thus far, only the Mycobacterium thermoresistibile EgtB (EgtB _Mth ) crystal structure is available, while the structural information for the more industrially attractive Egt1 enzyme is not. Herein, we reported the crystal structure of the ergothioneine sulfoxide synthase (EgtB _Cth ) from Candidatus Chloracidobacterium thermophilum. EgtB _Cth has both EgtB- and Egt1-type of activities. Guided by the structural information, we conducted Rosetta Enzyme Design calculations, and we biochemically demonstrated that EgtB _Cth can be engineered more toward Egt1-type of activity. This study provides information regarding the factors governing the substrate selectivity in Egt1- and EgtB-catalysis and lays the groundwork for future sulfoxide synthase engineering toward the development of an effective ergothioneine process through a synthetic biology approach.

On ovothiol biosynthesis and biological roles: from life in the ocean to therapeutic potential

Covering: up to 2018 Ovothiols are sulfur-containing natural products biosynthesized by marine invertebrates, microalgae, and bacteria. These compounds are characterized by unique chemical properties suggestive of numerous cellular functions. For example, ovothiols may be cytoprotectants against oxidative stress, serve as building blocks of more complex structures and may act as molecular messengers for inter- and intracellular signaling. Detailed understanding of ovothiol physiological role in marine organisms may unearth novel concepts in cellular redox biochemistry and highlight the therapeutic potential of this antioxidant. The recent discovery of ovothiol biosynthetic genes has paved the way for a systematic investigation of ovothiol-modulated cellular processes. In this highlight we review the early research on ovothiol and we discuss key questions that may now be addressed using genome-based approaches. This highlight article provides an overview of recent progress towards elucidating the biosynthesis, function and potential application of ovothiols.

Ovothiol A is the Main Antioxidant in Fish Lens

Tissue protection from oxidative stress by antioxidants is of vital importance for cellular metabolism. The lens mostly consists of fiber cells lacking nuclei and organelles, having minimal metabolic activity; therefore, the defense of the lens tissue from the oxidative stress strongly relies on metabolites. Protein-free extracts from lenses and gills of freshwater fish, Sander lucioperca and Rutilus rutilus lacustris, were subjected to analysis using high-field 1H NMR spectroscopy and HPLC with optical and high-resolution mass spectrometric detection. It was found that the eye lenses of freshwater fish contain high concentrations of ovothiol A (OSH), i.e., one of the most powerful antioxidants exciting in nature. OSH was identified and quantified in millimolar concentrations. The concentration of OSH in the lens and gills depends on the fish genus and on the season. A possible mechanism of the reactive oxygen species deactivation in fish lenses is discussed. This work is the first to report on the presence of OSH in vertebrates. The presence of ovothiol in the fish tissue implies that it may be a significantly more common antioxidant in freshwater and marine animals than was previously thought.

An Alternative Active Site Architecture for O2 Activation in the Ergothioneine Biosynthetic EgtB from Chloracidobacterium thermophilum

Sulfoxide synthases are nonheme iron enzymes that catalyze oxidative carbon-sulfur bond formation between cysteine derivatives and N-α-trimethylhistidine as a key step in the biosynthesis of thiohistidines. The complex catalytic mechanism of this enzyme reaction has emerged as the controversial subject of several biochemical and computational studies. These studies all used the structure of the γ-glutamyl cysteine utilizing sulfoxide synthase, MthEgtB from Mycobacterium thermophilum (EC 1.14.99.50), as a structural basis. To provide an alternative model system, we have solved the crystal structure of CthEgtB from Chloracidobacterium thermophilum (EC 1.14.99.51) that utilizes cysteine as a sulfur donor. This structure reveals a completely different configuration of active site residues that are involved in oxygen binding and activation. Furthermore, comparison of the two EgtB structures enables a classification of all ergothioneine biosynthetic EgtBs into five subtypes, each characterized by unique active-site features. This active site diversity provides an excellent platform to examine the catalytic mechanism of sulfoxide synthases by comparative enzymology, but also raises the question as to why so many different solutions to the same biosynthetic problem have emerged.