Use of a Tyrosine Analogue To Modulate the Two Activities of a Nonheme Iron Enzyme OvoA in Ovothiol Biosynthesis, Cysteine Oxidation versus Oxidative C-S Bond Formation

Noncanonical Amino Acids in Biocatalysis

In recent years, powerful genetic code reprogramming methods have emerged that allow new functional components to be embedded into proteins as noncanonical amino acid (ncAA) side chains. In this review, we will illustrate how the availability of an expanded set of amino acid building blocks has opened a wealth of new opportunities in enzymology and biocatalysis research. Genetic code reprogramming has provided new insights into enzyme mechanisms by allowing introduction of new spectroscopic probes and the targeted replacement of individual atoms or functional groups. NcAAs have also been used to develop engineered biocatalysts with improved activity, selectivity, and stability, as well as enzymes with artificial regulatory elements that are responsive to external stimuli. Perhaps most ambitiously, the combination of genetic code reprogramming and laboratory evolution has given rise to new classes of enzymes that use ncAAs as key catalytic elements. With the framework for developing ncAA-containing biocatalysts now firmly established, we are optimistic that genetic code reprogramming will become a progressively more powerful tool in the armory of enzyme designers and engineers in the coming years.

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.

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.

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.

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.

Recent Advances in Biocatalysis with Chemical Modification and Expanded Amino Acid Alphabet

The two main strategies for enzyme engineering, directed evolution and rational design, have found widespread applications in improving the intrinsic activities of proteins. Although numerous advances have been achieved using these ground-breaking methods, the limited chemical diversity of the biopolymers, restricted to the 20 canonical amino acids, hampers creation of novel enzymes that Nature has never made thus far. To address this, much research has been devoted to expanding the protein sequence space via chemical modifications and/or incorporation of noncanonical amino acids (ncAAs). This review provides a balanced discussion and critical evaluation of the applications, recent advances, and technical breakthroughs in biocatalysis for three approaches: (i) chemical modification of cAAs, (ii) incorporation of ncAAs, and (iii) chemical modification of incorporated ncAAs. Furthermore, the applications of these approaches and the result on the functional properties and mechanistic study of the enzymes are extensively reviewed. We also discuss the design of artificial enzymes and directed evolution strategies for enzymes with ncAAs incorporated. Finally, we discuss the current challenges and future perspectives for biocatalysis using the expanded amino acid alphabet.

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.

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.

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.

Convergent Evolution of Fungal Cysteine Dioxygenases

Cupin-type cysteine dioxygenases (CDOs) are non-heme iron enzymes that occur in animals, plants, bacteria and in filamentous fungi. In this report, we show that agaricomycetes contain an entirely unrelated type of CDO that emerged by convergent evolution from enzymes involved in the biosynthesis of ergothioneine. The activity of this CDO type is dependent on the ergothioneine precursor N-α-trimethylhistidine. The metabolic link between ergothioneine production and cysteine oxidation suggests that the two processes might be part of the same chemical response in fungi, for example against oxidative stress.

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.

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.

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.

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.

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.

A Nonheme Thiolate-Ligated Cobalt Superoxo Complex: Synthesis and Spectroscopic Characterization, Computational Studies, and Hydrogen Atom Abstraction Reactivity

The synthesis and characterization of a Co(II) dithiolato complex Co(Me₃TACN)(S₂SiMe₂) (1) are reported. Reaction of 1 with O₂ generates a rare thiolate-ligated cobalt-superoxo species Co(O₂)(Me₃TACN)(S₂SiMe₂) (2) that was characterized spectroscopically and structurally by resonance Raman, EPR, and X-ray absorption spectroscopies as well as density functional theory. Metal-superoxo species are proposed to S-oxygenate metal-bound thiolate donors in nonheme thiol dioxygenases, but 2 does not lead to S-oxygenation of the intramolecular thiolate donors and does not react with exogenous sulfur donors. However, complex 2 is capable of oxidizing the O-H bonds of 2,2,6,6-tetramethylpiperidin-1-ol derivatives via H atom abstraction. Complementary proton-coupled electron-transfer reactivity is seen for 2 with separated proton/reductant pairs. The reactivity studies indicate that 2 can abstract H atoms from weak X-H bonds with bond dissociation free energy (BDFE) ≤ 70 kcal mol^-1. DFT calculations predict that the putative Co(OOH) product has an O-H BDFE = 67 kcal mol^-1, which matches the observed pattern of reactivity seen for 2. These data provide new information regarding the selectivity of S-oxygenation versus H atom abstraction in thiolate-ligated nonheme metalloenzymes that react with O₂.

The proteobacterial species Burkholderia pseudomallei produces ergothioneine, which enhances virulence in mammalian infection

Bacteria use various endogenous antioxidants for protection against oxidative stress associated with environmental survival or host infection. Although glutathione (GSH) is the most abundant and widely used antioxidant in Proteobacteria, ergothioneine (EGT) is another microbial antioxidant, mainly produced by fungi and Actinobacteria. The Burkholderia genus is found in diverse environmental niches. We observed that gene homologs required for the synthesis of EGT are widely distributed throughout the genus. By generating gene-deletion mutants and monitoring production with isotope-labeled substrates, we show that pathogenic Burkholderia pseudomallei and environmental B. thailandensis are able to synthesize EGT de novo. Unlike most other bacterial EGT synthesis pathways described, Burkholderia spp. use cysteine rather than γ-glutamyl cysteine as the thiol donor. Analysis of recombinant EgtB indicated that it is a proficient sulfoxide synthase, despite divergence in the active site architecture from that of mycobacteria. The absence of GSH, but not EGT, increased bacterial susceptibility to oxidative stresses in vitro. However, deletion of EGT synthesis conferred a reduced fitness to B. pseudomallei, with a delay in organ colonization and time to death during mouse infection. Therefore, despite the lack of an apparent antioxidant role in vitro, EGT is important for optimal bacterial pathogenesis in the mammalian host.-Gamage, A. M., Liao, C., Cheah, I. K., Chen, Y., Lim, D. R. X., Ku, J. W. K., Chee, R. S. L., Gengenbacher, M., Seebeck, F. P., Halliwell, B., Gan, Y.-H. The proteobacterial species Burkholderia pseudomallei produces ergothioneine, which enhances virulence in mammalian infection.

Mechanistic Insight on the Activity and Substrate Selectivity of Nonheme Iron Dioxygenases

Nonheme iron dioxygenases catalyze vital reactions for human health particularly related to aging processes. They are involved in the biosynthesis of amino acids, but also the biodegradation of toxic compounds. Typically they react with their substrate(s) through oxygen atom transfer, although often with the assistance of a co-substrate like α-ketoglutarate that is converted to succinate and CO₂ . Many reaction processes catalyzed by the nonheme iron dioxygenases are stereoselective or regiospecific and hence understanding the mechanism and protein involvement in the selectivity is important for the design of biotechnological applications of these enzymes. To this end, I will review recent work of our group on nonheme iron dioxygenases and include background information on their general structure and catalytic cycle. Examples of stereoselective and regiospecific reaction mechanisms we elucidated are for the AlkB repair enzyme, prolyl-4-hydroxylase and the ergothioneine biosynthesis enzyme. Finally, I cover an example where we bioengineered S-p-hydroxymandelate synthase into the R-p-hydroxymandelate synthase.

Mini-Review: Ergothioneine and Ovothiol Biosyntheses, an Unprecedented Trans-Sulfur Strategy in Natural Product Biosynthesis

As one of the most abundant elements on earth, sulfur is part of many small molecular metabolites and is key to their biological activities. Over the past few decades, some general strategies have been discovered for the incorporation of sulfur into natural products. In this review, we summarize recent efforts in elucidating the biosynthetic details for two sulfur-containing metabolites, ergothioneine and ovothiol. Their biosyntheses involve an unprecedented trans-sulfur strategy, a combination of a mononuclear non-heme iron enzyme-catalyzed oxidative C-S bond formation reaction and a PLP enzyme-mediated C-S lyase reaction.