Structure of the sulfoxide synthase EgtB from the ergothioneine biosynthetic pathway

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.

Making routine native SAD a reality: lessons from beamline X06DA at the Swiss Light Source

Native single-wavelength anomalous dispersion (SAD) is the most attractive de novo phasing method in macromolecular crystallography, as it directly utilizes intrinsic anomalous scattering from native crystals. However, the success of such an experiment depends on accurate measurements of the reflection intensities and therefore on careful data-collection protocols. Here, the low-dose, multiple-orientation data-collection protocol for native SAD phasing developed at beamline X06DA (PXIII) at the Swiss Light Source is reviewed, and its usage over the last four years on conventional crystals (>50 µm) is reported. Being experimentally very simple and fast, this method has gained popularity and has delivered 45 de novo structures to date (13 of which have been published). Native SAD is currently the primary choice for experimental phasing among X06DA users. The method can address challenging cases: here, native SAD phasing performed on a streptavidin-biotin crystal with P21 symmetry and a low Bijvoet ratio of 0.6% is highlighted. The use of intrinsic anomalous signals as sequence markers for model building and the assignment of ions is also briefly described.

Gram-scale fermentative production of ergothioneine driven by overproduction of cysteine in Escherichia coli

Ergothioneine (ERG), a unique thiol compound, is suggested to function as an antioxidant and cytoprotectant. Despite several recent attempts to produce ERG using various organisms, its yield was still very low and the costs remained high. Since the level of ERG produced depends strictly on the availability of three distinct precursor amino acids (l-cysteine (Cys), l-histidine, and l-methionine (Met)), metabolic engineering for enhancement of the flux toward ERG biosynthesis is required. Herein, we took advantage of a high-Cys production system using Escherichia coli cells, in which Cys biosynthesis and excretion were activated, and applied it to the fermentative production of ERG from glucose. The Cys overproduction in E. coli cells carrying the egtBCDE genes from Mycobacterium smegmatis was effective for ERG production. Furthermore, coexpression of the egtA gene, which encodes γ-glutamylcysteine synthetase that synthesizes the γ-glutamylcysteine used as a sulfur source of ERG biosynthesis, enhanced ERG production even though E. coli intrinsically has γ-glutamylcysteine synthetase. Additionally, disruption of the metJ gene that encodes the transcriptional repressor involved in Met metabolism was effective in further increasing the production of ERG. Finally, we succeeded in the high-level production of 1.31 g/L ERG in a fed-batch culture process using a jar fermenter.

P450-Catalyzed Tailoring Steps in Leinamycin Biosynthesis Featuring Regio- and Stereoselective Hydroxylations and Substrate Promiscuities

Leinamycin (LNM) is a potent antitumor antibiotic produced by Streptomyces atroolivaceus S-140. Both in vivo and in vitro characterization of the LNM biosynthetic machinery have established the formation of the 18-membered macrolactam backbone and the C-3 alkyl branch; the nascent product, LNM E1, of the hybrid nonribosomal peptide synthetase (NRPS)-acyltransferase (AT)-less type I polyketide synthase (PKS); and the generation of the thiol moiety at C-3 of LNM E1. However, the tailoring steps converting LNM E1 to LNM are still unknown. Based on gene inactivation and chemical investigation of three mutant strains, we investigated the tailoring steps catalyzed by two cytochromes P450 (P450s), LnmA and LnmZ, in LNM biosynthesis. Our studies revealed that (i) LnmA and LnmZ regio- and stereoselectively hydroxylate the C-8 and C-4' positions, respectively, on the scaffold of LNM; (ii) both LnmA and LnmZ exhibit substrate promiscuity, resulting in multiple LNM analogs from several shunt pathways; and (iii) the C-8 and C-4' hydroxyl groups play important roles in the cytotoxicity of LNM analogs against different cancer cell lines, shedding light on the structure-activity relationships of the LNM scaffold and the LNM-type natural products in general. These studies set the stage for future biosynthetic pathway engineering and combinatorial biosynthesis of the LNM family of natural products for structure diversity and drug discovery.

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.

Inhibition and Regulation of the Ergothioneine Biosynthetic Methyltransferase EgtD

Ergothioneine is an emerging factor in cellular redox homeostasis in bacteria, fungi, plants, and animals. Reports that ergothioneine biosynthesis may be important for the pathogenicity of bacteria and fungi raise the question as to how this pathway is regulated and whether the corresponding enzymes may be therapeutic targets. The first step in ergothioneine biosynthesis is catalyzed by the methyltransferase EgtD that converts histidine into N-α-trimethylhistidine. This report examines the kinetic, thermodynamic and structural basis for substrate, product, and inhibitor binding by EgtD from Mycobacterium smegmatis. This study reveals an unprecedented substrate binding mechanism and a fine-tuned affinity landscape as determinants for product specificity and product inhibition. Both properties are evolved features that optimize the function of EgtD in the context of cellular ergothioneine production. On the basis of these findings, we developed a series of simple histidine derivatives that inhibit methyltransferase activity at low micromolar concentrations. Crystal structures of inhibited complexes validate this structure- and mechanism-based design strategy.

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.

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

Ovothiol is a histidine thiol derivative. The biosynthesis of ovothiol involves an extremely efficient trans-sulfuration strategy. The nonheme iron enzyme OvoA catalyzed oxidative coupling between cysteine and histidine is one of the key steps. Besides catalyzing the oxidative coupling between cysteine and histidine, OvoA also catalyzes the oxidation of cysteine to cysteine sulfinic acid (cysteine dioxygenase activity). Thus far, very little mechanistic information is available for OvoA-catalysis. In this report, we measured the kinetic isotope effect (KIE) in OvoA-catalysis using the isotopically sensitive branching method. In addition, by replacing an active site tyrosine (Tyr417) with 2-amino-3-(4-hydroxy-3-(methylthio)phenyl)propanoic acid (MtTyr) through the amber suppressor mediated unnatural amino acid incorporation method, the two OvoA activities (oxidative coupling between cysteine and histidine, and cysteine dioxygenase activity) can be modulated. These results suggest that the two OvoA activities branch out from a common intermediate and that the active site tyrosine residue plays some key roles in controlling the partitioning between these two pathways.

Evolutionary novelty in gravity sensing through horizontal gene transfer and high-order protein assembly

Horizontal gene transfer (HGT) can promote evolutionary adaptation by transforming a species' relationship to the environment. In most well-understood cases of HGT, acquired and donor functions appear to remain closely related. Thus, the degree to which HGT can lead to evolutionary novelties remains unclear. Mucorales fungi sense gravity through the sedimentation of vacuolar protein crystals. Here, we identify the octahedral crystal matrix protein (OCTIN). Phylogenetic analysis strongly supports acquisition of octin by HGT from bacteria. A bacterial OCTIN forms high-order periplasmic oligomers, and inter-molecular disulphide bonds are formed by both fungal and bacterial OCTINs, suggesting that they share elements of a conserved assembly mechanism. However, estimated sedimentation velocities preclude a gravity-sensing function for the bacterial structures. Together, our data suggest that HGT from bacteria into the Mucorales allowed a dramatic increase in assembly scale and emergence of the gravity-sensing function. We conclude that HGT can lead to evolutionary novelties that emerge depending on the physiological and cellular context of protein assembly.

Ultraviolet radiation significantly enhances the molecular response to dispersant and sweet crude oil exposure in Nematostella vectensis

Estuarine organisms are subjected to combinations of anthropogenic and natural stressors, which together can reduce an organisms' ability to respond to either stress or can potentiate or synergize the cellular impacts for individual stressors. Nematostella vectensis (starlet sea anemone) is a useful model for investigating novel and evolutionarily conserved cellular and molecular responses to environmental stress. Using RNA-seq, we assessed global changes in gene expression in Nematostella in response to dispersant and/or sweet crude oil exposure alone or combined with ultraviolet radiation (UV). A total of 110 transcripts were differentially expressed by dispersant and/or crude oil exposure, primarily dominated by the down-regulation of 74 unique transcripts in the dispersant treatment. In contrast, UV exposure alone or combined with dispersant and/or oil resulted in the differential expression of 1133 transcripts, of which 436 were shared between all four treatment combinations. Most significant was the differential expression of 531 transcripts unique to one or more of the combined UV/chemical exposures. Main categories of genes affected by one or more of the treatments included enzymes involved in xenobiotic metabolism and transport, DNA repair enzymes, and general stress response genes conserved among vertebrates and invertebrates. However, the most interesting observation was the induction of several transcripts indicating de novo synthesis of mycosporine-like amino acids and other novel cellular antioxidants. Together, our data suggest that the toxicity of oil and/or dispersant and the complexity of the molecular response are significantly enhanced by UV exposure, which may co-occur for shallow water species like Nematostella.

Role of Ergothioneine in Microbial Physiology and Pathogenesis

SIGNIFICANCE

L-ergothioneine is synthesized in actinomycetes, cyanobacteria, methylobacteria, and some fungi. In contrast to other low-molecular-weight redox buffers, glutathione and mycothiol, ergothioneine is primarily present as a thione rather than a thiol at physiological pH, which makes it resistant to autoxidation. Ergothioneine regulates microbial physiology and enables the survival of microbes under stressful conditions encountered in their natural environments. In particular, ergothioneine enables pathogenic microbes, such as Mycobacterium tuberculosis (Mtb), to withstand hostile environments within the host to establish infection. Recent Advances: Ergothioneine has been reported to maintain bioenergetic homeostasis in Mtb and protect Mtb against oxidative stresses, thereby enhancing the virulence of Mtb in a mouse model. Furthermore, ergothioneine augments the resistance of Mtb to current frontline anti-TB drugs. Recently, an opportunistic fungus, Aspergillus fumigatus, which infects immunocompromised individuals, has been found to produce ergothioneine, which is important in conidial health and germination, and contributes to the fungal resistance against redox stresses.

CRITICAL ISSUES

The molecular mechanisms of the functions of ergothioneine in microbial physiology and pathogenesis are poorly understood. It is currently not known if ergothioneine is used in detoxification or antioxidant enzymatic pathways. As ergothioneine is involved in bioenergetic and redox homeostasis and antibiotic susceptibility of Mtb, it is of utmost importance to advance our understanding of these mechanisms.

FUTURE DIRECTIONS

A clear understanding of the role of ergothioneine in microbes will advance our knowledge of how this thione enhances microbial virulence and resistance to the host's defense mechanisms to avoid complete eradication. Antioxid. Redox Signal. 28, 431-444.

Identification of the S-transferase like superfamily bacillithiol transferases encoded by Bacillus subtilis

Bacillithiol is a low molecular weight thiol found in Firmicutes that is analogous to glutathione, which is absent in these bacteria. Bacillithiol transferases catalyze the transfer of bacillithiol to various substrates. The S-transferase-like (STL) superfamily contains over 30,000 putative members, including bacillithiol transferases. Proteins in this family are extremely divergent and are related by structural rather than sequence similarity, leaving it unclear if all share the same biochemical activity. Bacillus subtilis encodes eight predicted STL superfamily members, only one of which has been shown to be a bacillithiol transferase. Here we find that the seven remaining proteins show varying levels of metal dependent bacillithiol transferase activity. We have renamed the eight enzymes BstA-H. Mass spectrometry and gene expression studies revealed that all of the enzymes are produced to varying levels during growth and sporulation, with BstB and BstE being the most abundant and BstF and BstH being the least abundant. Interestingly, several bacillithiol transferases are induced in the mother cell during sporulation. A strain lacking all eight bacillithiol transferases showed normal growth in the presence of stressors that adversely affect growth of bacillithiol-deficient strains, such as paraquat and CdCl₂. Thus, the STL bacillithiol transferases represent a new group of proteins that play currently unknown, but potentially significant roles in bacillithiol-dependent reactions. We conclude that these enzymes are highly divergent, perhaps to cope with an equally diverse array of endogenous or exogenous toxic metabolites and oxidants.

Ergothioneine Antioxidant Function: From Chemistry to Cardiovascular Therapeutic Potential

Ergothioneine (ESH), the betaine of 2-mercapto-L-histidine, is a water-soluble naturally occurring amino acid with antioxidant properties. ESH accumulates in several human and animal tissues up to millimolar concentration through its high affinity transporter, namely the organic cation transporter 1 (OCTN1). ESH, first isolated from the ergot fungus (Claviceps purpurea), is synthesized only by Actinomycetales and non-yeast-like fungi. Plants absorb ESH via symbiotic associations between their roots and soil fungi, whereas mammals acquire it solely from dietary sources. Numerous evidence demonstrated the antioxidant and cytoprotective effects of ESH, including protection against cardiovascular diseases, chronic inflammatory conditions, ultraviolet radiation damages, and neuronal injuries. Although more than a century after its discovery has gone by, our understanding on the in vivo ESH mechanism is limited and this compound still intrigues researchers. However, recent evidence about differences in chemical redox behavior between ESH and alkylthiols, such as cysteine and glutathione, has opened new perspectives on the role of ESH during oxidative damage. In this short review, we discuss the role of ESH in the complex machinery of the cellular antioxidant defense focusing on the current knowledge on its chemical mechanism of action in the protection against cardiovascular disease.

Sulfoxide Synthase versus Cysteine Dioxygenase Reactivity in a Nonheme Iron Enzyme

The sulfoxide synthase EgtB represents a unique family of nonheme iron enzymes that catalyze the formation of a C-S bond between N-α-trimethyl histidine and γ-glutamyl cysteine, which is the key step in the biosynthesis of ergothioneine, an important amino acid related to aging. A controversy has arisen regarding its catalytic mechanism related to the function of the active-site Tyr₃₇₇ residue. The biosynthesis of ergothioneine in EgtB shows structural similarities to cysteine dioxygenase which transfers two oxygen atoms to the thiolate group of cysteine. The question, therefore, is how do EgtB enzymes catalyze the C-S bond-formation reaction, while also preventing a dioxygenation of its cysteinate substrate? In this work we present a quantum mechanics/molecular mechanics study into the mechanism of sulfoxide synthase enzymes as compared to cysteine dioxygenase enzymes and present pathways for both reaction channels in EgtB. We show that EgtB contains a conserved tyrosine residue that reacts via proton-coupled electron transfer with the iron(III)-superoxo species and creates an iron(III)-hydroperoxo intermediate, thereby preventing the possible thiolate dioxygenation side reaction. The nucleophilic C-S bond-formation step happens subsequently concomitant to relay of the proton of the iron(II)-hydroperoxo back to Tyr₃₇₇. This is the rate-determining step in the reaction cycle and is followed by hydrogen-atom transfer from the CE1-H group of trimethyl histidine substrate to iron(II)-superoxo. In the final step, a quick and almost barrierless sulfoxidation leads to the sulfoxide product complexes. The work highlights a unique machinery and active-site setup of the enzyme that drives the sulfoxide synthase reaction.

A bioinspired and biocompatible ortho-sulfiliminyl phenol synthesis

Synthetic methods inspired by Nature often offer unique advantages including mild conditions and biocompatibility with aqueous media. Inspired by an ergothioneine biosynthesis protein EgtB, a mononuclear non-haem iron enzyme capable of catalysing the C-S bond formation and sulfoxidation, herein, we discovered a mild and metal-free C-H sulfenylation/intramolecular rearrangement cascade reaction employing an internally oxidizing O-N bond as a directing group. Our strategy accommodates a variety of oxyamines with good site selectivity and intrinsic oxidative properties. Combining an O-N bond with an X-S bond generates a C-S bond and an S=N bond rapidly. The newly discovered cascade reaction showed excellent chemoselectivity and a wide substrate scope for both oxyamines and sulfenylation reagents. We demonstrated the biocompatibility of the C-S bond coupling reaction by applying a coumarin-based fluorogenic probe in bacterial lysates. Finally, the C-S bond coupling reaction enabled the first fluorogenic formation of phospholipids, which self-assembled to fluorescent vesicles in situ.

Heteroatom-Heteroatom Bond Formation in Natural Product Biosynthesis

Natural products that contain functional groups with heteroatom-heteroatom linkages (X-X, where X = N, O, S, and P) are a small yet intriguing group of metabolites. The reactivity and diversity of these structural motifs has captured the interest of synthetic and biological chemists alike. Functional groups containing X-X bonds are found in all major classes of natural products and often impart significant biological activity. This review presents our current understanding of the biosynthetic logic and enzymatic chemistry involved in the construction of X-X bond containing functional groups within natural products. Elucidating and characterizing biosynthetic pathways that generate X-X bonds could both provide tools for biocatalysis and synthetic biology, as well as guide efforts to uncover new natural products containing these structural features.

Enzymatic Carbon-Sulfur Bond Formation in Natural Product Biosynthesis

Sulfur plays a critical role for the development and maintenance of life on earth, which is reflected by the wealth of primary metabolites, macromolecules, and cofactors bearing this element. Whereas a large body of knowledge has existed for sulfur trafficking in primary metabolism, the secondary metabolism involving sulfur has long been neglected. Yet, diverse sulfur functionalities have a major impact on the biological activities of natural products. Recent research at the genetic, biochemical, and chemical levels has unearthed a broad range of enzymes, sulfur shuttles, and chemical mechanisms for generating carbon-sulfur bonds. This Review will give the first systematic overview on enzymes catalyzing the formation of organosulfur natural products.

Theoretical Study of the Mechanism of the Nonheme Iron Enzyme EgtB

EgtB is a nonheme iron enzyme catalyzing the C-S bond formation between γ-glutamyl cysteine (γGC) and N-α-trimethyl histidine (TMH) in the ergothioneine biosynthesis. Density functional calculations were performed to elucidate and delineate the reaction mechanism of this enzyme. Two different mechanisms were considered, depending on whether the sulfoxidation or the S-C bond formation takes place first. The calculations suggest that the S-O bond formation occurs first between the thiolate and the ferric superoxide, followed by homolytic O-O bond cleavage, very similar to the case of cysteine dioxygenase. Subsequently, proton transfer from a second-shell residue Tyr377 to the newly generated iron-oxo moiety takes place, which is followed by proton transfer from the TMH imidazole to Tyr377, facilitated by two crystallographically observed water molecules. Next, the S-C bond is formed between γGC and TMH, followed by proton transfer from the imidazole CH moiety to Tyr377, which was calculated to be the rate-limiting step for the whole reaction, with a barrier of 17.9 kcal/mol in the quintet state. The calculated barrier for the rate-limiting step agrees quite well with experimental kinetic data. Finally, this proton is transferred back to the imidazole nitrogen to form the product. The alternative thiyl radical attack mechanism has a very high barrier, being 25.8 kcal/mol, ruling out this possibility.

Go it alone: four-electron oxidations by mononuclear non-heme iron enzymes

This review discusses the current mechanistic understanding of a group of mononuclear non-heme iron-dependent enzymes that catalyze four-electron oxidation of their organic substrates without the use of any cofactors or cosubstrates. One set of enzymes acts on α-ketoacid-containing substrates, coupling decarboxylation to oxygen activation. This group includes 4-hydroxyphenylpyruvate dioxygenase, 4-hydroxymandelate synthase, and CloR involved in clorobiocin biosynthesis. A second set of enzymes acts on substrates containing a thiol group that coordinates to the iron. This group is comprised of isopenicillin N synthase, thiol dioxygenases, and enzymes involved in the biosynthesis of ergothioneine and ovothiol. The final group of enzymes includes HEPD and MPnS that both carry out the oxidative cleavage of the carbon-carbon bond of 2-hydroxyethylphosphonate but generate different products. Commonalities amongst many of these enzymes are discussed and include the initial substrate oxidation by a ferric-superoxo-intermediate and a second oxidation by a ferryl species.

COST Action CM1201 "Biomimetic Radical Chemistry": free radical chemistry successfully meets many disciplines

The COST Action CM1201 "Biomimetic Radical Chemistry" has been active since December 2012 for 4 years, developing research topics organized into four working groups: WG1 - Radical Enzymes, WG2 - Models of DNA damage and consequences, WG3 - Membrane stress, signalling and defenses, and WG4 - Bio-inspired synthetic strategies. International collaborations have been established among the participating 80 research groups with brilliant interdisciplinary achievements. Free radical research with a biomimetic approach has been realized in the COST Action and are summarized in this overview by the four WG leaders.

Ergothioneine Biosynthesis and Functionality in the Opportunistic Fungal Pathogen, Aspergillus fumigatus

Ergothioneine (EGT; 2-mercaptohistidine trimethylbetaine) is a trimethylated and sulphurised histidine derivative which exhibits antioxidant properties. Here we report that deletion of Aspergillus fumigatus egtA (AFUA_2G15650), which encodes a trimodular enzyme, abrogated EGT biosynthesis in this opportunistic pathogen. EGT biosynthetic deficiency in A. fumigatus significantly reduced resistance to elevated H₂O₂ and menadione, respectively, impaired gliotoxin production and resulted in attenuated conidiation. Quantitative proteomic analysis revealed substantial proteomic remodelling in ΔegtA compared to wild-type under both basal and ROS conditions, whereby the abundance of 290 proteins was altered. Specifically, the reciprocal differential abundance of cystathionine γ-synthase and β-lyase, respectively, influenced cystathionine availability to effect EGT biosynthesis. A combined deficiency in EGT biosynthesis and the oxidative stress response regulator Yap1, which led to extreme oxidative stress susceptibility, decreased resistance to heavy metals and production of the extracellular siderophore triacetylfusarinine C and increased accumulation of the intracellular siderophore ferricrocin. EGT dissipated H₂O₂ in vitro, and elevated intracellular GSH levels accompanied abrogation of EGT biosynthesis. EGT deficiency only decreased resistance to high H₂O₂ levels which suggests functionality as an auxiliary antioxidant, required for growth at elevated oxidative stress conditions. Combined, these data reveal new interactions between cellular redox homeostasis, secondary metabolism and metal ion homeostasis.

Conservation of the C-type lectin fold for accommodating massive sequence variation in archaeal diversity-generating retroelements

Background

Diversity-generating retroelements (DGRs) provide organisms with a unique means for adaptation to a dynamic environment through massive protein sequence variation. The potential scope of this variation exceeds that of the vertebrate adaptive immune system. DGRs were known to exist only in viruses and bacteria until their recent discovery in archaea belonging to the ‘microbial dark matter’, specifically in organisms closely related to Nanoarchaeota. However, Nanoarchaeota DGR variable proteins were unassignable to known protein folds and apparently unrelated to characterized DGR variable proteins.

Results

To address the issue of how Nanoarchaeota DGR variable proteins accommodate massive sequence variation, we determined the 2.52 Å resolution limit crystal structure of one such protein, AvpA, which revealed a C-type lectin (CLec)-fold that organizes a putative ligand-binding site that is capable of accommodating 10¹³ sequences. This fold is surprisingly reminiscent of the CLec-folds of viral and bacterial DGR variable protein, but differs sufficiently to define a new CLec-fold subclass, which is consistent with early divergence between bacterial and archaeal DGRs. The structure also enabled identification of a group of AvpA-like proteins in multiple putative DGRs from uncultivated archaea. These variable proteins may aid Nanoarchaeota and these uncultivated archaea in symbiotic relationships.

Conclusions

Our results have uncovered the widespread conservation of the CLec-fold in viruses, bacteria, and archaea for accommodating massive sequence variation. In addition, to our knowledge, this is the first report of an archaeal CLec-fold protein.

A widespread bacterial phenazine forms S-conjugates with biogenic thiols and crosslinks proteins

Phenazines are redox-active compounds produced by a range of bacteria, including many pathogens. Endowed with various biological activities, these ubiquitous N-heterocycles are well known for their ability to generate reactive oxygen species by redox cycling. Phenazines may lead to an irreversible depletion of glutathione, but a detailed mechanism has remained elusive. Furthermore, it is not understood why phenazines have so many protein targets and cause protein misfolding as well as their aggregation. Here we report the discovery of unprecedented conjugates (panphenazines A, B) of panthetheine and phenazine-1-carboxylic (PCA) acid from a Kitasatospora sp., which prompted us to investigate their biogenesis. We found that PCA reacts with diverse biogenic thiols under radical-forming conditions, which provides a plausible model for irreversible glutathione depletion. To evaluate the scope of the reaction in cells we designed biotin and rhodamine conjugates for protein labelling and examined their covalent fusion with model proteins (ketosynthase, carbonic anhydrase III, albumin). Our results reveal important, yet overlooked biological roles of phenazines and show for the first time their function in protein conjugation and crosslinking.

Shedding light on ovothiol biosynthesis in marine metazoans

Ovothiol, isolated from marine invertebrate eggs, is considered one of the most powerful antioxidant with potential for drug development. However, its biological functions in marine organisms still represent a matter of debate. In sea urchins, the most accepted view is that ovothiol protects the eggs by the high oxidative burst at fertilization. In this work we address the role of ovothiol during sea urchin development to give new insights on ovothiol biosynthesis in metazoans. The gene involved in ovothiol biosynthesis OvoA was identified in Paracentrotus lividus genome (PlOvoA). PlOvoA embryo expression significantly increased at the pluteus stage and was up-regulated by metals at concentrations mimicking polluted sea-water and by cyclic toxic algal blooms, leading to ovothiol biosynthesis. In silico analyses of the PlOvoA upstream region revealed metal and stress responsive elements. Structural protein models highlighted conserved active site residues likely responsible for ovothiol biosynthesis. Phylogenetic analyses indicated that OvoA evolved in most marine metazoans and was lost in bony vertebrates during the transition from the aquatic to terrestrial environment. These results highlight the crucial role of OvoA in protecting embryos released in seawater from environmental cues, thus allowing the survival under different conditions.

Ergothioneine, an adaptive antioxidant for the protection of injured tissues? A hypothesis

Ergothioneine (ET) is a diet-derived, thiolated derivative of histidine with antioxidant properties. Although ET is produced only by certain fungi and bacteria, it can be found at high concentrations in certain human and animal tissues and is absorbed through a specific, high affinity transporter (OCTN1). In liver, heart, joint and intestinal injury, elevated ET concentrations have been observed in injured tissues. The physiological role of ET remains unclear. We thus review current literature to generate a specific hypothesis: that the accumulation of ET in vivo is an adaptive mechanism, involving the regulated uptake and concentration of an exogenous natural compound to minimize oxidative damage.

Conversion of a non-heme iron-dependent sulfoxide synthase into a thiol dioxygenase by a single point mutation

EgtB from Mycobacterium thermoresistibile catalyzes O₂-dependent sulfur–carbon bond formation between the side chains of Nα-trimethyl histidine and γ-glutamyl cysteine as a central step in ergothioneine biosynthesis.