Dimethyl Sulfoxide Reductase Research Articles

Novel Photoelectron-Assisted Microbial Reduction of Arsenate Driven by Photosensitive Dissolved Organic Matter in Mine Stream Sediments.

The microbial reduction of arsenate (As(V)) significantly contributes to arsenic migration in mine stream sediment, primarily driven by heterotrophic microorganisms using dissolved organic matter (DOM) as a carbon source. This study reveals a novel reduction pathway in sediments that photosensitive DOM generates photoelectrons to stimulate diverse nonphototrophic microorganisms to reduce As(V). This microbial photoelectrophic As(V) reduction (PEAsR) was investigated using microcosm incubation, which showed the transfer of photoelectrons from DOM to indigenous sediment microorganisms, thereby leading to a 50% higher microbial reduction rate of As(V). The abundance of two marker genes for As(V) reduction, arrA and arsC, increased substantially, confirming the microbial nature of PEAsR rather than a photoelectrochemical process. Photoelectron ion is unlikely to stimulate photolithoautotrophic growth. Instead, diverse nonphototrophic genera, e.g., Cupriavidus, Sphingopyxis, Mycobacterium, and Bradyrhizobium, spanning 13 orders became enriched by 10-50 folds. Metagenomic binning revealed their genetic potential to mediate the photoelectron-assisted reduction of As(V). These microorganisms contain essential genes involved in respiratory As(V) reduction, detoxification As(V) reduction, dimethyl sulfoxide reductase family, c-type cytochromes, and multiple heavy-metal resistance but lack a complete photosynthesis system. The novel microbial PEAsR pathway offers new insights into the interaction between photoelectron utilization and nonphototrophic As(V)-reducing microorganisms, which may have profound implications for arsenic pollution transportation in mine stream sediment.

Nitrate-dependent antimony oxidase in an uncultured Symbiobacteriaceae member.

Autotrophic antimony (Sb) oxidation coupled to nitrate reduction plays an important role in the transformation and detoxification of Sb. However, the specific oxidase involved in this process has yet to be identified. Herein, we enriched the microbiota capable of nitrate-dependent Sb(III) oxidation and identified a new Sb(III) oxidase in an uncultured member of Symbiobacteriaceae. Incubation experiments demonstrated that nitrate-dependent Sb(III) oxidation occurred in the microcosm supplemented with Sb(III) and nitrate. Both the 16S rRNA gene and metagenomic analyses indicated that a species within Symbiobacteriaceae played a crucial role in this process. Furthermore, carbon-13 isotope labelling with carbon dioxide-fixing Rhodopseudomonas palustris in combination with nanoscale secondary ion mass spectrometry revealed that a newly characterized oxidase from the dimethylsulfoxide reductase family, designated as NaoABC, was responsible for autotrophic Sb(III) oxidation coupled with nitrate reduction. The NaoABC complex functions in conjunction with the nitrate reductase NarGHI, forming a redox loop that transfers electrons from Sb(III) to nitrate, thereby generating the energy necessary for autotrophic growth. This research offers new insights into the understanding of how microbes link Sb and nitrogen biogeochemical cycles in the environment.

Mo than meets the eye: genomic insights into molybdoenzyme diversity of Seleniivibrio woodruffii strain S4T.

Seleniivibrio woodruffii strain S4T is an obligate anaerobe belonging to the phylum Deferribacterota. It was isolated for its ability to respire selenate and was also found to respire arsenate. The high-quality draft genome of this bacterium is 2.9 Mbp, has a G+C content of 48%, 2762 predicted genes of which 2709 are protein-coding, and 53 RNA genes. An analysis of the genome focusing on the genes encoding for molybdenum-containing enzymes (molybdoenzymes) uncovered a remarkable number of genes encoding for members of the dimethylsulfoxide reductase family of proteins (DMSOR), including putative reductases for selenate and arsenate respiration, as well as genes for nitrogen fixation. Respiratory molybdoenzymes catalyze redox reactions that transfer electrons to a variety of substrates that can act as terminal electron acceptors for energy generation. Seleniivibrio woodruffii strain S4T also has essential genes for molybdate transporters and the biosynthesis of the molybdopterin guanine dinucleotide cofactors characteristic of the active centers of DMSORs. Phylogenetic analysis revealed candidate respiratory DMSORs spanning nine subfamilies encoded within the genome. Our analysis revealed the untapped potential of this interesting microorganism and expanded our knowledge of molybdoenzyme co-occurrence.

Identification of bacterial dissimilatory antimonate reductase AnrA: genes and proteins involved in antimonate respiration and resistance in Geobacter sp. strain SVR.

Geobacter sp. strain SVR uses antimonate [Sb(V)] as a terminal electron acceptor for anaerobic respiration. Here, we visualized a possible key enzyme, periplasmic Sb(V) reductase (Anr), via active staining and non-denaturing gel electrophoresis. Liquid chromatography-tandem mass spectrometry analysis revealed that a novel dimethyl sulfoxide (DMSO) reductase family protein, WP_173201954.1, is involved in Anr. This protein was closely related with AnrA, a protein suggested to be the catalytic subunit of a respiratory Sb(V) reductase in Desulfuribacillus stibiiarsenatis. The anr genes of strain SVR (anrXSRBAD) formed an operon-like structure, and their transcription was upregulated under Sb(V)-respiring conditions. The expression of anrA gene was induced by more than 1 µM of antimonite [Sb(III)]; however, arsenite [As(III)] did not induce the expression of anrA gene. Tandem mass tag-based proteomic analysis revealed that, in addition to Anr proteins, proteins in the following categories were upregulated under Sb(V)-respiring conditions: (i) Sb(III) efflux systems such as Ant and Ars; (ii) antioxidizing proteins such as ferritin, rubredoxin, and thioredoxin; (iii) protein quality control systems such as HspA, HslO, and DnaK; and (iv) DNA repair proteins such as UspA and UvrB. These results suggest that strain SVR copes with antimony stress by modulating pleiotropic processes to resist and actively metabolize antimony. To the best of our knowledge, this is the first report to demonstrate the involvement of AnrA in Sb(V) respiration at the protein level. Furthermore, this is the first example to show high expression of the Ant system proteins in the Sb(V)-respiring bacterium.IMPORTANCEAntimony (Sb) exists mainly as antimonite [Sb(III)] or antimonate [Sb(V)] in the environment, and Sb(III) is more toxic than Sb(V). Recently, microbial involvement in Sb redox reactions has received attention. Although more than 90 Sb(III)-oxidizing bacteria have been reported, information on Sb(V)-reducing bacteria is limited. Especially, the enzyme involved in dissimilatory Sb(V) reduction, or Sb(V) respiration, is unclear, despite this pathway being very important for the circulation of Sb in nature. In this study, we demonstrated that the Sb(V) reductase (Anr) of an Sb(V)-respiring bacterium (Geobacter sp. SVR) is a novel member of the dimethyl sulfoxide (DMSO) reductase family. In addition, we found that strain SVR copes with Sb stress by modulating pleiotropic processes, including the Ant and Ars systems, and upregulating the antioxidant and quality control protein levels. Considering the abundance and diversity of putative anr genes in the environment, Anr may play a significant role in global Sb cycling in both marine and terrestrial environments.

The crucial and versatile roles of bacteria in global biogeochemical cycling of iodine

Abstract Iodine (I) is a trace element with health and environmental significance. Iodate (IO3-), iodide (I-) and organic iodine (org-I) are the major species of iodine that exist in the environment. Dissimilatory IO3--reducing bacteria reduce IO3- to I- directly under anoxic conditions via their IO3- reductases that include periplasmic iodate reductase IdrABP1P2, extracellular DMSO reductase DmsEFAB and metal reductase MtrCAB. IdrAB and DmsEFAB reduce IO3- to hypoiodous acid (HIO) and H2O2. The reaction intermediate HIO is proposed to be disproportionated abiotically into I- and IO3- at a ratio of 2:1. The H2O2 is reduced to H2O by IdrP1P2 and MtrCAB as a detoxification mechanism. Additionally, dissimilatory Fe(III)- and sulfate-reducing bacteria reduce IO3- to I- directly via their IO3- reductases and indirectly via the reduction products Fe(II) and sulfide in the presence of Fe(III) and sulfate, respectively. I--oxidizing bacteria oxidize I- to molecular iodine (I2) directly under oxic conditions via their extracellular multicopper iodide oxidases IoxAC. In addition to I2, a variety of org-I compounds are also produced by the I--oxidizing bacteria during I- oxidation. Furthermore, ammonia-oxidizing bacteria oxidize I- to IO3- directly under oxic conditions, probably via their intracellular ammonia-oxidizing enzymes. Many bacteria produce extracellular reactive oxygen species that can oxidize I- to triiodide (I3-). Bacteria also accumulate I- during which I- is oxidized to HIO by their extracellular vanadium iodoperoxidases. The HIO is then transported into the bacterial cells. Finally, bacteria methylate I- to org-I CH3I, probably via their methyltransferases. Thus, bacteria play crucial and versatile roles in the global biogeochemical cycling of iodine via IO3- reduction, I- oxidation and accumulation and org-I formation.

Microbial drivers of DMSO reduction and DMS-dependent methanogenesis in saltmarsh sediments

Saltmarshes are highly productive environments, exhibiting high abundances of organosulfur compounds. Dimethylsulfoniopropionate (DMSP) is produced in large quantities by algae, plants, and bacteria and is a potential precursor for dimethylsulfoxide (DMSO) and dimethylsulfide (DMS). DMSO serves as electron acceptor for anaerobic respiration leading to DMS formation, which is either emitted or can be degraded by methylotrophic prokaryotes. Major products of these reactions are trace gases with positive (CO2, CH4) or negative (DMS) radiative forcing with contrasting effects on the global climate. Here, we investigated organic sulfur cycling in saltmarsh sediments and followed DMSO reduction in anoxic batch experiments. Compared to previous measurements from marine waters, DMSO concentrations in the saltmarsh sediments were up to ~300 fold higher. In batch experiments, DMSO was reduced to DMS and subsequently consumed with concomitant CH4 production. Changes in prokaryotic communities and DMSO reductase gene counts indicated a dominance of organisms containing the Dms-type DMSO reductases (e.g., Desulfobulbales, Enterobacterales). In contrast, when sulfate reduction was inhibited by molybdate, Tor-type DMSO reductases (e.g., Rhodobacterales) increased. Vibrionales increased in relative abundance in both treatments, and metagenome assembled genomes (MAGs) affiliated to Vibrio had all genes encoding the subunits of DMSO reductases. Molar conversion ratios of <1.3 CH4 per added DMSO were accompanied by a predominance of the methylotrophic methanogens Methanosarcinales. Enrichment of mtsDH genes, encoding for DMS methyl transferases in metagenomes of batch incubations indicate their role in DMS-dependent methanogenesis. MAGs affiliated to Methanolobus carried the complete set of genes encoding for the enzymes in methylotrophic methanogenesis.

The Reversible Electrochemical Interconversion of Formate and CO2 by Formate Dehydrogenase from Cupriavidus necator.

The bacterial molybdenum (Mo)-containing formate dehydrogenase (FdsDABG) from Cupriavidus necator is a soluble NAD+-dependent enzyme belonging to the DMSO reductase family. The holoenzyme is complex and possesses nine redox-active cofactors including a bis(molybdopterin guanine dinucleotide) (bis-MGD) active site, seven iron-sulfur clusters, and 1 equiv of flavin mononucleotide (FMN). FdsDABG catalyzes the two-electron oxidation of HCOO- (formate) to CO2 and reversibly reduces CO2 to HCOO- under physiological conditions close to its thermodynamic redox potential. Here we develop an electrocatalytically active formate oxidation/CO2 reduction system by immobilizing FdsDABG on a glassy carbon electrode in the presence of coadsorbents such as chitosan and glutaraldehyde. The reversible enzymatic interconversion between HCOO- and CO2 by FdsDABG has been realized with cyclic voltammetry using a range of artificial electron transfer mediators, with methylene blue (MB) and phenazine methosulfate (PMS) being particularly effective as electron acceptors for FdsDABG in formate oxidation. Methyl viologen (MV) acts as both an electron acceptor (MV2+) in formate oxidation and an electron donor (MV+•) for CO2 reduction. The catalytic voltammetry was reproduced by electrochemical simulation across a range of sweep rates and concentrations of formate and mediators to provide new insights into the kinetics of the FdsDABG catalytic mechanism.

Bringing Nitric Oxide to the Molybdenum World-A Personal Perspective.

Molybdenum-containing enzymes of the xanthine oxidase (XO) family are well known to catalyse oxygen atom transfer reactions, with the great majority of the characterised enzymes catalysing the insertion of an oxygen atom into the substrate. Although some family members are known to catalyse the "reverse" reaction, the capability to abstract an oxygen atom from the substrate molecule is not generally recognised for these enzymes. Hence, it was with surprise and scepticism that the "molybdenum community" noticed the reports on the mammalian XO capability to catalyse the oxygen atom abstraction of nitrite to form nitric oxide (NO). The lack of precedent for a molybdenum- (or tungsten) containing nitrite reductase on the nitrogen biogeochemical cycle contributed also to the scepticism. It took several kinetic, spectroscopic and mechanistic studies on enzymes of the XO family and also of sulfite oxidase and DMSO reductase families to finally have wide recognition of the molybdoenzymes' ability to form NO from nitrite. Herein, integrated in a collection of "personal views" edited by Professor Ralf Mendel, is an overview of my personal journey on the XO and aldehyde oxidase-catalysed nitrite reduction to NO. The main research findings and the path followed to establish XO and AO as competent nitrite reductases are reviewed. The evidence suggesting that these enzymes are probable players of the mammalian NO metabolism is also discussed.

Metal-Containing Formate Dehydrogenases, a Personal View.

Mo/W-containing formate dehydrogenases (FDH) catalyzes the reversible oxidation of formate to carbon dioxide at their molybdenum or tungsten active sites. The metal-containing FDHs are members of the dimethylsulfoxide reductase family of mononuclear molybdenum cofactor (Moco)- or tungsten cofactor (Wco)-containing enzymes. In these enzymes, the active site in the oxidized state comprises a Mo or W atom present in the bis-Moco, which is coordinated by the two dithiolene groups from the two MGD moieties, a protein-derived SeCys or Cys, and a sixth ligand that is now accepted as being a sulfido group. SeCys-containing enzymes have a generally higher turnover number than Cys-containing enzymes. The analogous chemical properties of W and Mo, the similar active sites of W- and Mo-containing enzymes, and the fact that W can replace Mo in some enzymes have led to the conclusion that Mo- and W-containing FDHs have the same reaction mechanism. Details of the catalytic mechanism of metal-containing formate dehydrogenases are still not completely understood and have been discussed here.

Anaerobic selenite-reducing bacteria and their metabolic potentials in Se-rich sediment revealed by the combination of DNA-stable isotope probing, metagenomic binning, and metatranscriptomics

Microorganisms play a critical role in the biogeochemical cycling of selenium (Se) in aquatic environments, particularly in reducing the toxicity and bioavailability of selenite (Se(IV)). This study aimed to identify putative Se(IV)-reducing bacteria (SeIVRB) and investigate the genetic mechanisms underlying Se(IV) reduction in anoxic Se-rich sediment. Initial microcosm incubation confirmed that Se(IV) reduction was driven by heterotrophic microorganisms. DNA stable-isotope probing (DNA-SIP) analysis identified Pseudomonas, Geobacter, Comamonas, and Anaeromyxobacter as putative SeIVRB. High-quality metagenome-assembled genomes (MAGs) affiliated with these four putative SeIVRB were retrieved. Annotation of functional gene indicated that these MAGs contained putative Se(IV)-reducing genes such as DMSO reductase family, fumarate and sulfite reductases. Metatranscriptomic analysis of active Se(IV)-reducing cultures revealed significantly higher transcriptional levels of genes associated with DMSO reductase (serA/PHGDH), fumarate reductase (sdhCD/frdCD), and sulfite reductase (cysDIH) compared to those in cultures not amended with Se(IV), suggesting that these genes played important roles in Se(IV) reduction. The current study expands our knowledge of the genetic mechanisms involved in less-understood anaerobic Se(IV) bio-reduction. Additinally, the complementary abilities of DNA-SIP, metagenomics, and metatranscriptomics analyses are demonstrated in elucidating the microbial mechanisms of biogeochemical processes in anoxic sediment.

Influence of the ligand-field on EPR parameters of cis- and trans-isomers in MoV systems relevant to molybdenum enzymes: Experimental and density functional theory study

The electron paramagnetic resonance (EPR) investigation of mononuclear cis- and trans-(L1O)MoOCl2 complexes [L1OH = bis(3,5-dimethylpyrazolyl)-3-tert-butyl-2-hydroxy-5-methylphenyl)methane] reveals a significant difference in their spin Hamiltonian parameters which reflect different equatorial and axial ligand fields created by the heteroscorpionate donor atoms. Density functional theory (DFT) was used to calculate the values of principal components and relative orientations of the g and A tensors, and the molecular framework in four pairs of isomeric mononuclear oxo‑molybdenum(V) complexes (cis- and trans-(L1O)MoOCl2, cis,cis- and cis,trans-(L-N2S2)MoOCl [L-N2S2H2 = N,N′-dimethyl-N,N′-bis(mercaptophenyl)ethylenediamine], cis,cis- and cis,trans-(L-N2S2)MoO(SCN), and cis- and trans-[(dt)2MoO(OMe)]2− [dtH2 = 2,3-dimercapto-2-butene]). Scalar relativistic DFT calculations were conducted using three different exchange-correlation functionals. It was found that the use of hybrid exchange-correlation functional with 25% of the Hartree-Fock exchange leads to the best quantitative agreement between theory and experiment. A simplified ligand-field approach was used to analyze the influence of the ligand fields in all cis- and trans-isomers on energies and contributions of molybdenum d-orbital manifold to g and A tensors and relative orientations. Specifically, contributions that originated from the spin-orbit coupling of the dxz, dyz, and dx2-y2 orbitals into the ground state have been discussed. The new findings are discussed in the context of the experimental data of mononuclear molybdoenzyme, DMSO reductase.

Impact of the Dimethyl Sulfoxide Reductase Superfamily on the Evolution of Biogeochemical Cycles.

The dimethyl sulfoxide reductase (or MopB) family is a diverse assemblage of enzymes found throughout Bacteria and Archaea. Many of these enzymes are believed to have been present in the last universal common ancestor (LUCA) of all cellular lineages. However, gaps in knowledge remain about how MopB enzymes evolved and how this diversification of functions impacted global biogeochemical cycles through geologic time. In this study, we perform maximum likelihood phylogenetic analyses on manually curated comparative genomic and metagenomic data sets containing over 47,000 distinct MopB homologs. We demonstrate that these enzymes constitute a catalytically and mechanistically diverse superfamily defined not by the molybdopterin- or tungstopterin-containing [molybdopterin or tungstopterin bis(pyranopterin guanine dinucleotide) (Mo/W-bisPGD)] cofactor but rather by the structural fold that binds it in the protein. Our results suggest that major metabolic innovations were the result of the loss of the metal cofactor or the gain or loss of protein domains. Phylogenetic analyses also demonstrated that formate oxidation and CO2 reduction were the ancestral functions of the superfamily, traits that have been vertically inherited from the LUCA. Nearly all of the other families, which drive all other biogeochemical cycles mediated by this superfamily, originated in the bacterial domain. Thus, organisms from Bacteria have been the key drivers of catalytic and biogeochemical innovations within the superfamily. The relative ordination of MopB families and their associated catalytic activities emphasize fundamental mechanisms of evolution in this superfamily. Furthermore, it underscores the importance of prokaryotic adaptability in response to the transition from an anoxic to an oxidized atmosphere. IMPORTANCE The MopB superfamily constitutes a repertoire of metalloenzymes that are central to enduring mysteries in microbiology, from the origin of life and how microorganisms and biogeochemical cycles have coevolved over deep time to how anaerobic life adapted to increasing concentrations of O2 during the transition from an anoxic to an oxic world. Our work emphasizes that phylogenetic analyses can reveal how domain gain or loss events, the acquisition of novel partner subunits, and the loss of metal cofactors can stimulate novel radiations of enzymes that dramatically increase the catalytic versatility of superfamilies. We also contend that the superfamily concept in protein evolution can uncover surprising kinships between enzymes that have remarkably different catalytic and physiological functions.

Genomic Analysis of the Deep-Sea Bacterium Shewanella sp. MTB7 Reveals Backgrounds Related to Its Deep-Sea Environment Adaptation

Shewanella species are widely distributed in various environments, especially deep-sea sediments, due to their remarkable ability to utilize multiple electron receptors and versatile metabolic capabilities. In this study, a novel facultatively anaerobic, psychrophilic, and piezotolerant bacterium, Shewanella sp. MTB7, was isolated from the Mariana Trench at a depth of 5900 m. Here, we report its complete genome sequence and adaptation strategies for survival in deep-sea environments. MTB7 contains what is currently the third-largest genome among all isolated Shewanella strains and shows higher coding density than neighboring strains. Metabolically, MTB7 is predicted to utilize various carbon and nitrogen sources. D-amino acid utilization and HGT-derived purine-degrading genes could contribute to its oligotrophic adaptation. For respiration, the cytochrome o ubiquinol oxidase genes cyoABCDE, typically expressed at high oxygen concentrations, are missing. Conversely, a series of anaerobic respiratory genes are employed, including fumarate reductase, polysulfide reductase, trimethylamine-N-oxide reductase, crotonobetaine reductase, and Mtr subunits. The glycine reductase genes and the triplication of dimethyl sulfoxide reductase genes absent in neighboring strains could also help MTB7 survive in low-oxygen environments. Many genes encoding cold-shock proteins, glycine betaine transporters and biosynthetic enzymes, and reactive oxygen species-scavenging proteins could contribute to its low-temperature adaptation. The genomic analysis of MTB7 will deepen our understanding of microbial adaptation strategies in deep-sea environments.

Nonredundant Dimethyl Sulfoxide Reductases Influence Salmonella enterica Serotype Typhimurium Anaerobic Growth and Virulence.

Facultative anaerobic enteric pathogens can utilize a diverse array of alternate electron acceptors to support anaerobic metabolism and thrive in the hypoxic conditions within the mammalian gut. Dimethyl sulfoxide (DMSO) is produced by methionine catabolism and can act as an alternate electron acceptor to support anaerobic respiration. The DMSO reductase complex consists of three subunits, DmsA, DmsB, and DmsC, and allows bacteria to grow anaerobically with DMSO as an electron acceptor. The genomes of nontyphoidal Salmonella enterica encode three putative dmsABC operons, but the impact of the apparent genetic redundancy in DMSO reduction on the fitness of nontyphoidal S. enterica during infection remains unknown. We hypothesized that DMSO reduction would be needed for S. enterica serotype Typhimurium to colonize the mammalian gut. We demonstrate that an S. Typhimurium mutant with loss of function in all three putative DMSO reductases (ΔdmsA3) poorly colonizes the mammalian intestine when the microbiota is intact and when inflammation is absent. DMSO reduction enhances anaerobic growth through nonredundant contributions of two of the DMSO reductases. Furthermore, DMSO reduction influences virulence by increasing expression of the type 3 secretion system 2 and reducing expression of the type 3 secretion system 1. Collectively, our data demonstrate that the DMSO reductases of S. Typhimurium are functionally nonredundant and suggest DMSO is a physiologically relevant electron acceptor that supports S. enterica fitness in the gut.

Metabolic engineering of Mannheimia succiniciproducens for malic acid production using dimethylsulfoxide as an electron acceptor.

Microbial production of various TCA intermediates and related chemicals through the reductive TCA cycle has been of great interest. However, rumen bacteria that naturally possess strong reductive TCA cycle have been rarely studied to produce these chemicals, except for succinic acid, due to their dependence on fumarate reduction to transport electrons for ATP synthesis. In this study, malic acid (MA), a dicarboxylic acid of industrial importance, was selected as a target chemical for mass production using Mannheimia succiniciproducens, a rumen bacterium possessing a strong reductive branch of the TCA cycle. The metabolic pathway was reconstructed by eliminating fumarase to prevent MA conversion to fumarate. The respiration system of M. succiniciproducens was reconstructed by introducing the Actinobacillus succinogenes dimethylsulfoxide (DMSO) reductase to improve cell growth using DMSO as an electron acceptor. Also, the cell membrane was engineered by employing Pseudomonas aeruginosa cis-trans isomerase to enhance MA tolerance. High inoculum fed-batch fermentation of the final engineered strain produced 61 g/L of MA with an overall productivity of 2.27 g/L/h, which is the highest MA productivity reported to date. The systems metabolic engineering strategies reported in this study will be useful for developing anaerobic bioprocesses for the production of various industrially important chemicals.

Bromate reduction by Shewanella oneidensis MR-1 is mediated by dimethylsulfoxide reductase.

Microbial bromate reduction plays an important role in remediating bromate-contaminated waters as well as biogeochemical cycling of bromine. However, little is known about the molecular mechanism of microbial bromate reduction so far. Since the model strain Shewanella oneidensis MR-1 is capable of reducing a variety of oxyanions such as iodate, which has a high similarity to bromate, we hypothesize that S. oneidensis MR-1 can reduce bromate. Here, we conducted an experiment to investigate whether S. oneidensis MR-1 can reduce bromate, and report bromate reduction mediated by a dimethylsulfoxide reductase encoded with dmsA. S. oneidensis MR-1 is not a bromate-respiring bacterium but can reduce bromate to bromide under microaerobic conditions. When exposed to 0.15, 0.2, 0.25, 0.5, and 1 mM bromate, S. oneidensis MR-1 reduced bromate by around 100, 75, 64, 48, and 23%, respectively, within 12 h. In vivo evidence from gene deletion mutants and complemented strains of S. oneidensis MR-1 indicates that MtrB, MtrC, CymA, GspD, and DmsA are involved in bromate reduction, but not NapA, FccA, or SYE4. Based on our results as well as previous findings, a proposed molecular mechanism for bromate reduction is presented in this study. Moreover, a genomic survey indicates that 9 of the other 56 reported Shewanella species encode proteins highly homologous to CymA, GspD, and DmsA of S. oneidensis MR-1 by sequence alignment. The results of this study contribute to understanding a pathway for microbial bromate reduction.

Replacement of Molybdenum by Tungsten in a Biomimetic Complex Leads to an Increase in Oxygen Atom Transfer Catalytic Activity.

Upon replacement of molybdenum by tungsten in DMSO reductase isolated from the Rhodobacteraceae family, the derived enzyme catalyzes DMSO reduction faster. To better understand this behavior, we synthesized two tungsten(VI) dioxido complexes [WVIO2L2] with pyridine- (PyS) and pyrimidine-2-thiolate (PymS) ligands, isostructural to analogous molybdenum complexes we reported recently. Higher oxygen atom transfer (OAT) catalytic activity was observed with [WO2(PyS)2] compared to the Mo species, independent of whether PMe3 or PPh3 was used as the oxygen acceptor. [WVIO2L2] complexes undergo reduction with an excess of PMe3, yielding the tungsten(IV) oxido species [WOL2(PMe3)2], while with PPh3, no reactions are observed. Although OAT reactions from DMSO to phosphines are known for tungsten complexes, [WOL2(PMe3)2] are the first fully characterized phosphine-stabilized intermediates. By following the reaction of these reduced species with excess DMSO via UV–vis spectroscopy, we observed that tungsten compounds directly react to WVIO2 complexes while the Mo analogues first form μ-oxo Mo(V) dimers [Mo2O3L4]. Density functional theory calculations confirm that the oxygen atom abstraction from WVIO2 is an endergonic process contrasting the respective reaction with molybdenum. Here, we suggest that depending on the sacrificial oxygen acceptor, the tungsten complex may participate in catalysis either via a redox reaction or as an electrophile.

Resonance Raman spectroscopy of pyranopterin molybdenum enzymes

Resonance Raman spectroscopy (rR) is a powerful spectroscopic probe that is widely used for studying the geometric and electronic structure of metalloproteins. In this focused review, we detail how resonance Raman spectroscopy has contributed to a greater understanding of electronic structure, geometric structure, and the reaction mechanisms of pyranopterin molybdenum enzymes. The review focuses on the enzymes sulfite oxidase (SO), dimethyl sulfoxide reductase (DMSOR), xanthine oxidase (XO), and carbon monoxide dehydrogenase. Specifically, we highlight how Mo-Ooxo, Mo-Ssulfido, Mo-Sdithiolene, and dithiolene CC vibrational modes, isotope and heavy atom perturbations, resonance enhancement, and associated Raman studies of small molecule analogs have provided detailed insight into the nature of these metalloenzyme active sites.

The Role of the Nucleotides in the Insertion of the bis-Molybdopterin Guanine Dinucleotide Cofactor into apo-Molybdoenzymes.

The role of the GMP nucleotides of the bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor of the DMSO reductase family has long been a subject of discussion. The recent characterization of the bis-molybdopterin (bis-Mo-MPT) cofactor present in the E. coli YdhV protein, which differs from bis-MGD solely by the absence of the nucleotides, now enables studying the role of the nucleotides of bis-MGD and bis-MPT cofactors in Moco insertion and the activity of molybdoenzymes in direct comparison. Using the well-known E. coli TMAO reductase TorA as a model enzyme for cofactor insertion, we were able to show that the GMP nucleotides of bis-MGD are crucial for the insertion of the bis-MGD cofactor into apo-TorA.

Iodate Reduction by Shewanella oneidensis Requires Genes Encoding an Extracellular Dimethylsulfoxide Reductase.

Microbial iodate (IO3–) reduction is a major component of the iodine biogeochemical reaction network in anaerobic marine basins and radioactive iodine-contaminated subsurface environments. Alternative iodine remediation technologies include microbial reduction of IO3– to iodide (I–) and microbial methylation of I– to volatile gases. The metal reduction pathway is required for anaerobic IO3– respiration by the gammaproteobacterium Shewanella oneidensis. However, the terminal IO3– reductase and additional enzymes involved in the S. oneidensis IO3– electron transport chain have not yet been identified. In this study, gene deletion mutants deficient in four extracellular electron conduits (EECs; ΔmtrA, ΔmtrA-ΔmtrDEF, ΔmtrA-ΔdmsEF, ΔmtrA-ΔSO4360) and DMSO reductase (ΔdmsB) of S. oneidensis were constructed and examined for anaerobic IO3– reduction activity with either 20 mM lactate or formate as an electron donor. IO3– reduction rate experiments were conducted under anaerobic conditions in defined minimal medium amended with 250 μM IO3– as anaerobic electron acceptor. Only the ΔmtrA mutant displayed a severe deficiency in IO3– reduction activity with lactate as the electron donor, which suggested that the EEC-associated decaheme cytochrome was required for lactate-dependent IO3– reduction. The ΔmtrA-ΔdmsEF triple mutant displayed a severe deficiency in IO3– reduction activity with formate as the electron donor, whereas ΔmtrA-ΔmtrDEF and ΔmtrA-ΔSO4360 retained moderate IO3– reduction activity, which suggested that the EEC-associated dimethylsulfoxide (DMSO) reductase membrane-spanning protein DmsE, but not MtrA, was required for formate-dependent IO3– reduction. Furthermore, gene deletion mutant ΔdmsB (deficient in the extracellular terminal DMSO reductase protein DmsB) and wild-type cells grown with tungsten replacing molybdenum (a required co-factor for DmsA catalytic activity) in defined growth medium were unable to reduce IO3– with either lactate or formate as the electron donor, which indicated that the DmsAB complex functions as an extracellular IO3– terminal reductase for both electron donors. Results of this study provide complementary genetic and phenotypic evidence that the extracellular DMSO reductase complex DmsAB of S. oneidensis displays broad substrate specificity and reduces IO3– as an alternate terminal electron acceptor.