The active site structure and catalytic mechanism of arsenite oxidase

Lasers efficacy in pain management after primary and secondary endodontic treatment: a systematic review and meta-analysis of randomized clinical trials

Postoperative pain is a common concern following root canal treatments (RCT), impacting both patients and oral health practitioners. This systematic review and meta-analysis aimed to evaluate the effectiveness of laser treatment modalities in reducing postoperative pain compared to conventional methods after primary and secondary RCT in permanent mature teeth. A search of three electronic databases (PubMed, ScienceDirect, and The Cochrane Library) was conducted, using a broad range of keywords and terms. Gray literature and manual searches were conducted to complement the search. The inclusion criteria included randomized clinical trials based on the objective of the secondary study. A minimum sample size of 10 participants per group and a clearly defined criterion for postoperative pain assessment were required. The characteristics of the included studies were presented as tables. The Cochrane collaboration tool RoB 2.0 was used to assess the risk of bias within each study. Two reviewers extracted the data and assessed the studies independently, and discrepancies were resolved through consultation with a third reviewer. A random-effects model was employed for meta-analysis to estimate the overall effect measure. Heterogeneity was evaluated using Cochran's Q test and the I2 index. Publication bias was explored via Funnel plots and Egger's test. Subgroup analyses and meta-regression were conducted to assess variations among laser methods and examine the influence of independent factors. The significance threshold for all analyses was set at 5% (α = 0.05). Intraoral laser therapy demonstrated no significant advantage over conventional treatments but consistently outperformed placebo, particularly from 4 to 72 h post-treatment. Low-level laser therapy provided slight pain reduction in the first 8 h, though its effectiveness diminished in retreatment scenarios. Photodynamic therapy and laser disinfection showed marginal benefits, especially shortly after treatment, with reduced efficacy in longer-term or retreatment contexts. Further research is needed to explore different applications of laser modalities and assess distinct prognostic factors in more detail.

Active Site Characterization of a Campylobacter jejuni Nitrate Reductase Variant Provides Insight into the Enzyme Mechanism

Mo K-edge X-ray absorption spectroscopy (XAS) is used to probe the structure of wild-type Campylobacter jejuni nitrate reductase NapA and the C176A variant. The results of extended X-ray absorption fine structure (EXAFS) experiments on wt NapA support an oxidized Mo(VI) hexacoordinate active site coordinated by a single terminal oxo donor, four sulfur atoms from two separate pyranopterin dithiolene ligands, and an additional S atom from a conserved cysteine amino acid residue. We found no evidence of a terminal sulfido ligand in wt NapA. EXAFS analysis shows the C176A active site to be a 6-coordinate structure, and this is supported by EPR studies on C176A and small molecule analogs of Mo(V) enzyme forms. The SCys is replaced by a hydroxide or water ligand in C176A, and we find no evidence of a coordinated sulfhydryl (SH) ligand. Kinetic studies show that this variant has completely lost its catalytic activity toward nitrate. Taken together, the results support a critical role for the conserved C176 in catalysis and an oxygen atom transfer mechanism for the catalytic reduction of nitrate to nitrite that does not employ a terminal sulfido ligand in the catalytic cycle.

Genomic comparison of deep-sea hydrothermal genera related to Aeropyrum, Thermodiscus and Caldisphaera, and proposed emended description of the family Acidilobaceae

Deep-sea hydrothermal vents host archaeal and bacterial thermophilic communities, including taxonomically and functionally diverse Thermoproteota. Despite their prevalence in high-temperature submarine communities, Thermoproteota are chronically under-represented in genomic databases and issues have emerged regarding their nomenclature, particularly within the Aeropyrum-Thermodiscus-Caldisphaera. To resolve some of these problems, we identified 47 metagenome-assembled genomes (MAGs) within this clade, from 20 previously published deep-sea hydrothermal vent and submarine volcano metagenomes, and 24 MAGs from public databases. Using phylogenomic analysis, Genome Taxonomy Database Toolkit (GTDB-Tk) taxonomic assessment, 16S rRNA gene phylogeny, average amino acid identity (AAI) and functional gene patterns, we re-evaluated of the taxonomy of the Aeropyrum-Thermodiscus-Caldisphaera. At least nine genus-level clades were identified with two or more MAGs. In accordance with SeqCode requirements and recommendations, we propose names for three novel genera, viz. Tiamatella incendiivivens, Hestiella acidicharens and Calypsonella navitae. A fourth genus was also identified related to Thermodiscus maritimus, for which no available sequenced genome exists. We propose the novel species Thermodiscus eudorianus to describe our high-quality Thermodiscus MAG, which represents the type genome for the genus. All three novel genera and T. eudorianus are likely anaerobic heterotrophs, capable of fermenting protein-rich carbon sources, while some Tiamatella, Calypsonella and T. eudorianus may also reduce polysulfides, thiosulfate, sulfur and/or selenite, and the likely acidophile, Hestiella, may reduce nitrate and/or perchlorate. Based on phylogenomic evidence, we also propose the family Acidilobaceae be amended to include Caldisphaera, Aeropyrum, Thermodiscus and Stetteria and the novel genera described here.

Bacterial biota associated with the invasive insect pest Tuta absoluta (Meyrick)

Tuta absoluta (the tomato pinworm) is an invasive insect pest with a highly damaging effect on tomatoes causing between 80 and 100% yield losses if left uncontrolled. Resistance to chemical pesticides have been reported in some T. absoluta populations. Insect microbiome plays an important role in the behavior, physiology, and survivability of their host. In a bid to explore and develop an alternative control method, the associated microbiome of this insect was studied. In this study, we unraveled the bacterial biota of T. absoluta larvae and adults by sequencing and analyzing the 16S rRNA V3-V4 gene regions using Illumina NovaSeq PE250. Out of 2,092,015 amplicon sequence variants (ASVs) recovered from 30 samples (15 larvae and 15 adults), 1,268,810 and 823,205 ASVs were obtained from the larvae and adults, respectively. A total of 433 bacterial genera were shared between the adults and larval samples while 264 and 139 genera were unique to the larvae and adults, respectively. Amplicon metagenomic analyses of the sequences showed the dominance of the phylum Proteobacteria in the adult samples while Firmicutes and Proteobacteria dominated in the larval samples. Linear discriminant analysis effect size (LEfSe) comparison revealed the genera Pseudomonas, Delftia and Ralstonia to be differentially enriched in the adult samples while Enterococcus, Enterobacter, Lactococcus, Klebsiella and Wiessella were differentially abundant in the larvae. The diversity indices showed that the bacterial communities were not different between the insect samples collected from different geographical regions. However, the bacterial communities significantly differed based on the sample type between larvae and adults. A co-occurrence network of significantly correlated taxa revealed a strong interaction between the microbial communities. The functional analysis of the microbiome using FAPROTAX showed that denitrification, arsenite oxidation, methylotrophy and methanotrophy as the active functional groups of the adult and larvae microbiomes. Our results have revealed the core taxonomic, functional, and interacting microbiota of T. absoluta and these indicate that the larvae and adults harbor a similar but transitory set of bacteria. The results provide a novel insight and a basis for exploring microbiome-based biocontrol strategy for this invasive insect pest as well as the ecological significance of some of the identified microbiota is discussed.

Functional features of a novel Sb(III)- and As(III)-oxidizing bacterium: Implications for the interactions between bacterial Sb(III) and As(III) oxidation pathways

Antimony (Sb) and arsenic (As) share similar chemical characteristics and commonly coexist in contaminated environments. It has been reported that the biogeochemical cycles of antimony and arsenic affect each other. However, there is limited understanding regarding microbial coupling between the biogeochemical processes of antimony and arsenic. Here, we aimed to solve this issue. We successfully isolated a novel bacterium, Shinella sp. SbAsOP1, which possesses both Sb(III) and As(III) oxidase, and can effectively oxidize both Sb(III) and As(III) under aerobic and anaerobic conditions. SbAsOP1 exhibits greater aerobic oxidation activity for the oxidation of As(III) or Sb(III) compared to its anaerobic activity. SbAsOP1 also significantly catalyzes the oxidative mobilization of solid-phase Sb(III) under aerobic conditions. The activity of SbAsOP1 in oxidizing solid Sb(III) is 3 times lower than its activity in oxidizing soluble form. It is noteworthy that, in the presence of both Sb(III) and As(III) under aerobic conditions, either As(III) or Sb(III) significantly inhibits the oxidation of Sb(III) or As(III), respectively. In comparison, under anaerobic conditions and in the coexistence of Sb(III) and As(III), As(III) significantly inhibits Sb(III) oxidation, whereas Sb(III) almost completely inhibits As(III) oxidation. These findings suggest that under both aerobic and anaerobic conditions, SbAsOP1 demonstrates a partial preference for Sb(III) oxidation. Additionally, bacterial oxidations of Sb(III) and As(III) mutually inhibit each other to varying degrees. These observations gain a novel understanding of the interplay between the biogeochemical processes of antimony and arsenic.

Arsenite oxidase in complex with antimonite and arsenite oxyanions: Insights into the catalytic mechanism

Arsenic contamination of groundwater is among one of the biggest health threats affecting millions of people in the world. There is an urgent need for efficient arsenic biosensors where the use of arsenic metabolizing enzymes can be explored. In this work, we have solved four crystal structures of arsenite oxidase (Aio) in complex with arsenic and antimony oxyanions and the structures determined correspond to intermediate states of the enzymatic mechanism. These structural data were complemented with density-functional theory calculations providing a unique view of the molybdenum active site at different time points that, together with mutagenesis data, enabled to clarify the enzymatic mechanism and the molecular determinants for the oxidation of As(III) to the less toxic As(V) species.

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.

Spectroscopic and Structural Characterization of Reduced Desulfovibrio vulgaris Hildenborough W-FdhAB Reveals Stable Metal Coordination during Catalysis

Metal-dependent formate dehydrogenases are important enzymes due to their activity of CO₂ reduction to formate. The tungsten-containing FdhAB formate dehydrogenase from Desulfovibrio vulgaris Hildenborough is a good example displaying high activity, simple composition, and a notable structural and catalytic robustness. Here, we report the first spectroscopic redox characterization of FdhAB metal centers by EPR. Titration with dithionite or formate leads to reduction of three [4Fe-4S]¹⁺ clusters, and full reduction requires Ti(III)-citrate. The redox potentials of the four [4Fe-4S]¹⁺ centers range between -250 and -530 mV. Two distinct W^V signals were detected, W_D^V and W_F^V, which differ in only the g₂-value. This difference can be explained by small variations in the twist angle of the two pyranopterins, as determined through DFT calculations of model compounds. The redox potential of W^VI/V was determined to be -370 mV when reduced by dithionite and -340 mV when reduced by formate. The crystal structure of dithionite-reduced FdhAB was determined at high resolution (1.5 Å), revealing the same structural alterations as reported for the formate-reduced structure. These results corroborate a stable six-ligand W coordination in the catalytic intermediate W^V state of FdhAB.

Different Regulatory Strategies of Arsenite Oxidation by Two Isolated Thermus tengchongensis Strains From Hot Springs

Arsenic is a ubiquitous constituent in geothermal fluids. Thermophiles represented by Thermus play vital roles in its transformation in geothermal fluids. In this study, two Thermus tengchongensis strains, named as 15Y and 15W, were isolated from arsenic-rich geothermal springs and found different arsenite oxidation behaviors with different oxidation strategies. Arsenite oxidation of both strains occurred at different growth stages, and two enzyme-catalyzed reaction kinetic models were observed. The arsenite oxidase of Thermus strain 15W performed better oxidation activity, exhibiting typical Michaelis–Menten kinetics. The kinetic parameter of arsenite oxidation in whole cell showed a V_max of 18.48 μM min^–1 and K_M of 343 μM. Both of them possessed the arsenite oxidase-coding genes aioB and aioA. However, the expression of gene aioBA was constitutive in strain 15W, whereas it was induced by arsenite in strain 15Y. Furthermore, strain 15Y harbored an intact aio operon including the regulatory gene of the ArsR family, whereas a genetic inversion of an around 128-kbp fragment produced the inactivation of this regulator in strain 15W, leading to the constitutive expression of aioBA genes. This study provides a valuable insight into the adaption of thermophiles to extreme environments.

Organoarsenic feed additives in biological wastewater treatment processes: Removal, biotransformation, and associated impacts

Aromatic organoarsenicals are widely used in animal feeding operations and cause arsenic contamination on livestock wastewater and manure, thereby raising the risk of surface water pollution. Biological wastewater treatment processes are often used for livestock wastewater treatment. Organoarsenic removal and biotransformation under aerobic and anaerobic conditions, and the associated impacts have received extensive attention due to the potential threat to water security. The removal efficiency and biotransformation of organoarsenicals in biological treatment processes are reviewed. The underlying mechanisms are discussed in terms of functional microorganisms and genes. The impacts associated with organoarsenicals and their degradation products on microbial activity and performance of bioreactors are also documented. Based on the current research advancement, knowledge gaps and potential research in this field are discussed. Overall, this work delivers a comprehensive understanding on organoarsenic behaviors in biological wastewater treatment processes, and provides valuable information on the control of arsenic contamination from the degradation of organoarsenicals in biological wastewater treatment processes.

Phylogenomics reveals the basis of adaptation of Pseudorhizobium species to extreme environments and supports a taxonomic revision of the genus

The family Rhizobiaceae includes many genera of soil bacteria, often isolated for their association with plants. Herein, we investigate the genomic diversity of a group of Rhizobium species and unclassified strains isolated from atypical environments, including seawater, rock matrix or polluted soil. Based on whole-genome similarity and core genome phylogeny, we show that this group corresponds to the genus Pseudorhizobium. We thus reclassify Rhizobium halotolerans, R. marinum, R. flavum and R. endolithicum as P. halotolerans sp. nov., P. marinum comb. nov., P. flavum comb. nov. and P. endolithicum comb. nov., respectively, and show that P. pelagicum is a synonym of P. marinum. We also delineate a new chemolithoautotroph species, P. banfieldiae sp. nov., whose type strain is NT-26^T (=DSM 106348^T=CFBP 8663^T). This genome-based classification was supported by a chemotaxonomic comparison, with increasing taxonomic resolution provided by fatty acid, protein and metabolic profiles. In addition, we used a phylogenetic approach to infer scenarios of duplication, horizontal transfer and loss for all genes in the Pseudorhizobium pangenome. We thus identify the key functions associated with the diversification of each species and higher clades, shedding light on the mechanisms of adaptation to their respective ecological niches. Respiratory proteins acquired at the origin of Pseudorhizobium were combined with clade-specific genes to enable different strategies for detoxification and nutrition in harsh, nutrient-poor environments.

Application of Next Generation Sequencing (NGS) in Phage Displayed Peptide Selection to Support the Identification of Arsenic-Binding Motifs

Next generation sequencing (NGS) in combination with phage surface display (PSD) are powerful tools in the newly equipped molecular biology toolbox for the identification of specific target binding biomolecules. Application of PSD led to the discovery of manifold ligands in clinical and material research. However, limitations of traditional phage display hinder the identification process. Growth-based library biases and target-unrelated peptides often result in the dominance of parasitic sequences and the collapse of library diversity. This study describes the effective enrichment of specific peptide motifs potentially binding to arsenic as proof-of-concept using the combination of PSD and NGS. Arsenic is an environmental toxin, which is applied in various semiconductors as gallium arsenide and selective recovery of this element is crucial for recycling and remediation. The development of biomolecules as specific arsenic-binding sorbents is a new approach for its recovery. Usage of NGS for all biopanning fractions allowed for evaluation of motif enrichment, in-depth insight into the selection process and the discrimination of biopanning artefacts, e.g., the amplification-induced library-wide reduction in hydrophobic amino acid proportion. Application of bioinformatics tools led to the identification of an SxHS and a carboxy-terminal QxQ motif, which are potentially involved in the binding of arsenic. To the best of our knowledge, this is the first report of PSD combined with NGS of all relevant biopanning fractions.

Microbial Oxidation of Arsenite: Regulation, Chemotaxis, Phosphate Metabolism and Energy Generation

Arsenic (As) is a metalloid that occurs widely in the environment. The biological oxidation of arsenite [As(III)] to arsenate [As(V)] is considered a strategy to reduce arsenic toxicity and provide energy. In recent years, research interests in microbial As(III) oxidation have been growing, and related new achievements have been revealed. This review focuses on the highlighting of the novel regulatory mechanisms of bacterial As(III) oxidation, the physiological relevance of different arsenic sensing systems and functional relationship between microbial As(III) oxidation and those of chemotaxis, phosphate uptake, carbon metabolism and energy generation. The implication to environmental bioremediation applications of As(III)-oxidizing strains, the knowledge gaps and perspectives are also discussed.

Methane, arsenic, selenium and the origins of the DMSO reductase family

Mononuclear molybdoenzymes of the dimethyl sulfoxide reductase (DMSOR) family catalyze a number of reactions essential to the carbon, nitrogen, sulfur, arsenic, and selenium biogeochemical cycles. These enzymes are also ancient, with many lineages likely predating the divergence of the last universal common ancestor into the Bacteria and Archaea domains. We have constructed rooted phylogenies for over 1,550 representatives of the DMSOR family using maximum likelihood methods to investigate the evolution of the arsenic biogeochemical cycle. The phylogenetic analysis provides compelling evidence that formylmethanofuran dehydrogenase B subunits, which catalyze the reduction of CO₂ to formate during hydrogenotrophic methanogenesis, constitutes the most ancient lineage. Our analysis also provides robust support for selenocysteine as the ancestral ligand for the Mo/W atom. Finally, we demonstrate that anaerobic arsenite oxidase and respiratory arsenate reductase catalytic subunits represent a more ancient lineage of DMSORs compared to aerobic arsenite oxidase catalytic subunits, which evolved from the assimilatory nitrate reductase lineage. This provides substantial support for an active arsenic biogeochemical cycle on the anoxic Archean Earth. Our work emphasizes that the use of chalcophilic elements as substrates as well as the Mo/W ligand in DMSORs has indelibly shaped the diversification of these enzymes through deep time.

Catalytic properties of the metal ion variants of mandelate racemase reveal alterations in the apparent electrophilicity of the metal cofactor

Mandalate racemase (MR) from Pseudomonas putida requires a divalent metal cation, usually Mg2+, to catalyse the interconversion of the enantiomers of mandelate. Although the active site Mg2+ may be replaced by Mn2+, Co2+, or Ni2+, substitution by these metal ions does not markedly (<10-fold) alter the kinetic parameters Kappm, kappcat, and (kcat/Km)app for the substrates (R)- and (S)-mandelate, and the alternative substrate (S)-trifluorolactate. Viscosity variation experiments with Mn2+-MR showed that the metal ion plays a role in the uniform binding of the transition states for enzyme-substrate association, the chemical step, and enzyme-product dissociation. Surprisingly, the competitive inhibition constants (Ki) for inhibition of each metalloenzyme variant by benzohydroxamate did not vary significantly with the identity of the metal ion unlike the marked variation of the stability constants (K1) observed for M2+·BzH complex formation in solution. A similar trend was observed for the inhibition of the metalloenzyme variants by F-, except for Mg2+-MR, which bound F- tighter than would be predicted based on the stability constants for formation of M2+·F- complexes in solution. Thus, the enzyme modifies the enatic state of the bound metal ion cofactor so that the apparent electrophilicity of Mg2+ is enhanced, while that of Ni2+ is attenuated, resulting in a levelling effect relative to the trends observed for the free metals in solution.

Functional mononuclear molybdenum enzymes: challenges and triumphs in molecular cloning, expression, and isolation

Mononuclear molybdenum enzymes catalyze a variety of reactions that are essential in the cycling of nitrogen, carbon, arsenic, and sulfur. For decades, the structure and function of these crucial enzymes have been investigated to develop a fundamental knowledge for this vast family of enzymes and the chemistries they carry out. Therefore, obtaining abundant quantities of active enzyme is necessary for exploring this family's biochemical capability. This mini-review summarizes the methods for overexpressing mononuclear molybdenum enzymes in the context of the challenges encountered in the process. Effective methods for molybdenum cofactor synthesis and incorporation, optimization of expression conditions, improving isolation of active vs. inactive enzyme, incorporation of additional prosthetic groups, and inclusion of redox enzyme maturation protein chaperones are discussed in relation to the current molybdenum enzyme literature. This article summarizes the heterologous and homologous expression studies providing underlying patterns and potential future directions.

Structural and mechanistic analysis of the arsenate respiratory reductase provides insight into environmental arsenic transformations

Arsenate respiration by bacteria was discovered over two decades ago and is catalyzed by diverse organisms using the well-conserved Arr enzyme complex. Until now, the mechanisms underpinning this metabolism have been relatively opaque. Here, we report the structure of an Arr complex (solved by X-ray crystallography to 1.6-Å resolution), which was enabled by an improved Arr expression method in the genetically tractable arsenate respirer Shewanella sp. ANA-3. We also obtained structures bound with the substrate arsenate (1.8 Å), the product arsenite (1.8 Å), and the natural inhibitor phosphate (1.7 Å). The structures reveal a conserved active-site motif that distinguishes Arr [(R/K)GRY] from the closely related arsenite respiratory oxidase (Arx) complex (XGRGWG). Arr activity assays using methyl viologen as the electron donor and arsenate as the electron acceptor display two-site ping-pong kinetics. A Mo(V) species was detected with EPR spectroscopy, which is typical for proteins with a pyranopterin guanine dinucleotide cofactor. Arr is an extraordinarily fast enzyme that approaches the diffusion limit (K_m = 44.6 ± 1.6 μM, k_cat = 9,810 ± 220 seconds^-1), and phosphate is a competitive inhibitor of arsenate reduction (K_i = 325 ± 12 μM). These observations, combined with knowledge of typical sedimentary arsenate and phosphate concentrations and known rates of arsenate desorption from minerals in the presence of phosphate, suggest that (i) arsenate desorption limits microbiologically induced arsenate reductive mobilization and (ii) phosphate enhances arsenic mobility by stimulating arsenate desorption rather than by inhibiting it at the enzymatic level.

Modulating the Molybdenum Coordination Sphere of Escherichia coli Trimethylamine N-Oxide Reductase

The well-studied enterobacterium Escherichia coli present in the human gut can reduce trimethylamine N-oxide (TMAO) to trimethylamine during anaerobic respiration. The TMAO reductase TorA is a monomeric, bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor-containing enzyme that belongs to the dimethyl sulfoxide reductase family of molybdoenzymes. We report on a system for the in vitro reconstitution of TorA with molybdenum cofactors (Moco) from different sources. Higher TMAO reductase activities for TorA were obtained when using Moco sources containing a sulfido ligand at the molybdenum atom. For the first time, we were able to isolate functional bis-MGD from Rhodobacter capsulatus formate dehydrogenase (FDH), which remained intact in its isolated state and after insertion into apo-TorA yielded a highly active enzyme. Combined characterizations of the reconstituted TorA enzymes by electron paramagnetic resonance spectroscopy and direct electrochemistry emphasize that TorA activity can be modified by changes in the Mo coordination sphere. The combination of these results together with studies of amino acid exchanges at the active site led us to propose a novel model for binding of the substrate to the molybdenum atom of TorA.