The molybdenum oxotransferases and related enzymes

Discovery of a Gut Bacterial Pathway for Ergothioneine Catabolism

Ergothioneine is a diet-derived micronutrient for humans. However, enzymes involved in the catabolism of ergothioneine in human gut bacteria have not yet been identified. Herein, we characterize a sulfidogenic pathway for gut bacterial catabolism of this micronutrient, which involves an unprecedented reductive desulfurization reaction catalyzed by members of the xanthine oxidoreductase family (XOR), a class of molybdenum-containing flavoproteins. Notably, this is the first C-S bond cleavage reaction known to be catalyzed by XORs. Evidence for operation of this pathway was gained through in vitro reconstruction using heterologously produced enzymes derived from the human gut bacterium Blautia producta ATCC 27340. This catabolic activity enables B. producta ATCC 27340 to use ergothioneine as an alternative electron acceptor source. Homologues of the pathway enzymes are shown to be present not only in human gut bacteria but also in many environmental bacteria, suggesting the wide distribution of this catabolic strategy. In relation to the sulfur-containing metabolite, this discovery provides significant insight into biogeochemical sulfur cycling in diverse anoxic habitats beyond the human gut and, moreover, the design of new approaches for controlling intestinal hydrogen sulfide (H2S) production.

Iron-sulfur cluster-dependent enzymes and molybdenum-dependent reductases in the anaerobic metabolism of human gut microbes

Metalloenzymes play central roles in the anaerobic metabolism of human gut microbes. They facilitate redox and radical-based chemistry that enables microbial degradation and modification of various endogenous, dietary, and xenobiotic nutrients in the anoxic gut environment. In this review, we highlight major families of iron-sulfur (Fe-S) cluster-dependent enzymes and molybdenum cofactor-containing enzymes used by human gut microbes. We describe the metabolic functions of 2-hydroxyacyl-CoA dehydratases, glycyl radical enzyme activating enzymes, Fe-S cluster-dependent flavoenzymes, U32 oxidases, and molybdenum-dependent reductases and catechol dehydroxylases in the human gut microbiota. We demonstrate the widespread distribution and prevalence of these metalloenzyme families across 5000 human gut microbial genomes. Lastly, we discuss opportunities for metalloenzyme discovery in the human gut microbiota to reveal new chemistry and biology in this important community.

New solvated Mo(VI) complexes of isatin based asymmetric bisthiocarbohydrazones as potent bioactive agent: synthesis, DFT-molecular docking studies, biological activity evaluation and crystal structures

New solvated Mo(VI) complexes were isolated from the reaction of [MoO2(acac)2] with asymmetric isatin bisthiocarbohydrazone ligands. The ligands were obtained from the reaction of isatin monothiocarbohydrazone with 3,5-dibromo salicylaldehyde (L1), 3,5-dichloro salicylaldehyde (L2) and 3-chloro-5-bromo salicylaldehyde (L3), respectively. In the complexes, the ligands serve as ONS donors and coordinate to the [MoO2]2+ nucleus. The bonding sites are azomethine nitrogen atom, phenolic oxygen atom and thiol sulfur atom. The sixth coordination site is completed by an oxygen atom from an ethanol solvent. The ethanol-coordinated Mo(VI) complexes, C1-C3, [MoO2L(EtOH)] (L: L1-L3), were characterized using elemental analysis, IR and 1H NMR spectroscopies, and conductivity measurements. By crystallizing ethanol-solvated solid complexes from an EtOH/DMSO mixture, DMSO-solvated complexes (C4-C6) suitable for X-ray crystallography were obtained. Crystal structure analysis supports the proposed complex structures and geometries, but the ethanol in the sixth coordination site has been replaced by DMSO. When the anticarcinogenic effects of the ligands and complexes (C1-C3) on the C6 cell line were examined, it was found that the complexes showed higher activity than the ligands. The C3 complex appears to have the best anti-cancer activity compared to doxorubicin. Additionally, all compounds were determined to have high total antioxidant capacity. Data obtained from theoretical studies (DFT and docking) support experimental studies.

Generation of Thiosulfate, Selenite, Dithiosulfite, Perthionitrite, Nitric Oxide, and Reactive Chalcogen Species by Binuclear Zinc(II)-Chalcogenolato/-Polychalcogenido Complexes

A comparative bioinspired reactivity study of new binuclear Zn(II) complexes featuring coordinated thiolate, selenolate, trisulfide and diselenide in relation with (i) the generation of reactive sulfur/selenium species (RSS/RSeS), (ii) the oxygen dependent oxidation and disproportionation of polysulfide (Sn2-) to produce sulfite (SO32-), thiosulfate (S2O32-) and sulfide (S2-) by sulfur oxygenase reductase (SOR), and (iii) the reaction of Sn2- with nitrite (NO2-) to generate thionitrite (SNO-), perthionitrite (SSNO-) and nitric oxide (NO), is presented. The binuclear Zn(II)-thiolate/selenolate complexes could react with elemental sulfur to generate RSS/RSeS while similar reactions involving elemental selenium could not generate RSeS. The dizinc(II)-S3 and the dizinc(II)-Se2 complexes could react with dioxygen (O2) to generate binuclear Zn(II) complexes featuring coordinated thiosulfate (S2O32-) and selenite (SeO32-), respectively. Finally, unlike the nonreactive nature of the dizinc(II)-Se2 complex toward NO2-, reaction of the dizinc(II)-S3 complex with NO2- produced a new binuclear Zn(II) complex featuring a coordinated dithiosulfite (S3O2-) along with the formation of perthionitrite (SSNO-), of which the latter subsequently produced nitric oxide (NO) and S42-. The present work, thus, demonstrates the comparative reactivity of a series of binuclear Zn(II)-chalcogenolato/-polychalcogenido complexes for the generation of S2O32-, SeO32-, S3O2-, SSNO-, NO and RSS/RSeS.

The different effects of molybdate on Hg(II) bio-methylation in aerobic and anaerobic bacteria

In nature, methylmercury (MeHg) is primarily generated through microbial metabolism, and the ability of bacteria to methylate Hg(II) depends on both bacterial properties and environmental factors. It is widely known that, as a metabolic analog, molybdate can inhibit the sulfate reduction process and affect the growth and methylation of sulfate-reducing bacteria (SRB). However, after it enters the cell, molybdate can be involved in various intracellular metabolic pathways as a molybdenum cofactor; whether fluctuations in its concentration affect the growth and methylation of aerobic mercury methylating strains remains unknown. To address this gap, aerobic γ-Proteobacteria strains Raoultella terrigena TGRB3 (B3) and Pseudomonas putida TGRB4 (B4), as well as an obligate anaerobic δ-Proteobacteria strain of the SRB Desulfomicrobium escambiense CGMCC 1.3481 (DE), were used as experimental strains. The growth and methylation ability of each strain were analyzed under conditions of 500 ng·L-1 Hg(II), 0 and 21% of oxygen, and 0, 0.25, 0.50, and 1 mM of MoO4 2-. In addition, in order to explore the metabolic specificity of aerobic strains, transcriptomic data of the facultative mercury-methylated strain B3 were further analyzed in an aerobic mercuric environment. The results indicated that: (a) molybdate significantly inhibited the growth of DE, while B3 and B4 exhibited normal growth. (b) Under anaerobic conditions, in DE, the MeHg content decreased significantly with increasing molybdate concentration, while in B3, MeHg production was unaffected. Furthermore, under aerobic conditions, the MeHg productions of B3 and B4 were not influenced by the molybdate concentration. (c) The transcriptomic analysis showed several genes that were annotated as members of the molybdenum oxidoreductase family of B3 and that exhibited significant differential expression. These findings suggest that the differential expression of molybdenum-binding proteins might be related to their involvement in energy metabolism pathways that utilize nitrate and dimethyl sulfoxide as electron acceptors. Aerobic bacteria, such as B3 and B4, might possess distinct Hg(II) biotransformation pathways from anaerobic SRB, rendering their growth and biomethylation abilities unaffected by molybdate.

Selective Reduction of Aqueous Nitrate to Ammonium with an Electropolymerized Chromium Molecular Catalyst

Nitrate (NO3-) is a common nitrogen-containing contaminant in agricultural, industrial, and low-level nuclear wastewater that causes significant environmental damage. In this work, we report a bioinspired Cr-based molecular catalyst incorporated into a redox polymer that selectively and efficiently reduces aqueous NO3- to ammonium (NH4+), a desirable value-added fertilizer component and industrial precursor, at rates of ∼0.36 mmol NH4+ mgcat-1 h-1 with >90% Faradaic efficiency for NH4+. The NO3- reduction reaction occurs through a cascade catalysis mechanism involving the stepwise reduction of NO3- to NH4+ via observed NO2- and NH2OH intermediates. To our knowledge, this is one of the first examples of a molecular catalyst, homogeneous or heterogenized, that is reported to reduce aqueous NO3- to NH4+ with rates and Faradaic efficiencies comparable to those of state-of-the-art solid-state electrocatalysts. This work highlights a promising and previously unexplored area of electrocatalyst research using polymer-catalyst composites containing complexes with oxophilic transition metal active sites for electrochemical nitrate remediation with nutrient recovery.

Complementary hydrophobic interaction of the redox enzyme maturation protein NarJ with the signal peptide of the respiratory nitrate reductase NarG

In bacteria, NarJ plays an essential role as a redox enzyme maturation protein in the assembly of the nitrate reductase NarGHI by interacting with the N-terminal signal peptide of NarG to facilitate cofactor incorporation into NarG. The purpose of our research was to elucidate the exact mechanism of NarG signal peptide recognition by NarJ. We determined the structures of NarJ alone and in complex with the signal peptide of NarG via X-ray crystallography and verified the NarJ-NarG interaction through mutational, binding, and molecular dynamics simulation studies. NarJ adopts a curved α-helix bundle structure with a U-shaped hydrophobic groove on its concave side. This groove accommodates the signal peptide of NarG via a dual binding mode in which the left and right parts of the NarJ groove each interact with two consecutive hydrophobic residues from the N- and C-terminal regions of the NarG signal peptide, respectively, through shape and chemical complementarity. This binding is accompanied by unwinding of the helical structure of the NarG signal peptide and by stabilization of the NarG-binding loop of NarJ. We conclude that NarJ recognizes the NarG signal peptide through a complementary hydrophobic interaction mechanism that mediates a structural rearrangement.

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.

Tungsten Enzyme Using Hydrogen as an Electron Donor to Reduce Carboxylic Acids and NAD. ACS Catal 2022;12:8707-8717. [PMID: 35874620 PMCID: PMC9295118 DOI: 10.1021/acscatal.2c02147]

Tungsten-dependent aldehyde oxidoreductases (AORs) catalyze the oxidation of aldehydes to acids and are the only known enzymes reducing non-activated acids using electron donors with low redox potentials. We report here that AOR from Aromatoleum aromaticum (AOR_Aa) catalyzes the reduction of organic acids not only with low-potential Eu(II) or Ti(III) complexes but also with H₂ as an electron donor. Additionally, AOR_Aa catalyzes the H₂-dependent reduction of NAD⁺ or benzyl viologen. The rate of H₂-dependent NAD⁺ reduction equals to 10% of that of aldehyde oxidation, representing the highest H₂ turnover rate observed among the Mo/W enzymes. As AOR_Aa simultaneously catalyzes the reduction of acids and NAD⁺, we designed a cascade reaction utilizing a NAD(P)H-dependent alcohol dehydrogenase to reduce organic acids to the corresponding alcohols with H₂ as the only reductant. The newly discovered W-hydrogenase side activity of AOR_Aa may find applications in either NADH recycling or conversion of carboxylic acids to more useful biochemicals.

Inspired by Nature-Functional Analogues of Molybdenum and Tungsten-Dependent Oxidoreductases

Throughout the previous ten years many scientists took inspiration from natural molybdenum and tungsten-dependent oxidoreductases to build functional active site analogues. These studies not only led to an ever more detailed mechanistic understanding of the biological template, but also paved the way to atypical selectivity and activity, such as catalytic hydrogen evolution. This review is aimed at representing the last decade’s progress in the research of and with molybdenum and tungsten functional model compounds. The portrayed systems, organized according to their ability to facilitate typical and artificial enzyme reactions, comprise complexes with non-innocent dithiolene ligands, resembling molybdopterin, as well as entirely non-natural nitrogen, oxygen, and/or sulfur bearing chelating donor ligands. All model compounds receive individual attention, highlighting the specific novelty that each provides for our understanding of the enzymatic mechanisms, such as oxygen atom transfer and proton-coupled electron transfer, or that each presents for exploiting new and useful catalytic capability. Overall, a shift in the application of these model compounds towards uncommon reactions is noted, the latter are comprehensively discussed.

Nanotechnology Tools Enabling Biological Discovery

Over the years, the engineering aspect of nanotechnology has been significantly exploited. Medical intervention strategies have been developed by leveraging existing molecular biology knowledge and combining it with nanotechnology tools to improve outcomes. However, little attention has been paid to harnessing the strengths of nanotechnology as a biological discovery tool. Fundamental understanding of controlling dynamic biological processes at the subcellular level is key to developing personalized therapeutic and diagnostic interventions. Single-cell analyses using intravital microscopy, expansion microscopy, and microfluidic-based platforms have been helping to better understand cell heterogeneity in healthy and diseased cells, a major challenge in oncology. Also, single-cell analysis has revealed critical signaling pathways and biological intracellular components with key biological functions. The physical manipulation enabled by nanotools can allow real-time monitoring of biological changes at a single-cell level by sampling intracellular fluid from the same cell. The formation of intercellular highways by nanotube-like structures has important clinical implications such as metastasis development. The integration of nanomaterials into optical and molecular imaging techniques has rendered valuable morphological, structural, and biological information. Nanoscale imaging unravels mechanisms of temporality by enabling the visualization of nanoscale dynamics never observed or measured between individual cells with standard biological techniques. The exceptional sensitivity of nanozymes, artificial enzymes, make them perfect components of the next-generation mobile diagnostics devices. Here, we highlight these impactful cancer-related biological discoveries enabled by nanotechnology and producing a paradigm shift in cancer research and oncology.

Can water act as a nucleophile in CO oxidation catalysed by Mo/Cu CO-dehydrogenase? Answers from theory

The aerobic CO dehydrogenase from Oligotropha carboxidovorans is an environmentally crucial bacterial enzyme for maintenance of subtoxic concentration of CO in the lower atmosphere, as it allows for the oxidation of CO to CO₂ which takes place at its Mo−Cu heterobimetallic active site. Despite extensive experimental and theoretical efforts, significant uncertainties still concern the reaction mechanism for the CO oxidation. In this work, we used the hybrid quantum mechanical/molecular mechanical approach to evaluate whether a water molecule present in the active site might act as a nucleophile upon formation of the new C−O bond, a hypothesis recently suggested in the literature. Our study shows that activation of H₂O can be favoured by the presence of the Mo=O_eq group. However, overall our results suggest that mechanisms other than the nucleophilic attack by Mo=O_eq to the activated carbon of the CO substrate are not likely to constitute reactive channels for the oxidation of CO by the enzyme.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Deoxygenation reactions in organic synthesis catalyzed by dioxomolybdenum(VI) complexes

Dioxomolybdenum(VI) complexes have been applied as efficient, inexpensive and benign catalysts to deoxygenation reactions of a diverse number of compounds in the last two decades. Dioxomolybdenum complexes have demonstrated wide applicability to the deoxygenation of sulfoxides into sulfides and reduction of N-O bonds. Even the challenging nitro functional group was efficiently deoxygenated, affording amines or diverse heterocycles after reductive cyclization reactions. More recently, carbon-based substrates like epoxides, alcohols and ketones have been successfully deoxygenated. Also, dioxomolybdenum complexes accomplished deoxydehydration (DODH) reactions of biomass-derived vicinal 1,2-diols, affording valuable alkenes. The choice of the catalytic systems and reductant is decisive to achieve the desired transformation. Commonly found reducing agents involved phosphorous-based compounds, silanes, molecular hydrogen, or even glycols and other alcohols.

Characterisation of the redox centers of ethylbenzene dehydrogenase

Ethylbenzene dehydrogenase (EbDH), the initial enzyme of anaerobic ethylbenzene degradation from the beta-proteobacterium Aromatoleum aromaticum, is a soluble periplasmic molybdenum enzyme consisting of three subunits. It contains a Mo-bis-molybdopterin guanine dinucleotide (Mo-bis-MGD) cofactor and an 4Fe-4S cluster (FS0) in the α-subunit, three 4Fe-4S clusters (FS1 to FS3) and a 3Fe-4S cluster (FS4) in the β-subunit and a heme b cofactor in the γ-subunit. Ethylbenzene is hydroxylated by a water molecule in an oxygen-independent manner at the Mo-bis-MGD cofactor, which is reduced from the Mo^VI to the Mo^IV state in two subsequent one-electron steps. The electrons are then transferred via the Fe-S clusters to the heme b cofactor. In this report, we determine the midpoint redox potentials of the Mo-bis-MGD cofactor and FS1-FS4 by EPR spectroscopy, and that of the heme b cofactor by electrochemically induced redox difference spectroscopy. We obtained relatively high values of > 250 mV both for the Mo^VI-Mo^V redox couple and the heme b cofactor, whereas FS2 is only reduced at a very low redox potential, causing magnetic coupling with the neighboring FS1 and FS3. We compare the results with the data on related enzymes and interpret their significance for the function of EbDH.

Oxo versus Sulfido Coordination at Tungsten: A Spectroscopic and Correlated Ab Initio Electronic Structure Study

The tungsten ion that resides at the active site of a unique class of enzymes only found in esoteric hyperthermophilic archaea bacteria is known to possess at least one terminal chalcogenide ligand. The identity of this as either an oxo or sulfido (or both) is difficult to ascertain from structural studies; therefore, small-molecule analogues are developed to calibrate and substantiate spectroscopic signatures obtained from native proteins. The electronic structures of Tp*WECl₂ (E = O, S; Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) have been scrutinized using electronic, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopy to assess the impact of terminal chalcogen on the adjacent cis chloride ligands. Examination at the Cl K-edge provides a direct probe of the bonding and therein lability of these chloride ligands, and in conjunction with density functional theoretical and multireference calculations reveals greater bond covalency in Tp*WOCl₂ compared to Tp*WSCl₂. The computational model and electronic structure assignment are corroborated by the reproduction of spin-Hamiltonian parameters, whose magnitude is dominated by the sizeable spin-orbit coupling of tungsten.

Mo@GAA-Fe3O4 MNPs: a highly efficient and environmentally friendly heterogeneous magnetic nanocatalyst for the synthesis of polyhydroquinoline derivatives

Polyhydroquinolines were efficiently obtained from a sequential four-component reaction between dimedone or 1,3-cyclohexandione, ethyl acetoacetate, or methyl acetoacetate as a β-ketoester, aldehydes, and ammonium acetate, under the catalysis of Mo@GAA-Fe3O4 MNPs as a green, effective, recyclable, and environmentally friendly nanocatalyst. Due to its magnetic nature the prepared catalyst can be easily separated from the reaction mixture by an external magnet and reused several times without significant changes in catalytic activity and reaction efficiency. The catalyst was characterized using energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), vibrating sample magnetometry (VSM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

Oxygen Atom Transfer Reactivity of Molybdenum(VI) Complexes Employing Pyrimidine- and Pyridine-2-thiolate Ligands

Four dioxidomolybdenum(VI) complexes of the general structure [MoO₂L₂] employing the S,N-bidentate ligands pyrimidine-2-thiolate (PymS, 1), pyridine-2-thiolate (PyS, 2), 4-methylpyridine-2-thiolate (4-MePyS, 3) and 6-methylpyridine-2-thiolate (6-MePyS, 4) were synthesized and characterized by spectroscopic means and single-crystal X-ray diffraction analysis (2-4). Complexes 1-4 were reacted with PPh₃ and PMe₃, respectively, to investigate their oxygen atom transfer (OAT) reactivity and catalytic applicability. Reduction with PPh₃ leads to symmetric molybdenum(V) dimers of the general structure [Mo₂O₃L₄] (6-9). Kinetic studies showed that the OAT from [MoO₂L₂] to PPh₃ is 5 times faster for the PymS system than for the PyS and 4-MePyS systems. The reaction of complexes 1-3 with PMe₃ gives stable molybdenum(IV) complexes of the structure [MoOL₂(PMe₃)₂] (10-12), while reduction of [MoO₂(6-MePyS)₂] (4) yields [MoO(6-MePyS)₂(PMe₃)] (13) with only one PMe₃ coordinated to the metal center. The activity of complexes 1-4 in catalytic OAT reactions involving Me₂SO and Ph₂SO as oxygen donors and PPh₃ as an oxygen acceptor has been investigated to assess the influence of the varied ligand frameworks on the OAT reaction rates. It was found that [MoO₂(PymS)₂] (1) and [MoO₂(6-MePyS)₂] (4) are similarly efficient catalysts, while complexes 2 and 3 are only moderately active. In the catalytic oxidation of PMe₃ with Me₂SO, complex 4 is the only efficient catalyst. Complexes 1-4 were also found to catalytically reduce NO₃^- with PPh₃, although their reactivity is inhibited by further reduced species such as NO, as exemplified by the formation of the nitrosyl complex [Mo(NO)(PymS)₃] (14), which was identified by single-crystal X-ray diffraction analysis. Computed ΔG^⧧ values for the very first step of the OAT were found to be lower for complexes 1 and 4 than for 2 and 3, explaining the difference in catalytic reactivity between the two pairs and revealing the requirement for an electron-deficient ligand system.

Inorganic nitrite bioactivation and role in physiological signaling and therapeutics

The bioactivation of inorganic nitrite refers to the conversion of otherwise 'inert' nitrite to the diatomic signaling molecule nitric oxide (NO), which plays important roles in human physiology and disease, notably in the regulation of vascular tone and blood flow. While the most well-known sources of NO are the nitric oxide synthase (NOS) enzymes, another source of NO is the nitrate-nitrite-NO pathway, whereby nitrite (obtained from reduction of dietary nitrate) is further reduced to form NO. The past few decades have seen extensive study of the mechanisms of NO generation through nitrate and nitrite bioactivation, as well as growing appreciation of the contribution of this pathway to NO signaling in vivo. This review, prepared for the volume 400 celebration issue of Biological Chemistry, summarizes some of the key reactions of the nitrate-nitrite-NO pathway such as reduction, disproportionation, dehydration, and oxidative denitrosylation, as well as current evidence for the contribution of the pathway to human cardiovascular physiology. Finally, ongoing efforts to develop novel medical therapies for multifarious conditions, especially those related to pathologic vasoconstriction and ischemia/reperfusion injury, are also explored.

Tungstoenzymes: Occurrence, Catalytic Diversity and Cofactor Synthesis

Tungsten is the heaviest element used in biological systems. It occurs in the active sites of several bacterial or archaeal enzymes and is ligated to an organic cofactor (metallopterin or metal binding pterin; MPT) which is referred to as tungsten cofactor (Wco). Wco-containing enzymes are found in the dimethyl sulfoxide reductase (DMSOR) and the aldehyde:ferredoxin oxidoreductase (AOR) families of MPT-containing enzymes. Some depend on Wco, such as aldehyde oxidoreductases (AORs), class II benzoyl-CoA reductases (BCRs) and acetylene hydratases (AHs), whereas others may incorporate either Wco or molybdenum cofactor (Moco), such as formate dehydrogenases, formylmethanofuran dehydrogenases or nitrate reductases. The obligately tungsten-dependent enzymes catalyze rather unusual reactions such as ones with extremely low-potential electron transfers (AOR, BCR) or an unusual hydration reaction (AH). In recent years, insights into the structure and function of many tungstoenzymes have been obtained. Though specific and unspecific ABC transporter uptake systems have been described for tungstate and molybdate, only little is known about further discriminative steps in Moco and Wco biosynthesis. In bacteria producing Moco- and Wco-containing enzymes simultaneously, paralogous isoforms of the metal insertase MoeA may be specifically involved in the molybdenum- and tungsten-insertion into MPT, and in targeting Moco or Wco to their respective apo-enzymes. Wco-containing enzymes are of emerging biotechnological interest for a number of applications such as the biocatalytic reduction of CO2, carboxylic acids and aromatic compounds, or the conversion of acetylene to acetaldehyde.

Genetic basis for the cooperative bioactivation of plant lignans by Eggerthella lenta and other human gut bacteria

Plant-derived lignans, consumed daily by most individuals, are thought to protect against cancer and other diseases¹; however, their bioactivity requires gut bacterial conversion to enterolignans². Here, we dissect a four-species bacterial consortium sufficient for all five reactions in this pathway. A single enzyme (benzyl ether reductase; ber), was sufficient for the first two biotransformations, variable between strains of Eggerthella lenta, critical for enterolignan production in gnotobiotic mice, and unique to Coriobacteriia. Transcriptional profiling (RNAseq) independently identified ber and genomic loci upregulated by each of the remaining substrates. Despite their low abundance in gut microbiomes and restricted phylogenetic range, all of the identified genes were detectable in the distal gut microbiomes of most individuals living in Northern California. Together, these results emphasize the importance of considering strain-level variations and bacterial co-occurrence to gain a mechanistic understanding of the bioactivation of plant secondary metabolites by the human gut microbiome.

An Asymmetrically Substituted Aliphatic Bis-Dithiolene Mono-Oxido Molybdenum(IV) Complex With Ester and Alcohol Functions as Structural and Functional Active Site Model of Molybdoenzymes

A Mo^IV mono-oxido bis-dithiolene complex, [MoO(mohdt)₂]²⁻ (mohdt = 1-methoxy-1-oxo-4-hydroxy-but-2-ene-2,3-bis-thiolate) was synthesized as a structural and functional model for molybdenum oxidoreductase enzymes of the DMSO reductase family. It was comprehensively characterized by inter alia various spectroscopic methods and employed as an oxygen atom transfer (OAT) catalyst. The ligand precursor of mohdt was readily prepared by a three-step synthesis starting from dimethyl-but-2-ynedioate. Crystallographic and ¹³C-NMR data support the rationale that by asymmetric substitution the electronic structure of the ene-dithio moiety can be fine-tuned. The Mo^IVO bis-dithiolene complex was obtained by in situ reaction of the de-protected ligand with the metal precursor complex trans-[MoO₂(CN)₄]⁴⁻. The catalytic oxygen atom transfer mediated by the complex was investigated by the model OAT reaction from DMSO to triphenylphosphine with the substrate transformation being monitored by ³¹P NMR spectroscopy. [MoO(mohdt)₂]²⁻ was found to be catalytically active reaching 93% conversion, albeit with a rather low reaction rate (reaction time 56 h). The observed overall catalytic activity is comparable to those of related complexes with aromatic dithiolene ligands despite the novel ligand being aliphatic in nature and originally perceived to perform more swiftly. The respective results are rationalized with respect to a potential intermolecular interaction between the hydroxyl and ester functions together with the electron-withdrawing functional groups of the dithiolene ligands of the molybdenum mono-oxido complex and equilibrium between the active monomeric Mo^IVO and Mo^VIO₂ and the unreactive dimeric Mo2VO₃ species.

Secondary-Sphere Effects in Molecular Electrocatalytic CO2 Reduction

The generation of fuels and value-added chemicals from carbon dioxide (CO2) using electrocatalysis is a promising approach to the eventual large-scale utilization of intermittent renewable energy sources. To mediate kinetically and thermodynamically challenging transformations of CO2, early reports of molecular catalysts focused primarily on precious metal centers. However, through careful ligand design, earth-abundant first-row transition metals have also demonstrated activity and selectivity for electrocatalytic CO2 reduction. A particularly effective and promising approach for enhancement of reaction rates and efficiencies of molecular electrocatalysts for CO2 reduction is the modulation of the secondary coordination sphere of the active site. In practice, this has been achieved through the mimicry of enzyme structures: incorporating pendent Brønsted acid/base sites, charged residues, sterically hindered environments, and bimetallic active sites have all proved to be valid strategies for iterative optimization. Herein, the development of secondary-sphere strategies to facilitate rapid and selective CO2 reduction is reviewed with an in-depth examination of the classic [Fe(tetraphenylporphyrin)]+, [Ni(cyclam)]2+, Mn(bpy)(CO)3X, and Re(bpy)(CO)3X (X = solvent or halide) systems, including relevant highlights from other recently developed ligand platforms.

Remote Charge Effects on the Oxygen-Atom-Transfer Reactivity and Their Relationship to Molybdenum Enzymes

We report the syntheses, crystal structures, and characterization of the novel cis-dioxomolybdenum(VI) complexes [Tpm*Mo^VIO₂Cl](MoO₂Cl₃) (1) and [Tpm*Mo^VIO₂Cl](ClO₄) (2), which are supported by the charge-neutral tris(3,5-dimethyl-1-pyrazolyl)methane (Tpm*) ligand. A comparison between isostructural [Tpm*Mo^VIO₂Cl]⁺ and Tp*Mo^VIO₂Cl [Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate] reveals the effects of one unit of overall charge difference on their spectroscopic and electrochemical properties, geometric and electronic structures, and O-atom-transfer (OAT) reactivities, providing new insight into pyranopterin molybdoenzyme OAT reactivity. Computational studies of these molecules indicate that the delocalized positive charge lowers the lowest unoccupied molecular orbital (LUMO) energy of cationic [Tpm*MoO₂Cl]⁺ relative to Tp*MoO₂Cl. Despite their virtually identical geometric structures revealed by crystal structures, the Mo^VI/Mo^V redox potential of 2 is increased by 350 mV relative to that of Tp*Mo^VIO₂Cl. This LUMO stabilization also contributes to an increased effective electrophilicity of [Tpm*MoO₂Cl]⁺ relative to that of Tp*MoO₂Cl, resulting in a more favorable resonant interaction between the molydenum complex LUMO and the highest occupied molecular orbital (HOMO) of the PPh₃ substrate. This leads to a greater thermodynamic driving force, an earlier transition state, and a lowered activation barrier for the orbitally controlled first step of the OAT reaction in the Tpm* system relative to the Tp* system. An Eyring plot analysis shows that this initial step yields an O≡Mo^IV-OPPh₃ intermediate via an associative transition state, and the reaction is ∼500-fold faster for 2 than for Tp*MoO₂Cl. The second step of the OAT reaction entails solvolysis of the O≡Mo^IV-OPPh₃ intermediate to afford the solvent-substituted Mo^IV product and is 750-fold faster for the Tpm* system at -15 °C compared to the Tp* system. The observed rate enhancement for the second step is ascribed to a switch of the reaction mechanism from a dissociative pathway for the Tp* system to an alternative associative pathway for the Tpm* system. This is due to a more Lewis acidic Mo^IV center in the Tpm* system.

The Challenging in silico Description of Carbon Monoxide Oxidation as Catalyzed by Molybdenum-Copper CO Dehydrogenase

Carbon monoxide (CO) is a highly toxic gas to many living organisms. However, some microorganisms are able to use this molecule as the sole source of carbon and energy. Soil bacteria such as the aerobic Oligotropha carboxidovorans are responsible for the annual removal of about 2x108 tons of CO from the atmosphere. Detoxification through oxidation of CO to CO2 is enabled by the MoCu-dependent CO-dehydrogenase enzyme (MoCu-CODH) which-differently from other enzyme classes with similar function-retains its catalytic activity in the presence of atmospheric O2. In the last few years, targeted advancements have been described in the field of bioengineering and biomimetics, which is functional for future technological exploitation of the catalytic properties of MoCu-CODH and for the reproduction of its reactivity in synthetic complexes. Notably, a growing interest for the quantum chemical investigation of this enzyme has recently also emerged. This mini-review compiles the current knowledge of the MoCu-CODH catalytic cycle, with a specific focus on the outcomes of theoretical studies on this enzyme class. Rather controversial aspects from different theoretical studies will be highlighted, thus illustrating the challenges posed by this system as far as the application of density functional theory and hybrid quantum-classical methods are concerned.

Bacterial steroid hydroxylases: enzyme classes, their functions and comparison of their catalytic mechanisms

The steroid superfamily includes a wide range of compounds that are essential for living organisms of the animal and plant kingdoms. Structural modifications of steroids highly affect their biological activity. In this review, we focus on hydroxylation of steroids by bacterial hydroxylases, which take part in steroid catabolic pathways and play an important role in steroid degradation. We compare three distinct classes of metalloenzymes responsible for aerobic or anaerobic hydroxylation of steroids, namely: cytochrome P450, Rieske-type monooxygenase 3-ketosteroid 9α-hydroxylase, and molybdenum-containing steroid C25 dehydrogenases. We analyze the available literature data on reactivity, regioselectivity, and potential application of these enzymes in organic synthesis of hydroxysteroids. Moreover, we describe mechanistic hypotheses proposed for all three classes of enzymes along with experimental and theoretical evidences, which have provided grounds for their formulation. In case of the 3-ketosteroid 9α-hydroxylase, such a mechanistic hypothesis is formulated for the first time in the literature based on studies conducted for other Rieske monooxygenases. Finally, we provide comparative analysis of similarities and differences in the reaction mechanisms utilized by bacterial steroid hydroxylases.

Mechanism of the Molybdenum-Mediated Cadogan Reaction

Oxygen atom transfer reactions are receiving increasing attention because they bring about paramount transformations in the current biomass processing industry. Significant efforts have therefore been made lately in the development of efficient and scalable methods to deoxygenate organic compounds. One recent alternative involves the modification of the Cadogan reaction in which a Mo(VI) core catalyzes the reduction of o-nitrostyrene derivatives to indoles in the presence of PPh₃. We have used density functional theory calculations to perform a comprehensive mechanistic study on this transformation, in which we find two clearly defined stages: an associative path from the nitro to the nitroso compound, characterized by the reduction of the catalyst in the first step, and a peculiar mechanism involving oxazaphosphiridine and nitrene intermediates leading to an indole product, where the metal catalyst does not participate.

Formation of Mono Oxo Molybdenum(IV) PNP Pincer Complexes: Interplay between Water and Molecular Oxygen

The synthesis of cationic mono oxo MoIV PNP pincer complexes of the type [Mo(PNPMe-iPr)(O)X]+ (X = I, Br) from [Mo(PNPMe-iPr)(CO)X2] is described. These compounds are coordinatively unsaturated and feature a strong Mo≡O triple bond. The formation of these complexes proceeds via cationic 14e intermediates [Mo(PNPMe-iPr)(CO)X]+ and requires both molecular oxygen and water. ESI MS measurements with 18O labeled water (H2 18O) and molecular oxygen (18O2) indicates that water plays a crucial role in the formation of the Mo≡O bond. A plausible mechanism based on DFT calculations is provided. The X-ray structure of [Mo(PNPMe-iPr)(O)I]SbF6 is presented.

Assembly pathway of a bacterial complex iron sulfur molybdoenzyme

Protein folding and assembly into macromolecule complexes within the living cell are complex processes requiring intimate coordination. The biogenesis of complex iron sulfur molybdoenzymes (CISM) requires use of a system specific chaperone - a redox enzyme maturation protein (REMP) - to help mediate final folding and assembly. The CISM dimethyl sulfoxide (DMSO) reductase is a bacterial oxidoreductase that utilizes DMSO as a final electron acceptor for anaerobic respiration. The REMP DmsD strongly interacts with DMSO reductase to facilitate folding, cofactor-insertion, subunit assembly and targeting of the multi-subunit enzyme prior to membrane translocation and final assembly and maturation into a bioenergetic catalytic unit. In this article, we discuss the biogenesis of DMSO reductase as an example of the participant network for bacterial CISM maturation pathways.

Oxygen atom transfer between DMSO and benzoin catalyzed by cis-dioxidomolybdenum(vi) complexes of tetradentate Mannich bases

Dioxidomolybdenum(vi) complexes of tetradentate ONNO donor Mannich base ligands for the catalytic oxygen atom transfer between benzoin and DMSO are reported.

Acid-facilitated product release from a Mo(IV) center: relevance to oxygen atom transfer reactivity of molybdenum oxotransferases

We report that pyridinium ions (HPyr⁺) accelerate the conversion of [Tp*Mo^IVOCl(OPMe₃)] (1) to [Tp*Mo^IVOCl(NCCH₃)] (2) by 10³-fold, affording 2 in near-quantitative yield; Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate. This novel reactivity and the mechanism of this reaction were investigated in detail. The formation of 2 followed pseudo-first-order kinetics, with the observed pseudo-first-order rate constant (k _obs) linearly correlated with [HPyr⁺]. An Eyring plot revealed that this HPyr⁺-facilitated reaction has a small positive value of ∆S ^‡ indicative of a dissociative interchange (I_d) mechanism, different from the slower associative interchange (I_a) mechanism in the absence of HPyr⁺ marked with a negative ∆S ^‡. Interestingly, log(k _obs) was found to be linearly correlated to the acidity of substituted pyridinium ions. This novel reactivity is further investigated using combined DFT and ab initio coupled cluster methods. Different reaction pathways, including I_d, I_a, and possible alternative routes in the absence or presence of HPyr⁺, were considered, and enthalpy and free energies were calculated for each pathway. Our computational results further underscored that the I_d route is energetically favored in the presence of HPyr⁺, in contrast with the preferred I_a-NNO pathway in the absence of HPyr⁺. Our computational results also revealed molecular-level details for the HPyr⁺-facilitated I_d route. Specifically, HPyr⁺ initially becomes hydrogen-bonded to the oxygen atom of the Mo(IV)-OPMe₃ moiety, which lowers the activation barrier for the Mo-OPMe₃ bond cleavage in a rate-limiting step to dissociate the OPMe₃ product. The implications of our results were discussed in the context of molybdoenzymes, particularly the reductive half-reaction of sulfite oxidase.

[MoO2]2+-Mediated Oxygen Atom Transfer via an Unusual Lewis Acid Mechanism

Density functional theory is applied to the study of the oxygen atom transfer reaction from sulfoxide (DMSO) to phosphine (PMe₃) catalyzed by the [MoO₂]²⁺ active core. In this work, two fundamentally different roles are explored for this dioxometal complex in the first step of the catalytic cycle: as an oxidizing agent and as a Lewis acid. The latter turns out to be the favored pathway for the oxygen atom transfer. This finding may have more general implications for similar reactions catalyzed by the same [MoO₂]²⁺ core.

Mild, Selective Sulfoxidation with Molybdenum(VI) cis-Dioxo Catalysts

Three molybdenum(VI) cis-dioxo catalysts (8-10) were synthesized with the goal of developing stable and selective oxidation catalysts for sulfoxidation. Their reactivities were investigated with a variety of substrates. We have demonstrated the usefulness of these catalysts for the chemoselective sulfoxidation of sulfides in the presence of reactive moieties, which has important applications for total synthesis processes. Notably, these catalysts are able to oxidize compounds analogous to sulfur mustard and can be used as an alternative to sodium periodate or meta-chloroperoxybenzoic acid (m-CPBA) for the oxidation of various organic sulfides without sacrificing total conversion. As the catalysts are tolerant of water and hydrogen peroxide, they allow for the design of completely green oxidation reactions, particularly for sulfur-containing amino acids.

Mononuclear bis(dithiolene) Mo(iv) and W(iv) complexes with P,P; S,S; O,S and O,O donor ligands: a comparative reactivity study

Comparative redox reactions of eight Mo^IV/W^IVcomplexes with P,P; S,S; S,O and O,O donor ligands are presented.