Molybdenum and Tungsten Cofactors and the Reactions They Catalyze

Relativistic effect behind the molybdenum vs. tungsten selectivity in enzymes

Molybdenum and tungsten, being congeners of the 6th group of d-block elements, are similar in many respects in terms of their properties. In fact, both participate in similar types of oxotransferase activity in their enzymes. Molybdenum is regarded as the heaviest essential trace metal in all forms of life; however, its next heavier congener, tungsten, as the heaviest metal, is found only in some prokaryotic organisms. Tungstoenzymes are generally selected by nature for carrying out low-potential redox activities under anaerobic conditions in prokaryotic organisms. This nature's molybdenum vs. tungsten selectivity for their biological functions under different working conditions (surrounding temperature and aerobic/anaerobic environment) is determined mainly by the relativistic effect, which is experienced to different extents by these two congeners. Understanding the mechanistic aspects of the relativistic effect-controlled enzymatic activities of tungstoenzymes is of immense biotechnological interest to develop eco-friendly and cost-effective methods for the commercial synthesis of acetaldehyde through the hydration of acetylene and commercial production of hydrogen (H2, a green fuel) by producing tungsten-incorporated nitrogenase (W-N2-ase) in CA6 (mutant strain) and to develop a biomimetic method to replace the hazardous Birch reduction in organic synthesis.

Understanding the Carbyne Formation from C2H2 Complexes

Nature chooses a high-valent tungsten center at the active site of the enzyme acetylene hydratase to facilitate acetylene hydration to acetaldehyde. However, the reactions of tungsten-coordinated acetylene are still not well understood, which prevents the development of sustainable bioinspired alkyne hydration catalysts. Here we report the reactivity of two bioinspired tungsten complexes with the acetylene ligand acting as a four-: [W(CO)(C2H2)(PymS)2] (1) and a two-electron donor: [WO(C2H2)(PymS)2] (3), with PMe3 as a nucleophile to simulate the enzyme's reactivity (PymS = 4-(trifluoromethyl)-6-methylpyrimidine-2-thiolate). In dichloromethane, compound 1 was found to react to the cationic carbyne [W≡CCH2PMe3(CO)(PMe3)2(PymS)]Cl (2-Cl) while 3 reacts to the vinyl compound [WO(CH═CHPMe3)(PMe3)3(PymS)]Cl (4-Cl). The formation of the latter follows the common rules applied to η2-alkyne complexes, whereas the carbyne formation was not expected due to the challenging 1,2-H shift. To understand these differences in behavior between seemingly similar acetylene complexes, stepwise addition of the nucleophile in various solvents was investigated by synthetic, spectroscopic, and computational approaches. In this manuscript, we describe that only a four-electron donor acetylene complex can react to the carbyne over the η1-vinyl intermediate and that 1,2-H shift can be assisted by an H-transfer reagent (in this case, the decoordinated PymS ligand). Furthermore, to favor the attack of PMe3 at W coordinated acetylene, the metal center needs to be electron-poor and crowded enough to prevent nucleophile coordination. Finally, the intricate role of the anionic PymS ligand in the vicinity of the first coordination sphere models the potential involvement of amino acid residues during acetylene transformations in AH.

Is There a Connection between the Metabolism of Copper, Sulfur, and Molybdenum in Alzheimer’s Disease? New Insights on Disease Etiology

Alzheimer’s disease (AD) and other forms of dementia was ranked 3rd in both the Americas and Europe in 2019 in a World Health Organization (WHO) publication listing the leading causes of death and disability worldwide. Copper (Cu) imbalance has been reported in AD and increasing evidence suggests metal imbalance, including molybdenum (Mo), as a potential link with AD occurrence.We conducted an extensive literature review of the last 60 years of research on AD and its relationship with Cu, sulfur (S), and Mo at out of range levels.Weanalyzed the interactions among metallic elements’ metabolisms;Cu and Mo are biological antagonists, Mo is a sulfite oxidase and xanthine oxidase co-factor, and their low activities impair S metabolism and reduce uric acid, respectively. We found significant evidence in the literature of a new potential mechanism linking Cu imbalance to Mo and S abnormalities in AD etiology: under certain circumstances, the accumulation of Cu not bound to ceruloplasmin might affect the transport of Mo outside the blood vessels, causing a mild Mo deficiency that might lowerthe activity of Mo and S enzymes essential for neuronal activity. The current review provides an updated discussion of the plausible mechanisms combining Cu, S, and Mo alterations in AD.

Influence of the Greater Protein Environment on the Electrostatic Potential in Metalloenzyme Active Sites: The Case of Formate Dehydrogenase

The Mo/W-containing metalloenzyme formate dehydrogenase (FDH) is an efficient and selective natural catalyst that reversibly converts CO₂ to formate under ambient conditions. In this study, we investigate the impact of the greater protein environment on the electrostatic potential (ESP) of the active site. To model the enzyme environment, we used a combination of classical molecular dynamics and multiscale quantum-mechanical (QM)/molecular-mechanical (MM) simulations. We leverage charge shift analysis to systematically construct QM regions and analyze the electronic environment of the active site by evaluating the degree of charge transfer between the core active site and the protein environment. The contribution of the terminal chalcogen ligand to the ESP of the metal center is substantial and dependent on the chalcogen identity, with similar, less negative ESPs for Se and S terminal chalcogens in comparison to O regardless of whether the metal is Mo or W. The orientation of the side chains and conformations of the cofactor also affect the ESP, highlighting the importance of sampling dynamic fluctuations in the protein. Overall, our observations suggest that the terminal chalcogen ligand identity plays an important role in the enzymatic activity of FDH, suggesting opportunities for a rational bioinspired catalyst design.

The Biochemical Landscape of Riboswitch Ligands

More than 55 distinct classes of riboswitches that respond to small metabolites or elemental ions have been experimentally validated to date. The ligands sensed by these riboswitches are biased in favor of fundamental compounds or ions that are likely to have been relevant to ancient forms of life, including those that might have populated the "RNA World", which is a proposed biochemical era that predates the evolutionary emergence of DNA and proteins. In the following text, I discuss the various types of ligands sensed by some of the most common riboswitches present in modern bacterial cells and consider implications for ancient biological processes centered on the proven capabilities of these RNA-based sensors. Although most major biochemical aspects of metabolism are represented by known riboswitch classes, there are striking sensory gaps in some key areas. These gaps could reveal weaknesses in the performance capabilities of RNA that might have hampered RNA World evolution, or these could highlight opportunities to discover additional riboswitch classes that sense essential metabolites.

Comparative outcomes of exposing human liver and kidney cell lines to tungstate and molybdate

Tungsten has no known function in humans and is a relatively new contaminant, whereas molybdenum, its congener in the periodic table, is a nutritionally essential element. In addition to early studies on molybdosis in ruminants, their toxic effects in the form of tungstate and molybdate have been addressed primarily in rodents and are predominantly mediated by inducing oxidative stress in various tissues. The purpose of this study was to evaluate the differences between tungstate and molybdate in human liver (HepG2) and kidney (HEK293) cell lines in terms of retention in cells, effect on reactive oxygen species, and activities of xanthine oxidase and phosphatases. The cell lines were exposed to tungstate or molybdate (1 µM to 10 mM) for 24 h, lysed and analyzed for the above biochemical parameters. Despite the chemical similarity of the two anions, cell-specific differential effects were observed. At all concentrations, tungstate was retained more in HEK293 cells while molybdate was retained more in HepG2 cells. HepG2 cells were more sensitive to tungstate than molybdate, showing reduced viability at concentrations as low as 10 µM. Exposure to either anion resulted in the inhibition of protein tyrosine phosphatases at 1 mM and an increased production of reactive oxygen species (ROS) at 100 µM despite their inhibition of the ROS-producing molybdenum enzyme xanthine oxidase. In conclusion, the results indicate that excess of nutritionally essential molybdate or non-essential tungstate causes toxicity by affecting ROS- and phosphorylation-dependent signaling pathways and ensuing gene expression.

Active site architecture reveals coordination sphere flexibility and specificity determinants in a group of closely related molybdoenzymes

MtsZ is a molybdenum-containing methionine sulfoxide reductase that supports virulence in the human respiratory pathogen Haemophilus influenzae (Hi). HiMtsZ belongs to a group of structurally and spectroscopically uncharacterized S-/N-oxide reductases, all of which are found in bacterial pathogens. Here, we have solved the crystal structure of HiMtsZ, which reveals that the HiMtsZ substrate-binding site encompasses a previously unrecognized part that accommodates the methionine sulfoxide side chain via interaction with His182 and Arg166. Charge and amino acid composition of this side chain–binding region vary and, as indicated by electrochemical, kinetic, and docking studies, could explain the diverse substrate specificity seen in closely related enzymes of this type. The HiMtsZ Mo active site has an underlying structural flexibility, where dissociation of the central Ser187 ligand affected catalysis at low pH. Unexpectedly, the two main HiMtsZ electron paramagnetic resonance (EPR) species resembled not only a related dimethyl sulfoxide reductase but also a structurally unrelated nitrate reductase that possesses an Asp–Mo ligand. This suggests that contrary to current views, the geometry of the Mo center and its primary ligands, rather than the specific amino acid environment, is the main determinant of the EPR properties of mononuclear Mo enzymes. The flexibility in the electronic structure of the Mo centers is also apparent in two of three HiMtsZ EPR-active Mo(V) species being catalytically incompetent off-pathway forms that could not be fully oxidized.