Molybdenum is the only second-row transition metal required by most living organisms, and is nearly universally distributed in biology. Enzymes containing molybdenum in their active sites have long been recognized,1 and at present over 50 molybdenum-containing enzymes have been purified and biochemically characterized; a great many more gene products have been annotated as putative molybdenum-containing proteins on the basis of genomic and bioinformatic analysis.2 In certain cases, our understanding of the relationship between enzyme structure and function is such that we can speak with confidence as to the detailed nature of the reaction mechanism and, with the availability of high-resolution X-ray crystal structures, the specific means by which transition states are stabilized and reaction rate is accelerated within the friendly confines of the active site. At the same time, our understanding of the biosynthesis of the organic cofactor that accompanies molybdenum (variously called molybdopterin or pyranopterin), the manner in which molybdenum is incorporated into it, and then further modified as necessary prior to insertion into apoprotein has also (in at least some cases) become increasingly well understood.

It is now well-established that all molybdenum-containing enzymes other than nitrogenase (in which molybdenum is incorporated into a [MoFe7S9] cluster of the active site) fall into three large and mutually exclusive families, as exemplified by the enzymes xanthine oxidase, sulfite oxidase, and DMSO reductase; these enzymes represent the focus of the present account.3 The structures of the three canonical molybdenum centers in their oxidized Mo(VI) states are shown in Figure 1, along with that for the pyranopterin cofactor. The active sites of members of the xanthine oxidase family have an LMoVIOS-(OH) structure with a square-pyramidal coordination geometry. The apical ligand is a Mo=O ligand, and the equatorial plane has two sulfurs from the enedithiolate side chain of the pyranopterin cofactor, a catalytically labile Mo–OH group, and most frequently a Mo=S. Nonfunctional forms of these enzymes exist in which the equatorial Mo=S is replaced with a second Mo=O; in at least one member of the family the Mo=S is replaced by a Mo=Se, and in others it is replaced by a more complex –S–Cu–S–Cys to give a binuclear center. Members of the sulfite oxidase family have a related LMoVIO2(S–Cys) active site, again square-pyramidal with an apical Mo=O and a bidentate enedithiolate Ligand (L) in the equatorial plane but with a second equatorial Mo=O (rather than Mo–OH) and a cysteine ligand contributed by the protein (rather than a Mo=S) completing the molybdenum coordination sphere. The final family is the most diverse structurally, although all members possess two (rather than just one) equiv of the pyranopterin cofactor and have an L2MoVIY(X) trigonal prismatic coordination geometry. DMSO reductase itself has a catalytically labile Mo=O as Y and a serinate ligand as X completing the metal coordination sphere of oxidized enzyme. Other family members have cysteine (the bacterial Nap periplasmic nitrate reductases), selenocysteine (formate dehydrogenase H), –OH (arsenite oxidase), or aspartate (the NarGHI dissimilatory nitrate reductases) in place of the serine. Some enzymes have S or even Se in place of the Mo=O group. Members of the DMSO reductase family exhibit a general structural homology to members of the aldehyde:ferredoxin oxidoreductase family of tungsten-containing enzymes;4 indeed, the first pyranopterin-containing enzyme to be crystallographically characterized was the tungsten-containing aldehyde:ferredoxin oxidoreductase from Pyrococcus furiosus,5 a fact accounting for why many workers in the field prefer “pyranopterin” (or, perhaps waggishly, “tungstopterin”) to “molybdopterin”. The term pyranopterin will generally be used in the present account.




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Figure 1


Active site structures for the three families of mononuclear molybdenum enzymes. The structures shown are, from left to right, for xanthine oxidase, sulfite oxidase, and DMSO reductase. The structure of the pyranopterin cofactor common to all of these enzymes (as well as the tungsten-containing enzymes) is given at the bottom.

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The Mononuclear Molybdenum Enzymes

Molybdenum(VI) speciation in sulfidic waters; stability and lability of thiomolybdates

Thiolato-complexes of the transition metals

The sulfidation of crystalline MoO3 and the thermal decomposition of (NH4)2MoO2S2 to MoS2 via an {MoOS2} oxysulfide intermediate have been studied by means of monochromatic X-ray photoelectron spectroscopy (XPS) and infrared emission spectroscopy (IRES) Several basic steps of the sulfidation reaction could be resolved and explained in terms of the structure of crystalline MoO3 The sulfidation reaction starts at low temperatures with an exchange of terminal O2- ligands of the oxide for S2- by reaction with H2S from the sulfiding atmosphere In subsequent Mo−S redox reactions, bridging S22- ligands and Mo5+ centers are formed Lattice relaxation and further sulfur uptake are the main processes before, at temperatures above 200 °C, direct reactions with H2 occur, during which the Mo5+ centers are converted into the 4+ oxidation state The decomposition experiments with (NH4)2MoO2S2 show that terminal O2- ligands serve as the reactive sites The conversion of terminal MoOt to MoS entities and the subsequent

/pdf/basic-reaction-steps-in-the-sulfidation-of-crystalline-moo3-ub1x9ubuyz.pdf

Basic reaction steps in the sulfidation of crystalline MoO3 to MoS2, as studied by X-ray photoelectron and infrared emission spectroscopy

The biologically relevant oxygen atom transfer chemistry of molybdenum: from synthetic analogue systems to enzymes

Oxidation of molybdenum dithiocarbamate complexes with elemental sulfur

Nitrogenase is a complex of two separately purifiable proteins, the molybdenum-iron protein [MoFe] and the iron protein [Fe].1,2 In addition to catalyzing the reduction of N2 to ammonia, nitro genase has ATP-hydrolyzing activity,3 ATP-dependent H2-evolution activity,4 and supports a reaction between D2 and protons (from H2O) to form HD.5 In the absence of other substrates, all the reductant consumed is used to reduce protons to H2. When N2 is added as a substrate, an apparent maximum of 75% of the electrons reduce N2 while the remainder still reduce protons.6

HD Formation by Nitrogenase: A Probe for N2 Reduction Intermediates

Structure and properties of [Fe(WS4)2]3−

Synthesis of the [(AcO)2FeS2MS2]2− (M = Mo, W) ions and observation of a new bonding mode for complexes of [MoO2S2]2−

The hetero metal sulfur aggregates [Co(WS₄)₂] ⁿ⁻ (with variable electron population: n = 2, 3) could be isolated as salts and characterized by physical measurements (in-cluding X-ray structure analysis). Experimental and theoretical investigations show that due to a strong metal → ligand electron delocalisation WS4²⁻(like MOS₄²⁻) is a non-innocent ligand and that the additional charge upon reduction is located mainly at the S atoms of the ligand system.

Notizen: Zum Verständnis der zentralen Rolle des Schwefels in Multi-Metall-Aggregaten mit verschiedener Elektronenpopulation: [S2WS2CoS2WS2]n- (n = 2, 3)/On the Central Role of Sulfur in Multi Metal Aggregates with Different Electron Populations: [S2WS2CoS2WS2]n-(n = 2, 3)

W. E. Newton

Papers

Oxidation of molybdenum dithiocarbamate complexes with elemental sulfur

HD Formation by Nitrogenase: A Probe for N2 Reduction Intermediates

Structure and properties of [Fe(WS4)2]3−

Synthesis of the [(AcO)2FeS2MS2]2− (M = Mo, W) ions and observation of a new bonding mode for complexes of [MoO2S2]2−

Notizen: Zum Verständnis der zentralen Rolle des Schwefels in Multi-Metall-Aggregaten mit verschiedener Elektronenpopulation: [S2WS2CoS2WS2]n- (n = 2, 3)/On the Central Role of Sulfur in Multi Metal Aggregates with Different Electron Populations: [S2WS2CoS2WS2]n-(n = 2, 3)