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

Publisher Summary This chapter discusses the chemistry of nitrogen fixation and models for the reactions of nitrogenase The interest in nitrogen fixation for the inorganic chemist is to try to understand, using simple compounds, what nature does so comparatively effortlessly within the enzyme, nitrogenase This is of particular value considering the increasing demand for nitrogenous fertilizers, and the vast industrial expenditure of energy in producing ammonia The literature of chemical nitrogen fixation, even excluding that related to the Haber process, is now considerable The mechanisms of reaction of coordinated dinitrogen are a matter of dispute There are several proposals extant, chemical and biological, and these are espoused by their progenitors with varying degrees of fervor However, there is now sufficient chemistry available for us to make preliminary judgments concerning the validity of the various proposals

The Chemistry of Nitrogen Fixation and Models for The Reactions of Nitrogenase

Use of preassembled iron/sulfur and iron/molybdenum/sulfur clusters in the stepwise synthesis of potential analogs for the Fe/Mo/S site in nitrogenase

Publisher Summary This chapter focuses on the metabolism of one-carbon (C1) compounds chemotrophic anaerobes. C1-metabolizing species possess novel physiologies that are distinctive in many ways from the autotrophic and heterotrophic microorganisms, which seemingly are more widespread in the microbial world. C1 compounds refer to any oxidizable one-carbon substrate that contains carbon-bound electrons. A C1 substrate differs noticeably from the C1 compound carbon dioxide because, in addition to being able to be reduced or assimilated, it can also be oxidized and can provide electrons for use in energy metabolism or cell synthesis. The term “chemotrophic anaerobe” refers to any obligately non-oxygen-catabolizing microbial species whose growth is solely dependent on the generation of metabolic energy from chemical substrates. The chapter examines the microbial physiology that accounts for unicarbonotrophy in anaerobes. The C1 metabolites enter anaerobic ecosystems of the biosphere either as pollutants from aerobic environments, volcanic or deep subsurface emanations, or via chemical transformation reactions performed by anaerobic microorganisms. The formation of C1 metabolites often necessitates their removal in biological elemental cycles because they can accumulate and alters normal carbon and electron flow within a cell, or becomes toxic and result in cell death.

Metabolism of one-carbon compounds by chemotrophic anaerobes.

Die elektronenarmen Ubergangsmetalle V, Nb, Ta, Mo, W und Re bilden in den hochsten Oxidationsstufen tetraederformige Thioanionen mit bemerkenswerten Eigenschaften. Die farbenprachtigen Thiometallate werden durch Festkorperreaktion oder in Losung aus den Oxometallaten erhalten. Aus Thiometallaten lassen sich durch neuartige intramolekulare Redox-Prozesse bei gleichzeitiger Kondensation Poly(thiometallate) mit gemischten Valenzen gewinnen. Die Metall-Schwefel-Bindung reagiert sowohl nucleophil als auch elektrophil; bei der Mo–S-Bindung ist diese Eigenschaft von biochemischem Interesse. Als Komplexliganden ermoglichen die Thiometallate die Erzeugung von Multi-Metall-Komplexen. Daruber hinaus zeichnen sie sich durch vielseitiges Koordinationsverhalten sowie einzigartige elektronische Eigenschaften aus; wegen der ausgepragten Elektronendelokalisation existieren Thiometallato-Komplexe mit unterschiedlicher Elektronenpopulation. MoS42- hat auserdem Bedeutung fur bioanorganische Fragen, z. B. das Nitrogenase-Problem und den Cu-Mo-Antagonismus.

W. E. Newton

Papers

Molybdenum Chemistry of Biological Significance