Tungsten complexes enzymes

Molybdenum and tungsten complexes as models for oxygen atom transfer enzymes have been deployed in the full catalytic cycle from Scheme 4.3 predominantly in the early days of this field of research. A selection of the respective determined Michaelis-Menten parameters were expertly reviewed by Holm et al. Since in some cases both forms of model complexes (M and M mimicking the fully reduced or fully oxidized active sites, respectively) are isolable and available in a sufficient amount, the isolated half-reactions are much more often investigated than the whole catalytic cycle. This means that either the reduced form of the enzyme model is oxidized by an oxygen donor substrate like TMAO or the oxidized form is reduced by an oxygen acceptor substrate like triphenylphosphine (PhgP). The observed kinetic behaviour is in some cases described to be of a saturation type. An observation which... 

J. H. Enemarkand J. A. Cooney, in Model Complexes for Molybdenum- and Tungsten- Containing Enzymes, eds. H.-B. Kraatz and N. Metzler-Nolte, Wiley-VCH, Weinheim, 2006, p. 237. 

Interest in the molecular structures of molybdenum and tungsten complexes, particularly in those containing N-donor ligands, is enhanced by the occurrence of molybdenum in the enzyme nitrogenase. It is possible that the stereochemistry of these metals may be of importance in understanding processes of in vivo and in vitro nitrogen-fixation. 

In some prokaryotes the FDHs are complex enzymes which contain molybdenum or tungsten cofactors to transfer the electrons from formate oxidation to an independent active site, to reduce quinone, protons, or NAD(P) [82, 92]. These enzymes are suitable for adsorption onto an electrode, so that the electrode accepts the electrons from formate oxidation, and it may also donate electrons and drive... 

Complexes of molybdenum and tungsten with bidentate sulfur ligands have been investigated extensively. In recent years, the work in this field has been escalated by the impetus of designing models of such bioinorganic enzymes as nitrogenase and xanthine oxidase (125). The early work reviewed by Coucouvanis (1) dealt exclusively with the isolation of oxomolybdenum(V) and -(VI) species. 

Chemical systems of relevance to the molybdenum and tungsten enzymes include synthetic pterins, a-phosphorylated ketones (as precursor models), and a variety of molybdenum and tungsten oxido, sulfido, and 1,2-enedithiolate complexes. These compounds have been used to (1) confirm the identity of MPT derivatives (2) define steps in MPT biosynthesis (3) calibrate spectroscopic observations (4) give precise geometries and reactivities that can be used as input for theoretical studies and (5) provide options for mechanistic consideration. 

The one non-redox reaction catalyzed by a tungsten enzyme involves the hydration of acetylene to acetaldehyde [5], This same reaction is catalyzed by WO(mnt)22- but not by W02(mnt)22- [186], The five-coordinate W(IV) complex should have an open site that may be available to coordinate and activate the acetylene substrate [194],... 

The bis(l,2-enedithiolate) complexes discussed closely resemble the metal centers found in the dmso reductase family of Mo enzymes and in the tungsten enzymes. The reactivity of mono(l,2-enedithiolate) complexes remains a continuing challenge as synthetic chemists pursue accurate models for the xanthine oxidase and sulfite oxidase families of metal sites. New 1,2-dithiolate ligands [70,71] and complexes are needed to demonstrate ligand effects to help elucidation reaction mechanism. 

The final topic addressed in this chapter is the biosynthesis of the dithiolene cofactor ligand and its coordination to molybdenum and tungsten in the enzymes. Nature has clearly devised a synthetic process to overcome the twin difficulties of building a reactive dithiolene unit bearing a complicated and equally reactive pterin substituent. Molecular biology has been the tool to elucidate the steps in this complex process. Although the dithiolene formation step remains mainly a subject of conjecture, definitive information about the reagent molecule that will eventually be converted to a dithiolene is known. 

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