Molybdenum-Containing Systems

Some interesting work has been reported using Zn and Mo systems in plenum cable formulations. The use of a zinc molybdenum complex in place of ammonium octamolybdate is described in Ref 61 and the use of ammonium octamolybdate in combination with zinc stannate as a smoke suppressant in plenum cable compounds is discussed in Ref [62]. 

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Sulfate reducers can use a wide range of terminal electron acceptors, and sulfate can be replaced by nitrate as a respiratory substrate. Molybdenum-containing enzymes have been discovered in SRB (also see later discussion) and, in particular, D. desulfuricans, grown in the presence of nitrate, generates a complex enzymatic system containing the following molybdenum enzymes (a) aldehyde oxidoreduc-tase (AOR), which reduces adehydes to carboxylic acids (b) formate dehydrogenase (FDH), which oxidizes formate to CO2 and (c) nitrate reductase (the first isolated from a SRB), which completes the enzy-... 

The serendipitous discovery of this compound has proven to be extremely important to our visions of possibilities for new metal-metal bonded structures in reduced oxide phases. In retrospect it is amazing that oxide phases containing molybdenum in oxidation states less than 4+ were essentially unknown and certainly structurally uncharacterized. The existence of the previously mentioned series M2 Mo30q and LiM Mo303 should have been a tip-off to an extensive chemistry for metal-metal bonded molybdenum oxide systems. Indeed, subsequent work has revealed a plethora of new compounds all of which (where structure has been determined) feature strong metal-metal bonding in either discrete cluster units or extended chain arrays. 

The N2-fixing enzyme used by the bacteria is nitrogenase. It consists of two components an Fe protein that contains an [Fe4S4] cluster as a redox system (see p. 106), accepts electrons from ferredoxin, and donates them to the second component, the Fe-Mo protein. This molybdenum-containing protein transfers the electrons to N2 and thus, via various intermediate steps, produces ammonia (NH3). Some of the reducing equivalents are transferred in a side-reaction to In addition to NH3, hydrogen is therefore always produced as well. 

Fe2S2] clusters are part of the molybdenum containing hydroxylases. Typically, apart from molybdenum and two EPR-distinct iron-sulfur centres there can be FAD as additional cofactor. In Chlostridium purinolyticum a selenium-dependent purine hydroxylase has been characterized as molybdenum hydroxylase. The EPR of the respective desulfo molybdenum (V) signal indicated that the Mo-ligands should differ from those of the well known mammalian corollary xanthine oxidase.197 For the bacterial molybdenum hydroxylase quinoline oxidoreductase from Pseudomonas putida an expression system was developed in order to be able to construct protein mutants for detailed analysis. EPR was used to control the correct insertion of the cofactors, specifically of the two [Fe2S2] clusters.198... 

E. F. Pai and T. Nishino, The Molybdenum-Containing Xanthine Oxidoreductases and Picolinate Dehydrogenases, in Metal Ions in Biological Systems , eds. A. Sigel and H. Sigel, Marcel Dekker, New York, 2002, Vol. 39, p. 431. 

In Chapter 1, the structure, synthesis and toxicity of ricin, a protein present in the endosperm cells of the seeds of the castor oil plant, are described and possible uses of this toxic agent in immunology as conjugates with antibodies are discussed. The biochemical and pharmacological evidence for the presence of functional histamine receptors in the mammalian central nervous system is set out in Chapter 2. Chapter 3 covers the chemical properties of molybdenum-containing enzymes such as aldehyde oxidase and xanthine oxidase, although their physiological roles are by no means clear. 

Molybdate is imported into the cells by highly specific uptake systems (ABC family). The imported molybdate is than used for the formation of a specific molybdenum-containing cofactor that is the Mo-con-taining element of most Mo-dependent enzymes such as nitrate reductase (Menen-... 

Organic sulfur metabolites are catabolized up to sulfate, which in turn is excreted via the urine. The final step of the pathway -the reduction of sulfite to sulfate - is dependent on the molybdenum-containing enzyme, sulfite oxidase. Both, a genetic disorder of sulfite oxidase synthesis or a molybdenum deficiency will reduce the urinary excretion of sulfate, but increase that of sulfite, S-sulfocysteine and thiosulfate. Due to the toxic effect of sulfite on the nervous system, this disease is characterized by mental retardation and dislocation of ocular lenses (Rajagopalan 1988). A marginal amount of molybdenum or sulfite oxidase worsen the sulfite detoxification of the... 

Dinitrogen Complexes and Nitrogen Fixation.—Nitrogenase from Azotobacter vinelandii has been purified and the molybdoferrodoxin protein crystallized as needles.A molybdenum-containing peptide of mol. wt. ca. 1000 has been isolated from this system and shown to contain at least ten naturally occurring amino-acids, with leucine and isoleucine being most prominent molybdenum is present as both Mo " and Tungsten has been incorporated into... 

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