Molybdenum-enzymes

Hydrodesulfurization Hydrodenitrogenation Molybdenum MPT-containing Enzymes Molybdenum Organometal-lic Chemistry Oxidation Catalysis by Transition Metal Complexes Polyoxometalates. 

Nonlegumes respond to added Mo because of the requirements for Mo by their nitrate reductase enzymes. Molybdenum additions in the presence of high amounts of N in the soil have prevented depressed crop yields due to excess N. Grains such as com, wheat, and rice have shown improved utilization of N following Mo application, but again, not all cultivars have the same Mo requirements. Many crops in the Brassica family respond to added Mo. Some of the early studies describing whiptail in cauliflower also found that different Brassica crops required different amounts of Mo. 

Xanthine Oxidase. One of the most thoroughly studied flavoproteins is xanthine oxidase. This enzyme has been purified from milk and from liver. Recently the milk enzyme, for many years known as the Schardin-ger enzyme, has been crystallized in a highly purified state. For many years the absorption spectrum of the purified enzyme was a source of concern, because instead of the two distinct characteristic flavin peaks, an end absorption was always found to obscure the flavin spectrum. This has now been found to be the true spectrum of the enzyme. In addition to FAD, the enzyme contains iron and molybdenum in the ratio, 8 iron 2 FAD 2 Mo per mole of enzyme. Molybdenum does not affect... 

Because of its position in the Periodic Table, molybdenum has sometimes been linked to chromium (see Chromiumand chromium alloys) or to other heavy metals. However, unlike those elements, molybdenum and its compounds have relatively low toxicity, as shown in Table 3. On the other hand, molybdenum has been identified as a micronutrient essential to plant life (11,12) (see Fertilizers), and plays a principal biochemical role in animal health as a constituent of several important enzyme systems (see Mineral nutrients). 

Molybdenum, recognized as an essential trace element for plants, animals, and most bacteria, is present in a variety of metaHo enzymes (44—46). Indeed, the absence of Mo, and in particular its co-factor, in humans leads to severe debility or early death (47,48). Molybdenum in the diet has been impHcated as having a role in lowering the incidence of dental caries and in the prevention of certain cancers (49,50). To aid the growth of plants. Mo has been used as a fertilizer and as a coating for legume seeds (51,52) (see FERTILIZERS Mineral NUTRIENTS). 

The clearest manifestation of molybdenum in biology is its presence in over 20 enzymes which participate in a wide variety of redox processes (44—46). Some of the Mo enzymes and their occurrence are as follows ... 

Most of the Moco enzymes catalyze oxygen atom addition or removal from their substrates. Molybdenum usually alternates between oxidation states VI and IV. The Mo(V) state forms as an intermediate as the active site is reconstituted by coupled proton—electron transfer processes (62). The working of the Moco enzymes depends on the 0x0 chemistry of Mo (VI), Mo(V), and Mo (TV). 

J. T. Bolin and co-workers, in E. I. Stiefel, D. Coucouvanis, and W. E. Newton, eds.. Molybdenum Enzymes, Cofaetors and Model Systems American... 

It appears that chromium(III) is an essential trace element in mammalian metabolism and, together with insulin, is responsible for the clearance of glucose from the blood-stream. Tungsten too has been found to have a role in some enzymes converting CO2 into formic acid but, from the point of view of biological activity, the focus of interest in this group is unquestionably on molybdenum. 

Metal chemistry relevant to the multinuclear molybdenum and tungsten pterine enzymes 97CC1251. 

Mononuclear molybdenum pterine-based enzymes 96CRV2757. 

Molybdenum enzymes a survey of structural information from EXAFS and EPR spectroscopy. S. P. Cramer, Adv. Inorg. Bioinorg. Mech., 1983, 2, 260 (137). 

The compounds of the t/block elements show a wide range of interesting properties. Some are vital to life. Iron is an essential component of mammalian blood. Compounds of cobalt, molybdenum, and zinc are found in vitamins and essential enzymes. Other compounds simply make life more interesting and colorful. The beautiful color of cobalt blue glass, the brilliant greens and blues of kiln-baked pottery, and many pigments used by artists make use of d-block compounds. 

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. 

Not surprisingly, only about 20 of the chemical elements found on Earth are used by living organisms (Chapters 3 and 8). Most of them are common elements. Rare elements are used, if at all, only at extremely low concentrations for specialized functions. An example of the latter is the use of molybdenum as an essential component of nitrogenase, the enzyme that catalyzes the fixation of elemental dinitrogen. Because they are composed of common elements, living organisms exert their most profound effects on the cycles of those elements. 

Romao MJ, Huber R (1998) Structure and Function of the Xanthine-Oxidase Family of Molybdenum Enzymes. 90 69-96 Rosenzweig A, see Penneman RA (1973) 13 1-52... 

Molybdenum nitrogenase has been the subject of intensive study for more than 30 years, but much less work has been done on the vanadium and iron-only nitrogenases. Consequently, we first review the properties of Mo nitrogenase, and then in later sections outline what is known of the other two enzymes. 

Fig. 9. The MoFe protein cycle of molybdenum nitrogenase. This cycle depicts a plausible sequence of events in the reduction of N2 to 2NH3 + H2. The scheme is based on well-characterized model chemistry (15, 105) and on the pre-steady-state kinetics of product formation by nitrogenase (102). The enzymic process has not been chsiracter-ized beyond M5 because the chemicals used to quench the reactions hydrolyze metal nitrides. As in Fig. 8, M represents an aji half of the MoFe protein. Subscripts 0-7 indicate the number of electrons trsmsferred to M from the Fe protein via the cycle of Fig. 8.

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