Molybdenum nitrogen fixation, role

MgATP hydrolysis and, 47 189-191 nitrogenase complex, 47 186-189 substrates, 47 192-202 molybdenum iron proteins, 47 161, 166-174, 176-183, 191-192 structure, 47 162-164, 166-170 nitrogen fixation role, 36 78 in nitrogen fixation systems, 27 265-266 noncomplementary reactions with Sn", 10 215... 

In animal metabolism, oxomolybdoenzymes catalyse a number of oxidation processes. These oxidases contain Mo coordinated to terminal O and S atoms, and their action appears to involve loss of an O or S atom along with reduction to Mo or Mo". It is, however, the role of molybdenum in nitrogen fixation which has received most attention. 

In addition to being important in industry, transition metal ions play a vital role in living organisms. For example, complexes of iron provide for the transport and storage of oxygen, molybdenum and iron compounds are catalysts in nitrogen fixation, zinc is found in more than 150 biomolecules in humans, copper and iron play a crucial role in the respiratory cycle, and cobalt is found in essential biomolecules such as vitamin Bi2-... 

Molybdenum has long been known to have a role in nitrogen fixation over 50 years ago Bortels showed that Mo stimulated the N2-dependent growth of Azotobacter (2). Subsequently, these observations were rationalized when all purified nitrogenases were shown to be separable into an Fe protein and a MoFe protein that contained Mo as part of an essential iron- and molybdenum-containing cofactor (Fe-Moco), the probable site at which N2 is reduced. 

To date, only the role of vanadium in biogenic nitrogen fixation has been established unambiguously. In addition, there are a few reports on a putative importance of vanadium in (bacterial) nitrate reductases, otherwise a domain of molybdenum. These reports are based on investigations which remain to be backed up. They are nonetheless briefly addressed, with a question mark, in Section 4.4.2. 

The function of molybdenum in enzymic mechanisms has been reviewed and the role of molybdenum in nitrogen fixation examined. ... 

In a perspective provided by structure of the Periodic Table, molybdenum and tungsten are distinct in being the only 4d- and 5d-transition metals that are required for the normal metabolism of biological systems. These metals play a vital role as the catalytic centres of a wide variety of enzymes. Mo was first identified as an essential trace element in the 1930s, because of its role in nitrogen fixation this metal is now known to be the catalytic centre of over 50 enzymes. Evidence for the involvement of W in biological systems has been obtained only relatively recently, especially for enzymes of hyperthermophilic archea that thrive near lOOX. ... 

Boron also plays a role in symbiosis but apparently not as a catalyst like molybdenum. Boron is essential for the growth of meristematic tissues. It is also essential in building up normal nodules with a well-developed vascular system, and may thus indirectly have a favorable effect on nitrogen fixation. 

Vanadium acts as a partial substitute for molybdenum in catalyzing nitrogen fixation by Azotobacter and probably other organisms. It is an essential element for the green alga Scenedesmus but there is no proof that it is essential for higher green plants. Its role, if any, in plant nutrition is unknown. 

Interest in complexes of this type, especially those containing iron, molybdenum, and vanadium is based on their structural relationships to the M4E4 clusters that occur in several biological systems.These clusters play a central role in a variety of biological processes including photosynthesis, nitrogen fixation, and respiration. The role of metal clusters in biology is covered in Volume 8, Chapters 8.2, 8.7, 8.17, and 8.23. 

Molybdenum is an essential element for several enzymes important in plant and animal metabolism. It has a unique role in nitrogenase, an enzyme that converts molecular nitrogen into ammonia in nitrogen fixation. Molybdenum enzymes provide the key steps in the fixation of nitrogen by microorganisms and its utilization by higher plants (Table 1). 

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