Nitrogen fixing bacteria, molybdenum

Molybdenum is essential for a number of enzymes, for example xanthine oxidases in mammals and nitrogenases in nitrogen-fixing bacteria. 

Uptake of molybdate by cells of E. coli is accomplished by an ABC-type transport system.681 In some bacteria, e.g., the nitrogen-fixing Azotobacter, molybdenum can be stored in protein-bound forms.682... 

Uptake and Role of Molybdenum in Nitrogen-Fixing Bacteria... 

Loveless, T.M., Saah J.R. and Bishop PE (1999) Isolation of nitrogen-fixing bacteria containing molybdenum-independent nitrogenases from natural environments, Applied and environmental microbiology, 65, 4223-4226. 

In nature, ammonia is produced by the action of nitrogen-fixing bacteria on atmospheric N2 under very mild conditions (room temperature and 0.8 atm N2 pressure). These bacteria contain nitrogenases, iron- and molybdenum-containing enzymes that catalyze the formation of NH3. Industrially, NH3 is synthesized from its elements by the Haber-Bosch process, which typically uses finely divided iron as catalyst ... 

Within nitrogen-fixing bacteria, an enzyme (composed of proteins) called nitrogenase is responsible for nitrogen fixation. Within both of these bacteria the active site for N2 binding involves a molybdenum atom. 

A further development of this research was the discovery that the active site in nitrogen fixing bacteria is also a related iron-sulfur cluster. The significant differences are that one iron is replaced by a molybdenum and that the molybdenums from two of these clusters are linked by three bridging sulfur atoms. It is not uncommon that natural systems are able to produce quite varied chemistry by using some basic molecules which differ from one another in relatively minor ways. 

Plants usually obtain their nitrogen by absorption of nitrate or ammonium ions from the soil (symbiotic associations between higher plants and nitrogen fixing bacteria are of course exceptions to this). Ammonium ions may be utilized directly in the synthesis of amino acids (see p. 169), but nitrate must first be reduced to ammonia. This is accomplished in two stages the reduction of nitrate to nitrite followed by the reduction of nitrite to ammonia. The first step— nitrate reduction—is catalysed by the flavo-protein enzyme complex nitrate reductase (Fig. 5.12) which contains molybdenum and FAD (flavin adenine dinucleotide) as a prosthetic group. Reduced FMN... 

Organisms also require very low levels of a number of micronutrients, which are usually used by essential enzymes that enable metabolic reactions to occur. For plants, essential micronutrients include the elements boron, chlorine, copper, iron, manganese, sodium, vanadium, and zinc. The bacteria that fix atmospheric nitrogen required by plants use trace levels of molybdenum. Animals require in their diet elemental micronutrients including iron and selenium as well as miaonutrient vitamins consisting of small organic molecules. 

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