Molybdenum is widely distributed in plants and animals. The metal exists in three valence states: Mo4+, Mo5+, and Mo6+. A limited number of redox reactions exploit the multivalence states. Molybdenum-dependent enzymes are found in pathways that metabolize purines, pyrimidines, pterins, aldehydes, and sulfites. A cofactor structure for molybdenum has been proposed (Figure 7) and is referred to as molybdopterrin. Enzymes that use the cofactor include xanthine oxidase, sulfite oxidase,

Figure 7 Proposed structure for the molybdenum cofactor in nitrogenase. This center consists of a special pterin cofactor, a relative of tetrahydrofolate. The molybdenum engages two sulfur atoms as a dithiolate complex.

and aldehyde oxidase. In microorganisms, molybdenum is a key metal for the fixation of nitrogen. Xanthine oxidase is the enzyme with importance relevance to a mammalian system.

Reactivity A major nutritional concern of molybdenum is its ability to antagonize copper. Indiscriminant spraying of soils with molybdenum has been shown to affect the growth and productivity of ruminants. The effect relates to the formation of thiomolybdates in the rumen. The thiomolybdates interact and bind copper preventing its absorption from the rumen. Thiomolybdates have a very high affinity for copper almost to the exclusion of other metal ions. Lately, thiomolybdates have been used to control copper toxicity in Wilson's disease, a genetic disease of copper poisoning in humans.

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