Molybdenum biological role

Molybdenum is the only transition element in the second row of the Periodic Table that is known to have a biological role. It is present in at least 15 enzymes, which are listed in Table 23,... 

The concentration of tungsten in natural waters is very low and the element is nontoxic. Until recently tungsten was supposed to have no biological role at all. A few enzymes containing tungsten have, however, been found in anaerobic bacteria, especially dostridium. In these, tungsten assists in reduction/oxidation reactions such as CO fixation. It is a mystery why nature uses tungsten and not molybdenum for these purposes. 

The quality of the experimental evidence for nutritional essentiality varies widely for the ultratrace elements. The evidence for the essentiality of three elements, iodine, molybdenum and selenium, is substantial and noncontroversial specific biochemical functions have been defined for these elements. The nutritional importance of iodine and selenium are such that they have separate entries in this encyclopedia. Molybdenum, however, is given very little nutritional attention, apparently because a deficiency of this element has not been unequivocally identified in humans other than individuals nourished by total parenteral nutrition or with genetic defects causing disturbances in metabolic pathways involving this element. Specific biochemical functions have not been defined for the other 15 ultratrace elements listed above. Thus, their essentiality is based on circumstantial evidence, which most often is that a dietary deprivation in an animal model results in a suboptimal biological function that is preventable or reversible by an intake of physiological amounts of the element in question. Often the circumstantial evidence includes an identified essential function in a lower form of life, and biochemical actions consistent with a biological role or beneficial action in humans. The circumstantial evidence for essentiality is substantial for arsenic, boron, chromium, nickel, silicon, and vanadium. The evidence for essentiality for the... 

There are some molybdenum-containing enzymes, e.g. nitrogenase, with clearly defined and obviously vital biological roles and other such enzymes with much more nebulous roles. From a mechanistic point of view all molybdenum enzymes seem closely related to one another (Stiefel, 1973). Even so, it is perhaps a little unfortunate that the enzyme xanthine oxidase, which has proved to be the most amenable to study by EPR of molybdenum and is thus the enzyme whose catalytic mechanism is the best understood, is also the enzyme whose biological role is the least clear. 

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. 

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