Molybdenum biological activity

Finally, in 1797, the Frenchman L. N. Vauquelin discovered the oxide of a new element in a Siberian mineral, now known as crocoite (PbCr04), and in the following year isolated the metal itself by charcoal reduction. This was subsequently named chromium (Greek xpco ia, chroma, colour) because of the variety of colours found in its compounds. Since their discoveries the metals and their compounds have become vitally important in many industries and, as one of the biologically active transition elements, molybdenum has been the subject of a great deal of attention in recent years, especially in the field of nitrogen fixation (p. 1035). 

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. 

Biotin contains three chiral centers and therefore has eight stereoisomers.1819 Of these, only one, the dextrorotatory (-i-)-biotin, is biologically active.19 20 The vitamin is readily oxidized to die sulfoxide and sul-fone. The sulfoxide can be reduced back to biotin by a molybdenum-containing reductase in some bacteria (see also Chapter 16, Section H).20a Biotin is synthesized from pimeloyl-CoA (see chapter banner, p. 719 and Eq. 24-39). Four enzymes are required. Two of them, a... 

The thermochromism in Scheme 1-54 represents valence tautomerism in molybdenum complexes and, at the same time, the inherent redox activity of a dithoilate ligand. This phenomenon may be significant with respect to the biological activity of various molybdenum enzymes. 

Group 6B includes both chromium and molybdenum as essential trace elements. The oxidation state of Cr is of critical importance to its biological activity. The most common oxidation state is Cr3+ or Cr(III). Chromium(VI) is used in electroplating and in other industrial applications and is recognized as a carcinogen when inhaled. The essentiality of Cr was discovered when weaning rats were fed Cr-deficient diets,... 

In addition to direct trafficking to target metalloproteins, some metals need to enter specialized biosynthetic pathways for metal cofactor assembly. For example, iron in metaUoenzymes is usually present as part of heme or Fe-S clusters, so it must be routed into the biosynthetic pathways for these cofactors. In a similar vein, molybdenum is not biologically active unless it is first incorporated with a pterin compound to form molybdenum... 

Molybdenum is one of the biologically active transition elements.1 It is believed to be a necessary trace element in animal diets, but function and minimum levels have not been established. It is well established that nitrogenfixing bacteria employ enzymes containing both Mo and Fe. Recently2 one such enzyme, or a part thereof, has been obtained in pure, crystalline form. One molecular unit appears to have a molecular weight in the range 270,000-300,000 Daltons and to contain two atoms of Mo and forty of Fe. This protein in association with another which contains only iron possesses the ability to catalyze the reduction of Nz and other substrates. Chemical and structural details are not yet known. 

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