Molybdenum enzymes.

There are many molybdenum-containing enzymes distributed throughout the biosphere. The availability of molybdenum to biological systems is due to the high water solubility of oxidized forms of the metal. Molybdenum enzymes can be grouped on the basis of the structure of the metal centre. Three principal families of enzyme exist, with active sites consisting of (ppt)MoOS(OH) (the molybdenum hydroxylases), (ppt)MoO2(S-Cys) (the eukaryotic oxotransferases) and (ppt)2MoOX (the bacterial oxotransferases). Here, ppt represents a unique ppt cofactor (pyranopterin) that co-ordinates to the metal, and X is a metalliganded serine, cysteine or selenocysteine. The molybdenum hydroxylases catalyse their reactions differently to other hydroxylase enzymes, with water rather than molecular oxygen as the ultimate source of the oxygen atom incorporated into product, and with the generation rather than consumption of reducing equivalents. The active sites possess a catalytically labile Mo-OH (or possibly Mo-OH2) group that is transferred to substrate in the course of the hydroxylation reaction. These enzymes invariably have other redoxactive centres. The eukaryotic oxotransferases consist of the sulphite oxidases and plant nitrate reductases. They catalyse the transfer of an oxygen atom to or from their substrate (to and from nitrate) in a manner that involves formal oxidation-state changes of the molybdenum. As with the molybdenum hydroxylases, the ultimate source of oxygen is water rather than molecular oxygen. The bacterial oxotransferases and related enzymes differ from the other two groups of molybdenum enzymes in having two equivalents of the ppt cofactor co-ordinated to the metal. This family is quite diverse, as reflected in the fact that serine, cysteine or selenocysteine may be found co-ordinated to the molybdenum, depending on the enzyme. As in the case of the molybdenum hydroxylases, both eukaryotic and bacterial oxotransferases utilize water (rather than molecular oxygen) as the source of the oxygen atom incorporated into product, although for these enzymes, the catalytically labile oxygen in the active site is an Mo = O group rather than an Mo-OH.