Iron-sulfur centers hydroxylases

Molybdenum hydroxylases (i.e., AO and XO) are flavoproteins that contain in addition to a FAD, a pterine cofactor coordinated to a molybdenum atom, and an iron sulfur center for their catalytic activity. They catalyze the two-electron oxidation of substrates with transfer to molecular oxygen to produce H2O2, and insert an atom of oxygen from water into a wide range of N-hctcrocycies and aldehydes via two-electron redox reaction as shown in equation 1.4 ... 

The rationale for studies on flavin semiquinone metal interactions stems from the presence of flavin coenzymes which participate in electron transfer in a number of metalloflavoproteins. Iron-containing redox centers such as the heme and nonheme iron sulfur prosthetic groups (Fe2/S2, Fe+ZS, or the rubredoxin-type of iron center) constitute the more common type of metal donor-acceptor found in metalloflavoproteins, although molybdenum is encountered in the molybdenum hydroxylases (e.g. xanthine oxidase, aldehyde dehydrogenase). 

This enzyme, as well as nicotinic acid hydroxylase was recently reported by Andreesan to be a selenoenzyme. The discovery of both these enzymes was based on the clever assumption that selenium might well be a component of multisubunit enzymes containing redox centers such as iron-sulfur, flavin, molybdenum, etc. When Clostridium acidiurici was cultured in media with supplemental selenium, an elevated activity of xanthine dehydrogenase was observed. The clostridial enzyme is comparable to mammalian xanthine oxidases in that it contains flavin adeninedinucleotide (FAD), molybdenum and nonheme iron. This enzyme functions in vivo under anaerobic conditions and appears to catalyze the reduction of uric acid to xanthine. Again it will be interesting to learn the form of selenium in this enzyme. 

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