It was not until the early 1960s that discoveries by

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It was not until the early 1960s that discoveries by several research groups 19 led to the isolation, recognition, and characterization of the ferredox- ins. The use of EPR spectroscopy and its application to biological systems had a profoundly stimulating effect on the field (see below). Although cytochromes were discovered first, the ferredoxins are likely to be the older proteins from an evolutionary perspective. 20 Ferredoxins have rela- tively low-molecular-weight polypeptide chains, require no organic prosthetic group, and often lack the more complex amino acids. In fact, the amino-acid composition in clostridial ferredoxin is close to that found in certain meteo- rites. 21 The various Fe-S sites found in electron-transfer proteins (ferredoxins) are also found in many enzymes, 6,11,22.23 where these centers are involved in intra- or interprotein electron transfer. For example, sulfite reductase contains a siro- heme and an Fe4S4 center,24 which are strongly coupled and involved in the six- electron reduction of S03 2 - to H 2 S. Xanthine oxidase (see Figure 7.1) has two identical subunits, each containing two different Fe2S2 sites plus molybdenum and FAD sites. In xanthine oxidase, the Mo(VI) site carries out the two-electron oxidation of xanthine to uric acid, being reduced to Mo(IV) in the process. 25 The Mo(VI) site is regenerated by transferring electrons, one at a time, to the Fe2S2 and flavin sites, thereby readying the Mo site for the next equivalent of xanthine. Although the Fe2S2 sites do not directly participate in substrate reac- tions, they are essential to the overall functioning of the enzyme system. The Fe2S2 centers in xanthine oxidase play the same simple electron-transfer role as the Fe2S2 ferredoxins play in photosynthesis.
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367 2W+ 02 Mol. WI. - 150,000 Fe 2 S z ~ = flavin ~ [FAD] = Mo cofactor [molybdopterin] Figure 7.1 A schematic drawing of xanthine oxidase illustrating the Mo, flavin, and FezSz sites and interaction of the enzyme with substrate and oxidant(s). Structurally, all the iron-sulfur sites characterized to date are built up of (approximately) tetrahedral iron units (see Figure 7.2). In rubredoxins the single iron atom is bound in tetrahedral coordination by four thiolate ligands provided by cysteine side chains. In two-iron ferredoxins the Fe2S2 site consists of two tetrahedra doubly bridged through a pair of sulfide ions, i.e., Fe2(JLrSh, with the tetrahedral coordination of each Fe completed by two cysteine thiolates. In four-iron or eight-iron ferredoxins, the 'thiocubane' Fe4S4 cluster consists of four tetrahedra sharing edges with triply bridging S2- ions, i.e., FeiJLrS)4' with each Fe completing its tetrahedron by binding to a single cysteine thiolate. Finally, for Fe3S4 clusters, which are now being found in more and more pro- teins, the well-established structure has one triply bridging and three doubly bridging sulfide ions, Fe3(JLrS)(JLrSh. The Fe3S4 unit can be thought of as derived from the 'thiocubane' Fe4S4 unit by the removal of a single iron atom.
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