The feiii and feii tetrathiolate species now serve as

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The Fe(III) and Fe(II) tetrathiolate species now serve as excellent structural models for the Fe sites of both oxidized and reduced Rd. 69 The structural parameters for the oxidized rubredoxin analogues are very similar to those of the oxidized Rd iron site. The reduced complexes reveal a lengthening of the average Fe-S bond from 2.27 to 2.36 A, consistent with the change in oxidation state from ferric to ferrous. The addition of an electron has a more profound structural effect in this single-iron center than in some of the multiiron clusters, where electrons are more delocalized. Clearly, for the single-Fe sites, the dominant structural feature is their near- tetrahedral tetrathiolate coordination. The dominant electronic structural feature is the presence of high-spin Fe3+ and Fe 2 + sites. The important mode of chem- ical reactivity is a simple one-electron transfer. Each of these features carries over to the 2Fe, 4Fe, and 3Fe sites discussed below. C. FezSz Ferredoxins The simple 2Fe-2S proteins are sometimes referred to as "plant" or "plant- type" ferredoxins. The protein from spinach, which serves as an electron accep-
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L IRON-SULFUR PROTEINS AND MODELS 379 tor in the photosynthetic apparatus,14,15,50,70 was among the first to be well- characterized and widely studied, and could be considered the prototypical 2Fe-2S ferredoxin, However, 2Fe-2S proteins are also well-known in bacteria. 4 The protein from the cyanobacterium (blue-green alga) Spirulina platensis has been structurally elucidated by x-ray crystallography. 47 Putidaredoxin, from Pseudomonas putida, which serves as a donor to the P-450 camphor monooxy- genase system, has been extensively studied. 28 Fe2S2 centers are also well-estab- lished in mammalian proteins. Adrenodoxin 29 serves as the electron donor to the P-450 monooxygenase system that carries out the 11-f3-hydroxylation of steroids, The so-called "Rieske proteins" are found in the bel complex of mitochondria 47 as well as in the bd complex of the photosynthetic apparatus of plants. 71 In addition, Fe2S2 centers are well-known constituents of such redox proteins as xanthine oxidase,25,72 CO oxidase,25 succinate dehydrogenase,73-75 and putidamonooxin. 76 Table 7.1 lists some of the Fe2S2 proteins and their prop- erties. The x-ray crystal structure of only the single 2Fe-2S protein mentioned above has been determined; 70a the 2Fe-2S ferredoxin from the blue-green alga Spiru- lina platensis 6 ,22,47,77,78 shows significant sequence identity with chloroplast fer- redoxins typical of higher plants. 79 ,80 As Figure 7.8 shows, the Fe2S2 unit in this ll-kDa protein is bound by Cys-41, Cys-46, Cys-49, and Cys-79, The binuclear iron cluster is found in a largely hydrophobic region of the protein, but is within 5 A of the protein surface. 6 The sulfur atoms of the cluster, both inorganic and cysteinyl, are hydrogen-bonded to six peptide NH groups and one serine OH group, which presumably stabilize the cluster/protein complex, The serine involved in the H-bonding, Ser-40, is conserved in all plant and algal
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