In this chapter we first review the chemistry of the

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In this chapter we first review the chemistry of the Fe-S sites that occur in relatively simple rubredoxins and ferredoxins, and make note of the ubiquity of these sites in other metalloenzymes. We use these relatively simple systems to show the usefulness of spectroscopy and model-system studies for deducing bioinorganic structure and reactivity. We then direct our attention to the hydro- genase and nitrogenase enzyme systems, both of which use transition-metal- sulfur clusters to activate and evolve molecular hydrogen. I. IRON-SULFUR PROTEINS AND MODELS Iron sulfide proteins involved in electron transfer are called ferredoxins and rub- redoxins. * The ferredoxins were discovered first, and were originally classified as bacterial (containing Fe4S4 clusters) and plant (containing FezSz clusters) fer- redoxins. This classification is now recognized as being not generally useful, since both FezSz and Fe4S4 ferredoxins are found in plants,14,15 animals, Z,6,16 and bacteria. 4 Ferredoxins are distinguished from rubredoxins by their posses- sion of acid-labile sulfide; i.e., an inorganic S z- ion that forms HzS gas upon denaturation at low pH. Rubredoxins have no acid-labile sulfide, and generally have a single iron in a more or less isolated site. Despite their lack of acid- labile sulfide, rubredoxins are included in this chapter because they have se- * For review articles, see References I-II. For a discussion of nomenclature, see References 12 and 13. 365
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366 7 / FERREDOXINS, HYDROGENASES, AND NITROGENASES: METAL-SULFIDE PROTEINS quences much like those of the ferredoxins, and because their simple mononu- clear Fe 2 + and Fe 3+ sites provide convenient illustrations of key structural and spectroscopic features. In most ferredoxins, and in all rubredoxins, the protein ligands are cys- teines, which provide four thiolate donors to the IFe, 2Fe, or 4Fe center. Ad- ditionally, the existence of 3Fe centers and of Fe-S sites that contain a second metal (i.e., heteronuclear clusters) make the Fe-S class a broad and multifunc- tional one. Simple cytochromes and simple iron-sulfide proteins are similar, in that both can undergo one-electron transfer processes that are generally uncoupled from proton-, atom-, or group-transfer processes. Some of these proteins, such as cytochrome C3 from Desulfovibrio with four hemes 17 or ferredoxin from Clos- tridium pasteurianum with two Fe4S4 centers,6 can transfer more than one elec- tron, because they have multiple copies of a one-electron transfer group. The cytochromes were discovered in 1886 by McMunn,18 and their role in metabo- lism was discovered in the 1920s by Keilin (Chapter 6). The intense optical absorbance of these heme-containing proteins contributed singularly to their dis- covery and biochemical characterization. In contrast, the iron-sulfur proteins, although red to red-brown, absorb far more weakly in the visible region than do the cytochromes. Their presence is sometimes obscured by the cytochromes, and their frequent air instability made their initial recognition and isolation more difficult.
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