7 Ferredoxins, Hydrogenases, and Nitrogenases: Metal-Sulfide Proteins

Transition-metal/sulfide sites, especially those containing iron, are present in all forms of life and are found at the active centers of a wide variety of redox and catalytic proteins. These proteins include simple soluble electron-transfer agents (the ferredoxins), membrane-bound components of electron-transfer chains, and some of the most complex metalloenzymes, such as nitrogenase, hydrogenase, and xanthine oxidase. 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 Sz- 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