Nitric Oxide Complexes of Iron-Sulfur Proteins

NITRIC OXIDE COMPLEXES OF IRON-SULFUR PROTEINS 

The iron-sulfur proteins include small proteins that function as remarkably simple electron carriers and large multisubunit complexes with multiple activities. Although many of these enzymes function in electron transfer in bioener-getic or biosynthetic pathways, it has become clear that iron-sulfur proteins catalyze a broad array of reactions not always involving electron transfer. 

The prosthetic groups of iron-sulfur proteins fall into several classes (Lov-enberg, 1977 Spiro 1982). Rubredoxins bind single iron atoms with four cys-teinyl sulfur ligands they function as electron carriers in some bacterial systems. Rubredoxins generally have two such centers per molecule in the ferric state each iron center is EPR detectable. 

The 2Fe2S (S, acid-labile sulfur) ferredoxins have a redox active binuclear center, with each of the two iron atoms attached to the protein by two cysteinyl sulfur ligands and connected by two inorganic acid-labile sulfur ligands. At cty-ogenic temperatures these clusters are EPR detectable, with characteristic features in the vicinity of g = 1.94. Spinach ferredoxin has principal g values of 2.03, 1.96, and 1.88 and a broad absorbance spectrum with a weak maximum around 420 nm, giving these proteins a reddish brown color which bleaches on reduction. Ferredoxins are low potential electron carriers chloroplast ferredoxins function in photosynthetic electron transfer, but related proteins such as adrenal ferredoxin are involved in steroidogenic electron transfer in mitochondria in tissues which produce steroid hormones. 

Ferredoxins with 4Fe4S clusters are small, low potential electron carriers that function in bacterial electron transfer. Like the binuclear clusters, each tetranu-clear cluster can reversibly accept a single electron. The tetranuclear prosthetic group is a cubane with iron and acid-labile inorganic sulfide groups at alternate vertices the four iron atoms and the four sulfides form interpenetrating tetrahedra of slightly different sizes. 

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V. NITRIC OXIDE COMPLEXES OF IRON-SULFUR PROTEINS... 

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