PMID:6257697

Citation	Christner, JA, Münck, E, Janick, PA and Siegel, LM (1981) Mössbauer spectroscopic studies of Escherichia coli sulfite reductase. Evidence for coupling between the siroheme and Fe4S4 cluster prosthetic groups. J. Biol. Chem. 256:2098-101
Abstract	Escherichia coli NADPH-sulfite reductase is a complex hemoflavoprotein with an alpha 8 beta 4 subunit structure. The beta-subunits each contain one siroheme and a tetranuclear iron-sulfur center (Fe4S4). Isolated beta-monomers can catalyze the 6-electron reduction of sulfite to sulfide. We have studied the beta-monomers with Mössbauer and EPR spectroscopy. The data show conclusively that the siroheme and the Fe4S4 cluster are strongly exchange-coupled. This is proven by the observations that (a) the two chromophores share a single electronic spin and (b) the addition of 1 electron to oxidized sulfite reductase changes the environments of 5 iron atoms. Spin-sharing is demonstrated in oxidized and 2-electron-reduced sulfite reductase and strongly implicated in 1-electron-reduced material. Thus, sulfite reductase provides the first example of an active site where a heme and an iron-sulfur cluster are closely linked as a functional unit, probably via a common bridging ligand.
Links	PubMed
Keywords	Electron Spin Resonance Spectroscopy; Escherichia coli/enzymology; Heme/analysis; Iron/analysis; Iron-Sulfur Proteins/metabolism; Metalloproteins/metabolism; Oxidation-Reduction; Oxidoreductases/metabolism; Oxidoreductases Acting on Sulfur Group Donors/metabolism; Protein Conformation; Spectrum Analysis; Sulfur/analysis
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