title: Mössbauer spectroscopic studies of Escherichia coli sulfite reductase. Evidence for coupling between the siroheme and Fe4S4 cluster prosthetic groups.
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.
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D004926:Escherichia coli / Q000201:enzymology*; D006418:Heme / Q000032:analysis; D007501:Iron / Q000032:analysis; D007506:Iron-Sulfur Proteins / Q000378:metabolism*; D008667:Metalloproteins / Q000378:metabolism*; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism*; D050862:Oxidoreductases Acting on Sulfur Group Donors / Q000378:metabolism*; D011487:Protein Conformation; D013057:Spectrum Analysis; D013455:Sulfur / Q000032:analysis
chemical_list: D007506:Iron-Sulfur Proteins; D008667:Metalloproteins; D006418:Heme; D013455:Sulfur; D007501:Iron; D010088:Oxidoreductases; D050862:Oxidoreductases Acting on Sulfur Group Donors