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J. Biol. Chem., Vol. 256, Issue 5, 2098-2101, Mar, 1981

Mossbauer spectroscopic studies of Escherichia coli sulfite reductase. Evidence for coupling between the siroheme and Fe4S4 cluster prosthetic groups

JA Christner, E Munck, PA Janick and LM Siegel

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 Mossbauer 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.
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