Mössbauer studies of Escherichia coli sulfite reductase complexes with carbon monoxide and cyanide. Exchange coupling and intrinsic properties of the [4Fe-4S] cluster.
Mössbauer studies of the hemoprotein subunit (SiR) of E. coli sulfite reductase have shown that the siroheme and the [4Fe-4S] cluster are exchange-coupled. Here we report Mössbauer studies of SiR complexed with either CO or CN- and of SiR in the presence of the chaotropic agent dimethyl sulfoxide (Me2SO). The spectra of one-electron-reduced SiR X CN show that all five iron atoms reside in a diamagnetic environment; the ferroheme X CN complex is low spin and the [4Fe-4S] cluster is in the 2+ oxidation state. Titration with ferricyanide affords a CN- complex of oxidized SiR in which the siroheme iron is low spin ferric, with the cluster remaining in the 2+ state. At low temperatures, paramagnetic hyperfine interactions are observed for the iron sites of the cluster, suggesting that it is exchange-coupled to the heme iron. Reduction of one-electron-reduced SiR X CN and SiR X CO yields complexes with "g = 1.94"-type EPR signals showing that the second electron is accommodated by the iron-sulfur cluster. The fully reduced complexes yield well resolved Mössbauer spectra which were analyzed in the spin Hamiltonian formalism. The analysis shows that the cluster subsites are equivalent in pairs, one pair having properties reminiscent of ferric sites whereas the other pair has features more typical of ferrous sites. The Mössbauer spectra of oxidized SiR kept in 60% (v/v) Me2SO are virtually identical with those observed for SiR in standard buffer, implying that the coupling is maintained in the presence of the chaotrope. Fully reduced SiR displays an EPR signal with g values of g = 2.53, 2.29, and 2.07. In 60% Me2SO, this signal vanishes and a g = 1.94 signal develops; this transition is accompanied by a change in the spin state of the heme iron from S = 1 (or 2) to S = O.
Christner, JA; Janick, PA; Siegel, LM; Münck, E
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