Exchange Coupling between Siroheme and [4Fe-4S] Cluster in E. Coli Sulfite Reductase. Mössbauer Studies and Coupling Models for a 2-Electron Reduced Enzyme State and Complexes with Sulfide
Recent Mössbauer and EPR studies of the hemoprotein subunit (SiR) of E. coli sulfite reductase have shown that the siroheme and the [4Fe-4S] cluster are exchange coupled in a variety of states. Mössbauer studies of 2-electron-reduced SiR in the presence of 0.1 M guanidinium sulfate, SiR2−/(Gdm)2SO4, and of complexes of SiR with sulfide are reported. SiR2−/(Gdm)2SO4exhibits an EPR signal with g values characteristic of a system with electronic spin [formula omitted]. Mössbauer studies had indicated problems with such an interpretation. It is shown here that the siroheme iron of SiR2−/(Gdm)2SO4is high-spin ferrous (Sh= 2). With this firm assignment a spin coupling model is developed which is in accord with the EPR and Massbauer data. In this model the reduced iron sulfur cluster, with cluster spin Sc= ½, is exchange coupled to the siroheme iron, [formula omitted], where J is an effective coupling constant. J/D is determined to be −0.22, where D is the zero-field splitting parameter of the heme (D > 0 and D ≃ 5–15 cm−1). It is discussed how J is related to the coupling constant which characterizes the presumed bond between the siroheme iron and one iron subsite of the cluster. The model fits the EPR data of a variety of SiR complexes and of 2-electron-reduced spinach nitrite reductase. The similarity of the observed g values with those of an [formula omitted] system turns out to be fortuitous. The Mössbauer spectra of oxidized SiR with sulfide show that the heme iron is low-spin ferric (Sh= ½) and that the [4Fe-4S] cluster is in the 2+ oxidation state. Upon addition of one electron per siroheme the [4Fe-4S] cluster is reduced to the 1+ oxidation state (Sc= 1/2) while the heme iron remains low-spin ferric. Mössbauer spectra recorded at 4.2 K in a 6.0-T magnetic field show that both the heme and the cluster reside in a diamagnetic environment. The diamagnetism results from exchange interactions, [formula omitted], between the heme iron and the cluster. The high-field studies show that J > 6 cm−1. © 1984, American Chemical Society. All rights reserved.
Christner, JA; Münck, E; Kent, TA; Janick, PA; Salerno, JC; Siegel, LM
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