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


Journal Article

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.

Full Text

Duke Authors

Cited Authors

  • Christner, JA; Münck, E; Kent, TA; Janick, PA; Salerno, JC; Siegel, LM

Published Date

  • January 1, 1984

Published In

Volume / Issue

  • 106 / 22

Start / End Page

  • 6786 - 6794

Electronic International Standard Serial Number (EISSN)

  • 1520-5126

International Standard Serial Number (ISSN)

  • 0002-7863

Digital Object Identifier (DOI)

  • 10.1021/ja00334a054

Citation Source

  • Scopus