Electron spin-echo envelope modulation and spin–spin interaction studies of the iron–sulphur clusters in fumarate reductase of Escherichia coli

Abstract

Electron spin-echo envelope modulation (ESEEM) spectroscopy has been performed in order to obtain structural information about the reduced [2Fe-2S], [4Fe-4S] and oxidized [3Fe-4S] clusters in purified fumarate reductase from Escherichia coli. Spin echoes were detected from the reduced iron–sulphur clusters (centres 1–3) and also from oxidized centre 3. The ESEEM studies showed that the reduced [2Fe-2S] cluster is coordinated by a peptide ¹⁴N. In this study, we also observed a weak interaction between the oxidized [3Fe-4S] cluster and a peptide ¹⁴N; there was no evidence for coordination to the ¹⁴N-atoms of imidazole rings. Spin–lattice and spin–spin relaxation rates for oxidized and reduced protein were measured, and the results confirmed the presence of a spin–spin interaction between reduced centres 1 and 2.