Electron spin-echo envelope modulation studies of the [3Fe–4S] cluster of Escherichia coli fumarate reductase

Abstract

The dimeric form of purified Escherichia coli fumarate reductase contains three different iron–sulfur clusters, termed centres 1, 2 and 3 ([2Fe–2S], [4Fe–4S] and [3Fe–4S] clusters, respectively). We have performed electron spin-echo envelope modulation (ESEEM) spectroscopy in order to obtain information about the environment of the [3Fe–4S] cluster. Modulations from ¹⁴N were detected in the three-pulse ESEEM spectra from the cluster in the oxidised state. The low-frequency lines observed in the three-pulse ESEEM pattern were similar to those observed for the [3Fe–4S] cluster of bovine heart succinate dehydrogenase (B. A. C. Ackrell, E. B. Kearney, W. B. Mims, J. Peisach and J. H. Beinert, J. Biol. Chem., 1984, 259, 4015). Determination of the hyperfine and quadrupolar coupling (A≈ 0.7 MHz, e²qQ≈ 3.4 MHz) gave values that are similar to those determined for the [2Fe–2S] cluster of centre 1 (R. Cammack, A. Chapman, J. McCracken, J. B. Cornelius, J. Peisach and J. H. Weiner, Biochim. Biophys. Acta, 1988, 956, 307; J. K. Shergill, PhD thesis, 1993). These coupling constants indicate that the ¹⁴N-nucleus giving rise to the modulation pattern in the ESEEM spectrum is weakly coupled to centre 3, rather than directly coordinated to the iron.