Detection of a tetranuclear iron-sulfur center in fumarate reductase from Escherichia coli by electron paramagnetic resonance spectroscopy.

Soluble fumarate reductase and fumarate reductase complex from Escherichia coli have been investigated by electron paramagnetic resonance spectroscopy. Both succinate- and dithionite-reduced samples show signals associated with a [2Fe-2S]1+ cluster that account maximally for slightly more than one spin/molecule. In addition, at temperatures below 20 K, dithionite-reduced samples exhibit broad and complex features, to high and low field of the [2Fe-2S]1+ signal, that are attributable to a spin coupled [4Fe-4S]1+ cluster. Preliminary attempts to quantify the signals indicate that the [4Fe-4S] cluster is present in an approximate 1:1 stoichiometry with the [2Fe-2S] cluster. The observed enhancement of the spin relaxation of the [2Fe-2S]1+ cluster on dithionite reduction is attributed to spin-spin interaction between the S = 1/2, reduced tetranuclear and binuclear clusters.