Electron Magnetic Resonance of Iron–Sulfur Proteins in Electron-Transfer Chains: Resolving Complexity

Abstract

Iron–sulfur clusters, some of the most abundant electron-transfer groups in biology, were first detected and investigated using EPR spectroscopy. They are commonly found in large, membrane-bound complexes that are essential for energy conversion in living cells. Here we describe the applications of multiple-frequency, pulsed and double-resonance elec- tron magnetic resonance (EMR†) methods to investigate the structure and function of these iron–sulfur proteins. Such spectra can be observed from macromolecular complexes and membranes, as well as from whole cells and tissues. A careful choice of sample preparation and measure- ment parameters is required to partially resolve overlapping spectra from multiple iron–sulfur clusters. Recently the REFINE technique has been presented, which can be used to select the spectra of individual centers having different relaxation rates, such as cluster N2 from NADH: ubiquinone oxidoreductase. Where proteins are difficult to crystallize, EMR methods can provide structural information; ESEEM and ENDOR especially can identify the types of clusters and the nature of their pro- tein ligands. Pulsed EPR and PELDOR are able to provide information about distances between clusters and other paramagnets such as semi- quinone radicals or other metals. When crystal structures are known, EMR provides additional information about electronic structures and the disposition of protons. Potentially, EMR techniques can show details of protein movements and the effects of transmembrane potentials. Future directions for research are discussed.