Novel electron paramagnetic resonance signals from an Fe/S protein containing six iron atoms

Abstract

An EPR study is presented of an unusual iron–sulphur protein recently isolated from Desulfovibrio vulgaris(Hildenborough). The protein is a 50 kDa monomer and contains six iron atoms. The EPR spectrum of the dithionite-reduced protein is very similar to those found for model compounds which contain the [6Fe–6S]³⁺ prismane core. The spectrum is from a rapidly relaxing S= 1/2 ground state with g= 2.004, 1.819, 1.32. Spin quantification reveals the presence of nearly one spin system per protein molecule. In a higher, intermediate oxidation state the protein exhibits another S= 1/2 ground-state signal with g= 1.968, 1.953, 1.903. The fully oxidized protein shows no EPR spectrum. We believe we have found a single prismane-containing protein, that can exist in the redox states 3 +(S= 1/2), 4 +(S= 0 or integer), 5 +(S= 1/2) and 6 +(S= 0).