Iron-sulfur proteins: spin-coupling model for three-iron clusters.

Abstract

Recent Moessbauer and EPR studies of 2 ferredoxins [from Azotobacter vinelandii and Desultovibrio gigas] and of [beef heart mitodrondrion] aconitase gave evidence for a 3-Fe cluster, probably of a [3Fe-3S] type. The studies of the oxidized EPR-active centers showed that the 3 Fe sites are characterized by significantly different magnetic hyperfine coupling constants. For the ferredoxin from A. vinelandii, A1 = 41 MHz, A2 = +18 MHz, and A3 A3 = 5 MHz was observed. The magnetic properties of the clusters can be explained with a simple model of 3 high-spin ferric ions (S = 5/2) exchange-coupled to a system spin S = 1/2. The model assumes isotropic exchange and different couplings between the Fe sites. The results show that the 3 sites have intrinsic hyperfine interactions similar to those of ferric rubredoxin; the differences in the observed interactions reflect the geometrical features of spin coupling. Furthermore, the 3 exchange coupling constants are equal within a factor of 2. This implies that the 3-Fe cluster is a single covalently linked structure and should not be considered as a [2Fe-2S] cluster weakly coupled to a 3rd Fe atom.