Peter A. Janick

A novel system involving exchange-coupling between a heme and an ironsulfur cluster

Abstract The β-subunits of E. coli sulfite reductase (SiR) contain an Fe-isobacteriochlorin, termed siroheme, and a [4Fe-4S] cluster. Mossbauer and EPR studies [1] of oxidized SiR have demonstrated that the siroheme and the ironsulfur cluster are exchange coupled. Such a coupling implies a covalent link between the two chromophores; it is reasonable to assume that the attached to the heme iron by an as yet unspecified bridging ligand. In oxidized SiR the iron-sulfur cluster is in the 2+ oxidation state, a state in which [4Fe-4S] clusters are normally diamagnetic. Through exchange interactions with the siroheme the cluster acquires paramagnetism; the experimental observations have been explained qualitatively in a model which involves isotropic exchange between the heme iron and one site of the cluster [2]. Recent studies [3] of one-electron and two-electron reduced SiR, a nitrate ‘turnover’ (ferroheme-NO) complex, and studies of SiR in chaotropic agents that the coupling is maintained in may oxidation-, spin-, and complexation-states of the enzyme. We have also studied SiR complexed to cyanide (in three oxidation states) and carbon monoxide. Exchange coupling is indicated in the oxidized cyano complex; in the reduced CN − and CO complexes the heme is low-spin ferrous and thus in a state unfavorable for the development of interatomic exchange.