Azotobacter chroococcum 7Fe ferredoxin. Two pH-dependent forms of the reduced 3Fe clusters and its conversion to a 4Fe cluster

Abstract

Ferredoxin from A. chroococcum was studied by low temperature magnetic circular dichroism and EPR spectroscopy. When aerobically isolated, ferredoxin contains a (3Fe-4S] and [4Fe-4S] cluster. Anaerobic treatment with dithionite in the presence of ethanediol reduces the [3Fe-4S] cluster to give 2 spectroscopically distinct forms RI and RII which are reversibly interconvertible with a pKa .apprx. 7.5. The higher pH form, RII, has a high affinity for Fe2+ and converts readily to a [4Fe-4S]1+ cluster, scavenging Fe from the medium. The presence of the Fe chelator EDTA inhibits this conversion.