Get the iron out

J. Am. Chem. Soc. 134, 13470–13481 (2012)

Credit: RITESH KUMAR

Ferritins and ferritin-like proteins such as bacterioferritin B (BfrB) in Pseudomonas aeruginosa store iron as the insoluble Fe³⁺ to prevent formation of reactive oxygen species by excess Fe²⁺. Cells must also be able to retrieve this iron when needed; in P. aeruginosa, both ferredoxin reductase and the [2Fe-2S]-containing bacterioferritin-associated ferredoxin (Bfd) are thought to be involved in this process, but the mechanistic details are unknown. Yao et al. investigate this question in their report of the crystal structure of Bfd bound to the 24-mer BfrB. This first structure of Bfd shows a three-helix fold that has only previously been seen within the context of larger structures; its stability as an isolated domain is shown to depend on a phosphate ion that coordinates three cationic residues. At the largely conserved interface, Bfd inserts into a cleft between two BfrB monomers such that the [2Fe-2S] cluster is 15.1 Å from a buried heme thought to relay electrons from the surface to the core Fe³⁺. Functional studies with purified proteins indicate that Bfd loaded with [2Fe-2S] is necessary and sufficient for Fe²⁺ release and show alternating oxidation states of the [2Fe-2S] cluster and heme, supporting a direct electron transfer mechanism. These results demonstrate that Bfd facilitates iron mobilization and sets the stage for further detailed study of this interesting system.