Fortuitous structure determination of `as‐isolated' Escherichia coli bacterioferritin in a novel crystal form

Escherichia coli bacterioferritin was serendipitously crystallized in a novel cubic crystal form and its structure could be determined to 2.5 Å resolution despite a high degree of merohedral twinning. This is the first report of crystallographic data on `as‐isolated' E. coli bacterioferritin. The ferroxidase active site contains positive difference density consistent with two metal ions that had co‐purified with the protein. X‐ray fluorescence studies suggest that the metal composition is different from that of previous structures and is a mix of zinc and native iron ions. The ferroxidase‐centre configuration displays a similar flexibility as previously noted for other bacterioferritins.