The 2.6 Å resolution structure of Rhodobacter capsulatus bacterioferritin with metal-free dinuclear site and heme iron in a crystallographic `special position'

The first structure of a bacterioferritin from a photosynthetic organism (R. capsulatus) was solved by the MIR method at 2.6 Å resolution in the I422 space group and compared with that from E. coli solved in the P2₁ and P4₂2₁2 space groups. The new crystals reveal for the first time the presence of the non-symmetric heme molecule on a twofold crystallographic axis. It is concluded that in the crystal each heme site contains a symmetrical mixture of heme in two orientations.