Structure of a Monoclinic Crystal Form of Cytochrome b1 (Bacterioferritin) from E. coli

Crystals of E. coli cytochrome b1, alias bacterioferritin, were grown from a low ionic strength solution. The resulting monoclinic P2₁ structure was solved by molecular replacement and refined using noncrystallographic symmetries applied to the fundamental unit, consisting of two protein subunits and a single haem. From the Patterson self-rotation results it was shown that the asymmetric unit of the monoclinic crystal consists of 12 such dimers and corresponds to a complete, nearly spherical, molecule of bacterioferritin (M_r = 450 kDa) of 432 point-group symmetry. It is thus the most symmetrical cytochrome. As previously determined for the tetragonal form, the haem is located in a special position on a local twofold axis of the dimer. A bimetal centre is also observed within the four-helix bundle of each monomer; a metal-binding site is located on the fourfold axis.