Zinc, haem and water in bacterioferritin

This structure is a beauty: Willies et al. solved the structure of bacterioferritin from Escherichia coli to 1.9 Å. The hollow shell of bacterioferritin is made up of 24 identical subunits organised as 12 dimers. The two metals that the ferroxidase centre binds have previously been assigned as manganese or as a mixture of iron and zinc; now it has been shown that both are zinc. The haem is coordinated by two Met-52 residues from two subunits forming a dimer. Interestingly,

At the resolution of the structure presented, the electron density clearly shows the binding of haem in both conformations, each at half occupancy with no preference for either conformation

(the authors consistently use “conformation” throughout the paper but they really should say “orientation”, as they do in the legend to Figure 4). For the first time, a link is described between the internal and the external environment of the bacterioferritin via the cluster of water molecules and the haem; the authors suggest that this could be an electron-transport pathway.