Bacterioferritin from Mycobacterium smegmatis contains zinc in its di‐nuclear site

Bacterioferritins, also known as cytochrome b 1, are oligomeric iron‐storage proteins consisting of 24 identical amino acid chains, which form spherical particles consisting of 24 subunits and exhibiting 432 point‐group symmetry. They contain one haem b molecule at the interface between two subunits and a di‐nuclear metal binding center. The X‐ray structure of bacterioferritin from Mycobacterium smegmatis (Ms‐Bfr) was determined to a resolution of 2.7 Å in the monoclinic space group C2. The asymmetric unit of the crystals contains 12 protein molecules: five dimers and two half‐dimers located along the crystallographic twofold axis. Unexpectedly, the di‐nuclear metal binding center contains zinc ions instead of the typically observed iron ions in other bacterioferritins.