Structure determination of bacterioferritin from Desulfovibrio desulfuricans by the MAD method at the FeK-edge

Bacterioferritins constitute a subfamily of heme ferritins, proteins involved in iron storage and homeostasis. The protein isolated from Desulfovibrio desulfuricans ATCC 27774 is a homodimer of mass 52 kDa. The monomers are linked by an iron-coproporphyrin group and each monomer contains a diferric center. The 24-monomer clusters found in the crystal are probably the functional particles. MAD data from cubic bacterioferritin crystals were collected at the K-shell iron edge. Preliminary phasing was performed using the positions of 23 of the 40 Fe atoms expected in the asymmetric unit. Further MAD phasing allowed the identification of individual iron sites. Clear and interpretable electron-density maps were obtained after density modification.