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

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