A CRYSTALLOGRAPHIC STUDY OF HEME-BINDING TO FERRITIN

Ferritin, the iron-storage protein, binds porphyrins, metolloporphyrin s and the fluorescent dyes ANS (8-anilino-1-naphthalenesulfonic acid) and TNS (2-p-toluidinyl-6-naphthalenesulfonic acid), similarly to apo- myoglobin. Octahedral crystals of horse-spleen apo-ferritin (HSF; 174 amino acids) complexes prepared by the addition of haem, hematoporphyr in or Sn-protoporphyrin IX to a solution of apo-ferritin crystallize i n space group F432 with cell parameter a = 184.0 angstrom. X-ray cryst allographic analysis of single crystals prepared from a mixture contai ning haem or Sn-protoporphyrin IX shows that the haem-binding sites in these crystals are occupied by protoporphyrin IX, which is free of me tal, rather than by the original metalloporphyrin. The present paper d escribes the structure of horse-spleen apo-ferritin cocrystallized wit h Sn-protoporphyrin IX. The 6797 reflections up to 2.6 angstrom resolu tion used in the refinement were obtained from a data set recorded on a Nicolet/Xentronics area detector with Cu Kalpha radiation from a Rig aku RU 200 rotating anode. The final structure comprises 1613 non-H at oms, two Cd atoms and 170 solvent molecules. Four residues are describ ed as disordered. The root-mean-square deviations from ideal bond leng ths and angles are 0.013 angstrom and 2.88-degrees, respectively. Prot oporphyrins are observed in special positions on the twofold axes of t he ferritin molecule with a stoichiometry of 0.4 per subunit.