Comparison of the Structures of the Cubic and Tetragonal Forms of Horse-Spleen Apoferritin

Horse‐spleen apoferritin is known to crystallize in three different space groups, cubic F432, tetragonal P4212 and orthorhombic P21212. A structure comparison of the cubic and tetragonal forms is presented here. Both crystal forms were obtained by the vapor‐diffusion technique and data were collected at 2.26 Å (cubic crystal) and 2.60 Å (tetragonal crystal) resolution. Two main differences were observed between these crystal structures: (i) whereas intermolecular contacts only involve salt‐bridge type interactions via cadmium ions in the cubic structure, two types of interactions are observed in the tetragonal crystal (cadmium‐ion‐mediated salt bridges and hydrogen‐bonding interactions) and (ii) cadmium ions bound in the threefold axes of ferritin molecules exhibit lower site‐occupation factors in the tetragonal structure than in the cubic one.