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.