Structure of oxalate-substituted diferric mare lactoferrin at 2.7 Å resolution

Lactoferrin binds two Fe³⁺ and two CO ions with high affinity. It can also bind other metal ions and anions. In order to determine the perturbations in the environments of the binding sites in the N and C lobes and elsewhere in the protein, the crystal structure of oxalate-substituted diferric mare lactoferrin has been determined at 2.7 Å resolution. The final model has a crystallographic R factor of 21.3% for all data in the resolution range 17.0-2.7 Å. The substitution of an oxalate anion does not perturb the overall structure of the protein, but produces several significant changes at the metal-binding and anion-binding sites. The binding of the oxalate anion is symmetrical in both the N and C lobes, unlike in diferric dioxalate human lactoferrin, where the oxalate anion binds the metal ion symmetrically in the C lobe and asymmetrically in the N lobe.