Structure of diferric duck ovotransferrin at 2.35 A resolution

The structure of diferric duck ovotransferrin (DOT) has been determined and refined at a resolution of 2.35 A. The DOT structure, which contains two iron binding sites, is similar to the known transferrin and lactoferrin structures. The two iron-binding sites, one in the N-terminal lobe and one in the C-terminal lobe of the molecule, are similar but not identical. The main differences between the three known structures lie in the relative orientations of the N- and C-lobes with respect to each other. In the DOT structure the large aromatic side chain of Phe322 in the N-lobe packs against the conserved residue Gly387 in the C-lobe. This interaction is at the centre of the interface between the two lobes and could play a crucial role in determining their relative orientation. Other differences between the structures occur in the surface loops and in the peptide connecting the two lobes. The final crystallographic model consists of 5309 protein atoms (686 residues), two Fe(3+) ions, two (bi)carbonate ions and three carbohydrate moities. 318 water molecules have been added to the model. The final R factor is 0.22 for 25 400 observed reflections between 10 and 2.35 A resolution.