Crystal structure of diferric hen ovotransferrin at 2.4 Å resolution

The three-dimensional structure of diferric hen ovotransferrin has been determined by X-ray crystallography at 2.4 Å resolution. The structure was solved by molecular replacement, using the coordinates of diferric human lactoferrin as a search model. Several rounds of simulated annealing and restrained least-squares refinement have resulted in a model structure with an R-factor of 0.171 for the data between 11.0 and 2.4 Å resolution. The model comprises 5284 protein atoms (residues 5 to 686), 2 Fe³⁺, 2 CO₃^2- and 132 water molecules. The overall structure of ovotransferrin is similar to those of human lactoferrin and rabbit serum transferrin, being folded into two homologous lobes, each containing two dissimilar domains with one F_e^3- and one CO₃^2- bound at a specific site in each interdomain cleft. However, the relative orientation of the two lobes, which may be related to the class specificity of transferrins to receptors, is different from either human lactoferrin or rabbit serum transferrin. The angle of the relative orientation in ovotransferrin is increased by 6.8° and 15.7° as compared with to those in rabbit serum transferrin and human lactoferrin, respectively. Interdomain Lys209-Lys301 and Gln541-Lys638 interactions are found near the metal binding site of each lobe. The interlobe interactions and their role in the stabilization of iron binding are discussed.