Metal substitution in transferrins: specific binding of cerium(IV) revealed by the crystal structure of cerium substituted human lactoferrin

Proteins of the transferrin family pray a key role in iron homeostasis thro ugh their extremely strong binding of iron, as Fe3+. They are nevertheless able to bind a surprisingly wide variety of other metal ions. To investigat e how metal ions of different size, charge and coordination characteristics are accommodated, we have determined the crystal structure of human lactof errin (Lf) complexed with Ce4+. The structure, refined at 2.2 Angstrom reso lution (R=20.2%, R-free=25.7%) shows that the two Ce4+ ions occupy essentia lly the same positions as do Fe3+, and that the overall protein structure i s unchanged; the same closed structure is formed for Ce(2)Lf as for Fe(2)Lf . The larger metal ion is accommodated by small shifts in the protein ligan ds, made possible by the presence of water molecules adjacent to each bindi ng site. The two Ce4+ sites an equally occupied, indicating that the known difference in the pH-dependent release of Ce4+ arises from a specific proto nation event, possibly of the His Ligand in one of the binding sites. Compa ring the effects of binding Ce4+ with those for the binding of other metal ions, we conclude that the ability of transferrins to accommodate metal ion s other than Fe3+ depends on an interplay of charge, size, coordination and geometrical preferences of the bound metal ion. However, it is the ability to accept the six-coordinate, approximately octahedral, site provided by t he protein that is of greatest importance.