Synergism and substitution in the lactoferrins

The anion binding properties of human lactoferrin (Lf), with Fe³⁺ or Cu²⁺ as the associated metal ion, highlight differences between the two sites, and in the anion binding behaviour when different metals are bound. Carbonate, oxalate and hybrid carbonate-oxalate complexes have been prepared and their characteristic electronic and EPR spectra recorded. Oxalate can displace carbonate from either one or both anion sites of Cu₂(CO₃)₂Lf, depending on the oxalate concentration, but no such displacement occurs for Fe₂(CO₃)₂Lf although it does for the bovine analogue. Addition of oxalate and the appropriate metal ion to apoLf under carbonate-free conditions gives dioxalate complexes with both Fe³⁺ and Cu²⁺. The anion sites as determined from the crystal structures of Fe₂(CO₃)₂Lf, Fe₂(C₂O₄)₂Lf, Cu₂(CO₃)₂Lf, and Cu₂(CO₃)(C₂O₄)Lf have been compared. Both the carbonate and oxalate ions bind in bidentate fashion to the metal, except that the carbonate ion in the N-lobe site of dicupric lactoferrin is monodentate. The hybrid copper lactoferrin complex shows that the oxalate ion binds preferentially in the C-lobe site in a bidentate mode. A series of complexes containing the synergistic anion O,N-chelates with increasing substitution on the N atom (glycinate, iminodiacetate and nitrilotriacetate) have been prepared with iron bovine lactoferrin for comparison with the O,O- chelate oxalate. Overall these observations lead to a generalised model for synergistic anion binding by transferrins and allow comparisons to be made with nonsynergistic anions such as citrate and succinate.