Unusual features for zirconium(IV) binding to human serum transferrin

Human serum apotransferrin (hTF) binds to Zr(IV) slowly in the presence of nitrilotriacetate (NTA), citrate or ethylenediaminetetraacetate (EDTA) as donor ligands. For Zr(NTA)₂ ^2– as donor, equilibrium was reached in ca. 2 h (pH 7.4, 298 K, 10 mM Hepes, 5 mM bicarbonate) and full loading of the N- and C-lobe sites was achievable to give Zr₂-hTF. ¹³C NMR data suggest that carbonate can bind as a synergistic anion. ¹H and 2D [¹H,¹³C] (using _ε-[¹³C]Met-hTF) NMR studies show that there is little lobe-selectively in the order of Zr(IV) uptake. Fe(III) displaced Zr(IV) from the C-lobe of Zr₂-hTF first, followed by the N-lobe. However, in the presence of a large excess of NTA, Zr(IV) binds to the N-lobe of holo-hTF (Fe₂-hTF) first followed by the C-lobe. The ¹H and ¹³C NMR chemical shift changes for _ε-[¹³CH₃] of Met464, which is close to the C-lobe site, are quite distinct from those observed previously for Al(III), Fe(III), Ti(IV), Ga(III) and Bi(III) binding to hTF, suggesting that Zr(IV) binding may not induce lobe closure [as observed previously for Hf(IV)]. This may affect receptor recognition and play a role in the different biological behaviour of Zr(IV) compared to Ti(IV).