Alternative Structural State of Transferrin

Abstract

10.1074/jbc.274.15.10190 Transferrins bind Fe very tightly in a closed interdomain cleft by the coordination of four protein ligands (Asp, Tyr, Tyr, and His in ovotransferrin N-lobe) and of a synergistic anion, physiologically bidentate CO . Upon Feuptake, transferrins undergo a large scale conformational transition: the apo structure with an opening of the interdomain cleft is transformed into the closed holo structure, implying initial Fe binding in the open form. To solve the Fe-loaded, domain-opened structure, an ovotransferrin N-lobe crystal that had been grown as the apo form was soaked with Fe-nitrilotriacetate, and its structure was solved at 2.1 Å resolution. The Fe-soaked form showed almost exactly the same overall open structure as the iron-free apo form. The electron density map unequivocally proved the presence of an iron atom with the coordination by the two protein ligands of Tyr-OH and Tyr-OH. Other Fe coordination sites are occupied by a nitrilotriacetate anion, which is stabilized through the hydrogen bonds with the peptide NH groups of Ser, Ala, and Gly and a side chain group of Thr. There is, however, no clear interaction between the nitrilotriacetate anion and the synergistic anion binding site, Arg.