Abstract
Transferrins bind Fe3+ very tightly in a closed interdomain cleft by the coordination of four protein ligands (Asp60, Tyr92, Tyr191, and His250 in ovotransferrin N-lobe) and of a synergistic anion, physiologically bidentate CO32-. Upon Fe3+ uptake, 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 Fe3+ binding in the open form. To solve the Fe3+-loaded, domain-opened structure, an ovotransferrin N-lobe crystal that had been grown as the apo form was soaked with Fe3+-nitrilotriacetate, and its structure was solved at 2.1 A resolution. The Fe3+-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 Tyr92-OH and Tyr191-OH. Other Fe3+ coordination sites are occupied by a nitrilotriacetate anion, which is stabilized through the hydrogen bonds with the peptide NH groups of Ser122, Ala123, and Gly124 and a side chain group of Thr117. There is, however, no clear interaction between the nitrilotriacetate anion and the synergistic anion binding site, Arg121.
Highlights
Transferrins bind Fe3؉ very tightly in a closed interdomain cleft by the coordination of four protein ligands (Asp60, Tyr92, Tyr191, and His250 in ovotransferrin N-lobe) and of a synergistic anion, physiologically bidentate CO32؊
Upon Fe3؉ uptake, 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 Fe3؉ binding in the open form
To solve the Fe3؉-loaded, domainopened structure, an ovotransferrin N-lobe crystal that had been grown as the apo form was soaked with Fe3؉nitrilotriacetate, and its structure was solved at 2.1 Å resolution
Summary
THE CRYSTALLOGRAPHIC ANALYSIS OF IRON-LOADED BUT DOMAIN-OPENED OVOTRANSFERRIN N-LOBE*. For the iron-free apo form, x-ray crystallographic [11,12,13] and solution scattering (14 –17) analyses have revealed that all of the transferrin lobes, except for the lactoferrin C-lobe in crystal, assume a conformation with an opening of the interdomain cleft This implies that transferrin initially binds the Fe3ϩ ion in the open form before being transformed into the closed holo form [18, 19]. We report here a novel structural state of transferrin: the Fe3ϩ-loaded structure of ovotransferrin N-lobe with essentially the same open conformation as the apo form In this structure, the bound iron atom is coordinated by the two protein ligands of Tyr92-OH and Tyr191-OH. The observation strongly suggests that the two tyrosine residues are the initial Fe3ϩbinding ligands in the open transferrin
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
