Alternative structural state of transferrin. The crystallographic analysis of iron-loaded but domain-opened ovotransferrin N-lobe.
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.
Mizutani
K
K
Research Institute for Food Science, Kyoto University, Uji, Kyoto 6110011, Japan.
Yamashita
H
H
Kurokawa
H
H
Mikami
B
B
Hirose
M
M
D000818:Animals; D001665:Binding Sites; D002645:Chickens; D003207:Conalbumin / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray; D005260:Female; D005290:Ferric Compounds / Q000378:metabolism; D006860:Hydrogen Bonding; D007501:Iron; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D011487:Protein Conformation; D014168:Transferrin / Q000737:chemistry* / Q000378:metabolism
D005290:Ferric Compounds; D014168:Transferrin; D003207:Conalbumin; D007501:Iron