The Structural Mechanism for Iron Uptake and Release by Transferrins
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概要
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Transferrins are a group of iron-binding proteins that control the levels of iron in the body fluids of vertebrates by their ability to bind two Fe^<3+> and two CO^<2->_3. The transferrin molecule, with a molecular mass of about 80 kDa, is folded into two similarly sized homologous N-and C-lobes that are stabilized by many intrachain disulfides. As observed by X-ray crystallography, each lobe is further divided into two similarly sized domains, domain 1 and domain 2,and an Fe^<3+> binding site is within the interdomain cleft. Four of the six Fe^<3+> coordination sites are occupied by protein ligands (2 Tyr residues, 1 Asp, and 1 His) and the other two by a bidentate CO^<2->_3. Upon uptake and release of Fe^<3+>, transferrins undergo a large-scale conformational change depending on a common structural mechanism : domains 1 and 2 rotate as rigid bodies around a rotation axis that passes through the two antiparallel β-strands linking the domains. The extent of the rotation is, however, variable for different transferrin species and lobes. As a Fe^<3+> release mechanisms at low pH from the N-lobes of serum transferrin and ovotransferrin, the structral evidence for 'dilysine trigger mechanism' is shown. A structural mechanism for the Fe^<3+> release in presence of a non-synergistic anion is proposed on the basis of the sulfate-bound apo crystal structure of the ovotransferrin N-lobe. Domain-opened structures with the coordinated Fe^<3+> by the two tyrosine residues are demonstrated in fragment and intact forms, and their functional implications as a possible intermediate for iron uptake and release are discussed.
- 社団法人日本農芸化学会の論文
- 2000-07-23
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