![]() ![]() For instance, in the lanthanum series, wherein the elements have very similar chemical properties, the binding stabilities of the metal ions are correlated with their radii, that is, a Tf complex with a lanthanoid ion with a large radius is more unstable than one with a small radius. In particular, regarding the stability of a Tf-metal complex, the metal ion radius has been thought to be one of the most important factors. Many studies have been carried out on the physicochemical properties of Tf-metal complexes. This adduct returns to the cell surface, and the apoTf is dissociated from the TfR1 at pH 7.4, which allows the recycling of both the proteins. The iron-free Tf (apoTf) remains bound to the TfR1 in the endosome. The binding of the Fe(III) 2Tf complex to TfR1 is followed by endocytosis and iron release in the acidic environment (pH 5.6) of the endosome. Diferric Tf (Fe(III) 2Tf) binds to Tf receptor 1 (TfR1) at the cell surface (pH 7.4). Among these, ferric ions most tightly bind to Tf, giving a binding constant of 10 20~23 M −1. In addition to ferric ions, the iron-binding sites are capable of binding various other metal ions. Iron bound at each site is octahedrally coordinated by six ligands: two tyrosine residues, one aspartic acid residue, one histidine residue, and a synergistic anion (a bidentate carbonate ion in a physiological condition), of which one oxygen atom is in interaction with η NH of arginine residue. Each lobe is further subdivided into two similarly sized domains (N1 and N2 C1 and C2) and possesses a single iron-binding site in the interdomain cleft (N- and C-site). It consists of two major lobes, N- and C-lobe, which have about 40% homology between their amino acid sequences. Serum transferrin (Tf), the iron-transport protein in vertebrates, is a glycosylated single polypeptide with a molecular mass of approximately 80 kDa. The results provide a theoretical explanation for the binding of Fe(III) to Tf, which produces sufficient energy to induce a conformational transition of the Tf molecule, making it possible to interact with Tf receptor 1. In the estimation of the binding energies, the solvation energies (solvent effect) of free metal ions were a very important factor. The calculated energies were well correlated with the logarithmic values of the reported metal-binding constants of Tf, which had been experimentally determined, with a correlation coefficient of 0.96. The absolute values of the metal-binding energies of human serum transferrin (Tf) N-lobe, | Δ E |, were calculated using the density functional theory and were found to increase in magnitude in the following order: Fe(III)>Ga(III)>Al(III)>Cu(II)>Zn(II)>Ni(II). ![]()
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