title: Flexibility of the coordination geometry at the N-site of Cu(II)2 human serum-transferrin induced by the different orientations of Arg124.
abstract: The ESR spectra of dicupric human serum-transferrin (serum-Tf) were measured from -20 to 37°C in the liquid state (56% glycerol at pH 7.6). Two coordination geometries (types B-1 and B-2) with different ESR parameters were present at the N-site. The contents of the coordination geometry of type B-1 at the N-site increased as the temperature increased. The equilibrium constant between the coordination geometries of types B-1 and B-2 was determined by ESR spectra. The enthalpy value from type B-2 to B-1 was +5.3 kcal/mol, as obtained from a van't Hoff plot. The two conformational energies of the cluster models of the copper-binding site at the N-site of dicupric human serum-Tf, where the Arg124 residue was oriented in two different directions (conformations I and II), were calculated by Density Functional Theory, and the enthalpy value from conformation II to I was +2.1 kcal/mol. The enthalpy value was similar to that (+5.3 kcal/mol) obtained by the coordination geometrical change from type B-2 to B-1 in Cu(II)2 serum-Tf. In conformations I and II, the residue of Arg124 at the N-site is located either far from or near the copper-binding site, respectively, and in both cases the coordination geometry of the cupric ions at the N-site has changed from a flattened tetrahedron to a trigonal bipyramid. This result implies that the ESR spectral change from type B-2 to B-1 is caused by the presence of two different orientations of Arg124 in the change from conformation II to I.
authors.affiliation: Faculty of Pharmacy and Pharmaceutical Sciences, Fukuyama University.
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Hata
authors.forename: Yu
authors.initials: Y
authors.lastname: Shibata
authors.forename: Miku
authors.initials: M
authors.lastname: Okano
authors.forename: Asako
authors.initials: A
authors.lastname: Kodera
authors.forename: Misato
authors.initials: M
authors.lastname: Ueda
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Iwamoto
authors.forename: Hisao
authors.initials: H
authors.lastname: Tomida
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Iwamoto
authors.forename: Junzo
authors.initials: J
authors.lastname: Hirose
mesh_terms: D001120:Arginine / Q000378:metabolism*; D001665:Binding Sites; D003300:Copper / Q000378:metabolism*; D006801:Humans; D011487:Protein Conformation; D013696:Temperature; D013816:Thermodynamics; D061250:Transferrins / Q000737:chemistry*
chemical_list: D061250:Transferrins; D003300:Copper; D001120:Arginine