X-ray absorption near-edge spectroscopy of transferrins: a theoretical and experimental probe of the metal site local structure.

Proteins of the transferrin (Tf) family have a role in metal transport in vertebrates and have been extensively studied. The results here reported provide, for the first time, a detailed systematic comparison of metal sites in Tf complexes involving several atoms in the whole protein and in two different types of Tfs. The high interest in the structural variations induced in a metalloprotein upon the uptake of different metals is related to the hypothesis of the metals' involvement in some neuropathologies. We propose a comparative study of the X-ray absorption spectra at the K-edge of iron, copper, zinc and nickel in serotransferrin and ovotransferrin. The experimental data are simulated using an algorithm of the full multiple scattering method. Our results show that: (1) the local structure of each site (N-terminal and C-terminal) is correlated to the ligation state of the other site; (2) the difference between the two proteins is related to site local structure and depends on the metal ion nature being greater in the case of copper and zinc with respect to iron and nickel ions; (3) X-ray spectroscopy is confirmed as a suitable technique able to discriminate between coordination models proposed by X-ray diffraction.