Abstract
A variety of metal ions can bind to the iron-transport protein, transferrin, at two specific sites. For each metal ion, a carboxylate anion is concomitantly bound. Six metal ions which were examined fall into two classes based on proton release and ultraviolet spectral changes which accompany binding to the protein. Class II ions, which include Cu2+ and Zn2+, release approximately 2 H+/metal bond. Class III ions, which include Fe3+, Ga3+, Al3+, and VO2+, release approximately 3 H+/metal bound. The increase in absorbance near 242 nm, characteristic of tyrosine ionization, has the ratio 0.55–0.75 for class II:class III ions. Both Fe3+ and Cu2+ form metal-transferrin-oxalate complexes in the presence of excess C2O42−. Fe3+ releases close to 3 H+/metal whether forming oxalate or bicarbonate complexes with transferrin. Binding of Cu2+ to transferrin releases 2 H+/metal in the presence of C2O2−4 or HCO3−. Since equal numbers of H+/metal are released for both anions, it is likely that the bicarbonate ion does not lose its proton, and remains as HCO3− in transferrin. These results are interpreted in terms of possible combinations of ligands at the metal binding sites.
Published Version
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