Binding of monovalent anions to human serum transferrin.

Serum transferrin is the protein whose primary function is to bind iron and transport it through the blood. Apotransferrin has two specific metal-binding sites that bind a variety of metal ions in addition to the ferric ion. The distinguishing feature of the transferrins is that a "synergistic" bicarbonate anion is bound along with the metal ion to form a stable Fe(3+)-CO3-Tf ternary complex. Previous research has shown that apotransferrin will also bind divalent anions such as phosphate and sulfate. Difference UV spectroscopy has now been used to show that a series of monovalent anions bind weakly to apotransferrin. Equilibrium constants for the binding of chloride, perchlorate, bromide, fluoride and Hepes have been calculated. A reaction scheme for the binding of anions is proposed which predicts that the binding of the nonsynergistic anions to apotransferrin will interfere with metal binding by competing directly with the binding of the synergistic bicarbonate anion. Difference UV data are presented which demonstrate this type of competition between nonsynergistic anions and Tb3+. Competition from the nonsynergistic anions follows the order HPO4(2-) > SO4(2-) approximately F- > ClO4- approximately Cl- approximately Br-. Speciation calculations have been performed to determine the concentrations of anion-apotransferrin complexes in Hepes and Tris buffers and in human serum and to estimate the extent to which competition from anions in the buffer will interfere with metal-binding to apotransferrin.