Contrasting Synergistic Anion Effects in Vanadium(V) Binding to Nicatransferrin versus Human Serum Transferrin

Some ascidians sequester vanadium and other metal ions that are bound and transported in higher organisms by transferrin. The ascidian Ciona intestinalis has a monolobal transferrin (nicatransferrin) in its plasma. The binding of vanadium(V) to nicatransferrin was investigated by using isothermal titration calorimetry and UV-vis spectroscopy, in the presence and absence of NaHCO3, and was compared with human serum transferrin. Nicatransferrin and serum transferrin bind V(V) with similar strengths [K = (2.0 +/- 0.6) x 10(5) M-1 for nicatransferrin]; however, nicatransferrin requires a synergistic anion for V(V) binding, whereas serum transferrin does not. Spectroscopy supports a different kind of coordination site.