Structural and Functional Flexibility of Lactoferrin

Summary

Lactof errin is a protein that binds iron with great affinity, yet is also able to release it. It also binds a variety of other metal ions and anions. In order to investigate its mechanisms of binding and release, and the reasons for its versatility in binding, we have undertaken X-ray crystallographic studies on various forms of lactoferrin. The structure of a new crystal form of apolactoferrin, at 3.5-Å resolution, has shown that in each lobe the binding cleft is in an open state, but that the size of the conformational change, compared with diferric lactoferrin, varies: a domain rotation of 54° in the N-lobe and 18° in the C-lobe. Comparison with the previously determined apolactoferrin structure, in which the C-lobe is closed, leads to a dynamic model for iron binding. The crystal structure of oxalate-substituted diferric lactoferrin shows that larger anions can be accommodated without affecting domain closure, although the two binding sites adjust differently Solution studies also indicate that larger cations, such as Ce⁴⁺, may also be able to bind within the same closed structure. In this case, Ce³⁺ is oxidized to Ce⁴⁺ when it binds to lactoferrin, with a visible spectrum similar to those of Fe³⁺, Mn³⁺, and Co³⁺. Crystallographic binding studies using ruthenium complexes with antitumor activity show that these bind with high affinity in the binding cleft of apolactoferrin and more weakly in nonspecific external sites. This suggests possible uses of lactoferrin in drug delivery.