The low pKa value of iron-binding ligand Tyr188 and its implication in iron release and anion binding of human transferrin.

2D NMR-pH titrations were used to determine pKa values for four conserved tyrosine residues, Tyr45, Tyr85, Tyr96 and Tyr188, in human transferrin. The low pKa of Tyr188 is due to the fact that the iron-binding ligand interacts with Lys206 in open-form and with Lys296 in the closed-form of the protein. Our current results also confirm the anion binding of sulfate and arsenate to transferrin and further suggest that Tyr188 is the actual binding site for the anions in solution.

Anion exchange in human serum transferrin N-lobe: a model study with variant His249Ala.

The removal of Fe(III) from human serum transferrin by chelators is thought to proceed through intermediate species in which the chelator becomes associated with the metal center of the protein. The visible spectral shifts associated with the formation of such intermediates in the wild-type (WT) protein are too small for reliable kinetic data to be obtained. Therefore, studies were undertaken with the recombinant N-terminal lobe variant H249A, a variant showing more pronounced spectral changes.