THE MECHANISM OF IRON RELEASE FROM TRANSFERRIN - SLOW-PROTON-TRANSFER- INDUCED LOSS OF NITRILOTRIACETATOIRON(III) COMPLEX IN ACIDIC MEDIA

The role of protonation of amino acid ligands involved in iron release from human serum transferrin, previously saturated with nitrilotriace tatoiron(III) complex, has been elucidated in acidic media. Iron loss occurs first from the N-terminal site at pH < 6 and is followed at pH < 4 by iron release from the C-terminal iron-binding site. Nitrilotria cetatoiron(III) release from the N-terminal site is controlled by the slow protonation of the mixed protein/nitrilotriacetatoiron(III) compl ex; the second-order rate constant was k(3a) = 9.95 +/- 0.35 X 10(4) M (-1) s(-1). Protonation of an amino acid ligand in the C-terminal site leads to a new protein-site-C-loaded mixed complex with dissociation constant K-4= 0.300 +/- 0.025 mM. Nitrilotriacetatoiron(III) release i s the result of mixed complex dissociation and the slow rate-limiting protonation of the iron-free protein with a proton dissociation consta nt K-5a = 0.100 +/- 0.010 mM and a second-order rate constant k(5a) = 4.20+/-0.40X10(3)M(-1).s(-1). The mechanism of iron uptake and release seems to imply that slow proton transfers can induce complex formatio n between iron and the amino acid ligands of each of the protein iron- binding sites. These slow proton transfers may be controlled by the ch ange of conformation of the binding sites upon iron loss.