TRANSFERRIN, A MECHANISM FOR IRON RELEASE

Iron release from transferrin has been investigated in mildly acidic a nd acidic media in the presence of formate, acetate and citrate. It oc curs first from the N-terminal iron-binding site (N-site) of the holop rotein. It is independent of the nature and the concentration of compe ting ligands and is controlled by a slow proton transfer; second-order rate constant k(1) = (7.4 +/- 0.5) X 10(4) M(-1) s(-1) which can be a ttributed to a rate-limiting slow proton gain by a protein Ligand subs equent to a fast decarbonation of the N-site. Iron loss from the C-ter minal iron-binding site (C-site) is slower than that from the N-site a nd occurs by two pathways. The first is favoured below pH 4 and does n ot involve the formation of an intermediate ternary complex. It can be controlled by a rate-limiting slow proton-triggered decarbonation of the binding site; second-order rate constant k(3) = (2.25 +/- 0.05) X 10(4) M(-1) s(-1). The second pathway is favoured above pH 4 and invol ves a mixed protein-ligand iron complex. It takes place through the sl ow protonation of the mixed ternary complex and depends on the nature of the competing ligand. It is faster in the presence of citrate than in that of acetate; second-order rate constant k(4) = (1.75 +/- 0.10) X 10(3) M(-1) s(-1) for citrate and (85 +/- 5) M(-1) s(-1) for acetate . All these phenomena can possibly describe proton-triggered changes o f conformation of the binding sites.