Apotransferrin proton dissociation and interactions with hydrogencarbonate in neutral media

A spectrofluorimetric titration analysis of the proton dissociation and the interactions of human serum transferrin with hydrogencarbonate was performed at pH 7·0–9·0. Apotransferrin loses a single proton probably per binding site with Ka = (6·80 ± 0·35) × 10−9M. This proton dissociation is independent of the hydrogencarbonate concentration. Apotransferrin does not interact with CO32−. However, it interacts with two HCO3−, exhibiting two different affinity constants; the dissociation constant presumably for the C-site is KC = (4·40 ± 0·15) × 10−3M and that for presumably the N-site is KN = (3·60 ± 0·30) × 10−2M. These interactions are independent of pH and occur with the unprotonated and protonated apotransferrin species with the same low affinities. Such affinities are probably induced by ionic interactions involving the side chain of the arginine residues in each of the two binding sites. As for the proton dissociation, it can occur with one of the other side-chains of the amino acid residues of these binding sites.