Transferrins. Hen ovo-transferrin, interaction with bicarbonate and iron uptake

Abstract

Fe(III) uptake by the iron-delivery and iron-scavenging protein, hen ovotransferrin has been investigated in vitro between pH 6.5 and 9. In the absence of any ferric chelate, apo-ovotransferrin loses two protons with K1a = 50 +/- 1 nM and K2a = 4.0 +/- 0.1 nM. These acid-base equilibria are independent of the interaction of the protein with bicarbonate. The interaction with bicarbonate occurs with two different affinity constants, KC = 9.95 +/- 0.15 mM and KN = 110 +/- 10 mM. FeNAc3 exchanges its Fe(III) with the C-site of the protein in interaction with bicarbonate, direct rate constants k1 = 650 +/- 25 M-1 s-1, reverse rate constant k-1 = (6.0 +/- 0.1) x 10(3) M-1 s-1 and equilibrium constant K1 = 0.11 +/- 0.01. This iron-protein intermediate loses then a single proton, K3a = 3.50 +/- 0.35 nM, and undergoes a first change in conformation followed by a two or three proton loss, first order rate constant k2 = 0.30 +/- 0.01 s-1. This induces a new modification in conformation followed by the loss of one or two protons, first order rate constant k3 = (1.50 +/- 0.05) x 10(-2) s-1. These modifications in the monoferric protein conformation are essential for iron uptake by the N-site of the protein. In the last step, the monoferric and diferric proteins attain their final state of equilibrium in about 15,000 s. The overall mechanism of iron uptake by ovotransferrin is similar but not identical to those of serum transferrin and lactoferrin. The rates involved are, however, closer to lactoferrin than serum transferrin, whereas the affinities for Fe(III) are lower than those of serum transferrin and lactoferrin. Does this imply that the metabolic function transferrins is more related to kinetics than to thermodynamics?