Gallium uptake by transferrin and interaction with receptor 1

The kinetics and thermodynamics of Ga(III) exchange between gallium mononitrilotriacetate and human serum transferrin as well as those of the interaction between gallium-loaded transferrin and the transferrin receptor 1 were investigated in neutral media. Gallium is exchanged between the chelate and the C-site of human serum apotransferrin in interaction with bicarbonate in about 50 s to yield an intermediate complex with an equilibrium constant K-1 = (3.9 +/- 1.2) x 10(-2), a direct second-order rate constant k(1) = 425 +/- 50 M-1 s(-1) and a reverse second-order rate constant k(-1) = (1.1 +/- 3) x 10(4) M-1 s(-1). The intermediate complex loses a single proton with proton dissociation constant K-1a = 80 +/- 40 nM to yield a first kinetic product. This product then undergoes a modification in its conformation which lasts about 500 s to produce a second kinetic intermediate, which in turn undergoes a final extremely slow (several hours) modification in its conformation to yield the gallium-saturated transferrin in its final state. The mechanism of gallium uptake differs from that of iron and does not involve the same transitions in conformation reported during iron uptake. The interaction of gallium-loaded transferrin with the transferrin receptor occurs in a single very fast kinetic step with a dissociation constant K-d = 1.10 +/- 0.12 mu M and a second-order rate constant k(d) = (1.15 +/- 0.3) x 10(10) M-1 s(-1). This mechanism is different from that observed with the ferric holotransferrin and suggests that the interaction between the receptor and gallium-loaded transferrin probably takes place on the helical domain of the receptor which is specific for the C-site of transferrin and HFE. The relevance of gallium incorporation by the transferrin receptor-mediated iron-acquisition pathway is discussed.