Protonation and anion binding control the kinetics of iron release from human transferrin.

Iron release in vitro from human serum diferric transferrin (hFe2Tf) in acidic media (4.2 ≤ pH ≤ 5.4) in the presence of nonsynergistic anions occurs in at least five kinetic steps. Step 1 (most rapid) involves proton assisted release of carbonate from the protein. In subsequent steps, iron release from both the N- and C-terminal lobes is controlled by slow proton transfers and anion binding. In step 2, the N-terminal lobe takes up one proton with kinetic linkage to the binding of one anion. In step 3, iron release from the anion-linked N-terminal lobe is controlled by slow uptake of two protons with rate-constant, k2N, of 2.6(6) × 107, 6.1(6) × 107, and 9(1) × 107 M–2 s–1 in the presence of Cl–, NO3–, and SO42–, respectively. In step 4, the C-terminal lobe takes up one proton with kinetic linkage to the binding of one anion. In step 5, iron release from the anion-linked C-terminal lobe is controlled by slow uptake of two protons with rate-constant, k2C, of 8.4(2) × 104, 4.4(6) × 105, and 8.1(2) × 105 M–2...