Protonation and Anion Binding Control the Kinetics of Iron Release from Human Transferrin

• Published in: The journal of physical chemistry. B 2012-03, Vol.116 (12), p.3795-3807
• Main Authors: Kumar, Rajesh, Mauk, A. Grant
• Format: Article
• Language: English
• Subjects: Anions; Anions - chemistry; Anions - metabolism; Binding; Human; Humans; Hydrogen-Ion Concentration; Iron; Iron - metabolism; Kinetics; Linkages; Lobes; Protein Binding; Protons; Transferrin; Transferrin - metabolism; Uptakes
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp205879h

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, k 2N, of 2.6(6) × 107, 6.1(6) × 107, and 9(1) × 107 M–2 s–1 in the presence of Cl–, NO3 –, and SO4 2–, 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, k 2C, of 8.4(2) × 104, 4.4(6) × 105, and 8.1(2) × 105 M–2 s–1 in the presence of Cl–, NO3 –, and SO4 2–, respectively.