title: Protonation and anion binding control the kinetics of iron release from human transferrin.
abstract: Iron release in vitro from human serum diferric transferrin (hFe(2)Tf) 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) × 10(7), 6.1(6) × 10(7), and 9(1) × 10(7) M(-2) s(-1) in the presence of Cl(-), NO(3)(-), and SO(4)(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) × 10(4), 4.4(6) × 10(5), and 8.1(2) × 10(5) M(-2) s(-1) in the presence of Cl(-), NO(3)(-), and SO(4)(2-), respectively.
authors.affiliation: Department of Biochemistry and Molecular Biology, Life Sciences Centre, University of British Columbia, Vancouver, BC, Canada.
authors.forename: Rajesh
authors.initials: R
authors.lastname: Kumar
authors.forename: A Grant
authors.initials: AG
authors.lastname: Mauk
mesh_terms: D000838:Anions / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000378:metabolism*; D007700:Kinetics; D011485:Protein Binding; D011522:Protons; D014168:Transferrin / Q000378:metabolism*
chemical_list: D000838:Anions; D011522:Protons; D014168:Transferrin; D007501:Iron