Anion-mediated Fe3+ release mechanism in ovotransferrin C-lobe: a structurally identified SO4(2-) binding site and its implications for the kinetic pathway

J Biol Chem. 2001 Sep 21;276(38):35940-6. doi: 10.1074/jbc.M102590200. Epub 2001 Jul 20.


The differential properties of anion-mediated Fe(3+) release between the N- and C-lobes of transferrins have been a focus in transferrin biochemistry. The structural and kinetic characteristics for isolated lobe have, however, been documented with the N-lobe only. Here we demonstrate for the first time the quantitative Fe(3+) release kinetics and the anion-binding structure for the isolated C-lobe of ovotransferrin. In the presence of pyrophosphate, sulfate, and nitrilotriacetate anions, the C-lobe released Fe(3+) with a decelerated rate in a single exponential progress curve, and the observed first order rate constants displayed a hyperbolic profile as a function of the anion concentration. The profile was consistent with a newly derived single-pathway Fe(3+) release model in which the holo form is converted depending on the anion concentration into a "mixed ligand" intermediate that releases Fe(3+). The apo C-lobe was crystallized in ammonium sulfate solution, and the structure determined at 2.3 A resolution demonstrated the existence of a single bound SO(4)(2-) in the interdomain cleft, which interacts directly with Thr(461)-OG1, Tyr(431)-OH, and His(592)-NE2 and indirectly with Tyr(524)-OH. The latter three groups are Fe(3+)-coordinating ligands, strongly suggesting the facilitated Fe(3+) release upon the anion occupation at this site. The SO(4)(2-) binding structure supported the single-pathway kinetic model.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anions
  • Binding Sites
  • Conalbumin / chemistry
  • Conalbumin / metabolism*
  • Ferric Compounds / metabolism*
  • Hydrogen Bonding
  • Kinetics
  • Models, Molecular
  • Sulfates / metabolism*


  • Anions
  • Ferric Compounds
  • Sulfates
  • Conalbumin

Associated data

  • PDB/1IQ7