Transferrin, a mechanism for iron release

Area of Science:

• Biochemistry and Biophysics
• Metal-protein interactions
• Protein conformational changes

Background:

• Transferrin is the primary iron transport protein in blood plasma.
• Understanding iron release mechanisms is crucial for iron metabolism and related diseases.
• The N-terminal (N-site) and C-terminal (C-site) iron-binding sites of transferrin exhibit distinct release kinetics.

Purpose of the Study:

• To investigate the kinetics and mechanisms of iron release from transferrin's N-site and C-site.
• To elucidate the role of pH and competing ligands in modulating iron release.
• To explore the influence of proton transfer and conformational changes on iron dissociation.

Main Methods:

• Kinetic studies of iron release from holotransferrin in mildly acidic and acidic media.
• Experiments conducted in the presence of competing ligands: formate, acetate, and citrate.
• Analysis of rate constants and reaction pathways to determine rate-limiting steps.

Main Results:

• Iron release from the N-site is rapid, proton-transfer-controlled, and independent of competing ligands (k1 = 7.4 x 10^4 M^-1 s^-1).
• Iron release from the C-site is slower and occurs via two pH-dependent pathways.
• Below pH 4, C-site release is proton-triggered (k3 = 2.25 x 10^4 M^-1 s^-1); above pH 4, it involves ligand-dependent ternary complexes (k4 = 1.75 x 10^3 M^-1 s^-1 for citrate, 85 M^-1 s^-1 for acetate).

Conclusions:

• Protonation events are key triggers for iron release from both transferrin sites.
• The C-site exhibits complex iron release mechanisms influenced by pH and ligand competition.
• Observed phenomena suggest proton-triggered conformational changes in transferrin's iron-binding sites.