Ternary Complex Formation Facilitates a Redox Mechanism for Iron Release from a Siderophore

While the naturally occurring reducing agents glutathione (GSH) and ascorbate (H₂A) alone are ineffective at reducing iron(III) sequestered by the siderophore ferrioxamine B, the addition of an iron(II) chelator, sulfonated bathophenanthroline (BPDS), facilitates reduction by either reducing agent. A mechanism is described in which a ternary complex is formed between ferrioxamine B and BPDS in a rapidly established pre-equilibrium step, which is followed by rate limiting reduction of the ternary complex by glutathione or ascorbate. Spectral, thermodynamic, and kinetic evidence are given for ternary complex formation. Ascorbate was found to be slightly more efficient at reducing the ternary complex than glutathione (k₄=2.1×10⁻³ M⁻¹ s⁻¹ and k₄=6.3×10⁻⁴ M⁻¹ s⁻¹, respectively) at pH 7. Reduction is followed by a rapid ligand exchange step where iron is released from ferrioxamine B to form tris-(BPDS)iron(II). The implications of these results for siderophore mediated iron transport and release are discussed.