Desferriferrioxamine B (H4DFB) is a linear trihydroxamic acid siderophore produced under iron deficient conditions by some species of the bacteria Nocardia and Streptomyces for the purpose of sequestering environmental iron. Upon solubilizing ferric iron, the highly stable, hexacoordinate iron-siderophore complex (FeHDFB+) is recognized at the cell membrane where iron is incorporated into the cell and released for use in vital intracellular processes. We are using desferrioxamine B as a prototypical siderophore to investigate the efficacy of a redox facilitated iron release mechanism. Interestingly, the naturally occurring reducing agent glutathione alone is unable to reduce FeHDFB+ due to thermodynamic restrictions; however, the presence of a strong ferrous chelator, sulfonated bathophenanthroline (bphen), enables glutathione to reduce ferrioxamine B. Upon reduction, iron is subsequently released from the highly stable FeHDFB+ complex and bphen acts as a thermodynamic sink by simultaneously forming the iron(II) tris-bphen complex (Fe(bphen)34-). Here, we give a detailed description of this reduction and release process demonstrating that reduction is a viable mechanism for iron release from highly stable siderophore complexes. The proposed mechanism involves the formation of a ternary complex followed by reduction, which allows for rapid ligand exchange and ultimately the release of iron from the siderophore, desferrioxamine B.
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