Figure 1. Yersiniabactin, pyochelin, and biliverdin.

Yersiniabactin, the siderophore produced by Yersinia spp., is the product of the biosynthetic pathway to which Irp3 belongs. Irp3 reduces a double bond in the center thiazoline ring to make the thiazolidine ring of the product. The substrate is reduced while covalently attached to a phosphopantetheinyl post-translational modified serine of the peptidyl carrier domain of the High Molecular Weight Protein 1 (HMWP1). (B) Pyochelin is a siderophore produced by Pseudomonas aeruginosa. PchG performs the analogous reaction as Irp3. The substrate is reduced while covalently attached to a phosphopantetheinyl post-translational modified serine of the peptidyl carrier domain of the nonribosomal peptide synthetase PchF. This ring is subsequently methylated by the methylation domain of PchF. (C) Biliverdin is the substrate of biliverdin reductase, an enzyme which catalyzes the final step in heme degradation, reducing the center methylene bridge to form bilirubin. Biliverdin reductase is a structural homologue of Irp3, as found herein.