FIGURE 3: The branched periplasmic pathways for pyoverdine tailoring. PvdN generates the succinamide residue and PtaA the α-ketoglutarate residue from glutamate. An unknown enzyme converts succinamide (most likely) or possibly glutamic acid to malamide in some species. The lower abundant succinic acid and malic acid pyoverdine forms are likely hydroly-sis products of the corresponding amides. Note that the exact position of the hydroxyl group in malamide and malic acid residues is not resolved.