Siderophores are synthesized via non-ribosomal peptide synthetases and contain functional groups such as carboxylates, catecholates, and hydroxamates, which are essential for binding the metal iron. Hydroxamate functional groups are commonly derived from the hydroxylation of the terminal amino group of the amino acids L-lysine or L-ornithine. This reaction is catalyzed by flavin-dependent monooxygenases. Deletion of MbtG, the flavoenzyme that catalyzes the hydroxylation of lysines present in mycobactin, the siderophore of Mycobacterium tuberculosis, has been shown to be essential for mycobacterial survival. Thus, MbtG is an attractive target for drug design. Similarly, in Aspergillus fumigatus, the N5-ornithine hydroxylase (SidA) has been shown to be essential for pathogenesis and survival in this fungus. We have selected the flavin-dependent monooxygenases from these organisms because they represent potential drug targets. In addition, studies on these enzymes will allow us to provide information about the mechanism of oxygen activation and substrate selectivity and will help us understand what determines the chemical outcome of the oxygenation reaction.