The siderophore biosynthetic enzyme A (SidA) ornithine hydroxylase fromAspergillus fumigatusis a fungal disease drug target involved in the production of hydroxamate-containing siderophores, which are used by the pathogen to sequester iron. SidA is anN-monooxygenase that catalyzes the NADPH-dependent hydroxylation ofl-ornithine through a multistep oxidative mechanism, utilizing a C4a-hydroperoxyflavin intermediate. Here we present four new crystal structures of SidA in various redox and ligation states, including the first structure of oxidized SidA without NADP(H) orl-ornithine bound (resting state). The resting state structure reveals a newoutactive site conformation characterized by large rotations of the FAD isoalloxazine around the C1-' C2 ' and N10-C1 ' bonds, coupled to a 10-angstrom movement of the Tyr-loop. Additional structures show that either flavin reduction or the binding of NADP(H) is sufficient to drive the FAD to theinconformation. The structures also reveal protein conformational changes associated with the binding of NADP(H) andl-ornithine. Some of these residues were probed using site-directed mutagenesis. Docking was used to explore the active site of theoutconformation. These calculations identified two potential ligand-binding sites. Altogether, our results provide new information about conformational dynamics in flavin-dependent monooxygenases. Understanding the different active site conformations that appear during the catalytic cycle may allow fine-tuning of inhibitor discovery efforts.