Pseudomonas aeruginosa is an opportunistic pathogen that produces the siderophore pyoverdine, which enables it to acquire the essential nutrient iron from its host. Formation of the iron-chelating hydroxamate functional group in pyoverdine requires the enzyme PvdA, a flavin-dependent monooxygenase that catalyzes the N 5 hydroxylation of L-ornithine. pvdA from P. aeruginosa was successfully overexpressed in Escherichia coli, and the enzyme was purified for the first time. The enzyme possessed its maximum activity at pH 8.0. In the absence of L-ornithine, PvdA has an NADPH oxidase activity of 0.24 ؎ 0.02 mol min ؊1 mg ؊1 . The substrate L-ornithine stimulated this activity by a factor of 5, and the reaction was tightly coupled to the formation of hydroxylamine. The enzyme is specific for NADPH and flavin adenine dinucleotide (FAD ؉ ) as cofactors, as it cannot utilize NADH and flavin mononucleotide. By fluorescence titration, the dissociation constants for NADPH and FAD ؉ were determined to be 105.6 ؎ 6.0 M and 9.9 ؎ 0.3 M, respectively. Steady-state kinetic analysis showed that the L-ornithine-dependent NADPH oxidation obeyed Michaelis-Menten kinetics with apparent K m and V max values of 0.58 mM and 1.34 mol min ؊1 mg ؊1 . L-Lysine was a nonsubstrate effector that stimulated NADPH oxidation, but uncoupling occurred and hydrogen peroxide instead of hydroxylated L-lysine was produced. L-2,4-Diaminobutyrate, L-homoserine, and 5-aminopentanoic acid were not substrates or effectors, but they were competitive inhibitors of the L-ornithine-dependent NADPH oxidation reaction, with K ic s of 3 to 8 mM. The results indicate that the chemical nature of effectors is important for simulation of the NADPH oxidation rate in PvdA.Iron is an essential nutrient for most organisms. In humans, iron is sequestered by iron binding proteins such as transferrin, lactoferrin, and hemoglobin and is therefore unavailable to invading microbial pathogens. Microorganisms can counter this nonspecific host defense mechanism by the production of low-molecular-weight, high iron(III) affinity molecules termed siderophores (4, 25). Siderophores are synthesized intracellularly and then secreted into the environment, where they chelate and deliver the complexed iron back into the cell via specific membrane transporters. Most siderophores are peptides, and the functional groups that chelate iron(III) include catecholates, carboxylates, and hydroxamates (15).The hydroxamate functional groups are usually derived from diamino acids such as L-lysine and L-ornithine. A flavin-dependent monooxygenase (EC 1.14.13) catalyzes the first step in the formation of hydroxamates by oxidizing the terminal amino group of the respective substrates to produce the corresponding hydroxylamines (Fig. 1). On the basis of studies with the Escherichia coli L-lysine N 6 -hydroxylase (IucD) and by analogy to other flavin monooxygenases (1), this reaction is proposed to proceed in two steps. In the reductive half reaction, flavin adenine dinucleotide (FAD ϩ ) is reduced by NADP...