Heterologous expression, purification, and characterization of an L-ornithine N5-hydroxylase involved in pyoverdine siderophore biosynthesis in Pseudomonas aeruginosa

Pseudonionas 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 mu 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 Havin adenine dinucleotide (FAD(+)) as cofactors, as it cannot utilize NADH and Havin mononucleotide. By fluorescence titration, the dissociation constants for NADPH and FAD(+) were determined to be 105.6 +/- 6.0 mu M and 9.9 +/- 0.3 mu 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 mu 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.