How pH Modulates the Reactivity and Selectivity of
a Siderophore-Associated Flavin Monooxygenase
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Abstract
Flavin-containing
monooxygenases (FMOs) catalyze the oxygenation
of diverse organic molecules using O<sub>2</sub>, NADPH, and the flavin
adenine dinucleotide (FAD) cofactor. The fungal FMO SidA initiates
peptidic siderophore biosynthesis via the highly selective hydroxylation
of l-ornithine, while the related amino acid l-lysine
is a potent effector of reaction uncoupling to generate H<sub>2</sub>O<sub>2</sub>. We hypothesized that protonation states could critically
influence both substrate-selective hydroxylation and H<sub>2</sub>O<sub>2</sub> release, and therefore undertook a study of SidA’s
pH-dependent reaction kinetics. Consistent with other FMOs that stabilize
a C4a-OO(H) intermediate, SidA’s reductive half reaction is
pH independent. The rate constant for the formation of the reactive
C4a-OO(H) intermediate from reduced SidA and O<sub>2</sub> is likewise
independent of pH. However, the rate constants for C4a-OO(H) reactions,
either to eliminate H<sub>2</sub>O<sub>2</sub> or to hydroxylate l-Orn, were strongly pH-dependent and influenced by the nature
of the bound amino acid. Solvent kinetic isotope effects of 6.6 ±
0.3 and 1.9 ± 0.2 were measured for the C4a-OOH/H<sub>2</sub>O<sub>2</sub> conversion in the presence and absence of l-Lys, respectively. A model is proposed in which l-Lys accelerates
H<sub>2</sub>O<sub>2</sub> release via an acid–base mechanism
and where side-chain position determines whether H<sub>2</sub>O<sub>2</sub> or the hydroxylation product is observed