Structural and Biochemical Characterization of the Flavin-Dependent Siderophore-Interacting Protein from <i>Acinetobacter baumannii</i>

Abstract

<i>Acinetobacter baumannii</i> is an opportunistic pathogen with a high mortality rate due to multi-drug-resistant strains. The synthesis and uptake of the iron-chelating siderophores acinetobactin (Acb) and preacinetobactin (pre-Acb) have been shown to be essential for virulence. Here, we report the kinetic and structural characterization of BauF, a flavin-dependent siderophore-interacting protein (SIP) required for the reduction of Fe(III) bound to Acb/pre-Acb and release of Fe(II). Stopped-flow spectrophotometric studies of the reductive half-reaction show that BauF forms a stable neutral flavin semiquinone intermediate. Reduction with NAD(P)H is very slow (<i>k</i> _obs, 0.001 s^-1) and commensurate with the rate of reduction by photobleaching, suggesting that NAD(P)H are not the physiological partners of BauF. The reduced BauF was oxidized by Acb-Fe (<i>k</i> _obs, 0.02 s^-1) and oxazole pre-Acb-Fe (ox-pre-Acb-Fe) (<i>k</i> _obs, 0.08 s^-1), a rigid analogue of pre-Acb, at a rate 3-11 times faster than that with molecular oxygen alone. The structure of FAD-bound BauF was solved at 2.85 Å and was found to share a similarity to <i>Shewanella</i> SIPs. The biochemical and structural data presented here validate the role of BauF in <i>A. baumannii</i> iron assimilation and provide information important for drug design.