Browsing by Author "Tanner, John J."

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    Kinetic and Structural Characterization of a Flavin-Dependent Putrescine N-Hydroxylase from Acinetobacter baumannii
    Lyons, Noah S.; Bogner, Alexandra N.; Tanner, John J.; Sobrado, Pablo (American Chemical Society, 2022-11-15)
    Acinetobacter baumannii is a Gram-negative opportunistic pathogen that causes nosocomial infections, especially among immunocompromised individuals. The rise of multidrug resistant strains of A. baumannii has limited the use of standard antibiotics, highlighting a need for new drugs that exploit novel mechanisms of pathogenicity. Disrupting iron acquisition by inhibiting the biosynthesis of iron-chelating molecules (siderophores) secreted by the pathogen is a potential strategy for developing new antibiotics. Here we investigated FbsI, an N-hydroxylating monooxygenase involved in the biosynthesis of fimsbactin A, the major siderophore produced by A. baumannii. FbsI was characterized using steady-state and transient-state kinetics, spectroscopy, X-ray crystallography, and small-angle X-ray scattering. FbsI was found to catalyze the N-hydroxylation of the aliphatic diamines putrescine and cadaverine. Maximum coupling of the reductive and oxidative half-reactions occurs with putrescine, suggesting it is the preferred (in vivo) substrate. FbsI uses both NADPH and NADH as the reducing cofactor with a slight preference for NADPH. The crystal structure of FbsI complexed with NADP+was determined at 2.2 Å resolution. The structure exhibits the protein fold characteristic of Class B flavin-dependent monooxygenases. FbsI is most similar in 3D structure to the cadaverine N-hydroxylases DesB and DfoA. Small-angle X-ray scattering shows that FbsI forms a tetramer in solution like the N-hydroxylating monooxygenases of the SidA/IucD/PvdA family. A model of putrescine docked into the active site provides insight into substrate recognition. A mechanism for the catalytic cycle is proposed where dehydration of the C4a-hydroxyflavin intermediate is partially rate-limiting, and the hydroxylated putrescine product is released before NADP+
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    Structural and Biochemical Characterization of the Flavin-Dependent Siderophore-Interacting Protein from Acinetobacter baumannii
    Valentino, Hannah; Korasick, David A.; Bohac, Tabbetha J.; Shapiro, Justin A.; Wencewicz, Timothy A.; Tanner, John J.; Sobrado, Pablo (American Chemical Society, 2021-07-06)
    Acinetobacter baumannii 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 (k 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 (k obs, 0.02 s-1) and oxazole pre-Acb-Fe (ox-pre-Acb-Fe) (k 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 Shewanella SIPs. The biochemical and structural data presented here validate the role of BauF in A. baumannii iron assimilation and provide information important for drug design.
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    Structural Determinants of Flavin Dynamics in a Class B Monooxygenase
    Campbell, Ashley C.; Robinson, Reeder; Mena-Aguilar, Didier; Sobrado, Pablo; Tanner, John J. (American Chemical Society, 2020-12-08)
    The ornithine hydroxylase known as SidA is a class B flavin monooxygenase that catalyzes the first step in the biosynthesis of hydroxamate-containing siderophores in Aspergillus fumigatus. Crystallographic studies of SidA revealed that the FAD undergoes dramatic conformational changes between out and in states during the catalytic cycle. We sought insight into the origins and purpose of flavin motion in class B monooxygenases by probing the function of Met101, a residue that contacts the pyrimidine ring of the in FAD. Steady-state kinetic measurements showed that the mutant variant M101A has a 25-fold lower turnover number. Pre-steady-state kinetic measurements, pH profiles, and solvent kinetic isotope effect measurements were used to isolate the microscopic step that is responsible for the reduced steady-state activity. The data are consistent with a bottleneck in the final step of the mechanism, which involves flavin dehydration and the release of hydroxy-l-ornithine and NADP+. Crystal structures were determined for M101A in the resting state and complexed with NADP+. The resting enzyme structure is similar to that of wild-type SidA, consistent with M101A exhibiting normal kinetics for flavin reduction by NADPH and wild-type affinity for NADPH. In contrast, the structure of the M101A-NADP+ complex unexpectedly shows the FAD adopting the out conformation and may represent a stalled conformation that is responsible for the slow kinetics. Altogether, our data support a previous proposal that one purpose of the FAD conformational change from in to out in class B flavin monooxygenases is to eject spent NADP+ in preparation for a new catalytic cycle.
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    Trapping conformational states of a flavin-dependent N-monooxygenase in crystallo reveals protein and flavin dynamics
    Campbell, Ashley C.; Stiers, Kyle M.; Del Campo, Julia S. Martin; Mehra-Chaudhary, Ritcha; Sobrado, Pablo; Tanner, John J. (2020-09-18)
    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.