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. 2006 Jul 12;128(27):8920-31.
doi: 10.1021/ja062046j.

Enterobactin protonation and iron release: structural characterization of the salicylate coordination shift in ferric enterobactin

Rebecca J Abergel  1 , Jeffrey A Warner, David K Shuh, Kenneth N Raymond
Affiliations

Enterobactin protonation and iron release: structural characterization of the salicylate coordination shift in ferric enterobactin

Rebecca J Abergel et al. J Am Chem Soc. .

Abstract

The siderophore enterobactin (Ent) is produced by many species of enteric bacteria to mediate iron uptake. This iron scavenger can be reincorporated by the bacteria as the ferric complex [Fe(III)(Ent)](3)(-) and is subsequently hydrolyzed by an esterase to facilitate intracellular iron release. Recent literature reports on altered protein recognition and binding of modified enterobactin increase the significance of understanding the structural features and solution chemistry of ferric enterobactin. The structure of the neutral protonated ferric enterobactin complex [Fe(III)(H(3)Ent)](0) has been the source of some controversy and confusion in the literature. To demonstrate the proposed change of coordination from the tris-catecholate [Fe(III)(Ent)](3)(-) to the tris-salicylate [Fe(III)(H(3)Ent)](0) upon protonation, the coordination chemistry of two new model compounds N,N',N''-tris[2-(hydroxybenzoyl)carbonyl]cyclotriseryl trilactone (SERSAM) and N,N',N''-tris[2-hydroxy,3-methoxy(benzoyl)carbonyl]cyclotriseryl trilactone (SER(3M)SAM) was examined in solution and solid state. Both SERSAM and SER(3M)SAM form tris-salicylate ferric complexes with spectroscopic and solution thermodynamic properties (with log beta(110)() values of 39 and 38 respectively) similar to those of [Fe(III)(H(3)Ent)](0). The fits of EXAFS spectra of the model ferric complexes and the two forms of ferric enterobactin provided bond distances and disorder factors in the metal coordination sphere for both coordination modes. The protonated [Fe(III)(H(3)Ent)](0) complex (d(Fe)(-)(O) = 1.98 A, sigma(2)(stat)(O) = 0.00351(10) A(2)) exhibits a shorter average Fe-O bond length but a much higher static Debye-Waller factor for the first oxygen shell than the catecholate [Fe(III)(Ent)](3)(-) complex (d(Fe)(-)(O) = 2.00 A, sigma(2)(stat)(O) = 0.00067(14) A(2)). (1)H NMR spectroscopy was used to monitor the amide bond rotation between the catecholate and salicylate geometries using the gallic complexes of enterobactin: [Ga(III)(Ent)](3)(-) and [Ga(III)(H(3)Ent)](0). The ferric salicylate complexes display quasi-reversible reduction potentials from -89 to -551 mV (relative to the normal hydrogen electrode NHE) which supports the feasibility of a low pH iron release mechanism facilitated by biological reductants.

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Figures

Figure 1
Figure 1
Ferric enterobactin (top) and the coordination shift from catecholate (bottom left) to salicylate (bottom right) upon protonation.
Figure 2
Figure 2
Enterobactin and its synthetic analogs: the catecholate TRENCAM and salicylate SERSAM, SER(3M)SAM, TRENSAM and TREN(3M)SAM ligands. The coordinating oxygen atoms are indicated in red.
Figure 3
Figure 3
Spectrophotometric titrations of SER(3M)SAM by KOH (top) and [FeIII(SER(3M)SAM)]0 by HCl (bottom) in water. I = 0.1 (KCl), T = 25.0 °C, l = 1 cm.
Figure 4
Figure 4
Calculated frontier molecular orbital energy level diagrams for [FeIII(Ent)]3- (left) and sal-[FeIII(H3Ent)]0 (right).
Figure 5
Figure 5
k-space spectra (left), Fourier transforms (right, solid lines) and R-space fits (right, dashed lines) of [FeIII(Ent)]3- at 30, 100, 200 and 300 K.
Figure 6
Figure 6
Einstein model fits of first-shell oxygens for: [FeIII(TREN(3M)SAM)]0, [FeIII(SERSAM)]0, [FeIII(SER(3M)SAM)]0, [FeIII(TRENSAM)]0, [FeIII(H3Ent)]0, [FeIII(Ent)]3- and [FeIII(TRENCAM)]3- from top to bottom.
Figure 7
Figure 7
Variation of the 1H NMR chemical shift as a function of pD for gallic enterobactin (symbols). Solid lines represent the fitted data. Dashed lines designate the pKa values of the complex.
Figure 8
Figure 8
The 2D 1H NMR NOESY spectrum of [GaIII(Ent)]3- (0.1 M) in DMSO-d6, τm = 0.7 s. Selected NOEs are indicated in red.
Figure 9
Figure 9
Cyclic voltammogram of [FeIII(TREN(3M)SAM]0 at 50 mV/s (top), reduction wave for [FeIII(SER(3M)SAM]0 at different scan rates – 2000 to 25 mV/s for spectra 1 to 8 - (middle), sweep rate dependence of the reduction current peak for all complexes (bottom).
Scheme 1
Scheme 1
Synthesis of the two salicylate analogs of enterobactin SERSAM and SER(3M)SAM.
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References

    1. Coordination Chemistry of Microbial Iron Transport. 75. Part 74: Dertz EA, Raymond KN. Inorg. Chem. 2006 In Press.

    1. Raymond KN, Dertz EA, Kim SS. Proc. Natl. Acad. Sci. USA. 2003;100:3584–3588. - PMC - PubMed
    1. Earhart CF. In: Iron Transport in Bacteria: Molecular Genetics, Biochemistry, Microbial Pathogenesis and Ecology. Crosa JH, Payne SM, editors. Washington, D.C.: ASM Press; 2004. pp. 133–146.
    1. Walsh CT, Marshall CG. In: Iron Transport in Bacteria: Molecular Genetics, Biochemistry, Microbial Pathogenesis and Ecology. Crosa JH, Payne SM, editors. Washington, D.C.: ASM Press; 2004. pp. 18–37.
    1. Boukhalfa H, Crumbliss AL. BioMetals. 2002;15:325–339. - PubMed

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