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The siderocalin/enterobactin interaction: a link between mammalian immunity and bacterial iron transport.

ABSTRACT: The siderophore enterobactin (Ent) is produced by enteric bacteria to mediate iron uptake. Ent scavenges iron and is taken up by the bacteria as the highly stable ferric complex [Fe (III)(Ent)] (3-). This complex is also a specific target of the mammalian innate immune system protein, Siderocalin (Scn), which acts as an antibacterial agent by specifically sequestering siderophores and their ferric complexes during infection. Recent literature suggesting that Scn may also be involved in cellular iron transport has increased the importance of understanding the mechanism of siderophore interception and clearance by Scn; Scn is observed to release iron in acidic endosomes and [Fe (III)(Ent)] (3-) is known to undergo a change from catecholate to salicylate coordination in acidic conditions, which is predicted to be sterically incompatible with the Scn binding pocket (also referred to as the calyx). To investigate the interactions between the ferric Ent complex and Scn at different pH values, two recombinant forms of Scn with mutations in three residues lining the calyx were prepared: Scn-W79A/R81A and Scn-Y106F. Binding studies and crystal structures of the Scn-W79A/R81A:[Fe (III)(Ent)] (3-) and Scn-Y106F:[Fe (III)(Ent)] (3-) complexes confirm that such mutations do not affect the overall conformation of the protein but do weaken significantly its affinity for [Fe (III)(Ent)] (3-). Fluorescence, UV-vis, and EXAFS spectroscopies were used to determine Scn/siderophore dissociation constants and to characterize the coordination mode of iron over a wide pH range, in the presence of both mutant proteins and synthetic salicylate analogues of Ent. While Scn binding hinders salicylate coordination transformation, strong acidification results in the release of iron and degraded siderophore. Iron release may therefore result from a combination of Ent degradation and coordination change.

SUBMITTER: Abergel RJ

PROVIDER: S-EPMC3188318 | biostudies-literature | 2008 Aug

REPOSITORIES: biostudies-literature

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The siderocalin/enterobactin interaction: a link between mammalian immunity and bacterial iron transport.

Abergel Rebecca J RJ Clifton Matthew C MC Pizarro Juan C JC Warner Jeffrey A JA Shuh David K DK Strong Roland K RK Raymond Kenneth N KN

Journal of the American Chemical Society 20080805 34

The siderophore enterobactin (Ent) is produced by enteric bacteria to mediate iron uptake. Ent scavenges iron and is taken up by the bacteria as the highly stable ferric complex [Fe (III)(Ent)] (3-). This complex is also a specific target of the mammalian innate immune system protein, Siderocalin (Scn), which acts as an antibacterial agent by specifically sequestering siderophores and their ferric complexes during infection. Recent literature suggesting that Scn may also be involved in cellular ...[more]

PMID: 18680288

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Project description: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.

Dataset Information

The siderocalin/enterobactin interaction: a link between mammalian immunity and bacterial iron transport.

Publications

The siderocalin/enterobactin interaction: a link between mammalian immunity and bacterial iron transport.

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