Enterobactin Protonation and Iron Release:  Structural Characterization of the Salicylate Coordination Shift in Ferric Enterobactin¹

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 (14) Å 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 mV 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. Keywords

Human Urinary Composition Controls Antibacterial Activity of Siderocalin*

“…Because of its insolubility in aqueous environments, "free" iron exists in a labile pool, for which aryl alcohols are well suited ligands. Indeed, the higher pH associated with restrictive urine closely parallels the trends in ferric complex stability and SCN binding observed for catecholate ligands (12,38). Because ferric complexes can form from multiple aryl alcohol chelators (e.g.…”

“…6b). Within the urinary pH range, this trend mirrors the greater ferric complex binding affinity to SCN and the greater ferric ion binding affinity to enterobactin and other catecholate ligands observed with increasing pH values (12,38).…”

The Solution Structure, Binding Properties, and Dynamics of the Bacterial Siderophore-binding Protein FepB

“…Second, we targeted the small, non-ribosomally synthesized ligand, Ent (669 Da), for irradiation and detected its interface with a large protein (FepB, ϳ34 kDa). Enterobactin is composed of three 2,3-catecholate moieties linked to a triserine backbone, and the proton signals of GaEnt have reported chemical shifts in the region between ϳ3.7 and 7 ppm (76,77). Specifically ILV methyl-protonated and otherwise deuterated holo-FepB was dissolved in ϳ100% D 2 O, and no 1 H signals originating from the protein were observed in the irradiated region (4 -7 ppm).…”

Siderophores and mussel foot proteins: the role of catechol, cations, and metal coordination in surface adhesion

“…Thus in the salicylate binding mode, Fe(III) is coordinated by the amide oxygen and the ortho hydroxyl oxygen of 2,3-DHBA [20,21].…”