Tricatecholate enterobactin

Catecholate Siderophores. Simple Catechol Complexes. As noted earlier, the common siderophore for enteric bacteria is the tricatechol, enterobactin (Figure 4). In order to perfect synthetic and... 

Figure 1. Schematic of the two iron transport systems of microorganisms. The high affinity system is comprised of specific carriers of ferric ion (siderophores) and their cognate membrane hound receptors. Both components of the system are regulated by iron repression through a mechanism which is still poorly understood. The high affinity system is invoked only when the available iron supply is limiting otherwise iron enters the cell via a nonspecific, low affinity uptake system. Ferri-chrome apparently delivers its iron by simple reduction. In contrasty the tricatechol siderophore enterobactin may require both reduction and ligand hydrolysis for release...

The Catechol-Containing Siderophore—Enterobactin. The isolation and characterization of the cyclic triester 2,3-dihydroxy-N-benzoyl-Z-serine, a tricatechol siderophore (Figure 4), were independently reported by both Pollack and Neilands (18) and O Brien and Gibson (19). The ligand was isolated from cultures of Salmonella typhimurium and Escherichia coli and given the names enterobactin and enterochelin, respectively. Enterobactin is an efficient cellular transport agent but, unlike ferrichrome, intracellular release of the iron involves enzymatic hydrolysis of the enterobactin to the monomer, 2,3-dihydroxy-N-benzoyl-Z-ser-ine (1). 

The ferrichromes, fusarinines, and ferrioxamines are typical trihydroxamate siderophores, while enterobactin is a cyclic tricatecholate siderophore. However, there are several exceptions which employ mixed forms of coordination. For example, aero-bactin, schizokinen and arthrobactin contain, in addition to two hydroxamate groups, a a-hydroxy-carboxylate unit which completes the hexadentate ligand structure 30). The recently isolated ferrioxamine H, with two hydroxamate and one carboxylate, is only pentadentate — the sixth coordination site presumably occupied by water 31). 

As with the hydroxamate siderophores, our initial approach has been to study simple tris(catecholato)metallate(III) complexes as models for the tricatecholate siderophore enterobactin. Unlike hydroxamates, catecholate is a symmetric, bidentate ligand. 

In the case of enterobactin, the complete reduction of the complex in a 1 3 Fe2SC>3 monomer solution, was achieved at a hydrogen (calomel) electrode potential of — 0.6 V, which should be contrasted with the irreducibility of the corresponding cyclic triester complex down to electrode potentials approaching hydrogen discharge ( — 1.8 V) (63). A plausible explanation is that the reducible species is a partially dissociated (i.e. hydrated) complex, present in equilibrium with the tricatechol chelate, which itself is practically irreducible. This is consistent with the observation that the potential for full reduction decreases as the concentration of monomeric ligand is increased beyond the stoichiometric requirement. 

Figure 2. Structural formula of the tricatecholate siderophore enterobactin...

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