Suraj Dhungana

Coordination Chemistry and Redox Processes in Siderophore-Mediated Iron Transport

In this mini-review we present an environmental iron mobility/transport scheme consisting of inter-related controls, whereby the first coordination shell of iron modulates the iron redox potential (E1/2), and the oxidation state of iron controls the chemistry of the first coordination sphere and therefore the immediate chemical environment of the iron. Siderophores (microbially generated iron specific chelators) may be viewed as iron redox mediators. Siderophore chelation of environmental iron in a reduced (Fe(II)) oxidation state results in facile air oxidation of iron due to the negative redox potentials observed for Fe-siderophore complexes. This solubilizes the iron and locks it into a specific coordination environment, thereby preventing hydrolysis and precipitation. The high-spin Fe3+ → Fe+ electron transfer process may be viewed as a switch that controls the thermodynamic stability and kinetic lability of the first coordination shell. Reduction of iron(III)-siderophore complexes to iron(II)-siderophore complexes decreases thermodynamic stability, increases the rate of siderophore ligand exchange, and increases the ease of siderophore donor atom protonation, thus facilitating a rapid turnover of the first coordination shell. Results are presented for iron-siderophore pH and oxidation state dependent speciation studies that are relevant to environmental and microbial iron mobility and transport.

The influence of the synergistic anion on iron chelation by ferric binding protein, a bacterial transferrin

Although the presence of an exogenous anion is a requirement for tight Fe3+ binding by the bacterial (Neisseria) transferrin nFbp, the identity of the exogenous anion is not specific in vitro. nFbp was reconstituted as a stable iron containing protein by using a number of different exogenous anions [arsenate, citrate, nitrilotriacetate, pyrophosphate, and oxalate (symbolized by X)] in addition to phosphate, predominantly present in the recombinant form of the protein. Spectroscopic characterization of the Fe3+/anion interaction in the reconstituted protein was accomplished by UV-visible and EPR spectroscopies. The affinity of the protein for Fe3+ is anion dependent, as evidenced by the effective Fe3+ binding constants (K′eff) observed, which range from 1 × 1017 M−1 to 4 × 1018 M−1 at pH 6.5 and 20°C. The redox potentials for Fe3+nFbpX/Fe2+nFbpX reduction are also found to depend on the identity of the synergistic anion required for Fe3+ sequestration. Facile exchange of exogenous anions (Fe3+nFbpX + X′ → Fe3+nFbpX′ + X) is established and provides a pathway for environmental modulation of the iron chelation and redox characteristics of nFbp. The affinity of the iron loaded protein for exogenous anion binding at pH 6.5 was found to decrease in the order phosphate > arsenate ∼ pyrophosphate > nitrilotriacetate > citrate ∼ oxalate ≫ carbonate. Anion influence on the iron primary coordination sphere through iron binding and redox potential modulation may have in vivo application as a mechanism for periplasmic control of iron delivery to the cytosol.

Iron chelation properties of an extracellular siderophore exochelin MN.

The coordination chemistry of an extracellular siderophore produced by Mycobacterium neoaurum, exochelin MN (ExoMN), is reported along with its pK(a) values, Fe(III) and Fe(II) chelation constants, and aqueous solution speciation as determined by spectrophotometric and potentiometric titration techniques. Exochelin MN is of particular interest as it can efficiently transport iron into pathogenic M. leprae, which is responsible for leprosy, in addition to its own parent cells. The Fe(III) coordination properties of ExoMN are important with respect to understanding the Fe(III) acquisition and uptake mechanism in pathogenic M. leprae, as the siderophores from this organism are very difficult to isolate. Exochelin MN has two hydroxamic acid groups and an unusual threo-beta-hydroxy-l-histidine available for Fe(III) chelation. The presence of threo-beta-hydroxy-l-histidine gives rise to a unique mode of Fe(III) coordination. The pK(a) values for the two hydroxamic acid moieties, the histidine imidazole ring and the alkylammonium groups on ExoMN, correspond well with the literature values for these moieties. Proton-dependent Fe(III)- and Fe(II)-ExoMN equilibrium constants were determined using a model involving sequential protonation of the Fe(III)- and Fe(II)-ExoMN complexes. These data were used to develop a model whereby deprotonation reactions on the surface of the complex in the second coordination shell result in first coordination shell isomerization. The overall formation constants were calculated: log beta(110) = 39.12 for Fe(III)-ExoMN and 16.7 for Fe(II)-ExoMN. The calculated pFe value of 31.1 is one of the highest among all siderophores and their synthetic analogues and indicates that ExoMN is thermodynamically capable of removing Fe(III) from transferrin. The E(1/2) for the Fe(III)ExoMN/Fe(II)ExoMN(-) couple was determined to be -595 mV from quasi-reversible cyclic voltammograms at pH = 10.8, and the pH-dependent E(1/2) profile was used to determine the Fe(II)-ExoMN protonation constants.

Iron Chelation Properties of an Extracellular Siderophore Exochelin MS

The coordination chemistry of an extracellular siderophore produced by Mycobacterium smegmatis, exochelin MS (ExoMS), is reported along with its pK(a) values, Fe(III) and Fe(II) chelation constants, and aqueous solution speciation as determined by spectrophotometric and potentiometric titrations. Exochelin MS has three hydroxamic acid groups for Fe(III) chelation and has four additional acidic protons from a carboxylic acid group and three primary amine groups, on the backbone of the molecule. The pK(a) values for the three hydroxamic acid moieties, the carboxylic acid group and the alkylammonium groups on ExoMS, correspond well with the literature values for these moieties. Equilibrium constants for proton-dependent Fe(III)-ExoMS equilibria were determined using a model involving the sequential protonation of the Fe(III)-ExoMS complexes at the first and second coordination shells. The equilibrium constants (beta) for the overall formation of Fe(III)ExoMS(H(3))(2+) and Fe(II)ExoMS(H(3))(+) from Fe((aq))(3+) or Fe((aq))(2+) and the deprotonated hydroxamate coordinating group form of the siderophore, ExoMS(H(3))(-), are calculated as log beta(III) = 28.9 and log beta(II) = 10.1. A calculated pFe value of 25.0 is very similar to that of other linear trihydroxamic acid siderophores, and indicates that ExoMS is thermodynamically capable of removing Fe(III) from transferrin. The E(1/2) for the Fe(III)-ExoMS/Fe(II)-ExoMS couple was determined from quasi reversible cyclic voltammograms at pH = 6.5 and found to be -380 mV.