Microbial iron transport via a siderophore shuttle: A membrane ion transport paradigm

Abstract

A mechanism of ion transport across membranes is reported. Microbial transport of Fe 3؉ generally delivers iron, a growthlimiting nutrient, to cells via highly specific siderophore-mediated transport systems. In contrast, iron transport in the fresh water bacterium Aeromonas hydrophila is found to occur by means of an indiscriminant siderophore transport system composed of a single multifunctional receptor. It is shown that (i) the siderophore and Fe 3؉ enter the bacterium together, (ii) a ligand exchange step occurs in the course of the transport, and (iii) a redox process is not involved in iron exchange. To the best of our knowledge, there have been no other reports of a ligand exchange mechanism in bacterial iron transport. The ligand exchange step occurs at the cell surface and involves the exchange of iron from a ferric siderophore to an iron-free siderophore already bound to the receptor. This ligand exchange mechanism is also found in Escherichia coli and seems likely to be widely distributed among microorganisms.

Identification of a Candida albicans Ferrichrome Transporter and Its Characterization by Expression inSaccharomyces cerevisiae

“…It is still unresolved whether the iron-siderophore complex is transported into the cell or whether iron is released from the siderophore as a consequence of binding to the transporter. Studies suggest that iron can be released from siderophores by either reduction through cell surface reductases, by intracellular-reducing agents, or as shown recently through ligand exchange with endogenous siderophores (34). A major question remaining is the mechanism of siderophore iron FIG.…”

Characterization of a Bacillus subtilis transporter for petrobactin, an anthrax stealth siderophore

“…Although the binding of the iron-free form of siderophores by receptor proteins was observed in other bacteria, the receptors typically have higher affinities for the ferric complex, and it is the binding of the ferric form that triggers receptor conformation changes leading to translocation of the siderophore complex (2). Therefore, our observation can be indicative of a different mechanism of PB-mediated iron uptake involving, for example, a siderophore shuttle (26). Alternatively, it can reflect a different change of the protein conformation or tryptophan environment imposed by the two forms of PB influencing quenching of protein fluorescence (20).…”

Novel Insights into the Basis for Escherichia coli Superoxide Dismutase's Metal Ion Specificity from Mn-Substituted FeSOD and Its Very High Em

“…Consistent with the tendency of Fe to be a limiting nutrient, especially in oxidizing environments (11,12), many highpotential Fe-containing enzymes have non-Fe-containing analogues. Several of the Mn-containing analogues are also homologues (13,14).…”