Recognition and transport of natural and synthetic siderophores by microbes

Iron transport represents a unique problem in biology. In general, organisms have large requirements for iron, creating a need for both transport and storage systems. For aerobic organisms this means that femc ion, which is completely insoluble as the hydroxide at neutral pH, must be complexed so that it can be solubilized for both mobilization and storage. For bacteria this problem is particularly acute since they must obtain their iron from a hostile environment where the available concentrations of [Fe3+] correspond to equilibrium concentrations lower than molar. To accomplish this microorganisms synthesize low-molecular-weight sequestering agents, siderophores, that are extraordinarily stable iron complexing agents. This paper gives an overview of the iron complexation, mobilization, and cellular transport of iron by siderophores. Particular focus is on the tri-catechol siderophore enterobactin (produced by Escherichia coli,), its mobilization of iron, and the relationship of the virulence of pathogenic microorganisms to their acquisition of iron utilizing siderophores. The recent crystal structure determination of enterobactin and the synthesis of unusually stable analogs provide a basis for understanding the unique stability, structure and function of this remarkable molecule.