Characterization of Fe(III) sequestration by an analog of the cytotoxic siderophore brasilibactin A: Implications for the iron transport mechanism in mycobacteria

Mycobacteria such as M. tuberculosis represent a significant health concern throughout much of the developing world. In mycobacteria and other pathogenic bacteria, an important virulence factor is the ability of the bacterium to obtain iron from its host. One means of obtaining iron is through the use of siderophores. Brasilibactin A is a membrane bound siderophore produced by Nocardia brasiliensis with structural similarity to the mycobactin class of siderophore in mycobacteria. A characterization of the protonation constants and Fe(III) affinity of a water soluble Brasilibactin A analog (Bbtan) has been performed. Using protonation constants and competition with EDTA, the stability constant of the 1 : 1 Fe(III)-Bbtan complex was found to be log beta(110) = 26.96. The pFe of Bbtan is 22.73, somewhat low for a proposed siderophore molecule. The redox potential of the Fe-Bbtan complex was found to be -300 mV vs. NHE, very high for an iron-siderophore complex. The combination of relatively low complex stability and ease of iron reduction may play a crucial role in the mechanism of mycobactin siderophore-mediated iron uptake in mycobacteria and related organisms.