3.A.1.14.1
Iron (Fe³⁺) or ferric-dicitrate porter, FecBCDE (Braun and Herrmann, 2007). The Mycobacterium tuberculosis (Mtb) ortholog has two substrate binding proteins, FecB and FecB2 (de Miranda et al. 2023).  The crystallographic structures of Mtb FecB and FecB2 were determined to 2.0 Å and 2.2 Å resolution, respectively, and they show distinct ligand binding pockets. In vitro ligand binding experiments for FecB and FecB2 were performed with heme and bacterial siderophores from Mtb and other species, revealing that both FecB and FecB2 bind heme, while only FecB binds the Mtb sideophore ferric-carboxymycobactin (Fe-cMB). Subsequent structure-guided mutagenesis of FecB identified a single glutamate residue-Glu339-that significantly contributes to Fe-cMB binding (de Miranda et al. 2023).