Ferrichrome- and Citrate-Mediated Iron Transport

Iron transport through the outer membrane as ferrichrome- and ferric citrate-siderophore complexes is catalyzed by the FhuA and FecA proteins and across the cytoplasmic membrane by the FhuCDB and FecBCDE proteins, respectively. The energy for transport across the outer membrane is provided by the proton motive force of the cytoplasmic membrane, and the energy for transport across the cytoplasmic membrane is provided by ATP hydrolysis. Energy transfer from the cytoplasmic membrane to the outer membrane is mediated by the TonB-ExbB-ExbD proteins. Knowledge about the proteins that transport ferric siderophores, their interaction, and their subcellular location provides insights into the transport mechanism, but much work remains to be done before an understanding of the molecular mechanism of substrate translocation across the outer membrane and the cytoplasmic membrane is gained. Although crystal structures of the proteins in the entire pathway are now available (FhuA in the outer membrane, FhuD in the periplasm, and BtuCD in the cytoplasmic membrane), they provide only a framework for a detailed biochemical and genetic analysis. Crystal structures are static, yet transport is a dynamic process involving many rearrangements of amino acid side chains and entire polypeptide regions.