The substrates handled by the ISVH family systems are quite different. Their common characteristic is to chelate iron (ferrichrome, enterobactin, achromobactin, anguibactin, citrate, exochelin, hemin, vibriobactin) or cobalt (vitamin B-12). All these systems are associated with high-affinity outer membrane receptors in Gram-negative bacteria, the activity of which is dependent on a transmembrane protein complex made of TonB, ExbC and ExbD whose function is to transduce energy to the outer membrane. Once released from the outer membrane receptor, the substrate is translocated through the inner membrane thanks to an ABC BPD importer ( Pubmed : 10203757 ) .