Metal Assimilation Pathways - Biological Inorganic Chemistry - page 139

pathogen, which exploits any break in host defenses), Serratiamarcescens (urinary tract and wound infections), Vi br io
cholera (cholera), Yersinia pestis (plague), and Yersinia entercolytica (gastroenteritis), to mention but a few!
Iron uptake from transferrin, siderophores, and haem by Gram-negative bacteria is represented schematically
in Fig. 7.5 . All three iron uptake pathways require an outer-membrane receptor, a periplasmic-binding protein
(PBP), and an inner-membrane ATP-binding cassette (ABC) transporter. Whereas Fe 3 þ -siderophores and haem

FIGURE 7.5 Schematic representation of iron uptake in Gram-negative bacteria. There are numerous iron uptake pathways in Gram-negative
bacteria which include iron uptake from transferrin, siderophores, or haem. All of these uptake pathways require an outer-membrane receptor,
a PBP, and an inner-membrane ABC transporter. Not all bacteria have all three systems; but some have more than one type. Transport through
the outer-membrane receptor requires the action of the TonB system (TonB, ExbB, and ExbD). (From Krewulak & Vogel, 2008 . Reproduced

with permission from Elsevier.)

are transported across the outer membrane by their receptors, the iron is stripped from transferrin and lactoferrin at
the outer membrane by their respective receptor complexes, the TbpA/TbpB and LbpA/LbpB proteins, attached to
the outer membrane by an N-terminal lipid anchor. It is thought that one of them, perhaps TbpB acts as a primary
binding protein for iron-saturated transferrin, facilitating its binding to TbpA. Both TbpA and LbpA are integral
membrane proteins, which are predicted to have large surface loops that bind transferrin and lactoferrin,
respectively, forcing the separation of the two domains surrounding the iron-binding sites and releasing the Fe 3 þ .
However, to date, the structures of TbpA/TbpB and LbpA/LbpB have not been determined.