Chemistry Reference
In-Depth Information
(a)
(b)
FIGURE 7.7
Ribbon representations of the crystal structures of (a) ligand-free and (b) FecA bound to ferric citrate. The 22-
b
strand barrel is
depicted in ribbon format and the N-terminal cork domain is in space-filling format. The binding of ferric citrate (coloured orange) induces
a conformational change in the extracellular loops L7 (cyan) and L8 (red) such that the solvent accessibility of ferric citrate is reduced.
(From
Krewulak & Vogel, 2008
.
Reproduced with permission from Elsevier.)
change in two extracellular loops, reducing the accessibility of ferric citrate. In order for the ferric siderophore to
move into the periplasmic space, the cork has to be at least partially displaced from the interior of the beta-barrel.
However, the outer membrane has neither an established ion gradient nor a source of ATP to provide energy for
this process. The energy is provided by the TonB protein,
7
together with the ExbB and ExbD proteins which are
anchored in the cytoplasmic membrane. In a still not well-understood manner, they harvest the energy of the
proton motive force of the cytoplasmic membrane. We know that the highly conserved C-terminal region of TonB
interacts directly with the OMT, as is illustrated in
Fig. 7.8
for the ferric enterobactin OMT, FepA.
Once the Fe
3
þ
-siderophore, haem, or Fe
3
þ
have been delivered into the periplasm by the OMTs and TonB, they
are transported by periplasmic-binding proteins across the periplasmic space. We already mentioned that the
7. So called because, together with the Ton A protein (now designated FhuA), they were originally identified as two membrane proteins
required for uptake and internalization of the T1 bacteriophage (yet another ironic twist of iron biology).
