Iron
is an essential element for all organisms, and microorganisms
produce small molecule iron-chelators, siderophores, to efficiently
acquire Fe(III). Gram-positive bacteria possess lipoprotein siderophore-binding
proteins (SBPs) on the membrane. Some of the SBPs bind both apo-siderophores
(iron-free) and Fe-siderophore (iron-chelated) and only import Fe-siderophores.
When the SBP initially binds an apo-siderophore, the SBP uses the
Gram-positive siderophore-shuttle mechanism (the SBPs exchange Fe(III)
from a Fe-siderophore to the apo-siderophore bound to the protein)
and/or displacement mechanism (the apo-siderophore bound to the SBP
is released and a Fe-siderophore is then bound to the protein) to
import the Fe-siderophore. Previously, we reported that the <i>Bacillus cereus</i> SBP, YxeB, exchanges Fe(III) from a ferrioxamine
B (FO) to a desferrioxamine B (DFO) bound to YxeB using the siderophore-shuttle
mechanism although the iron exchange was indirectly elucidated. Synthetic
Cr-DFO (inert metal FO analog) and Ga-DFO (nonreducible FO analog)
are bound to YxeB and imported via YxeB and the corresponding permeases
and ATPase. YxeB exchanges Fe(III) from FO and Ga(III) from Ga-DFO
to DFO bound to the protein, indicating that the metal-exchange occurs
without metal reduction. YxeB also binds DFO derivatives including
acetylated DFO (apo-siderophore) and acetylated FO (AcFO, Fe-siderophore).
The iron from AcFO is transferred to DFO when bound to YxeB, giving
direct evidence of iron exchange. Moreover, YxeB also uses the displacement
mechanism when ferrichrome (Fch) is added to the DFO:YxeB complex.
Uptake by the displacement mechanism is a minor pathway compared to
the shuttle mechanism
