Metalloproteins and Metallopeptides –Natural Metallofoldamers - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 19

Figure 1.13 Stereo relaxed eye view of the crystal structures of one channel of gramicidin A
dimer (top, viewed through the channel; bottom, side view; PDB ID 1AV2). Cs þ ions located
in the channel are shown in green spheres, water molecules are shown in small red spheres.

this obstacle, microorganisms excrete Fe 3þ -specific siderophores which bind Fe 3þ with
extraordinarily high affinity constants in the range of 10 30 -10 52 M 1 and transport Fe 3þ

into cells via specific receptors [156,171]. There are three families of siderophores, differ-
ing from each other by their iron-binding sites: hydroximate- (such as ferrichrome from
Penicillium and the edible Ustilago ), catechol-containing (e.g., enterobactin from E. coli ),
carboxylate-containing (like the simple citrate), and their combinations (such as aerobac-
tin) [172]. Upon Fe 3þ binding, the ferrichromes fold to afford an octahedral metal coordi-
nation sphere with a more compact conformation than their metal-free apo-forms
(Figure 1.14, top). The complexes are recognized and transported into the cells by spe-
cies-specific receptors. For example, although ferrichrome A serves as an iron carrier for
fungi, it does not in bacteria [173]. The folding is also stereo-specific. While the iron
complexes of ferrioxamines B [174], D 1 [175], and E [176] and desferrioxamine E [177]
fold into a mixture of L- and D- cis isomers, ferrichrome complexes [178] are exclusively
L- cis isomers. A few structures of the transporter protein FhuA (ferric hydroxamate