Ferrichrome, crystal structure

Figure 3.3 Comparison of the FepA and FhuA crystal structures. A portion of the 13-barrel (in violet is removed to show the globular cork domain (in yellow) that inserts from the periplasm into the channel of the 11-barrel. FhuA is loaded with ferrichrome (iron is shown as a green ball) (Ferguson et ah, 1998 Locher et ah, 1998). The FepA crystal structure does not reveal Fe3+-enterobactin, but the FepA structure shown might be partially occupied by enterobactin (Buchanan et ah, 1999). 

The crystal structure of FhuA, with and without bound ferrichrome, has been determined (Ferguson et ah, 1998 Locher et al, 1998). FhuA consists of 22 antiparallel transmembrane 3-strands extending from residue 161 to residue 723, which form a (3-barrel (Figure 3.3, Plate 4). The -barrel strands are interconnected by large loops at the cell surface and small turns in the periplasm. Such a 3-barrel structure is the... 

Locher, K. P., Rees, B., Koebnik, R., Mitschler, A., Moulinier, L., Rosenbusch, J. P. and Moras, D. (1998). Transmembrane signaling across the ligand-gated FhuA receptor crystal structures of free and ferrichrome-bound states reveal allosteric changes, Cell, 95, 771-778. 

The crystal structure of FhuA bound to the iron ligand ferrichrome has also been determined. This shows that the ferric-siderophore complex binds inside the barrel above the plug, forming hydrogen bonds and van der Waals interactions with residues within the plug domain and with the /3-strands of the barrel (Figure 2). 

Crystal structures have been determined of several ferrichromes. Most of this work was performed by van der Helm and coworkers. The crystal structures of the members of this siderophore family can be superposed and a root-mean-square deviation of the 49 atoms, which are common to all members, is obtained of approximately 0.30 A. A comparison of the conformational angles around the cyclic hexapeptide ring shows differences of not more than 25°. This indicates that the structures and conformations are not the same but that they are similar with some conformational freedom. In all structures, the iron coordination site is on one side of the molecule, the coordination of the metal is K-cis, and the conformation of the amino acids is L. A (II) bend and a (I) bend of the cyclic peptide skeleton is found. In addition, extensive conformational analyses of siderophores in solution were performed by Lhnas and coworkers employing H andNMR. ... 

Figure 11 Crystal structure of the OM-siderophore receptor FhuA with the bound siderophore ferrichrome (emphasized by a space filling atom presentation). The plot is based on PDB file IQFF, taken from the NCBl database using the program PYMOL. Resolution of the structure is 2.70 A, and the space group is P6i. The upper side of the figure corresponds to the extracellular side of the outer membrane. See Section 5.2 for a detailed discussion...

A number of substances of the ferrichrome family, containing three hydroxamic acid groups coordinated to a central ferric ion, have been isolated, mainly from microbial sources. These compounds contain a cyclic hexapeptide. Crystal structures for ferrichrome A, cyc/o[LSer-LSer-Gly-Tri ((5-N-methylglutaconyl-5-N-hydroxy-Orn)] (Figs. 11 and 12) (Zalkin et al, 1966), the isomorphous alumichrome A (van der Helm et ai, 1980), and more recently ferrichrysin [LSer-LSer-Gly-tri(5-JV-acetyl- -N-hydroxy-Orn)] (Norrestam et al, 1975) and ferrichrome (Loghry and van der Helm, 1978) have been determined and show that the molecules are essentially isostruc-tural. In these molecules, the ferric ion has approximate octahedral coordination to the six oxygen atoms of the three 5-N-acetyl-5-N-hydroxy or the... 

Scheme 13.9. On the left, a representation of the work of Fischer, E. Raske, K. Chem. Ber., 1907, 40, 3717) demonstrating the chemical relationship between (S)-(-)-serme and (S)-(+)-alanine. On the right, a representation of the X-ray crystal structure of ferrichrome A from the information presented by Zalkin, A. Forrester, J. D. Templeton, D. Yi.J.Am. Chem. Soc., 1966,88,1810.

Figure 3.5 Structures of FhuA ligands as determined by X-ray analysis of co-crystals with FhuA. Albomycin adopts an extended and a compact conformation in the FhuA crystal, and rifamycin CGP 4832 binds to the same FhuA site as ferrichrome and albomycin although it assumes a different conformation.

Zalkin A, Forrester JD, Templeton DH (1966) Ferrichrome A Tetrahydrate. Determination of Crystal and Molecular Structure. J Am Chem Soc 88 1810... 

Since siderophore receptors, being membrane-bound, are hard to crystallize, there are to date few structures in the Protein Data Bank (PDB). One such structure is the E. call FhuA entries IbyS, with and without ferrichrome, Ifcp and 2fcp , respectively. ... 

In some instances the microbes have made some innovations. The various members of the ferrichrome group have to be regarded as very sophisticated ligands and their existence underlines once more the fabulous synthetic capability of microorganisms. Fortunately, in the case of ferrichrome A a crystallographic structure has been worked out. This is certain to aid in the interpretation of the biological mechanism of action of these growth factors. At the same time the very detailed crystal... 

Figure 2. Crystal and solution structure of the ferrichrome siderophores as determined by x-ray diffraction (11) and high resolution NMR (12). The ferrichrome peptides differ in the nature of the acyl substituent at the metal hydroxamate (R) and in the side chains of the three small, neutral, spacer amino acids (R1, R2, and R3). Ferrichrome M = Fe R = CHa R R2 = R3 = H (see also Figure 6 and Refs.

Finally, our assignment of configuration is confirmed by the CD spectra and structure of chromium desferriferrichrome. This compound is found to be isomorphous with ferrichrome (42) which crystallizes in the A-cis configuration as has been determined by x-ray crystallography (43). In addition, its CD spectrum (Figure 10) is similar to those of the model A-cis-Cr(hydroxamate)3 complexes. 

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