Ferrichrome conformation

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.

Figure 3.2 Chemical structures of selected siderophores to demonstrate the four major structural classes and the different solutions of microorganisms to scavenge iron. See for comparison the conformations of the Fe3+-complexes of ferrichrome and albomycin shown in Figure 3.5.

Figure 3.6 Comparison of the chemical structures of rifamycin (Rifampicin ) and rifamycin CGP 4832 the latter is transported by FhuA. Note the entirely different chemical structures (Figure 3.2) and conformations (Figure 3.5) of the ferrichrome and albomycin FhuA transport substrates.

The EFF calculations yielded a single Cs-symmetric conformation for each type of ferrichrome analog (Figure 4), both with a A-cis configuration of the hydroxamates about the metal when L-amino acids were used. Taken together with the spectroscopic data, pronounced differences were observed for the conformations of these iron complexes. Inspection of the calculated conformations showed that the backbone amide groups may... 

Fe111 complexes of ferrichrome,1189 ferrichrome A,1190 ferrichrysin1191 and ferricrocin.1192 These ferrichromes all show similar conformations of the peptide backbone and similar stereochemistry. Together with two fusarinine siderophores,1193,1194 they all contain ornithine in the l configuration. Surprisingly the d isomer of N -hydroxyomithine is present in neurosporin.1195... 

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. ... 

Microorganisms typically express uptake systems specific for the siderophores that they synthesize as well as siderophores synthesized by other microbes, a strategy that allows them to compete for iron in an environment containing multiple microbial species. The structures of three OM receptors (FecA, FepA, and FhuA) are very similar to one another in that they are all -barrel proteins containing 22 antiparallel /3-strands that form a tube. The N-terminus forms a mobile globular domain that can occupy and occlude the chaimel formed in the center of the tube. When FhuA binds the ferrichrome near the outer surface of the central chaimel, major conformational changes occur both at the outer surface and on the periplasmic face of the receptor. The position of the globular cork domain then shifts to allow the bound siderophore to translocate across the membrane. 

All outer-membrane transporters (OMTs) involved in iron uptake are made up of a 22-stranded p—barrel, which is occluded by an independently folded mixed a P globular cork domain of around 160 amino acid residues. This is illustrated for a vitamin B12 receptor and the ferric siderophore receptors for citrate, enterobactin, ferrichrome, pyochelin, and pyoverdin from E. coli and P. aeruginosa in Fig. 7.6. The ferric siderophore sits on top of the cork domain, as can be seen in Fig. 7.7 for FecA. The binding of the ligand induces a conformational... 

All the ferrichromes whose conformations have been studied are des-scribedin Fig. 1. This structural model is based on the X-ray crystallographic study of ferrichrome A by Zalkin et al. (27, 28) and in most respects, it is confirmed by PMR solution studies (29, 30). The structure exhibited by the model in Fig. 1 is globular with the three substituted ornithyl side chains embracing the metal ion in octahedral coordination while the cyclohexapeptide backbone assumes a distorted antiparallel /3-pleated sheet structure. 

On the basis of chemical evidence suggesting cyclodimerization of linear tripeptides ( doubling reaction ) Schwyzer has proposed (31) a conformational model for cyclohexapaptides involving two transannular hydrogen bonds which pair the carbonyl oxygen and amide nitrogen atoms of opposed residues, e.g., in ferrichrome it would be... 

We have extensively studied the solution conformation of the metal-free ferrichromes, deferriferrichrome, deferriferricrocin and deferriferri-chrysin (29, 40). By plotting the PMR chemical shift versus temperature for the different amides, an estimate of the relative extent of steric shielding or intramolecular H-bonding can be obtained from their linear slopes. Fig. 2, a and b, show this kind of plot for deferriferrichrome both in H20 and in (CDa SO. It is evident from such graphs that while little distinction can be made between the extent of shielding of the amides in water, at least two amides, namely those denoted by G3 (a glycyl... 

The conformational studies on the ferrichromes and on ferrioxamine B indicate that a number of intramolecular hydrogen bonds are formed in the process of metal-chelation and that these contribute to the overall stability of the coordinated conformation relative to that of the free species. Consistent with this view, it should be mentioned that extensive hydrogen bonding has also been observed in the low molecular weight monovalent cation complexes of the antibiotics monensin, nigericin, dianemycin, the enniatins and valinomycin by NMR spectroscopy (69, 70), X-ray crystallography (71, 72, 73), or both. Like the siderochromes, these compounds act by mediating cation fluxes across membranes. 

As clearly shown by the ferrichromes and the bacterial ferredoxins, the stability of the coordinated globule immediately reflects the single, non-essential amino acid substitutions. These further introduce positive and negative strains that are not strong enough to overcome the tight conformational control imposed by the iron complexation but which... 

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