Ferri-siderophores

As mentioned previously, siderophores must selectively bind iron tightly in order to solubilize the metal ion and prevent hydrolysis, as well as effectively compete with other chelators in the system. The following discussion will address in more detail the effect of siderophore structure on the thermodynamics of iron binding, as well as different methods for measuring and comparing iron-siderophore complex stability. The redox potentials of the ferri-siderophore complexes will also be addressed, as ferri-siderophore reduction may be important in the iron uptake process in biological systems. 

Formal Fe3+/Fe2+ Redox Potentials for Ferri-Siderophore and Siderophore Mimic Complexes at Potential-Limiting, High-pH Conditions... 

Another factor that will affect the complex redox potential is the architecture of the siderophore. A plot of ferri-siderophore redox potentials as a function of pFe for a series of hydroxamate complexes of differing denticity (shown in Fig. 19) exhibits a trend. The trend demonstrates that hydroxamate siderophores of higher denticity will form complexes with more negative E1/2 values than analogous siderophores of lower denticity. 

The implications of these mechanistic studies for our understanding of environmental iron sequestration by siderophores is as follows. The hydroxyl containing aqua ferric ions will tend to form ferri-siderophore complexes more rapidly than the hexaaqua ion and ferrous ion will be sequestered more rapidly than the ferric ion. However, once in a siderophore binding site the ferrous ion will be air oxidized to the ferric ion, due to the negative redox potentials (see Section III.D). This also means that Fe dissolution from rocks will be influenced by mineral composition (other donors in the first coordination shell) as well as surface reductases in contact with the rock, and of course surface area (4,13). 

One paradigm for membrane transport of iron is the binding of the receptor protein to an iron-free siderophore molecule, followed by exchange of iron from an external ferri-siderophore to the receptor bound iron-free siderophore, and subsequent transfer across the cellular membrane. This shuttle mechanism has been explored in the transport system of ferric pyoverdine in P. aeruginosa (215,216). It is unclear why the bacterial system behaves in this manner, but mutagenesis studies of the protein suggest that residues involved in the closure of the P-barrel will not interact in the same way with the iron-free siderophore as they do with the ferri-siderophore. A similar mechanism has been suggested for A hydrophila and E. coli (182). 

The schizokinen-mediated Fe " transport in Bacillus megaterium was studied by double labelling with e and (8). At 37°C, uptake of Fe and of are parallel during the first 30 sec, then that of e continues until it levels off after 2 min, while that of [ H]-schizokinen drops to a low constant level. At 0°C, uptake of both labels reaches this low level which is obviously due to the binding of the ferri-siderophore to the cell surface. At 37°C, transport into the cell, release of iron, and re-export of the ligand follow. Apparently a shuttle mechanism takes place, cf. the experimental results obtained with parabactin (Sect. 3.2) indicative of a taxi mechanism. 

In contrast to the tris-catecholate siderophores, which form charged iron(III) complexes, the hydroxamate-based ferri-siderophore complexes are electrically neutral, which may influence their transport through biological membranes. 

Liermann LJ, Kalinowski BE, Brantley SL, Ferry JG (2000) Role of bacterial siderophores in dissolution of hornblende. Geochim Cosmochim Acta 64 587-602... 

The transport system of Bacillus subtilis accommodates the Fe " complexes of enterobactin (A-configured), enanfio-D-enterobactin and of corynebactin (bacilli-bactin) (both A). Since only A complexes can be bound to the receptor a configurational change from A to A is induced. Only the natural ferri-L-siderophores can be degraded enzymatically (399, 408). 

Figure 1. Schematic of the two iron transport systems of microorganisms. The high affinity system is comprised of specific carriers of ferric ion (siderophores) and their cognate membrane hound receptors. Both components of the system are regulated by iron repression through a mechanism which is still poorly understood. The high affinity system is invoked only when the available iron supply is limiting otherwise iron enters the cell via a nonspecific, low affinity uptake system. Ferri-chrome apparently delivers its iron by simple reduction. In contrasty the tricatechol siderophore enterobactin may require both reduction and ligand hydrolysis for release...

Figure 11. Uptake of the radioactive label of 55Fe-ferri-chrome (O), a cis-chromic 3H-deferriferrichrome 3H-fer-richrome (A), and 3H-ferrichrome with excess ferric NTA (A) in Escherichia coli RW193. Cell optical density at 650 nm was 1.2, and siderophore concentration was 0.5 fiM (62).

Chromic Ferrichrome Complexes. The spectra for the model chromic hydroxamate complexes are reproduced in Figure 6. Since the visible and CD spectra of the isomers are wholly dominated by the metal complex chromophore, these data can be used to characterize and to identify coordination isomers of complexes formed by the siderophores. The preparation and characterization of the chromic complexes of des-ferriferrichrome and desferriferrichrysin have been reported (3). Although an examination of molecular models for both complexes shows two coordination isomers are possible (A-cis and A-cis), both chromic complexes consist exclusively of the A-cis isomer. These results agree with x-ray crystallographic investigations which have shown that both ferri-chrysin and ferrichrome A crystallize as only the A-cis isomer (14, 15). Both chromic complexes have identical CD spectra which are the same as the A-cis Cr(men)3 spectrum (Figure 6). 

The first synthesis of a siderophore was the preparation of ferrioxamine B over 20 years ago in order to confirm the chemical structure of this natural product67). Synthesis of the other hydroxamate containing siderophores has as a central problem preparation of the constituent to-N-hydroxy amino acid in an optically pure form. The most important such subunit in hydroxamate siderophores is Ns-hydroxy ornithine. This is a chiral building block of the diketopiperazine-containing siderophores (rhodo-torulic acid 68), dimerum acid 69), coprogen 70) and coprogen B 69>), the cyclic hexa-peptides of the ferrichrome family27), the fusarinines 71 -73) and the antibiotic ferri-chrome derivatives albomycines Sl5 S2 and e 61-62). 

In common with most prokaryotes, many fungi have siderophore-dependent iron uptake systems. The ferri-chrome-type siderophores are often employed, although other types of siderophore are also used. Indeed, even if, like the quintessential scavenger baker s yeast (Saccharomyces cerevisiae), they produce no siderophores of their own, they nonetheless have several distinct facilitators for the uptake of ferric siderophores, including ferric enterobactin, which is produced by many bacteria, but not by fungi. 

Sifleramines. Obsolete name for microbial iron(III) complexes of the trihydroxamate type with growth promoting properties (e.g., ferrioxamines, ferri-chromes). Tlie name siderophores is now established for the low-molecular weight compounds participating in iron transport. 

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