Siderophore ferric complexes

Table V shows redox potentials (Ey2) for ferric complexes of a series of natural and synthetic siderophores. The first coordination shell of the complex formed between iron and siderophore...

Using linear regression, it is possible to estimate the protonation constants of the Fe(II) complexes of siderophore complexes where the redox potentials have been measured over a range of pH values (59). This also explains the variation in reversibility of reduction as the pH changes, as the stability of the ferro-siderophore complex is much lower than the ferric complex, and the increased lability of ligand exchange and increased binding site competition from H+ may result in dissociation of the complex before the iron center can be reoxidized. 

Jalal MAF, Love SK, van der Hehn D (1986) Siderophore Mediated Iron(III) Uptake in Gliocladium virens. 1. Properties of cw-Fusarinine, trani-Fusarinine, Dimerum Acid, and their Ferric Complexes. J Inorg Biochem 28 417... 

Konetschny-Rapp S, Jung G, Raymond KN, Meiwes J, Zahner H (1992) Solution Thermodynamics of the Ferric Complexes of New Desferrioxamine Siderophores by Directed Fermentation. J Am Chem Soc 114 2224... 

When deficient in iron, bacteria and fungi produce and excrete to the extracellular medium low molecular weight, specific iron-carrier molecules, called siderophores. These siderophores bind ferric ions, to form soluble complexes. The complexed ferric ions are transported into the cell through high-affinity and energy-dependent receptor proteins located on the outer membrane. In Gram-negative bacteria, such as E. coli, the most studied system, siderophore-iron complexes are transported initially to the periplasm. 

Once the siderophore-iron complexes are inside the bacteria, the iron is released and utilized for vital cell functions. The iron-free hydroxamate siderophores are commonly re-excreted to bring in an additional iron load (Enterobactin is at least partially degraded by a cytoplasmic esterase This cycle is repeated until specific intracellular ferric uptake regulation proteins (Fur proteins) bind iron, and signal that the intracellular iron level is satisfactory, at -which point ne-w siderophore and siderophore-receptor biosynthesis are halted and the iron-uptake process stops. This intricate feedback mechanism allows a meticulous control over iron(III) uptake and accumulation against an unfavorable concentration gradient so as to maintain the intracellular iron(III) level within the required narrow window. Several excellent reviews concerning siderophore-iron transport mechanisms have been recently published i.3,i6, is,40,45,60-62 ... 

In the LI CAM series lipophilicity, hence ability to cross cell membranes, was introduced by substitution of alkyl groups on the terminal nitrogens262. Rastetter et al.256 have also produced a linear polycatecholamide. However, unlike the LICAM series their siderophore is a chiral analogue, synthesized from L-asparagine, for which a formation constant of io46-5 1-2 has been calculated for its ferric complex. In contrast the formation constant for ferric enterobactinate is 1052 263. ... 

The chromic-substituted siderophore complexes can be prepared and, in contrast to the naturally occurring ferric complexes, are kinetic-ally inert to isomerization or ligand substitution. 

Certain secondary metabolites act as metal transport agents. One group is composed of the siderophores (also known as sideramines) which function in uptake, transport, and solubilization of iron. Siderophores are complex molecules which solubilize ferric ion which has a solubility of only 10 18 mo 1/1 at pH 7.4... 

Eukaryotic phytoplankton do not appear to produce siderophores and there is little evidence for direct cellular uptake of ferric siderophore chelates. Instead there is mounting evidence for the utilization of a high-affinity transport system that accesses ferric complexes via their reduction at the cell surface and subsequent dissociation of the resulting ferrous-ligand complexes. The released ferrous ions bind to iron(ii) receptors on iron transport proteins located on the outer cell membrane, which transport the iron into the cell. This intracellular transport involves the reoxidation of bound iron(ii) to iron(iii) by a copper protein, and thus copper is required for cellular iron uptake. The availability of iron to this transport... 

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