Ferrous iron, chelation

Rahhal S, Richter FIW. Reduction of hydrogen peroxide by the ferrous iron chelate of diethy]enetnaminc-/V,/V,/V, /V",/V7 -pentaacetate. J Am Chem Soc 1988 110 3126-3133. 

The po.ssible role of a chelate reductase for iron uptake from microbial siderophores has been examined for several plant species (30,47). With certain microbial siderophores such as rhizoferrin and rhodotorulic acid, the reductase may easily cleave iron from the siderophore to allow subsequent uptake by the ferrous iron transporter. However, with the hydroxamate siderophore, ferrioxamine B, which is produced by actinomycetes and u.sed by diverse bacteria and fungi, it has been shown that the iron stress-regulated reductase is not capable... 

While much is known about siderophore-mediated ferric-iron transport, very little is known about ferrous-iron transport and iron metabolism inside the cell. It is generally assumed that Fe3+ chelated to the siderophore must be reduced to allow removal from the strong claws of the chelator. Indeed, in some cases the siderophore transported iron was found 30 minutes later in the intracellular Fe2+ pool of the cells (Matzanke et ah, 1991). 

Aconitase was first determined to be an Fe-S protein in 1972 by Kennedy, Rauner and Gawron (23). Chemical analyses of inactive enzyme gave values of 2 Fe and 3 S /protein of 66,000 daltons. The observed molar relaxivity of water protons by this preparation of aconitase was 473 M s l (25). This value was an order of magnitude lower than measured in the earlier preparation of Villafranca and Mildvan (21) and much closer to that of Fe-S proteins (26). One mole of Fe + per mole of protein was taken up by the enzyme upon activation in the presence of cysteine and ascorbate, or lost upon inactivation in the presence of the iron chelator ferrozine (27). Gawron s group also demonstrated a correlation between loss of one Fe and loss of enzyme activity, as well as the protection afforded by citrate against both losses. However, the presence of an Fe-S cluster in aconitase remained for the moment a curiosity, in particular because of the unusual Fe/S= stoichiometries. The essential Fe that is correlated with activity continued to be interpreted in terms of the "ferrous-wheel" model. 

This apparent contradiction may be related to UV absorbance contributed by mineral forms. The gel permeation technique used in this laboratory has been observed to concentrate mineral components. For example, nitrate in this fraction has been found to exceed 1 g/L. Furthermore, some mineral forms such as ferrous iron have been observed to absorb at 260 nm. Chelates have been found to quench fluorescence. Thus, the concentrations of all minerals and their contributions to UV absorbance or fluorescence quenching should be carefully examined. High concentrations of metallic ions also may play a catalytic role during pyrolysis and further contribute to the absence of pyrolysis products (27). 

Absorption of iron by the individual varies with age, iron status, the amount and chemical form of the iron ingested, and with conditions in the gastrointestinal tract, only about 5—15% of iron in the diet being normally absorbed. Ferrous iron, as the sulphate, gluconate, fumarate or lactate or as ferrous ammonium sulphate, is appreciably taken up into the bloodstream from the duodenum, especially in the presence of ascorbic acid, a reducing agent. Little difference was found in the extent of their absorption between ferrous sulphate and the various chelates, but ferric ammonium citrate or polysaccharide complexes were only very poorly absorbed22)... 

Iron is a nonamphoteric, transition element with the ability to exist in two oxidation states—Fe2+ (ferrous) and Fe3+ (ferric). A positive reaction to alkaline ferric chloride is an indication of the presence of hydroxyl groups with which Fe2+ forms colored complexes. Stable copper and iron chelates... 

The incubation of spinach ferredoxin (Fry and San Pietro (46)) or bacterial ferredoxin (Lovenberg, Buchanan, and Rabinowitz (65)) with the iron chelating agent, o-phenanthroline, results in removal of the iron from the protein and in the formation of a ferrous triphenanthrolate complex. Under these conditions, all of the iron appears to be in the ferrous state, but this does not constitute proof that iron of the native protein is also in the ferrous state. Reduction of bound ferric iron could occur... 

The individual reactions affected by iron stress can be considered as regulated biochemical pathways, although regulation by iron is not understood. The mechanism of iron absorption and transport involves the release of hydrogen ions by the root, which lowers the pH of the root zone. This favors Fe3+ solubility and reduction of Fe3 to Fe2+. Reductants are released by roots or accumulate in roots of plants that are under iron stress. These "reductants, along with Fe3+ reduction by the root, reduce Fe3+ to Fe2+, and Fe2+ can enter the root. Ferrous iron has been detected throughout the protoxylem of the young lateral roots. The Fe2+ is probably kept reduced by the reductant in the root, and it may or may not have entered the root by a carrier mechanism. The root-absorbed Fe2+ is believed to be oxidized to Fe3, chelated by citrate, and transported in the metaxylem to the tops of the plant for use. We assume Fe2+ is oxidized as it enters the metaxylem because there is no measureable Fe2+ there (13), and Fe3+ citrate is transported in the xylem exudate (30, 31,32). 

Ferrous Glycinate occurs as a fine, free-flowing powder. It has an octahedral structure with two water molecules and two chelated glycinate ions coordinated to the central ferrous iron. 

Another hypothesis was recently proposed by Afanas ev [63] to explain the inability of various OH scavengers to prevent deoxyribose oxidation by xanthine oxidase in the absence of iron-chelating EDTA. His hypothesis is that ferrous peroxy complexes, Fe(OOH)+, may be transiently formed, to decompose into two different reactive species, FeO+ and OH, which should both be very strong free radical initiators. There is no experimental data to support this speculation however. 

Proline hydroxylase has been isolated and characterized and is known to contain a mononuclear nonheme ferrous iron center that is the catalytic active site of the enzyme, coordinated by two histidine and one aspartate side chains. The requirement for Fe(II) is reflected in the in vivo sensitivity of collagen formation to chelators specific for ferrous ion (e g. 2,2 -dipyridyl). In addition to the catalytic metal cofactor, the reaction requires a reducing cosubstrate, 2-oxoglutarate, dioxygen, and the procollagen peptide (equation 1). 

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