Iron-bound phosphate

Iron-bound phosphate Calcium-bound phosphate Acid-soluble organic phosphate Sodium hydroxide-extractable phosphorus Fe(OOH) P CaCOj P ASOP NaOH3,-P 0.02 M Ca-NTA/dithionite, pH 7.8-8.0 0.05 M Na-EDTA, pH about 8.0 0.5 M HCI or 0.25 M HjSO (30 min) 2.0 M NaOH (OO C, 30 min)... 

Sinha (60,61) has suggested that humic and fulvic acids play a major role in mobilising iron and transporting it from the soil to plant roots. At the normal soil pH it is believed that iron bound by the fulvic acid is partially hydroxylated as Fe(OH)2 (62). These complexes interact with phosphate to give an organicmetallic phosphate which may be taken up by plants (60). It has been suggested that the entire humic-iron-phosphate complex is taken up by the roots of plants and not just the iron and phosphate (60, 63). Jorgensen (64) has observed that soil humates suppress the uptake of Pb2+ into plants it is possible that they will also suppress actinide concentration in plants. 

It has been shown8263 that the analysis of iron in uteroferrin is dependent on the presence of phosphate. Analysis of the native enzyme with one tightly bound phosphate gives an incorrect low answer suggesting the presence of only one iron. 

Features due to one Fe-P component per iron at 3.1-3.2 A can be observed. Since only one phosphate is bound to each enzyme molecule, the EXAFS fits require that it be bridging the diiron unit. Que and Scarrow favored an 0,0 -bridging mode for the phosphate in the porcine enzyme (la, 46), by analogy to the several examples of 0,0 -bridging phosphates in synthetic complexes. In contrast, Kauzlarich et at. (48) proposed a terminally bound phosphate for the bovine enzyme, but did not address the discrepancy between the model and the coordination number determined for P in the fit. 

The purple acid phosphatases can occur in two diferric forms—one as the tightly bound phosphate complex (characterized for the bovine and porcine enzymes) (45, 171, 203) and the other derived from peroxide or ferricyanide oxidation of the reduced enzyme (thus far accessible for only the porcine enzyme) (206). These oxidized forms are catalytically inactive. They are EPR silent because of antiferromagnetic coupling of the two Fe(IIl) ions and exhibit visible absorption maxima near 550-570 nm associated with the tyrosinate-to-Fe(III) charge-transfer transition. The unchanging value of the molar extinction coefficient between the oxidized and reduced enzymes indicates that the redox-active iron does not contribute to the visible chromophore and that tyrosine is coordinated only to the iron that remains ferric in agreement with the NMR spectrum of Uf, (45). 

In soils and sediments, complexation can increase organic phosphorus stabilization, especially with iron (III) and calcium ions and their minerals (Harrison, 1987 House and Denison, 2002). The interaction with iron (III) was reported to transform a large part of the labile and moderately labile organic phosphorus forms supplied with manure to paddy soils into more resistant organic phosphorus, possibly because inositol phosphates initially bound to calcium or magnesium were transformed into iron-bound compounds (Zhang et aL, 1994). In the presence of calcium, myo-inositol hexakisphosphate can form two soluble calcium complexes with one or two calcium ions (Ca - or Ca2-phytate), but when three calcium ions are involved (Cag-phytate), the complex precipitates at all pH values (Graf, 1983). This enhances the interaction of myo-... 

Figure 4. Fe(2p3y2) spectra collected in an area tribostressed under 1N load. It is possible to observe a contribution at 712.4eV, which can be attributed to iron bound to phosphate.

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

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