Ferrous iron reduction intermediate

Dissolved iron(III) is (i) an intermediate of the oxidative hydrolysis of Fe(II), and (ii) results from the thermal non-reductive dissolution of iron(III)(hydr)oxides, a reaction that is catalyzed by iron(II) as discussed in Chapter 9. Hence, iron(II) formation in the photic zone may occur as an autocatalytic process (see Chapter 10.4). This is also true for the oxidation of iron(II). As has been discussed in Chapter 9.4, the oxidation of iron(II) by oxygen is greatly enhanced if the ferrous iron is adsorbed at a mineral (or biological) surface. Since mineral surfaces are formed via the oxidative hydrolysis of Fe(II), this reaction proceeds as an autocatalytic process (Sung and Morgan, 1980). Both the rate of photochemical iron(II) formation and the rate of oxidation of iron(II) are strongly pH-dependent the latter increases with... 

This generalization has some qualifications. Recently, nitrile hydratase was reported to be a low-spin Fe " enzyme. Demonstration of the spin state is incomplete at this time and the low-spin claim for the enzyme may be revised. Also, the binding of NO to high-spin ferrous iron (S = 2) results in a one-electron reduction and intermediate spin (5 = i) ferric iron. Finally, low-spin ferric iron has been observed for cyano 3,4-protochatechuate dioxygenase (Whittaker and Lipscomb, 1984). 

And, of special relevance to cytochrome c oxidase, the reactions of hemes (57, 39, 40) as well as simple aquo ferrous iron (55, 41) with dioxygen seem to proceed through two-electron reduction of bridged intermediates. Thus, dipyridine hemes react cleanly with dioxygen to form... 

After the reduction of Fe(III) by zinc blend concentrate and after the separation of the solute noble metals, ferrous iron is oxidized to ferric iron which then precipitates in the form of hematite. The intermediate noble metal yields from the Jarosite and Hematite processes are compared in Figure 9.7. The quality of the hematite produced permits its utilization in the cement industry as a coloring material. For use in the steel industry, the zinc content of hematite must be reduced from 1% to 0.1%. 

If both MT-1 and HO-1 mRNA induction by heme-hemopexin involves a copper-redox enzyme in both heme transport (and consequent induction of HO-1 mRNA) and the signaling pathway for MT-1 expression, a plausible working model can be formulated by analogy with aspects of the yeast iron uptake processes and with redox reactions in transport (Figure 5-6). First, the ferric heme-iron bound to hemopexin can act as an electron acceptor, and reduction is proposed to be required for heme release. The ferrous heme and oxygen are substrates for an oxidase, possibly NADH-dependent, in the system for heme transport. Like ferrous iron, ferrous heme is more water soluble than ferric heme and thus more suitable as a transport intermediate between the heme-binding site on hemopexin and the next protein in the overall uptake process. The hemopexin system would also include a copper-redox protein in which the copper electrons would be available to produce Cu(I), either as the copper oxidase or for Cu(I) transport across the plasma membrane to cytosolic copper carrier proteins for incorporation into copper-requiring proteins [145]. The copper requirement for iron transport in yeast is detectable only under low levels of extracellular copper as occur in the serum-free experimental conditions often used. 

Water. Based on the overall balanced equation for this reaction, a minimum of one mole of water per mole of nitro compound is required for the reduction to take place. In practice, however, 4 to 5 moles of water per mole of nitro compound are used to ensure that enough water is present to convert all of the iron to the intermediate ferrous and ferric hydroxides. In some cases, much larger amounts of water are used to dissolve the amino compound and help separate it from the iron oxide sludge after the reaction is complete. 

The mercaptoacetate dependence and the effects of the scavengers indicate that competition exists between Fe(III) (OH) (RS)2 2 and RSH- for the intermediates H02 and HO The reactions of these radicals result in their reduction and, also, when Fe(III) (OH) (RS)2 2 is involved, in the reduction of the iron to the ferrous state. The best source of electrons in this latter reaction is the ligands coordinated to the iron ion. Thus, it appears that H02 and H02 react with Fe(III) (OH)(RS)2 2 to give disulfide in a manner similar to that suggested by Lamfrom and Nielsen for RS ... 

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