Fe -S complexes

Fe-S complexes have important functions in today s living systems, in enzymes such as the ferredoxins and oxidoreductases, as well as in electron transport proteins. It is striking that these redox reactions mainly involve elements and compounds such as CO, H2 and N2, which were probably also components of the primeval Earth s atmosphere. Thus, the assumption of an active involvement of Fe-S clusters in a (hypothetical) Fe-S world in processes which finally led to biogenesis appears completely reasonable We now have a background to the theory of the chemoau-totrophic origin of life . 

Other Iron Compounds of Biological Interest.—The valency of iron in a range of ferredoxin extracts has been determined by ESCA. The structure of the Fe—S complex in a bacterial ferredoxin has been determined. The iron and sulphur atoms occupy alternate corners of a cube and four more sulphur atoms project from the iron atoms.Admission of oxygen to a neutral solution of Fe and excess penicillamine gives a red bis-complex, which is relatively stable in aqueous solution at room temperature. Quantitative... 

Figure 4. Mixed Mo-Fe-S complexes (a) structure of [ (RSFe)3S4Mo 2-(SR)3p as determined in Ref. 126 for R = Ph (b) structure of [(PhS)2-FeS2MoS2]2 as described in Ref. 129. In both cases, S represents the thiolate sulfur atom.

A Hypothetical Cycle for N2 Reduction with [Fe(S )] Complexes and Reversible Redox Reactions of Diazene Complexes / 662... 

Scheme 37 shows a hypothetical cycle for the N2 reduction in the coordination sphere of [Fe(S )] complexes (7e-7g) (47), emphasizing the dominance of iron and sulfur in all nitrogenase cofactors. 

The structure of the hexanuclear [Fe6S,(SR)2]3 cluster (50) Each of the eight sulfur atoms triply bridges an octahedral 44.2.6.1.6 Mo-Fe-S complexes... 

D. CoQ does not accept electrons from an Fe-S complex, but from FMNH2... 

Experimental data for Fe-S complexes could be fit by an equation of the form ... 

For sulfide or hydrogen sulfide removal (the species depends on the solution pH), found, for example, in wastewater networks or industrial aqueous effluents, activated carbon preloaded with iron (AC-Fe) or copper was used. In this case, an AC—Fe—S complex is formed on the activated carbon surface following the simple reaction ... 

Fig. 20.6. Succinate dehydrogenase contains covalently bound FAD. As a consequence, succinate dehydrogenase and similar flavopro-teins reside in the inner mitochondrial membrane where they can directly transfer elechons into the electron transport chain. The elechons are hansferred from the covalently bound FAD to an Fe-S complex on the enzyme, and then to coenzyme Q in the electron hansport chain (see Chapter 21). Thus, FAD does not have to dissociate from the enzyme to transfer its electrons. All the other enzymes of the TCA cycle are found in the mitochondrial mahix.

The peptides cystein and methionine contain sulfur in their side groups. Sulfides bind easily to metal ions, such as Fe U Cu +, Zn +, and Pb ". In the case of Fe, Fe-S complexes are formed, which are capable of easy reduction or oxidation. Pb + is poisonous and binds strongly at Zn + sites, but the structure is very different from the tetrahedral zinc complex. 

Iron-sulfur proteins are also found in Complex III and Complex IV and in fer-rodoxins, cyt c reductase, nitrogenase, and a number of other systems. In most cases, they have a role in mitochondrial redox reactions. The iron-sulfur complex is easily oxidized or reduced. In fact, the HOMO-LUMO gap is small, so that Fe-S complexes work as small pieces of metal. Solid iron sulfide may become a metal if high pressure is applied. 

Complex III (bCj) is docked with ubiquinol (QHj), which is oxidized to ubiquinone (Q) at the site Qo, close to the intermembrane space. Qo delivers one electron to the Fe-S complex and another to cyt Fl. On the matrix side of the membrane, the opposite reaction takes place, but only half the amount can be produced. There is a net delivery of four protons to the intermembrane space, for each two electrons delivered to cyt c. 

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