Fe3S4 centers

A summary of the Fe-Fe and Fe-S distances in cuboidal [Fe3S4] ° centers as deduced by X-ray crystallography and EXAFS, is given in... 

Bacterial assimilatory nitrate reductases have similar properties.86/86a In addition, many bacteria, including E. coli, are able to use nitrate ions as an oxidant for nitrate respiration under anaerobic conditions (Chapter 18). Tire dissimilatory nitrate reductases involved also contain molybdenum as well as Fe-S centers.85 Tire E. coli enzyme receives electrons from reduced quinones in the plasma membrane, passing them through cytochrome b, Fe-S centers, and molybdopterin to nitrate. The three-subunit aPy enzyme contains cytochrome b in one subunit, an Fe3S4 center as well as three Fe4S4 clusters in another, and the molybdenum cofactor in the third.87 Nitrate reduction to nitrite is also on the pathway of denitrification, which can lead to release of nitrogen as NO, NzO, and N2 by the action of dissimi-latory nitrite reductases. These enzymes873 have been discussed in Chapters 16 and 18. 

D. gigas Fdll shown in Figure 7.15 confirms the partial (apoFe) thiocubane Fe3S4 center. The iron atoms are ligated by three cysteinyl residues from protein side chains. The cube missing an iron is now firmly established as a viable structural type. 

Fig. 17. Comparison of the VTMCD spectra (4.5 K and 5 T) of the [Fe3S4] and [Fe3Se4] centers in bovine heart aconitase. Taken from Ref. (188).

Linear Fe3S4(SR)4 clusters have been found in partially unfolded proteins, but are not known to exist as functional centers to date. Synthetic analogs comprise the all-ferric [Fe3S4(SPh)4] species (a [Fe3S4] + core) that may be reduced to the corresponding [Fe3 S4] core at —1.35 V versus SCE in acetonitrile (—1.66 V for the ethyl thiolate derivative). 

Figure 5 The most commonly encountered FeS centers (a) the monoiron center of rubredoxin, (b) the FeySy cluster of plant-type ferredoxins, (c) the FeySy cluster of Rieske proteins, (d) the Fe3S4 cluster of ferredoxins, and (e) the Fe4 4 cluster of ferredoxins and high potential iron-sulfur proteins (FliPiPs).

Figure 16-26 (A) Stereoscopic view of the structure of the Desulfovibrio gigas hydrogenase as an a-carbon plot. The electron density map at the high level of 8o is superimposed and consists of dark spheres representing the Fe and Ni atoms. The iron atoms of the two Fe4S4 and one Fe3S4 clusters are seen clearly forming a chain from the surface of the protein to the Ni-Fe center. (B) The structure of the active site Ni-Fe pair. The two metals are bridged by two cysteine sulfur atoms and an unidentified atom, perhaps O, and the nickel is also coordinated by two additional cysteine sulfurs. Unidentified small molecules LI, L2, and L3 are also present. From Volbeda et Courtesy of M. Frey.

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