Mossbauer spectroscopy iron-sulfur clusters

This key enzyme of the dissimilatory sulfate reduction was isolated from all Desulfovibrio strains studied until now 135), and from some sulfur oxidizing bacteria and thermophilic Archaea 136, 137). The enzymes isolated from sulfate-reducing bacteria contain two [4Fe-4S] clusters and a flavin group (FAD) as demonstrated by visible, EPR, and Mossbauer spectroscopies. With a total molecular mass ranging from 150 to 220 kDa, APS reductases have a subunit composition of the type 012)32 or 02)3. The subunit molecular mass is approximately 70 and 20 kDa for the a and )3 subunits, respectively. Amino-acid sequence data suggest that both iron-sulfur clusters are located in the (3 subunit... 

EPR and Mossbauer spectroscopies have been successfully used to characterize iron-sulfur clusters. Hydrogenases are no exception. Here, we will describe the knowledge gained from applying these spectroscopies to the study of [NiFe] hydrogenase. 

One large class of non-heme iron-containing biomolecules involves proteins and enzymes containing iron-sulfur clusters. Iron-sulfur clusters are described in Sections 1.7 (Bioorganometallic Chemistry) and 1.8 (Electron Transfer) as well as in Section 3.6 (Mossbauer Spectroscopy). See especially Table 3.2 and the descriptive examples discussed in Section 3.6.4. Iron-sulfur proteins include rubredoxins, ferrodoxins, and the enzymes aconitase and nitrogenase. The nitrogenase enzyme was the subject of Chapter 6 in the hrst edition of this text—see especially Section 6.3 for a discussion of iron-sulfur clusters. In this... 

Mossbauer spectroscopy. The earlier Mossbauer work of (HE) Evans et had already provided support for the [4Fe 4S]-type structure for FeS-A, FeS-B andFeS-X. AfteraPS-I core complex containing only the iron-sulfur cluster FeS-X became available, however, renewed Mossbauer studies of the core complex were undertaken by Petrouleas, Brand, Parrett and Golbeck ". ... 

Pyruvate formate-lyase activating enzyme is the member of the radical-SAM family whose cluster properties are most similar to those of aconitase. The cluster in pyruvate formate-lyase activating enzyme is quite labile, and in fact until 1997 it was not known that the enzyme contained an iron-sulfur cluster, as all preparations to that time had been done aerobically, under which conditions the cluster falls apart. It was initially reported that PFL-AE contained a mixture of [2Fe-2S] and [4Fe-4S] clusters, and subsequent reconstitution studies of the apo enzyme provided evidence for a [4Fe-4S] cluster. Further studies showed that anaerobic isolation resulted in purification of a form of PFL-AE that contained primarily [3Fe-4S] clusters, which upon reduction converted to [4Fe-4S] clusters.This reductive cluster conversion from [3Fe S] to [4Fe-4S] clusters even in the absence of added iron was remarkably reminiscent of aconitase (see Section 8.27.2.2), and suggested a labile cluster site. Adding to the similarity to aconitase, Mossbauer spectroscopy provided evidence for a linear [3Fe-4S] cluster in PFL-AE isolated under appropriate conditions.Therefore all of the cluster forms previously identified in aconitase were also found in PFL-AE, and like aconitase it appeared to be relatively simple to interconvert between these cluster forms. 

The conclusion that Fx is a [4Fe-4S] cluster is also supported by a recent Mossbauer study of the P700 and Fx-containing Photosystem I core protein. Mossbauer spectroscopy is one of the best methods for determining the identity of an iron-sulfur cluster, and offers the added advantage that the degree of delocalization of the electron over the iron-sulfur cluster can be determined. 

Iron-sulfur clusters are versatile electron transfer cofactors, which are ubiquitous in many metalloenzymes. In the Bacillus subtilis, redox regulator (Fnr) that controls genes of the anaerobic metabolism in response to low oxygen tension, an unusual structure for the oxygen-sensing [4Fe-4S] " cluster was detected by a combination of genetic experiments with UV-visible and Mossbauer spectroscopy [79]. Asp-141 was identified as the fourth iron-sulfur cluster ligand besides three Cys residues. 

Layer et al. employed UV visible absorption speetroscopy and Mossbauer speetroscopy to characterize Escherichia coli CYaY as an iron donor for the assembly of [Fc2S2] cluster in the scalfold IscU." In this study, the author provided iron in the form of CyaY-Fe ". At time intervals, formation of iron-sulfur clusters on IscU was monitored by UV-visible absorption spectroscopy from the increase of characteristic absorption bands in the 300-700 nm region (Figure 5.11)." The absorption bands at 320,410, and 456 nm and the shoulder at 510 nm present in the UV-visible spectrum at the end of the reaction are characteristic for 6282] clusters assembled in IscU. The author also provided iron in the form of CyaY- Fe , and recorded the Mossbauer spectrum... 

The association of sulfur and iron into simple to more complex molecular assemblies allows a great flexibility of electron transfer relays and catalysis in metalloproteins. Indeed, the array of different structures, the interactions with amino-acid residues and solvent and their effect on redox potential and spectroscopic signatures is both inspiring for chemists and electrochemists, and of paramount importance for the study of these centers in native conditions. Most of the simpler natural clusters have been synthesized and studied in the laboratory. Particularly, the multiple redox and spin states can be studied on pure synthetic samples with electrochemical and spectroscopic techniques such as EPR or Fe Mossbauer spectroscopy. More complex assembhes still resist structural... 

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