Iron-sulfur proteins, Mossbauer spectroscopy

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... 

The second class of iron-containing proteins which have been well-studied by Mossbauer spectroscopy, and by other resonance techniques, are the iron-sulfur proteins. These molecules are also known by the name, ferredoxins. Iron-sulfur proteins in several varieties serve as electron-transport agents for processes in plants, bacteria, and mammals. Perhaps the most-studied physiological process involving the iron-sulfur proteins is the study of their role in photosynthesis. This subject has been extensively reviewed by Arnon 126,135), Hind and Olson 127), Hall and... 

Table 1 lists some of the properties of the plant-type iron sulfur-proteins for which extensive study by EPR and Mossbauer spectroscopy has been reported. The physical properties summarized show that the plant-type iron sulfur proteins have molecular weights in the range from 12,000 to 24,000 and have EPR g-values (gx, gy, gz) all of the g = 1.94" type shown in Fig. 6 but with minor variations reflecting axial or nonaxial symmetry of the paramagnetic center. The amino-acid sequences of four plant-type iron-sulfur-proteins are known alfalfa (136), L. glauca (137), Scenedesmus (138), and spinach (139). Each protein has about 97 residues, all in a single peptide chain these are shown in Table 2. 

The report-in-depth on Mossbauer spectroscopy of the iron-sulfur proteins includes collaborative research, also published elsewhere, with W. H. Onne-Johnson, Graham Palmer, R. H. Sands, and I. Salmeen. 

Cammack, R., Dickson, D. P. E., Johnson, C. E. Evidence from Mossbauer spectroscopy and magnetic resonance on the active centers of the iron-sulfur proteins, in Ref. 12, p. 283... 

The fully characterization of both simple or more complex (but well defined in terms of active center composition) iron-sulfur proteins leads to a well of information. The compilation of typical spectroscopic features of the known basic iron-sulfur structures enables a preliminary characterization of centers in a new simple situation or even in some more complex ones. EPR spectroscopy of the iron-sulfur cores in the appropriated oxidation states have characteristics that can be used to readily distinguish certain type of centers58). This technique has also been used to analyse components in complex systems. However the use of EPR as the sole technique can be misleading, when applied to new situations as we have seen for the case of the [3 Fe—xS] core. In this particular case only the conjunction of EPR and Mossbauer can lead to a proper characterization27 33 34 55). 

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. 

A number of chemical structures for Fd have been suggested. The chelate structure [Fig.(l)] is based on chemical analysis, ESR and Mossbauer spectroscopy (see also Iron-sulfur proteins). Model experiments led to the construction of metal-atom island structures, or clusters. In this model, the labile sulfur... 

Besides heme and iron-sulfur proteins, other types of metalloproteins could also been Investigated by Mossbauer spectroscopy. For instance, ferritin, transferirin, ovotransferrin, lactoferrin, siderochromes, and hemerythrin. 

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