Iron-sulfur proteins model compounds

Up to now, two types of iron compounds have been studied with ENDOR, namely heme compounds (hemoproteins and some heme model compounds) and iron-sulfur proteins. For comprehensive summaries of the corresponding EPR work, the reader is referred to the literature234-2371. 

Fe-S proteins contain four basic core structures, which have been characterized crystal-lographically both in model compounds (Rao and Holm, 2004) and in iron-sulfur proteins. These are (Figure 3.6), respectively, (A) rubredoxins found only in bacteria, in which the [Fe-S] cluster consists of a single Fe atom liganded to four Cys residues—the iron atom... 

This account will deal in turn with structural features of the one-iron, two-iron, four-iron and three-iron proteins, and also with the most important topic of interconversion between the three-iron and four-iron clusters. Model compounds will be described briefly. Finally, kinetic studies and the role of iron-sulfur proteins in complex enzymes will be discussed. 

Figure 30 Structures of the one-iron, two-iron and four-iron centres in the iron-sulfur proteins and of a model compound...

This cluster formally contains three iron(III) and one iron(E). It is present in a class of proteins called high potential iron-sulfur proteins (HiPIP). It has also been prepared through oxidation of [(RS)4Fe4S4]2 model compounds [57]. Both in the model compound at low temperatures and in proteins there is electron delocalization on one mixed valence pair [58-62]. Therefore, the polymetallic center is constituted by two iron ions at the oxidation state +2.5 and two iron ions at the oxidation state +3. Hamiltonian (6.20), or a more complicated one [40, 41,43], can be used to describe the electronic structure. Indeed, a delocalization operator is sometimes needed in the Hamiltonian [40,41,43]. Consistently with magnetic Mossbauer data the S M subspin involving the mixed valence pair is 9/2, whereas the S n subspin involving the iron(IH) ions is 4. Mossbauer and EPR data do not exclude % and 3, respectively, for the two pairs [57] in any case, the... 

Averill, B. A., Orme-Johnson, W. H. Iron-sulfur proteins and their synthetic analogs, in Metal Ions in Biological Systems, V. 7, Iron in Model and Natural Compounds (ed.) Sigel, H., p. 127, New York, Marcel Dekker, 1978... 

The very large number of known iron enzymes makes an exhaustive treatment of their chemistry unrealistic for a book such as this one. We arbitrarily decided to include only a few examples starting with heme proteins, followed by non-heme oxo-bridged proteins, iron storage proteins, and finally iron sulfur proteins. In all cases a very small segment is dedicated to the discussion of model compounds the reader is advised to review the references given in the text for further details. 

Model compounds are often used to model iron-sulfur proteins. Comment on the applicability of the following models, and on the data given. 

The most simple iron-sulfur protein is rubredoxin (Rd) which contains one Fe atom per protein. The Fe atom is coordinated by four sulfur atoms of cysteinyl residue in a tetrahedral environment. Figure V-21 shows the crystal structure of a model compound, [FefSi-o-xyl),]" (S2-o-xyl = o-xylene-of,a -diihiolate). Long et al." first obtained the RR spectrum of... 

The ability to selectively enhance the modes of the resonant chromophore has fueled the steady development of RR spectroscopy and its transient RR and TR variants as exquisite probes of active site structure and dynamics in a wide range of metalloproteins and enzymes. Inasmuch as this remarkable potential has been more fully realized in the study of heme proteins than for any other class of metalloproteins, it is natural that the illustrative applications presented here be focused on these systems. However, excellent summaries of applications to other metalloproteins and their model compounds are available, including copper proteins,iron-sulfur proteins, and non-heme oxo-iron clusters. ... 

Y. Guo, Initial studies on iron sulfur proteins and related model compounds via 57Fe and 6INi synchrotron radiation based perturbed angular correlations (SRPAC), in nuclear resonant scattering on nitrogenase, hydrogenase and model systems, Ph.D. thesis, 2009. 

For updating the information presented in this chapter, a literature search on the keyword nitrogenase modified with structure, X ray, Mossbauer, iron sulfur cluster, or model compound will generate citations referring to the newest research results. A search of the Protein Data Bank (PDB) at the website address http //www.rcsb.org/pdb/ will yield the latest updates on X ray, NMR, and other submitted structural data. 

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