Rubredoxin iron ligands

In this review we will deal with iron-sulfur proteins where the iron atoms are coordinated only by cysteine ligands and bridging sulfurs, as well as rubredoxin (Rd hereafter), which is the initial building block in all subsequent discussions. 

In recent years, several model complexes have been synthesized and studied to understand the properties of these complexes, for example, the influence of S- or N-ligands or NO-releasing abilities [119]. It is not always easy to determine the electronic character of the NO-ligands in nitrosyliron complexes thus, forms of NO [120], neutral NO, or NO [121] have been postulated depending on each complex. Similarly, it is difficult to determine the oxidation state of Fe therefore, these complexes are categorized in the Enemark-Feltham notation [122], where the number of rf-electrons of Fe is indicated. In studies on the nitrosylation pathway of thiolate complexes, Liaw et al. could show that the nitrosylation of complexes [Fe(SR)4] (R = Ph, Et) led to the formation of air- and light-sensitive mono-nitrosyl complexes [Fe(NO)(SR)3] in which tetrathiolate iron(+3) complexes were reduced to Fe(+2) under formation of (SR)2. Further nitrosylation by NO yields the dinitrosyl complexes [(SR)2Fe(NO)2], while nitrosylation by NO forms the neutral complex [Fe(NO)2(SR)2] and subsequently Roussin s red ester [Fe2(p-SR)2(NO)4] under reductive elimination forming (SR)2. Thus, nitrosylation of biomimetic oxidized- and reduced-form rubredoxin was mimicked [121]. Lip-pard et al. showed that dinuclear Fe-clusters are susceptible to disassembly in the presence of NO [123]. 

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... 

C) cuboidal three-iron-four-sulfide [Fe3-S4] clusters—stable oxidation states are 0 and + 1 and (D) cubane four-iron-four-sulfide [Fe4-S4] clusters—stable oxidation states are + 1 and +2 for ferredoxin-type clusters and +2 and +3 for HIPIP clusters. Electrons can be delocalized, such that the valences of individual iron atoms lie between ferrous and ferric forms. Low-molecular-weight proteins containing the first and the last three types are referred to as rubredoxins (Rd) and ferredoxins (Fd), respectively. The protein ligands are frequently Cys residues, but a number of others are found, notably His, which replaces two of the thiol ligands in the [Fe2-S2] Rieske proteins. In addition to these, discrete Rd... 

Fig. 58. Stereo drawing of the rubredoxin backbone with the iron (filled circle) and its cysteine sulfur ligands and all the water molecules (open circles) identified during refinement of the structure at 1.2 A resolution. Adapted from Watenpaugh et al. (1979), Fig. 11, with permission.

Active Site Structure of Rubredoxin There are several non-heme iron-sulphur proteins that are involved in electron transfer. They contain distinct iron-sulphur clusters composed of iron atoms, sulphydryl groups from cysteine residues and inorganic or labile sulphur atoms or sulphide ions. The labile sulphur is readily removed by washing with acid. The cysteine moieties are incorporated within the protein chain and are thus not labile. The simplest type of cluster is bacteria rubredoxin, (Cys-S)4 Fe (often abbreviated FelSO where S stands for inorganic sulphur), and contains only non labile sulphur. It is a bacterial protein of uncertain function with a molecular weight of 6000. The single iron atom is at the centre of a tetrahedron of four cysteine ligands (Fig.). 

The prosthetic groups of iron-sulfur proteins fall into several classes (Lov-enberg, 1977 Spiro 1982). Rubredoxins bind single iron atoms with four cys-teinyl sulfur ligands they function as electron carriers in some bacterial systems. Rubredoxins generally have two such centers per molecule in the ferric state each iron center is EPR detectable. 

Iron is the most abundant metal on earth and the commonest electron transfer agents involve iron complexes. Life is thought to have evolved in reductive conditions, in which the dominant form of iron would be as iron sulfide, not iron oxide. The simplest forms of electron transfer agents (found in plants and bacteria) involve iron with thiolate ligands. Some simple electron transfer proteins, such as rubredoxin, contain a single iron centre in an S4 donor environment within a protein (Fig. 10-7). 

The active site of rubredoxin (Rd), (I) (Figure 1), consists of one Fe ion tetrahedraUy coordinated by four cysteine thiolate ligands. The iron core serves as the electron-transport site of rubredoxin proteins. 

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