Iron-sulphur clusters Rubredoxins

Fig. 1. Schematic representation of the four basic types of iron-sulphur clusters. The rubredoxin-type and the [2Fe-2S], [3Fe—4S] and [4Fe-4S] clusters are shown in (a) to (d), respectively. Reprint from Prog. Biophys. Mol. Biol., Vol. 70, H. Sticht and P. Rosch, The structure of iron-sulfur proteins , pp. 95-136, Copyright 1998, with permission from Elsevier Science.

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.). 

Iron-sulphur clusters are the third type of the widely available electron-transfer sites in biology. They consist of iron ions surrounded by four sulphur ions, either thiolate groups from cysteine residues or inorganic sulphide ions. Regular clusters with one (rubredoxins), two, three, or four (ferredoxins) iron ions are known, as well as a number of more irregular clusters, also with other ligands than cysteine [112,181]. Their reduction potentials vary between -700 and +400 mV [112]. 

The electronic stmcture, spectroscopy, and reduction potential have been thoroughly studied for all common classes of iron-sulphur clusters [52,89,182-191]. In particular, Noodleman and coworkers have performed detailed quantum chemical calculations on iron-sulphur clusters in various spin states [192-198]. It is now settled that rubredoxin contains an iron ion in the high-spin state (quintet for Fe , sextet for Fe ), whereas in the [2Fe-2S] clusters, the two iron ions are both in the high-spin state, but antiferromagntically coupled to form a singlet or doublet state for the oxidised (Ill+ni) and reduced (mixed-valence n+ni) forms, respectively [112,162]. In variance to the Cua site, the unpaired spin is trapped at one of the iron ions in the mixed-valence state. 

At first, the increase in reorganisation energy for the dimeric iron-sulphur clusters (coii jtu ed to the monomeric rubredoxin site) may seem a bit strange. 

FIGURE 9.8. Structures of iron-sulphur centers of rubredoxin and ferredoxin. Rubredoxins are the simplest iron-sulphur proteins, and these contain one Fe-S center with the iron in a tetrahedral environment. Ferredoxins contain clusters of two [2Fe-2S] or four [4Fe-4S] iron atoms. 

---