Rubredoxins bacterial

Figure 8.39 Fourier transformed Fe extended X-ray absorption fine structure (EXAFS) and retransformation, after applying a 0.9-3.5 A filter window, of (a) a rubredoxin, (b) a plant ferredoxin and (c) a bacterial ferredoxin, whose structures are also shown. (Reproduced, with permission, Ifom Teo, B. K. and Joy, D. C. (Eds), EXAFS Spectroscopy, p. 15, Plenum, New York, 1981)...

Figure 25.9 Some non-haem iron proteins (a) rubredoxin in which the single Fe is coordinated, almost tetra-hedrally, to 4 cysteine-sulfurs, (b) plant ferredoxin, [Fe2S2(S-Cys)4], (c) [Fe4S4(S-Cys)4] cube of bacterial ferredoxins. (This is in fact distorted, the Fe4 and S4 making up the two interpenetrating tetrahedra, of which the latter is larger than the former).

Fig. 1. Iron-sulfur clusters basic building blocks. In most cases the iron is tetrahe-drally coordinated by sulfur from cysteinyl residues (and labile sulfur). Variability on coordination is allowed (see text). A, Rubredoxin type FeS4 (simplest cluster, no labile sulfur) B, plant-type ferredoxin [2Fe-2S] C, bacterial ferredoxin [3Fe-4S] D, bacterial ferredoxin and HiPIP [4Fe-4S] E, novel cluster [4Fe-2S, 20] ( hybrid cluster ).

The simplest of these proteins are rubredoxins, which are bacterial proteins having a characteristic red colour (from which their name is derived) containing an FeS4 assembly, consisting of an Fe(III) ion coordinated to four cysteine groups. The typical tetrahedral structure of this group is illustrated in Figure 17 for the rubredoxin isolated from Clostridium pasteurianum (FW 6100).35... 

At this point it may be valuable to digress a moment and discuss the state-of-the-knowledge in the field of Fe-S proteins by the ntid-1970 s. At this time there were three known structures found in nature, IFe as represented by rubredoxin, the [2Fe-2S] cluster as represented by plant ferredoxins, and the [4Fe-4S] cluster as found in many bacterial ferredoxins (24). The schematic structures and selected properties are listed in Table I. 

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 Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedrally coordinated by a combination of thiolate and sulfide donors. Of the 10 or more structurally characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein rubredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane structures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane structure (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH3 at a MoFe S8 homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

From the biological area, iron-sulfur clusters in biomolecules such as rubredoxin mononuclear Fe-S clusters (Rao et at., 1972), plant-type ferredoxin 2Fe-2S clusters (Johnson, 1975) and bacterial-type ferredoxin 4Fe-2S clusters (Thompson et at., 1974) are readily distinguished from one another by their Mossbauer spectra. The temperature dependence of relaxation effects can provide information about the types of internuclear interaction and can even lead to estimates of the distance between paramagnetic sites, for example, the two 4Fe-4S clusters in ferredoxin in Peptococcus aerogenes (Adman etal., 1973). 

Recently, another red protein Buchanan, Lovenberg, and Rabinowitz 32) Mortenson 72) has been isolated in crystalline form from C. pasteurianum and certain of its properties determined Lovenberg and Sobel 67)). Lovenberg and Sobel 67) named it rubredoxin because of its color and properties of an electron carrier. They showed rubredoxin differed from ferredoxin in absorption spectrum, composition and redox potential. Rubredoxin contained no inorganic sulfide the recent demonstration 49) of the similarity of the optical rotatory dispersion spectra of rubredoxin and bacterial ferredoxin makes a further comparison of the properties of these proteins particularly interesting. 

Fig. 19.13 Iron-sulfur clusters in ferredoxins (a) FC S in bacterial rubredoxin (b) Fe S photosynihetic ferredoxin (c) Fc4S4 in cubane-like ferredoxin.

ORD spectra (300—600 nm) of bacterial ferredoxin and rubredoxin suggested similar asymmetric environments for both non-heme iron proteins. Spectral differences with plant ferredoxin were attributed to transitions involving the iron-sulfur chromophores (116). Subsequent CD measurements in the range 215—800 nm resolved more differences between oxidized bacterial ferredoxin and rubredoxin however, the spectra were considered to be sufficiently alike to support the existence... 

Iron-sulfur proteins occur in animal, plant, and bacterial cells. The proteins are characterized by the presence of 1-0, 2-2, 4rA, 6-6, or 8-8 atoms of iroursulfide. Only the structure of clostridial rubredoxin, a 1-0 protein, is knoum. It contains iron ligated to four sulfur atoms of cysteine residues of the polypeptide. With the exception of the ""high potential iron protein, all the proteins show unexpectedly low redox potentials and function in biological oxidationr-reduc-tion reactions. 

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