Rubredoxin redox potential

One example of a sequence determinant of redox potentials that has been identified in this manner is an Ala-to-Val mutation at residue 44, which causes a 50 mV decrease in redox potential (and vice versa) in the rubredoxins [68]. The mutation was identified because the sum of the backbone contributions to ( ) of residues 43 and 44 change by 40 mV due to an —0.5 A backbone shift away from the redox site. This example points out the importance of examining the backbone contributions. The corresponding site-specific mutants have confirmed both the redox potential shift [75] and the structural shift [75]. 

While the oxidation reduction potential of the ferredoxins is —0.2 V to —0.4 V and that of the rubredoxins is about —0.05 V, a protein from the photosynthetic bacterium Chromatium has a redox potential of +0.35 V. This is the high potential iron protein, or HIPIP. 

V) are more biologically relevant because of the solvent and associated hydrogen-bonding properties. However, specific interactions from the protein cannot be fully reproduced. Indeed, the redox potential of [Fe(SCH2CH20H)4] f in water is close to the lower limit of FeS4 sites in rubredoxin proteins (—0.1 to +0.1 V versus standard hydrogen electrode (SHE)). 

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. 

The difference in redox potential and in thermal stability between native rubredoxin and the simple model complexes has been suggested to be brought about by the different protein environments (18). This is as yet unproved, however. The amino acid sequences of many rubredoxins isolated from various sources have been determined, as shown in Fig. 5 (19). A sequence around the Fe active site, Cys-X-Y-Cys, is an invariant fragment and primarily determines the chemical and physical properties. For example, C. pasteurianum rubredoxin has such sequences, Cys6-Thr-Val-Cys9 and Cys39-Pro-Leu-Cys42. 

Low DW, Hill MG. Rational fine-mning of the redox potentials in chemically synthesized rubredoxins. J. Am. Chem. Soc. 1998 120 11536-11537. 

Rubredoxin type proteins have a narrow range of redox potentials around 0 mV. The value determined for D.gigas desulforedoxin is within this range (— 35 mV)50). 

Notice the large difference in the potential of cytochrome c and rubredoxin (Figure 1.5), 0.25 volts vs. —0.06 volts, respectively. In polynuclear ferredox-ins, in which each iron is tetrahedrally coordinated by sulfur, reduction potentials are near —0.4 volts. Thus, the entire range of redox potentials, as illustrated in Table 1.4, is more than one volt. 

The redox potential determined for both iron sites of Rr is over 200 mV, which is too high for Rr to be involved in electron transfer reactions in the cytoplasm of Desulfovibrio. The much higher redox potential of the Rr Rd-like center in comparison to those exhibited by rubredoxins (around OmV) is intriguing if one considers the similar spectroscopic characteristics of both centers [58]. 

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. 

The complete structure of clostridial rubredoxin has been solved by Jensen and his associates using x-ray analysis (49), and the structure of the iron-sulfur center of the high potential iron protein, but not of the polypeptide, has been determined by x-ray analysis by Kraut and associates (50). Both of these are relatively atypical proteins in this class, either having no inorganic sulfide or having a very high redox potential. So far, we still have no solution from x-ray work for the 2-iron-2-sulfur or the 8-iron-8-sulfur type protein. 

Rubredoxin a redoxin, functionally similar to Fer-redoxin (see). M, 6,000. R. was isolat from Clostridium pasteurianum synthesis of R. appears to be promoted by a relative deficien<7 of iron. It contains one Fe atom/molecule of protein, which is less than the Fe content of ferredoxin. Under acid conditions it is more stable than ferredoxin, and it has a more positive redox potential (E o -0.057 V) thus when R. replaces ferredoxin in a feiredoxin-dependent reaction the reaction rate is decreased. The iron is bound by coordination with 4 cysteinyl residues other possible ligands are tyrosine and lysine. Redoxins similar to R. have been isolated from Peptostreptococcus elsde-nii and other baeteria. R. from Micrococcus aerogenes contains 53 amino acid residues of known sequence. 

Redox Proteins.—The study of redox proteins by ilf-ray analysis has been very productive during the year. High-resolution structures of ferri- and ferro-cytochrome c, cytochrome b, cytochrome c, ferredoxin, rubredoxin, high-potential iron protein, and two flavodoxins have now been worked out, although not all were published in 1971. 

Figure 7.20 shows the ranges of redox behavior known for Fe-S centers. Clearly, the Fe-S systems can cany out low-potential processes. The rubredoxins cover the mid-potential range, and the HiPIPs are active in the high-potential region. 

The advantage of ferredoxins over rubredoxins in terms of redox chemistry is that by combining several Fe centres in close proximity, it is possible to access a greater range of reduction potentials. Different conformations of the protein pockets which surround the Fe S clusters affect the detailed structural features of the cluster cores and, thus, their reduction potentials, e.g. —420 mV for spinach [2Fe-2S] ferredoxin, and —270 mV for adrenal [2Fe-2S] ferredoxin. A [2Fe-2S] ferredoxin acts as a one-electron transfer centre, going from an Fe(II)/Fe(II) state in the reduced form to an Fe(II)/Fe(III) state when oxidized and vice versa. Evidence for the localized, mixed valence species comes from EPR spectroscopic data. 

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