Rubredoxin effect

The effective distances obtained by Nordenskiold et al. (40) are compared with the internuclear distances in Table I. Clearly, the point dipole approximation is reasonable for the hydrogen nuclei in these complexes, while substantial deviations are observed for the oxygen nuclei. The findings of these early quantum chemical studies were confirmed by Sahoo and Das (41-43). Wilkens et al. have reported DFT calculations using Eq. (16) for a 104 atom model for high-spin Fe(III) rubredoxin (44). Large discrepancies between the effective distances and the input distances for the calculations were found for the hyperfine-shifted nitrogen-15 resonances, as well as for proton and carbon-13 nuclei in cysteines bound to the iron center. 

Fe-xS] ferredoxins, 33 54-55 rubredoxin, 33 44-51 hydrophobic effect, 33 60-62 significance, 33 40-44 metal complexes of, 7 218-220 model complexes, catalysis by, 33 61-62 Peptococcus aerogenes ferredoxin, structure, 38 242, 244-245... 

FIGURE 1.27 Sulfur K-pre-edge XAS59 for (a) Fe "(SR)4 model complex, (b) the model compared to several rubredoxins to illustrate the effect of the protein environment in reducing covalency, and (c) Fe2S2-type complexes with RS4 (black) replaced by Cl- (red) or lS2 by pSe2- (blue). (See the color version of this figure in Color Plates section.)... 

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). 

The hydroxylation reaction is directly effected by an enzyme-hemoprotein, monooxigenase, cytochrome P450 containing protocheme IX. The reduction of the enzyme involves flavin reductases and electron carriers, such as adrenodoxin, rubredoxin, and cytochrome b5. Dioxygen, being a weak one-electron oxidant, is activated after the reduction in the enzyme heme coordination sphere. The various forms of cytochrome P450 from liver microsomes and from Pseudomonas putida have a molecular mass of about 49000. One of the subunits of the enzyme from mitochondria of... 

III. Chelating Effects of Peptide Ligands A. Rubredoxin Peptide Model Complexes... 

The chelation effect also brings about a stabilization of the — 1 state of the peptide model complexes as indicated by the thermal stability and redox behavior. Only [Fe(Z-cys-Pro-Leu-cys-OMe)2] exhibits a relatively reversible redox couple in the cyclic voltammogram measurement, but the others do not (20). The bulkiness of side chains of the X and Y residues in Cys-X-Y-Cys probably restricts the adoption of the inherent by preferable conformation (ift = 0°), resulting in a more restricted orientation of Fe-S-C. In fact, the X-ray analysis of native rubredoxin shows that two of the Fe-S torsion angles are restricted and the other two are normal, i.e., conformationally more stable. 

There are certain practical aspects that need to be taken into account in assessing the significance of temperature factors. Errors in measurement of intensities, arising for example from incomplete correction of radiation damage or absorption, will have more serious effects on temperature factors than on atomic positions. The data must extend to a resolution of better than 2 A, otherwise temperature factors tend to be underestimated. Restraints in the refinement which assume that positional disorders of bonded atoms are highly correlated may bias the results. However, it is encouraging that in several structures (e.g., rubredoxin [193] and avian pancreatic polypeptide [194]) where the restraints were relaxed these assumptions were found to be valid. 

Lode and Coon (238) have found no significant optical or EPR spectroscopic difference between the iron-saturated (2-Fe) and partially iron-saturated (1-Fe) Ps. oleovorans rubredoxin suggesting that both iron atoms are located in similar environments. The two species of rubredoxin exhibit the characteristic g = 9.4 and g=4.3 spin resonances observed in the anaerobic bacteria rubredoxins and these have been attributed to transitions within the lowest and middle Kramer s doublets of the high spin ferric ion, respectively (227). The optical absorption, ORD and CD spectra of the reduced protein show bands and Cotton effects, respectively, which are similar to those of the 6,000 dalton rubredoxins. 

The structural parameters for the oxidized rubredoxin analogues are very similar to those of the oxidized Rd iron site. The reduced complexes reveal a lengthening of the average Fe-S bond from 2.27 to 2.36 A, consistent with the change in oxidation state from ferric to ferrous. The addition of an electron has a more profound structural effect in this single-iron center than in some of the multiiron clusters, where electrons are more delocalized. 

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