Hydrogen, redox potential sulfur

A second example is that of an Ala-to-Cys mutation, which causes the fonnation of a rare SH S hydrogen bond between the cysteine and a redox site sulfur and a 50 mV decrease in redox potential (and vice versa) in the bacterial ferredoxins [73]. Here, the side chain contribution of the cysteine is significant however, a backbone shift can also contribute depending on whether the nearby residues allow it to happen. Site-specific mutants have confirmed the redox potential shift [76,77] and the side chain conformation of cysteine but not the backbone shift in the case with crystal structures of both the native and mutant species [78] the latter can be attributed to the specific sequence of the ferre-doxin studied [73]. 

The lower redox potential in menaquinone-oxidizing be complexes can be attributed to the absence of the hydrogen bond from the Oy of Ser 163 (ISF)/Ser 130 (RFS) to the bridging sulfur S-1 (see Section III,B,2). This serine is completely conserved in Rieske proteins of bci and bef complexes, but in all sequences of Rieske proteins from menaquinone-oxidizing be complexes, alanine or glycine is found in... 

Fig. 4. Representation of the ligand sphere of the [2Fe-2S] cluster of the Rieske protein from spinach and the attribution of g-tensor to moleculEir axes as discussed in the text. Ser 130 has been observed to influence the redox potentiEd of the cluster via hydrogen interactions with the acid-labile bridging sulfur.

Some detailed comparisons of the protein environments around the HiPIP and Fd clusters have been made.769,770 It is noteworthy that the HiPIP cluster is more deeply buried (about 4.5 A) than is the case for the clusters in the other iron-sulfur proteins. All iron-sulfur proteins for which structural data are available, with the exception of the three-iron protein from Azotobacter vinelandii, have hydrogen bonding between the cysteine sulfur in the iron-sulfur cluster and the backbone peptide link. It appears that there is an approximate correlation between the number of NH S hydrogen bonds in the environment of a cluster and its redox potential. In HiPIP, these hydrogen bonds become more linear and shorten on reduction of the cluster. It is possible, therefore, that the oxidation states of the cluster may be controlled by the geometries of the hydrogen bonds.770... 

The clusters thus generated have been found to have properties closely matching those of native proteins. Further studies have shown that N-H- -S hydrogen bonding to sulfur atoms within the cluster creates a small positive shift in redox potentials. The sequence Cys-(X)3-Cys-(X)2-Cys-(X)2-Cys, which includes sequence (6) and is often found in ferredoxins, has been shown to support efficient cluster reconstitution. ... 

The behavior of the redox potential oscillations under the effects of hydrogen peroxide, iodate ions, Mn(II) ions, acetone, and sulfuric acid was examined. The interrelationship between redox potential oscillations and iodine oscillations was studied. Both the chloride ion and Cu(II) inhibit the iodate-hydrogen peroxide reaction where Mn(II) catalyzed production of iodine is the key step. 

The simple constitution of the active center, iron and sulfur, contrasts with the diversified role played by these proteins in key biological oxidation-reduction processes, such as carbon, hydrogen, sulfur and nitrogen metabolism, using a very wide range of redox potentials (+ 350 mV in photosynthetic bacteria to — 600 mV in chloro-plasts). 

Iron-Sulfur Electron Transfer Proteins.31 Ferredoxins. These relatively small proteins (6,000-12,000) which contain non-heme iron, cysteine-sulfur, and so-called inorganic sulfur have redox potentials close to that of the standard hydrogen electrode. They appear to occur in all green plants, including algae, in all photosynthetic bacteria and protozoa and in some fermentative anaerobic bacteria. These molecules play an essential role as electron-transfer agents at the low-potential end of the photosynthetic process, but an exact chemical specification of their activity is still lacking. 

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