Rubredoxin reduction potential

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. 

FIGURE 3.6 Determination of the reduction potential of rubredoxin by electrochemistry and by EPR monitored bulk titration. The (+) data are from cyclic voltammograms taken at different temperatures the ( ) point is from low temperature EPR monitored titrations at ambient temperature or at 80°C. (Data from Hagedoorn et al. 1998.)... 

Are biomacromolecules of greater mass than that of rubredoxin (6 kDa), in particular enzymes (typically, >50 kDa), capable of such rapid conformational adjustment with decreasing temperature At present the answer appears to be We do not know. Unfortunately, the reduction potentials) of enzymes in solution is not usually determinable with direct electrochemistry, so you are invited to find and explore other molecular properties to probe as a function of temperature, for example, (de) protonations near paramagnetic sites that can be followed both by optical and by EPR spectroscopy. 

Blue copper proteins, 36 323, 377-378, see also Azurin Plastocyanin active site protonations, 36 396-398 charge, 36 398-401 classification, 36 378-379 comparison with rubredoxin, 36 404 coordinated amino acid spacing, 36 399 cucumber basic protein, 36 390 electron transfer routes, 36 403-404 electron transport, 36 378 EXAFS studies, 36 390-391 functional role, 36 382-383 occurrence, 36 379-382 properties, 36 380 pseudoazurin, 36 389-390 reduction potentials, 36 393-396 self-exchange rate constants, 36 401-403 UV-VIS spectra, 36 391-393 Blue species... 

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. 

UV-VIS Spectra Reduction Potentials Active-Site Protonations Charge on Proteins Self-Exchange Rate Constants Electron Transfer Routes Comparison with Rubredoxin Summary References... 

Lin IJ, Gebel EB, Machonkin TE, Westler WM, Markley JL (2003) Correlation between hydrogen bond lengths and reduction potentials in Clostridium pasteurianum rubredoxin. J Am Chem Soc 125 1464-1465... 

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 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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