Reduction potential cytochromes

Hess measured the apparent equilibrium constant for reduction of cytochrome by ferroeyanide as a function of pH 185), and found that the actual reduction step did not involve a proton and was entirely independent of pH. The free energy of reduction, and hence the reduction potential, change with pH only because the ratio of amounts of state III and IV changes with pH. The observed decrease of cytochrome reduction potential of 60 mV per pH unit above pH 8 is exactly what would be calculated from the simple Nernst equation. 

A decade after the discovery of the Rieske protein in mitochondria (90), a similar FeS protein was identified in spinach chloroplasts (91) on the basis of its unique EPR spectrum and its unusually high reduction potential. In 1981, the Rieske protein was shown to be present in purified cytochrome Sg/complex from spinach (92) and cyanobacteria (93). In addition to the discovery in oxygenic photosynthesis, Rieske centers have been detected in both single-RC photosynthetic systems [2] (e.g., R. sphaeroides (94), Chloroflexus (95)) and [1] (Chlo-robium limicola (96, 97), H. chlorum (98)). They form the subject of a review in this volume. 

Superoxide generation was detected via the NADPH-dependent SOD-inhibitable epinephrine oxidation and spin trapping [15,16], Grover and Piette [17] proposed that superoxide is produced equally by both FAD and FMN of cytochrome P-450 reductase. However, from comparison of the reduction potentials of FAD (-328 mV) and FMN (190 mV) one might expect FAD to be the most efficient superoxide producer. Recently, the importance of the microsomal cytochrome h558 reductase-catalyzed superoxide production has been shown in bovine cardiac myocytes [18]. 

However, to be a quantitative assay of superoxide detection, Reaction (1) had to be an exothermic reaction, i.e., the difference between the one-electron reduction potentials of reagents AE° = / °[02 /02] / °[A /A] must be <0. In this case the rate constants of Reaction (1) will be sufficiently high (10s—109 1 mol 1 s ). Among traditionally applied assays, three compounds satisfy this condition cytochrome c, lucigenin, and tetranitromethane (Table 32.1). 

The resulting hydroquinone (QH2) then diffuses to the cytochrome be complex, which oxidises QH2 back to Q, using the resulting reduction potential, via cytochrome c, to reduce the special pair and hence regenerate the reaction centre. 

A variety of physical methods has been used to ascertain whether or not surface ruthenation alters the structure of a protein. UV-vis, CD, EPR, and resonance Raman spectroscopies have demonstrated that myoglobin [14, 18], cytochrome c [5, 16, 19, 21], and azurin [13] are not perturbed structurally by the attachment of a ruthenium complex to a surface histidine. The reduction potential of the metal redox center of a protein and its temperature dependence are indicators of protein structure as well. Cyclic voltammetry [5, 13], differential pulse polarography [14,21], and spectroelectrochemistry [12,14,22] are commonly used for the determination of the ruthenium and protein redox center potentials in modified proteins. 

The midpoint reduction potential of cytochrome c and the kinetics of its reduction by Fe(EDTA) are also significantly influenced by substitutions at Phe-82. As measured by direct electrochemistry at pH 6 to eliminate any... 

To evaluate the functional role(s) of this residue, six mutations, Lys, His, Glu, Asn, Leu, and Ala, were introduced at this site, and the electrochemical and NMR properties of the resulting proteins examined [134]. Contrary to expectation, removal of Arg-38 did not result in a change in the dependence of the cytochrome c reduction potential on pH. Instead, as the electron-withdrawing ability of the residue substituted at position-38 decreased, the reduction potential also decreased, with the greatest decrease (50 mV) observed for the Ala mutant. The variation of reduction potential with pH, however, remained essentially the same as that previously observed for the wild-type protein. 

The kinetics of the reactions of horse cytochrome c(II), M, 12,400, (charge 8+) reduction potential 260 mV, with parsley and French bean plastocyanins PCu(II) (charges — 7 and — 8 respectively), have been studied. As in the case of HIPIP, cytochrome c is not a physiologically relevant protein. It is nevertheless important in assessing different approaches prior to investigating the reactions of physiological redox partners. In the case of the reaction of parsley PCu(II) with cytochrome c(II), the rate constant (25 °C) is 1.5 X 10 s at pH7.6, 1 = 0.10 M(NaCl) [141]. There is no evidence... 

Qrunones can accept one or two electrons to form the semiquinone anion (Q ") and the hydroquinone dianion (Q ). Single-electron reduction of a quinone is catalyzed by flavoenzymes with relatively low substrate selectivity (Kappus, 1986), for instance NADPH cytochrome P-450 reductase (E.C. 1.6.2.3), NADPH cytochrome b5 reductase (E.C. 1.6.2.2), and NADPH ubiquinone oxidoreductase (E.C. 1.6.5.3). The rate of reduction depends on several interrelated chemical properties of a quinone, including the single-electron reduction potential, as well as the number, position, and chemical characteristics of the substituent(s). The flavoenzyme DT-diphorase (NAD(P)H quinone acceptor oxidoreductase E.C. 1.6.99.2) catalyzes the two-electron reduction of a quinone to a hydroquinone. 

In vitro studies established that electrons are transferred efficiently from both NiFe- and iron-only hydrogenases to cytochrome C3. This small 13 kDa c-type cytochrome has four hemes (reduction potential -200 to -300 mV)... 

Care should be exercised that excess of one reactant does in fact promote irreversible reaction if this is the desired object, otherwise invalid kinetics and mechanistic conclusions will result. Consideration of the reduction potentials for cytochrome-c Fe(III) and Fe(CN)g (0.273 V and 0.420V respectively) indicates that even by using a 10 -10 fold excess of Fe(CN)j , reduction of cytochrome-c Fe(III) will still not be complete. An equilibrium kinetic treatment is therefore necessary. ... 

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