Semiquinone-iron interactions

For purposes of discussing the EPR of iron-semiquinone interactions in reaction centers, simplified simulations of the spectra are given in Figure 14. The parameters chosen for the simulation are similar to those of Calvo et al. [36], The spin Hamiltonian relevant to a one-electron reduced Qa-Fc-Qb system has terms involving the iron zero-field splitting and the semiquinone-iron electron spin-spin interactions ... 

Although a low potential iron-semiquinone was not observed in EPR titrations of P. laminosum changes in the appearance of the g=1.9 signal occurred at potentials below -200mV. At a similar potential a rise of about 20 to 25% of the maximum signal was also observed in the split radical signal (the pheophytin doublet radical thought to be due to interaction between the semiquinone and reduced pheophytin). 

There are various pathways for free radical-mediated processes in microsomes. Microsomes can stimulate free radical oxidation of various substrates through the formation of superoxide and hydroxyl radicals (the latter in the presence of iron) or by the direct interaction of chain electron carriers with these compounds. One-electron reduction of numerous electron acceptors has been extensively studied in connection with the conversion of quinone drugs and xenobiotics in microsomes into reactive semiquinones, capable of inducing damaging effects in humans. (In 1980s, the microsomal reduction of anticancer anthracycline antibiotics and related compounds were studied in detail due to possible mechanism of their cardiotoxic activity and was discussed by us earlier [37], It has been shown that semiquinones of... 

The rationale for studies on flavin semiquinone metal interactions stems from the presence of flavin coenzymes which participate in electron transfer in a number of metalloflavoproteins. Iron-containing redox centers such as the heme and nonheme iron sulfur prosthetic groups (Fe2/S2, Fe+ZS, or the rubredoxin-type of iron center) constitute the more common type of metal donor-acceptor found in metalloflavoproteins, although molybdenum is encountered in the molybdenum hydroxylases (e.g. xanthine oxidase, aldehyde dehydrogenase). 

Although at pH 8 the electron distribution favours the formation of flavin semiquinone and reduced iron-sulfur center, the magnetic moments of the two redox centers do not interact. At pH 10, however, 2-electron-reduced TMADH exhibits the EPR spectrum diagnostic of the spin-mteracting state. In a more detailed analysis using the pH-jump technique, the interconversion of three states of TMADH [state 1, dihy-droflavin-oxidised 4Fe-4S center (formed at pH 6) state 2, flavin semi-quinone-reduced 4Fe-4S center (formed at pH 8) state 3, spin interacting state (formed at pH 10)] were studied in both H2O and D2O (Rohlfs et al., 1995). The kinetics were found to be consistent with a reaction mechanism that involves sequential protonation/deprotonation and electron transfer events (Figure 6). Normal solvent kinetic isotope effects were observed and proton inventory analysis revealed that at least one proton is involved in the reaction between pH 6 and 8 and at least two protons are involved between pH 8 and 10. At least three protonation/... 

The primary acceptor in PS II is a plastoquinone, PQ, as ascertained from optical absorbance difference spectroscopy [46], Until recently, the EPR spectrum of the semiquinone escaped observation, and only the advent of preparation methods for PS II subchloroplast particles made its recording possible. As surmised earlier, the spectrum of the intact acceptor [47] very much resembled the very broad qui-none-iron acceptor complex in purple bacteria, whereas in iron-depleted PS II particles the narrow spectrum typical of an immobilized semiquinone was found [48], As in the bacterial photosystem, flash-induced reduction of Q, of the second quinone, Qb, or of both resulted in somewhat different EPR spectra, indicative of structural changes that influence the magnetic interaction between the semiquinone and the non, and/or between the two semiquinones [49],... 

Cytochromes, as components of electron transfer chains, must interact with the other components, accepting electrons from reduced donor molecules and transferring them to appropriate acceptors. In the respiratory chain of the mitochondria, the ubiquinolxytochrome c oxidoreductase, QCR or cytochrome bc complex, transfers electrons coming from Complexes 1 and 11 to cytochrome c. The bc complex oxidises a membrane-localised ubiquinol the redox process is coupled to the translocation of protons across the membrane, in the so-called proton-motive Q cycle, which is presented in a simplified form in Figure 13.14. This cycle was first proposed by Peter Mitchell 30 years ago and substantially confirmed experimentally since then. The Q cycle in fact consists of two turnovers of QH2 (Figure 13.14). In both turnovers, the lipid-soluble ubiquinol (QH2) is oxidized in a two-step reoxidation in which the semiquinone CoQ is a stable intermediate, at the intermembrane face of the mitochondrial inner membrane. It transfers one electron to the Rieske iron—sulfur protein (ISP), one electron to one of the two cytochrome b haems (bi), while two protons are transferred to the intermembrane space. In both of the Q cycles, the cytochrome bi reduces cytochrome bfj while the Reiske iron—sulfur cluster reduces cytochrome c/. The cytochrome ci in turn reduces the water-soluble cytochrome c, which transfers its electrons to the terminal oxidase, cytochrome c oxidase, described above. In one of the two Q cycles, reduced cytochrome bf reduces Q to the semiquinone, which is then reduced to QH2 by the second reduced cytochrome bn- The protons required for this step are derived from the matrix side of the membrane. The overall outcome of the two CoQ cycles (10) (/ — matrix o — intermembrane space) is... 

In photosynthetic bacterial reaction centers, an iron atom is located between the two quinones and interacts magnetically with the reduced semiquinone to yield a broad EPR signal with a pronounced peak at g=1.8, as discussed in the previous chapter. This EPR signal upon flash excitation displays an oscillatory pattern with a periodicity of 2, similar to that in Pig. 3 (B). We modify the equation shown in the previous section by incorporating the iron atom into the representation of the reaction center and indicate the observed magnetic interaction of iron with the semiquinone by placing these two species inside a pair of braces, , as shown in Fig. 4, top. 

The main absorption band of benzoquinones appears around 260 nm in nonpolar solvents and at 280 nm iu water. Extinction coefficients are 1.3-1.5 x 10 M Upon reduction to hydroquinones, a four times smaller band at 290 nm is found. The most important property of quinones and related molecules is the relative stability of their one-electron reduction products, the semiquinone radicals. The parent compound 1,4-benzoquinone is reduced by FeCl, ascorbic acid, and many other reductants to the semiquinone anion radical which becomes protonated in aqueous media (pk = 5.1). Comparisons of the benzaldehyde reduction potential with some of the model quinones given below show that carbonyl anion radicals are much stronger reductants than semiquinone radicals and that ortho- and para-benzoquinones themselves are even relatively strong oxidants comparable to iron(III) ions in water (Table 7.2.1). This is presumably caused by the repulsive interactions between two electropositive keto oxygen atms, which are separated only by a carbon-carbon double bond. When this positive charge can be distributed into neighboring n systems, the oxidation potential drops significantly (Lenaz, 1985). 

Due to the fact that the arylutea type herbicides, such as diuron and monuron, did not inhibit the wild type bacterial reaction centers, the predictions have been based mainly upon mutations of the Qp-binding domain, which was affected by interaction with diuron. For example, the characterization of the herbicide-resistant mutants from Bps. viridis has revealed that one of the mutants, T4 (Tyr L222 to Phe) was sensitive to the urea type inhibitors similar to the D1 protein of PSII reaction centre. The semiquinone-iron electron paramagnetic resonance (EPR) signal of Qp in viridisTA mutants was also similar to that reported for photosystem II. 

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