Electron and proton transfers through quinones and cytochrome bc complexes

Vitamin E (α-tocopherol), the principal chain-breaking antioxidant in biological membranes, prevents toxicant- and carcinogen-induced oxidative damage by trapping reactive oxyradicals. Although α-t...

The role of metabolism in the antioxidant function of vitamin E.

The electron transfer reactions involving quinones, hydroquinones, and catechols are very important in many areas of chemistry, especially in biological systems. The therapeutic efficiency as well as toxicity of anthracycline anticancer drugs, a class of anthraquinones, is governed by their electrochemical properties. Other quinones serve as important functional moiety in various biological systems like electron-proton carriers in the respiratory chain and their involvement in photosynthetic electron flow systems. The present paper summarizes literatures on the reduction of quinones in different solvents under various conditions using different electrochemical methods. The influence of different reaction conditions including pH of the media, nature of supporting electrolytes, nature of other additives, intramolecular or intermolecular hydrogen bonding, ion pair formation, polarity of the solvents, stabilization of the semiquinone and quinone dianion, catalytic property, and adsorption at the electrode surface, are discussed and relationships between reaction conditions and products formed have been presented.

https://downloads.hindawi.com/journals/ijelc/2011/816202.pdf

Electrochemical Reduction of Quinones in Different Media: A Review

The role of ubiquinone (Q) in the respiratory chain is quantitatively analyzed by correlating both the steady-state redox level of Q and the kinetics of oxidation and reduction with the flux of the overall electron transport in uncoupled submitochondrial particles. This is achieved by experimentally defining donor and acceptor activities for Q.



1
The degree of reduction of Q in the steady state is proportional to the respiratory activity with NADH and succinate, if the respiratory activity is varied by titrating the donor side for Q with rotenone and malonate, respectively. The proportionality constant (acceptor activity, Vox) is independent of the substrate used.

2
The degree of oxidation of Q in the steady state is proportional to the respiratory activity as varied by titration of the acceptor side for Q with antimycin. The proportionality constant (donor activity, Vred) is independent of the acceptor activity and depends on the dehydrogenase activity for NADH and succinate.

3
From these experimental relations, the redox state of Q in the steady state and the respiratory activity can be described as functions of the donor and the acceptor activity only. The equations are valid with both NADH and succinate as the substrates.

4
The kinetics of oxidation of Q on the addition of oxygen as measured by the quench-flow method are in agreement with that measured by direct absorption recording in a mixing chamber. The reaction is first order with a rate constant equal to the acceptor activity divided by the amount of redox-active Q (Vox/Qa). The final steady-state level (in the present case 15% reduction) is a result also of the reduction reaction with the first-order rate constant, VredQa.

5
The acceptor activity can also be measured as maximum respiratory activity with duro-hydroquinone. This activity is independent of the presence of Q but sensitive to antimycin. Thus, the acceptor activity for ubiquinone can be measured by three independent methods.

6
It is concluded, that also in the steady state the reduction and oxidation of the active Q-pool follow pseudo-first-order reactions, the rates of which are equal to the respiratory rate. The total amount of redox-active Q is kinetically and functionally homogeneous and is not divided into substrate-specific compartments.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1973.tb02767.x

The kinetics of the redox reactions of ubiquinone related to the electron-transport activity in the respiratory chain.

The quenching of fluorescence of n-(9-anthroyloxy)stearic acids and other probes by different ubiquinone homologues and analogues has been exploited to assess the localization and lateral mobility of the quinones in lipid bilayers of model and mitochondrial membranes. The true bimolecular collisional quenching constants in the lipids together with the lipid/water partition coefficients were obtained from Stern-Volmer plots at different membrane concentrations. A monomeric localization of the quinone in the phospholipid bilayer is suggested for the short side-chain ubiquinone homologues and for the longer derivatives when cosonicated with the phospholipids. The diffusion coefficients of the ubiquinones, calculated from the quenching constants either in three dimensions or in two dimensions, are in the range of (1-6) X 10(-6) cm2 s-1, both in phospholipid vesicles and in mitochondrial membranes. A careful analysis of different possible locations of ubiquinones in the phospholipid bilayer, accounting for the calculated diffusion coefficients and the viscosities derived therefrom, strongly suggests that the ubiquinone 10 molecule is located within the lipid bilayer with the quinone ring preferentially adjacent to the polar head groups of the phospholipids and the hydrophobic tail largely accommodated in the bilayer midplane. The steady-state rates of either ubiquinol 1-cytochrome c reductase or NADH:ubiquinone 1 reductase are proportional to the concentration of the quinol or quinone substrate in the membrane. The second-order rate constants appear to be at least 3 orders of magnitude lower than the second-order constants for quenching of the fluorescent probes; this is taken as a clear indication that ubiquinone diffusion is not the rate-determining step in the quinone-enzyme interaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of partition and lateral diffusion coefficients of ubiquinones by fluorescence quenching of n-(9-anthroyloxy)stearic acids in phospholipid vesicles and mitochondrial membranes.

Mark F. Marcus

Papers

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The electrochemical oxidation of p-dimethylaminophenol in aqueous solution