The function of ubiquinone in mitochondrial electron transport

Reed, Jacquelyn Sandra (1987) The function of ubiquinone in mitochondrial electron transport. University of Southampton, Doctoral Thesis.

Abstract

The kinetic behaviour of the quinone-pool was studied by direct measurement of the quinone redox-state. Deviations from so-called Q-pool behaviour were also investigated. Experiments were carried out on isolated enzymes, reconstituted electron transport systems and the natural membrane system as found in submitochondrial particles. Inactive or enzymically non-reducible ubiquinone was found in a 1:1 stoichiometry with Complex I and Compex III. In Complex I this was found to be exchangeable with the bulk quinone pool but it could not be removed by dilution into lipid suggesting a specific association, perhaps with a quinone-binding protien. In isolated Complex I ubiquinone reduction kinetcs were found to be biphasic under all conditions but in reconstituted Complex I - Complex III the kinetics paralleled the cytochrome b reduction, that is, biphasic or monophasic depending on the membrane fluidity. Under conditions of limited availability of cytochrome c a deviation from first-order oxidation of quinol despite the presence of a homogeneous Q-pool was found. The results were fitted to the prediction of the kinetic model of Ragan and Cottingham (1985). Further predictions of this model were used to determine dissociation constants for Complex I and Complex III with ubiquinone both in SMP and in a reconstituted system. The results for the two systems were found to be comparable when expressed per mol of phospholipid present. Experiments to produce non-random protein distribution in SMP failed to show deviations from Q-pool behaviour as had been hoped. Possible explanations for this result were put forward. (D74417/87)

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