The mechanism of mitochondrial superoxide production by the cytochrome bc1 complex

Production of reactive oxygen species (ROS) by the mitochondrial respiratory chain is considered to be one of the major causes of degenerative processes associated with oxidative stress. Mitochondrial ROS has also been shown to be involved in cellular signaling. It is generally assumed that ubisemiquinone formed at the ubiquinol oxidation center of the cytochrome bc(1) complex is one of two sources of electrons for superoxide formation in mitochondria. Here we show that superoxide formation at the ubiquinol oxidation center of the membrane-bound or purified cytochrome bc(1) complex is stimulated by the presence of oxidized ubiquinone indicating that in a reverse reaction the electron is transferred onto oxygen from reduced cytochrome b(L) via ubiquinone rather than during the forward ubiquinone cycle reaction. In fact, from mechanistic studies it seems unlikely that during normal catalysis the ubisemiquinone intermediate reaches significant occupancies at the ubiquinol oxidation site. We conclude that cytochrome bc(1) complex-linked ROS production is primarily promoted by a partially oxidized rather than by a fully reduced ubiquinone pool. The resulting mechanism of ROS production offers a straightforward explanation of how the redox state of the ubiquinone pool could play a central role in mitochondrial redox signaling.