Suppression of superoxide production by a spin-spin coupling between semiquinone and the Rieske cluster in cytochrome bc 1 .

Catalytic reactions of quinol oxidoreductases may lead to the generation of superoxide due to electron leaks from unstable semiquinone intermediates (SQ). For cytochrome bc(1), the mechanism of suppression of superoxide generation remains unknown. We analyzed conditions of formation of a spin-spin-coupled state between SQ and the Rieske cluster (SQ-FeS) associated with catalysis of the quinol oxidation site of cytochrome bc(1). We reveal that mutants that preclude direct interaction between SQ and the Rieske cluster do not form SQ-FeS and release enhanced superoxide. In the enzymes generating SQ-FeS, little or no superoxide is detected. We propose that SQ-FeS suppresses superoxide generation, becoming an element modulating superoxide release under physiologically relevant conditions slowing electron flow through the enzyme.