Loss of a conserved tyrosine residue of cytochrome b induces reactive oxygen species production by cytochrome bc₁

Production of reactive oxygen species (ROS) induces oxidative damages, decreases cellular energy conversion efficiencies, and induces metabolic diseases in humans. During respiration, cytochrome bc₁ efficiently oxidizes hydroquinone to quinone, buthowit performs this reaction without any leak of electrons to O2 to yield ROS is not understood. Using the bacterial enzyme, here we show that a conserved Tyr residue of the cytochrome b subunit of cytochrome bc₁ is critical for this process. Substitution of this residue with other amino acids decreases cytochrome bc1 activity and enhances ROS production. Moreover, the Tyr to Cys mutation cross-links together the cytochrome b and iron-sulfur subunits and renders the bacterial enzyme sensitive to O₂ by oxidative disruption of its catalytic [2Fe-2S] cluster. Hence, this Tyr residue is essential in controlling unproductive encounters between O₂ and catalytic intermediates at the quinol oxidation site of cytochrome bc₁ to prevent ROS generation. Remarkably, the same Tyr to Cys mutation is encountered in humans with mitochondrial disorders and in Plasmodium species that are resistant to the anti-malarial drug atovaquone. These findings illustrate the harmful consequences of this mutation in human diseases.