Fig. 7. Schematic of the mitochondrial respiratory chain showing the effect of CO binding (x) at the terminal oxidase on hydrogen peroxide (H₂O₂) production. CO slows the rate of electron transport (solid arrows), enabling electrons to accumulate, including at complex III. Complex III and the proton motive Q cycle have been expanded to show sites of ubi-semiquinone (Q·-) formation from which electrons can be donated to produce superoxide (·O2-) (dotted arrows). Complex III O₂ availability is also increased by CO, which promotes ·O₂-production and its conversion to H₂O₂ by SOD2 (MnSOD). SOD2 induction may increase the extra-mitochondrial H₂O₂ leak rate. Cyt, cytochrome; ISP, iron sulfur protein; Q, oxidized coenzyme; QH₂, reduced coenzyme Q. The details of the Q cycle, including the transmembrane proton flux, have been omitted for clarity.