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Deletion of COQ10 in Saccharomyces cerevisiae elicits a respiratory defect characterized by the absence of cytochrome c reduction, which is correctable by the addition of exogenous diffusible coenzyme Q2. Unlike other coq mutants with hampered coenzyme Q6 (Q6) synthesis, coq10 mutants have near wild-type concentrations of Q6. In the present study, we used Q-cycle inhibitors of the coenzyme QH2–cytochrome c reductase complex to assess the electron transfer properties of coq10 cells. Our results show that coq10 mutants respond to antimycin A, indicating an active Q-cycle in these mutants, even though they are unable to transport electrons through cytochrome c and are not responsive to myxothiazol. EPR spectroscopic analysis also suggests that wild-type and coq10 mitochondria accumulate similar amounts of Q6 semiquinone, despite a lower steady-state level of coenzyme QH2–cytochrome c reductase complex in the coq10 cells. Confirming the reduced respiratory chain state in coq10 cells, we found that the expression of the Aspergillus fumigatus alternative oxidase in these cells leads to a decrease in antimycin-dependent H2O2 release and improves their respiratory growth.