Catalyzes the cyanide-resistant oxidation of ubiquinol and the reduction of molecular oxygen to water, but does not translocate protons and consequently is not linked to oxidative phosphorylation. May increase respiration when the cytochrome respiratory pathway is restricted, or in response to low temperatures. 2 a ubiquinol + O2 = 2 a ubiquinone + 2 H2O Binds 2 iron ions per subunit. When the two monomeric subunits are covalently linked by a S-S bond, the enzyme is essentially inactive. When the disulfide bond is reduced, its component sulfhydryls can associate with K-keto acids through formation of a thiohemiacetal, resulting in enzyme activation. Pyruvate increases Vmax, but not the substrate affinity. Optimum pH is 7.0-7.5. Homodimer; disulfide-linked. Mitochondrial, possibly in the inner surface of the inner mitochondrial membrane. By salicylic acid. Belongs to the alternative oxidase family.