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. Increases 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. Activated by glyoxylate, irrespective to the substitution found at Cys-127. That suggests the presence of a second activation site, possibly Cys-177. Homodimer; disulfide-linked. Mitochondrial, possibly in the inner surface of the inner mitochondrial membrane. Expressed in roots, stems, cotyledons, leaves and flowers. High expression in sepals. Expressed throughout development. Low expression during 48 hours after imbibition and then increases. Up-regulated by antimycin A, low-nitrogen and salt stresses, high light, H(2)O(2), ethylene, paraquat, rotenone, salicylic acid, malonate,erythromycin and cold treatments. No visible phenotype under normal growth conditions. Decreased operating efficiency of photosystem II and an enhanced activity of cyclic electron transport around photosystem I. Altered photosynthetic carbon metabolism when grown under high CO(2) concentrations. Cys-127 is involved in the sulfhydryl/disulfide regulation system, but is not required for subunit dimerization. Presence of a positive charge at this residue 127 confers activity while an uncharged substitution creates an inactive enzyme (PubMed:9804851, PubMed:12034477). Belongs to the alternative oxidase family.