The alternative oxidase (AOX) is an enzyme that forms part of the electron transport chain in mitochondria of different organisms. The alternative oxidase is an integral monotopic membrane protein that is tightly bound to the inner mitochondrial membrane from matrix side. The enzyme has been predicted to contain a coupled diiron center on the basis of a conserved sequence motif consisting of the proposed iron ligands, four glutamate and two histidine amino acid residues. A catalytic cycle has been proposed that involves this di-iron center and at least one transient protein-derived free radical, which is probably formed on a tyrosine residue
Proteins homologous to the mitochondrial oxidase have also been identified in bacterial genomes. Multiple copies have been detected in plants which are not linked to oxidative phosphorylation and are located in the chloroplast.
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