Alternative oxidase, ferritin-like diiron-binding domain
Alternative oxidase (AOX) is a mitochondrial ubiquinol oxidase found in plants and some fungi and protists. AOX is a member of the ferritin-like diiron-carboxylate superfamily. The plant mitochondrial protein alternative oxidase catalyses dioxygen dependent ubiquinol oxidation to yield ubiquinone and water. AOX is a cyanide-resistant, salicylhydroxamic acid-sensitive oxidase that transfers electrons from ubiquinol to oxygen, bypassing the cytochrome chain. AOX has been proposed to contain a hydroxo-bridged diiron center within a four-helix bundle and a proximal redox-active tyrosine residue. AOX is proposed to be peripherally associated with the matrix side of the inner mitochondrial membrane. Fungal and protozoan AOXs generally exist as monomers. In plants, AOX is dimeric. Pyruvate is an allosteric activator of plant AOX involved in the reversible inactivation of the enzyme though the formation of an intermolecular disulfide bridge between monomeric subunits. The enzyme is non-proton-motive and does not contribute to the conservation of energy. The heat that dissipates from AOX activity is used in thermogenic plants to volatilize primary amines to attract pollinating insects. Other functions have been proposed: i) that the alternative oxidase allows Krebs-cycle turnover when the energy charge of the cell is high, and ii) that the enzyme protects against oxidative stress. The expression of AOX is induced when plants are exposed to a variety of stresses including chilling, pathogen attack, senescence and fruit ripening.