Day D A, Wiskich J T
Division of Biochemistry and Molecular Biology, Australian National University, Canberra.
J Bioenerg Biomembr. 1995 Aug;27(4):379-85. doi: 10.1007/BF02110000.
Plant mitochondria contain two terminal oxidases: cytochrome oxidase and the cyanide-insensitive alternative oxidase. Electron partioning between the two pathways is regulated by the redox poise of the ubiquinone pool and the activation state of the alternative oxidase. The alternative oxidase appears to exist as a dimer which is active in the reduced, noncovalently linked form and inactive when in the oxidized, covalently linked form. Reduction of the oxidase in isolated tobacco mitochondria occurs upon oxidation of isocitrate or malate and may be mediated by matrix NAD(P)H. The activity of the reduced oxidase is governed by certain other organic acids, notably pyruvate, which appear to interact directly with the enzyme. Pyruvate alters the interaction between the alternative oxidase and ubiquinol so that the oxidase becomes active at much lower levels of ubiquinol and competes with the cytochrome pathway for electrons. These requirements for activation of the alternative oxidase constitute a sophisticated feed-forward control mechanism which determines the extent to which electrons are directed away from the energy-conserving cytochrome pathway to the non-energy conserving alternative oxidase. Such a mechanism fits well with the proposed role of the alternative oxidase as a protective enzyme which prevents over-reduction of the cytochrome chain and fermentation of accumulated pyruvate.
细胞色素氧化酶和对氰化物不敏感的交替氧化酶。两条途径之间的电子分配受泛醌池的氧化还原状态和交替氧化酶的激活状态调节。交替氧化酶似乎以二聚体形式存在，以还原的、非共价连接的形式具有活性，而以氧化的、共价连接的形式则无活性。在分离的烟草线粒体中，异柠檬酸或苹果酸氧化时会发生氧化酶的还原，这可能由基质NAD(P)H介导。还原型氧化酶的活性受某些其他有机酸的控制，特别是丙酮酸，丙酮酸似乎直接与该酶相互作用。丙酮酸改变交替氧化酶与泛醇之间的相互作用，使氧化酶在泛醇水平低得多时变得活跃，并与细胞色素途径竞争电子。这些激活交替氧化酶的条件构成了一种复杂的前馈控制机制，该机制决定了电子从能量守恒的细胞色素途径转向非能量守恒的交替氧化酶的程度。这样一种机制与交替氧化酶作为一种保护酶的作用非常契合，这种保护酶可防止细胞色素链过度还原和积累的丙酮酸发酵。
