Abstract
Plants and some other organisms including protists possess a complex branched respiratory network in their mitochondria. Some pathways of this network are not energy-conserving and allow sites of energy conservation to be bypassed, leading to a decrease of the energy yield in the cells. It is a challenge to understand the regulation of the partitioning of electrons between the various energy-dissipating and -conserving pathways. This review is focused on the oxidase side of the respiratory chain that presents a cyanide-resistant energy-dissipating alternative oxidase (AOX) besides the cytochrome pathway. The known structural properties of AOX are described including transmembrane topology, dimerization, and active sites. Regulation of the alternative oxidase activity is presented in detail because of its complexity. The alternative oxidase activity is dependent on substrate availability: total ubiquinone concentration and its redox state in the membrane and O2 concentration in the cell. The alternative oxidase activity can be long-term regulated (gene expression) or short-term (post-translational modification, allosteric activation) regulated. Electron distribution (partitioning) between the alternative and cytochrome pathways during steady-state respiration is a crucial measurement to quantitatively analyze the effects of the various levels of regulation of the alternative oxidase. Three approaches are described with their specific domain of application and limitations: kinetic approach, oxygen isotope differential discrimination, and ADP/O method (thermokinetic approach). Lastly, the role of the alternative oxidase in non-thermogenic tissues is discussed in relation to the energy metabolism balance of the cell (supply in reducing equivalents/demand in energy and carbon) and with harmful reactive oxygen species formation.
Highlights
Cyanide-insensitive respiration was first observed in plants in 1929 by Genevois in sweet pea seedlings [1]
This review is focused on the oxidase side of the respiratory chain that presents a cyanide-resistant energy-dissipating alternative oxidase (AOX) besides the cytochrome pathway
In 1978, a cyanide-resistant quinol oxidase was solubilized from Arum maculatum mitochondria [4,5], and the alternative cyanide-resistant respiration was attributed to an enzyme called the alternative oxidase (AOX)
Summary
Cyanide-insensitive respiration was first observed in plants in 1929 by Genevois in sweet pea seedlings [1]. When isolated mitochondria were used, studies were focused on the relative activities of both cyanide-insensitive and cytochrome pathways by measuring rates of respiration in the presence of an inhibitor of each pathway. In 1986, monoclonal antibodies to three induced proteins from Sauromatum guttatum responsible for cyanideresistant respiration were obtained [8], strongly stimulating the field of the alternative oxidase research. These monoclonal antibodies were used to identify AOX proteins in a wide variety of plants [9,10], in fungi [11,12], in trypanosomes [13], and amoebae [14]. For more detailed comprehensive reviews of the alternative oxidase field, the reader is referred to reviews by Moore and Siedow [16], McIntosh [19], Siedow and Umbach [10], Day and Wiskich [20], Day et al [9], Wagner and Krab [21], Krab [22], and Vanlerberghe and McIntosh [23]
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