ALTERNATIVE OXIDASE IN THE BRANCHED MITOCHONDRIAL RESPIRATORY NETWORK- AN OVERVIEW ON STRUCTURE, FUNCTION, REGULATION, AND ROLE

Plants and some other organisms including protists possess a complex b ranched respiratory network in their mitochondria. Some pathways of th is network are not enaergy-conserving and allow sites of energy conser vation to be bypassed, leading to a decrease of the energy yield in th e cells. It is a challenge to understand the regulation of the partiti oning of electrons between the various energy-dissipating and -conserv ing pathways. This review is focused on the oxidase side of the respir atory chain that presents a cyanide-resistant energy-dissipating alter native oxidase (AOX) besides the cytochrome pathway. The known structu ral 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 alterna tive oxidase activity is dependent on substrate availability: total ub iquinone concentration and its redox state in the membrane and O-2 con centration in the cell. The alternative oxidase activity can be long-t erm regulated (gene expression) or short-term (post-translational modi fication, allosteric activation) regulated. Electron distribution (par titioning) between the alternative and cytochrome pathways during stea dy-state respiration is a crucial measurement to quantitatively analyz e the effects of the various levels of regulation of the alternative o xidase. Three approaches are described with their specific domain of a pplication and limitations: kinetic approach, oxygen isotope different ial discrimination, and ADP/O method (thermokinetic approach). Lastly, the role of the alternative oxidase in non-thermogenic tissues is dis cussed in relation to the energy metabolism balance of the cell (suppl y in reducing equivalents/demand in energy and carbon) and with harmfu l reactive oxygen species formation.