Figure 1.

Schematic diagram of the respiratory chain, illustrating the effect of alternative oxidase (AOX). (A) AOX accepts electrons from reduced ubiquinone (CoQ) and reduces oxygen to water, thus bypassing complexes III and IV. At baseline without stress, AOX expression has little effect on the activities of the endogenous respiratory chain and thus the physiological level of reactive oxygen species (ROS). C = cytochrome c. (B) When complex III is dysfunctional, complex III cannot accept electrons from CoQ efficiently, and therefore the CoQ pool becomes overreduced. Reverse electron transport occurs when electrons from overreduced CoQ are transferred back to complex I. This process generates a significant amount of superoxide. (C) AOX can reoxidize the CoQ pool and prevent electrons from being transferred back to complex I, thus decreasing reverse electron transport–associated ROS production. Also, electrons from the ubiquinone pool are transferred to AOX rather than to complex III, thus decreasing the superoxide production from complex III. FAD = flavin adenine dinucleotide; FADH₂ = FAD reduced; NAD⁺ = nicotinamide adenine dinucleotide; NADH = NAD⁺ reduced.