The physiological role, structure, and regulation of animal alternative oxidases

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All organisms use electron transport systems (ETS) to generate energy for growth and reproduction. Eukaryotic organisms use ETS in mitochondria to produce ATP and the components of these ETS are highly variable in terms of the complexes that put electrons into the chain and the complexes that pull electrons out. In addition to the typical terminal oxidase in ETS, cytochrome c oxidase (COX), many organisms contain a second terminal oxidase called alternative oxidase (AOX). While AOX also utilizes oxygen as the final electron carrier and produces water, in contrast to COX, AOX does not pump protons and does not contribute to the proton motive force and proton gradient used to generate ATP. While at first glance this reaction appears to be energetically wasteful, AOX mRNA and protein levels greatly increase during environmental stress in many organisms. This implies that AOX plays a role in the successful acclimation to stress conditions, perhaps by conferring flexibility to the ETS.