Molecular Biology of the Alternative Oxidase

Abstract

Possibly the most easily recognized component of mitochondrial respiration specific to plants is a second terminal oxidase that has been variously described as being responsible for cyanide-insensitive 'cyanide-resistant or pathway respiration.' This type of respiration has been best defined in thermogenic inflorescences of the Araceae, such as Sauromatum guttatum, the lily' (Meeuse, 1975). Alternative pathway respiration has also been identified in other types of organisms; for example, in protista such as trypanosomes, in fungi such as Neurospora crassa and Hansenula anomala, and in green algae such as Chlamydomonas (Moore and Siedow, 1991). Although some reports exist for such activity in animal tissues, they have never been reliably confirmed. What is the alternative pathway of respiration, how does it work, and, perhaps more importantly, what is its purpose? Alternative pathway respiration branches from the Cyt pathway at ubiquinone and donates electrons directly to oxygen to form water, apparently in a single four-electron step (Siedow and Moore, 1993). An important feature of this pathway is that it does not contribute to a transmembrane potential; electrons that flow down this pathway lose two of three potential coupling sites for proton transport, and thus ATP production (Fig. 1). The only confirmed function for alternative pathway respiration is to supply heat in thermogenic blooms of the Araceae, such as S. guttatum, the voodoo lily (Meeuse, 1975). In this organism heat is produced in ephemeral inflorescences during a single day of flowering to better volatilize scents to attract insect pollinators.