AOX2

AOX2, short for Alternative oxidase 2, is a mitochondrial enzyme that forms part of the alternative respiratory pathway in plants, fungi, and some protists. In Arabidopsis thaliana, AOX2 refers to the AOX2 gene, one member of a small gene family that also includes AOX1a, AOX1b, and AOX1c. AOX2-encoded proteins reside in the inner mitochondrial membrane and can function as homodimers or higher-order oligomers as part of their action on respiration.

The enzyme catalyzes the oxidation of ubiquinol with the simultaneous reduction of oxygen to water, bypassing the cytochrome bc1 complex (complex III) and complex IV. This provides an alternative route for electrons that is not coupled to proton pumping, thus reducing ATP yield but alleviating electron pressure on the main respiratory chain. It can help limit reactive oxygen species under stress and maintain metabolic flux during adverse conditions.

Expression of AOX2 is responsive to stress. In Arabidopsis, AOX2 transcription is often low under non-stressed conditions but can be rapidly upregulated by drought, cold, salinity, wounding, and certain pathogen challenges. Its regulation overlaps with other stress-responsive genes and may involve abscisic acid signaling and mitochondrial retrograde signaling.

Biological significance and research: AOX2 contributes to stress tolerance and metabolic flexibility, particularly when the main electron transport chain is compromised. It is not essential for normal growth in tightly controlled environments, but altering AOX2 expression can affect stress responses in plants and crop species. In animals, AOX genes are typically absent, but researchers have explored heterologous expression of plant AOXs as potential therapies for mitochondrial dysfunction.