Alternative oxidase (AOX) constitutes a small family of proteins in Citrus clementina and Citrus sinensis L. Osb. [Poster-B186]

The alternative oxidase (AOX) protein is present in plants, fungi and some invertebrates. It is involved in the mitochondrial respiratory chain, providing an alternative route for the transport of electrons, leading to the reduction of oxygen to form water. The present study aimed to characterize the family of AOX genes in mandarin and sweet orange. Four AOX genes were identified in each citrus species. The 1500 bp-upstream region of each AOX gene contained regulatory cis-elements related to internal and external response factors. CsAOX genes showed a differential expression in citrus tissues. AOX proteins contained conserved motifs as well as several putative post-translational modification sites. The CcAOXd protein was modeled by homology and its 3-D structure showed two hydrophobic helices probably involved in the anchoring in the inner mitochondrial membrane. The active site of the protein is located in a hydrophobic environment deep inside the AOX structure and contains a diiron center. The molecular docking of CcAOXd with ubiquinone showed that the binding site is a recessed pocket formed by the helices and submerged in the membrane. These data are important for future functional studies of citrus AOX, as well as for biotechnological approaches leading to AOX inhibition using ubiquinone homologs.