New insight into the structure and function of the alternative oxidase

The alternative oxidase is a ubiquinol oxidase found in plant mitochondria, as well as in the mitochondria of some fungi and protists. It catalyzes a cyanide-resistant reduction of oxygen to water without translocation of protons across the inner mitochondrial membrane, and thus functions as a non-energy-conserving member of the respiratory electron transfer chain. The active site of the alternative oxidase has been modelled as a diiron center within a four-helix bundle by Siedow et al. (FEBS Lett. 362 (1995) 10-14) and more recently by Andersson and Nordlund (FEBS Lett. 449 (1999) 17-22). The cloning of the Arabidopsis thaliana IMMUTANS (Im) gene, which encodes a plastid enzyme distantly related to the mitochondrial alternative oxidases (Wu et al. Plant Cell 11 (1999) 43-55; Carol et al. Plant Cell 11 (1999) 57-68), has now narrowed the range of possible ligands to the diiron center of the alternative oxidase. The Im protein sequence suggests a minor modification to the recent model of the active site of the alternative oxidase. This change moves an invariant tyrosine into a conserved hydrophobic pocket in the vicinity of the active site, in a position analogous to the long-lived tyrosine radical at the diiron center of ribonucleotide reductase, and similar to the tyrosines near the diiron center of bacterioferritin and rubrerythrin. The Im sequence and modified structural model yield a compelling picture of the alternative oxidase as a diiron carboxylate protein. The current status of the relationship of structure to function in the alternative oxidase is reviewed.