Molecular evolution of cytochrome C oxidase underlies high-altitude adaptation in the bar-headed goose

Bar-headed geese (Anser indicus) fly at up to 9,000 m elevation during their migration over the Himalayas, sustaining high metabolic rates in the severe hypoxia at these altitudes. We investigated the evolution of cardiac energy metabolism and O(2) transport in this species to better understand the molecular and physiological mechanisms of high-altitude adaptation. Compared with low-altitude geese (pink-footed geese and barnacle geese), bar-headed geese had larger lungs and higher capillary densities in the left ventricle of the heart, both of which should improve O(2) diffusion during hypoxia. Although myoglobin abundance and the activities of many metabolic enzymes (carnitine palmitoyltransferase, citrate synthase, 3-hydroxyacyl-coA dehydrogenase, lactate dehydrogenase, and pyruvate kinase) showed only minor variation between species, bar-headed geese had a striking alteration in the kinetics of cytochrome c oxidase (COX), the heteromeric enzyme that catalyzes O(2) reduction in oxidative phosphorylation. This was reflected by a lower maximum catalytic activity and a higher affinity for reduced cytochrome c. There were small differences between species in messenger RNA and protein expression of COX subunits 3 and 4, but these were inconsistent with the divergence in enzyme kinetics. However, the COX3 gene of bar-headed geese contained a nonsynonymous substitution at a site that is otherwise conserved across vertebrates and resulted in a major functional change of amino acid class (Trp-116 → Arg). This mutation was predicted by structural modeling to alter the interaction between COX3 and COX1. Adaptations in mitochondrial enzyme kinetics and O(2) transport capacity may therefore contribute to the exceptional ability of bar-headed geese to fly high.