• haemoglobin.

The greylag goose (Anser anser), which lives on lowlands and cannot tolerate hypoxic conditions, presents a striking contrast to its close relative the bar-headed goose (A. indicus), which lives at high altitude and possesses high-altitude hypoxia adaptation. There are only four amino-acid residue differences at α18, α63, α119 and β125 between the haemoglobins of the two species. The crystal structure of greylag goose oxy haemoglobin was determined at 3.09 Å resolution. Its quaternary structure is slightly different from that of the bar-headed goose oxy haemoglobin, with a rotation of 2.8° in relative orientation of the two dimers. Of the four mutations, those at α119 and β125 produce contact changes in the α1β1 interface and may be responsible for the differences in intrinsic oxygen affinity between the two species; those at α18 and α63 may be responsible for the differences in quaternary structure between the two species.