• haemoglobin;
  • molecular mechanism;
  • oxygen affinity;
  • high-altitude respiration;
  • bar-headed goose.

Haemoglobin from the bar-headed goose (Anser indicus) has higher oxygen affinity than that from its lowland relatives such as greylag goose (A. anser). The crystal structure of bar-headed goose aquomet haemoglobin was determined at 2.3 Å resolution and compared with the structures of the goose oxy, human, horse and other avian haemoglobins and the sequences of other avian haemoglobins. Four amino-acid residues differ between greylag goose and bar-headed goose haemoglobins, among which Alaα119 and Aspβ125 in bar-headed goose haemoglobin reduces the contacts between the α1 and β1 subunits compared with Pro and Glu, respectively, and therefore may increase the oxygen affinity by loosening the α1β1 interface. Compared with human oxy haemoglobin, the relative orientation of two αβ dimers in the bar-headed goose aquomet and oxy Hbs are rotated by about 4°, indicating a unique quaternary structural difference from the typical R state. This new `RH' state is probably correlated with the higher oxygen affinity of bar-headed goose haemoglobin.