Avian haemoglobins and structural basis of high affinity for oxygen: structure of bar-headed goose aquomet haemoglobin

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 `R_H' state is probably correlated with the higher oxygen affinity of bar-headed goose haemoglobin.