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 aff inity than that from its lowland relatives such as greylag goose (A. anser) . The crystal structure of bar-headed goose aquomet haemoglobin was determi ned at 2.3 Angstrom resolution and compared with the structures of the goos e 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 Ala alpha 119 and Asp beta 1 25 in bar-headed goose haemoglobin reduces the contacts between the alpha ( 1) and beta (1) subunits compared with Pro and Glu, respectively, and there fore may increase the oxygen affinity by loosening the alpha (1)beta (1) in terface. Compared with human oxy haemoglobin, the relative orientation of t wo alpha beta dimers in the bar-headed goose aquomet and oxy Hbs are rotate d by about 4 degrees, 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.