The crystal structure of bar-headed goose hemoglobin in deoxy form: the allosteric mechanism of a hemoglobin species with high oxygen affinity

The crystal structure of a high oxygen affinity species of hemoglobin, bar-headed goose hemoglobin in deoxy form, has been determined to a resolution of 2.8 Å. The R and Rfree factor of the model are 0.197 and 0.243, respectively. The structure reported here is a special deoxy state of hemoglobin and indicates the differences in allosteric mechanisms between the goose and human hemoglobins. The quaternary structure of the goose deoxy hemoglobin shows obvious differences from that of human deoxy hemoglobin. The rotation angle of one αβ dimer relative to its partner in a tetramer molecule from the goose oxy to deoxy hemoglobin is only 4.6°, and the translation is only 0.3 Å, which are much smaller than those in human hemoglobin. In the α1β2 switch region of the goose deoxy hemoglobin, the imidazole ring of His β297 does not span the side-chain of Thr α141 relative to the oxy hemoglobin as in human hemoglobin. And the tertiary structure changes of heme pocket and FG corner are also smaller than that in human hemoglobin. A unique mutation among avian and mammalian Hbs of α119 from proline to alanine at the α1β1interface in bar-headed goose hemoglobin brings a gap between Ala α119 and Leu β55, the minimum distance between the two residues is 4.66 Å. At the entrance to the central cavity around the molecular dyad, some residues of two β chains form a positively charged groove where the inositol pentaphosphate binds to the hemoglobin. The His β146 is at the inositol pentaphosphate binding site and the salt-bridge between His β146 and Asp β94 does not exist in the deoxy hemoglobin, which brings the weak chloride-independent Bohr effect to bar-headed goose hemoglobin.