Allosteric mechanisms underlying the adaptive increase in hemoglobin–oxygen affinity of the bar-headed goose

The high blood–O₂affinity of the bar-headed goose (Anser indicus) is an integral component of the biochemical and physiological adaptations that allow this hypoxia-tolerant species to undertake migratory flights over the Himalayas. The high blood–O₂affinity of this species was originally attributed to a single amino acid substitution of the major hemoglobin (Hb) isoform, HbA, which was thought to destabilize the low-affinity T state, thereby shifting the T–R allosteric equilibrium towards the high-affinity R state. Surprisingly, this mechanistic hypothesis has never been addressed using native proteins purified from blood. Here, we report a detailed analysis of O₂equilibria and kinetics of native major HbA and minor HbD isoforms from bar-headed goose and greylag goose (Anser anser), a strictly lowland species, to identify and characterize the mechanistic basis for the adaptive change in Hb function. We find that HbA and HbD of bar-headed goose have consistently higher O₂affinities than those of the greylag goose. The corresponding Hb isoforms of the two species are equally responsive to physiological allosteric cofactors and have similar Bohr effects. Thermodynamic analyses of O₂equilibrium curves according to the two-state Monod–Wyman–Changeaux model revealed higher R-state O₂affinities in the bar-headed goose Hbs, associated with lower O₂dissociation rates, compared with the greylag goose. Conversely, the T state was not destabilized and the T–R allosteric equilibrium was unaltered in bar-headed goose Hbs. The physiological implication of these results is that increased R-state affinity allows for enhanced O₂saturation in the lungs during hypoxia, but without impairing O₂delivery to tissues.