Fig 4. HbA isoforms of the high-altitude Andean goose and the low-altitude Orinoco goose are distinguished by four amino acid substitutions, one of which (β86Ala→Ser) appears to be mainly responsible for the observed species difference in intrinsic O2-affinity.
Replacing the ancestral Ala with Ser at β86 (the 2nd residue of the F-helix) results in the addition of a helix-capping hydrogen bond (shown in magenta) between the carbonyl oxygen of β86Ser and the γ-oxygen of β89Ser. The resultant stabilization of the F-helix is predicted to increase O2-affinity via subtle displacements of covalent and noncovalent β-heme contacts.