Fig. 3: The bicarbonate ion binding site.

a Symmetry-related bicarbonate ions are shown (as CPK models) in the deoxy HbAM tetramer, and side-chains bonding to them are shown as sticks. The carbon atoms of the α₁ subunit are shown in magenta, the β₁ subunit in cyan, the α₂ subunit in lime, and the β₂ subunit in orange. b, c Cryo-EM density around the bicarbonate ion and (d, e) the bonding interactions. The hydrogen bonds between the bicarbonate ion and side-chains are shown as grey dotted lines (with distances). f An overlay of deoxy HbAM and human HbA at the bicarbonate ion binding site. For deoxy human HbA, the carbon atoms of the β₁ subunit are coloured salmon, and the α₂ subunit is shown in white. g α₁β₁ overlay of deoxy HbAM and carbonmonoxy HbAM at the bicarbonate ion binding site, showing relative displacements of Lys β₁38 and Arg β₁40. For carbonmonoxy HbAM, the carbon atoms of the β₁ subunit are shown in purple, and those of the α₂ subunit are shown in brown. h Another view of the overlay in g, showing Met β₁33. i Interactions formed by Arg β₁39, which points away from the bicarbonate ion. j A different view of the overlay in g, showing Asp α₂94 and Tyr β₁41.