Figure 3.

Coordinate difference plot (CDP) showing the differences between corresponding Cα atoms of deoxyhemoglobin (blue), COHbA (red), α₂β⁸²CA⁸²β (yellow), and α₂β⁸²ND⁸²β (green) after their α₁β₁ interfaces have been superimposed onto that of R2 hemoglobin as described in Fig. 2. The ordinate indicates the displacement of the corresponding Cα atoms, and the abscissa shows the residue number where the A:B dimer corresponds to α₁β₁ and the C:D dimer corresponds to α₂β₂. The region labeled tertiary differences (α₁β₁) corresponds to structural alterations between tertiary elements, such as slight helical displacements. The largest structural difference is confined to the first few residues of the A helix of the α subunit (labeled αA), which, in the R, α₂β⁸²CA⁸²β, and α₂β⁸²ND⁸²β structures, are identical (see Fig. 2). The region labeled quaternary differences (α₂β₂) visualizes the large quaternary rotational differences between these structures. From this plot, it is clear that the structures of α₂β⁸²CA⁸²β and α₂β⁸²ND⁸²β lie directly between those of the R and R2 liganded structures.