Globins have remarkable sequence diversity, and yet maintain a common fold. In spite of the diversity, there are highly-conserved residues at several sites. The conserved residues were examined in terms of the structural stability, by employing the pseudo-energy functions of the structure/sequence compatibility method. The fitness of each residue type to the structural environment was evaluated at seven highly-conserved sites: the Leu (at the B10 site), Phe (CD1), and Leu (F4) residues were found to fit their respective sites due to hydrophobic interactions; Pro (C2) stabilizes the N-terminal edge of an α-helical structure; and Phe (CD4) is stabilized by backbone hydrogen-bonding to Phe (CD1). On the other hand, the other two residues, His (E7) and His (F8), are poorly suited to the sites from a structural viewpoint, suggesting that their conservation clearly results from a heme-related functional requirement. The invariant Phe residue (CD1) has been suggested to be important for supporting the heme. The present analysis revealed that this residue is also well suited to the site in terms of energy.
