Although the amino acid sequences of myoglobin and hemoglobin are homologous, and they adopt the same globin fold, there are important differences in patches where the polypeptide chains in hemoglobin make contact with one another. In particular, some of the surface residues that are polar in myoglobin are hydrophobic in hemoglobin; e.g.,residue B15 (the 15th residue in helix B) is a lysine in myoglobin and a leucine or valine in hemoglobin; and residue FG2 (the second residue in the loop between helices F and G) is a histidine in myoglobin and leucine in both chains of hemoglobin. In hemoglobin, both these residues are part of the contact patch between the α and β chains.
