Fig. 1.

Schematic structures of myoglobin and hemoglobin. Images on right show polypeptide backbone and hemes. Images on left are adapted from Dickerson and Geis [3]. The straight and almost straight segments of the Dickerson and Geis images consist of α-helices. The heme groups are red and the iron atom to which oxygen binds is the grey sphere. In both molecules, the iron atom is covalently bonded to 4 nitrogens of the planar porphyrin ring and the imidazole nitrogen of a histidine side chain of the globin. Hemoglobin has one exact two-fold symmetry axis (y) that interchanges α₁β₁ and α₂β₂ and, because of the similarity in structure of the α and β subunits, has 2 pseudo-two-fold symmetry axes (x and z)