FIGURE 3-14 Evolution of the globin protein family. (a) Hemoglobin is a tetramer of two α and two β subunits. The structural similarity of these subunits to leghemoglobin and myoglobin, both of which are monomers, is evident. A heme molecule (red) noncovalently associated with each globin polypeptide is directly responsible for oxygen binding in these proteins. (b) A primitive monomeric oxygen-binding globin is thought to be the ancestor of modern-day blood hemoglobins, muscle myoglobins, and plant leghemoglobins. Sequence comparisons have revealed that the evolution of the globin proteins parallels the evolution of animals and plants. Major changes occurred with the divergence of plant globins from animal globins and of myoglobin from hemoglobin. Later, gene duplication gave rise to the α and β subunits of hemoglobin. See R. C. Hardison, 1996, P. Natl. Acad. Sci. USA 93:5675.

[Part (a) data from G. Fermi et al., 1984, J. Mol. Biol. 175:159–174, PDB ID 2hbb (hemoglobin), H. C. Watson, 1969, Prog. Stereochem. 4:299, PDB ID 1mbn (myoglobin), and M. S. Hargrove et al., 1997, J. Mol. Biol. 266:1032–1042, PDB ID 1bin (leghemoglobin).]