On the Evolutionary Relationship of the 4-a-Helical Heme Proteins

helices of each nearly coincide. Four aromatic side chains, including the heme ligands, and a methionine occur in spatially equivalent positions in contact with the heme groups. This structural evidence suggests that the two cytochrome families may have diverged from a common molecular ancestor. Proteins exhibiting similarities in amino acid sequence and function are generally assumed to have arisen through proc- esses of divergent evolution from a common molecular ances- tor. X-ray structural studies