Derivation of the globins from type b cytochromes.

Similarities in the amino acid sequences of vertebrate and invertebrate globins, b5 and b2 cytochromes and chicken sulfite oxidase point to a common ancestry for all of these proteins. The distal heme ligand (histidine or its equivalent) is common to both sets of proteins, but the proximal histidine ligand of the cytochromes is replaced by another histidine residue in the globins. This explains why the heme is reversed between globins and b5 cytochromes. It seems likely that the genes for primitive globins contained three exons, the first two of which were derived from a cytochromelike DNA sequence. A model is presented to show how globins may have evolved from a pre-existing type b cytochrome; the complexity of the required changes is an indication that all globins are monophyletic.