Comparison of the evolution of heme binding and NAD binding proteins.

Of the 85 three-dimensionally characterized residues of cytochrome b5, 51 are structurally and topologically equivalent to the globin fold. When these proteins have been superimposed, the heme irons are found to be less than 1.4 A separated and the heme normals are inclined by less than 9.5 degrees. Comparison of minimum base changes per codon between heme binding and NAD binding proteins are of the same order.