Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Globular Proteins: Structure and Function of Hemoglobin
Myoglobins isolated from different species share a similar conformation

We have already seen that in Homologous Proteins from different species, such as the cytochrome c series, certain positions in The polypeptide chains contain invariant—i.e., always identical—amino acid residues, whereas other positions can accommodate different residues (see Fig. 6-14). The same is true for myoglobins isolated from various species of whales, seals, and some terrestrial vertebrates. This in itself provides strong evidence that all myoglobins share a common ancestor and therefore possess a similar polypeptide chain folding. But an even more compelling confirmation of the common origin hypothesis comes from the X-Ray Diffraction Analysis of myoglobins from other species, which demonstrated that all these proteins share a tertiary Structure similar to that of sperm whale Myoglobin. The similarity in tertiary structure among various myoglobins and the Homology of their Amino acid sequences suggest that the Amino Acid Sequence of myoglobin must somehow determine the three-dimensional folding of its polypeptide chain. A similar situation is observed in other homologous proteins: each group of such proteins exhibits both amino acid Sequence homology and similarity in tertiary structure.