Evolution and Protein Structure - II
Cytochrome is a single polypeptide chain of about 104 residues:
- Found in all aerobic organisms, where it plays a key role in mitochondrial electron transport.
- Like essentially all proteins involved in oxygen transport or redox reactions it contains a heme unit.
|Horse Cytochrome C (1hrc)
Comparison of cytochrome c from a large number of species:
The sequences are remarkably similar across species, especially at certain positions.
- For example, every one has a phenylalanine at position 10, a histidine at 18, a proline at 30 and a methionine at 80. Clearly, evolution selects against any change at these positions.
- In addition, numerous sites display a limited number of changes, in most cases allowing only residues with similar properties.
- For example, residue 98 can be only leucine, isoleucine, or valine - all lipophilic residues.
- Likewise, His18 (shown in the graphic above), which coordinates to the heme iron, is conserved throughout.
The implication is that the strongly conserved residues contribute significantly to the structure and function of the protein.
Other positions contribute very little and are highly variable; evolutionary drift randomizes these residues.
Note that human and chimpanzee sequences are identical, and macaque and spider monkey are very similar to the human/chimpanzee sequence. This implies a common and relatively recent evolutionary divergence.
Backbone alignment likewise shows similarities:
|Backbone Alignment of Horse Cytochrome C (1hrc) with Tuna Cytochrome C (5cyt)
RMSD = 0.47 Å
A useful way to present these similarities and differences is to draw an evolutionary tree [Schwartz and Dayhoff, Science, 1978, 199, 395]:
- The lengths of the branches in this tree are drawn proportional to the number of differences in the primary sequence.
- The tree clearly reveals the three main kingdoms of eukaryotes: fungi, animals, and plants.
- Such trees tend to agree closely with those constructed by evolutionary biologists using morphological data, and provide independent evidence of common descent.
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