The structure of cytochromec and the rates of molecular evolution

SummaryThe x-ray structure analysis of ferricytochromec shows the reasons for the evolutionary conservatism of hydrophobic and aromatic side chains, lysines, and glycines, which had been observed from comparisons of amino acid sequences from over 30 species. It also shows that the negative character of one portion of the molecular surface is conserved, even though individual acidic side chains are not, and that positive charges are localized around two hydrophobic “channels” leading from the interior to the surface.The reason for the unusual evolutionary conservation of surface features in cytochromesc is probably the interaction of the molecule with two other large macromolecular complexes, its reductase and oxidase. This conservation of surface structure also explains the relatively slow rate of change of cytochromec sequences in comparison with the globins and enzymes of similar size.The rate of evolution of a protein is the rate of occurrence of mutations in the genome modified by the probability that a random change in amino acid sequence will be tolerable in a functioning protein. The observed rates of change in fibrinopeptides, the globins, cytochromec, and several enzymes are interpreted in terms of the proteins' biological roles.

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