The lysozymes of hen egg-white and bacteriophage T4 have similar catalytic properties but their amino acid sequences are not homologous. The question therefore arises whether they are derived from a common ancestral protein or have arisen independently. On the basis of the data we have gathered, it is shown here that the two enzymes are similar in the conformation of their backbones, in their modes of binding substrates, in specific protein-substrate interactions and in their presumed modes of action. We conclude that the two enzymes have diverged from a common precursor. This seems to be the most convincing example to date of the divergence of proteins with nonhomologous amino acid sequences.
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