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The evolution of lysozyme and alpha-lactalbumin

K Nitta and S Sugai
Department of Polymer Science, Faculty of Science, Hokkaido University, Japan.

From the analysis of phylogenetic trees constructed from the amino acid sequences and metal-binding properties of various lysozymes c and alpha- lactalbumins, it was found that before the divergence of the lineages of birds and mammals, calcium-binding lysozyme diverged from non- calcium-binding lysozyme. alpha-Lactalbumin evolved from the calcium- binding lysozyme along the mammalian lineage after the divergence of birds and mammals.

Rapid evolution took place, not in the process of acquisition of the activity of alpha-lactalbumin, but after the loss of lysozyme activity, due to the change in the distribution of selective pressure on each amino acid site.

A general process for the change in function of a protein during evolution is suggested to be as follows: after duplication of the gene, one of their protein products acquires a new function, besides that already present; the old function is eventually lost.