They share no discernable amino acid sequence identity and yet they posess common secondary and tertiary structures. Lysozyme C gene also gave rise, after gene duplication 300 to 400 million years ago, to a gene that currently codes for a-lactalbumin, a preotin expressed only in the lactating mammary gland of all but a few species of mammals. It is required for the synthesis of lactose, the sugar secreted in milk. Although structurally similar, functionally they are quite distinct. Specific amino acid substitutions in a-lactalbumin account for the loss of the enzyme activity of lysozyme and the acquisition of the features necessary for its role in lactose synthesis.
Evolutionary implications are as yet unclear but are being unraveled in many laboratories.
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