MOLECULAR DIVERGENCE OF LYSOZYMES AND ALPHA-LACTALBUMIN

The vast number of proteins that sustain the currently living organism s have been generated from a relatively small number of ancestral gene s that has involved a variety of processes. Lysozyme is an ancient pro tein whose origin goes back an estimated 400 to 600 million years. Thi s protein was originally a bacteriolytic defensive agent and has been adapted to serve a digestive function on at least two occasions, separ ated by nearly 40 million years. The origins of the related goose type and T4 phage lysozyme that are distinct from the more common C type a re obscure. They share no discernable amino acid sequence identity and yet they possess 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 alpha-lactalbumin, a protein e xpressed only in the lactating mammary gland of all but a few species of mammals. It is required for the synthesis of lactose, the sugar sec reted in milk. alpha-lactalbumin shares only 40% identity in amino aci d sequence with lysozyme C, but it has a closer spatial structure and gene organization. Although structurally similar, functionally they ar e quite distinct. Specific amino acid substitutions in alpha-lactalbum in account for the loss of the enzyme activity of lysozyme and the acq uisition of the features necessary for its role in lactose synthesis. Evolutionary implications are as yet unclear but are being unraveled i n many laboratories.