alpha-Lactalbumins and lysozymes

Lysozyme is ubiquitous in a variety of tissues and secretions. Chick-type (c-type) lysozymes lyse the cell walls of certain bacteria. In contrast, alpha-lactalbumin appears to occur only in mammalian milk and colostrum. It has the unusual property of acting as a modifier protein to modify the action of galactosyl transferase to a lactose synthase. Both proteins have diverged from a common ancestor. This is reflected in the striking relationship between their amino acid sequences, and the high conservation of disulfide bridges, their intron-exon organization, and three-dimensional structures. In studying their evolutionary relationships some important differences are noted, e.g., all alpha-lactalbumins strongly bind Ca(II), but only some c-type lysozymes do so. These properties point the way to future investigations that are necessary before firm conclusions can be made about their evolutionary history.