lysozyme

n.
An enzyme occurring naturally in egg white, human tears, saliva, and other body fluids, capable of destroying the cell walls of certain bacteria and thereby acting as a mild antiseptic.

An enyme that was first identified and named by Alexander Fleming, who recognized its bacteriolytic properties. It has been designated muramidase, since it is known to facilitate the hydrolysis of a β-1-4-glycosidic bond between N-acetylglucosamine and N-acetylmuramic acid in bacterial cell walls; it also hydrolyzes similar glycosidic bonds in fragments of chitin. The most detailed studies have been performed on hen egg-white lysozyme, because this product is readily available. However, enzymes possessing lysozyme activity have been found in bacteria, bacteriophages, and plants and in human leukocytes, nasal secretions, saliva, and tears. The three-dimensional structure of the protein has been defined by x-ray crystallography. Additional data are available for the amino acid sequence of human lysozyme and also for a bacteriophage lysozyme. These results have given rise to speculation concerning the origin of the lysozyme gene during evolution.

Certain enzyme functions appear to be widely distributed in nature. The amino acid sequences of proteins possessing these functions reflect changes that have occurred in the course of evolution. The structures of lysozymes from three sources, distant in evolution, have been carefully examined. Hen egg lysozyme has no structural elements in common with bacteriophage lysozyme. Thus it must be concluded that these two enzymes emerged in evolution completely independent of each other. Preliminary studies of the structure of human lysozyme reveal considerable similarity to the structure of hen egg lysozyme. In fact, the resemblance is so great that it can be concluded that these proteins evolved from the same gene and have an essentially identical mechanism of action.

The amino acid composition of α-lactalbumin, a protein in cow's milk, is quite similar to that of hen egg lysozyme; nearly half of the amino acid positions in these two proteins are identical. It is postulated from a comparison of the amino acid sequences of hen egg lysozyme, human lysozyme, and α-lactalbumin that a “deletion” occurred during evolution in the α-lactalbumin gene with a resulting loss of information for two amino acids near position 13. In addition, positions 10, 12, and 19 in human lysozyme and α-lactalbumin are identical, so it is possible to see remnants of a common ancestral gene in all three proteins. These data illustrate the manner in which amino acid sequence information is being used as a molecular reflection of the paths of evolution. See also Enzyme; Proteins, evolution of.

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(lī'sōzîm)
n

An enzyme in major salivary secretions that may rupture bacterial cell walls and may regulate the oral flora.

lyso—lyses bacteria; zyme—an enzyme naturally present in body fluids but ordinarily obtained from egg white for in vitro work. It hydrolyzes a specific glycoside bond in the peptidoglycan that forms bacterial cell walls, yielding the disaccharide N-acetylglucosamine-N-acetylmuramate. An important component of innate resistance.

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