muramidase

Muramidase is an enzyme that hydrolyzes the β-1,4-glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine in peptidoglycan, the essential component of bacterial cell walls. The best known muramidase is lysozyme, particularly hen egg-white lysozyme, which serves as a widely used model enzyme in biochemistry and structural biology. In enzymology, muramidase activity is commonly described by the enzyme commission number EC 3.2.1.17.

Muramidases occur in a variety of organisms. Humans and other animals secrete lysozymes in tears, saliva, mucus and milk; plants and some microorganisms also produce muramidases. Bacteriophages often encode muramidases (lysin) used to degrade bacterial walls during infection.

Structure and mechanism: Lysozymes are compact globular proteins of roughly 14 kDa with about 120-130 amino acids and several disulfide bonds. In hen egg-white lysozyme, the catalytic machinery includes two conserved residues, typically a glutamate and an aspartate. The mechanism is commonly described as a retaining double-displacement, in which a glycosyl-enzyme intermediate forms and is followed by hydrolysis of the acetal linkage, releasing shorter peptidoglycan chains.

Applications and significance: Muramidase activity is used in scientific research to digest cell walls and extract contents; in food and cosmetic industries as an antimicrobial additive; in clinical research, muramidases and engineered lysins are explored as antibacterial therapies. The term muramidase reflects the substrate muramic acid; lysozyme remains the more widely used common name.