Lysozyme is a muramidase enzyme that cleaves the β(1→4) glycosidic bond in peptidoglycan, weakening bacterial cell walls and causing lysis. It is found in bodily fluids like tears, saliva and mucus and contributes to innate immune defense.
Biochemical Role and Biological Distribution
As one of the first antimicrobial factors described by Alexander Fleming, lysozyme is ubiquitous across species. The enzyme is a small cationic protein that hydrolyzes the bond between N -acetylmuramic acid and N -acetylglucosamine in peptidoglycan, destabilizing the rigid polymer that gives bacterial cells their shape. High concentrations are secreted by mucosal glands, exocrine glands, neutrophils and macrophages, providing rapid defenses at epithelial surfaces and within phagosomes. In humans, tears contain up to 2 mg ml⁻¹ of lysozyme, and colostrum and saliva provide additional protection for newborns and oral tissues. The enzyme is particularly active against Gram positive bacteria because their thick peptidoglycan layer is directly exposed, whereas the outer membrane of Gram negative bacteria shields the target bond. Lysozyme also acts synergistically with antimicrobial peptides, complement proteins and other enzymes to create an inhospitable environment for microbes. Some bacteria produce inhibitors or modify their peptidoglycan to resist lysozyme, highlighting its evolutionary significance.
Presence and Applications
Outside the human body, lysozyme is abundant in hen egg white, providing protection to the developing embryo. In research laboratories, purified lysozyme is routinely used to break open Gram positive cells during genomic DNA isolation or to gently lyse Escherichia coli for protein purification. In the food industry, it can prevent spoilage of cheese and wine by targeting lactic acid bacteria and other contaminants. Lysozyme derived from chicken egg white is approved as a natural preservative in some countries, reflecting its safety and efficacy.
Lysozyme exemplifies an ancient and versatile antimicrobial strategy. Its ability to breach the peptidoglycan sacculus makes it a key player in mucosal immunity and a useful tool in molecular biology and food preservation.
Related Terms: Peptidoglycan, Innate Immunity, Gram Positive Bacteria, Antimicrobial Peptide, Lytic Cycle