Hen egg white lysozyme is a small protein found in the albumen of chicken eggs, constituting about 3.5% of the total egg white proteins. This enzyme serves as a defense system, acting as a muramidase that targets and breaks down bacterial cell walls. Its role is to protect the developing chick embryo from microbial contamination.
Identification and Biological Role
The antibacterial properties of lysozyme were first observed by Alexander Fleming in 1921, several years before his discovery of penicillin. Fleming noticed a clear zone of bacterial dissolution after a drop of his nasal mucus fell onto a culture plate. He named the substance “lysozyme,” combining the Greek word for dissolution, lysis, with the suffix enzyme.
Lysozyme is present as an innate immune defense mechanism in various biological secretions across many species, including human tears, saliva, and milk. In chickens, the enzyme’s high concentration in egg white provides a first line of defense, safeguarding the egg’s interior from bacteria. The protein consists of a single polypeptide chain of 129 amino acids, cross-linked by four disulfide bridges.
The Enzyme’s Lytic Mechanism
Hen egg white lysozyme’s destructive power stems from its specific muramidase activity. Lysozyme functions as a hydrolytic enzyme, using water to break specific chemical bonds in the bacterial cell wall. The enzyme’s primary target is peptidoglycan, a rigid polymer that forms the structural exoskeleton of most bacterial cells.
Peptidoglycan is composed of alternating sugar molecules, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), which are linked together in long chains. Lysozyme specifically recognizes and cleaves the \(beta\)-(1,4)-glycosidic bond that connects the NAM molecule to the NAG molecule. This enzymatic cleavage rapidly weakens the structural integrity of the bacterial cell wall, which is especially effective against Gram-positive bacteria that possess a thick, exposed peptidoglycan layer.
Once the rigid cell wall is compromised, the bacterial cell becomes osmotically unstable. The high internal pressure inside the cell forces water to rush in. This influx of water causes the cell membrane to rupture, a process called lysis, which leads to the death of the bacterium. Lysozyme’s effect is less pronounced on Gram-negative bacteria because their peptidoglycan layer is protected by an outer lipopolysaccharide membrane.
Industrial and Pharmaceutical Applications
Hen egg white lysozyme is widely utilized due to its potent antibacterial action. The enzyme is classified as a Generally Recognized As Safe (GRAS) substance and is employed extensively in the food industry as a preservative. In Europe, lysozyme is designated as the food additive E1105.
Its primary use in food processing is to control the growth of spoilage and pathogenic bacteria, particularly Gram-positive species. For instance, lysozyme is added during cheese production to inhibit the growth of Clostridium tyrobutyricum, a bacterium responsible for “late blowing” and off-flavors in semi-hard cheeses. It is also used in the wine industry to limit the proliferation of lactic acid bacteria, which can cause undesirable secondary fermentation and alter the wine’s flavor profile.
Lysozyme also has growing applications in medicine and animal health. Its inherent antibacterial and anti-inflammatory properties make it a component in some topical pharmaceuticals, such as wound care products and oral treatments. Furthermore, lysozyme is included in animal feed as an alternative to traditional antibiotics, promoting gut health and providing an effective control measure against bacterial pathogens in livestock.