Lysozyme is a naturally occurring enzyme produced by animals that functions as a non-specific defense against microorganisms, forming part of the innate immune system. This protein acts as a biological catalyst to break down foreign materials. Discovered in 1921 by Alexander Fleming, who also discovered penicillin, lysozyme is sometimes referred to as the body’s own antibiotic for its ability to rapidly dissolve certain bacteria. Its primary importance lies in providing a protective barrier on various body surfaces, guarding against environmental pathogens before they can establish an infection.
The Structure and Function of Lysozyme
Lysozyme is classified as a glycoside hydrolase, a type of enzyme that uses water to break specific chemical bonds. It is a relatively small, globular protein possessing a compact structure with a deep cleft on its surface. This cleft functions as the active site where the enzyme binds to its specific target molecule.
The target of lysozyme is peptidoglycan, a large molecule that forms the rigid mesh-like structure of bacterial cell walls, particularly in Gram-positive bacteria. Peptidoglycan chains are made of alternating sugar molecules, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). Lysozyme specifically targets and cleaves the $\beta$-(1,4)-glycosidic bond connecting the NAM and NAG units in this chain.
This process is known as hydrolysis, where the enzyme inserts a water molecule to break the bond in the peptidoglycan backbone. By breaking these structural links, the enzyme compromises the integrity of the bacterial cell wall. Without its supporting cell wall, the bacterial cell cannot withstand its internal osmotic pressure and rapidly bursts, a process termed cell lysis. This mechanical destruction is highly effective and represents the enzyme’s primary mechanism of action against susceptible bacteria.
Where Lysozyme is Found in Nature
Lysozyme is found in animals, plants, and microorganisms, indicating its conserved role in defense. In humans, the enzyme is present in various bodily secretions exposed to the external environment, acting as a chemical barrier. High concentrations are found in tears, protecting the delicate surface of the eye from bacterial invasion.
It is also abundant in saliva and mucus, providing a first line of defense in the mouth, nasal passages, and respiratory tract against inhaled or ingested pathogens. Lysozyme is a significant component of human breast milk, where its concentration is substantially higher than in livestock milk. This provides passive immunity to infants, helping to protect their digestive system from bacterial infections.
Outside of the human body, a historically significant source is the egg white of birds, such as chickens. The lysozyme in egg whites serves to protect the developing embryo from microbial contamination. Hen egg-white lysozyme has been extensively studied and is often used as a model for understanding protein structure and function due to its abundance and stability.
Lysozyme’s Role in Immune Defense
Lysozyme is a fundamental component of the innate immune system. Its constant presence in mucosal surfaces and bodily fluids ensures it can immediately attack foreign bacteria upon contact. This ubiquitous distribution acts as a chemical sentinel, preventing the initial establishment of infection at common entry points.
Beyond its direct bacteriolytic activity, lysozyme works closely with professional immune cells, such as phagocytes. Phagocytes, including macrophages and neutrophils, engulf and destroy foreign invaders. Once a bacterium is internalized by a phagocyte, lysozyme is delivered to the internal compartment to assist in the degradation of the engulfed pathogen.
The enzyme’s action inside the phagocyte helps break down the bacterial cell wall, releasing bacterial components like peptidoglycan fragments. These fragments can then activate internal cellular receptors, stimulating a robust immune response that recruits other immune cells to the site of infection. Therefore, lysozyme not only kills bacteria directly but also modulates the immune response, helping the body to fight invading organisms more effectively.