Enzymes are specialized proteins that accelerate biochemical reactions within living organisms. Some enzymes serve as powerful natural defenses against harmful bacteria, acting as a first line of protection in various biological systems. These enzymatic defenses are found across the animal and plant kingdoms, highlighting their widespread importance in maintaining health and combating microbial threats.
The Primary Antibacterial Enzyme
The antibacterial enzyme is lysozyme, also known as muramidase or N-acetylmuramide glycanhydrolase. Its discovery dates back to 1921 when Alexander Fleming observed that a drop of his nasal secretions caused the dissolution of bacteria in a Petri dish. He published his findings in 1922, naming the substance “lysozyme” from “lysis” (dissolution) and “enzyme.”
Fleming continued his research, demonstrating this bacteriolytic substance was present in other body fluids. Lysozyme is a globular protein with a molecular weight of approximately 14.3 to 14.6 kDa, composed of 129 amino acids. Despite his later, more famous discovery of penicillin, Fleming believed lysozyme’s significance would eventually be recognized.
How Lysozyme Disarms Bacteria
Lysozyme disarms bacteria by targeting their cell walls, specifically the peptidoglycan layer. This mesh-like structure, composed of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) sugar molecules cross-linked by peptides, provides strength and rigidity to bacterial cells. Lysozyme acts as a glycoside hydrolase, catalyzing the hydrolysis of the β-1,4 glycosidic bonds that link NAM and NAG residues within the peptidoglycan chains.
The enzyme’s active site binds to a hexasaccharide unit of the peptidoglycan. Upon binding, lysozyme induces a distortion in the substrate, making the bond susceptible to cleavage. This enzymatic action weakens the bacterial cell wall. The compromised cell wall integrity leads to osmotic lysis, where the bacterial cell bursts due to internal pressure.
Where Lysozyme is Found
Lysozyme is widely distributed in nature, serving as a natural defense mechanism in numerous organisms. In humans, it is found in various bodily secretions that form part of the innate immune system. These include tears, saliva, mucus, and breast milk.
Beyond secretions, lysozyme is also present in cytoplasmic granules of phagocytes, such as macrophages and polymorphonuclear neutrophils, which are immune cells involved in engulfing and destroying pathogens. Outside the human body, a significant source of lysozyme is chicken egg whites, where it constitutes about 3.5% of the total protein content. Lysozyme can also be found in the milk of various mammals and has been identified in certain plants and microorganisms.
Lysozyme’s Broader Significance
Lysozyme’s significance extends beyond its direct antibacterial action, playing a role in broader host defense and offering potential applications in various fields. As a component of the innate immune system, it provides protection against a wide range of bacterial pathogens, particularly Gram-positive bacteria due to their more accessible peptidoglycan layer. Its ability to degrade bacterial cell walls can also enhance the release of bacterial products, which then activate host immune responses.
The enzyme’s properties have led to its exploration in therapeutic applications, including the treatment of infections, wound healing, and anti-biofilm strategies. For example, human lysozyme in milk has shown promise in helping piglets recover from diarrheal disease caused by E. coli. Lysozyme is also utilized in the food industry as a natural preservative, inhibiting microbial growth to extend the shelf life of products like cheese and wine. Its role in animal nutrition, by supporting gut health and reducing reliance on antibiotics in livestock farming, also addresses concerns about antibiotic resistance.