Lysozymes

Lysozymes are a family of small, water-soluble enzymes that catalyze the hydrolysis of beta-1,4 glycosidic bonds in peptidoglycan, specifically between N-acetylmuramic acid and N-acetylglucosamine. This action disrupts bacterial cell walls and can lead to cell lysis. Lysozymes are found in a wide range of organisms and tissues, including secretions such as tears, milk, saliva, and mucus, as well as in phagocytic cells, eggs (notably hen egg white), plants, and microorganisms. The best-known member is hen egg-white lysozyme (HEWL), a compact protein of about 130 amino acids stabilized by multiple disulfide bonds.

Mechanism and structure: The active site features two catalytic residues, typically a glutamate and an aspartate, arranged for a retaining hydrolysis mechanism that proceeds via a glycosyl-enzyme intermediate. Substrate binding occurs in a shallow groove that recognizes several sugar units along the peptidoglycan chain. The enzyme is especially effective against Gram-positive bacteria, where the peptidoglycan is readily accessible; Gram-negative bacteria are more resistant due to the outer membrane limiting substrate access.

Biological role and applications: In animals, lysozymes contribute to innate immunity by directly lysing bacteria and by modulating inflammatory responses. In humans, lysozyme is abundant in tears and mucus and present in neutrophils. Beyond biology, lysozymes are used in food preservation, biotechnology, and as model enzymes for studying protein folding, stability, and enzyme mechanics.

History: Lysozyme was discovered by Alexander Fleming in 1922 in egg white. Its structure and function have been extensively studied, making it a classic paradigm for enzyme structure–function relationships.