Lysozyme

Lysozyme, also called muramidase, is a 14.4 kilodalton enzyme that damages bacterial cells by catalyzing the hydrolysis of beta 1,4 bonds between residues of N-acetylmuramic acid and N-acetyl-D-glucosamine in a peptidoglycan. Lysozyme is abundant in many secretions such as saliva, tears, and mucus. It is abundant in human milk (about 40mg/100ml) and mare's, where it constitutes one of the defense factors, while it is practically absent in ruminant and guinea pig milk. It is also present in the cytoplasmic granules of PMN polymorphonuclear neutrophils. Lysozyme deficiency, due to mutations in the LYZ gene located on chromosome 12, has been associated with skeletal dysplasias and an increased susceptibility to infections.

Egg white contains a large amount of this enzyme. Lysozyme was discovered by Fleming, the same person who discovered penicillin. It acts as a barrier against infections. In industry, that obtained from egg white is used to control lactic bacteria in wines. It is also used in the manufacture of cheeses, to protect them from alterations by Clostridium.

Physiology

Many of the bacteria affected by lysozyme are not pathogenic. In some cases, lysozyme is the main reason why these organisms do not become pathogenic. Lysozyme also disrupts the cell wall of pathogenic bacteria by transforming them into spheroplasts or protoplasts, called L forms. Lysozyme can act as an innate opsonin or as a catalytic enzyme. Lysozymes serve as innate opsonins by binding to the bacterial surface, reducing the negative charge and facilitating phagocytosis of the bacteria, all before the arrival of opsonins from the immune system. In other words, lysozyme makes it easier for phagocytic cells to take up bacteria. As an enzyme, it works by attacking peptidoglycans, which explains its location in the cell wall of bacteria, especially gram positive ones. Lysozyme hydrolyzes the glycosidic bond between carbon 1 (C1) of the N-acetylmuramic acid (NAM) residue and carbon 4 (C4) of N-acetylglucosamine (NAG).

It does this by binding the peptidoglycan molecule at the active site, a prominent cleft between its two domains, causing the substrate molecule to assume a very tense, transition-state-like conformation. According to the Phillips mechanism, the lyzozyme binds to a hexasaccharide deforming the fourth sugar (the D ring) into a half-chair conformation. In this state of tension the glycosidic bond is easily broken.

Glutamic acid 35 (Glu35) and aspartic acid 52 (Asp52) have been found to be essential for the activity of this enzyme. Glu35 acts as a proton donor for the glycosidic bond, cutting the C-O bond in the substrate. Asp52 acts as a nucleophile to generate the enzyme-glycosidic intermediate (enzyme-substrate complex). This intermediate then reacts with a water molecule to leave the enzyme intact again and release the hydrolysis product.

Role in pathologies

Some forms of amyloidosis are caused by a mutation of the lysozyme gene, which leads to the accumulation of this enzyme in various tissues.

History

Alexander Fleming (1881-1955), who discovered penicillin, discovered lysozyme in 1922. Its structure was described by David Chilton Phillips (1924-1999) in 1965, when he obtained an image with a resolution of 2 angstroms (200 pm). This work led Phillips to provide an explanation for how enzymes speed up reactions. in terms of its physical structure. The original mechanism proposed by Phillips was later revised.

Howard Florey (1898-1968) and Ernst B. Chain (1906-1979) also investigated lysozymes. Although they never made much progress in this field, they, along with Fleming, developed penicillin.