Lysozyme is a protein found in tears, saliva, and other secretions. It can degrade the cell walls of certain kinds of bacteria and acts as a natural antibiotic. This protein is also found in egg white and is one of the major contributors to egg allergies.
While most of the body is protected by a covering of skin, the eyes and mucus membranes do not have this protection. Lysozyme is produced in tears and mucus secretions to protect these areas against invasion by bacteria. It is also present in the blood to help keep bacteria from being spread throughout the body. This protein is part of the immune system and is also found in human milk. It is passed to children through breast-feeding, helping them to establish their immune system.
Eggs from hens have a large quantity of this protein in their whites, which are more prone to cause allergies than the yolks. It is one of the top four proteins in the egg white to which people who have food allergies from eggs have the strongest allergic response. There is no treatment for this, except for scrupulously avoiding egg products, which can be a difficult process. Even the anesthetic propofol contains egg lecithin.
Lysozyme is of historical interest, since it was the first antibacterial substance discovered. Sir Alexander Fleming, who went on to discover penicillin, had a cold and put a drop of mucus on a plate of bacteria. He discovered that the mucus killed the bacteria, but the molecule is too large to be practical as a drug. Many years later, this protein was the first enzyme to have its three-dimensional structure solved.
The manner in which lysozyme protects against bacterial invasion is through its activity as an enzyme — an agent that speeds up reactions. The bacteria affected have polysaccharides in their cell walls that are chains of sugars with side chains that contain amine, NH2, groups. The protein degrades these polysaccharides by adding a molecule of water to the sugar linkage, causing it to break open. This is known as glycoside hydrolase, or water breakdown of sugar. Once the polysaccharide chain is disrupted, the bacterial cells burst.
There are several types of lysozymes found in a variety of different organisms, forming a family of enzymes. In humans, the gene is known as the LYZ gene. Another term for the enzyme is muramidase, because it cleaves the bond that connects N-acetylmuramic acid to its adjoining sugar molecule. The technical name for the enzyme is N-acetylmuramide glycanhydrolyase.