Lysozyme

Lysozyme
Hen egg-white lysozyme (PDB 1LYZ) rising from an animated eye, a frame from the front-page video of Molecular Reality Corporation, 2026
Identifiers
Organism	Widespread in animals; the best studied form is from hen egg white
Symbol	LYZ
PDB structure	1LYZ
Properties
Subunits	Monomer; 129 amino acids (hen egg white)
Function	Hydrolysis of peptidoglycan in bacterial cell walls

Lysozyme is an enzyme that kills bacteria by cutting through their cell walls. It breaks the bond between two sugars, N-acetylmuramic acid and N-acetylglucosamine, that alternate in the long chains of peptidoglycan, the mesh that holds a bacterial cell together against its own internal pressure.^[1] It is abundant in tears, saliva, mucus and milk, and especially in egg white. It was named by Alexander Fleming in 1922, and in 1965 the enzyme from hen egg white became the first enzyme whose three-dimensional structure was determined by X-ray crystallography.

An antibacterial property of egg white had been reported in 1909 by Laschtschenko.^[1] Fleming found the enzyme by accident. In late 1921, suffering from a cold, he cultured some of his own nasal mucus and saw that it dissolved the colonies of a yellow bacterium, which he named Micrococcus lysodeikticus (now Micrococcus luteus). He went on to test other fluids, among them tears contributed by colleagues, which were especially rich in the substance. His paper, received on February 13, 1922, was titled "On a remarkable bacteriolytic element found in tissues and secretions", and explained the name: "As this substance has properties akin to those of ferments I have called it a 'Lysozyme'."^[2] Lysozyme attacked mostly harmless bacteria, and it was of little use as a medicine. In 1928 Fleming noticed that a mold contaminating a culture plate had killed the bacteria around it, and penicillin made him famous instead.

Hen egg-white lysozyme is a single chain of 129 amino acids, with a mass of about 14.3 kilodaltons, held in shape by four disulfide bonds. Its structure was solved at a resolution of 2 ångströms in 1965 by David Phillips's group at the Royal Institution in London. It was among the first protein structures to be solved and the first of an enzyme.^[3] The model showed a deep cleft across the molecule in which a chain of six sugars can lie. The bond is cut between the fourth and fifth, beside two acidic side chains: glutamic acid 35, which donates a proton, and aspartic acid 52.^[1]

How the cut is made was argued for decades. Phillips proposed that the enzyme strains the sugar and stabilizes a short-lived positive ion until water completes the reaction. A competing view held that aspartic acid 52 bonds to the sugar for a moment. In 2001 Stephen Withers's group slowed the reaction with a fluorinated substrate until the intermediate accumulated, and detected it by mass spectrometry: it was covalent.^[4] The entry 1LYZ in the Protein Data Bank, a refinement of the hen structure published in 1974, was deposited in February 1975, four years after the archive opened.^[5]

A different enzyme with the same activity, the lysozyme of the bacteriophage T4, which a virus uses to break out of the bacterium it has infected, became a standard test protein for studies of how mutations change a protein's stability.

Egg-white lysozyme is cheap to purify, crystallizes readily and is stable, and generations of students and crystallographers have grown their first protein crystals from it. It is also a food additive, E1105, used in hard cheeses to stop the growth of bacteria that would otherwise produce gas and split the wheels as they age. In people, some mutations in the gene for human lysozyme make the protein unstable, and the misfolded enzyme builds up as amyloid fibers in the kidneys and other organs, a hereditary form of amyloidosis.

In August 2026 Molecular Reality Corporation reduced its front page to one sentence, "everybody's racing to build the brain, we're building the eye", above a twelve-second animation of an eye that opens and releases a protein from its pupil.^[6] The protein is hen egg-white lysozyme, rendered from the entry 1LYZ.^[7] The company has not said why it chose lysozyme. It is the protein Fleming found in tears. The animation is described in the article The Seeing Eye.

The enzyme turns up elsewhere in the company's work. One of the sender personas of its members' software, added on March 16, 2026, is named Moleculart Fleming, after Mozart and the discoverer; its portrait is a quiz host in black and white beside a large sign reading GAME, and it has sent no messages.^[7] The quiz JeoPOREdy! has a clue whose answer is lysozyme, filed under "Doge Bio" after the company's planned game about dogs, DogeBio; its commentary says that the game "lets you molecularly stream right from your pet's playful slobber!" The monsters of Moleculoids include two mutants of T4 lysozyme.^[8] One of the 197 animated eyes in the gallery Iridian is titled Lysozyme Phase-Shift.^[9]

• Green fluorescent protein, the protein released by Iridian's first eye
• Everybody's racing to build the brain