Lysozyme

Based on Wikipedia: Lysozyme

In 1922, Alexander Fleming—yes, that Alexander Fleming, the man who would later discover penicillin—was suffering from a cold. Being a microbiologist, he did what any scientist would do: he let some of his nasal mucus drip onto a petri dish full of bacteria. What happened next surprised him. The bacteria dissolved. Something in human snot was killing microbes.

Fleming had discovered lysozyme, though he didn't know its structure or exactly how it worked. He just knew that our bodies produce a substance capable of dissolving bacterial cells like sugar in hot tea. "As this substance has properties akin to those of ferments," he wrote, "I have called it a 'Lysozyme'"—combining the Greek "lysis" (to loosen or dissolve) with "enzyme."

This was six years before Fleming's more famous discovery of penicillin. And in some ways, lysozyme is the more remarkable find. Penicillin is something we extract from mold and turn into medicine. Lysozyme is something your body manufactures continuously, right now, as you read this sentence. It's flowing in your tears, coating your mouth in saliva, present in your nasal passages, and—if you're a nursing mother—saturating the milk you feed your infant.

Your Body's Built-In Antibiotic

Lysozyme belongs to your innate immune system—the ancient, always-on defense network that doesn't need to "learn" about threats the way your adaptive immune system does. While antibodies are custom-made weapons against specific invaders, lysozyme is more like a standing army, perpetually on patrol.

Its targets are primarily gram-positive bacteria. This classification, named after Hans Christian Gram who developed the staining technique in 1884, divides bacteria based on their cell wall structure. Gram-positive bacteria have thick walls made largely of a molecule called peptidoglycan—a mesh-like structure that gives bacterial cells their shape and protects them from bursting.

Lysozyme is a molecular lockpick. It finds the glycosidic bonds holding peptidoglycan together and breaks them. Without its structural support, the bacterial cell wall fails. The cell's internal pressure pushes outward against a weakened shell, and the bacterium essentially explodes. Microbiologists call this process lysis—the same root word Fleming used when naming the enzyme.

Think of it this way: bacteria live under constant osmotic pressure, like balloons that are slightly over-inflated. Their cell walls are what keep them from popping. Lysozyme pokes holes in that wall.

Where Your Tears and Egg Whites Agree

One of the highest concentrations of lysozyme in nature occurs in a seemingly unrelated place: chicken egg whites. This makes perfect biological sense once you think about it. A developing chick embryo sits for weeks inside a warm, moist shell—ideal conditions for bacterial growth. The egg white serves as a chemical moat, with lysozyme acting as one of its primary defenders.

Human tears contain lysozyme for similar reasons. Your eyes are exposed, vulnerable, constantly facing a world teeming with microorganisms. The conjunctiva—the thin membrane covering your eye—doesn't have the advantage of being dry and acidic like your skin. Instead, it relies on secreted enzymes for protection. When these defenses fail, you get conjunctivitis: pink eye.

Human milk is even more concentrated in lysozyme. Breast milk contains somewhere between 1,600 and 3,000 times more lysozyme than cow's milk. This isn't surprising when you consider that human infants, unlike calves, are born with relatively underdeveloped immune systems and need all the help they can get.

Researchers have actually created transgenic goats—the founder of the line was named Artemis—that produce milk containing human lysozyme. The goal was to create a milk that could help protect children from diarrheal diseases in regions where breastfeeding isn't always possible. Piglets fed human lysozyme milk recovered faster from E. coli infections in laboratory studies.

The First Enzyme We Truly Saw

Lysozyme holds a special place in the history of biochemistry. In 1937, Edward Abraham—who would later play a crucial role in developing penicillin for clinical use—crystallized it for the first time. Crystallization was essential because it allowed scientists to use X-ray diffraction to determine the molecule's three-dimensional structure.

The breakthrough came in 1965, when David Chilton Phillips obtained the first high-resolution X-ray crystallography model of lysozyme. He presented the structure at a Royal Institution lecture that year. This was only the second protein structure ever solved, and the very first enzyme structure. It was also the first enzyme to be fully sequenced containing all twenty of the common amino acids.

Why does this matter? Because seeing the structure of an enzyme allowed scientists to understand how it actually worked. Phillips could look at lysozyme's shape—a cleft between two domains where the target molecule fits—and propose a specific mechanism for how it cuts peptidoglycan bonds. For the first time, biochemists could explain enzyme function in terms of physical architecture rather than abstract chemical notation.

The Debate Over How It Actually Works

Phillips proposed what became known as the Phillips mechanism. He suggested that lysozyme worked through two processes: physically straining the target molecule by bending it into an uncomfortable position, and then stabilizing a charged intermediate state during the cutting reaction.

In his model, a hexasaccharide—a chain of six sugar units—binds into the enzyme's cleft. The enzyme distorts the fourth sugar unit, forcing it from its normal chair-like shape into what chemists call a half-chair conformation. This is like bending a stick until it's stressed and ready to snap. The physical strain weakens the glycosidic bond that needs to break.

For decades, this ionic mechanism was widely accepted. But science is a conversation, not a pronouncement, and in 2001 a researcher named Vocadlo proposed a revision. Using mass spectrometry to analyze the reaction, his team found evidence for a covalent intermediate—meaning the enzyme actually forms a temporary chemical bond with part of the substrate during the reaction, rather than just stabilizing a charged state.

More recent quantum mechanics calculations have supported this revised mechanism, suggesting that the covalent pathway is about 30 kilocalories per mole more stable than the ionic pathway Phillips originally proposed. The science continues to evolve, and our understanding of this enzyme discovered over a century ago is still being refined.

A Molecular Machine with Two Gears

Recent work with carbon nanotube sensors has revealed something elegant about lysozyme's operation. The enzyme exists in two conformational states: an open active form and a closed inactive form.

When it's working, lysozyme is remarkably efficient. It can processively hydrolyze its substrate—meaning it doesn't release and rebind for each cut but instead slides along, snipping bonds in sequence. On average, it breaks about 100 bonds at a rate of 15 per second.

Transitioning from the closed inactive state to the open active state requires two conformational changes—two gear shifts, if you will. Inactivation requires only one. This asymmetry may help the enzyme stay engaged with its work once it starts.

Beyond Just Breaking Bacteria

For years, researchers assumed that lysozyme's antimicrobial power came entirely from its ability to digest bacterial cell walls. But evidence has emerged for non-enzymatic activity as well.

In one experiment, scientists mutated a critical amino acid in lysozyme's active site—changing aspartate at position 52 to serine. This mutation should have disabled the enzyme's catalytic function. And it did. But remarkably, the mutant lysozyme still showed antimicrobial activity. Something besides its wall-breaking ability was contributing to bacterial killing.

Lysozyme appears to have what researchers call lectin-like properties—the ability to recognize and bind to bacterial carbohydrate molecules even without destroying them. It also interacts with antibodies and T-cell receptors, suggesting deeper integration with the broader immune system than the simple "bacterial wall dissolver" label implies.

When Lysozyme Goes Wrong

Like most biological molecules, lysozyme can cause problems when its levels are abnormal. In certain cancers—particularly myelomonocytic leukemia—cancer cells produce excessive amounts of lysozyme. These toxic blood levels can lead to kidney failure and dangerously low potassium. Treating the underlying cancer often resolves these complications.

Serum lysozyme levels are also used as a marker for sarcoidosis, an inflammatory disease that causes granulomas to form in various organs. While it's less specific than measuring angiotensin converting enzyme (another diagnostic marker), lysozyme is more sensitive, making it useful for monitoring disease activity in patients already diagnosed with the condition.

A Laboratory Workhorse

Beyond its biological role, lysozyme has become an essential tool in molecular biology laboratories. When researchers need to break open bacterial cells to extract proteins or other molecules, lysozyme offers a gentle alternative to harsher mechanical methods.

The technique is particularly useful for what's called periplasmic extraction. Bacteria like E. coli have a space between their inner and outer membranes called the periplasm, where certain proteins accumulate. Lysozyme can digest the outer wall, releasing periplasmic contents while leaving the inner membrane intact as sealed vesicles called spheroplasts.

The optimal conditions are fairly specific: temperatures up to 60 degrees Celsius, a pH range of 6.0 to 7.0, and careful attention to salt concentrations. Sodium chloride actually promotes lysis at low concentrations but inhibits it at high concentrations—a useful dial for controlling the reaction.

There's an ironic complication, though. When scientists use lysozyme to help crystallize other proteins, some lysozyme molecules can contaminate the final crystal. In fact, certain proteins seem unable to crystallize without such contamination—a reminder that even our most refined laboratory techniques carry fingerprints of the tools we use.

A Century of Discovery

The story of lysozyme spans over a hundred years, from Laschtschenko's 1909 observation that egg whites could kill bacteria, through Fleming's characterization of the enzyme in 1922, to the present day where quantum mechanical simulations probe its atomic-level mechanisms.

In 2007, researchers finally achieved the complete chemical synthesis of a functional lysozyme molecule—a goal that scientists at the University of Liverpool had been pursuing for decades. This wasn't just a technical achievement; it demonstrated that we could build from scratch a molecule that evolution had refined over millions of years.

Fleming went on to discover penicillin, which revolutionized medicine and saved countless millions of lives. But lysozyme, his earlier discovery, reminds us that our bodies have been fighting bacteria since long before we knew bacteria existed. Every time you cry, every time you nurse an infant, every time you swallow, you deploy an enzyme that Alexander Fleming first noticed because he had a runny nose and the curiosity to see what it might do to bacteria in a dish.

The molecule in your tears is the same one that made scientific history—the first enzyme whose structure we solved, the first for which we proposed a detailed mechanism. It connects you to Fleming in his laboratory, to Phillips presenting at the Royal Institution, to the millions of years of evolution that shaped this elegant molecular machine.

Your body makes it constantly, without any conscious effort on your part. It's been protecting you since before you were born.