chitin
Meaning of chitin: The tough material that forms insect shells and fungal cell walls, and a useful drug target precisely because humans do not make it.
Definition of chitin
Chitin is a tough, fibrous carbohydrate that forms the hard outer skeleton of insects, crabs, shrimp and lobsters, and the cell walls of fungi. Long chains of a modified sugar lie side by side and bond to one another, producing a material that is strong, flexible and almost insoluble in water. Humans and other mammals do not make it.
What is it made of?
The building block is N-acetylglucosamine, a glucose molecule carrying a small nitrogen-containing group. Thousands of these link into a long chain, and the chains stack in sheets held together by many weak bonds. The arrangement resembles that of cellulose in plants, which is why the two materials behave similarly, but the added nitrogen group makes chitin tougher. As a polysaccharide it is a structural material rather than a fuel, and the animals that build with it cannot digest it either.
Where is it found, and where is it not?
| Organism | Role of chitin |
|---|---|
| Insects and spiders | The rigid external skeleton |
| Crabs, shrimp, lobsters | The shell, stiffened further with calcium |
| Fungi | A key component of the cell wall |
| Plants | Absent; plants use cellulose instead |
| Mammals | Absent, though enzymes that break it down are present |
Why does it matter in medicine?
Its absence from human tissue is the point. Fungal cells depend on chitin for the wall that holds them together, so an enzyme that builds it is an attractive drug target: blocking it damages the fungus and leaves human cells alone. The same principle underlies several agricultural fungicides and insect growth regulators. Chitin fragments in the air are also recognized by the immune system as a signal of fungal or insect presence, and that recognition is under study for its part in asthma and allergic airway disease. Humans do produce enzymes capable of breaking chitin down, a curious survival in a body that builds none of it.
What is chitosan?
Chitosan is chitin that has been chemically stripped of most of its nitrogen-containing groups, which makes it soluble in acid and easier to work with. It is used in wound dressings, where it helps blood clot and forms a barrier, and it is sold as a weight loss supplement on the claim that it binds dietary fat, a claim with weak supporting evidence. Both chitin and chitosan are usually manufactured from shellfish waste, which is why products carry a shellfish warning.
Used in a sentence
The antifungal agent acts by interfering with chitin synthesis in the fungal cell wall.
Good to know
Chitin occurs in fungi and arthropods but not in plants or in mammalian tissue, and that absence is what makes its assembly a selective target for antifungal and insecticidal agents.
Word origin
Where “chitin” comes from
From the Greek chiton, meaning a tunic or covering, a reference to the outer casing it forms.