An enzyme is a large molecule that acts as a selective catalyst, accelerating metabolic reactions in an organism. In enzymatic reactions, the initial molecule is called a "substrate" and a molecule resulting from the reaction is called a "product."

Biological enzymes are highly substrate-specific, and every enzyme-substrate pair fits into one another like a lock and key. Some enzymes require additional components ("cofactors") to begin a reaction, such as metal ions or complex organic compounds like ATP.

Most enzymes are proteins, although some catalytic molecules ("ribozymes") were discovered in the early 1980s.