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Enzymes are biological catalysts that accelerate chemical reactions in living organisms. Most are proteins, though some RNA molecules have catalytic activity (ribozymes). They act by binding substrates at their active sites, forming an enzyme-substrate complex that lowers the activation energy. Enzymes are highly specific for their substrates and typically catalyze a single type of reaction. They are not consumed in the reaction and can be used repeatedly.

Enzymes have intricate structures, with active sites formed by a region of the protein where substrates bind. The induced-fit model describes how shape changes during binding. Many enzymes require cofactors, such as metal ions or organic coenzymes (often vitamins). The protein portion without its cofactor is an apoenzyme; with its cofactor it is a holoenzyme. Activity is influenced by temperature and pH, and by inhibitors or activators; each enzyme has an optimum temperature and pH before denaturation or loss of function.

Classification follows the Enzyme Commission (EC) system into six main classes: oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. Kinetic behavior is commonly described by Michaelis-Menten parameters such as Vmax and Km. Inhibition can be competitive or noncompetitive and may be reversible or irreversible.

Applications are broad, including digestion of carbohydrates, proteins, and fats; industrial processes such as fermentation and biocatalysis; and medical contexts as drug targets or diagnostic tools. Well-known examples include amylase, protease, lipase, lactase, and DNA polymerases.