Enzyme, any of the more than 1,000 proteins that act as catalysts in chemical reactions in life processes. Generally, enzymes speed up chemical reactions in cells that otherwise would occur too slowly to sustain life. The enzyme itself is not changed by the reaction that it stimulates. Some enzymes cannot function without accessory substances, called coenzymes, which are supplied by food. Many minerals and vitamins act as coenzymes, which is why their absence in a diet may be harmful.
See also: Protein.