enzyme /en″zīm/ [Gk, en, in, zyme, ferment] , a protein produced by living cells that catalyzes chemical reactions in organic matter. Most enzymes are produced in tiny quantities and catalyze reactions that take place within the cells. Digestive enzymes, however, are produced in relatively large quantities and act outside the cells in the lumen of the digestive tract. The substance that is acted upon by an enzyme is called a substrate. See also substrate.
