Enzymes are protein molecules in cells which work as catalysts. Enzymes speed up chemical reactions in the body, but do not get used up in the process.
Almost all biochemical reactions in living things need enzymes. With an enzyme, reactions work very much faster than they would without.
The substances at the start of the reaction are called substrates. The substances at the end of the reaction are the products. Enzymes work on the substrates, and turn them into products.
Anselme Payen found the first enzyme in 1833.