Enzyme molecules function by altering other molecules. Enzymes combine with the altered molecules to form a complex molecular structure in which chemical reactions take place. The enzyme, which remains unchanged, then separates from the product of the reaction. A single enzyme molecule can perform its entire function a million times a minute. The chemical reactions occur thousands or even millions of times faster with enzymes than without them.
Most enzymes are proteins.
The human body has thousands of kinds of enzymes. Each kind does one specific job. Without enzymes, a person could not breathe, see, move, or digest food. Photosynthesis in plants also depends on enzymes. Many enzymes break down complex substances into simpler ones.