Enzymes are complex proteins that act as catalysts for the countless biochemical reactions that keep humans, animals, plants, and microorganisms alive. Constituents of every living cell, enzymes have relatively large molecules that contain one or more amino acid chains. The sequence of amino acids within the chains and the distinctive way each chain folds into its own characteristic three-dimensional shape help determine the enzyme's particular activity. In order for them to act, many enzymes also need to be attached to a nonprotein substance called a coenzyme. In most cases, these coenzymes are composed wholly or partially of vitamins, especially those in the water-soluble B family. The typical animal cell (roughly one-billionth the size of a drop of water) contains about three thousand different enzymes, almost all programmed to perform specific chemical reactions necessary for metabolism.

For example, in the digestive tract certain enzymes are involved in breaking down...