Vast group of protein molecules, produced by all living things, that act as catalysts in chemical and biological reactions, including photosynthesis, helping cells communicate, inhibiting free-radical damage, and much more. Enzymes are used in skin-care products to facilitate exfoliation, to help overall biological processes in skin that have slowed down because of age or sun damage, and to inhibit free-radical damage. Enzymes accelerate biochemical reactions in a cell that would proceed minimally or not at all if the enzymes weren’t present. Most enzymes are finicky about how and under what conditions they will act. Sometimes several enzymes are required to carry out a particular chemical reaction, and their actions are affected by temperature and pH. Some enzymes depend on the presence of other enzymes, called coenzymes, to function, or they depend on a specific body temperature. It would require an exceptionally complicated process to stimulate enzyme activity via topical application to the skin. Enzymes are divided into six main categories, including oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. The names of most individual enzymes end in –ase.