Enzymes are macromolecular biological catalysts. They are responsible for thousands of metabolic processes that sustain life. The study of enzymes is called enzymology. Enzymes are highly selective catalysts, greatly accelerating both the rate and the specificity of metabolic chemical reactions: from the digestion of food to the synthesis of DNA. Most enzymes are proteins, although a few are catalytic RNA molecules, such as theribosome. Enzymes fold into a specific three-dimensional structure, and may use organic (e.g. biotin) and inorganic (e.g. magnesium ion) cofactors to aid catalysis.

Enzymes work by converting starting molecules (substrates) into different molecules (products). Almost all chemical reactions in a biological cell need enzymes to happen at rates high enough to sustain life. Because enzymes are selective for their substrates, they speed up only a few reactions from among many possibilities. The set of enzymes made in a cell determines which metabolic pathways occur in that cell.

Like all catalysts, enzymes increase the rate of a reaction by lowering its activation energy. Most enzyme reaction rates are millions of times faster than the un-catalyzed reactions, and some are so fast that they are limited only by how fast substrates can diffuse to the enzyme. As with all catalysts, enzymes are not consumed by the reactions they catalyze, nor do they alter the equilibrium of these reactions. However, enzymes do differ from most other catalysts in that they are highly specific for their substrates. Enzymes are known to catalyze about 4,000 biochemical reactions.

Enzyme activity can be affected by other molecules: decreased by inhibitors or increased by activators. Manydrugs and poisons are enzyme inhibitors. Activity is also affected by temperature, pressure, chemical environment (e.g., pH), and the concentration of substrate. Some enzymes are used commercially, for example, in the synthesis of antibiotics. In addition, some household products use enzymes to speed up biochemical reactions (e.g., enzymes in biological washing powders break down protein or fat stains on clothes; enzymes in meat tenderizers break down proteins into smaller molecules, making the meat easier to chew).