An enzyme – formerly called ferment – is a substance that is able to accelerate a chemical reaction (i.e. acts as a catalyst). Enzymes consist of very large biomolecules. Almost all enzymes are proteins; only the catalytically active RNA is an exception. Like other proteins, enzymes are formed by protein biosynthesis on the ribosomes. They take over important functions in the metabolism of organisms. In particular, they control the majority of all biochemical reactions – from digestion to transcription (RNA polymerase) and replication (DNA polymerase) of genetic information.