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An enzyme is a biological molecule that catalyzes a chemical reaction or causes a chemical change in another substance. The molecules transformed by enzymes are called substrates. Some enzymes act independently but others require the assistance of coenzymes in order to function correctly. Enzymes are generally globular proteins and range from just 62 amino acid residues in size. A small number of RNA-based biological catalysts exist, with the most common being the ribosome; these are referred to as either RNA-enzymes or ribozymes.
An enzyme's name is often derived from its substrate or the chemical reaction it catalyzes. The International Union of Biochemistry and Molecular Biology have developed a nomenclature for enzymes based broadly on the mechanism of action. Each enzyme is described by a sequence of four numbers preceded by "EC".
Enzymes are usually very selective as to which reactions they catalyze and the substrates that are involved in these reactions. This selectivity is extremely important as enzymes serve a wide variety of key functions inside living organisms. They are essential for metabolic function and signal transduction pathways. Tight control of enzyme activity is essential for homeostasis. Malfunction of a single enzyme by mutation, overproduction, underproduction or deletion, can lead to a genetic disease. For example, mutations in genes coding for DNA repair enzymes cause hereditary cancer syndromes such as Xeroderma Pigmentosum. Enzyme inhibitors, substrates and activators are commonly used in the treatment of many diseases, for example, cyclooxygenase inhibitors and the treatment of inflammation.