Enzymes are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. Almost all metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life.
The International Union of Biochemistry and Molecular Biology have developed a nomenclature for enzymes, the EC numbers. Each enzyme is described by "EC" followed by a sequence of four numbers which represent the hierarchy of enzymatic activity.