Enzymes are protein catalysts, the latter being substances that change the speed of chemical reactions. About 90% of cellular proteins are enzymes and some structural proteins are also enzymes. A protein is classified as an enzyme if it is known to catalyse a reaction, although it is always possible that a given protein, which is currently not classified as an enzyme, may catalyse an as yet un-recognized reaction. The Nomenclature Committee of IUBMB (International Union of Biochemistry and Molecular Biology) lists thousands of enzymes, which are classified according to the reactions they catalyse. The Nomenclature Committee publishes an enzyme catalogue, abbreviated as EC, in which each enzyme receives a four-part number. The first part indicates the general category of the reaction it catalyses as one of the following: Oxidoreductase, Transferase, Hydrolase, Lyase, Isomerase, Ligase (synthetase). The second and third parts of the EC number indicate...