Classification and Nomenclature of Enzymes

Classification and Nomenclature of Enzymes

Because thousands of different enzymes exist, the International Union of Biochemistry created a standard scheme that sorts every enzyme into one of six major classes based purely on the type of reaction it catalyses. Oxidoreductases catalyse oxidation-reduction reactions, in which electrons or hydrogen atoms are transferred between two substrates. Transferases catalyse the transfer of a chemical group (such as a phosphate, amino or methyl group) from one molecule to another. Hydrolases catalyse the breaking of a bond by the addition of a water molecule (hydrolysis), as seen in most digestive enzymes. Lyases remove a group from a substrate without hydrolysis, often creating a double bond, or add a group across an existing double bond. Isomerases rearrange the atoms within a single molecule to convert it into one of its isomers, without changing its overall molecular formula. Ligases join two separate molecules together into one, a reaction that generally requires energy supplied by ATP.

Every enzyme is additionally assigned a unique Enzyme Commission (EC) number made up of four digits separated by full stops. The first digit states which of the six major classes the enzyme belongs to; the second digit gives its sub-class, narrowing down the type of group or bond involved; the third digit gives the sub-subclass, specifying the reaction still more precisely; and the fourth digit is simply the serial number of that particular enzyme within its sub-subclass, distinguishing it from other enzymes that share the same first three digits. Alongside this systematic numbering, enzymes are also given working names for everyday use, usually formed by adding the suffix '-ase' to the name of the substrate they act on (for example, an enzyme acting on urea is called urease) or to the type of reaction they perform. …