Enzyme Commission number

The Enzyme Commission number (EC number) is a numerical classification scheme for enzymes, based on the chemical reactions they catalyze. As a system of enzyme nomenclature, every EC number is associated with a recommended name for the respective enzyme.
Strictly speaking, EC numbers do not specify enzymes, but enzyme-catalyzed reactions. If different enzymes (for instance from different organisms) catalyze the same reaction, then they receive the same EC number. Furthermore, through convergent evolution, completely different protein folds can catalyze an identical reaction and therefore would be assigned an identical EC number (these are called non-homologous isofunctional enzymes, or NISE). By contrast, UniProt identifiers uniquely specify a protein by its amino acid sequence.
Format of number.
Every enzyme code consists of the letters "EC" followed by four numbers separated by periods. Those numbers represent a progressively finer classification of the enzyme. Preliminary EC numbers exist and have an 'n' as part of the fourth (serial) digit (e.g. EC 3.5.1.n3).
For example, the tripeptide aminopeptidases have the code "EC 3.4.11.4", whose components indicate the following groups of enzymes:
Reaction similarity.
Similarity between enzymatic reactions (EC) can be calculated by using bond changes, reaction centres or substructure metrics (formerly EC-BLAST, now the EMBL-EBI Enzyme Portal).
History.
The enzyme nomenclature scheme was developed starting in 1955, when the International Congress of Biochemistry in Brussels set up an Enzyme Commission. The first version was published in 1961. The current sixth edition, published by the International Union of Biochemistry and Molecular Biology in 1992, contains 3196 different enzymes. Supplements 1-4 were published 1993-1999. Subsequent supplements have been published electronically, at the website of the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology. In August 2018, the Enzyme Commission reorganized the nomenclature by adding the top-level EC 7 category describing translocases.