Nomenclature and Classification of Enzymes

Nomenclature and Classification of Enzymes

  • Enzymes were classified and named earlier based on different features. Most enzymes have been named by adding the suffix ‘ase’ to the name of their substrate or to a word or phrase describing their activity. Thus, urease catalyse the hydrolysis of urea and lipase helps in the hydrolysis of lipids.

  • Enzymes were also named by their discoverers for a broad function

        Eg: pepsin. In greek ‘pepsis’ means digestion and lysozyme was named for its ability to lyse (meaning breakdown) bacterial cell walls

  • Enzymes were also named based on their source

        Eg: trypsin which came from Greek tryein means to wear down and the enzyme was                obtained by rubbing pancreatic tissue with glycerine.

  • Next naming is two word naming.

Eg: pyruvate decarboxylase, First word indicates the substrate and the second word represents the reaction which is catalysed.

Sometimes the same enzyme has two or more names or two enzymes have the same name. To avoid such ambiguities and increasing number of new enzyme discoveries, an international agreement has been made for naming and classification of enzymes. In 1961 the International Union of Biochemistry and Molecular Biology established an Enzyme Commission to develop a nomenclature for enzymes    

According to the type of reactions that the enzymes catalyze, enzymes are classified into seven categories, which are oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases, and translocases. These groups were subdivided and further subdivided, so that a four-digit number preceded by the letters EC for Enzyme Commission could precisely identify all enzymes.

  • The first digit indicates membership of one of the seven major classes. The next two indicate subclasses and sub-subclasses. The last digit indicates where the enzyme belongs in the sub-subclass.

  • Oxidoreductases, transferases and hydrolases are the most abundant forms of enzymes.

  • •Eg  Nucleoside monophosphate (NMP) kinase. It catalyzes the following reaction:

     

NMP ( nucleotide monophosphate ) kinase transfers a phosphoryl group from ATP to NMP to form a nucleoside diphosphate (NDP) and ADP. Consequently, it is a transferase, or member of group 2.

Many groups in addition to phosphoryl groups, such as sugars and carbon units, can be transferred. Transferases that shift a phosphoryl group are designated 2.7(ie, 7th subclass). Various functional groups can accept the phosphoryl group. If a phosphate is the acceptor, the transferase is designated 2.7.4. The final number designates the acceptor more precisely. In regard to NMP kinase, a nucleoside monophosphate is the acceptor, and the enzyme's designation is EC 2.7.4.4

Seven major classes of Enzymes

Enzymes with similar reaction specificities are grouped together in each group.

The oxidoreductases (class 1)-Catalyse the transfer of reducing equivalents from one redox system to another.

The transferases (class 2)-Catalyse the transfer of other groups from one molecule to another. Oxidoreductases and transferases generally require coenzymes

The hydrolases (class3)-Are also involved in group transfer, but the acceptor is always a water molecule.

Lyases (class 4, often also referred to as “synthases”) -Catalyse reactions involving either the cleavage or formation of chemical bonds, with double bonds either arising or disappearing.

The isomerases (class 5) -Move groups within a molecule, without changing the gross composition of the substrate.

The ligases (class 6) or synthetases,”-The ligation reactions catalysed by ligases are energy-dependent and are therefore always coupled to the hydrolysis of nucleoside triphosphates.

Translocases (class 7)-Translocases (class 7)-These are group of enzymes that catalyse the movement of ions or molecules across membranes or their separation within membranes.



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