Classification and naming of enzymes

There is a formal system of enzyme nomenclature, in which each enzyme has a number, and the various enzymes are classified according to the type of reaction catalysed and the substrates, products and coenzymes of the reaction. This is used in research publications, when there is a need to identify an enzyme unambiguously, but for general use there is a less formal system of naming enzymes. Almost all enzyme names end in -ase, and many are derived simply from the name of the substrate acted on, with the suffix -ase. In some cases, the type of reaction catalysed is also included.

Table 2.2 Classification of enzyme-catalysed reactions

1

Oxidoreductases

Oxidation and reduction reactions

Dehydrogenases

Addition or removal of H

Oxidases

Two-electron transfer to O2, forming H2O2 Two-electron transfer to '/2O2, forming H2O

Oxygenases

Incorporate O2 into product

Hydroxylases

Incorporate '/2O2 into product as -OH and form H2O

Peroxidases

Use as H2O2 as oxygen donor, forming H2O

2

Transferases

Transfer a chemical group from one substrate to the other

Kinases

Transfer phosphate from ATP onto substrate

3

Hydrolases

Hydrolysis of C-O, C-N, O-P and C-S bonds (e.g. esterases, proteases, phosphatases, deamidases)

4

Lyases

Addition across a C-C double bond

(e.g. dehydratases, hydratases, decarboxylases)

5

Isomerases

Intramolecular rearrangements

6

Ligases (synthetases)

Formation of bonds between two substrates Frequently linked to utilization of ATP with intermediate formation of phosphorylated enzyme or substrate

Altogether there are some 5—10,000 enzymes in human tissues. However, they can be classified into only six groups, depending on the types of chemical reaction they catalyse:

1 oxidation and reduction reactions;

2 transfer of a reactive group from one substrate onto another;

3 hydrolysis of bonds;

4 addition across carbon—carbon double bonds;

5 rearrangement of groups within a single molecule of substrate;

6 formation of bonds between two substrates, frequently linked to the hydrolysis of ATP ^ ADP + phosphate.

This classification of enzymes is expanded in Table 2.2, to give some examples of the types of reactions catalysed.

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