Structural Biochemistry/Specific Enzymes and Catalytic Mechanisms/Enzyme Classification

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Since many enzymes have common names that do not refer to their function or what kind of reaction they catalyze, a enzyme classification system was established. There were six classes of enzymes that were created so that enzymes could easily be named. These classes are:Oxidoreductases,Transferases, Hydrolases, Lyases, Isomerases, and Ligases. This is the internation classification used for enzymes. Enzymes are normally used for catalyzing the transfer of functional groups, electrons, or atoms. Since this is the case, they are assigned names by the type of reaction they catalyze. The enzymes were numbered 1-6 and from here, they were divided into subdivisions. This allowed for the addition of a four-digit number that would precede EC(Enzyme Commission) and each enzyme could be identified. The reaction that an enzyme catalyzes must be know before it can be classified.[1]

Oxidoreductases catalyze oxidation-reduction reactions where electrons are transferred. These electrons are usually in the form of hydride ions or hydrogen atoms. When a substrate is being oxidized it is the hydrogen donor. The most common name used is a dehydrogenase and sometimes reductase will be used. An oxidase is referred to when the oxygen atom is the acceptor.

Glutathione S-transferase

Transferases catalyze group transfer reactions. The transfer occurs from one molecule that will be the donor to another molecule that will be the acceptor. Most of the time, the donor is a cofactor that is charged with the group about to be transferred. Example: Hexokinase used in glycolysis.

Hydrolases catalyze reactions that involve hydrolysis. This cases usually involves the transfer of functional groups to water. When the hydrolase acts on amide, glycosyl, peptide, ester, or other bonds, they not only catalyze the hydrolytic removal of a group from the substrate but also a transfer of the group to an acceptor compound. These enzymes could also be classified under transferaes since hydrolysis can be viewed as a transfer of a functional group to water as an acceptor. However, as the acceptor's reaction with water was discovered very early, it's considered the main function of the enzyme which allows it to fall under this classification. For example: Chymotrypsin.

Lyases catalyze reactions where functional groups are added to break double bonds in molecules or the reverse where double bonds are formed by the removal of functional groups. For example: Fructose bisphosphate aldolase used in converting fructose 1,6-bisphospate to G3P and DHAP by cutting C-C bond.

Isomerases catalyze reactions that transfer functional groups within a molecule so that isomeric forms are produced. These enzymes allow for structural or geometric changes within a compound. Sometime the interconverstion is carried out by an intramolecular oxidoreduction. In this case, one molecule is both the hydrogen acceptor and donor, so there's no oxidized product. The lack of a oxidized product is the reason this enzyme falls under this classification. The subclasses are created under this category by the type of isomerism. For example: phosphoglucose isomerase for converting glucose 6-phosphate to fructose 6-phosphate. Moving chemical group inside same substrate.

Ligases are used in catalysis where two substrates are litigated and the formation of carbon-carbon, carbon-sulfide, carbon-nitrogen, and carbon-oxygen bonds due to condensation reactions. These reactions are couple to the cleavage of ATP.

Subcategories

Each of the six main categories holds the following subcategories[2].

Oxidoreductases

Transferases

Hydrolases

Lyases


Isomerases

Ligases


References

  1. ↑ Berg, Jeremy M., John L. Tymoczko, and Lubert Stryer. BIOCHEMISTRY. 6th ed. New York: W. H. FREEMAN AND COMPANY, 2007.
  2. ↑ Webb, Edwin C. (1992). Enzyme nomenclature 1992: recommendations of the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology on the nomenclature and classification of enzymes. San Diego: Published for the International Union of Biochemistry and Molecular Biology by Academic Press. ISBN 0-12-227164-5. http://www.chem.qmul.ac.uk/iubmb/enzyme/.
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