Essay on the Naming and Classification of Enzymes

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Enzymes are commonly named by appending the suffix -ase to the name of the substrate or the catalytic action of the enzyme.

Thus the enzyme which catalyzes the hydrolysis of urea is named as urease and the enzyme, which catalyzes the oxidation of glucose, is named as glucose oxidizes. But there are many enzyme names which convey little or nothing about the nature of the reactions they catalyze. For example, the enzyme which catalyzes the decomposition of H20.) To H2O and 02, was named as catalase, the name that tells nothing about its nature or function.

Many enzymes are named even without the suffix -ase. For example, the proteolytic enzyme secreted by the pancreas is named as trypsin. With the discovery of large number of enzymes, their systematic nomenclature and classification was needed, so that any given enzyme can be precisely identified.

A scheme for the systematic functional classification and nomenclature of enzymes was adopted in 1961 by the Enzyme Commission set up under the auspices of the International Union of Biochemistry (IUB) and the International Union of Pure and Applied Chemistry (IUPAC).

The list of enzymes has since been modified, some enzymes renamed a: new enzymes added. At present there are about 3196 enzymes and the responsibility reviewing and adding new enzymes to the list now lies with the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC IUBMB).

Enzymes are now named and classified according to the nature of the biochemical reaction they catalyze. The Commission on Enzymes has recognized major classes of enzymes basing on the type of reactions they catalyze.

Class 1. Oxidoreductases:

These enzymes catalyze reactions in which one substrate is oxidized and another substrate is reduced. Thus, these enzymes catalyze transfer of hydrogen, oxygen or electrons between the substrates for oxidation reduction reactions. The enzymes alcohol dehydrogenase and catalase belong to this class.

Class 2. Transferases:

These are enzymes involved in the transfer of chemical group such as one-carbon groups (e.g., methyl group), aldehyde or ketonic group phosphoryl groups etc from one substrate to another. The enzyme hexokinase and citrate synthesis belong to this class.

Class 3. Hydrolases:

These enzymes catalyze the hydrolytic cleavage of C-0, C-N, C-C, P-0 and other single bonds. The enzymes sucrose, urease and phosphatase belong to this class.

Class 4. Lyases:

These enzymes catalyze the removal of groups, other than hydrolysis or oxidation-reduction reactions, often leaving a double bond. The enzymes fumarase and isocitrate lyase belong to this class.

Class 5. Isomerizes:

These enzymes catalyze reactions involving intermolecular rearrangement of groups in the substrate molecule to form a product which is a different isomeric form of the substrate. The enzymes triose phosphate isomerase and phosphoglyeerate mutase belong to this class.

Class 6. Ligases (also known as Synthetics):

These enzymes catalyze the joining or ligation of two substrates at the expense of ATP (or other nucleoside triphosphates) hydrolysis.

The enzymes aminoacyl-tRNA synthetase and Glutamine synthetase belong to this class. Note here that some enzymes] named as synthases (e.g. citrate synthase, glutamate synthase) do not belong to this class. Note here that some enzymes named as synthases (e.g. citrate synthase, glutamate synthase) do not belong to this class.


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