Structural Biochemistry/Enzyme Catalytic Mechanism/Enzyme Classification

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Class Type of Reduction Examples
Hydrolases Catalyze hydrolysis reactions Estrases Digestive enzymes
Isomerases Catalyze isomerization (changing of a molecule into its isomer) Phospho hexo isomerase, Fumarase
Ligases Catalyze bond formation coupled with ATP hydrolysis. Citric acid synthetase
Lyases Catalyze a group elimination in order to form double bonds (or a ring structure). Decarboxylases Aldolases
Oxidoreductases Catalyze oxidation-reduction reactions Dehydrogenases Oxidases
Transferases Catalyze the transfer of functional groups among molecules. Transaminase Kinases

The classification of an enzyme is shown within the table as it's class and the type of reduction the enzyme goes through. An example of a name is glucose phosphotransferase. In this reaction ATP transfers one of its phosphates to glucose: ATP + D-glucose -> ADP + D-glucose 6-phosphate. Since this process "transfers" a phosphate group to glucose, it is within the classification of transferases, hence the name "glucose phosphotransferase." 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 are six classes of enzymes that were created with subclasses based on what they catalyze so that enzymes could easily be named. Depending on the type of reaction catalyzed, an enzyme can have various names. These classes are Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, and Ligases. This is the internation classification used for enzymes. For example, a common oxidoreductase is dehydrogenase. Dehydrogenase is known as an enzyme that oxidizes a substrate and transferring protons. 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. 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.

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.

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.

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.

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.

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.

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.