Enzymes can be classified based on the kind of chemical reaction they catalyze, as following:
- Addition or removal of water. E.g. Hydrolases (esterases, carbohydrases, nucleases, deaminases, amidases, and proteases), Hydrases (fumarase, enolase, aconitase and carbonic anhydrase) etc.
- Transfer of electrons. E.g. Oxidases, Dehydrogenases
- Transfer of a radical. E.g. Transglycosidases (monosaccharides), Transphosphorylases and phosphomutases (phosphate group), Transaminases (amino group), Transmethylases (methyl group), Transacetylases (acetyl group)
- Splitting or forming a C-C bond. E.g. Desmolases
- Changing geometry or structure of a molecule. E.g. Isomerases
- Joining two molecules through hydrolysis of pyrophosphate bond in ATP or other tri-phosphate. E.g. Ligases
Broadly enzymes can be divided into six classes and they are:-
Oxidoreductases catalyze oxidation or reduction reactions. These reactions involve transfer of electrons from one molecule (reductant) to another (oxidant). These reactions are vital to life for their role in essential metabolic processes like glycolysis, which occurs in nearly every living organism.
A– + B → A + B–
Where, A is t oxidant and B is reductant. E.g. alcohol dehydrogenase converts primary alcohols to aldehydes.
These enzymes catalyze transfer of a functional group (e.g. methyl) from one molecule to another. The molecule which loses functional group is called donor and molecule accepting the functional group is called acceptor. These transfer processes are some of the most basic and vital reactions in life. E.g. transaminase, which transfers an amine group from one molecule to another.
They bring about hydrolysis i.e. hydrolytic cleavage of bonds such as C–O, C–N, C–C and some other bonds, including phosphoric anhydride bonds with addition of water. There is a wide variety of identified hydrolases, over 200 of them, from those that break down proteins to those that cleave ester bonds and more. Exohydrolase enzymes cleaves molecules at end of the chain and endohydrolase enzymes do so in middle of the chain.
E.g. Phosphatases break the oxygen‐phosphorus bond of phosphate esters. Pepsin hydrolyzes peptide bonds in proteins.
These enzymes cleave bonds by means other than hydrolysis or oxidation and often generate a double bond. Lysis reactions are kind of elimination reactions that are not hydrolytic or oxidative. The lyases are also sometimes called synthase enzymes. Reverse reaction is also possible. However, two substrates are required for the reverse reaction to happen, whereas one substrate is required for lysis reaction. This makes lyases unique among enzymes.
E.g. Deaminases remove ammonia in the removal of amino groups from amino acids.
The isomerase enzymes catalyze structural changes within a molecule which brings about a change in shape since there is only one substrate and one product with nothing gained or lost. Within this category, there are a few sub-categories depending upon their effect. There are geometric, structural, enantiomer, and stereoisomer isomerases. E.g. triose phosphate isomerase catalyze interconversion of Glyceraldehyde-3- phosphate and Dihydroxy acetone phosphate in glycolysis.
Ligases or Synthases
Ligation is brought about by ligase enzymes. Ligation occurs when two substrates are joined together. Chemical potential energy is usually required for this reaction to occur, so it is often paired with hydrolysis of a diphosphate bond. E.g. DNA ligase which catalyzes ligation or repair of breaks in DNA.
E.g. Aminoacyl‐transfer RNA synthetases join amino acids to their respective tRNAs in preparation for protein synthesis.
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