Classification of Enzymes

The International Union of Biochemistry classified Enzymes in 1964. The enzymes are categorized into six divisions as follows: 1) Oxidoreductases 2) Transferases 3) Hydrolases 4) Lyases 5) Isomerases 6) Synthetases or ligases.
Oxidoreductases: This group of enzymes is associated with the oxidation-reduction reaction in the cells. The enzymes which remove hydrogen from the substrate is known as dehydrogenase and the one which adds oxygen to the substrate is known as oxidase. This group of enzymes plays an important role in oxidative phosphorylation associated with Krebs cycle e.g. Succinic dehydrogenase, cytochrome oxidase.
Transferases: The enzymes which bring about the transfer of one group (except hydrogen) form one substrate to another substrate are termed as Transferases e.g. One phosphate group of the ATP molecules gets linked to the hexose sugar in presence of hexokinase. Hence hexokinase is classified under the group transferases.
Hydrolases: An enzyme which splits any complex organic substance by adding a molecule of water is known as hydrolase.
Maltase
Maltose + H2O ————? Glucose + Glucose
Lyases: These enzymes catalyse the cleavage of macromolecules into smaller molecules without adding H2O molecules. E.g. Fructose 1, 6-diphosphate (having 6 carbons) is split into two molecules of triose phosphate in the presence of enzyme aldolase during the process of glycolysis. Thus aldolase is an enzyme of the group lyases.
Isomerases: Isomerases are enzymes which catatyse changes only in the molecular structure of the substrate. Transfer of atoms within the molecule results in the formation of a new molecule.e.g.
Fructose isomerase
—————?
Glucose 6-phosphate ?————— Fructose 6-phosphate

Synthetases or ligases : These enzymes synthesize a new molecule by joining 2 molecules with the help of energy from pyrophosphate bond of ATP. E.g. Acetyl co-A. synthetase.
enzyme
Acetic acid + ATP + Co-A ————? Acetyl co-A + AMP +Pyrophosphate(H4P2O7)