Most of the enzymes consist of proteins. They catalyze biological reactions without themselves being used up. Edward Buchner was the first to isolate an enzyme, i.e. ‘zymase’. J.B. Sumner crystallized urease enzyme.
Sumner defines enzyme as a protein with catalytic activity. An enzyme shows a 3D structure containing ‘R’ (alkyl) group of amino acid that forms active sites during the folding of the polypeptide chain. Usually, 3-12 amino acids form an active site. The enzyme reacts with the substrate only at these active sites. Enzymes can speed up the reaction in both directions. An enzyme is specific for both the binding of substrate and type of reaction. Duclaux (1883) provided a system for naming enzymes by using the suffix –ase. According to IUB (International Union of Biochemistry), enzymes are classified into six classes based on their action.
(1) Oxidoreductase: In this class, the enzymes that induce oxidation-reduction reaction are included (Cytochrome oxidase).
(2) Transferase: This class of enzymes is responsible for group (other than ‘H’) transfer (Transaminase). The group includes amino, phosphate, methyl, etc.
(3) Hydrolase: This enzyme induces the addition or removal of water molecule from a reaction (Amylase).
(4) Lyase: Lyase catalyzes the cleavage of a specific covalent bond and the removal of a group without hydrolysis (Aldolase).
(5) Isomerase: Isomerase catalyzes the rearrangement of the molecular structure to form an isomer (Phosphohexose isomerase).
(6) Ligase: Ligase induces the joining of two molecules by a covalent bond (Pyruvate carboxylase).