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Enzymes are natural thermolabile, biological catalysts. They are protein or polypeptide in nature, except for a few, i.e., ribozyme is not a protein but a RNA molecule with catalytic activity. Enzymes are secreted by living cells.
Important Precursor form (zymogen) : Trypsinogen, chymotrypsinogen, Proelastase, Procarboxypeptidase, Procolipase, Prophospholipase A2, pepsinogen.
Some protein enzymes contain no chemical group other than amino acid residue (e.g., trypsin, chymotrypsin, elastase etc). However, most enzymes are associated with non-protein chemical components which are required for their catalytic activity. The complete enzyme (with its non-protein component) is called holoenzyme which is made of the protein portion (apoenzyme) and the cofactor / coenzyme. Apoenzyme is inactive alone (without cofactor/coenzyme).

  • According to international Union of Biochemistry (IUB), each enzyme is characterized by a code number called enzyme code number or EC-number. According to the IUB system, enzymes are classified into six major classes.
  1. EC-1: Oxidoreductases: - These enzyme catalyze oxidation-reduction reactions, i.e., transfer of electrons. One of the substrates is oxidized (loses electron) and the other is reduced (accepts electron). Enzymes in this category include dehydrogenases, Oxidases, Hydroperoxidases, Oxygenases, reductases and hydroxylases.          
  2. EC-2 : Transferases : - These enzymes catalyze the transfer of functional group either from a donor molecule to an acceptor or within a single molecule. Examples are amino transferase (transaminase), kinase, transketolase, transaldolase, transcarboxylase etc.
  3. EC-3: Hydrolases : - Hydrolases make use of water to cleave a single molecule into two molecules. These hydrolytic enzymes catalyze the cleavage of C - 0, C - N, C - C and some other bonds with addition of water. These enzymes are commonly found in the digestive secretions and lysosomes. Enzymes in this category include esterases, glycosidases, peptidases (trypin, pepsin), amidases, acid phosphatases etc. All digestive enzymes belong to this category.
  4. EC-4: Lyases : - Lyases split the substrate molecule by non-hydrolytic process. Examples are decarboxylases, aldolases, hydratases, enolase, fumarase etc.
  5. EC - 5: Isomerases : - These enzymes catalyse interconversion of isomers of a compound. They include racemases, epimerases, cis- and trans -isomerases and intramolecular transferases (mutases).
  6. EC - 6 : Ligases : - Ligases ligate or bind two compounds together by creating a new chemical bond. A source of energy is required, usually ATP. Example are synthetases, carboxylases.  
Enzyme class Important Examples
  • Oxidases : - Cytochrome oxidase, L-amino acid oxidase, xanthine oxidase, Tyrosinase.
  • Dehydrogenases (DH):
    • NAD+ - linked: - Pyruvate DH, Isocitrate DH, malate DH, a-ketoglutrate DH, glutamate DH, Lactate DH, glyceruldehyde-3-P-DR, b-hydroxy acyl CoA DR, glycerol-3-P DH (cytoplasmic).
    • NADP+- linked: - Glucose-6-P-DH, 6-phosphogluconate DR, 3-ketoacyl reductase, Enoyl reductase
    • FAD -linked: - Succinate DR, Fatty acyl CoADH, glycerol-3-p-DH (mitochondrial).
  • Hydroperoxidases : - Peroxidases, catalases
  • Oxygenases: 
    • Monooxygenase: - Phenylalanine hydroxylase. cytochrome P-450.
    • Dioxygenase : - Homogentisate oxidase, L-tryptophan dioxygenase, cyclooxygenase.
Transferase Amino transferase or transaminase, e.g., SGOT (AST) and SGPT (ALT), kinases (Hexokinase glucokinase, pyruvate kinase etc), Transketolases, transaldolases, transcarboxy lases.
Hydrolases All digestive enzymes (Pepsin. trypsin, lipases, esterases), lysosomal enzymes, urease, and phosphatase
Lyases Decarboxylases, aldolases, hydratases, enolase, fumarase, Arginosuccinase
Isomerases Racemases,epimerases,cis-trans- isomerases, mutases
Ligases Synthetases Carboxylases, DNA ligase

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