Enzyme Classification

Classification of Enzymes

• Early enzymes were given whimsical names, such as Pepsin, Trypsin, and Chymotrypsin, which are still used today.
• Later, enzymes were named by adding the suffix "-ase" to the substrate, for example, Lactase acts on lactose to form glucose and galactose.

IUBMB System of Classification

• The International Union of Biochemistry and Molecular Biology (IUBMB) developed a system of nomenclature for enzymes in 1964, modified in 1972 and 1978.
• The system uses a four-digit code starting with EC (enzyme class), followed by:
  • First digit: class
  • Second digit: subclass
  • Third digit: sub-subclass or subgroup
  • Fourth digit: number of the particular enzyme in the list

Six Major Classes of Enzymes

• Class 1: Oxidoreductases
  • Catalyze oxidation of one substrate with simultaneous reduction of another substrate or co-enzyme
  • Example: Alcohol dehydrogenase (Alcohol-NAD-oxidoreductase; EC.1.1.1.1)
• Class 2: Transferases
  • Transfer one group (other than hydrogen) from the substrate to another substrate
  • Example: Hexokinase (ATP-Hexose-6-phosphatetransferase)
• Class 3: Hydrolases
  • Hydrolyze ester, ether, peptide, or glycosidic bonds by adding water
  • Example: Acetyl choline esterase or Acetyl choline hydrolase
• Class 4: Lyases
  • Remove groups from substrates or break bonds by mechanisms other than hydrolysis
  • Example: Aldolase
• Class 5: Isomerases
  • Produce optical, geometric, or positional isomers of substrates
  • Example: Triose phosphate isomerase
• Class 6: Ligases
  • Link two substrates together, usually with simultaneous hydrolysis of ATP
  • Example: Acetyl CoA carboxylase