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

Learn about the history of enzyme naming, from early whimsical names to the modern IUBMB system of classification. Understand how enzymes are named based on their substrates and functions.