Enzyme Classification (ch6)

Enzyme Classes

The Enzyme Commission of the International Union of Biochemistry and Molecular Biology categorizes enzymes into six main classes based on the reactions they catalyze.

1. Oxidoreductases

• This class involves oxidation-reduction reactions.

2. Transferases

• Catalyze the transfer of functional groups between two compounds.
  • Transaminases: Transfer an amino group from one substrate to another.
    • Example:
      $\text{Alanine} + \alpha\text{-Ketoglutarate} \rightleftharpoons \text{Pyruvate} + \text{Glutamate}$
  • Kinases: Transfer phosphate groups from one substrate to another.

3. Hydrolases

• Catalyze hydrolysis reactions (addition of H₂O) that split a compound into two products.
  • Proteases: Hydrolyze peptide bonds in proteins.
    • Example:
      $\text{Polypeptide} + H_2O \rightarrow \text{Shorter polypeptide} + \text{Amino acid}$
  • Lipases: Hydrolyze ester bonds in lipids.
  • Nucleases: Hydrolyze phosphate ester bonds in nucleic acids.

4. Lyases

• Catalyze the addition or removal of a group without hydrolysis.
  • Decarboxylases: Remove $CO_2$ from a substrate.
  • Deaminases: Remove $NH_3$ from a substrate.
  • Dehydratases: Remove $H_2O$ from a substrate.
  • Hydratases: Add $H_2O$ to a substrate.
    • Example:
      $\text{Pyruvate} \rightarrow \text{Ethanal} + CO_2$

5. Isomerases

• Catalyze the rearrangement (isomerization) of atoms within a substrate.
  • Isomerases: Catalyze rearrangement reactions.
  • Epimerases: Catalyze rearrangements of carbohydrates.
    • Example:
      $\text{Maleate} \rightleftharpoons \text{Fumarate}$

6. Ligases

• Catalyze the joining of two substrates, using ATP energy.
  • Synthetases: Form a bond between two substrates, utilizing the energy of ATP.
  • Carboxylases: Catalyze the formation of a bond between $CO_2$ and a substrate, utilizing the energy of ATP.
    • Example:
      $\text{Pyruvate} + CO<em>2 + ATP \rightarrow \text{Oxaloacetate} + ADP + P</em>i + H^+$