MCAT Content / Enzyme Structure And Function / Enzyme Classification By Reaction Type

Enzyme Classification by Reaction Type

Topic: Enzyme Structure And Function

Based on the type of catalyzed biochemical reaction, enzymes are classified into one of six classes: oxidoreductases, transferases, hydrolases, lyases, isomerases, or ligases.

Enzymes are biological catalysts, and nearly all of them are proteins. Enzymes are highly specific in their action; that is, each enzyme catalyzes only one type of reaction in only one compound or a group of structurally related compounds due to their specificity. Consequently, enzymes are classified by reaction type. The names for classes of enzymes are generally descriptive of the type of reaction they catalyze and usually end in the suffix -ase.

There are six classes of enzymes:

  1. Oxidoreductases: These enzymes catalyze oxidation and reduction reactions by transferring electrons or hydrogen from one molecule to another.
    Example: Alcohol dehydrogenase catalyzes the oxidation reaction of ethanol to form an aldehyde.
  2. Transferases: These enzymes move a functional group from one molecule to the other.
    Example: Peptidyl transferase transfers lysine residue from tRNA during translation.
  3. Hydrolases: These enzymes catalyze a reaction in which a molecule reacts with water to break and form two different molecules by hydrolysis.
    Example: Serine hydrolases causes hydrolysis of the peptide bond.
  4. Lyases: These enzymes catalyze a reaction in which a molecule breaks to form two different molecules without reacting with water.
    Example: Arginosuccinate lyase breaks argininosuccinate into arginine and fumarate.
  5. Isomerases: These enzymes catalyze the reaction in which a molecule is converted into its isomer.
    Example: Phosphoglucoisomerase converts glucose-6-phosphate to fructose-6-phosphate during glycolysis.
  6. Ligases: These enzymes catalyze the reaction in which two molecules join to form one molecule.
    Example: DNA ligase joins two strands of DNA.

Practice Questions


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Key Points

• Based on the type of reactions catalyzed by an enzyme, the enzymes are classified into six major classes.

• The six major classes of enzymes are oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases.

• Oxidoreductases enzymes catalyze oxidation and reduction reactions.

• Transferases enzymes move a functional group from one molecule to the other.

• Hydrolases enzymes catalyze hydrolysis reactions.

• Lyases enzymes catalyze reactions in which a molecule breaks to form two different molecules without reacting with water.

• Isomerases enzymes catalyze reactions in which a molecule is converted into its isomer.

• Ligases enzymes catalyze reactions in which two molecules join to form one molecule.


Key Terms

oxidation: a reaction in which loss of the electrons occurs

reduction: a reaction in which gain of electrons occurs

catalyze: accelerate or speed up

hydrolysis: a reaction in which a molecule breakdown when it reacts with water

isomer: molecules with the same formula but a different arrangement of atoms.



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