MCAT Content / Enzymes / Classification By Reaction Type

Classification by Reaction Type

Topic: Enzymes

Enzymes are named for the reactions they catalyze; there are six categories of enzymes.

Often, the suffix “-ase” is added to the end of the substrate upon which the enzyme acts. According to systematic naming conventions, enzymes are classified into six categories:

  1. Oxidoreductases catalyze the transfer of electrons or hydrogen ions (i.e. oxidation-reduction reactions).
  2. Transferases catalyze reactions in which groups are transferred from one location to another.
  3. Hydrolases regulate hydrolysis reactions (the addition of water).
  4. Lyases catalyze reactions where functional groups are added to double bonds, or double bonds are formed via the removal of functional groups.
  5. Isomerases catalyze the transfer of groups within a molecule, with the effect of producing isomers.
  6. Ligases catalyze condensation reactions coupled with the hydrolysis of high energy molecules.

There is a major distinction between classifications that occurs between lyases and ligases. The particular type of lyase that catalyzes the addition of one substrate to the double bond a second substrate is sometimes called a synthase (e.g. ATP synthase). Ligase enzymes require energy input from ATP or some other nucleotide. Ligases are sometimes called synthetases and do not require ATP to catalyze their reactions.

Other enzymes to know are kinases and phosphatases. A kinase phosphorylates a molecule while a phosphatase dephosphorylates a molecule. A kinase often phosphorylates another enzyme in order to activate or deactivate it (e.g. hexokinase phosphorylates glucose when it enters a cell).


Practice Questions

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MCAT Official Prep (AAMC)

Practice Exam 1 B/B Section Passage 4 Question 22

Sample Test B/B Section Passage 1 Question 3


Key Points

• Enzymes are named after the reactions they catalyze.

• Oxidoreductases catalyze oxidation-reduction reactions where electrons are transferred.

• Transferases catalyze the transfer of functional groups (like phosphate and amino groups).

• Hydrolases catalyze reactions that involve hydrolysis (addition of water).

• Lyases catalyze reactions where functional groups are added to double bonds, or double bonds are formed via the removal of functional groups.

• Isomerases catalyze the transfer of groups within a molecule, with the effect of producing isomers.

• Ligases catalyze condensation reactions coupled with the hydrolysis of high energy molecules.

• Synthases catalyze the addition of one substrate to the double bond a second substrate.

• Synthetases and do not require ATP to catalyze their reactions.

• A kinase phosphorylates a molecule.

• A phosphatase dephosphorylates a molecule.


Key Terms

Catalyze: to bring about the catalysis of a chemical reaction.

Phosphorylate: to introduce a phosphate group into a molecule or compound

Dephosphorylate: remove a phosphate group from a molecule or compound

Kinase: is an enzyme that catalyzes the transfer of phosphate groups

Ligases: enzymes that are capable of catalyzing the reaction of joining two large molecules by establishing a new chemical bond

ATP: adenosine triphosphate the common energy unit in the cell



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