General Chemistry
Behavioral Sciences
Lab Techniques

Enzyme Classes

As we have already seen enzymes are like nature’s own speedsters, zipping up chemical reactions in our bodies. They’re essential for a slew of biological processes and are really specific about which substrates they work with and the reactions they help along. But here’s the cool part: even with all that specificity, we can group them into some broad classes. The MCAT is pretty fond of testing your know-how on this, pushing you to figure out which class an enzyme belongs to. Don’t stress though, we’ve got some tips coming up for these very questions. First up, let’s walk through the essential details for each class.

Enzyme Profiles:


  • Reaction Type: Catalyze oxidation-reduction reactions (transfer of electrons)
    • If they show a reaction look for a gain or loss of oxygen bonds, as well as the disappearance or appearance of carbon carbon double bonds
  • Keywords: NAD+, NADH, FAD, redox, dehydrogenase, oxidase
  • Examples: Any of the dehydrogenases in metabolism, such as lactate dehydrogenase converting lactate to pyruvate


  • Reaction Type: Transfer functional groups between molecules
    • If you they show you a reaction look for a group that was previously on one molecule to show up on a different molecule
  • Keywords: Kinase, phosphorylase, phosphorylation, methyl transfer
  • Examples: The MCAT loves to test kinases and phosphorylases such as, tyrosine kinases


  • Reaction Type: Catalyze the cleavage of bonds with the addition of water
    • If they show you a reaction make sure water is a reactant and you have some larger molecule being broken into multiple smaller components
  • Keywords: Phosphatase, peptidase, protease, lipase
    • Commonly hydrolyses break down larger biomolecules in cells (ex: protease, lipase, etc.) so if you see something named biomolecule-ase there is a very high chance that thing is a hydrolase
  • Examples: Proteases breaking down proteins


  • Reaction Type: Catalyze the addition of groups to double bonds or the formation of double bonds by removal of groups
    • If they show a reaction make sure you see a larger molecule being broken down into smaller parts
  • Keywords: Decarboxylase, synthase, aldolase, dehydratase
  • MCAT Examples: Fumarase catalyzing the hydration of fumarate to malate in the Krebs cycle


  • Reaction Type: Catalyze the rearrangement of atoms within a molecule
    • If you they show you a reaction look for rearrangement and make sure nothing is being gained or lost in the process
  • Keywords: Epimerase, racemase, mutase
  • MCAT Examples: Phosphoglucomutase rearranging glucose-1-phosphate to glucose-6-phosphate


  • Reaction Type: Catalyze the joining of two molecules
  • If they show you a reaction look for two molecules that become one, generally you will see more individual molecules on the reactant side and less on the product side
  • Keywords: Synthetase, carboxylase, DNA ligase
  • MCAT Examples: Acetyl-CoA carboxylase involved in fatty acid biosynthesis

Special Feature Enzymes

In addition to knowing the classes we just outlined you will also want to know some more specific information about kinases, phosphorylases, and phosphatases, since the MCAT likes to ask more specific questions about these in particular.

Kinases (Class: Transferases): These are the enzymes that specifically attach a phosphate group to a molecule, usually using ATP as the phosphate donor. They’re like the ‘on’ switch for many cellular processes, signaling for activities to commence. Even though they tend to turn processes on phosphorylation can also be used to turn off processes as well.

For kinases, you can use the (rather bad) mnemonic “Key-nase.” Think of kinases as a key, with ATP attached as a keychain. This way, you’ll remember that kinases always carry ATP along to unlock the process of adding phosphate groups to other molecules.

Phosphorylases (Class: Transferases): These enzymes add a phosphate group too, but they typically do it without using ATP. Instead, they often use inorganic phosphate to add onto specific substrates like glucose, which is part of the process of breaking down glycogen in our cells.

To remember the phosphate source think of phosphorylase as “FOX-phorylase” since it can help you imagine a fox sitting on a rock, indicating that the phosphate source is inorganic (like a mineral, which could be found in our rock).

Phosphatases (Class: Hydrolases): In contrast to the two previous enzymes, these enzymes remove a phosphate group from the molecule.

The mnemonic “Phospha-take-away-ase” works well to remember this.

Approaching Enzyme-Related Problems

Enzyme class questions are pretty easy to spot – usually, there’s a mention of a specific enzyme class. But here’s the thing: the AAMC can be quite crafty. They might tuck away crucial details in the passage, making you work a little harder to spot them.

So, let’s roll up our sleeves and break down the specifics and get you ready to spot and solve these enzyme class puzzles on the MCAT by going over what to look.

  1. Enzyme Names
    • Look to identify any specific names mentioned and match them up with the class they correspond to using the enzyme profiles. Keep in mind memorizing a wide variety of different enzyme names with in each class is generally unnecessary.
  2. Given Reactions
    • If the passage or question gave you reaction sequences see if they were discussing the action of the enzyme. If they were match up the changes in the reaction with the expected changes caused by each class.
  3. Changes In Substrate Names
    • This is the trickiest one and the one that students typically miss. Instead of directly discussing the enzyme names or showing a reaction the AAMC might only mention the name of a substrate and the name of the product after reacting. In this case think about what that name change indicates and match the change up with an enzyme.