Enzyme Classes

Enzyme Naming

Enzymes are named in a very consistent fashion. Outside of a couple of exceptions, they will start with the name of their substrate and end with the suffix -ase. So a protease is an enzyme the breaks down proteins and a methyltransferase transfers a methyl from one substrate to another. This is important to notice because we can use the name of an enzyme to figure out what it does. This will help later on when learning metabolism and allows us to infer what an enzyme can do even if we haven’t learned about it.

Furthermore, enzymes are named for the type of reaction they can catalyze. For instance, a methyltransferase is a transferase it transfers stuff from place to place. While a protease is a hydrolase and breaks proteins apart using water. The MCAT will often ask us to categorize enzyme by type so let’s dive into the different classifications and the names most frequently used to describe them.


Oxidoreductases catalyze oxidation and reduction reactions these are frequently seen in metabolism and our biological electron carriers (ex: NAD+, NADP+, etc.) When spotting an oxidoreductase lookout for the presence of the electron carriers and see if any of the molecules are losing or gaining any new hydrogen or oxygen bonds.


On the MCAT we will run into the dehydrogenases most frequently including pyruvate dehydrogenase, succinate dehydrogenase, and ⍺-ketoglutarate dehydrogenase.


Transferases as their name suggests transfer functional groups from one molecule to another. Typically what they transfer is listed in the name. Such as an acetyltransferase which moves an acetyl group from one molecule to the next or a transaminase which move a whole amino group from one molecule to another.

Kinase and Phosphorylase

There are two notable transferases we should be able to spot on sight. The kinase and the phosphorylase both of which transfer a phosphate group from one place to another. Kinases transfer phosphates to and from organic phosphate sources including ATP and GTP. While phosphorylase transfer phosphates from inorganic phosphates usually phosphoric acid.


Hydrolases break apart molecules using water to cleave bonds in two. These are the enzymes of our lysosome and include proteases, lipases, and nucleases among others. Typically they simply end in -ase and don’t have carry the hydro anywhere in the name.


We can see that trend continue with phosphatase which cleaves the phosphate groups off of other molecules using water. It can be a bit tricky to remember that phosphatases remove phosphates and phosphorylases add them. I remember the distinction by thinking of the t in phosphatase as standing for taking away a phosphate.


Lyases like hydrolases break molecules however they do so without the use of water. The easiest way to spot these it to recognize the breaking apart of a molecule and no water in sight.


Decarboxylases are one of the most common examples of lyases and involve the removal of CO2 from a molecule.


Isomerases don’t add or remove anything like the other enzymes we have explored. Instead they rearrange molecules generating isomers of the original molecule. These can be structural isomers such as glucose-6-phosphate and fructose-6-phosphate or stereoisomers such as L-alanine and D-alanine.


When an isomerase only changes a functional group around within in a molecule it is called a mutase. For example phosphoglycerate mutase converts 3-phosphoglycerate to 2-phosphoglycerate. Here the position of the phosphate molecule is the only that has changed.


Ligases join together molecules forming new bonds between the two molecules. For example, DNA ligase which joins together the backbone of DNA by forming a new phosphodiester bonds.


One of the other common ligases are the synthetases which synthesize new molecules. For example argininosuccinate synthetase combines an arginine and a succinate to form argininosuccinate.

Reverse Reactions

When looking at a reaction or enzyme name we have to be careful not to assume the name of an enzyme describes the exact reaction involved. Why? Enzymes catalyze reverse reactions too. So while enolase is classified as a hydrolase the forward reaction which is normally seen in glycolysis involves the loss of water and formation of a new carbon carbon bond. We see another example of this in glycolysis with the kinase catalyzed reactions.

While hexokinase and phosphofructokinase add a phosphate from ATP to a glucose derivative both phosphoglycerate kinase and pyruvate kinase add a phosphate group back to ADP from our altered glucose molecule.