Types of Enzymes: Study with Video Lessons, Practice Problems & Examples

Enzymes are categorized into six major classes: oxidoreductases, transferases, hydrolyases, isomerases, lyases, and ligases. Oxidoreductases facilitate oxidation-reduction reactions by transferring electrons, while transferases catalyze the transfer of functional groups. Hydrolyases break bonds using water, and isomerases rearrange atoms to form isomers. Lyases create or break bonds without redox reactions or water, and ligases join molecules using energy, often from ATP. Understanding these classes is crucial for grasping metabolic pathways and enzyme functions in biological systems.

Types of Enzymes

In this video, we're going to begin our discussion on the different types of enzymes. It turns out that there are six major classes of enzymes, and most enzymes fall under one of these six major classes. The six major classes of enzymes are oxidoreductases, transferases, hydrolases, isomerases, lyases, and ligases. Moving forward in our course, we're going to briefly talk about each one of these major classes of enzymes in their own separate videos. Because most of your professors expect you to recognize the different reactions that these enzymes catalyze. One way that helps me remember all of these six major classes of enzymes is to remember that they all help reactions get over the hill, where each of these red letters in "over the hill" represents the first letter of one of the major classes of enzymes. Down below, we have a little energy diagram, one that we've seen before in our previous lesson videos, just to remind us that enzymes literally help reactions get over the hill. The hill here is literally the energy of activation, and all of these enzymes do this in part by stabilizing the transition state. Moving forward, in our next lesson video, we're going to talk about our first major class of enzyme, the oxidoreductases. I'll see you guys in that video.

Types of Enzymes

So in our last lesson video, we said that there are 6 major classes of enzymes, and the mnemonic that helps us memorize those 6 major classes of enzymes is "just over the pill," where each of these letters represents the first letter of one of the major classes of enzymes. And so, in this video, we're going to focus on our first major class of enzyme, which is the "o" in our mnemonic, which stands for oxidoreductases. And so, oxidoreductases are enzymes that pretty much do exactly what they sound like they do, and that is they catalyze oxidation-reduction reactions or redox reactions for short. And they do that by transferring electrons between molecules during a reaction. And so recall from your previous chemistry courses that substances that lose electrons during a reaction are oxidized, whereas substances that gain electrons during a reaction are reduced because they're gaining negatively charged electrons, and so their overall charge is being reduced. And so notice down below in our example, we have a little mnemonic to help you guys memorize oxidation-reduction reactions. And that mnemonic is "Leo the lion goes grrr," which I'm sure you guys have probably heard before in your previous chemistry courses. And so the "LEO" here just stands for "lose electrons is oxidation," whereas the "GER" here stands for "gain electrons is reduction."

Down below, we have a little image of a lion that has a nametag that says, "hello, my name is Leo," and he's shouting, "grrr." And so hopefully, by memorizing "Leo the lion goes grrr," then you guys will be able to understand all of your oxidation-reduction reactions. And so whenever you see that hydrogen or oxygen atoms are being transferred between different molecules in a reaction, that's a good indication that that could be an oxidation-reduction reaction. And so, what I also want you guys to note is that a classic example of an oxidoreductase is the enzyme lactate dehydrogenase or LDH, which is an enzyme that is incredibly important for anaerobic respiration, and we'll be able to talk a lot more about this enzyme much later in our course. But for now, again, I just want you guys to know that lactate dehydrogenase or LDH is an example of an oxidoreductase.

Below, we have the chemical reaction that lactate dehydrogenase catalyzes, which is the conversion of L-lactate and NAD⁺ into pyruvate, NADH, and an acid. Above this, we have a question asking which molecule is oxidized and which molecule is reduced. Well, thankfully, we have our mnemonic here, "Leo the lion goes GER." And taking a closer look at this reaction, notice that the NAD⁺, as it goes from left to right, it becomes NADH and the positive charge is missing, which means that its overall charge has been reduced, and it must have gained electrons in the process of being reduced. And so, essentially, what we can say is, from the left here to the right, NADH must have been reduced, it must have gained electrons, and NAD⁺ must have gained electrons. And of course, if it's gaining electrons, that means that it must have gained the electrons from the only other substrate here, L-lactate, which means L-lactate must have been oxidized or it must have lost electrons. And so, essentially, what we can say is that L-lactate, as it goes from left to right and breaks up into pyruvate and this high acid right here, it must have been oxidized. And so, essentially, there are some clues in here that can help us determine that this is an oxidation-reduction reaction. One of the clues is that NAD⁺ here is gaining a hydrogen over here. And we said previously that hydrogen transfers between molecules are a sign that electrons are being transferred, and that is a sign that it is an oxidation-reduction reaction. And so because NAD⁺ is gaining this hydrogen here, it's actually gaining the electrons that are associated with that hydrogen, and that is why it's gaining the electrons and becoming reduced here. And so, in the process, notice that the L-lactate, on the other hand, is going to be losing this set of electrons, and because it's losing the electrons, it's getting oxidized. Now, notice that the L-lactate, if you compare its overall charge to the overall charge of the pyruvate, they're exactly the same. And that's not what we would expect if a molecule is losing electrons. If it's losing negatively charged electrons, its overall charge should be getting more positive. But, we don't really see that reflected when we compare the pyruvate to the L-lactate. But, actually being broken up into not just pyruvate, it's also being broken up into this acid over here. And really, the positive charge that we feel like should be on the pyruvate is being reflected by this acid here. And so, another thing that you could have realized is that notice that the pyruvate here actually has more bonds to the oxygen atom. And so, over here there are only 3 bonds to oxygen atoms, whereas over here in the pyruvate structure, there are 4 bonds. And the more bonds that you have to the oxygen atom, the more likely you're going to be oxidized. More oxygen, more oxidation. And so, essentially, this concludes our lesson on oxidoreductases, and we'll be able to get some practice in our next video. So I'll see you guys there.

Which of the following is an indicator that a reaction is catalyzed by an oxidoreductase?

• A. Loss of a functional group.
• B. Loss of H₂O.
• C. Loss of a hydrogen.
• D. Loss of a substrate.

Types of Enzymes

So now that we've covered our 1st major class of enzyme, the oxidoreductases, we can move on to our 2nd major class of enzyme, which in our mnemonic, Over the Hill, is the t. And of course, the t stands for transferases. And transferases are enzymes that pretty much do exactly what they sound like they do and that is they catalyze the transfer of functional groups. An example of a transferase is an aminotransferase, which of course transfers amino groups between molecules. A very specific aminotransferase is the enzyme alanine transaminase, which is very important for the metabolism of a very specific amino acid, alanine.

Alanine transaminase converts L-alanine into pyruvate. In the process, it transfers the amino group from L-alanine over to alpha-ketoglutarate, and it also transfers the carbonyl group on alpha-ketoglutarate over to L-alanine. In the process, it converts L-alanine into pyruvate, and it converts alpha-ketoglutarate into L-glutamate. Moving forward, we're going to talk about a lot of different types of transferases. But for now, all I want you guys to be able to do is recognize that an enzyme that catalyzes the transfer of functional groups is likely to be a transferase. We'll be able to get a little bit of practice in our next video, so I'll see you guys there.

Kinases add phosphate groups to molecules. Kinases are an example of which class of enzyme?

• A. Oxidoreductase.
• B. Phosphorylase.
• C. Hydrolyase.
• D. Transferase.

Types of Enzymes

So now that we've covered the first two major classes of enzymes, Oxidoreductases and Transferases, we can move on to our 3rd major class of enzyme, which in our mnemonic, Over the Hill, is the h. And, of course, we know the h stands for hydrolases. And so, hydrolases are enzymes that catalyze hydrolysis reactions. Recall from our previous lesson videos that hydrolysis reactions are just reactions that break bonds by adding water. We can see that in the prefix hydro, which means water. It turns out there are many types of hydrolases, including peptidases, lipases, and nucleases, which respectively hydrolyze proteins, lipids, and nucleic acids. Recall from our previous lesson videos that trypsin is an example of a peptidase. Down below in our example, we can see that we have a polypeptide chain that extends on both sides, and we're treating our polypeptide chain with trypsin. Recall from our previous lesson videos that trypsin acts like a knight's sword. It cleaves after amino acids that have a lysine or an arginine as the r group. You can see here that this pink bond is the peptide bond that's being cleaved, and it's only cleaved in the presence of water. Over here on the right with the products, you can see that the water is incorporated into the amino group with 2 hydrogens and the carboxylate group with 1 oxygen, and that cleaves the bond and generates polypeptide fragments. Moving forward in our course, we're going to talk about a lot of different types of hydrolases, and we'll be able to get some practice with these concepts here in our next practice video. So I'll see you guys there.

Below is a reaction in the oxidative phase of the pentose phosphate pathway catalyzed by the enzyme gluconolactonase. This enzyme would be classified as what type?

• A. Transferase.
• B. Hydrolyase.
• C. Lyase.
• D. Isomerase.

Types of Enzymes

Alright. So now that we've covered the first three major classes of enzymes, oxidoreductases, transferases, and hydrolases, we can move on to our 4th major class of enzyme, which in our mnemonic Over the Hill is the I. And of course, the I stands for isomerases. And isomerases are enzymes that pretty much do exactly what they sound like, which is catalyzing the creation of an isomer. And of course, they do that by shifting functional groups around to different locations within the same exact molecule. And so recall from our previous lessons that isomers are just molecules that have the same exact chemical formula with just a different arrangement of atoms. And so a classic example of an isomerase is the enzyme triose phosphate isomerase, which produces an isomer during glycolysis. And later in our course when we cover glycolysis in more detail, we'll revisit this triose phosphate isomerase enzyme.

Now, below in our example, we're showing the reaction that triose phosphate isomerase catalyzes, and it catalyzes the conversion of dihydroxyacetone phosphate, or DHAP, into glyceraldehyde 3-phosphate, or G3P. Essentially, what you'll notice is the only difference between DHAP and G3P is the arrangement of the atoms because they have the same exact chemical formula, and therefore, they are isomers of one another. Dihydroxyacetone has the hydroxyl group on the end, and it has a carbonyl group in the middle, whereas glyceraldehyde 3-phosphate has the carbonyl group on the end and the hydroxyl group in the middle. And so again, because there is this creation of an isomer, that classifies triose phosphate isomerase as an isomerase. And so again, we'll be able to get a little bit of practice in our next practice video, so I'll see you guys there.

What class of enzyme is required to convert a cis-fatty acid into a trans-fatty acid?

• A. Oxidoreductase.
• B. Isomerase.
• C. Hydrolyase.
• D. Lyase.

Types of Enzymes

Alright. So now we're moving on to our 5th major class of enzyme, which in our mnemonic, Over the Hill, is the first l, which, of course, stands for lyase. And lyases are enzymes that either form or break bonds, and they do that without redox reactions or the addition of water. And so the way that lyases form or break bonds is via addition elimination reactions, which should sound familiar to you guys from your previous organic chemistry courses. And so one thing to note that will help you recognize reactions that are catalyzed by lyases is to know that lyases typically either create or remove double bonds or rings in the process. And so a classic example of a lyase is the enzyme pyruvate decarboxylase, which catalyzes the conversion of pyruvate into acetaldehyde and carbon dioxide. And so one thing to note here is that there is a bond being broken. So this bond right here is being broken, and in the process, this molecule right here, this portion of the molecule is being released as a carbon dioxide molecule. And what I want you guys to notice is on the left hand side of our equation over here, there are a total of 2 double bonds. But on the right hand side of our equation, notice that there are a total of 3 double bonds, which means that a double bond has been created in the process and that is a good hint that this reaction is catalyzed by a lyase. Now another thing that I want you guys to notice is that the reaction here is balanced. So if we count the total number and types of atoms on the left hand side of the equation, it will equal the total number and types of atoms on the right hand side of the equation. And so we know that water is not being added, to break this bond right here, because if water were being added, then we would expect there to be an additional oxygen atom. But on the left, we have a total of 3 oxygen atoms, and on the right, we still have a total of 3 oxygen atoms, so no water molecule was added to break this bond. And another thing that you'll notice is that, really, we're adding this hydrogen into this molecule over here, and then, again, we're releasing this part of the molecule as a CO₂. So there's no transfer of electrons from one molecule to another molecule. So this is not an oxidation-reduction reaction. And so, of course, these are the signs of, knowing that pyruvate decarboxylase is a lyase, and we'll be able to apply the concepts that we've learned here moving forward. So I'll see you guys in our next video.

Which of the following best describes the function of a lyase enzyme?

• A. To facilitate a reaction where two substrates form one product without the use of water.
• B. To facilitate a reaction where one substrate forms two products with the use of water.
• C. To cleave bonds via the transfer of electrons between molecules.
• D. To tell fibs.

Types of Enzymes

In this video, we're going to talk about our 6th and final major class of enzyme, which in our mnemonic, Over the Hill, is the second l, which, of course, stands for ligases. And ligases are enzymes that use energies, such as ATP, in order to catalyze the ligation, or the covalent joining, of 2 separate molecules together as one single molecule. And so a classic example of a ligase is DNA ligase, which is critical for DNA replication, a process that we'll talk a lot more about later in our course. Now, down below in our example image, notice that we have 2 separate DNA molecules. We've got this DNA molecule over here on the left and this DNA molecule over here on the right. And also notice that DNA ligase is able to use energy such as ATP in order to catalyze the reaction between these appropriate functional groups on these different DNA molecules, in order to covalently join these separate molecules together as one single molecule. And so in our next practice video, we'll be able to get some practice utilizing these concepts. Now, some of you may have realized that magnesium is also being utilized by DNA ligase, and that's because magnesium is a cofactor for DNA ligase. And that transitions us into our next lesson video, where we'll talk about cofactors. So, looking forward to seeing you guys there.

Types of Enzymes

Alright. In this video, we're going to do a quick recap on the 6 major classes of enzymes. And the mnemonic that helps us memorize those 6 major classes is just over the hill, where each of these letters represents the first letter of one of the major classes of enzymes. And so the o is for oxidoreductases, the t is for transferases, the h is for hydrolyases, the i is for isomerases, the first l is for lyases, and the second l is for ligases. And so remember that enzymes literally help reactions get over the hill because they all decrease the energy of activation.

Now, for the first class of enzymes, the oxidoreductases, they do pretty much exactly what they sound like, which is catalyze oxidation-reduction reactions or redox reactions. And they do that by the transfer of electrons between molecules. And so over here on the right left, what you'll notice is we have molecule A here, which has 2 pairs of electrons, these 2 blue balls, and it's transferring them over to molecule B. And so molecule B ends up with the 2 electrons and molecule A is lacking electrons. And so a mnemonic to help us memorize oxidation-reduction reactions is just Leo the lion goes ger, where Leo stands for lose electrons oxidation, and ger stands for gain electrons reduction. And so because molecule A here is losing electrons as it goes from this side over to this side here, we can say that molecule A is being oxidized. And then molecule B, on the other hand, which is going from here over to here, is gaining 2 pairs of electrons, and so for that reason, it's being reduced. So we'll write reduced over here.

Alright. So moving on to our second class of enzymes, we have the transferases which also do pretty much exactly what they sound like. They transfer things between molecules and the things that they transfer are functional groups. And so you can see that molecule A here is transferring its orange functional group here over to molecule B, and so molecule B ends up with the functional group.

So moving on to our 3rd class, we have hydrolyases, and recall that the root hydro means water. And so hydrolyases break bonds by adding water to the bond. And notice here that we have a bond here between substance A and B, a covalent bond, and through the addition of water, hydrolyases are able to cleave that bond, and notice that water is being added across this bond and now molecule A is separate from molecule B.

So moving on to isomerases, they also do pretty much exactly what they sound like, which is they create isomers. And the way that they create isomers is by just rearranging atoms within the same molecule. And so notice that the functional group here on substance A, this hydroxyl group, is on the right side of molecule A. But after the reaction, notice that the hydroxyl group is now on the left side of molecule A. So all that's happened is a rearrangement of the atoms and a creation of the isomer, and those are the types of reactions that isomerases catalyze.

Now moving on to our first l here, we have lyases, and lyases either break or form bonds, but they do so without redox reactions or water. And so here, from left to right, you can see that there's a covalent bond holding molecule A and B together, and lyases are able to catalyze the cleavage of this bond here to separate A from B. But, lyases are also able to catalyze the reverse, reaction, and we'll see that in some of our examples moving forward in our course as well. And so that's why we're saying that lyases are also able to form bonds. Now, good indications of lyases are the change in the number of double bonds that are present from the reactant to the product side and the formation or the loss of a ring. And so, those are signs that lyases could potentially be present.

And so moving on to our next, and final, major class of enzymes, we have the ligases, which use energy in order to covalently join molecules together. And so here we can see that ATP is being utilized by the ligase to covalently join molecule A and B together. You can see that a new covalent bond has been formed holding them together. And so moving forward in our course, we're going to see examples of enzymes that fall into all 6 of these classes. So it's important to familiarize yourself with the types of reactions that they catalyze.

So that concludes this recap, and I'll see you guys in our next video.

The presence of an electron acceptor such as NAD ⁺ indicates which class of enzyme?

• A. Oxidoreductase.
• B. Isomerase.
• C. Hydrolyase.
• D. Transferase.

Digestive enzymes are all examples of which class of enzyme?

• A. Ligase.
• B. Lyase.
• C. Hydrolyase.
• D. Transferase.

The reaction below is catalyzed by the enzyme arginase. What is the classification of this enzyme?

a) Hydrolyase. 

b) Oxidoreductase. 

c) Lyase. 

d) Ligase.

• A. Hydrolyase.
• B. Oxidoreductase.
• C. Lyase.
• D. Ligase.

Which class of enzyme catalyzes a reaction where the chemical formula does not change?

• A. Isomerase.
• B. Oxidoreductase.
• C. Transferase.
• D. Ligase.

What class of enzyme is required to combine two molecules using ATP?

• A. Lyase.
• B. Transferase.
• C. Oxidoreductase.
• D. Ligase.

Here’s what students ask on this topic:

What are the six major classes of enzymes?

The six major classes of enzymes are oxidoreductases, transferases, hydrolyases, isomerases, lyases, and ligases. Oxidoreductases facilitate oxidation-reduction reactions by transferring electrons. Transferases catalyze the transfer of functional groups between molecules. Hydrolyases break bonds using water, a process known as hydrolysis. Isomerases rearrange atoms within a molecule to form isomers. Lyases create or break bonds without the involvement of redox reactions or water, often forming or removing double bonds or rings. Ligases use energy, typically from ATP, to covalently join two molecules together. Understanding these classes is crucial for grasping metabolic pathways and enzyme functions in biological systems.

How do oxidoreductases function in biochemical reactions?

Oxidoreductases function by catalyzing oxidation-reduction (redox) reactions, which involve the transfer of electrons between molecules. In these reactions, one molecule loses electrons (is oxidized) while another gains electrons (is reduced). A common mnemonic to remember this is 'LEO the lion says GER,' where LEO stands for 'Lose Electrons Oxidation' and GER stands for 'Gain Electrons Reduction.' An example of an oxidoreductase is lactate dehydrogenase, which converts lactate and NAD⁺ into pyruvate, NADH, and H⁺. This enzyme is crucial for anaerobic respiration. Recognizing the role of oxidoreductases helps in understanding various metabolic pathways and energy production in cells.

What is the role of transferases in metabolic processes?

Transferases play a crucial role in metabolic processes by catalyzing the transfer of functional groups from one molecule to another. This transfer is essential for various biochemical pathways, including amino acid metabolism and nucleotide synthesis. For example, aminotransferases transfer amino groups between amino acids and keto acids, facilitating the synthesis and degradation of amino acids. A specific example is alanine transaminase, which converts L-alanine to pyruvate while transferring the amino group to α-ketoglutarate, forming L-glutamate. Understanding transferases is vital for comprehending how cells regulate and execute complex biochemical reactions.

How do hydrolyases catalyze reactions?

Hydrolyases catalyze reactions by breaking chemical bonds through the addition of water, a process known as hydrolysis. These enzymes are essential for the degradation of various biomolecules. For instance, peptidases hydrolyze peptide bonds in proteins, lipases break down lipids, and nucleases cleave nucleic acids. An example is the enzyme trypsin, a peptidase that cleaves peptide bonds at the carboxyl side of lysine or arginine residues. The reaction involves the incorporation of water, resulting in the formation of two separate fragments. Hydrolyases are crucial for digestion, cellular metabolism, and the recycling of biomolecules.

What is the function of ligases in DNA replication?

Ligases play a critical role in DNA replication by catalyzing the covalent joining of DNA fragments. DNA ligase, for example, is essential for sealing nicks in the DNA backbone, which occur during the replication process. This enzyme uses energy from ATP to form a phosphodiester bond between the 3'-hydroxyl end of one nucleotide and the 5'-phosphate end of another. This action is crucial for creating a continuous DNA strand from Okazaki fragments on the lagging strand. Without ligases, the DNA replication process would be incomplete, leading to fragmented and non-functional DNA molecules.