Activation Energy: Videos & Practice Problems

The activation energy barrier is influenced by four primary factors: entropy, proper orientation of substrates, distortion of substrates, and solvation. Enzymes reduce entropy by restricting substrate motion, enhancing collision likelihood. They also ensure proper orientation, increasing reaction chances. Distortion into a high-energy transition state is stabilized by enzymes, lowering activation energy. Lastly, enzymes desolvate substrates, removing hydration shells that hinder reactions. Together, these mechanisms significantly accelerate chemical reactions, demonstrating the crucial role of enzymes in biological systems.

Activation Energy

In this video, we're going to begin our discussion on the primary factors that contribute to the activation energy. Recall from our previous lesson videos that the activation energy, or the energy of activation, can be symbolized with either of these two symbols, and really it is the energy barrier that exists between the substrates or the reactants and the transition state. This energy of activation must be overcome in order to initiate the reaction and to convert the substrates into the products.

Just to briefly refresh our memories, let's take a look at this little energy diagram over here on the right. Notice that we have the reactants or the substrates at this position and then we have the transition state way up here, which is a transient entity that exists at a local maximum peak energy point of a reaction. Literally, the activation energy is just the difference in energy between the two here. This activation energy is what controls the kinetics or the speed of a reaction. The greater the energy of activation barrier is, the longer it will take for the reaction to take place, so the slower the reaction will go.

Here we have a question and it's asking us what factors actually contribute to the energy of activation barrier? It turns out that there are four factors that primarily contribute to the energy of activation barrier. The first is entropy. The second is the proper orientation of the substrates. The third is the distortion of the substrate. And then, of course, the fourth is salvation. Moving forward in our course, we're going to briefly describe each of these four factors in their own separate videos. The major takeaway is that the binding energy that's released when an enzyme forms non-covalent interactions with its substrate will actually influence each of these four factors in order to decrease the energy of activation.

In our next lesson video, we're going to start with the first factor, which is a reduction in entropy. I'll see you guys in that next video.

Which of the following best describes the energy of activation for a reaction?

• A. The difference in free energy between the substrate and product.
• B. The difference in entropy between the substrate and transition state.
• C. The difference in free energy between the substrate and the transition state.
• D. The difference in entropy between the product and transition state.

Answer: (C)

Activation Energy

So in our last lesson video, we said that there are 4 factors that primarily contribute to the activation energy barrier. And in this video, we're going to focus on our first factor, which has to do with entropy. Enzymes are able to reduce the entropy and the random motion of their substrates. And so here's the setup for how this works. When there's no enzyme present, molecules in solution require collisions in order for the reactions to take place. But the problem is that molecules in solution also have lots of random motion. And so this random motion leads to a high amount of local entropy in the system. Now, what we mean by random motion is that these substrates are moving in random directions, and the chances that they're going to be moving in a direction that allows for a collision to take place is much lower. And so the more random motion that there is, the lower the likelihood is that the substrates are actually going to collide and the lower the likelihood is of the reaction taking place, and that will ultimately increase the energy of activation and slow down the entire reaction. Now, guys, this is exactly where enzymes come into play because enzymes are able to decrease the local entropy of the substrates in the reaction system by restricting the random motion of the substrates and bringing them closer together. And this will ultimately lead to the increased likelihood of the substrates reacting, which will decrease the energy of activation and speed up the entire chemical reaction.

So let's take a look at our example below to get a better visual of how enzymes reduce entropy. And so notice on the left over here in this blue box, we have a scenario where there's no enzyme present. In this scenario, notice that our substrates are moving in random directions, so we have the random motion of the substrates in solution. And notice that substrate A is moving in a random direction toward the left and substrate B is moving in a random direction towards the bottom right. And so remember that these substrates need to be moving towards each other for a collision to take place in order for the reaction to occur. And so when there's lots of random motion, the chances of the reaction occurring decreases. And so what we're saying is that there's higher randomness in this scenario and this higher randomness leads to a higher energy of activation and that will ultimately slow down the reaction.

Now over here on the right, we have a scenario where there is an enzyme present, and the enzyme is this red structure that we see here. And so we can see that the substrates in this scenario are restricted, and so the restricted motion of the substrates in the enzyme-substrate complex will ultimately lower the randomness of the substrates, which will lower the energy of activation and speed up the entire chemical reaction.

Now, some of you might be thinking, wait a second, Jason. Didn't you say way back in our previous lessons that processes should increase universal entropy? So how is it that enzymes can get away with reducing entropy? Well, that's a great thought. And so essentially, the process is very similar to the hydrophobic effect. And so you can see that the molecules in solution are interacting with the solvent, and there is a hydration shell around these solvent molecules. And so notice that when the enzyme-substrate complex forms, the hydration shell is removed. And so the water molecules that are removed in the hydration shell are able to increase the universal entropy even though the enzyme is decreasing the entropy of the local substrates. And so, that's a great thought. And that essentially wraps up our lesson on how enzymes are able to reduce entropy and random motion in order to decrease the energy of activation and speed up chemical reactions. And so in our next lesson video, we'll talk about the second factor that contributes to the activation energy barrier. So I'll see you guys in that video.

Activation Energy

So in this video, we're going to talk about our second primary factor that contributes to the activation energy barrier, and that is the proper orientation of substrates. Now in our last lesson video, we said that when enzymes are not present, substrates in solution require a collision in order to react. But, not only do substrates in solution require a collision in order to react, they also require collision in a proper orientation. And so that's very important to keep in mind. Properly oriented random collisions between substrates in solution can be a pretty rare event, and that can ultimately increase the energy of activation and slow down the entire chemical reaction. Now, this is exactly where enzymes come into play because enzymes can use their binding energy to properly orient the functional groups of substrates and increase the chances for a reaction to take place, and that will ultimately decrease the energy of activation and speed up the entire chemical reaction. So let's take a look at our example down below just to get a visual on how enzymes properly orient their substrates. Now over here on the left side of our image in this blue box, we have a scenario where there is no enzyme that's present. And so in this scenario, notice that there is an improper orientation of the random collision. And so notice that substrate a has its functional group pointing in a downwards direction, and substrate b has its functional group pointing in a backwards direction. And so even though these two substrates, their random motion allows them to collide with one another, they do not have the proper orientation of the collision. And so the functional groups, because they are not properly oriented, they will not react, so there will be no reaction. Now, over here on the right, we have a scenario where there is an enzyme that's present, and, again, the enzyme is the red structure. And so you can see that with the enzyme, there is the proper orientation of the substrates in the enzyme substrate complex. And so notice that the functional groups are now in the proper position to be able to react with one another, and so the functional groups are going to actually react. And so now we can see how enzymes are able to properly orient their substrates in order to decrease the energy of activation and speed up chemical reactions. So, in our next lesson video, we're going to talk about the third factor that contributes to the activation energy barrier, and so I'll see you guys in that video.

Which of the following statements is false with respect to an enzyme's ability to catalyze a reaction?

• A. An enzyme provides a reaction surface & suitable environment for a reaction to take place.
• B. An enzyme binds substrates to position them correctly & allow them to attain transition state configurations.
• C. An enzyme allows a reaction to go through a less stable transition state than normal.
• D. a & c are false.

Answer: (C)

Activation Energy

In this video, we're going to talk about the 3rd primary factor that contributes to the activation energy barrier, and that is the distortion of a substrate. Many reactions require the distortion of a substrate into an unstable, high energy transition state. These distortions lead to an increased energy of activation, which ultimately slows down the entire chemical reaction. And again, this is exactly where enzymes come into play because through the induced fit model, enzymes are able to form interactions with the transition state throughout the reaction, and the released binding energy can be used to stabilize distortions in the transition state and decrease the energy of activation, allowing for the entire reaction to occur faster. Now, down below in our example images, we can better visualize how enzymes can stabilize transition state distortions.

Over here on the far left in this blue box, we have a scenario where there is no enzyme present. Notice that our substrate group over here is going to interact with the carboxyl group. But as it is right now, they are so far apart that they cannot interact. Therefore, the only way that they can get close enough to each other is if our substrate becomes distorted and takes on a high energy unstable transition state, where it bends in such a way that the amino group can be in close proximity to the carboxyl group. But again, this is going to lead to a high energy of activation, and that's going to slow down the entire chemical reaction.

The remaining boxes over here with the yellow background are scenarios where an enzyme is present, and, of course, the enzyme is the red structures that are shown. When the enzyme is first introduced, we know that the enzyme-substrate complex is going to form. From our previous lessons on the induced fit model, we know that the active site of the enzyme is not going to be perfectly shaped for the substrate, and instead, the active side of the enzyme is better suited for the transition state because we want to stabilize the transition state. Over here, we can see that the enzyme induced fit model allows for the enzyme to change confirmations, and it also allows for the substrate to change confirmations into the transition state. The enzyme induced fit model actually stabilizes the distortions in the transition state. You can see we have this really distorted transition state here where the amino group is close to the carboxyl group, but our bar is bent in a way that it creates an unstable high-energy molecule. However, our enzyme, through these non-covalent bonds that form, is able to stabilize this transition state. This lowers the energy of activation and allows for the reaction to proceed faster. Therefore, it allows for the products to be released at a faster rate.

You can see that a new bond was able to form between the amino and carboxyl group over here, and we were able to split our substrate bar in half here, to break it open, and we were also able to release water. Here, you can see how enzymes are indeed able to stabilize transition state distortions in order to decrease the energy of activation and allow the reaction to occur faster. Really, this is one of the major ways that enzymes influence the energy of activation, by stabilizing the distortions of a substrate. In our next lesson video, we'll talk about the 4th and final primary factor that contributes to the activation energy barrier. So, I'll see you guys in that video.

Which of the following is not a way that enzymes increase rates of reactions?

• A. Binding of substrates in close proximity.
• B. Covalently binding to the substrate.
• C. Stabilization of the transition state.
• D. Conformational changes in binding site slightly increases E_A.

Answer: (D)

Activation Energy

In this video, we're going to talk about our 4th and final factor that contributes to the activation energy barrier, and that is solvation, or solvent interactions. And of course, in biological systems, the solvent is water. And so the solvation or the hydration shells that surround substrates in aqueous solutions can actually interfere with substrate reactions. And of course, this interference is going to increase the energy of activation, which will ultimately slow down the chemical reaction. And of course, this is where enzymes come into play again because enzymes are capable of removing the potentially hydration shells and replacing the hydration shell interactions with enzyme substrate interactions that decrease the energy of activation and speed up the chemical reaction. And so down below in our example, we can better visualize how enzymes desolvate substrates. And so on the left-hand side of our image in the light blue box, we have a scenario where there is no enzyme present. And notice that even though the substrates are properly oriented and they are randomly moving towards each other, that the hydration shells that surround the substrates are capable of acting as blockers and essentially interfere with the reaction. And so the interference of the reaction is going to lead to a high energy of activation, which is going to slow down the chemical reaction. And so over here on the right, in the image with a little yellow background, this is a scenario where an enzyme is present. And of course, the enzyme is this red structure here. And we can see that the hydration shells are actually removed in the enzyme substrate complex, and now the hydration shells cannot interfere with the reaction, and so that leads to a lower energy of activation, which will speed up the chemical reaction. And so what you'll notice is that even though the enzyme is lowering the entropy of the system, the local entropy of the substrates, the universal entropy is still increasing because these water molecules that break free from the hydration shell are going to increase the universal entropy, similar to the way that we saw it happen in the hydrophobic effect in our previous lesson videos. And so, essentially, what we can see here is that enzymes are capable of decreasing the energy of activation simply by desolvating substrates and removing the hydration shells. And so that concludes this lesson here, and we'll be able to get some practice utilizing these concepts in our next video. So I'll see you guys there.

Here’s what students ask on this topic:

What is activation energy and why is it important in chemical reactions?

Activation energy (E_a) is the minimum amount of energy required for a chemical reaction to occur. It represents the energy barrier that must be overcome for reactants to be converted into products. This concept is crucial because it determines the rate at which a reaction proceeds. Higher activation energy means a slower reaction rate, as fewer molecules have the necessary energy to reach the transition state. Enzymes play a vital role in biological systems by lowering the activation energy, thereby speeding up reactions and making them more efficient.

How do enzymes lower the activation energy of a reaction?

Enzymes lower the activation energy of a reaction through several mechanisms: 1) Reducing entropy by restricting substrate motion, which increases the likelihood of collisions. 2) Properly orienting substrates to enhance the chances of a reaction. 3) Stabilizing the high-energy transition state through the induced fit model, which reduces the energy required for the reaction. 4) Desolvating substrates by removing hydration shells, which can interfere with reactions. These actions collectively decrease the activation energy, allowing the reaction to proceed faster.

What role does entropy play in the activation energy of a reaction?

Entropy, in the context of activation energy, refers to the randomness or disorder of the substrates in a reaction. High entropy means substrates are moving randomly, reducing the likelihood of effective collisions. Enzymes reduce local entropy by restricting substrate motion and bringing them closer together, which increases the chances of a successful reaction. This reduction in entropy lowers the activation energy, thereby speeding up the reaction. However, the overall universal entropy still increases, as the process releases water molecules from hydration shells, similar to the hydrophobic effect.

How does the proper orientation of substrates affect the activation energy?

Proper orientation of substrates is crucial for reducing activation energy. In solution, substrates must collide in a specific orientation for a reaction to occur. Random collisions often do not meet this requirement, leading to a higher activation energy and slower reaction rate. Enzymes facilitate proper orientation by binding substrates in their active sites, aligning functional groups correctly. This increases the likelihood of a successful reaction, thereby lowering the activation energy and speeding up the reaction.

What is the induced fit model and how does it relate to activation energy?

The induced fit model describes how enzymes change shape to better fit the substrate upon binding. This model is crucial for lowering activation energy. When the enzyme and substrate form a complex, the enzyme undergoes a conformational change that stabilizes the transition state. This stabilization reduces the energy required to reach the transition state, thereby lowering the activation energy. The induced fit model ensures that the enzyme is most effective at stabilizing the high-energy, distorted transition state, facilitating a faster reaction.