Enzyme Kinetics - Online Tutor, Practice Problems & Exam Prep

Enzyme kinetics studies the rate of enzyme-catalyzed reactions, symbolized by reaction velocity (v). Key factors influencing reaction rates include temperature, substrate concentration, and the presence of catalysts. Increasing temperature can enhance reaction rates but risks protein denaturation. Substrate concentration can also boost rates, though it requires significant energy and may lead to overcrowding. Enzymes serve as effective catalysts, allowing higher reaction rates with minimal amounts. Understanding variables like total enzyme concentration (E_T), free substrate concentration ([S]), and reaction rates (v_max) is crucial for grasping enzyme kinetics.

Enzyme Kinetics

In this video, we're going to begin our discussions on enzyme kinetics. So enzyme kinetics is just a branch of biochemistry that's related to the rate or the velocity of an enzyme catalyzed reaction. And so, of course, it's going to be measured by the reaction rate or the reaction velocity, which we refreshed our memories on in our last lesson video. So we know that it's symbolized with the lowercase letter v. And so enzyme kinetics actually has a lot of different factors that are incorporated into it. And so moving forward in our next couple of videos, we're going to slowly introduce the most important factors when it comes to enzyme kinetics that you guys need to be familiar with, and we're going to start with how to increase the reaction rate and the first way is to increase the temperature of the system.

Now this method actually provides an issue to cells because increasing the temperature can nonspecifically increase all the reaction rates in the system, and that is typically not what cells want to do. Typically, cells want to have control over exactly what reaction is going to increase the reaction rate. And if you increase the temperature too much, that could eventually lead to the denaturation of proteins. And so if we take a look at our first method down below in our example, notice that we have a graph that has the reaction rate on the y-axis and the temperature on the x-axis. And notice that in the beginning, as the temperature increases, so does the reaction rate. But again, the reaction rate of all the reactions in the systems is going to increase, not just the very particular reaction that a cell might be interested in increasing. Now the reaction rate will only increase up to a point where proteins in the cell begin to denature. And once proteins in the cell begin to denature, the cell is going to become stressed and eventually die. And so over time, the reaction go to method go to method that cells typically use to increase their reaction rates.

Now the second main way to increase the rate of reaction is to increase the substrate concentration. Now this method also provides an issue for cells because increasing the substrate concentration takes lots of time and energy in order to make enough substrate that will actually increase the reaction rate enough. And so when we make so much substrate, that could potentially create overcrowding in the already limited space within the cellular environment. And so if we take a look at our second method down below in our example, notice we have a graph again that has the reaction rate on the y-axis, but this time we have the substrate concentration on the x-axis. And notice that as the substrate concentration increases, so does the reaction rate. But notice that in order to get the reaction rate up to a point that's significantly high enough, we would have to add so much substrate where the amount of substrate is going to go off the graph, and that is a lot of substrate. If we were to add this much substrate, that could create overcrowding within the cell. And so for these reasons, this is not the typical go-to method for how cells increase their reaction rates.

So moving on to our third method that increases reaction rates, you can add a catalyst to the reaction. And so this method here provides a solution for cells on how to increase their reaction rates because living systems only need to use very small amounts of enzymes in order to increase their reaction rates. And so if we take a look at our third method down below in our example, notice we have a graph again that has the reaction rate on the y-axis, and again, the substrate concentration on the x-axis. So we have this same curve over here for the uncatalyzed reaction that we had before. And so the point here is that when we add an enzyme, so this red curve represents the enzyme catalyzed reaction, notice that with lower concentrations of substrate, we're able to get much higher reaction rates. And so just by adding small amounts of an enzyme, we're able to increase the reaction rates drastically. And that's why, utilizing enzymes is the go-to method for cells and living systems to increase their reaction rates. And so, moving forward, we're going to be able to get some practice utilizing these concepts, so I'll see you guys in those videos.

Assuming the [S] is always saturating the enzymes (E), which of the plots below is correct?

• A.
• B.
• C.
• D.

Which of the following is not a method of increasing a reaction's rate?

• A. Increase the [substrate].
• B. Increase the [enzyme].
• C. Increase the [product].
• D. Slightly increase the temperature.

Enzyme Kinetics

So in our last lesson video, we said that enzyme kinetics is measured by the reaction rate or the speed of a reaction. And we also said that there are many factors or variables to consider when it comes to enzyme kinetics. And so in this video, we're going to introduce all of the enzyme kinetics variables. And notice down below in our example, we have all of these enzyme kinetics variables. And that's a lot to remember. But don't worry because moving forward in our course, we're going to slowly break down each one of these new enzyme kinetic variables one by one so that you guys can better understand how they relate to enzyme kinetics. Getting started here, what I want you guys to first recognize is that the first four enzyme kinetics variables are review from our previous lesson videos, and so not all of these variables are new information. Some of it is review information.

We already know that the E stands for enzyme. And we also already know that brackets mean the concentration. And so this symbol [E] is the concentration of free enzyme molecules. Now we also know that s is for substrate, and so this symbol [S] represents the concentration of free substrate molecules. ES is, of course, going to be the enzyme-substrate complex, and so this is going to be the concentration of the enzyme-substrate complex. And then last but not least, p, we already know stands for the product. And so this symbol [P] is the concentration of free product molecules. Again, this is almost half of our table vertically, so that is a good sign that it's all review, and there's not that much new information here.

But our first new variable is this variable right here, which has the letters E and T in it. So what could this possibly represent? It's going to be the total amount of enzyme, the total concentration of enzyme. So the t here is for total, and so what this means is it's going to include both the enzyme that is free, that is not bound to the substrate, and it's also going to include the enzyme that is bound to the substrate in the enzyme-substrate complex. The sum of both of these variables that we already covered up above is going to be this new variable right here ET. This one here doesn't take a lot of memory effort. It's pretty simple. Just remember, the t is for total.

Here we have these variables that are new, and again, these are going to be the ones that, moving forward in our course, we're going to break down one by one so that we can better understand how they work and connect to enzyme kinetics. The first one that we're going to talk about here is this lowercase k, which stands for the rate constant. The next one that we have here is this k with a little m, which stands for the Michaelis constant km. Then this one over here that we have is kcat, and cat stands for the catalytic constant. Moving in our course, we're going to talk about each one of these variables.

Over here on this side, we already know that v is the variable for the reaction rate. And so these v's here are also for the reaction rate or the reaction velocities. What makes them different are the little subscripts that they have. So this one has a 0 and is termed v₀. This is going to be the initial reaction velocity at the very beginning of a reaction. And then this one here, which is v_max, is of course going to be the maximum reaction velocity. Kinetic variables, and again, moving forward in our course, we're going to be breaking each one of these down starting with the rate constant. So I'll see you guys in our next lesson video where we'll cover that variable.

Which of the following options represents the total concentration of enzyme?

• A. [E][ES].
• B. [ES][E].
• C. [E] + [ES].
• D. [ES]/[E].
• E. [E]/[ES].

Enzyme Kinetics

So before we actually talk about the rate constant variable in our next topic, I first want to point out something that's really important to note about when biochemists are trying to study enzymes in a laboratory. And so this video might get a little bit confusing for you guys, but that's okay because there's only one thing that I want you guys to take away from this video. Not only are we going to cover that one thing throughout the video, but by the end of the video, I'll do a quick recap to make sure you guys know exactly what that one thing is that I want you to take away from this video. That being said, let's go on and get started.

First, I want you guys to recall from our previous lesson video that we defined the total enzyme concentration or ET as the sum of the free enzyme concentration plus the concentration of enzyme that's present in the enzyme substrate complex. We distinguished the total enzyme concentration from the free enzyme concentration, and we did that because we have this enzyme substrate complex that forms. Now, however, do we also need to consider the same for the substrate? Do we need to consider that the total substrate concentration is equal to the sum of the free substrate concentration plus the concentration of substrate in the enzyme substrate complex?

Well, it turns out that even though we know that enzyme kinetics is affected by both the total enzyme concentration and the total substrate concentration, the answer to this question is no. We do not need to consider this expression. That means that we do not need to define the total enzyme concentration as a separate variable from the free enzyme concentration. That's exactly why in our last lesson video we did not include the total substrate concentration.

Now, why is it exactly that we don't need to consider this expression? The answer turns out to be that under typical laboratory conditions when we're studying enzymes in a lab, the total substrate concentration is in great excess over the total enzyme concentration. So there's much, much more substrate than there is enzyme. We can say that the total substrate concentration is going to be much greater than the total enzyme concentration. This means that the amount of substrate that's going to be bound to the enzyme substrate complex at any given time throughout the reaction is going to be pretty much negligible in comparison to the total substrate concentration. If the substrate concentration that's bound to the enzyme substrate complex is negligible, this means that the enzyme substrate complex portion is super small in comparison to the total substrate concentration, which means that in this expression we can pretty much eliminate this entire concentration.

What that means is we can say that the total substrate concentration is going to be equal to the free substrate concentration. Down below we're saying the same exact thing: the total substrate concentration can really be expressed as the free substrate concentration. So, they're pretty much the same exact thing which is why we didn't define this variable in our last lesson video. The reason that this portion is true is that the substrate concentration is always so much greater than the total enzyme concentration, which means that the substrate concentration will also be much greater than the concentration of the enzyme substrate complex.

A little graph below will help remind you guys that the substrate concentration, here in blue, is going to be much greater, notice it's much greater, than the enzyme concentration here in red. The total enzyme concentration remains small and relatively stable under laboratory conditions. Notice here that we have a star, and this star is going to come back up later in our course when we're talking about Michaelis-Menten enzyme kinetics and the assumptions of Michaelis-Menten enzyme kinetics. But again, we're not going to talk about that until a little later in our course. But for now, what I want you guys to know is that this star is going to represent something later in our course. So keep that in mind.

Like I said, there's only one thing that I really want you guys to take away from this video, and that is the conclusion here. Moving forward, in our course, we are going to say that the substrate concentration, the free substrate concentration, is actually going to represent the total amount of substrate. That's pretty much exactly what we've been saying, that the substrate concentration, the free substrate concentration, is about equal to the total substrate concentration. You’re not going to see us use this variable moving forward; you'll only see us using this variable and that is the only takeaway that I want you guys to take from this video. So that concludes this video and I'll see you guys in our next one.

Here’s what students ask on this topic:

What is enzyme kinetics and why is it important?

Enzyme kinetics is the study of the rates of enzyme-catalyzed reactions. It is important because it helps us understand how enzymes work, how they can be regulated, and how they interact with substrates and inhibitors. This knowledge is crucial for various applications, including drug development, metabolic engineering, and understanding disease mechanisms. By studying enzyme kinetics, scientists can determine the efficiency of enzymes, identify potential targets for pharmaceuticals, and design better industrial catalysts.

How does temperature affect enzyme activity?

Temperature affects enzyme activity by influencing the reaction rate. As temperature increases, the reaction rate typically increases due to higher kinetic energy, which leads to more frequent collisions between enzyme and substrate molecules. However, if the temperature becomes too high, it can cause the enzyme to denature, losing its functional shape and thus its activity. Therefore, there is an optimal temperature range where the enzyme activity is at its peak, beyond which the activity declines sharply due to denaturation.

What is the Michaelis-Menten constant (Km) and what does it signify?

The Michaelis-Menten constant (Km) is a key parameter in enzyme kinetics that represents the substrate concentration at which the reaction rate is half of its maximum velocity (Vmax). It provides insight into the affinity of the enzyme for its substrate; a low Km indicates high affinity, meaning the enzyme can achieve half-maximal activity at a low substrate concentration. Conversely, a high Km suggests low affinity, requiring a higher substrate concentration to reach half-maximal activity. Understanding Km helps in characterizing enzyme efficiency and designing inhibitors.

What is the difference between Vmax and V0 in enzyme kinetics?

In enzyme kinetics, Vmax represents the maximum reaction velocity achieved by the enzyme when the substrate concentration is saturating, meaning all enzyme active sites are occupied. It reflects the enzyme's catalytic capacity. On the other hand, V0 (initial reaction velocity) is the reaction rate measured at the very beginning of the reaction, when the substrate concentration is not yet depleted and product formation is minimal. V0 is used to determine kinetic parameters like Km and Vmax through initial rate experiments.

Why is substrate concentration important in enzyme kinetics?

Substrate concentration is crucial in enzyme kinetics because it directly influences the reaction rate. At low substrate concentrations, the reaction rate increases linearly with substrate concentration. As substrate concentration continues to rise, the reaction rate approaches a maximum velocity (Vmax) where all enzyme active sites are saturated with substrate. Understanding the relationship between substrate concentration and reaction rate helps in determining key kinetic parameters like Km and Vmax, which are essential for characterizing enzyme behavior and designing effective inhibitors.