Noncompetitive Inhibition - Online Tutor, Practice Problems & Exam Prep

Noncompetitive inhibitors are a specific type of mixed inhibitor that bind to allosteric sites on both the free enzyme and the enzyme-substrate complex, leading to a decrease in the apparent V_max without affecting the apparent K_m. This results in a constant K_m while the V_max decreases, as the substrate cannot outcompete the inhibitor. In Lineweaver-Burk plots, the slope increases due to the decreased V_max while the x-intercept remains unchanged, indicating no effect on K_m.

Noncompetitive Inhibition

In this video, we're going to talk about our 4th type of reversible inhibition, which is noncompetitive inhibition. And so, of course, noncompetitive inhibition is going to be caused by noncompetitive enzyme inhibitors. And, as you may have already assumed, noncompetitive enzyme inhibitors do not compete with the substrate for binding. That's because noncompetitive enzyme inhibitors, as we've mentioned before in some of our previous lesson videos, are really just a specific type of mixed inhibitor, and mixed inhibitors don't even mention competition whatsoever. So, of course, they're not going to compete. And so, because again, noncompetitive enzyme inhibitors are just a specific type of mixed inhibitor, noncompetitive enzyme inhibitors also have mixed binding to the enzyme and bind to allosteric sites on either the free enzyme or the enzyme-substrate complex. And as we know from our previous lesson videos, all enzyme inhibitors, regardless of what type they are, are going to lower or decrease the initial reaction velocity or the \( v_0 \) of an enzyme-catalyzed reaction. So again, no surprise here. And so, of course, ultimately, the binding of a noncompetitive inhibitor to either the free enzyme or the enzyme-substrate complex is going to prevent the conversion of the substrate into the product. And, again, we already knew this from our previous lesson videos because anytime an inhibitor is bound to the enzyme, it's going to inhibit the reaction and prevent the enzyme from catalyzing the reaction. Now, one of the defining features of noncompetitive enzyme inhibitors is that they actually bind with the same exact binding affinity to the free enzyme and to the enzyme-substrate complex, which is something different than other typical mixed inhibitors do. And so what this means is that for noncompetitive enzyme inhibitors, the inhibition constant of the free enzyme \( k_i \) is going to be exactly equal to the inhibition constant of the enzyme-substrate complex \( k'_i \), which is again, this is something different that was not true with other types of mixed inhibitors in our previous lesson videos. And so if we take a look at our example down below of noncompetitive inhibition, this image should look pretty familiar to you guys because it's actually the same exact image that we used for mixed enzyme inhibitors. And so, again, that's because, again, noncompetitive inhibitors are just a type of mixed inhibitor. And so over here on the left-hand side, we have the same exact enzyme-catalyzed reaction, and notice that the noncompetitive enzyme inhibitor can bind to the free enzyme or to the enzyme-substrate complex. And really, the main difference between mixed inhibitors and noncompetitive inhibitors is that mixed inhibitors bind with the same exact affinity to the free enzyme and to the enzyme-substrate complex. And so, again, what this means is that the mixed inhibitor will bind to the free enzyme, and the mixed inhibitor will bind to the ES with the same exact affinity. And again, we'll talk more about the effects of noncompetitive inhibitors, in our next lesson video. And we'll also relate it back to our mnemonic. So I'll see you guys in our next video.

Noncompetitive Inhibitor Effects

In this video, we're going to discuss the effects that noncompetitive enzyme inhibitors have on enzymes. Noncompetitive enzyme inhibitors do not affect the apparent Kilometers of an enzyme. This means that the apparent Kilometers of an enzyme does not change in the presence of a noncompetitive enzyme inhibitor. However, noncompetitive enzyme inhibitors do decrease the apparent v_max of an enzyme. The real question is exactly how and why do noncompetitive enzyme inhibitors have these particular effects on an enzyme?

To begin to understand this, let's take a look at our image down below. This image should look familiar to you from our previous lesson videos because it is almost exactly identical to the corresponding image from our previous lessons on mixed enzyme inhibition. Recall that noncompetitive enzyme inhibitors are just a specific type of mixed inhibitor. Noncompetitive enzyme inhibitors affect the free enzyme equally as much as they affect the enzyme substrate complex. The degree of inhibition on the free enzyme or α is going to be exactly equal to the degree of inhibition on the enzyme substrate substrate complex or α'. When α is equal to α', indicated in this region of our image, it is characteristic of a noncompetitive enzyme inhibitor. This region of the image is the only portion that differs from the corresponding mixed inhibition image from our previous lessons.

Let's go back to our text to discuss exactly how and why noncompetitive enzyme inhibitors do not affect the apparent Kilometers. This is due to Le Chatelier's principle of equilibrium between the free enzyme and the enzyme substrate complex. Noncompetitive enzyme inhibitors equally affect the free enzyme and the enzyme substrate complex. The degree of inhibition on the free enzyme, α, is going to be exactly equal to the degree of inhibition on the enzyme substrate complex, α'. The equilibrium between the two will shift to the left, but it will also equally shift to the right, meaning that overall there's no net shift of this equilibrium.

The "no" in front of noncompetitive inhibitor is quite unique among these reversible inhibitors. We can use this "no" as a memory tool to remind us that noncompetitive enzyme inhibitors do not affect the apparent Kilometers because there's no overall reaction shift in this particular equilibrium.

Now, let's move on to discuss how they decrease the apparent v_max. The noncompetitive enzyme inhibitor does not compete with the substrate. Recall here that we are using the owner of our puppy to represent the noncompetitive enzyme inhibitor. The owner is not a very competitive person, wearing a shirt that says "we are all winners". He has no interest in competing with anyone or anything, including the substrate. If the noncompetitive enzyme inhibitor does not compete with the substrate for a binding position to the enzyme's active site, the substrate cannot outcompete the noncompetitive inhibitor. The effects of the noncompetitive inhibitor will not be reversed simply by increasing the substrate concentration to saturating levels, which means that the apparent v_max is going to be decreased, and consequently, the k_cat, or the catalytic constant, or the turnover number will also decrease.

With the decrease in the k_cat, there is a decrease in the v_max. The total enzyme concentration will remain the same and will not be affected by the noncompetitive enzyme inhibitor.

Down below in this box is a little memory tool to help you remember these particular effects that noncompetitive enzyme inhibitors have. The "no" in noncompetitive inhibitor is unique, and we can use it as a memory tool to remember that noncompetitive enzyme inhibitors do not affect the Kilometers. Noncompetitive enzyme inhibitors do not allow any change in Kilometers, and if the substrate can't compete, the v_max cannot be maintained. Hopefully, this can help you remember that noncompetitive enzyme inhibitors do not affect the Kilometers, however, they do decrease the v_max.

This concludes our brief lesson on the effects of noncompetitive enzyme inhibitors. We will be able to get some practice applying these concepts and learn more as we move forward in our course. So I'll see you all in our next video.

Noncompetitive Inhibition & Michaelis-Menten Plots

So now that we know that noncompetitive enzyme inhibitors do not affect the apparent Km, but they do decrease the apparent V_max, in this video, we're going to talk about how noncompetitive inhibitors affect the Michaelis-Menten plot. Recall from our previous lesson videos that noncompetitive enzyme inhibitors are really just a specific type of mixed inhibitor, which means that noncompetitive enzyme inhibitors will have mixed binding to the enzyme and can bind either to the free enzyme or to the enzyme-substrate complex, which means that the degree of inhibition on the free enzyme and the degree of inhibition on the enzyme-substrate complex, alpha and alpha prime, will measure its degree of inhibition. And so when it comes down to it, a noncompetitive inhibitor is a mixed inhibitor where the degree of inhibition on the free enzyme alpha is exactly equal to the degree of inhibition on the enzyme-substrate complex, alpha prime. Since alpha is equal to alpha prime with a noncompetitive inhibitor, as we already know from our previous lessons, this leads to no change to the apparent Km, meaning that the apparent Km is going to be exactly equal to the Km. And, of course, the degree of inhibition on the enzyme-substrate complex alpha prime is always going to lead to a decreased V_max, a decreased apparent V_max. Here, you can see that the apparent V_max is defined as the V_max over alpha prime.

If we take a look down below at our image in our example, notice over here on the left-hand side, we're showing you the equation in the presence of a noncompetitive inhibitor. Notice that this equation is exactly the same as the equation in the presence of a mixed inhibitor, which goes to show that really a noncompetitive inhibitor is a type of mixed inhibitor, where the degree of inhibition on the free enzyme alpha is exactly equal to the degree of inhibition on the enzyme-substrate complex alpha prime. If alpha is exactly equal to alpha prime, this ratio right here is going to cancel out, and we're just going to have the Km, which shows that the Km is not being changed, and really it's just the V_max here that's going to be decreased. If we take a look over here at this Michaelis-Menten plot, notice again that all enzyme inhibitors, regardless of what type, are going to decrease the initial reaction rate or the initial reaction velocity V₀. Notice that with these two curves, this black curve here is the enzyme catalyzed reaction in the absence of inhibitor, and this blue curve right here is the enzyme catalyzed reaction in the presence of a noncompetitive inhibitor. And, of course, notice that there is this decrease in the V_max. The V_max of the enzyme catalyzed reaction in the presence of a noncompetitive inhibitor is being decreased. However, notice that with a noncompetitive enzyme inhibitor, there is not a change to the Km. The Km is exactly equal to the apparent Km. We can see that here in this Michaelis-Menten plot.

This here concludes our lesson on how noncompetitive inhibitors affect the Michaelis-Menten plot, and in our next lesson video, we'll talk about how noncompetitive inhibitors affect the Lineweaver-Burk plot. So, I'll see you guys in that video.

Noncompetitive Inhibition & Lineweaver-Burk Plots

In this video, we're going to talk about how non-competitive inhibitors affect the Lineweaver-Burk plot. And so, again, recall that way back in some of our previous lesson videos, we talked about shifting Lineweaver-Burk plots. A lot of the skills that we developed in those older videos are going to be very useful here in this lesson. If you don't remember much about shifting Lineweaver-Burk plots, make sure to go back and check out those older videos before you continue here. Now that being said, it turns out that the slope of the line on a Lineweaver-Burk plot, which is, of course, the ratio of the Kilometers/Vmmax, is actually going to increase. So it increases with more non-competitive inhibitor. And so the reason for this is because recall from our previous lesson videos that non-competitive enzyme inhibitors always decrease the apparent Vmax, but they have no effect on the apparent Kilometers. And so if the Kilometers is unchanged but the Vmax is actually being decreased, a smaller number in the bottom half of this denominator is actually going to increase the slope as we've already mentioned. And so also recall that a Lineweaver-Burk plot is known as a double reciprocal plot. And so even though the apparent Vmax decreases, the y-intercept, which is the reciprocal of the Vmax, is going to increase in the presence of a non-competitive inhibitor. But, of course, since there's no effect on the apparent Kilometers in the presence of a non-competitive inhibitor, the x-intercept, which is the negative reciprocal of the Kilometers, is also not going to be affected. So it's going to stay exactly the same. And so, if we take a look down below at our image, notice on the left-hand side, we have the Lineweaver-Burk equation in the presence of a non-competitive inhibitor. And so really all we need to do is take the Michaelis-Menten equation in the presence of a non-competitive inhibitor, which we covered in our last lesson video, and take the reciprocal of that and that's what will give us this equation here. And so over here on the right, what we have is a Lineweaver-Burk plot. The black line here represents the enzyme catalyzed reaction in the absence of inhibitor, whereas, this purple line here represents the enzyme catalyzed reaction in the presence of non-competitive inhibitor. And so, notice that even in the presence of the non-competitive inhibitor, the Kilometers is not being affected, and we can tell because the x-intercept is not changing. However, notice that the y-intercept of the line in the presence of inhibitor is actually being increased as we mentioned up above. The y-intercept is increasing. However, an increased y-intercept is getting further away from this 0 marker, which we know acts as the infinity marker for the Vmax. And so because it's getting further away from the infinity marker, the Vmax is actually being decreased as we mentioned up above. And so, if we were to add even more non-competitive inhibitor, if we were to go ahead and add plus 2, concentration of inhibitor, of course, the Kilometers is still not going to change, so we're gonna have the same exact x-intercept. But the, Vmax is going to decrease even further. And so we're gonna have a line that looks something along the lines of this. And so notice that in the presence of more non-competitive inhibitor, the slope actually increases further. And so this here concludes our lesson on how non-competitive inhibitors affect Lineweaver-Burk Plots. In our next video, we'll be able to get some practice utilizing all these concepts. So I'll see you guys there.

Indicate with an 'x' which of the kinetic parameters would be altered in the presence of the given inhibitor.

Alright. So, this practice problem wants us to indicate with an x in the table down below, which of the kinetic parameters would be altered in the presence of the given inhibitors. And so, we already know from our previous lesson videos that competitive enzyme inhibitors are going to increase the KM. So, with competitive enzyme inhibitors, KM is increased. And so, we know that KM is going to be increased, but because there is competition between the competitive inhibitor and the substrate, it's possible for the substrate to out compete the inhibitor and therefore it can keep the Vmax. And so what that means is that the Vmax is not going to be altered in the presence of a competitive inhibitor, but the KM is going to be increased. And so, we can go ahead and mark with an x right here that only the km is going to be increased by a competitive inhibitor. Now, with non-competitive inhibitors, recall what we want to focus on is this 'no', and the 'no' tells us that there is no change to the KM. And so the KM is going to stay the same. And so, if the KM stays the same, then the KM is not going to be affected by non-competitive inhibitors. However, we know that all enzyme inhibitors, regardless of what type, are going to decrease the initial reaction velocity, which will lead to a decrease in the Vmax as well if the substrate can't compete with the inhibitor. And since non-competitive inhibitors are not competitive, that means that the Vmax is going to decrease. And so, what we can say is that the Vmax is going to decrease, but the KM is going to be unaltered. So, this here concludes our practice problem, and I'll see you guys in our next video.

What can be determined from the following Lineweaver Burk plot?

• A. Data collected in the absence (solid line) & presence (dashed line) of a competitive inhibitor.
• B. Data collected in the absence (solid line) & presence (dashed line) of a noncompetitive inhibitor.
• C. Data collected in the absence (dashed line) & presence (solid line) of a competitive inhibitor.
• D. Data collected in the absence (dashed line) & presence (solid line) of a noncompetitive inhibitor.

How would you expect the line on a Lineweaver-Burk plot to change if the enzyme was treated with a noncompetitive inhibitor?

• A. The y-intercept would move up (away from the origin).
• B. The x-intercept would move left (away from the origin).
• C. The y-intercept would move down (toward the origin).
• D. The x-intercept would move right (toward the origin).

Here’s what students ask on this topic:

What is noncompetitive inhibition in enzymes?

Noncompetitive inhibition is a type of enzyme inhibition where the inhibitor binds to an allosteric site, which is a site other than the enzyme's active site. This binding can occur whether the enzyme is free or bound to the substrate. Noncompetitive inhibitors do not compete with the substrate for the active site, and their binding decreases the enzyme's maximum reaction velocity (V_max) without affecting the Michaelis constant (K_m). This means that the enzyme's affinity for the substrate remains unchanged, but the overall rate of the reaction is reduced.

How do noncompetitive inhibitors affect the Michaelis-Menten plot?

In a Michaelis-Menten plot, noncompetitive inhibitors decrease the maximum reaction velocity (V_max) without changing the Michaelis constant (K_m). This results in a lower plateau for the reaction rate at high substrate concentrations. The curve in the presence of a noncompetitive inhibitor will show a reduced V_max but the same K_m as the uninhibited reaction. This indicates that the enzyme's affinity for the substrate is unchanged, but the overall catalytic efficiency is reduced.

What is the effect of noncompetitive inhibitors on Lineweaver-Burk plots?

In Lineweaver-Burk plots, noncompetitive inhibitors increase the slope of the line because they decrease the apparent V_max while leaving the K_m unchanged. The y-intercept, which is the reciprocal of V_max, increases, indicating a lower V_max. The x-intercept, which represents -1/K_m, remains the same, showing that K_m is unaffected. This results in a steeper line in the presence of a noncompetitive inhibitor compared to the uninhibited reaction.

Why do noncompetitive inhibitors not affect the Michaelis constant (K_m)?

Noncompetitive inhibitors do not affect the Michaelis constant (K_m) because they bind to an allosteric site on the enzyme, not the active site where the substrate binds. This means that the inhibitor does not interfere with the substrate's ability to bind to the enzyme. As a result, the enzyme's affinity for the substrate remains unchanged, and thus the K_m remains constant. The primary effect of noncompetitive inhibitors is to decrease the maximum reaction velocity (V_max).

How do noncompetitive inhibitors differ from competitive inhibitors?

Noncompetitive inhibitors differ from competitive inhibitors in their binding sites and effects on enzyme kinetics. Noncompetitive inhibitors bind to an allosteric site, not the active site, and can bind to both the free enzyme and the enzyme-substrate complex. This decreases the maximum reaction velocity (V_max) without affecting the Michaelis constant (K_m). In contrast, competitive inhibitors bind to the active site, directly competing with the substrate. This increases the apparent K_m (indicating decreased affinity) but does not change the V_max since the inhibition can be overcome by increasing substrate concentration.