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 Inhibition - Online Tutor, Practice Problems & Exam Prep
Noncompetitive Inhibition
Video transcript
Noncompetitive Inhibitor Effects
Video transcript
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 vmax 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 vmax. 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 vmax is going to be decreased, and consequently, the kcat, or the catalytic constant, or the turnover number will also decrease.
With the decrease in the kcat, there is a decrease in the vmax. 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 vmax cannot be maintained. Hopefully, this can help you remember that noncompetitive enzyme inhibitors do not affect the Kilometers, however, they do decrease the vmax.
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
Video transcript
So now that we know that noncompetitive enzyme inhibitors do not affect the apparent Km, but they do decrease the apparent Vmax, 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 Vmax, a decreased apparent Vmax. Here, you can see that the apparent Vmax is defined as the Vmax 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 Vmax 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 Vmax. The Vmax 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
Video transcript
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.
Problem Transcript
What can be determined from the following Lineweaver Burk plot?
How would you expect the line on a Lineweaver-Burk plot to change if the enzyme was treated with a noncompetitive inhibitor?