7. Enzyme Inhibition and Regulation

Degree of Inhibition

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concept

## Degree of Inhibition

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all right. So now that we've introduced the inhibition constant, the next topic that we're going to cover before we actually get into the details of the different types of reversible inhibitors is the degree of inhibition. So recall from our previous lesson videos, we said that enzyme inhibitors by definition will decrease the initial reaction rate, or velocity V not of an enzyme catalyzed reaction. But here we have a question, and really, it's just asking how Combine oh chemists measure the inhibitor effects on the initial reaction Velocity V, not oven enzyme catalyzed reaction. And it turns out that the answer to this question is the degree of inhibition. And so the degree of inhibition is really just a factor that measures and quantifies how much an enzyme is actually being inhibited by the inhibitor. And so recall from our previous lesson videos that enzyme inhibitors can either bind to the free enzyme to form the E I complex or enzyme inhibitors could also bind to the enzyme substrate complex toe form, the E s I complex, And because enzyme inhibitors can bind to either the free enzyme or the enzyme substrate complex, biochemist can actually separately measure the degree of inhibition on the free enzyme and the enzyme substrate complex. And so, in our next lesson video, we're going to focus on the degree of inhibition on the free enzyme. But later, in a separate video, we'll talk about the degree of inhibition on the enzyme substrate complex, and so I'll see you guys in our next video.

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concept

## Degree of Inhibition

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All right, so in this video, we're going to talk about the degree of inhibition on the free enzyme. And so the degree of inhibition on the free enzyme is represented with the Greek variable Alfa. And so the degree of inhibition on the free enzyme Alfa is just equal to one plus the ratio of the concentration of inhibitor over the inhibition constant that we talked about in our previous lesson videos. And so this degree of inhibition on the free enzyme Alfa is a unit lis factor that is always greater than or equal to one. And so here you can see that Alfa will always be greater than or equal to one. Now, when there's no inhibitor present at all, that means that Alfa is going to equal exactly one. And then as Alfa begins to increase, then the concentration of inhibitor will also increase. So, ah, value of Alfa greater than one means that there is inhibitor present. And of course, the greater Alfa is the Mawr inhibitor is present. And so if we take a look at our image down below notice over here on the left hand side, what we have is the same enzyme catalyzed reaction that we've seen so many times before in our previous lesson videos. And so here in this video we're specifically focusing on again the degree of inhibition on the free enzyme Alfa. And so that means that the inhibitor here is gonna be on Lee inhibiting the free enzyme as we see here. And so, of course, we know that when the inhibitors bound to the enzyme, no reaction is able to take place. And so here what we can see again, What we're emphasizing is that the degree of inhibition on the free enzyme is equal to the variable Alfa and Alfa is just equal to again one plus this ratio here of the concentration of inhibitor over the inhibition constant. And so, if we take a second and think about what this here actually means, we know that of course, the concentration of inhibitor is either gonna be zero or some positive number. We're never gonna have a negative number for the concentration of inhibitor. And we know from our previous lesson videos that the inhibition constant K I also represents a specific concentration of inhibitor. It's the exact concentration of inhibitor that allows for half of maxim maximum inhibition. And so if we have to concentrations here that are both positive, then that means that this ratio here is going to be positive. And of course, any positive number here plus one, is going to be greater than one. And in this equation right here, there's no way that Alfa is going to be less than one. And so here in our chart below where it says Alfa is less than one, what we can write is that it's simply not possible. Toe have an Alfa less than one. And we're never going to see that, uh, in our course and so also noticed that, uh, if Alfa is exactly equal toe one, as we mentioned up above in our lesson, Uh, if Alfa is equal to one, that means that there's going to be no inhibitor present. So essentially the inhibitor is going to be absent. Or we could just say that there is no inhibitor present when alphas equal to one. Now, in our last scenario here, where you can see if Alfa is greater than one. That means that the inhibitor is indeed present and it will be inhibiting the enzyme and so we'll be able to get some. I'll be able to show you guys an example of how to utilize this degree of inhibition on the free enzyme Alfa on our next video. So I will see you guys there.

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example

## Degree of Inhibition Example 1

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All right. So here we have an example. Problem that's asking. According to the data in the table below, shown right here which enzyme is affected most by the inhibitor is an enzyme, a enzyme be or enzyme See. And so, taking a look at this table over here, notice that for each of these, three enzymes were given the degree of inhibition that the inhibitor has on each enzyme. And so, of course, the enzyme that has the greatest degree of inhibition is gonna be the one that's going to be inhibited the most and so enzyme be because it has the greatest degree of inhibition here. It's going to be the one that's inhibited the most. And so we could go ahead and say that it's enzyme be here. That is, uh, the correct answer for this example. Now, of course, we know that if the degree of inhibition is equal to one, then what that means is Alfa will be equal toe one. And if this is the case, that means that there will be essentially no inhibitor present, essentially as if the inhibitor were absent. And so notice here that enzyme C has an Alfa that is really close to equaling toe one. It's 1.2 And so what this means is that really the inhibitor has a very, very small, small effect on, uh, enzyme see, And it doesn't really inhibit enzyme. See that much. And so the main take away here that we want to get is that the greater the degree of inhibition, the mawr, that enzyme will be inhibited by that enzyme inhibitor. And so this here concludes our example and I'll see you guys in our next video, where we'll be able to get some practice.

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Problem

ProblemCalculate the degree of inhibition of an inhibitor on the free enzyme if the [I] = 3 Î¼M and the K_{I} = 6 Î¼M.

A

Î± = 1.5

B

Î± = 2

C

Î± = 4

D

Î± = 3.5

E

Î± = 0.5

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concept

## Degree of Inhibition

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All right. So now that we've covered the degree of inhibition on the free enzyme in this video, we're going to specifically talk about the degree of inhibition on the enzyme substrate complex. And so the degree of inhibition on the enzyme substrate complex or the E s complex is represented as Alfa Prime in the prime is this little apostrophe that we see here. And so the degree of inhibition on the enzyme substrate complex Alfa Prime off course is going to quantify the effect that an inhibitor has on the enzyme substrate complex. And so, looking down at our image below notice, what we have is the same enzyme catalyzed reaction that we've seen so many times before in our previous lessons and notice that this time the inhibitor is affecting the enzyme substrate complex toe form, the E s I complex. And of course, we know that any time the inhibitor is bound to the enzyme like it is here, the reaction is not going to be able to get take place. And so here in this video, what we're emphasizing is that the degree of inhibition on the enzyme substrate complex is represented as Alfa Prime And of course, it's going to be very, very similar to the degree of inhibition on the free enzyme. And so it could be represented as one plus this ratio of the concentration of inhibitor over the inhibition constant but notice that it actually uses the inhibition constant on the enzyme substrate complex. K prime I instead of the inhibition complex Thean addition constant on the free enzyme K I and so this k prime I it is going to correspond with the Alfa Prime. And so you might be wondering, Why is it that we're talking about these degree of inhibitions Alfa and Alfa Prime? Well, it turns out that both Alfa and Alfa Brian uh, degree of inhibitions are going to be used as factors to modify. The McHale is meant in and the line Weaver Burke equations that were already familiar with from our previous lesson videos. And so Alfa and Alfa Prime will modify these two equations specifically in the presence of inhibitors, and we'll be able to talk more about how Alfa and Alfa Prime can be used as factors in these two equations. Ah, little bit later in our course, but for now this concludes our introduction to the degree of inhibition on the enzyme substrate complex, Alfa Prime, and we'll be able to get some practice utilizing these concepts in our next couple of videos, so I'll see you guys there.

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Problem

ProblemCalculate the degree of inhibition of an inhibitor on the ES-complex if the [I] = 8 Î¼M and the K'_{I} = 0.03 mM.

A

Î± = 0.87

B

Î± = 1.08

C

Î± = 1.27

D

Î± = 2.39

E

Î± = 0.27