Physical Chemistry : Inhibitors

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #16 : Equilibrium And Kinetics

Which of the following is true regarding competitive inhibition?

I. They form covalent bonds with the active site

II. They are reversible

III. They are similar to allosteric inhibitors

Possible Answers:

II and III

II only

III only

I and II

Correct answer:

II only

Explanation:

Competitive inhibition decreases enzyme activity by binding to the active site of the enzyme. Recall that active sites are sites on enzymes where the substrates bind. Upon binding to the enzyme, the substrates undergo changes that facilitate and speed up the chemical reaction. A competitive inhibitor binds to this active site and prevents the substrate from binding. With no binding, the substrate will not undergo the necessary changes and, subsequently, the chemical reaction. A key characterisitic of competitive inhibitors is that the bond between the inhibitor and the active site is reversible. This means that the chemical bonds involved here are weak, reversible noncovalent bonds such as hydrogen bonds and van der Waals forces. Covalent bonds are very strong and are usually found in irreversible interactions.

Allosteric inhibitors are molecules that bind to enzymes at their allosteric site(s). In this way, the allosteric inhibiton is very similar to, and is a subset of, another type of enzyme inhibition, noncompetitive inhibition.

Example Question #17 : Equilibrium And Kinetics

A researcher is analyzing the effects of an unknown inhibitor. He observes that the inhibition can be overcome by increasing the concentration of the substrate. What can you conclude about this inhibitor?

Possible Answers:

The inhibitor increases the 

The inhibitor decreases the 

The inhibitor decreases the Michaelis constant

The inhibitor increases the Michaelis constant

Correct answer:

The inhibitor increases the Michaelis constant

Explanation:

Recall that competitive inhibition can be overcome by increasing substrate concentration. Competitive inhibitors alter the Michaelis constant, , but maintain the  (maximum reaction rate). Inhibitors act to decrease the reaction rate. To figure out the effect of competitive inhibitors on the Michaelis constant, we need to look at the Michaelis-Menten equation.

where  is reaction rate,  is maximum reaction rate,  is substrate concentration, and  is the Michaelis constant. Since reaction rate is inversely proportional to the , competitive inhibitors will increase  and, thereby, decrease reaction rate.

Example Question #18 : Equilibrium And Kinetics

The slope of a Lineweaver-Burk plot is  and the x-intercept is . Upon addition of a noncompetitive inhibitor the slope increases to . Which of the following is the correct value of  (Michaelis constant) after the addition of the inhibitor? 

Possible Answers:

Cannot be determined from the given information

Correct answer:

Explanation:

To answer this question we need to first figure out the equation for slope and x-intercept of Lineweaver-Burk plot. The Linweaver-Burk plot is a graphical way to plot the Michaelis-Menten equation. It is defined as the reciprocal of Michaelis-Menten equation. Michaelis-Menten equation is as follows.

where  is reaction rate,  is maximum reaction rate,  is substrate concentration, and  is the Michaelis constant. Taking the reciprocal of this gives us

The slope, therefore, is . The x-intercept can be found by plugging in zero for the Y value (the reaction rate, ). The x-intercept is .

 The question states that the slope is  and the x-intercept is . Using the equation for x-intercept we can solve for .

Using the equation for slope we can solve for 

Recall that the addition of a noncompetitive inhibitor alters the  but not the ; therefore,  is still  after the addition of noncompetitive inhibitior.

Note that if we were asked to solve for the , we would have had to use the new slope () and the same  value ().

Example Question #19 : Equilibrium And Kinetics

Which of the following is true regarding noncompetitive inhibitors?

I. They do not form covalent bonds with the active site

II. They alter both Km and Vmax

III. They alter the shape of the active site

Possible Answers:

III only

II and III

I and III

I only

Correct answer:

III only

Explanation:

Noncompetitive inhibitors bind to enzymes and prevent the formation of the enzyme-substrate complex. These inhibitors bind to a location other than the active site. Upon binding, the inhibitors alter the conformation of the active site and prevent the binding of substrate. They form covalent bonds with the enzyme; therefore, these are irreversible inhibitors and are hard to remove.  and  are altered by both types of inhibitors (competitive and noncompetitive). Competitive inhibitors alter the  whereas the noncompetitive inhibitors alter the .

This implies that competitive inhibition can be overcome by increasing substrate concentration whereas noncompetitive inhibition cannot. Molecularly this makes sense. Competitive inhibitors bind reversibly to the active site and prevent binding of substrate. If we were to drastically increase its concentration, substrate will compete with and remove the competitive inhibitor from the active site. Noncompetitive inhibitors, on the other hand, alter the conformation of the active site, making it hard for substrates to bind to the active site; therefore, the substrate will not be able bind, regardless of the substrate concentration.  

Example Question #20 : Equilibrium And Kinetics

Upon addition of an inhibitor, which of the following is expected to happen in a catalytic reaction?

Possible Answers:

The amount of products will decrease because the equilibrium is shifted to the left

It will take longer to reach equilibrium

The amount of reactants will increase because the reverse reaction will be favored

The amount of reactants will increase because the the forward reaction is halted

Correct answer:

It will take longer to reach equilibrium

Explanation:

Inhibitors are molecules that prevent the action of catalysts. They bind to catalysts and prevent substrate binding, thereby halting the catalytic action. Since catalysts increase the speed of a reaction, addition of an inhibitor will lower the speed of the reaction. This does not mean that the reaction will stop proceeding; it simple means that it will take longer for the reaction to complete (reach equilibrium).

Remember that a catalyst speeds up both the forward and the reverse reaction; therefore, inhibitors will slow down both reactions. As mentioned, inhibitors will only slow down the reaction. The amount of products produced (equilibrium) will not change, although it will take longer for products to form.

Example Question #1 : Inhibitors

__________ inhibitors bind to the active site and __________ inhibitors alter the binding affinity of substrate and catalyst.

Possible Answers:

Noncompetitive . . . noncompetitive

Competitive . . . competitive

Competitive . . . noncompetitive

Noncompetitive . . . competitive

Correct answer:

Competitive . . . competitive

Explanation:

There are two main types of inhibitors. Competitive inhibitors bind to the active site of the catalyst and prevent substrate from binding. This phenomenon causes a decreasing in the binding affinity of substrate and catalyst. However, competitive inhibitors can be overcome by adding excess substrates. The substrates will dissociate the competitive inhibitor and carry out the reaction; therefore, the reaction can still be carried out at a faster rate and the maximum rate of reaction is not altered.

Noncompetitive inhibitors bind to the catalyst at an allosteric site. They alter the conformation of the active site and prevent substrate binding. They cannot be overcome by addition of excess substrate; therefore, they lower the maximum rate of reaction.

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