Biochemistry : Competitive Inhibition

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Enzyme Kinetics And Inhibition

A biochemist finds a bottle labeled "competitive inhibitor of trypsin" in his refrigerator. He finds that the  of this enzyme for trypsin is

If the biochemist uses  of this inhibitor in a solution of trypsin, what is the apparent  of this enzyme? 

Possible Answers:

Correct answer:

Explanation:

A competitive inhibitor has no effect on the  of an enzyme. 

Example Question #2 : Enzyme Kinetics And Inhibition

What happens to the  of an enzyme-catalyzed reaction when a competitive inhibitor of the enzyme is added?

Possible Answers:

 decreases

 increases by a factor of two

Cannot be determined

 does not change

 increases by a factor of 10

Correct answer:

 does not change

Explanation:

To answer this question, we need to understand what competitive inhibition is. When a competitive inhibitor is present, an enzyme's active site will be able to bind either substrate or the inhibitor. Thus, for a given substrate concentration, the reaction will be slower in the presence of the inhibitor because sometimes the inhibitor will interfere with the binding of the substrate to the enzyme's active site. However, we're not looking for reaction rate at a given substrate concentration. Instead, we are looking at the maximum possible reaction rate given any amount of substrate concentration. If we keep adding more and more substrate in the presence of the inhibitor, eventually we will get to a point where there is so much substrate present that having the inhibitor around doesn't make a difference on the reaction rate. Therefore, the addition of a competitive inhibitor has no effect on the  of an enzyme catalyzed reaction.

Example Question #3 : Enzyme Kinetics And Inhibition

What happens to  and/or  if a competitive inhibitor is added to an enzyme?

Possible Answers:

 stays the same

 increases

 remains the same

 decreases

Correct answer:

 remains the same

Explanation:

Competitive inhibitors bind the active site of enzymes, and compete with the substrate for this binding site. Thus, the  does not change since if enough substrate is added, regardless of the differential affinities between the substrate and inhibitor for the active site, the substrate will outcompete the inhibitor. However,  increases upon the addition of a competitive inhibitor.

Example Question #1 : Competitive Inhibition

A researcher adds  of competitive inhibitors to an existing solution of substrate and enzyme. The researcher notices that the effect of the enzyme decreases. What can the researcher do to increase the effect of the enzyme back to normal levels (to levels before inhibitors were added)?

Possible Answers:

Increase the concentration of substrate

Increase the volume of the solution

Decrease the volume of the solution

Nothing can be done to bring enzyme activity to normal levels

Correct answer:

Increase the concentration of substrate

Explanation:

Competitive inhibitors bind to the active site of the enzyme and prevent substrates from binding to enzyme. This prevents the enzyme-substrate reaction from happening, thereby decreasing the activity of enzymes; however, competitive inhibitors can be overcome by increasing the concentration of substrates. Increase in the amount of substrates will displace the inhibitors from the active site and allow for substrates to bind. This will bring the efficacy of the enzyme back up to normal levels.

Increasing and decreasing the volume of the solution will concentrate or dilute all species in the solution, respectively. This will not decrease the effects of competitive inhibitors on the enzyme.

Example Question #5 : Enzyme Kinetics And Inhibition

Competitive inhibitors bind to __________ on enzyme and form __________ bonds.

Possible Answers:

active site . . . weak

active site . . . strong

allosteric site . . . strong

allosteric site . . . weak

Correct answer:

active site . . . weak

Explanation:

There are two types of sites on the enzyme where molecules can bind. Active sites are the main location for substrate-enzyme binding. These sites usually involve weak, reversible bonds (such as hydrogen bonds) between substrate and enzyme. Allosteric site, on the other hand, are found at a different location on the enzyme and bind certain types of inhibitors and modulators of the enzyme. These are usually more permanent bonds (covalent bonds) and are irreversible.

Competitive inhibitors bind to active sites and form weak, reversible bonds. This is why we can dissociate competitive inhibitors from the active site by increasing the concentration of substrates. Substrates will compete for the active site and displace bound competitive inhibitors.

Example Question #6 : Enzyme Kinetics And Inhibition

Which of the following is true regarding competitive inhibition?

Possible Answers:

None of these

It decreases the maximum effect of the enzyme and it decreases the attraction between enzyme and substrate

It decreases the attraction between enzyme and substrate only

It decreases the maximum effect of the enzyme only

Correct answer:

It decreases the attraction between enzyme and substrate only

Explanation:

Competitive inhibition is characterized by an increase in the Michaelis-Menten constant, . Note that this constant represents the substrate concentration at which half the enzymes are occupied with substrate. If  increases, then it suggests that a higher concentration of substrate is needed to occupy half the enzymes.  is also a measure of the affinity between substrate and enzyme. As  increases, the affinity decreases and more substrate is required to bind 50% of the enzyme. Competitive inhibitors bind to the active site of the enzyme and compete with the substrate for the binding site on the enzyme, thereby decreasing the affinity and increasing .

Competitive inhibitors do not alter the maximum velocity of an enzyme-substrate reaction. Recall that enzymes speed up reactions; therefore, the velocity of a reaction is a direct measure of its efficacy. This means that competitive inhibitors do not alter the efficacy of the enzyme.

Example Question #2 : Competitive Inhibition

Which of the following best describes the function of competitive inhibitors?

Possible Answers:

No Change in 

Decrease the 

Increase the 

No change in 

Decrease the 

Increase in 

No change in 

Increase the 

Increase the 

Increase the 

Correct answer:

Increase the 

No change in 

Explanation:

Competitive inhibitors bind to the substrate binding site of an enzyme and have the following effect: Increase , No change in .

Noncompetitive inhibitors bind to a site other than the substrate binding site and have the following effect: No change in , Decrease in .

Increasing the , lowers the affinity since the  is the substrate concentration at which the reaction proceeds as one-half of .

Example Question #8 : Enzyme Kinetics And Inhibition

Carbon monoxide binds to hemoglobin at the same site as oxygen, and it does so with a much higher affinity - carboxyhemoglobin results.  The type of inhibition by carbon monoxide on hemoglobin is which of the following?

Possible Answers:

Competitive inhibition

Uncompetitive inhibition

No inhibition

Mixed-inhibition

Noncompetitive inhibition

Correct answer:

Competitive inhibition

Explanation:

Because carbon monoxide binds at the same site as oxygen, this is a form of competitive inhibition. In order to overcome this type of inhibition, the concentration of substrate (oxygen) needs to be increased. 

Example Question #9 : Enzyme Kinetics And Inhibition

Carbon monoxide binds to hemoglobin at the same site as oxygen, and with a much higher affinity - carboxyhemoglobin results.  What is true about the type of inhibition occurring here?

Possible Answers:

 is raised for this type of inhibition

 remains unchanged for this type of inhibition

 is lowered for this type of inhibition

This type of inhibition can be overcome by decreasing the amount of substate available

This type of inhibition can be overcome by increasing the amount of substrate available

Correct answer:

This type of inhibition can be overcome by increasing the amount of substrate available

Explanation:

The type of inhibition being described here is competitive.  The carbon monoxide binds to the same site that oxygen does.  Therefore, by increasing the amount of substrate available, the inhibitor can be outcompeted.  This is why Vmax for competitive inhibition is unchanged.  Km on the other hand, is decreased for competitive inhibition.

Example Question #10 : Enzyme Kinetics And Inhibition

Which of the following molecules is most likely to competitively inhibit an enzyme that catalyzes the reaction of ?

Possible Answers:

Glucose

Correct answer:

Explanation:

A molecule can only competitively inhibit another molecule if it fits into the same active site in the enzyme.  In the reaction,  goes into the active site of the enzyme, and so only a molecule with its similar structure can competitively inhibit it - .

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