Biochemistry : Noncompetitive Inhibition

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Noncompetitive Inhibition

When inhibition takes place at a site that is not the active site, this is called __________ inhibition.

Possible Answers:

trans-competitive

None of these

competitive

substrate

allosteric

Correct answer:

allosteric

Explanation:

An allosteric (meaning "other site") inhibition will involve binding of a molecule to a site other than the active site. Competitive inhibition involves the binding of an inhibitor molecule to the active site of an enzyme. Both forms of inhibition decrease the rate of an enzyme-catalyzed reaction.

Example Question #2 : Noncompetitive Inhibition

There are at least four types of glucose transporter in the body. GLUT1 and GLUT3 are located in most tissues including the brain and the red blood cells. These glucose transporters rapidly take up glucose from the blood but have the lowest  value. GLUT2 is commonly found in the liver and the pancreas. GLUT2 has a lower affinity for glucose but has the highest  value. GLUT4 is common in skeletal tissues and in adipose tissues. This transporter is normally not active for uptake unless stimulated by insulin or during exercise.  

Suppose there is a molecule that is able to lower the  of the enzyme. What type of regulation would this molecule exhibit? 

Possible Answers:

Noncompetitive inhibition

Cooperative stimulation

Cooperative inhibition 

Uncompetitive stimulation 

Competitive inhibition 

Correct answer:

Noncompetitive inhibition

Explanation:

Molecules that are able to bind to an enzyme and prevent it from reaching  are noncompetitive inhibitors. Take carbon monoxide for example. The molecule binds to the hemoglobin but stays attached to it. Oxygen is not able to break the covalent bond between the carbon monoxide and the heme group and is therefore a noncompetitive. With carbon monoxide permanently attached to the heme, the hemoglobin cannot reach full saturation of oxygen and therefore  is lowered.

Example Question #3 : Noncompetitive Inhibition

A molecule binds to the allosteric site of an enzyme. What can you conclude about this molecule?

I. The molecule cannot be a competitive inhibitor

II. The molecule can increase the activity of the enzyme

III. The molecule will decrease the affinity between enzyme and substrate

Possible Answers:

I and III

III only

I only

I and II

Correct answer:

I and II

Explanation:

Allosteric sites on an enzyme bind both enhancers and inhibitors. Since the question only states that it binds to allosteric site, the molecule could be an inhibitor or an enhancer of the enzyme. Recall that inhibitors inhibit the activity of the enzyme whereas enhancers increase the activity of the enzyme.

Noncompetitive inhibitors and/or activators bind to the allosteric site. Competitive inhibitors, on the other hand, bind to the active site; therefore, this molecule cannot be a competitive inhibitor. Competitive inhibitors decrease the affinity between enzyme and substrate whereas noncompetitive inhibitors do not alter the affinity. Since we already determined it cannot be a competitive inhibitor, this molecule cannot decrease the affinity.

Example Question #4 : Noncompetitive Inhibition

Which of the following is true regarding noncompetitive inhibition?

Possible Answers:

More that one of these are true

Noncompetitive inhibition decreases the maximum efficacy of the enzymes

Substrate can never bind to the enzyme in the presence of a noncompetitive inhibitor

Noncompetitive inhibition decreases the affinity of the enzyme to the substrate

Correct answer:

Noncompetitive inhibition decreases the maximum efficacy of the enzymes

Explanation:

Noncompetitive inhibition is characterized by a decrease in the maximum velocity (or efficacy) of an enzyme. Noncompetitive inhibitors bind irreversibly to the enzyme and prevent the substrate-enzyme activity. This decreases the efficacy of the enzyme. Unlike competitive inhibition, noncompetitive inhibition cannot be overcome by increasing the concentration of substrates because of the irreversible interaction between inhibitor and enzyme.

Noncompetitive inhibition does not alter the Michaelis-Menten constant, . This means that the affinity between enzyme and substrate is not altered in noncompetitive inhibition. Noncompetitive inhibitors bind to allosteric sites on the enzyme and prevent the substrate-enzyme interaction by altering the active site. Sometimes, noncompetitive inhibitors allow for substrate-enzyme interaction but inactivate the activity of the enzyme; therefore, noncompetitive inhibitors allow for substrate binding sometimes, but they always prevent the enzyme activity and the enzymatic reaction.

Example Question #5 : Noncompetitive Inhibition

Upon analysis, it is determined that the interaction between an inhibitor and an enzyme involves the formation of bonds between nitrogen and hydrogen atoms in adjacent molecules. Which of the following is true regarding this molecule?

Possible Answers:

More than one of these

It binds to the allosteric site of the enzyme

It decreases the affinity between the substrate and enzyme

Its effects can be overcome by increasing substrate concentration

Correct answer:

More than one of these

Explanation:

The question states that the bond between nitrogen and hydrogen atoms occur between adjacent molecules; therefore, this is an intermolecular bond. Recall that hydrogen bonds are intermolecular bonds that occur between hydrogen atoms and either nitrogen, oxygen, or fluorine atoms. This means that the intermolecular bond involved in this question is a hydrogen bond. Hydrogen bonds (like other intermolecular bonds) are reversible and can be broken by applying some energy. Since the bond between the inhibitor and the enzyme is reversible, the inhibitor must be a competitive inhibitor. Noncompetitive inhibitors, on the other hand, bind irreversibly (via covalent bonds) to the allosteric site on the enzyme.

Competitive inhibitors can be overcome by increasing substrate concentration. This occurs because the reversible, weak bonds between the inhibitor and enzyme can be broken when there is excess substrate present (substrate competes with the competitive inhibitors for the enzyme). Competitive inhibitors also increase the Michaelis-Menton constant, . Increasing  decreases affinity between substrate and enzyme.

Example Question #6 : Noncompetitive Inhibition

If an enzymatic reaction is interrupted by the presence of a non-competitive inhibitor, which of the following best describes how the reaction kinetics will be effected? 

Possible Answers:

Only  will be decreased

Only  will be increased

Only  will decrease

Both  and  decrease

 decreases but  increases

Correct answer:

Only  will be decreased

Explanation:

Non-competitive inhibitors work by binding the enzyme without hindering the substrate's access to the active site. Therefore, the affinity of the enzyme to its substrate is not impacted , however it does negatively impact the enzyme's ability to form the final product. Therefore, the maximum velocity  of the reaction is decreased. 

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