GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with Inhibitors

Study concepts, example questions & explanations for GRE Subject Test: Biochemistry, Cell, and Molecular Biology

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Example Questions

Example Question #1 : Help With Inhibitors

A researcher has designed a new type of inhibitor that binds at the active site of an enzyme. What type of inhibition does this molecule display?

Possible Answers:

Noncompetitive inhibition

Suicide inhibition

Uncompetitive inhibition

Competitive inhibition

Correct answer:

Competitive inhibition

Explanation:

Because the inhibitor binds at the active site, it is actively competing with the ligand for access to the enzyme. This type of inhibitor displays competitive inhibition. Competitive inhibition can be overcome by adding excessive amounts of substrate. If the amount of substrate greatly out-measures the amount of inhibitor, then the substrate will still bind the enzyme very frequently and allow the reaction to proceed.

Noncompetitive inhibitors bind an enzyme at a spot that is not the active site. Uncompetitive inhibitors bind the enzyme-substrate complex, once the substrate has already entered the active site. Suicide inhibitors "kill" enzymes, typically by making permanent modifications to amino acids in the active site.

Example Question #2 : Help With Inhibitors

On a Lineweaver-Burk plot, an inhibited enzyme is shown to have a less negative x-intercept than the uninhibited enzyme, but the y-intercept remains the same. The type of inhibition displayed is __________ and the inhibited reaction has a __________ value.

Possible Answers:

competitive . . . larger

non-competitive . . . larger

non-competitive . . . smaller

competitive . . . smaller

Correct answer:

competitive . . . larger

Explanation:

The x-intercept on a Lineweaver-Burk plot tells us the negative reciprocal of .

Because the x-intercept is less negative, this tells us that the inhibited reaction has a larger . Having a different x-intercept but the same y-intercept is characteristic of competitive inhibition. The inhibitor and the substrate are competing for the same binding site. 

Example Question #3 : Help With Inhibitors

Which of the following choices describes a way to graphically determine the type of inhibition being displayed by an inhibitor?

I. Plot initial reaction rate versus the concentration of substrate for the uninhibited enzyme, and then compare to the inhibited enzyme

II. Plot the inverse of the initial reaction rate versus the inverse of the substrate concentration for the uninhibited enzyme, and then compare to the inhibited enzyme

III. Plot the concentration of the inhibitor versus the concentration of substrate

Possible Answers:

I, II, and III

I and II

II only

I only

Correct answer:

I and II

Explanation:

Plotting the concentration of the inhibitor versus the concentration of the substrate will not give you any useful information because the reaction rate is essential in determining the type of inhibitor present.

Plotting initial reaction rate versus substrate concentration, or plotting the inverses, describes the graphical representation of Michaelis-Menten kinetics and a Lineweaver-Burk plot, respectively. Both of these are excellent methods to visually determine the type of inhibition displayed. On the graph, the line representing the inhibited enzyme will shift in predictable fashions depending on the type of inhibition. 

Example Question #4 : Help With Inhibitors

You have an enzyme solution and you add an inhibitor molecule and observe a marked decrease in enzyme activity. You increase the substrate concentration but this does not lead to any observable increase in enzyme activity. What can you conclude about your inhibitor?

Possible Answers:

That it is a noncompetitive inhibitor

That is it an inorganic inhibitor

That it is a competitive inhibitor

That it binds the enzyme's active site

That it is a kinase

Correct answer:

That it is a noncompetitive inhibitor

Explanation:

Noncompetitive inhibitors bind to enzymes away from the active site (allosteric) and distort it, reducing its affinity for substrate. Since they do not directly compete with substrate for enzyme binding, increasing the substrate concentration in the presence of a noncompetitive inhibitor will have no affect. While enzyme inhibitors include both organic and inorganic molecules, there is not enough information in the question stem to conclude the chemical classification of the inhibitor.

All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

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