GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Enzyme Regulation

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Example Questions

Example Question #11 : Enzymes

Which of the following is an example of allosteric regulation of enzymes?

Possible Answers:

The non-covalent binding of cAMP somewhere other than the active site

Phosphorylation of an amino acid in the active site

Phosphorylation of an amino acid somewhere other than the active site

The non-covalent binding of cAMP to the active site

Correct answer:

The non-covalent binding of cAMP somewhere other than the active site

Explanation:

The difference between the binding of cAMP and phosphorylation is that the latter is a covalent modification. Covalent modifications are a different way to regulate proteins, and do not fall under the category of allosteric regulation. Allosteric regulation only occurs outside of the active site, often simply called an allosteric site. The non-covalent binding of cAMP to a region of an enzyme outside of the active site thus qualifies as allosteric regulation.

Example Question #12 : Enzymes

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

Competitive inhibition

Uncompetitive 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 #13 : Enzymes

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

competitive . . . smaller

non-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 #14 : Enzymes

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 #2 : 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 competitive inhibitor

That it binds the enzyme's active site

That it is a noncompetitive inhibitor

That it is a kinase

That is it an inorganic inhibitor

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.

Example Question #21 : Enzymes

How do competitive inhibitors affect enzyme efficiency?

Possible Answers:

Raise the maximum rate of the enzymatic reaction

Lower the maximum rate of the enzymatic reaction

Raise the Michaelis constant

Lower the Michaelis constant

Correct answer:

Raise the Michaelis constant

Explanation:

Competitive inhibitors can be overpowered by introducing excess substrate, so they do not affect the maximum rate of the enzyme. They do, however, make it so that more substrate is required in order to get the enzyme working at half of its maximum rate. As a result, competitive inhibitors act by raising the Michaelis constant of enzymes.

Example Question #21 : Enzymes

How does a noncomeptitive inhibitor affect an enzyme?

Possible Answers:

Lowers the Michaelis constant of the enzyme

Lowers the maximum rate of the enzymatic reaction

Raises the maximum rate of the enzymatic reaction

Raises the Michaelis constant of the enzyme

Correct answer:

Lowers the maximum rate of the enzymatic reaction

Explanation:

A noncompetitive inhibitor acts to decrease how fast the enzyme can act on substrates. It accomplishes this by lowering the maximum rate at which it can create products. Noncompetitive inhibitors do not alter the enzyme's Michaelis constant.

Example Question #23 : Enzymes

How is pepsinogen activated in the stomach?

Possible Answers:

A portion is cleaved, activating the enzyme

Cofactors bind to the enzyme, increasing its efficiency

It is phosphorylated by another enzyme

It is activated by the temperature change in the stomach lumen

Correct answer:

A portion is cleaved, activating the enzyme

Explanation:

Once in the stomach lumen, pepsinogen finds itself in a very acidic environment. The acidic environment cleaves an amino acid sequence from pepsinogen, turning it into the active enzyme pepsin. This type of activation causes pepsin to only activate in the stomach lumen where it is needed.

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