Types of Inhibition

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Biochemistry › Types of Inhibition

Questions 1 - 10
1

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?

More than one of these

It binds to the allosteric site of the enzyme

Its effects can be overcome by increasing substrate concentration

It decreases the affinity between the substrate and enzyme

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.

2

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?

Only will be decreased

Only will be increased

Only will decrease

Both and decrease

decreases but increases

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.

3

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?

Only will be decreased

Only will be increased

Only will decrease

Both and decrease

decreases but increases

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.

4

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?

More than one of these

It binds to the allosteric site of the enzyme

Its effects can be overcome by increasing substrate concentration

It decreases the affinity between the substrate and enzyme

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.

5

Which of the following is true about noncompetitive inhibition?

The inhibitor binds independently of substrate binding however km does not change

The inhibitor binds to the same site as the substrate, dropping the Km

The inhibitor competes with the substrate to bind to the active site, and drops the Vmax

The inhibitor binds to a separate site from the substrate and enhances enzyme activity

Vmax stays the same, however Km increases

Explanation

With uncompetitive inhibitors, the inhibitor binds to a site separate from the binding site of the substrate. This can occur even while the substrate is bound to the enzyme, blocking the process and reduce the catalysis of the enzyme.

This will result in the reduction of Vmax because the enzymes ability for catalysis is being reduced by the binding of inhibitor to the enzyme-substrate complex. Km does not change because the substrate and the uncompetitive inhibitor bind to different sites.

6

Which of the following is true about noncompetitive inhibition?

The inhibitor binds independently of substrate binding however km does not change

The inhibitor binds to the same site as the substrate, dropping the Km

The inhibitor competes with the substrate to bind to the active site, and drops the Vmax

The inhibitor binds to a separate site from the substrate and enhances enzyme activity

Vmax stays the same, however Km increases

Explanation

With uncompetitive inhibitors, the inhibitor binds to a site separate from the binding site of the substrate. This can occur even while the substrate is bound to the enzyme, blocking the process and reduce the catalysis of the enzyme.

This will result in the reduction of Vmax because the enzymes ability for catalysis is being reduced by the binding of inhibitor to the enzyme-substrate complex. Km does not change because the substrate and the uncompetitive inhibitor bind to different sites.

7

The oxidation of glucose to two molecules of pyruvate produces a net two molecules of ATP during glycolysis. ATP allosterically inhibits the enzyme, PFK-1, that catalyzes the third step of glycolysis. This is an example of which fo the following mechanisms?

Feedback inhibition

Feed-forward activation

Negative cooperativity

Competitive inhibition

Noncompetitive inhibition

Explanation

This is an example of feedback inhibition, as feedback inhibition is a mechanism in which a molecule binds to an enzyme to decrease its activity. The mechanism is now balanced. Blocking an enzyme typically helps correct a metabolic imbalanace or assists in destruction of a pathogen. In this case the ATP binds to a site other than the protein's active site, and since it blocks PFK-1, a feedback inhibition has occurred.

8

Which of the following is true of uncompetitive inhibitors of enzymes?

They only affect enzymes that act on multiple substrates.

They bind to both the enzyme-substrate complex and the free enzyme.

They lower the concentration of free enzyme available to bind to substrate.

They decrease the apparent KM and increase the apparent VM on a Lineweaver-Burke plot.

There effect can be countered by adding more substrate.

Explanation

The correct answer is that uncompetitive inhibitors of enzymes only affect enzymes that act on multiple substrates. Uncompetitive inhibitors bind to the enzyme-substrate complex only, not to the free enzyme. They distort the active site to prevent the enzyme from being catalytically active without actually blocking the binding of the substrate. This cannot occur with an enzyme that only acts on a single substrate at a time. Adding more substrate and lowering the amount of free enzyme available both apply to competitive inhibitors, which bind to free enzymes and block the substrate-binding site of the enzyme. Uncompetitive inhibitors do decrease the apparent KM on a Lineweaver-Burke plot, but they also lower the apparent VM.

9

Which of the following is true of uncompetitive inhibitors of enzymes?

They only affect enzymes that act on multiple substrates.

They bind to both the enzyme-substrate complex and the free enzyme.

They lower the concentration of free enzyme available to bind to substrate.

They decrease the apparent KM and increase the apparent VM on a Lineweaver-Burke plot.

There effect can be countered by adding more substrate.

Explanation

The correct answer is that uncompetitive inhibitors of enzymes only affect enzymes that act on multiple substrates. Uncompetitive inhibitors bind to the enzyme-substrate complex only, not to the free enzyme. They distort the active site to prevent the enzyme from being catalytically active without actually blocking the binding of the substrate. This cannot occur with an enzyme that only acts on a single substrate at a time. Adding more substrate and lowering the amount of free enzyme available both apply to competitive inhibitors, which bind to free enzymes and block the substrate-binding site of the enzyme. Uncompetitive inhibitors do decrease the apparent KM on a Lineweaver-Burke plot, but they also lower the apparent VM.

10

The oxidation of glucose to two molecules of pyruvate produces a net two molecules of ATP during glycolysis. ATP allosterically inhibits the enzyme, PFK-1, that catalyzes the third step of glycolysis. This is an example of which fo the following mechanisms?

Feedback inhibition

Feed-forward activation

Negative cooperativity

Competitive inhibition

Noncompetitive inhibition

Explanation

This is an example of feedback inhibition, as feedback inhibition is a mechanism in which a molecule binds to an enzyme to decrease its activity. The mechanism is now balanced. Blocking an enzyme typically helps correct a metabolic imbalanace or assists in destruction of a pathogen. In this case the ATP binds to a site other than the protein's active site, and since it blocks PFK-1, a feedback inhibition has occurred.

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