Biochemistry : Uncompetitive Inhibition

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Uncompetitive Inhibition

An uncompetitive inhibitor binds to which of the following?

Possible Answers:

The active site of the enzyme before the substrate has a chance to bind

An allosteric site on the enzyme, regardless of whether or not the substrate is already bound to the active site

An allosteric site on the enzyme, only when the substrate is already bound to the active site

An allosteric site on the enzyme only when the substrate has not yet bound to the active site

The active site of the enzyme at the same time as the substrate

Correct answer:

An allosteric site on the enzyme, only when the substrate is already bound to the active site

Explanation:

Uncompetitive inhibition occurs when an inhibitor binds to an allosteric site of a enzyme, but only when the substrate is already bound to the active site. In other words, an uncompetitive inhibitor can only bind to the enzyme-substrate complex.

Example Question #2 : Uncompetitive Inhibition

Which of the following changes occurs when an uncompetitive inhibitor binds to the enzyme-substrate complex?

Possible Answers:

 increases

 remains unchanged

 decreases

 remains unchanged

 increases

Correct answer:

 decreases

Explanation:

Uncompetitive inhibition occurs when an inhibitor binds to an allosteric site on the enzyme, but only when it is an enzyme-substrate complex. Because the inhibitor binds to the enzyme-substrate complex and then changes the enzyme's conformation, it makes it incredibly difficult for the substrate to become unbound from the enzyme. Thus, the apparent affinity of the substrate for the enzyme is dramatically increased. A decrease in  represents an increase in affinity.  still decreases when an uncompetitive inhibitor binds.

Example Question #3 : Uncompetitive Inhibition

Which of the following is true about noncompetitive inhibition?

Possible Answers:

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

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

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

Vmax stays the same, however Km increases

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

Correct answer:

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

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. 

Example Question #4 : Uncompetitive Inhibition

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

Possible Answers:

There effect can be countered by adding more substrate.

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

They only affect enzymes that act on multiple substrates.

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

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

Correct answer:

They only affect enzymes that act on multiple substrates.

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.

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